Below you will find our publications.

74.
N. Yeow, R. F. Tabor and G. Garnier
Mapping the distribution of specific antibody interaction forces on individual red blood cells.
Scientific Reports 7 (2017) 41956.

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Current blood typing methods rely on the agglutination of red blood cells (RBCs) to macroscopically indicate a positive result. An indirect agglutination mechanism is required when blood typing with IgG forms of antibodies. To date, the interaction forces between anti-IgG and IgG antibodies have been poorly quantified, and blood group related antigens have never been quantified with the atomic force microscope (AFM). Instead, the total intensity resulting from fluorescent-tagged antibodies adsorbed on RBC has been measured to calculate an average antigen density on a series of RBCs. In this study we mapped specific antibody interaction forces on the RBC surface. AFM cantilever tips functionalized with anti-IgG were used to probe RBCs incubated with specific IgG antibodies. This work provides unique insight into antibody-antigen interactions in their native cell-bound location, and crucially, on a per-cell basis rather than an ensemble average set of properties. Force profiles obtained from the AFM directly provide not only the anti-IgG – IgG antibody interaction force, but also the spatial distribution and density of antigens over a single cell. This new understanding might be translated into the development of very selective and quantitative interactions that underpin the action of drugs in the treatment of frontier illnesses.

73.
V. S. Raghuwanshi, J. Su, C. J. Garvey, S. A. Holt, W. Raverty, R. F. Tabor, P. J. Holden, M. Gillon, W. Batchelor and G. Garnier
Bio-deuterated cellulose thin films for enhanced contrast in neutron reflectometry.
Cellulose 24 (2017) 11-20.

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Novel molecularly smooth, flat and thin films of regenerated bio-deuterated cellulose were produced for enhanced contrast with adsorbed molecules in neutron reflectivity (NR) and for cellulose structure studies. The cellulose films were produced to study both the solid/air interface and the solid/liquid interface. Cellulose films with a wide range of scattering contrast were achieved by combining exchange of 1H for deuterium on hydroxyl groups via water in the liquid phase and via biosynthesis of deuterated bacterial cellulose by Gluconacetobacter xylinus which can deuterate the hydrogens bonded to carbon atoms in cellulose. The deuterated cellulose combined with NR will help to provide new information on the interaction of various (bio)-macromolecules and cellulose. This includes quantifying and visualizing the density profile of polymers and biomolecules adsorbed onto cellulose surface. The potential of this material for IR studies of materials adsorbed to cellulose films is briefly discussed.

72.
R. Prathapan, R. Thapa, G. Garnier, R. F. Tabor
Modulating the zeta potential of cellulose nanocrystals using salts and surfactants.
Colloids and Surfaces A: Physicochemical and Engineering Aspects 509 (2016) 11-18.

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The zeta potential of cellulose nanocrystal (CNC) aqueous dispersions was studied as a function of solution conditions, including changing pH and different electrolyte identities and concentrations. A range of electrolytes that spans typical Hofmeister/hydrophobic effects was explored, along with both cationic and anionic surfactants. A subtle interplay of electrostatic and hydrophobic effects in ion adsorption was uncovered, including evidence of charge reversal and supercharging when hydrophobic surfactants are added to aqueous CNC dispersions. The apparent effects of zeta potential on dispersion stability were explored by using atomic force microscopy (AFM) to determine the roughness of resulting CNC films. The root mean square roughness (RMS) of these cellulose films was unaffected by the presence of surfactants (achieving a constant value of ∼9 nm), but scaled inversely and non-linearly with the zeta potential of the CNC suspension while using the ionic salts from ∼2 nm to 10 nm, indicating a facile method for the control of cellulose film roughness.

71.
J. Su, V. S. Raghuwanshi, W. Raverty, C. J. Garvey, P. J. Holden, M. Gillon, S. A. Holt, R. Tabor, W. Batchelor and G. Garnier
Smooth deuterated cellulose films for the visualisation of adsorbed bio-macromolecules.
Scientific Reports 6 (2016) 36119.

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Novel thin and smooth deuterated cellulose films were synthesised to visualize adsorbed bio-macromolecules using contrast variation neutron reflectivity (NR) measurements. Incorporation of varying degrees of deuteration into cellulose was achieved by growing Gluconacetobacter xylinus in deuterated glycerol as carbon source dissolved in growth media containing D2O. The derivative of deuterated cellulose was prepared by trimethylsilylation(TMS) in ionic liquid(1-butyl-3-methylimidazolium chloride). The TMS derivative was dissolved in toluene for thin film preparation by spin-coating. The resulting film was regenerated into deuterated cellulose by exposure to acidic vapour. A common enzyme, horseradish peroxidase (HRP), was adsorbed from solution onto the deuterated cellulose films and visualized by NR. The scattering length density contrast of the deuterated cellulose enabled accurate visualization and quantification of the adsorbed HRP, which would have been impossible to achieve with non-deuterated cellulose. The procedure described enables preparing deuterated cellulose films that allows differentiation of cellulose and non-deuterated bio-macromolecules using NR.

70.
T. M. McCoy, A. Valiakhmetova, M. J. Pottage, C. J. Garvey, L. de Campo, C. Rehm, D. A. Kuryashov and R. F. Tabor
Structural Evolution of Wormlike Micellar Fluids Formed by Erucyl Amidopropyl Betaine with Oil, Salts, and Surfactants.
Langmuir 32 (2016) 12423-12433.

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Solutions of extended, flexible cylindrical micelles, often known as wormlike micelles, have great potential as the base for viscoelastic complex fluids in oil recovery, drilling, and lubrication. Here, we study the morphology and nanostructural characteristics of a model wormlike micellar fluid formed from erucyl amidopropyl betaine (EAPB) in water as a function of a diverse range of additives relevant to complex fluid formulation. The wormlike micellar dispersions are extremely oleo-responsive, with even as little as 0.1% hydrocarbon oil causing a significant disruption of the network and a decrease in zero-shear viscosity of around 100-fold. Simple salts have little effect on the local structure of the wormlike micelles but result in the formation of fractal networks at larger length scales, whereas even tiny amounts of small organic species such as phenol can cause unexpected phase transitions. When forming mixtures with other surfactants, a vast array of self-assembled structures are formed, from spheres to ellipsoids, lamellae, and vesicles, offering the ultimate sensitivity in designing formulations with specific nanostructural characteristics.

Click for publications 60-69...

69.
M. S. F. Tan, S. C. Moore, R. F. Tabor, N. Fegan, S. Rahman, G. A. Dykes
Attachment of Salmonella strains to a plant cell wall model is modulated by surface characteristics and not by specific carbohydrate interactions.
BMC microbiology 16 (2016) 212.

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Background: Processing of fresh produce exposes cut surfaces of plant cell walls that then become vulnerable to human foodborne pathogen attachment and contamination, particularly by Salmonella enterica. Plant cell walls are mainly composed of the polysaccharides cellulose, pectin and hemicelluloses (predominantly xyloglucan). Our previous work used bacterial cellulose-based plant cell wall models to study the interaction between Salmonella and the various plant cell wall components. We demonstrated that Salmonella attachment was favoured in the presence of pectin while xyloglucan had no effect on its attachment. Xyloglucan significantly increased the attachment of Salmonella cells to the plant cell wall model only when it was in association with pectin. In this study, we investigate whether the plant cell wall polysaccharides mediate Salmonella attachment to the bacterial cellulose-based plant cell wall models through specific carbohydrate interactions or through the effects of carbohydrates on the physical characteristics of the attachment surface.
Results: We found that none of the monosaccharides that make up the plant cell wall polysaccharides specifically inhibit Salmonella attachment to the bacterial cellulose-based plant cell wall models. Confocal laser scanning microscopy showed that Salmonella cells can penetrate and attach within the tightly arranged bacterial cellulose network. Analysis of images obtained from atomic force microscopy revealed that the bacterial cellulose-pectinxyloglucan composite with 0.3 % (w/v) xyloglucan, previously shown to have the highest number of Salmonella cells attached to it, had significantly thicker cellulose fibrils compared to other composites. Scanning electron microscopy images also showed that the bacterial cellulose and bacterial cellulose-xyloglucan composites were more porous when compared to the other composites containing pectin.
Conclusions: Our study found that the attachment of Salmonella cells to cut plant cell walls was not mediated by specific carbohydrate interactions. This suggests that the attachment of Salmonella strains to the plant cell wall models were more dependent on the structural characteristics of the attachment surface. Pectin reduces the porosity and space between cellulose fibrils, which then forms a matrix that is able to retain Salmonella cells within the bacterial cellulose network. When present with pectin, xyloglucan provides a greater surface for Salmonella cells to attach through the thickening of cellulose fibrils.

68.
Y. Hu, W. Zou, V. Julita, R. Ramanathan, R. F. Tabor, R. Nixon-Luke, G. Bryant, V. Bansal and B. L. Wilkinson
Photomodulation of bacterial growth and biofilm formation using carbohydrate-based surfactants
Chemical Science 7 (2016) 6628-6634.

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Naturally occurring and synthetic carbohydrate amphiphiles have emerged as a promising class of antimicrobial and antiadhesive agents that act through a number of dynamic and often poorly understood mechanisms. In this paper, we provide the first report on the application of azobenzene trans–cis photoisomerization for effecting spatial and temporal control over bacterial growth and biofilm formation using carbohydrate-based surfactants. Photocontrollable surface tension studies and small angle neutron scattering (SANS) revealed the diverse geometries and dimensions of self-assemblies (micelles) made possible through variation of the head group and UV-visible light irradiation. Using these light-addressable amphiphiles, we demonstrate optical control over the antibacterial activity and formation of biofilms against multi-drug resistant (MDR) Pseudomonas aeruginosa, methicillin-resistant Staphylococcus aureus (MRSA) and Gram-negative Escherichia coli. To probe the mechanism of bioactivity further, we evaluated the impact of trans–cis photoisomerization in these surfactants on bacterial motility and revealed photomodulated enhancement in swarming motility in P. aeruginosa. These light-responsive amphiphiles should attract significant interest as a new class of antibacterial agents and as investigational tools for probing the complex mechanisms underpinning bacterial adhesion and biofilm formation.

67.
S. Tripathi and R. F. Tabor
Modeling two-rate adsorption kinetics: Two-site, two-species, bilayer and rearrangement adsorption processes
Journal of Colloid and Interface Science 476 (2016) 119-131.

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The adsorption kinetics of many systems show apparent two-rate processes, where there appears to be resolved fast and slow adsorption steps. Such non-standard adsorption processes cannot be accounted for by conventional modeling methods, motivating new approaches. In this work, we present four different models that can account for two-rate adsorption and are based upon physically realistic processes – two adsorbing species, two surface sites having different energies, bilayer formation and molecular rearrangement modes. Each model is tested using a range of conditions, and the characteristic behavior is explored and compared. In these models, the effects of mass transport and bulk concentration are also accounted for, making them applicable in systems which are transport-limited or attachment-limited, or intermediate between the two. The applicability of these models is demonstrated by fitting exemplar experimental data for each of the four models, selecting the model on the basis of the known physical behavior of the adsorption kinetics. These models can be applied in a wide range of systems, from stagnant adsorption in large volume water treatment to highly dynamic flow conditions relevant to printing, coating and processing applications.

66.
M. J. Pottage, T. L. Greaves, C. J. Garvey and R. F. Tabor
The effects of alkylammonium counterions on the aggregation of fluorinated surfactants and surfactant ionic liquids
Journal of Colloid and Interface Science 475 (2016) 72-81.

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The effects of organic counterions with varying carbon number on surfactant aggregation have been analysed by coupling perfluorooctanoate surfactant anions with various alkylammonium counterions. Both the degree of substitution (primary to tertiary) and alkyl chain length (0–3 carbons) of the counterions were varied to provide a comprehensive matrix of geometries and lipophilicities. Surface activity was measured using pendant drop tensiometry, while temperature-controlled small-angle neutron scattering was used to probe changes in aggregation morphology. It was found that the use of such alkylammonium counterions resulted in a strong preference for bilayer formation even at low surfactant concentration (<2 wt%), when compared to simple inorganic counterions such as sodium which favour near-spherical micelles. At increased temperatures, some counterions led to unique phase behaviour wherein a transition between two structurally different lamellar phases is seen, rationalised as a transition into a microscopic phase separation wherein a surfactant-rich lamellar phase coexists with a dilute micellar phase. The results indicate that aggregation is controlled by a delicate balance of counterion size, hydrophilicity and diffuseness of charge, providing new methods for the subtle control of surfactant solutions.

65.
A. C. Y. Liu, R . F. Tabor, L. Bourgeois, M. D. de Jonge, S. T. Mudie and T. C. Petersen
Calculation of projected bond­orientational order parameters to quantify local symmetries from transmission diffraction data
Physical Review Letters 116 (2016) 205501.

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The bond-orientational order parameters introduced by Steinhardt et al. [Phys. Rev. B 28, 784 (1983)] have been an invaluable measurement tool for assessing short-range order in disordered, close-packed assemblies of particles in which the particle positions are known. In many glassy systems the measurement of particle position is not possible or limited (field of view, thickness, resolution) and the bond-orientational order parameters cannot be measured, or adequately sampled. Here we calculate a set of rotationally averaged, projected bond-orientational order parameters that reflect the symmetries of close-packed particle clusters when projected onto a plane. We show by simulation that these parameters are unique fingerprints that can be directly compared to angular correlations in limited-volume, transmission geometry, diffraction patterns from close-packed glassy assemblies.

64.
H. Kulhari, D. Pooja, R. Kota, T. S. Reddy, R . F. Tabor, R. Shukla, D. Adams, R. Sistla and V Bansal
Cyclic RGDfK Peptide Functionalized Polymeric Nanocarriers for Targeting Gemcitabine to Ovarian Cancer Cells
Molecular Pharmaceutics 13 (2016) 1491-1500.

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Current cancer chemotherapies commonly suffer from nonspecificity, drug resistance, poor bioavailability, and narrow therapeutic indices. To achieve the optimum drug efficacy, we designed a polymeric drug delivery system for targeted intracellular delivery of a clinically approved, water-soluble anticancer drug, gemcitabine hydrochloride (GEM). We utilized the unique ability of a cyclic pentapeptide cRGDfK to specifically target αvβ3 integrin receptors that are overexpressed on SKOV-3 human ovarian cancer cells. This significantly increased the effective intracellular drug concentration even at low doses, thereby remarkably improving the chemotherapeutic potential of GEM. cRGDfK-conjugated, GEM-loaded nanoparticles reduced the nonspecific hemolytic cytotoxicity of the drug, simultaneously influencing intracellular processes such as mitochondrial membrane potential (DΨm), reactive oxygen species (ROS) levels, and apoptosis, thereby favorably influencing drug antiproliferative efficacy.

63.
D. B. Shinde, J. Brenker, C. D. Easton, R . F. Tabor, A. Neild and M. Majumder
Shear Assisted Electrochemical Exfoliation of Graphite to Graphene
Langmuir 32 (2016) 3552-3559.

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The exfoliation characteristics of graphite as a function of applied anodic potential (1-10 V) in combination with shear field (400-74 400 s–1) have been studied in a custom-designed microfluidic reactor. Systematic investigation by atomic force microscopy (AFM) indicates that at higher potentials thicker and more fragmented graphene sheets are obtained, while at potentials as low as 1 V, pronounced exfoliation is triggered by the influence of shear. The shear-assisted electrochemical exfoliation process yields large (∼10 μm) graphene flakes with a high proportion of single, bilayer, and trilayer graphene and small ID/IG ratio (0.21-0.32) with only a small contribution from carbon-oxygen species as demonstrated by X-ray photoelectron spectroscopy measurements. This method comprises intercalation of sulfate ions followed by exfoliation using shear induced by a flowing electrolyte. Our findings on the crucial role of hydrodynamics in accentuating the exfoliation efficiency suggest a safer, greener, and more automated method for production of high quality graphene from graphite.

62.
T. M. McCoy, A. C. Y. Liu and R. F. Tabor
Light-controllable dispersion and recovery of graphenes and carbon nanotubes using a photo-switchable surfactant
Nanoscale 8 (2016) 6969-6974.

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The aqueous dispersibility of carbon-based nanomaterials, namely graphene oxide (GO), reduced graphene oxide (rGO) and carbon nanotubes (CNTs), can be controlled by light via the photoisomerisation of a photoswitchable surfactant molecule adsorbed to the surface of these materials. By incorporating a cationic azobenzene photosurfactant into these systems, GO, rGO and CNT dispersions can be separated and redispersed on command utilising UV radiation at 365 nm, whereby the surfactant molecules change from the trans to the cis isomer. This increases their aqueous solubility and in turn, alters their adsorption affinity for the GO and rGO sheets such that the ratio of free to adsorbed surfactant molecules changes significantly, allowing for reversible phase separation of the colloids. These effects present a unique method for controlling the dispersion behaviour of two-dimensional nanomaterials using light as a clean and low energy external stimulus.

61.
A. N. Calabrese, Y. Liu, T. Wang, I. F. Musgrave, T. L. Pukala, R. F. Tabor, L. L. Martin, J. A. Carver and J. H. Bowie
The Amyloid Fibril-Forming Properties of the Amphibian Antimicrobial Peptide Uperin 3.5
ChemBioChem 17 (2016) 239-246.

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The amphibian skin is a vast resource for bioactive peptides, which form the basis of the animals' innate immune system. Key components of the secretions of the cutaneous glands are antimicrobial peptides (AMPs), which exert their cytotoxic effects often as a result of membrane disruption. It is becoming increasingly evident that there is a link between the mechanism of action of AMPs and amyloidogenic peptides and proteins. In this work, we demonstrate that the broad-spectrum amphibian AMP uperin 3.5, which has a random-coil structure in solution but adopts an α-helical structure in membrane-like environments, forms amyloid fibrils rapidly in solution at neutral pH. These fibrils are cytotoxic to model neuronal cells in a similar fashion to those formed by the proteins implicated in neurodegenerative diseases. The addition of small quantities of 2,2,2-trifluoroethanol accelerates fibril formation by uperin 3.5, and is correlated with a structural stabilisation induced by this co-solvent. Uperin 3.5 fibril formation and the associated cellular toxicity are inhibited by the polyphenol (−)-epigallocatechin-3-gallate (EGCG). Furthermore, EGCG rapidly dissociates fully formed uperin 3.5 fibrils. Ion mobility–mass spectrometry reveals that uperin 3.5 adopts various oligomeric states in solution. Combined, these observations imply that the mechanism of membrane permeability by uperin 3.5 is related to its fibril-forming properties.

60.
Y. Hu, J. B. Marlow, R. Ramanathan, W. Zou, H. G. Tiew, M. J. Pottage, V. Bansal, R. F. Tabor and B. L. Wilkinson
Synthesis and Properties of Photoswitchable Carbohydrate Fluorosurfactants
Australian Journal of Chemistry 68 (2016) 1880-1884.

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We describe the parallel synthesis, photocontrollable surface tension, and antibacterial performance of a new class of carbohydrate fluorosurfactant. Novel fluorosurfactants comprised a mono- or disaccharide head group linked to an azobenzene unit that was variably substituted with a trifluoromethyl group. Fluorosurfactants were rapidly assembled using the venerable CuI-catalysed azide–alkyne cycloaddition reaction and exhibited light-addressable surface activity, excellent water solubility, and selective antibacterial activity against Gram-positive Staphylococcus aureus. Notably, the physicochemical and biological activity of these novel materials was heavily dependent on the nature of the head group and the position of the trifluoromethyl substituent on the azobenzene ring. The UV-adapted cis-isomer of fluorosurfactants displayed good thermal stability at ambient temperature, with little reversion to the stable trans isomer after 16 h. These novel, light-responsive materials should find broad interest in a range of biomedical and technological fields, including drug and gene delivery, self-cleaning oleophobic surfaces, and antibacterial coatings for medical devices.

Click for publications 50-59...

59.
C. S. Butler, Z. L. E. Seeger, T. D. M. Bell, A. I. Bishop, R. F. Tabor
Local determination of thin liquid film profiles using colour interferometry
European Physical Journal E 39 (2016) 1-7.

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We explore theoretically the interference of white light between two interfaces as a function of the optical conditions, using separately: a) idealised conditions where the light is composed of three discrete wavelengths; b) a more typically experimentally realisable case where light comprises a sum of three Gaussian wavelength distributions; and c) unfiltered white light from a broadband source comprising a broad distribution of wavelengths. It is demonstrated that the latter case is not only optically simple to arrange, but also provides unambiguous absolute separation information over the range 0-1μm --a useful range in studies of cell adhesion, thin liquid films and lubrication-- when coupled to detection using a typical colour camera. The utility of this technique is verified experimentally by exploring the air film between a cylinder and surface, as well as arbitrary liquid films beneath air bubbles that are interacting with solid surfaces.

58.
G. Hazell, M. Hinojosa-Navarro, T. M. McCoy, R. F. Tabor and J. Eastoe
Responsive materials based on magnetic polyelectrolytes and graphene oxide for water clean-up
Journal of colloid and interface science 464 (2016) 285-290.

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Hypothesis: Owing to attractive interactions between negatively charged graphene oxide (GO) and a paramagnetic cationic polyelectrolyte (polyallydimethylammonium chloride with a FeCl4− counterion (Fe-polyDADMAC) it should be possible to generate magnetic materials. The benefit of using charge-based adsorption is that the need to form covalently linked magnetic materials is offset, which is expected to significantly reduce the time, energy and cost to make such responsive materials. These systems could have a wide use and application in water treatment. Experiments: Non-covalent magnetic materials were formed through the mixing of Fe-pDADMAC and GO. A systematic study was conducted by varying polymer concentration at a fixed GO concentration. UV–Vis was used to confirm and quantify polymer adsorption onto GO sheets. The potential uses of the systems for water purification were demonstrated. Findings: Fe-polyDADMAC adsorbs to the surface of GO and induces flocculation. Low concentrations of the polymer (<9 mmol/L) favour flocculation, whereas higher concentrations (>20 mmol/L) induce restabilization. Difficult-to-recover gold nanoparticles can be separated from suspensions as well as the pollutant antibiotic tetracycline. Both harmful materials can be magnetically recovered from the dispersions. This system therefore has economical and practical applications in decontamination and water treatment.

57.
S. E. Stewart, C. H. Bird, R. F. Tabor, M. E. D'Angelo, S. Piantavigna, J. C. Whisstock, J. A. Trapani, L. L. Martin and P. I. Bird
Analysis of Perforin Assembly by Quartz Crystal Microbalance Reveals a Role for Cholesterol and Calcium-independent Membrane Binding
The Journal of Biological Chemistry 290 (2015) 31101-31112.

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Perforin is an essential component in the cytotoxic lymphocyte-mediated cell death pathway. The traditional view holds that perforin monomers assemble into pores in the target cell membrane via a calcium-dependent process and facilitate translocation of cytotoxic proteases into the cytoplasm to induce apoptosis. Although many studies have examined the structure and role of perforin, the mechanics of pore assembly and granzyme delivery remain unclear. Here we have employed quartz crystal microbalance with dissipation monitoring (QCM-D) to investigate binding and assembly of perforin on lipid membranes, and show that perforin monomers bind to the membrane in a cooperative manner. We also found that cholesterol influences perforin binding and activity on intact cells and model membranes. Finally, contrary to current thinking, perforin efficiently binds membranes in the absence of calcium. When calcium is added to perforin already on the membrane, the QCM-D response changes significantly, indicating that perforin becomes membranolytic only after calcium binding.

56.
A. Pathak, V. L. Blair, R. L. Ferrero, P. C. Junk, R. F. Tabor and P. C. Andrews
Synthesis and structural characterisation of bismuth(III) hydroxamates and their activity against Helicobacter pyloris
Dalton Transactions 44 (2015) 16903-16913.

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Seven new bismuth(III) hydroxamate complexes derived from the hydroxamic acids N-methylfurohydroxamic acid (H-MFHA), N-benzoyl-N-phenylhydroxamic acid (H-BPHA), salicylhydroxamic acid (H2-SHA), benzohydroxamic acid (H2-BHA), and acetohydroxamic acid (H2-AHA) have been synthesized and characterized. The complexes formed are either tris-hydroxamato complexes containing only mono-anionic ligands, [Bi(H-SHA)3], [Bi(MFHA)3] and [Bi(BPHA)3]; mixed-anion complexes, [Bi(SHA)(H-SHA)] and [Bi(AHA)(H-AHA)]; and potassium bismuthate complexes, K[Bi(SHA)2] and K[Bi(BHA)2]. The solid-state structure of three complexes has been determined through single crystal X-ray diffraction; [Bi(MFHA)3]2·Me2C[double bond, length as m-dash]O, {[Bi(SHA)(H-SHA)(DMSO)2][Bi(SHA)(H-SHA)(DMSO)]·DMSO}∞ and [Bi(BPHA)3]2·2EtOH. All the complexes and their parent acids were assessed for the bactericidal activity against three strains of Helicobacter pylori (26695, B128 and 251). Of the acids, only acetohydroxamic acid showed any activity at low concentrations (MIC 6.25 μg mL−1; 83.26 μM) while the others were not toxic below 25 μg mL−1. In contrast, their bismuth(III) complexes all showed excellent activity across all three strains (e.g. 0.28 μM for [Bi(H-SHA)3] to 6.01 μM for K[Bi(BHA)2] against strain 251) with only minor variations in activity being both ligand and composition dependant.

55.
J. D. Berry, M. J. Neeson, R. R. Dagastine, D. Y. C. Chan and R. F. Tabor
Measurement of surface and interfacial tension using pendant drop tensiometry
Journal of Colloid and Interface Science 454 (2015) 226-237.

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Pendant drop tensiometry offers a simple and elegant solution to determining surface and interfacial tension – a central parameter in many colloidal systems including emulsions, foams and wetting phenomena. The technique involves the acquisition of a silhouette of an axisymmetric fluid droplet, and iterative fitting of the Young-Laplace equation that balances gravitational deformation of the drop with the restorative interfacial tension. Since the advent of high-quality digital cameras and desktop computers, this process has been automated with high speed and precision. However, despite its beguiling simplicity, there are complications and limitations that accompany pendant drop tensiometry connected with both Bond number (the balance between interfacial tension and gravitational forces) and drop volume. Here, we discuss the process involved with going from a captured experimental image to a fitted interfacial tension value, highlighting pertinent features and limitations along the way. We introduce a new parameter, the Worthington number, Wo, to characterise the measurement precision. A fully functional, open-source acquisition and fitting software is provided to enable the reader to test and develop the technique further.

54.
C. I. Browne, R. F. Tabor, F. Grieser and R. R. Dagastine
Direct AFM force measurements between air bubbles in aqueous monodisperse sodium poly(styrene sulfonate) solutions
Journal of Colloid and Interface Science 451 (2015) 69-77.

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Structural forces play an important role in the rheology, processing and stability of colloidal systems and complex fluids, with polyelectrolytes representing a key class of structuring colloids. Here, we explore the interactions between soft colloids, in the form of air bubbles, in solutions of monodisperse sodium poly(styrene sulfonate) as a model polyelectrolyte. It is found that by self-consistently modelling the force oscillations due to structuring of the polymer chains along with deformation of the drops, it is possible to precisely predict the interaction potential between approaching bubbles. In line with polyelectrolyte scaling theory, two distinct regimes of behaviour are seen, corresponding to dilute and semi-dilute polymer solutions. It is also seen that by blending monodisperse systems to give a bidisperse sample, the interaction forces between soft colloids can be controlled with a high degree of precision. At increasing bubble collision velocity, it is revealed that hydrodynamic flow overwhelms oscillatory structural interactions, showing the important disparity between equilibrium behaviour and dynamic interactions.

53.
G. W. Greene, L. L. Martin, R. F. Tabor, A. Michalczyk, L. Ackland and R. Horn
Lubricin: a versatile, biological anti-adhesive with properties comparable to polyethylene glycol
Biomaterials 53 (2015) 127-136.

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Lubricin is a glycoprotein found in articular joints which has been recognized as being an important biological boundary lubricant molecule. Besides providing lubrication, we demonstrate, using a quartz crystal microbalance, that lubricin also exhibits anti-adhesive properties and is highly effective at pre- venting the non-specific adsorption of representative globular proteins and constituents of blood plasma. This impressive anti-adhesive property, combined with lubricin's ability to readily self-assemble to form dense, highly stable telechelic polymer brush layers on virtually any substrates, and its innate biocompatibility, makes it an attractive candidate for anti-adhesive and anti-fouling coatings. We show that coatings of lubricin protein are as effective as, or better than, self-assembled monolayers of polyethylene glycol over a wide range of pH and that this provides a simple, versatile, highly stable, and highly effective method of controlling unwanted adhesion to surfaces.

52.
J. Su, C. J. Garvey, S. Holt, R. F. Tabor, B. Winther-Jensen, W. Batchelor and G. Garnier
Adsorption of cationic polyacrylamide at the cellulose-liquid interface: a neutron reflectometry study
Journal of Colloid and Interface Science, 448 (2015) 88–99.

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The layer thickness and density of high molecular weight cationic polyacrylamide (CPAM) adsorbed at the cellulose-water interface was quantified by neutron reflectometry. The thickness of a full monolayer of CPAM of constant molecular weight (13 MD) but different charge densities, adsorbed with or without NaCl (10-3 M), was studied. Thin cellulose films (40 +/- 7 A) of roughness <10 A were produced by spin coating a cellulose acetate-acetone solution and regenerating by alkaline hydrolysis. Film smoothness was greatly improved by controlling the concentration of cellulose acetate (0.13 wt%) and the hydrolysis time in sodium methoxide. The adsorption thickness of CPAM (40% charge 13 MD) at the solid-D2O interface was 43 +/- 4 A on cellulose and 13 +/- 2 A on silicon, an order of magnitude smaller than the CPAM radius of gyration. At constant molecular weight, the thickness of the CPAM layer adsorbed on cellulose increases with polymer charge density (10 +/- 1 A at 5%). Addition of 10-3 M NaCl decreased the thickness of CPAM layer already adsorbed on cellulose. However, the adsorption layer on cellulose of a CPAM solution equilibrated in 10-3 M NaCl is much thicker (89 +/- 11 A for 40% CPAM). For high molecular weight CPAMs adsorbed from solution under constant conditions, the adsorption layer can be varied by 1 order of magnitude via control of the variables affecting electrostatic intra- and inter-polymer chain interactions.

51.
T. M. McCoy, P. Brown, J. Eastoe and R. F. Tabor
Non-covalent magnetic control and reversible recovery of graphene oxide using iron oxide and magnetic surfactants
ACS Applied Materials & Interfaces 7 (2015) 2124-2133.

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The unique charging properties of graphene oxide (GO) are exploited in the preparation of a range of non-covalent magnetic GO materials, using microparticles, nanoparticles and magnetic surfactants. Adsorption and desorption is controlled by modification of pH within a narrow window of < 2 pH units. The benefit conferred by using charge- based adsorption is that the process is reversible, and the GO can be captured and separated from the magnetic nanomaterial, such that both components can be recycled. Iron oxide (Fe2O3) microparticles form a loosely flocculated gel network with GO, which is demonstrated to undergo magnetic compressional dewatering in the presence of an external magnetic field. For composites formed from GO and Fe2O3 nanoparticles, it is found that low Fe2O3:GO mass ratios (< 5 : 1) favour flocculation of GO, whereas higher ratios (> 5 : 1) cause overcharging of the surfaces resulting in restabilization. The effectiveness of the GO adsorption and magnetic capture process is demonstrated by separating traditionally difficult-to-recover gold nanoparticles (d ~ 10 nm) from water. The fully recyclable nature of the assembly and capture process, combined with the vast adsorption capacity of GO, presents obvious and appealing advantages for applications in decontamination and water treatment.

50.
Y. Hu, R. F. Tabor and B. L. Wilkinson,
Sweetness and Light: Design and Applications of Photoswitchable Glycoconjugates
Organic and Biomolecular Chemistry 13 (2015) 2216-2225.

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Carbohydrate-protein binding is supramolecular recognition process that underpins myriad biological events. However, the precise conformational and configurational requirements for biomolecular recognition are often poorly understood, since such phenomena often occur in a strongly spatiotemporal manner. Such challenges have prompted the development of photoswitchable glycoconjugates for modulating glycan structure and conformation and have emerged as promising investigational tools for probing carbohydrate-protein recognition processes and modulating binding affinities for whole bacterial cells. Reversible photoisomerisation, in particular that of azobenzene glycoconjugates, has also been exploited as a promising strategy for controlling supramolecular self-assembly and macroscopic properties, and has emerged as a promising strategy for the development of responsive carbohydrate-based materials. The following review will highlight the recent advances in the design and applications of photoswitchable glycoconjugates, paying particular attention to the application of light as a stimulus for modulating protein and cellular adhesion, amphiphilicity and supramolecular assembly of carbohydrate-based materials.

Click for publications 40-49...

49.
C. J. Fewkes, R. F. Tabor and R. R. Dagastine,
Sphere to rod transitions in self assembled systems probed using direct force measurement
Soft Matter 11 (2015) 1303-1314.

Click for abstract

The influence of nanoparticle shape, in particular the sphere to rod transition, on surface forces and consequently the properties of colloidal fluids is an interesting but not well investigated phenomenon. Here, the surface force behaviour of concentrated surfactant solutions containing cetyltrimethylammonium bromide and sodium salicylate with micelle shapes varying from slightly prolate to high aspect ratio rods was measured. Atomic force microscopy (AFM) with both rigid particle and soft droplet probes was used with comparisons and analysis made using the Chan-Dagastine-White model. It is observed that small changes to the micelle shape result in no discernable differences to the surface force behaviour, however, once the micelles are elongated significantly the long range forces adjust in nature from oscillatory to that of a single attractive force well. This highlights the importance that nanocolloid shape has on the behaviour and properties of emulsions and other colloidal fluids; specifically for emulsion flocculation and handling in systems of rod and worm like micelles.

48.
L. Zhang, X. Li, L. Ong, R. F. Tabor, B. A. Bowen, A. I. Fernando, A. Nilghaz, G. Garnier, S. L. Gras, X. Wang and W. Shen
Cellulose nanofibre textured SERS substrate
Colloids and Surfaces A 468 (2015) 309-314.

Click for abstract

Poor reproducibility limits the wide uptake of low-cost surface enhanced Raman spectroscopy (SERS) substrates. This study reports a relatively low-cost and reproducible cellulose nanofibre (CNF) textured SERS substrate. Utilizing a layer of CNFs deposited onto glass slides, nanoscale roughness was achieved, which facilitated effective aggregation of gold nanoparticles (AuNPs) to form a novel CNF textured SERS substrate. This substrate meets the critical roughness requirements to control the distribution of AuNPs to provide 'hot spots' for SERS detection, offering significant signal enhancement. The reproducibility and accuracy of low-cost cellulosic SERS substrates were significantly improved on a model SERS molecule of 4-aminothiophenol.

47.
C. I. Browne, R. F. Tabor, F. Grieser and R. R. Dagastine,
Direct AFM force measurements between air bubbles in aqueous polydisperse sodium poly(styrene sulfonate) solutions: effect of collision speed, polyelectrolyte concentration and molar mass
Journal of Colloid and Interface Science 449 (2015) 236-245.

Click for abstract

Interactions between colliding air bubbles in aqueous solutions of polydisperse sodium poly(styrene sulfonate) (NaPSS) using direct force measurements were studied. The forces measured with deformable interfaces were shown to be more sensitive to the presence of the polyelectrolytes when compared to similar measurements using rigid surfaces. The experimental factors that were examined were NaPSS concentration, bubble collision velocity and polyelectrolyte molecular weight. These measurements were then compared with an analytical model based on polyelectrolyte scaling theory in order to explain the effects of concentration and bubble deformation on the depletion interaction between bubbles. It was found that an increase in number density of NaPSS molecules saw an increase in the magnitude of the depletion interaction. Conversely this interaction was overwhelmed by an increase in the fluid flow in the system at higher bubbles collision velocities. It was found that the dispersity in the molecular weight caused the structural forces to be smoothed and only one force oscillation was able to be measured between bubbles. Dispersity plays a significant role in the interactions present between the bubbles and has implications that also affect the polyelectrolyte overlap concentration of the solution. Further understanding of these implications could play a role in the improvement in industries such as water treatment and mineral processing where polyelectrolytes are used extensively.

46.
R. F. Tabor, M. J. Pottage, C. J. Garvey and B. L. Wilkinson,
Light-induced structural evolution of photoswitchable carbohydrate-based surfactant micelles
Chemical Communications 51 (2015) 5509-5512.

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We report the light-induced structural evolution of photoresponsive carbohydrate-based surfactant micelles incorporating azobenzene units, using time-resolved small angled neutron scattering (TR-SANS). Using this technique we have monitored the structural changes in micellisation in situ by following the intermediate shapes and aggregation numbers over time, demonstrating for the first time the course and implications of this process. These changes are fully reversible, with ambient or blue light returning the system to its initial aggregation state. Such control of micellisation and aggregate morphology has clear applications in photo-stimulated release and photo-rheological fluids.

45.
M. J. Pottage, T. L. Greaves, C. J. Garvey, S. T. Mudie and R. F. Tabor,
Controlling the characteristics of lamellar liquid crystals using counterion choice, fluorination and temperature
Soft Matter 11 (2015) 261-268.

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The characteristics of robust and highly ordered fluorinated lamellar phases were explored as a function of temperature, counterion identity and fluorination of the surfactant and co-surfactant. Structural and composition effects were probed using a combination of small-angle scattering of X-rays and neutrons, polarising microscopy and calorimetry. It was found that in general, the phases remained remarkably stable with increasing temperature, showing only moderate loss of order and increased membrane flexibility. By changing the surfactant's cationic counterion, it was possible to exert influence on both the shape of micelles formed and the inter-layer spacing of the lamellar phases obtained. Ordering and crystallinity of the lamellar membranes could be controlled by the level of fluorination of both the surfactant and co-surfactant. These results suggest that subtle manipulations of selected control parameters including co-surfactant selection and counterion choice can provide a high level of control over membrane spacing and local order within lamellar phases, providing guidance where these materials are used as templates.

44.
I. Ullah, A. Shah, A. Badshah, A. Shah, N. A. Shah and R. Tabor,
Surface, aggregation properties and antimicrobial activity of four novel thiourea-based non-ionic surfactants
Colloids and Surfaces A, 464 (2015) 104-109.

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Four novel thiourea-based non-ionic surfactants, 1-decanoyl-3-phenylthiourea, 1-decanoyl-3-ethylthiourea, 1-butyl-3-decanoylthiourea and 1-cyclohexyl-3-decanoylthioureawere synthesized from readily available synthetic building blocks in high yield. The compounds were characterized by 1H NMR, 13C NMR, FTIR and UV-Visible spectrophotometry. These surfactants show very low solubility in water, and interestingly low, but well-defined sub-millimolar critical micelle concentrations (CMCs) in ethanol and toluene, indicating that they are moderately amphiphobic. The antibacterial activity of the synthesized thiourea-based surfactants was tested against five bacterial strains, as well as the antifungal activity against five fungal strains. In all cases, the inhibition of bacterial and fungal growth was significant for all of the synthesized molecules. These molecules are particularly desirable for antimicrobial functionalization of surfaces due to their facile synthesis and their low water solubility, providing robust coatings in aqueous environments.

43.
S. E. Stewart, M. E. d'Angelo, S. Piantavigna, R. F. Tabor, L. L. Martin and P. I. Bird,
Assembly of Streptolysin O Pores Assessed by Quartz Crystal Microbalance and Atomic Force Microscopy Provides Evidence for the Formation of Anchored but Incomplete Oligomers
BBA Biomembranes, 1848 (2015) 115-126.

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Streptolysin O (SLO) is a bacterial pore forming protein that is part of the cholesterol dependent cytolysin (CDC) family. We have used quartz crystal microbalance with dissipation monitoring (QCM-D) to examine SLO membrane binding and pore formation. In this system, SLO binds tightly to cholesterol-containing membranes, and assembles into partial and complete pores confirmed by atomic force microscopy. SLO binds to the lipid bilayer at a single rate consistent with the Langmuir isotherm model of adsorption, indicating there is no cooperation between unbound and bound monomers. Changes in dissipation illustrate that SLO alters the viscoelastic properties of the bilayer during pore formation, but there is no loss of material from the bilayer as reported for small membrane-penetrating peptides. SLO mutants were used to further dissect the assembly and insertion processes by QCM-D. This shows the signature of SLO in QCM-D changes when pore formation is inhibited, and that bound and inserted SLO forms can be distinguished. Furthermore a pre-pore locked SLO mutant binds reversibly to lipid, suggesting that the partially complete wt SLO forms observed by AFM are anchored to the membrane.

42.
M. J. Neeson, D. Y. C. Chan and R. F. Tabor,
Compound Pendant Drop Tensiometry for Surface Tension Measurement at Zero Bond Number
Lanmguir 30 (2014) 15388-15391.

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A widely used method to determine the interfacial tension between fluids is to quantify the pendant drop shape that is determined by gravity and interfacial tension forces. Failure of this method for small drops or small fluid density differences is a critical limitation in microfluidic applications and when only small fluid samples are available. By adding a small spherical particle to the interface to apply an axisymmetric deformation, both the particle density and the interfacial tension can be simultaneously and precisely determined, providing an accurate and elegant solution to a long-standing problem.

41.
M. J. Neeson, R. R. Dagastine, D. Y. C. Chan and R. F. Tabor,
Evaporation of a capillary bridge between a particle and a surface
Soft Matter, 10 (2014) 8489-8499.

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The liquid bridge that forms between a particle and a flat surface, and the dynamics of its evaporation are pertinent to a range of physical processes including paint and ink deposition, spray drying, evaporative lithography and the flow and processing of powders. Here, using time-lapse photography, we investigate the evaporative dynamics of a sessile liquid bridge between a particle and a planar substrate. Different wetting characteristics of the particle and substrate are explored, as well as the effects of contact line pinning and stick-slip boundary conditions. A theoretical framework is developed to quantify and analyse the experimental observations. For the size range of particles and drops used in this study, gravity is by far the smallest force in the system when compared to the surface tension and capillary interactions that are present, but in certain circumstances it dictates the key evolution stages of the geometry of the particle-drop-substrate systems. Analysis of evaporation dynamics and capillary forces indicate that at low Bond numbers, surface tension forces dominate and provide unique opportunities for the control of particles on surfaces.

40.
R. F. Tabor, D. D. Tan, S. S. Han, S. A. Young, Z. L. E. Seeger, M. J. Pottage, C. J. Garvey, and B. L. Wilkinson.
Reversible pH- and Photo-controllable Carbohydrate-based Surfactants
Chemistry - A European Journal, 20 (2014) 13881-13884.

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We report the parallel synthesis and properties of a library of photo-switchable surfactants comprising a hydrophobic butylazobenzene tail group and a hydrophilic carbohydrate head group, including the first surfactants to exhibit dual photo- and pH-responsive behaviour. This new generation of surfactants show varying micelle morphologies, photo-controllable surface tension, and pH-induced aggregation and adsorption.

Click for publications 30-39...

39.
R. F. Tabor, F. Grieser, R. R. Dagastine and D. Y. C. Chan,
The hydrophobic force: measurements and methods
Physical Chemistry Chemical Physics, 16 (2014) 18065-18075.

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The hydrophobic force describes the attraction between water-hating molecules (and surfaces) that draws them together, causing aggregation, phase separation, protein folding and many other inherent physical phenomena. Attempts have been made to isolate the range and magnitude of this interaction between extended surfaces for more than four decades, with wildly varying results. In this perspective, we critically analyse the application of common force-measuring techniques to the hydrophobic force conundrum. In doing so, we highlight possible interferences to these measurements and provide physical rationalisation where possible. By analysing the most recent measurements, new approaches to establishing the form of this force become apparent, and we suggest potential future directions to further refine our understanding of this vital, physical force.

38.
M. J. Pottage, T. Kusuma, I. Grillo, C. J. Garvey, A. D. Stickland and R. F. Tabor,
Fluorinated Lamellar Phases: Structural Characterisation and Use as Templates for Highly Ordered Silica Materials
Soft Matter, 10 (2014) 4902-4912.

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Highly ordered silica was synthesised by using a lamellar phase comprising the anionic fluorinated surfactant sodium perfluorooctanoate and the partially-fluorinated co-surfactant/oil 1H,1H,2H,2H-perfluorooctan-1-ol in water. The phase behaviour of this system was thoroughly analysed, and it was found that even low levels of the alcohol (<0.5 mol %) were sufficient to induce a phase change from normal micelles to a lamellar phase, rationalised as a result of geometric and electrostatic effects. The properties of these phases were compared to their hydrocarbon analogues, demonstrating the unique and valuable properties exhibited by fluorocarbons, directly related with the observed nanostructure. Small-angle neutron scattering was used to analyse the inter- nal structure of the systems, providing information on the inter-lamellar spacing, bilayer thickness and membrane elasticity. The potential for these phases to act as shear-thinning lubricants was assessed using oscillatory rheology, obtaining shear-dependent viscosity along with storage and loss moduli.

37.
J. Eastoe and R. F. Tabor,
Surfactants and Nanoscience
in Colloidal Foundations of Nanoscience (D. Berti and G. Palazzo, eds) Elsevier 2014, pp. 135-157.

Click for abstract

Surfactants are amphiphilic molecules -- that is, they have one part that loves water and another part that loves oil. These dual characteristics give them a range of properties connected to two key behaviors -- adsorption at interfaces and self-assembly in solution. The former explains the multifarious applications of surfactants in processes such as detergency, wetting, spreading and stabilization of particles, droplets and bubbles in dispersions, emulsions, and foams. The latter underpins the formation of nanoscale structures from biological cells to micelles, microemulsions, and liquid crystals. This chapter introduces surfactants and their unusual and useful properties and characteristics. Examples of uses in emerging and nanotechnologies are covered, where these properties prove central for control over properties over nanometer length scales.

36.
S. Soleimani, A. Salabat and R. F. Tabor,
Effect of surfactant type on platinum nanoparticle size of composite Pt/a-Al2O3 catalysts synthesized by a microemulsion method
Journal of Colloid and Interface Science, 426 (2014) 287-292

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A facile method for the generation of platinum-on-alumina hybrid materials with high-surface area is presented, employing a microemulsion-based synthesis of metal nanoparticles. Pt nanoparticles (NPs) supported on a-Al2O3 were prepared by the reduction of metal ions in water-in-oil microemulsion systems stabilized by a range of different surfactants with cationic, anionic and nonionic headgroups, namely AOT, CTAB, Tween80 and TX-100. The synthesized materials were characterized using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is demonstrated that choice of surfactant can be used to tailor the size of the generated Pt nanoparticles, and seen that surfactant charge has a determining role in this process. Pt NPs formed in microemulsion systems based on charged surfactants (AOT and CTAB) are smaller than those prepared in nonionic microemulsion systems (TX-100 and Tween80). A solvent-induced demixing process was used to cleanly obtain the hybrid materials from the reaction medium at low energy cost.

35.
C. Wu, D. Y. C. Chan and R. F. Tabor,
A simple and accurate method for calculation of the structure factor of interacting charged spheres
Journal of Colloid and Interface Science, 426 (2014) 80-82

Click for abstract

Calculation of the structure factor of a system of interacting charged spheres based on the Ginoza solution of the Ornstein-Zernike equation has been developed and implemented on a stand-alone spreadsheet. This facilitates direct interactive numerical and graphical comparisons between experimental structure factors with the pioneering theoretical model of Hayter-Penfold that uses the Hansen-Hayter renormalisation correction. The method is used to fit example experimental structure factors obtained from the small-angle neutron scattering of a well-characterised charged micelle system, demonstrating that this implementation, available in the Supplementary Information, gives identical results to the Hayter-Penfold-Hansen approach for the structure factor, S(q) and provides direct access to the pair correlation function, g(r). Additionally, the intermediate calculations and outputs can be readily accessed and modified within the familiar spreadsheet environment, along with information on the normalisation procedure.

34.
T. M. McCoy, M. J. Pottage and R. F. Tabor,
Graphene Oxide-Stabilised Oil-in-Water Emulsions: pH-Controlled Dispersion and Flocculation
Journal of Physical Chemistry C 118 (2014) 4529-4535

Click for abstract

Oil-in-water emulsions stabilised by graphene oxide (GO) can be flocculated by either an increase or decrease in pH. At highly acidic pH, fully reversible flocculation of emul- sion droplets can be achieved, whereas when adjusted to high pH, the flocculation is irreversible, which we interpret as a permanent chemical change in the GO. We corre- late the effectiveness of GO as a stabiliser to its aqueous aggregation state, and explore the effects of the GO surface charge in determining emulsion properties. By directly measuring the interaction forces between two emulsion droplets coated in GO using the atomic force microscope, we demonstrate the basis for the pH dependent flocculation behaviour. It is shown that interfacial charge of the GO and oil-water interface in the overriding drive for the exceptional stability of acidic GO emulsions.

33.
R. F. Tabor, C. Wu, F. Greiser, R. R. Dagastine and D. Y. C. Chan
Measurement of the Hydrophobic Force in a Soft Matter System
Journal of Physical Chemistry Letters 4 (2013) 3872-3877

Click for abstract

The hydrophobic attraction describes the well-known tendency for nonpolar molecules and surfaces to agglomerate in water, controlled by the re-organisation of intervening water molecules to minimize disruption to their hydrogen bonding network. Measurements of the attraction between chemically-hydrophobised solid surfaces have reported ranges varying from tens to hundreds of nanometers, all attributed to hydrophobic forces. Here, by studying the interaction between two hydrophobic oil drops in water under well-controlled conditions where all known surface forces are suppressed, we observe only a strong, short-ranged attraction with an exponential decay length of 0.30 ± 0.03 nm – comparable to molecular correlations of water molecules. This attraction is implicated in a range of fundamental phenomena from self-assembled monolayer formation to the action of membrane proteins and non-stick surface coatings.

32.
C. Wu, H. S. Leese, D. Mattia, R. R. Dagastine, D. Y. C. Chan and R. F. Tabor,
Study of Fluid and Transport Properties of Porous Anodic Aluminum Membranes by Dynamic Atomic Force Microscopy
Langmuir 29 (2013) 8969-8977

Click for abstract

Recent work on carbon nanotubes (CNT) has focused on their potential application in water treatment due to their predicted and observed enhanced flow rates. However some recent work on the lesser known porous anodic alumina membranes (PAAMs) has also shown that these membranes demonstrate similar characteristics and thus are a possible alternative to CNTs. Despite their potential applications, not much research has been conducted on PAAMs hydrodynamical properties and in this paper, we present experimental results and theoretical models that explore the fluid flow characteristics of these membranes. The experiments were conducted using an atomic force microscope (AFM) that pushed a solid silica particle against PAAMs that were characterised with different pore diameters. Furthermore the PAAMs were classified as either 'closed' or 'open' with the latter allowing fluid to pass through. The theoretical model developed to describe the experimental data incorporates Derjaguin-Landau-Verwey-Overbeek (DLVO) effects, cantilever drag and hydrodynamic forces. By using the slip boundary condition for the hydrodynamic forces, we were able to fit the model to the experimental and also demonstrated that the difference between the 'closed' and 'open' PAAMs are negligible. The slip lengths did not correspond to any physical feature of the PAAMs but our model does provide a simple yet effective means of describing the hydrodynamics of not only PAAMs but for membranes in general.

31.
R. F. Tabor, M. I. Zaveer, R. R. Dagastine, I. Grillo and C. J. Garvey
Phase behaviour, small-angle neutron scattering and rheology of ternary nonionic surfactant-oil-water systems: a comparison of oils
Langmuir 29 (2013) 3575-3582

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The phase behaviour of the nonionic surfactant Triton X-100 (polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether) was studied in two three-component systems: Triton-water-p-xylene and Triton-water-trichloroethylene. It was found that the aromatic solvent was able to produce monophasic soft matter systems at a significantly greater range of compositions. The structural characteristics of the phases generated were analysed by small-angle neutron scattering, showing evidence for microemulsion, lamellar and reverse-microemulsion phases. In addition, for the Triton-water-p-xylene system, an L3 `sponge' phase was found in a water-rich region of the phase diagram and the properties of this were examined using rheological measurements. The differences in phase behaviour are discussed in light of the solvation properties of the surfactant in the different solvents studied. Most notably, xylene appears to favour phases with low-curvature interfaces, suggesting preferential solvation of the central phenyl group of Triton.

30.
R. F. Tabor, C. Wu, F. Grieser, D. Y. C. Chan and R. R. Dagastine
Non-linear and cyclical collisions between drops and bubbles: Using AFM to understand droplet interactions in micro-scale flows
Soft Matter 9 (2013) 2426-2433

Click for abstract

Understanding the mechanics and outcome of droplet and bubble collisions is central to a range of processes from emulsion stability to mineral flotation. The atomic force microscope has been shown to be sensitive and accurate in measuring the forces during such interactions; in combination with a suitable model framework, a powerful tool is obtained for understanding surface forces and droplet stability in dynamic systems. Here we demonstrate for the first time that this process is not limited to linear motion, and that accelerating, decelerating and cyclical droplet velocities can be used to explore the collisions between droplets and bubbles in ways that much more closely mimic real systems. In particular, the motion of droplets experiencing oscillating pumping pressures is explored, providing insight into fluid handling for microfluidics. By modelling a range of processes in which drops collide and deform, and sometimes coalesce, the validity of the theoretical model - which accounts for deformation, surface forces and dynamic lubrication - is demonstrated. Further, it is shown how this model can be used as a predictive tool to determine whether a given droplet collision will be stable or colaescent.

Click for publications 20-29...

29.
S. Cui, R. Manica, R. F. Tabor and D. Y. C. Chan
Interpreting atomic force microscopy measurements of hydrodynamic and surface forces with nonlinear parametric estimation
Review of Scientific Instruments 83 (2012) 103702

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A nonlinear parameter estimation method has been developed to extract the separation-dependent surface force and cantilever spring constant from atomic force microscope data taken at different speeds for the interaction between a silica colloidal probe and plate in aqueous solution. The distinguishing feature of this approach is that it exploits information from the velocity dependence of the force-displacement data due to hydrodynamic interaction to provide an unbiased estimate of the functional form of the separation-dependent surface force. An assumed function for the surface force with unknown parameters is not required. In addition, the analysis also yields a consistent estimate of the in situ cantilever spring constant. In combination with data from static force measurements, this approach can further be used to quantify the extent of hydrodynamic slip.

28.
M. J. Neeson, R. F. Tabor, F. Grieser, R. R. Dagastine and D. Y. C. Chan
Compound sessile drops
Soft Matter 8 (2012) 11042-11050

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Compound drops arise from the contact of three immiscible fluids and can assume various geometric forms based on the interfacial chemistry of the phases involved. Here we present a study of a new class of compound drops that is sessile on a solid surface. The possible geometries are demonstrated experimentally with appropriate fluid combinations and accounted for with a quantitative theoretical description. Although such systems are broadly controlled by relative interfacial energies, subtleties such as the van der Waals force and effects of micro-gravity, despite drop sizes being well below the capillary length, come into play in determining the equilibrium state that is achieved. The drying of a compound sessile drop was measured experimentally, and the process revealed a novel transition between different characteristic configurations of compound sessile drops. Such drops may prove to be useful as the first step towards development of functional surfaces in applications such as soft optics, photonics and surface encapsulation.

27.
H. E. Lockie, R. Manica, R. F. Tabor, G. W. Stevens, F. Grieser, D. Y. C. Chan, and R. R. Dagastine
Anomalous Pull-Off Forces Between Surfactant Free Emulsion Drops in Different Aqueous Electrolytes
Langmuir 28 (2012) 4259-4266

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A systematic study of collisions between surfactant free organic drops in aqueous electrolyte solutions reveal the threshold at which continuum models provide a complete description of thin-film interactions. For collision velocities above around 1 micron/s, continuum models of hydrodynamics and surface forces provide a complete description of the interaction, despite the absence of surfactant. This includes accurate prediction of coalescence at high salt concentration (500 mM). In electrolyte solutions at intermediate salt concentration (50 mM) drop-drop collisions at lower velocity (less than 1 micron) or extended time of forced drop-drop interaction, exhibit a strong pull-off force of systematically varying magnitude. The observations have implications on the effects of ion-specificity and time-dependence in drop-drop interactions where kinetic stability is marginal.

26.
S.-Y. Tan, R. F. Tabor, L. Ong, G. W. Stevens, and R. R. Dagastine;
Nano-mechanical properties of clay-armoured emulsion droplets.
Soft Matter 8 (2012) 3112-3121

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There has been a growing interest in understanding the stabilization mechanism of particle-armoured emulsion droplets over the last few decades due to their importance in many everyday products. Here, the mechanical properties of clay-armoured emulsion droplets were investigated using laser scanning confocal microscopy with an in situ atomic force microscopy measurement. This combination allows the visualization of droplet shape as a function of applied force. The emulsion droplets were found to be mechanically robust, stable against coalescence during drop collisions and able to recover from large deformations without disintegration. A Hookean constitutive law was used to extract the surface Young's modulus of the clay-armoured droplets as a function of a range of solution conditions. The clay-armoured droplets were relatively insensitive to changes in solution ionic strength and pH. However, in the presence of cationic surfactants, the surface Young's modulus decreases and shows significant reduction well above the critical micelle concentration. These changes are most likely due to desorption of clay platelets from the oil-water interface after charge neutralisation and the eventual solubilisation of the oil droplet. The elasticity measurements in this study should help illuminate the impact of the clay-armoured droplets on macroscopic properties of emulsions including rheological properties and emulsion stability.

25.
M. Hopkins Hatzopoulos, R. F. Tabor and J. Eastoe;
Microemulsions, in Encyclopedia of Colloid and Interface Science, Ed. T. Tadros,
Springer, New York, 2013. (Invited Chapter)

Click for abstract

No abstract data available

24.
R. F. Tabor, F. Grieser, R. R. Dagastine and D. Y. C. Chan;
Measurement and analysis of forces in bubble and droplet systems using AFM.
Journal of Colloid and Interface Science 371 (2012) 1-14

Click for abstract

The use of atomic force microscopy to measure and understand the interactions between deformable colloids - particularly bubbles and drops - has grown to prominence over the last decade. Insight into surface and structural forces, hydrodynamic drainage and coalescence events has been obtained, aiding in the understanding of emulsions, foams and other soft matter systems. This article provides information on experimental techniques and considerations unique to performing such measurements. The theoretical modelling frameworks which have proven crucial to quantitative analysis are presented briefly, along with a summary of the most significant results from drop and bubble AFM measurements. The advantages and limitations of such measurements are noted in the context of other experimental force measurement techniques.

23.
R. F. Tabor, H. Lockie, D. Y. C. Chan, F. Grieser, I. Grillo, K. J. Mutch and R. R. Dagastine;
Structural forces in soft matter systems: unique flocculation pathways between deformable droplets.
Soft Matter 7 (2011) 11334-11344

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Oscillatory structural forces caused by colloidal additives including micelles, microemulsion droplets and particles were explored between rigid and deformable interfaces using direct force measurements with the atomic force microscope. The observed oscillations from rigid surfaces become distorted when confinement occurs between deformable interfaces, giving rise to a force hysteresis between approaching and separating interfaces. Small-angle neutron scattering was used to determine the bulk structure of the colloidal additives, as a basis for comparison with their behaviour when confined in thin films. It is seen that confinement itself does not appear to significantly alter the structure of the colloidal additive when compared to the bulk; however, at small separations, interactions with the confining interfaces may become important. The combined approach uncovered an unique flocculation pathway that is available to deformable emulsion droplets, and that the strength of this flocculation can be tuned by changes in the size and concentration of the structuring colloid, the emulsion droplet size, and the ionic strength of the solution.

22.
R. F. Tabor, R. Manica, D. Y. C. Chan, F. Grieser and R. R. Dagastine;
Repulsive Van der Waals forces in soft matter: why bubbles do not stick to walls.
Physical Review Letters 106 (2011) 064501/1-4

Click for abstract

Measurements of nonequilibrium hydrodynamic interactions between bubbles and solid surfaces in water provide direct evidence that repulsive van der Waals forces of quantum origin control the behavior of liquid films on solids in air. In addition to being the simplest and most universal 3-phase system, the deformable air-water interface greatly enhances the sensitivity of force measurements compared with rigid systems. The strength of the repulsive interaction, controlled by the choice of solid, is sufficient to prevent coalescence (sticking) on separation due to hydrodynamic interactions.

21.
R. F. Tabor, D. Y. C. Chan, F. Grieser and R. R. Dagastine;
Anomalous stability of CO2 in pH-controlled bubble coalescence.
Angewandte Chemie International Edition 50 (2011) 3454-3456

Click for abstract

Bubbles represent one of the simplest and most pervasive entities in the world around us, occurring in almost all liquid systems. The controlled collision between two microbubbles in water was investigated to understand the effect of gas type and solution pH on their stability, using ubiquitous gases-pure CO2, air, nitrogen, and argon.

20.
C. Browne, R. F. Tabor, D. Y. C. Chan, R. R. Dagastine, M. Ashokkumar and F. Grieser;
Bubble coalescence during acoustic cavitation in aqueous electrolyte solutions.
Langmuir 27 (2011) 12025-12032

Click for abstract

Bubble coalescence behavior in aqueous electrolyte (MgSO4, NaCl, KCl, HCl, H2SO4) solutions exposed to an ultrasound field (213 kHz) has been examined. The extent of coalescence was found to be dependent on electrolyte type and concentration, and could be directly linked to the amount of solubilized gas (He, Ar, air) in solution for the conditions used. No evidence of specific ion effects in acoustic bubble coalescence was found. The results have been compared with several previous coalescence studies on bubbles in aqueous electrolyte and aliphatic alcohol solutions in the absence of an ultrasound field. It is concluded that the impedance of bubble coalescence by electrolytes observed in a number of studies is the result of dynamic processes involving several key steps. First, ions (or more likely, ion-pairs) are required to adsorb at the gas/solution interface, a process that takes longer than 0.5 ms and probably fractions of a second. At a sufficient interfacial loading (estimated to be less than 1-2% monolayer coverage) of the adsorbed species, the hydrodynamic boundary condition at the bubble/solution interface switches from tangentially mobile (with zero shear stress) to tangentially immobile, commensurate with that of a solid-liquid interface. This condition is the result of spatially nonuniform coverage of the surface by solute molecules and the ensuing generation of surface tension gradients. This change reduces the film drainage rate between interacting bubbles, thereby reducing the relative rate of bubble coalescence. We have identified this point of immobilization of tangential interfacial fluid flow with the 'critical transition concentration' that has been widely observed for electrolytes and nonelectrolytes. We also present arguments to support the speculation that in aqueous electrolyte solutions the adsorbed surface species responsible for the immobilization of the interface is an ion-pair complex.

Click for publications 10-19...

19.
R. F. Tabor, C. Wu, H. Lockie, R. Manica, D. Y. C. Chan, F. Grieser and R. R. Dagastine;
Homo- and hetero-interactions between air bubbles and oil droplets measured by atomic force microscopy.
Soft Matter 7 (2011) 8977-8983

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The atomic force microscope was used to analyse the interactions between bubbles and oil droplets in surfactant-free aqueous solutions. Both homo- (bubble-bubble and drop-drop) and hetero- (bubble-drop) interactions were examined to elucidate the role of colloidal and hydrodynamic forces which, together with interfacial deformations dictate the stability in these systems. It is shown that electrical double-layer forces can be rendered attractive within a small pH range, and that the Van der Waals force can be switched from attractive to repulsive by material choice and ionic strength through salt effects on the so-called 'zero-frequency' term of the Lifshitz theory. By measuring interaction events between bubbles and drops at higher velocities, it is seen that deformation of the bodies and lubrication in the film generated between them can be predicted with a continuum hydrodynamic theory. These results suggest that solution pH and droplet material choice can be used to enhance or inhibit coalescence in such multi-component and multi-phase systems, and this may prove useful in controlling the behaviour of systems in microfluidics, as well as dispersion and formulation science.

18.
R. F. Tabor, H. Lockie, D. Mair, R. Manica, D. Y. C. Chan, F. Grieser and R. R. Dagastine;
Combined AFM-confocal microscopy of oil droplets: absolute separations and forces in nano-films.
Journal of Physical Chemistry Letters 2 (2011) 961-965

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Quantitative interpretation of the dynamic forces between micrometer-sized deformable droplets and bubbles has previously been limited by the lack of an independent measurement of their absolute separation. Here, we use in situ confocal fluorescence microscopy to directly image the position and separation of oil droplets in an atomic force microscopy experiment. Comparison with predicted force vs. separation behavior to describe the interplay of force and deformation showed excellent agreement with continuum hydrodynamic lubrication theory in aqueous films less than 30 nm thick. The combination of force measurement and 3D visualization of geometric separation and surface deformation is applicable to interactions between other deformable bodies.

17.
R. F. Tabor, A. J. Morfa, F. Grieser, D. Y. C. Chan, and R. R. Dagastine;
The effect of gold oxide in measurements of colloidal force.
Langmuir 27 (2011) 6026-6030

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Atomic force microscopy, contact-angle, and spectroscopic ellipsometry measurements were employed to investigate the presence and properties of gold oxide on the surface of gold metal. It was found that, in agreement with available literature, unoxidized gold surfaces were hydrophobic, whereas oxidation rendered the surface highly hydrophilic. The oxide could be removed with ethanol or base but appeared to be stable over long periods in water or salt solutions between pH 3 and 7. After oxidation, the oxide layer thickness, determined using ellipsometry, was consistent with an approximate monolayer of Au-O bonds at the gold surface. The presence of gold oxide was found to alter significantly the electrical double-layer characteristics of the gold surface below pH 6 and may explain the apparent inconsistencies in observed force behavior where gold is employed as well as aiding in design of future microfluidic systems which incorporate gold as a coating.

16.
S. Wu, L. Shi, L. B. Garfield, R. F. Tabor, A. Striolo and B. P. Grady;
Influence of surface roughness on cetyltrimethylammonium bromide (CTAB) adsorption from aqueous solution.
Langmuir 27 (2011) 6091-6098

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The influence of surface roughness on surfactant adsorption was studied using a quartz crystal microbalance with dissipation (QCM-D). The sensors employed had root-mean-square (R) roughness values of 2.3, 3.1, and 5.8 nm, corresponding to fractal-calculated surface area ratios (actual/nominal) of 1.13, 1.73, and 2.53, respectively. Adsorption isotherms measured at 25 °C showed that adsorbed mass of cetyltrimethylammonium bromide per unit of actual surface area below 0.8 cmc, or above 1.2 cmc, decreases as the surface roughness increases. At the cmc, both the measured adsorbed amount and the measured dissipation increased dramatically on the rougher surfaces. These results are consistent with the presence of impurities, suggesting that roughness exacerbates well-known phenomena reported in the literature of peak impurity-related adsorption at the cmc. The magnitude of the increase, especially in dissipation, suggests that changes in adsorbed amount may not be the only reason for the observed results, as aggregates at the cmc on rougher surfaces are more flexible and likely contain larger amounts of solvent. Differences in adsorption kinetics were also found as a function of surface roughness, with data showing a second, slower adsorption rate after rapid initial adsorption. A two-rate Langmuir model was used to further examine this effect. Although adsorption completes faster on the smoother surfaces, initial adsorption at zero surface coverage is faster on the rougher surfaces, suggesting the presence of more high-energy sites on the rougher surfaces.

15.
R. F. Tabor, D. Y. C. Chan, F. Grieser and R. R. Dagastine;
Structural forces in soft matter systems.
Journal of Physical Chemistry Letters 2 (2011) 434-437

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Oscillating structural forces arise when nanoscale colloids are confined at high concentration between two approaching surfaces. As layers of colloid are squeezed out, changes in osmotic pressure cause alternating regions of repulsion and attraction. Here, we provide direct measurements of such oscillatory structural forces between the soft interfaces of two emulsion droplets. Quantitative comparison indicates that the deformable nature of droplets allows them to act as far more sensitive probes than solid spheres. In addition, the responsive nature of soft surfaces can give rise to unexpected behaviors not encountered in rigid systems including reversible aggregation/flocculation for emulsion droplets and, potentially, spatial ordering within concentrated emulsion phases.

14.
R. F. Tabor, J. Eastoe and P. Dowding;
A two-step model for surfactant adsorption at solid surfaces.
Journal of Colloid and Interface Science 346 (2010) 424-428

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A theoretical model is presented which accounts for two stages of surfactant adsorption onto a solid surface. The model incorporates both mass transfer (diffusion) and attachment terms, making it applicable to systems where mass transport and adsorption may occur on similar timescales. The model is tested against example systems which appear to show two-step adsorption processes, consisting of cationic ammonium bromide surfactants adsorbing onto silica from water and organic solvents. Kinetic parameters suggest that adsorption may occur in a broadly similar fashion from both water and low-dielectric solvents, and that the fast adsorption step appears to be transport-limited for organic solvents but may experience an adsorption barrier in aqueous systems.

13.
R. F. Tabor, J. Eastoe, P. Dowding, I. Grillo and S. E. Rogers;
Bidisperse colloids: nanoparticles and microemulsions in coexistence.
Journal of Colloid and Interface Science 344 (2010) 447-450

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Mixed 'hard-soft' colloidal systems have been generated in which the 'hard' components (80 nm diameter silica nanoparticles) coexist with a population of 'soft' microemulsion droplets, both structures stabilised by the anionic surfactant sodium bis(ethylhexyl)sulfosuccinate (AOT) with toluene as solvent. The addition of water to swell the inverse micelles to form microemulsion droplets appears to increase attractive interactions between the silica particles (determined by DLS), possibly due to adsorption of some water at the silica-toluene interface; however, long-term stability of the dispersions is maintained. Small-angle neutron scattering was used to examine the structures present in these new colloidal systems.

12.
R. F. Tabor, J. Eastoe and P. Dowding;
Adsorption and desorption of cationic surfactants onto silica from toluene studied by ATR-FTIR.
Langmuir 26 (2010) 671-677

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The adsorption and desorption behavior of cationic dialkyldimethylammonium bromide surfactants (Di-CnDABs where n = 10, 12, 14) at the silica-toluene interface has been studied. Adsorption is a rapid process, consistent with transport control, whereas desorption appears to occur in a two-stage process, with varying proportions of surfactant desorbing in fast and slow modes. These proportions appear to be affected by trace moisture present in the adjacent toluene solvent, possibly owing to competition between surfactant and water molecules for surface sites. Surprisingly, the surfactant tail length (n) has a significant impact on solubility in toluene, and this appears to affect bulk-surface partitioning. The results are compared with previous experiments utilizing nonionic surfactants (Tabor, R. F.; Eastoe, J.; Dowding, P. Langmuir 2009, 25, 9785), and also with work on surfactant-stabilized silica in nonpolar solvents (Tabor, R. F.; Eastoe, J.; Dowding, P. J.; Grillo, I.; Heenan, R. K.; Hollamby, M. Langmuir 2008, 24, 12793). Observations are explained in terms of the balance of interactions between the surfactant, solvent, and surface.

11.
L. J. Wang, R. F. Tabor, J. Eastoe, X. F. Li, R. K. Heenan and J. F. Dong;
Formation and stability of nanoemulsions with mixed ionic-nonionic surfactants.
Physical Chemistry Chemical Physics 11 (2009) 9772-9778

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A simple, low-energy two-step dilution process has been applied with binary mixtures of ionic-nonionic surfactants to prepare nanoemulsions. The systems consist of water/DDAB-C(12)E(5)/decane. Nanoemulsions were obtained by dilution of concentrates located in bicontinuous microemulsion or lamellar liquid crystal phase regions. The nanoemulsions generated were investigated both by contrast-variation small-angle neutron scattering (SANS) and dynamic light scattering (DLS). The SANS profiles show that C(12)E(5) nanodroplets suffer essentially no structural change on incorporation of the cationic DDAB surfactant, except for increased electrostatic repulsive interactions. Interestingly, SANS indicated that the preferred droplet sizes were hardly affected by the surfactant mixture composition (up to a DDAB molar ratio (m(DDAB)/(m(DDAB) + m(C(12)E(5))) of 0.40) and droplet volume fraction, phi, between 0.006 and 0.120. No notable changes in the structure or radius of nanoemulsion droplets were observed by SANS over the test period of 1 d, although the droplet number intensity decreased significantly in systems stabilized by C(12)E(5) only. However, the DLS sizing shows a marked increase with time, with higher droplet volume fractions giving rise to the largest changes. The discrepancy between apparent nanoemulsion droplet size determined by DLS and SANS data can be attributed to long-range droplet interactions occurring outside of the SANS sensitivity range. The combined SANS and DLS results suggest flocculation is the main mechanism of instability for these nanoemulsions. The flocculation rate is shown to be significantly retarded by addition of the charged DDAB, which may be due to enhanced electrostatic repulsive forces between droplets, leading to improved stability of the nanoemulsions.

10.
R. F. Tabor, J. Eastoe and P. Dowding;
Adsorption and desorption of nonionic surfactants on silica from toluene studied by ATR-FTIR.
Langmuir 25 (2009) 9785-9791

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The adsorption and desorption kinetics and equilibrium surface coverages for a range of poly(ethylene glycol) monoalkyl ether nonionic surfactants (CnEms) from toluene onto silica were studied using attentuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) coupled with a constant-flow liquid cell. The initial adsorption rate appears to be transport controlled, and broadly independent of molecular structure, whereas equilibrium adsorbed amounts are dependent on surfactant head group size (which controls both surface interactions and the surface−bulk partitioning). Desorption kinetic data are fitted to a simple two-step model based upon Langmuir-type kinetics. The results are interpreted in terms of a simple model comprising two modes of surfactant adsorption, each with a characteristic adsorbed amount and desorption rate constant; a possible physical basis for this observation is presented. Trace water levels appear to have a strong impact on the equilibrium surface coverages in these systems, possibly by influencing both surfactant interactions and solubility.

Click for publications 1-9...

9.
M. J. Hollamby, K. Trickett, A. Mohamed, S. Cummings, R. F. Tabor, O. Myakonkaya, S. Gold, S. Rogers, R. K. Heenan and J. Eastoe;
Tri-chain hydrocarbon surfactants as designed micellar modifiers for supercritical CO2.
Angewandte Chemie - International Edition 48 (2009) 4993-4995

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Low-cost hydrocarbon surfactants act as fluid modifiers for supercritical carbon dioxide (scCO2). Increased terminal branching of the surfactant chains aids micelle formation, and more chains allows water to be incorporated.

8.
R. F. Tabor, J. Eastoe and I. Grillo;
Time-resolved small-angle neutron scattering as a lamellar phase evolves into a microemulsion.
Soft Matter 5 (2009) 2125-2129

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Time-resolved small-angle neutron scattering (SANS) was used to follow the formation of a microemulsion after rapid stopped-flow mixing of a lamellar phase (L_alpha), comprising the non-ionic surfactant TX-100 and water, with toluene. Initially after mixing, an oil-swollen L_alpha phase was formed. Between 4 and 160 s the L_alpha system evolved into a dense water-in-oil (w/o) microemulsion, during which time the two phases coexisted. After 160 s, the system was a pure single-phase microemulsion. SANS spectra were fitted using a linear combination of the phenomenological Teubner-Stey microemulsion and the Nallet lamellar models. The characteristic repeat distance of water domains in the microemulsion was seen to decrease over the first 70 s of the transformation as more of the microemulsion formed; conversely the domain correlation length appeared to increase.

7.
R. F. Tabor, R. J. Oakley, J. Eastoe, C. F. J. Faul, I. Grillo and R. K. Heenan;
Reversible light-induced critical separation.
Soft Matter 5 (2009) 78-80

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A reversible, critical phase separation induced by UV light at 350 nm has been achieved by doping a small amount of photosensitive surfactant into an AOT-stabilised water-in-decane microemulsion. The initial and separated phases were analysed by small-angle neutron scattering (SANS), which suggests irradiation produces two coexisting water-in-oil microemulsions with relative droplet concentrations of 1 : 3. To better understand the effect of the photosurfactant, drop shape analysis tensiometry was used, showing a change in the effectiveness of the interfacial film under different irradiation conditions.

6.
R. F. Tabor, J. Eastoe, P. J. Dowding, I. Grillo, R. K. Heenan and M. Hollamby;
Formation of surfactant-stabilized silica organosols.
Langmuir 24 (2008) 12793-12797

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Organosols comprising silica nanoparticles, stabilized by adsorbed surfactant layers in low dielectric organic solvents were formulated, and their properties studied. A range of different methods for organosol formation starting from aqueous sols were evaluated and compared, in order to determine the most reliable and reproducible approach. To understand the influence of surfactant type and solvent on stability, samples were prepared with a range of surfactants and in different solvents and solvent blends. Structural properties and interparticle interactions were probed using dynamic light scattering (DLS), zeta potentials were determined, and the surfactant layers were investigated with contrast-variation SANS. SANS data suggest that for systems stabilized by ionic surfactants, the nanoparticles are in equilibrium with a population of reverse micelles, but this is apparently not the case for those stabilized by nonionic surfactants. Low zeta potentials show evidence of a small amount of surface charging in these nonaqueous systems, although it is unlikely to have any significant effect on their overall stability.

5.
M. J. Hollamby, R. Tabor, K. J. Mutch, K. Trickett, J. Eastoe, R. K. Heenan and I. Grillo;
Effect of solvent quality on aggregate structures of common surfactants.
Langmuir 24 (2008) 12235-12240

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Aggregate structures of two model surfactants, AOT and C12E5 are studied in pure solvents D2O, dioxane-d8 (d-diox) and cyclohexane-d12 (C6D12) as well as in formulated D2O/d-diox and d-diox/C6D12 mixtures. As such these solvents and mixtures span a wide and continuous range of polarities. Small-angle neutron scattering (SANS) has been employed to follow an evolution of the preferred aggregate curvature, from normal micelles in high polarity solvents, through to reversed micelles in low polarity media. SANS has also been used to elucidate the micellar size, shape as well as to highlight intermicellar interactions. The results shed new light on the nature of aggregation structures in intermediate polarity solvents, and point to a region of solvent quality (as characterized by Hildebrand Solubility Parameter, Snyder polarity parameter or dielectric constant) in which aggregation is not favored. Finally these observed trends in aggregation as a function of solvent quality are successfully used to predict the self-assembly behavior of C12E5 in a different solvent, hexane-d14 (C6D14).

4.
A. Salabat, J. Eastoe, A. Vesperinas, R. F. Tabor and K. J. Mutch;
Photorecovery of nanoparticles from an organic solvent.
Langmuir 24 (2008) 1829-1832

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Commercial silica nanoparticles were dispersed in toluene, stabilized by a mixture of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and a photolyzable anionic surfactant sodium hexylphenylazosulfonate (C6PAS). Selective photolysis of the interfacial C6PAS component induces colloid instability, resulting in flocculation and eventual phase separation of the silica nanoparticles. UV−vis spectroscopy was used to follow the photochemical breakdown of C6PAS; diffusion coefficient measurements by dynamic light scattering were employed to monitor the photoinduced flocculation; and silica contents in the toluene, before and after UV light irradiation, were determined gravimetrically. The results show that light can be used to trigger separation and recovery of nanoparticles stabilized by photolabile interfacial layers.

3.
R. F. Tabor, J. Eastoe and P. J. Dowding;
Surfactant adsorption and aggregation kinetics: relevance to tribological phenomena, in Surfactants in Tribology, Eds. Girma Biresaw and K. L. Mittal;
CRC Press, Boca Raton (2008)

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No abstract data available

2.
A. Salabat, J. Eastoe, K. J. Mutch and R. F. Tabor;
Tuning aggregation of microemulsion droplets and silica nanoparticles using solvent mixtures.
Journal of Colloid and Interface Science 318 (2008) 244-251

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The effect of solvent on stability of water-in-oil microemulsions has been studied with AOT (sodium bis(2-ethylhexyl)sulfosuccinate) and different solvent mixtures of n-heptane, toluene and dodecane. Dynamic light scattering DLS was used to monitor the apparent diffusion coefficient DA and effective microemulsion droplet diameter on changing composition of the solvent. Interdroplet attractive interactions, as indicated by variations in DA, can be tuned by formulation of appropriate solvent mixtures using heptane, toluene, and dodecane. In extreme cases, solvent mixtures can be used to induce phase transitions in the microemulsions. Aggregation and stability of model AOT-stabilized silica nanoparticles in different solvents were also investigated to explore further these solvent effects. For both systems the state of aggregation can be correlated with Veff, the effective molecular volume of the solvent Vmol mixture.

1.
R. F. Tabor, S. Gold and J. Eastoe;
Electron density matching as a guide to surfactant design.
Langmuir 22 (2006) 963-968

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The effectiveness at reducing interfacial tension between water and different organic solvents was studied, with 14 structurally different dichain sulfosuccinate surfactants. Variations in chemical structure ranged from linear/branched alkyl tail groups, to phenyl-tipped tail units, to partially and fully fluorinated tails. The solvents n-heptane, toluene, and perfluoroheptane were used as example oil phases. Interfacial activity was measured in terms of a reduced interfacial tension scale, RIFT, based on the value in the presence of surfactants compared to that for the pure solvent- water interface. Overall surfactant chain structure was determined to be the key factor affecting RIFT. Furthermore, a strong correlation was observed between RIFT and the electron density Fe of the different surfactants: with any given oil, the most effective surfactants have Fe values closest to that for the solvent. For example, phenyl-tipped surfactants were shown to be comparatively more effective at the interface with an aromatic solvent (toluene) than with an aliphatic n-alkane (heptane). Furthermore, fluorination of the tail groups decreased effectiveness at the hydrocarbon/water interface, which was substantially increased at the fluorocarbon/water interface: this too followed the electron density-matching pattern. The importance of chain-tip chemical structure was also noted, with regard to the introduction of phenyl, CF3-, and H-CF2- terminal moieties. For branched alkyl-tailed surfactants, it was found that effectiveness could be linked to an empirical 'branching factor'. The significance of the electron density matching of organic solvent and surfactant for the prediction of interfacial activities is highlighted, and this concept may prove useful for the future The effectiveness at reducing interfacial tension between water and different organic solvents was studied, with 14 structurally different dichain sulfosuccinate surfactants. Variations in chemical structure ranged from linear/branched alkyl tail groups, to phenyl-tipped tail units, to partially and fully fluorinated tails. The solvents n-heptane, toluene, and perfluoroheptane were used as example oil phases. Interfacial activity was measured in terms of a reduced interfacial tension scale, RIFT, based on the value in the presence of surfactants compared to that for the pure solvent- design of new high-efficiency surfactants.