Browsing by Author "Yaron, PN"

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    Effects of surfactant transport on high internal phase emulsions under shear: a combined rheological and structural study
    (American Institute of Chemical Engineers, 2021-10-19) Yaron, PN; Reynolds, PJ; Mata, JP; White, JW
    We report the change in rheological behavior of high-internal phase emulsions (HIPEs) under shear of a polyisobutylene-based (PIBSA) oil-soluble surfactant and with and without the addition of a water-soluble polyacrylamide (PAM) co-surfactant. We used a series of contrast-matched small-angle and ultra-small angle neutron scattering (SANS and USANS) coupled with in situ rheological measurements to track the locations of the surfactant and co-surfactant as a function of shear. This work follows a series of papers analyzing the structural variation and stability of emulsions stabilized with PIBSA and various mixtures of PIBSA/PAM under static and shear conditions. The emulsions’ sensitivity to aqueous/oil phase ratios, surfactant concentration, surfactant molecular weight, and polydispersity has been defined. The emulsions consist of almost spherical micron scale, highly polydisperse, aqueous droplets dispersed in a continuous oil phase with aqueous/oil phase ratios of about 9:1. The emulsions are rheologically unexceptional and follow previously established predictions and theory. The emulsions show refinement to higher viscosity after high shear, and shear thinning. The structural basis for its rheological behavior however does not follow theory. Shear dependent changes observed in the SANS data were tracked by SANS model parameters using a convolution of two well-established models. Shear thinning is explained by SANS observed shear disruption of inter-droplet bilayer links, causing deflocculation to more spherical, less linked, aqueous droplets. Refinement to higher viscosity is accompanied by droplet size reduction and loss of surfactant from the oil continuous phase. Refinement occurs because of shear-induced droplet anisotropy, which we have also observed in the SANS experiment. The observed anisotropy and emulsion refinement cannot be reproduced by either isolated molecule or mean-field models and require a more detailed consideration of interdroplet forces in the sheared fluid. Even at concentrations by the stability limit, a large percentage of surfactant(s) is dissolved as small n-mers or as larger reverse micelles that play an important part in stabilization of the emulsion under shear. Steady shear on PIBSA/PAM emulsions reduces the number of reverse surfactant micelles present in the both the continuous oil phase to provide the surfactant needed to cover the newly formed surface area as the emulsion refines to smaller droplet sizes. Surfactant adsorption to droplet interfaces is accomplished by two processes. The first draws soluble surfactant from the oil phase to the interface and lowers the concentration of dissolved n-meric surfactant. In parallel, oil-soluble reverse micelles begin to break up, allowing a shear rate dependent steady state to establish between the surfactant reservoirs and aqueous droplets. The application of low-shear rate recovery intervals allowed the recovery dynamics of the surfactant distribution to be observed. The results showed little reduction of the emulsion interfacial area upon return to its quiescent state, but a large recovery of the reverse micelle volume fraction that indicates the continuous phase acts as the reservoir of surfactant when the emulsion is under shear. Drastic changes in the rheological behavior of emulsions with PAM co-surfactant indicate different kinetics dictate surfactant and co-surfactant droplet adsorption. We report the effect of altering the chain length and concentration of PAM based co-surfactants (C12-PAM, C14-PAM, and C16-PAM) on the properties of the high-internal phase PIBSA emulsions under shear.
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    High internal phase emulsions under Shear. Co-surfactancy and shear stability
    (American Chemical Society, 2011-05-19) Yaron, PN; Scott, AJ; Reynolds, PA; Mata, JP; White, JW
    Large changes in the rheology of high-internal phase aqueous-in-oil emulsions (HIPEs) using an oil-soluble polyisobutylene-based primary surfactant (PIBSA) are provoked by very small quantities of water-soluble polyamide-based cosurfactants (PAM with C-12, C-14, and C-16 tails). The structural origin of this was studied using small-angle neutron scattering (SANS) from sheared emulsions, with simultaneous in situ rheology measurements. The PAM drastically lowers the droplet oil interfacial tension by displacing PIBSA, causing large droplet deformation under shear and much lowered emulsion yield stress. With PAM, the surfactant monolayer at the droplet surface becomes more responsive to droplet shape change and redistributes in response to shear which the PIBSA-only system does not. Although it is oil-insoluble, PAM also reaches the nanoscale PIBSA micelles in the oil phase, changing micelle size and content in ways predictable from the hydrophilicity of the different PAMs. PAM does not, however, strongly affect the viscosities at high shear rates; shear thinning and thickening are unaffected. Droplet size, droplet-droplet flattening, and linkage determine the viscosities observed, more so than, droplet-oil interfacial tension. We infer from this that the droplet motion under shear does not involve much transient droplet deformation as the droplets move by each other. Author: Dikundwar, AG; Venkateswarlu, C; Piltz, RO; Chandrasekaran, S and Row, TNG Year: 2011 Journal: Crystengcomm Title: Crystal structures of fluorinated aryl biscarbonates and a biscarbamate: a counterpoise between weak intermolecular interactions and molecular symmetry Volume: 13 Pages: 1531-1538 Abstract: Conformational features and supramolecular structural organization in three aryl biscarbonates and an aryl biscarbamate with rigid acetylenic unit providing variable spacer lengths have been probed to gain insights into the packing features associated with molecular symmetry and the intermolecular interactions involving 'organic' fluorine. Four structures but-2-yne-1,4-diyl bis(2,3,4,5,6-pentafluorophenylcarbonate), 1; but-2-yne-1,4-diyl bis(4-fluorophenylcarbonate), 2; but-2-yne-1,4-diyl bis(2,3,4,5,6-pentafluorophenylcarbamate), 3 and hexa-2,4-diyne-1,6-diyl bis(2,3,4,5,6-pentafluorophenylcarbonate), 4 have been analyzed in this context. Compound 1 adopts a non-centrosymmetric "twisted'' (syn) conformation, whereas 2, 3 and 4 acquire a centrosymmetric "extended'' (anti) conformation. Weak intermolecular interactions and in particular those involving fluorine are found to dictate this conformational variation in the crystal structure of 1. A single-crystal neutron diffraction study at 90 K was performed on 1 to obtain further insights into these interactions involving 'organic' fluorine.© 2011, American Chemical Society
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    Human serum albumin binding to silica nanoparticles - effect of protein fatty acid ligand
    (Royal Society of Chemistry, 2014-02-19) Ang, JC; Henderson, MJ; Campbell, RA; Lin, JM; Yaron, PN; Nelson, A; White, JW
    Neutron reflectivity shows that fatted (F-HSA) and defatted (DF-HSA) versions of human serum albumin behave differently in their interaction with silica nanoparticles premixed in buffer solutions although these proteins have close to the same surface excess when the silica is absent. In both cases a silica containing film is quickly established at the air-water interface. This film is stable for F-HSA at all relative protein-silica concentrations measured. This behaviour has been verified for two small silica nanoparticle radii (42 Å and 48 Å). Contrast variation and co-refinement have been used to find the film composition for the F-HSA-silica system. The film structure changes with protein concentration only for the DF-HSA-silica system. The different behaviour of the two proteins is interpreted as a combination of three factors: increased structural stability of F-HSA induced by the fatty acid ligand, differences in the electrostatic interactions, and the higher propensity of defatted albumin to self-aggregate. The interfacial structures of the proteins alone in buffer are also reported and discussed. © 2015, Royal Society of Chemistry.
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    Nano- and microstructure of high-internal phase emulsions under shear
    (American Chemical Society, 2010-03-18) Yaron, PN; Reynolds, PA; McGillivray, DJ; Mata, JP; White, JW
    High-internal phase aqueous-in-oil emulsions of two surfactant concentrations were studied using small-angle neutron scattering (SANS) and simultaneous in situ rheology measurements. They contained a continuous oil phase with differing amounts of hexadecane and d-hexadecane (for contrast matching experiments), a deuteroaqueous phase almost saturated with ammonium nitrate, and an oil-soluble stabilizing polyisobutylene-based surfactant. The emulsions’ macroscopic rheological behavior has been related to quantify changes in microscale and nanoscale structures observed in the SANS measurements. The emulsions are rheologically unexceptional and show, inter alia, refinement to higher viscosity after high shear, and shear thinning. These are explained by changes observed in the SANS model parameters. Shear thinning is explained by SANS-observed shear disruption of interdroplet bilayer links, causing deflocculation to more spherical, less linked, aqueous droplets. Refinement to higher viscosity is accompanied by droplet size reduction and loss of surfactant from the oil continuous phase. Refinement occurs because of shear-induced droplet anisotropy, which we have also observed in the SANS experiment. This observed anisotropy and the emulsion refinement cannot be reproduced by either isolated molecule or mean-field models but require a more detailed consideration of interdroplet forces in the sheared fluid. © 2010, American Chemical Society
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    Stability of high internal phase emulsions at low surfactant concentration studied by small angle neutron scattering
    (Elsevier, 2010-09-15) Reynolds, PA; McGillivray, DJ; Mata, JP; Yaron, PN; White, JW
    The changes in structure of high internal phase emulsions at low concentrations and at elevated temperature are reported for comparison with the same emulsions under conditions well away from instability. Small angle neutron scattering measurements on aqueous ammonium nitrate droplets dispersed in hexadecane and stabilized by very small quantities of a polyisobutylene-based surfactant (PIBSA) as well as related inverse micellar solutions in hexadecane, have been made as a function of temperature and surfactant concentration. Experimental conditions here favour larger and more deformable droplets than in previous studies. Besides the expected micelles and adsorbed surfactant, planar bilayers of micron lateral extent between touching droplets cover 20% of the droplet surface. Another difference from previous experiments is that the oil phase in the emulsions, and corresponding inverse micellar solutions are different in micellar radii and composition. The differences, and changes with surfactant concentration and temperature, are attributed to fractionation of the polydisperse PIBSA in the emulsions, but not the inverse micellar solutions. At low PIBSA concentration and high temperature the SANS shows emulsion decomposing into separate oil and aqueous phases. This occurs when the micelle concentration reaches a very small but measurable value. The inverse micelles may suppress by steric action long wavelength unstable capillary waves in the bilayers. Depletion repulsion forces here have a minor role in the emulsion stabilization. © 2010, Elsevier Ltd.