Andrew M. Howe

The kinetics of solubilisate exchange between water droplets of a water-in-oil microemulsion

Exchange rates of aqueous solubilisates between water droplets in a water-in-oil microemulsion stabilised by sodium bis(2-ethyl-hexyl) sulphosuccinate (AOT) have been measured as a function of droplet size, temperature and the chain length of the oil. The effects of additives (e.g. alcohols) on the exchange kinetics have also been investigated. Exchange rates were measured using very fast chemical reactions as indicators for exchange. Three types of reaction were investigated: proton transfer, metal–ligand complexation and electron transfer. Similar exchange rates were found for all three reactions. The indicator reactions involve the exchange of reactant ions of differing size and charge type; exchange rates were, however, independent of the ion transferred, but dependent on droplet size and temperature. For AOT as dispersant, exchange occurs with a second-order rate constant of 106–108 dm3 mol–1 s–1, two to four orders of magnitude slower than the droplet encounter rate as predicted from simple diffusion theory. The apparent activation enthalpy is high (and increases with droplet size) but is largely compensated by a positive activation entropy. Exchange, on balance, is a relatively facile process which typically takes place on a millisecond timescale (depending on the droplet concentration).The exchange mechanism involves transient water droplet coalescence and separation. This is the dynamic process whereby the equilibrium properties of the microemulsion, e.g. droplet size and polydispersity, are maintained. There is a correlation between the exchange rate constants and the stability of the single-phase microemulsion. This relationship between the kinetic and equilibrium properties is discussed in terms of the ‘natural curvature’ of the surfactant interface and inter-droplet interactions.

TECHNIQUE TO MEASURE EMULSION CREAMING BY VELOCITY OF ULTRASOUND

ABSTRACT A new non-intrusive, Instantaneous technique to monitor the creaming/sedimentation of colloidal dispersions is reported. Here the technique is used to determine the detailed concentration profiles of water-continuous emulsions as a function of height and time. The emulsions consist of hexadecane stabilised by nonionic surfactant (Brij 35) In water In the presence and absence of a polysaccharide thickener (hydroxyethylcellulose). The technique involves the measurement of the velocity of ultrasound through the emulsion, which depends simply on the velocity through the individual phases and the bulk composition of the emulsion (provided the individual droplets are smaller than the wavelength of the ultrasound). Applications of the technique may include In-line monitoring of composition during the processing of dispersions and predictive testing of dispersion stability (shelf-life).

Structure and stability of microemulsion-based organo-gels

The phase stability, structure and physical (mechanical) properties of gelatin-containing, AOT-stabilised microemulsion-based organo-gels have been studied as a function of their composition. Phase-stability studies indicate that the presence of small amounts of gelatin changes the shape and extent of the single-phase microemulsion region with respect to temperature. Small-angle neutron scattering shows that the gel structure is based on coexisting gelatin–water networks and microemulsion droplets. The gel structure is sensitive to the AOT and gelatin concentrations, water content, added salts and the nature of the oil used as the continuous phase. The likelihood of gel formation is linked to the proximity of the microemulsion upper-temperature phase boundary and the gelatin helix–coil transition temperature at ca. 32 °C. The frequency dependence of the elastic moduli of the organo-gels is similar to that of aqueous gelatin gels, being characteristic of a viscoelastic solid. The network strand density, derived from the relaxed elastic modulus, is consistent with the same parameter calculated from neutron scattering measurements.

Gravitational destabilization of emulsions flocculated by non-adsorbed xanthan

Abstract The anionic polysaccharide xanthan, widely used as a stabilizer in food dispersions, induces flocculation by a depletion mechanism. The effect of flocculation, induced by the presence of xanthan, on the gravitational destabilization (creaming and compaction) of 20% oil-in-water emulsions is presented. The detailed gravitational destabilization processes are studied by following changes in the concentration profile with time. In the absence of flocculant the polydisperse droplets rise as a diffuse meniscus and form a cream of ~70% oil at the top of the sample. Such behaviour is consistent with creaming in a dispersion of non-interacting polydisperse particles. In contrast the flocculated droplets form a sharp meniscus and cream rapidly. At the top of the sample the floes form a less concentrated cream which undergoes compaction under gravity. The compaction is characterized by a concentration-dependent bulk modulus.

Microemulsion-based gels: A small-angle neutron scattering study

Abstract Fluid water-in-oil microemulsions stablished by the surfactant aerosol-OT (OAT) may be gelled on addition of gelatin. A small angle neutron-scattering study from AOT-stablished microemulsions and microemulsion-based gels (MBGs) show that the addition of gelatin to the microemulsion broadens and shifts to a higher wave vector the characteristics single broad peak and reduces the absolute scattering intensity. The behaviour is consistent with a picture strongly coupled microemulsion and gel systems, indicating that the MBG forms hydrated networks of gelatin.

Rheology of polystyrene latexes with adsorbed and free gelatin

Abstract The rheology of monodisperse polystyrene latex particles of two different particle radii (26 and 67 nm) has been studied with a range of concentrations of the polyampholyte gelatin. Gelatin contributes to the rheology by adsorption to the particles and by thickening the continuous phase. High viscosities and strong shear thinning are measured for low volume fractions of latex. A procedure is presented to deconvolute the effects of free and bound gelatin by applying simple hard-sphere models. This procedure allows us to estimate the effective size of the gelatin-covered particles as well as the continuous-phase gelatin concentration and viscosity. The layer thicknesses from rheology agree well with those from PCS. The effect of varying particle volume fraction, ionic strength, pH and gelatin and surfactant concentration on the rheology of these suspensions is presented. For the smaller latex, the adsorbed layer occupies a greater fraction of the effective volume. Increasing free polymer concentration reduces the adsorbed-layer thickness. The reduced critical shear stress increases with the suspension viscosity for suspensions of the 26 nm latex but is constant for the 67 nm latex. At very high shear (>2000 s −1 ), the suspensions show excess shear thinning over that expected from a hard-sphere model. This excess thinning is attributed to deformation of the adsorbed gelatin layer under high shear stress and interpreted in terms of an empirical interparticle potential.

Small-angle neutron-scattering studies of microemulsions stabilized by aerosol-OT. Part 2.—Critical scattering and phase stability

Small-angle neutron scattering has been used to characterise the structural properties of various microemulsions containing D2O water droplets, stabilised by the surfactant aerosol-OT in decane, undecane and dodecane solvents. Phase diagrams have also been obtained for corresponding systems containing H2O. The SANS intensity profile exhibits a critical scattering component in the low-Q region which increases in magnitude as the temperature approaches the limit of stability. The observations may be fitted to an Ornstein–Zernike expression to determine a correlation length ξ, and this is found to vary systematically with respect to the reduced temperature difference Iµ=(Tc–T)/Tc. The critical constants v and γ are found to be 0.67±0.10 and 1.65±0.15, respectively.

Determination of gravitational separation in dispersions from concentration profiles

Abstract A non-intrusive technique is used to investigate the gravitational sedimentation/creaming of particles in a dispersion. The velocity of ultrasound is measured over a series of heights in the dispersion, and the data are converted to concentration profiles which are collected over a period of time. Often the method detects sedimentation/creaming at a very early stage and can therefore predict long-term stability. Data are presented on a model suspension, and compared with theoretical predictions. In a polydisperse emulsion the creaming behaviour gives a good estimate of the particle size distribution. In an emulsion containing a depletion flocculant, coexistence of two phases is detected, and the compaction of the flocculated cream layer gives a measure of the strength of the network.

Stability of oil-in-water emulsions. The effect of dispersed phase and polysaccharide on creaming

Abstract The creaming of concentrated alkane-in-water emulsions stabilised by a non-ionic surfactant has been measured in the presence of hydroxyethylcellulose polysaccharide. Creaming rates under gravity were determined by visual observation. In the emulsions containing polysaccharide the creaming was much more rapid than expected from consideration of the oil droplet size, density difference, the rheological properties of the continuous phase and the current oil concentration. Flocculation effects would appear to be the primary mechanism for the anomalous behaviour, although other effects cannot be entirely ruled out at this stage. The final oil concentrations in the creamed emulsion phases have also been measured, and an interpretation is given in terms of the pressure in the cream, the droplet size and the interfacial tension.

Structure and dynamics of water-in-oil microemulsions stabilised by Aerosol-OT

Abstract Small-angle scattering techniques are used in conjunction with dynamic studies of collision processes to investigate the mechanism of droplet aggregation in AOT-stabilised water-in-oil microemulsions. The presence of ‘almost dry’ reversed micelles co-existing with the water droplets is postulated to explain the scattering and kinetics data. The equilibrium droplet distribution function is shown to be dependent on the dynamic processes occurring in the microemulsion.