Brian H. Robinson

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.

Kinetics and mechanism of formation of quantum-sized cadmium sulphide particles in water–aerosol-OT–oil microemulsions

Quantum-sized cadmium sulphide particles have been prepared in water-in-oil microemulsions stabilised by Aerosol-OT. The equilibrium particle size has been varied by changing the droplet size of the parent microemulsion in which the particles are prepared. The UV–VIS spectrum of such particles is sensitive to particle size and hence the kinetics of particle growth can be readily followed using UV–VIS spectrophotometric detection. Using the stopped-flow method, particle growth has been monitored at 280 nm as a function of droplet size, temperature and the nature of the oil phase. The rates are consistent with inter-droplet exchange of solubilizates being the rate-determining step. The apparent activation enthalpies calculated for the growth process agree well with those previously found for the exchange of small ions between microemulsion droplets. To interpret the kinetics, a relatively simple model is proposed based on the rapid coagulation mechanism first proposed by Smoluchowski. In the treatment the usual diffusion-controlled rate constant is replaced with the rate constant for inter-droplet communication.

Activity of lipase in water-in-oil microemulsions

The lipase-catalysed hydrolysis rates of several nitrophenyl alkanoate esters of varying alkyl chain length (C4–C16) have been measured both in aqueous solution and in water-in-oil (w/o) microemulsions (which are known to contain discrete droplets). Lipase retains its activity in w/o microemulsions of water, heptane and sodium bis-2-ethylhexyl sulphosuccinate (AOT); the observed rates are consistent with the intrinsic activity of the enzyme (i.e. kcat/Km) being the same as in water. However, the observed conversion rates for C4 and C6 substrates are slower in the microemulsion system because of substrate partitioning to the oil-continuous phase, which results in a reduced concentration in the aqueous pseudophase. This conclusion is reached by comparing lipase and non-enzymic-(i.e. buffer) catalysed rates in both solution media. Again for the C4 and C6 substrate, partition coefficients for the substrates in the limit of high molar ratio of H2O:AOT, as determined from the kinetic results, show good agreement with measured values in heptane + water mixtures. This suggests that lipase functions effectively in the water pseudophase of the microemulsion. Lipase in the microemulsion can also catalyse the hydrolysis of longer chain alkanoates (up to C16). It can be inferred from the kinetics that such substrates partition to the interface where the lipase must also be active. In the case of AOT microemulsions, the pH profile of enzyme activity is not significantly altered compared with bulk water. The lipase retains > 60% activity in the microemulsion after incubation at 35 °C for 6 days. In w/o microemulsions of water, heptane, chloroform and cetyltrimethylammonium bromide (CTAB), the observed hydrolysis rates are significantly reduced and the intrinsic activity is reduced by a factor of twenty as compared with the AOT system. This is thought to be caused by inhibitory binding of CTAB to the protein.

Characterisation of water-containing reversed micelles by viscosity and dynamic light scattering methods

The size and aggregation number of reversed micelles formed by the system aerosol-OT + H2O + organic solvent have been determined by viscosity and dynamic light scattering methods. For the viscosity method, a procedure for deriving values of the aggregation number from particles of variable density is described. Measurements were made in cyclohexane, toluene and chlorobenzene. The dynamic light scattering method, based on photon correlation spectroscopy, yields single exponential correlation functions from which values of the translational diffusion coefficient and the micelle radius can be derived. The droplet size was found to depend primarily on the ratio of surfactant to water concentrations, but was essentially independent of solvent and concentration at a fixed surfactant to water concentration ratio. Satisfactory agreement was obtained among the two methods discussed in this paper and one (sedimentation ultracentrifugation) described previously.

Variation of surfactant counterion and its effect on the structure and properties of Aerosol-OT-based water-in-oil microemulsions

The sodium salt of the di-chained anionic surfactant bis-2-ethylhexylsulfosuccinate [Aerosol-OT or Na(AOT)] stabilises essentially monodisperse, spherical water-in-oil microemulsion (w/o) droplets in alkanes over a wide range of pressure, temperature and composition. In order to investigate the effect of change in counterion charge and size on the microemulsion properties, we have replaced the Na+ counterion by doubly charged ions. The surfactant is then M2+(AOT)2·nH2O: M is from the series Mg2+, Ca2+, Co2+, Ni2+, Cu2+ and Zn2+, and n is the number of water ligands associated with the surfactant molecule. The value of n was determined by FTIR and depends on M2+, but can be between 2 and 8 per molecule of M2+(AOT)2 depending on the nature of M. The ion replacement, assessed by UV–VIS spectrophotometry, is ca. 100% efficient. The effect of temperature on the phase stability of the single-phase M2+(AOT)2 water-in-oil (w/o) microemulsion systems is negligible, in contrast to that observed for the corresponding Na(AOT) system. The structure and properties of the microemulsion are found to be dependent on the counterion identity. Small-angle neutron scattering (SANS) and viscosity measurements provide evidence for the existence of rod-shaped aggregates for Co2+, Ni2+, Cu2+ and Zn2+ at low water constants given by w=[H2O]/[AOT]≈ 5, whilst for Mg2+ and Ca2+ spherical aggregates are present as for Na+. On further addition of water at constant surfactant concentration (w > 10) with Co2+, Ni2+, Cu2+ and Zn2+ the aggregates undergo a shape change, and a more spherical structure is favoured. The results may be explained in terms of the interaction of the different counterions with the SO–3 head group of the surfactant.

Reactivity of α-chymotrypsin in water-in-oil microemulsions

Abstract Detailed kinetic studies have been carried out for the reaction of a range of substrates with a-chymotrypsin in water-in-oil (w/o) microemulsion systems. The main conclusions are as follows: (a) α-chymotrypsin is active when dispersed in w/o microemulsions consisting of water/ aerosol-OT (AOT)/heptane and water/tetraethyleneglycolmono-n-dodecylether (C 12 E 4 )/ heptane. In both systems, the turnover number ( k cat ) is similar to that measured in bulk water. (b) The maximum enzyme activity (as a function of pH of the solubilised water) is found at pH ≈ 8 in bulk water and also in the microemulsion systems. (c) For three substrates in the water/AOT/heptane system, k cat values are similar to those found in bulk aqueous solution, whereas Michaelis constant ( K M ) values (with units of mol dm −3 , and expressed per dm 3 of dispersed water) are found to be increased by a factor of approximately 100. (d) For the substrate N -glutaryl- L -phenylalanine- p -nitroanilide (GPNA) in the AOT system, values of Δ H ‡ cat and Δ H O M were found to be similar to the bulk water values. (c) At a pH where the enzyme is active, it is found that the enzyme slowly loses its activity with time, due to autolysis. The rate of inactivation of α-chymotrypsin was increased in both the AOT and C 12 E 4 systems as compared with bulk water. For the ACT system, a minimum in the enzyme inactivation rate (i.e., maximum stability with respect to autolysis) was found at R = [H 2 O]/[surfactant] ≈ 12. In the C 12 E 4 system, the rate of inactivation was essentially independent of R .

REVERSE ENZYME SYNTHESIS IN MICROEMULSION-BASED ORGANO-GELS

Lipase from three different sources has been immobilised in microemulsion-based gels (MBGs) with retention of catalytic activity. Such lipase-containing MBGs prove to be novel solid-phase catalysts for use in apolar organic solvents such as n-heptane. Using these systems, preparative-scale synthesis of a wide variety of esters under mild conditions was possible with products easily isolated and obtained in high yield. Stereoselective esterification of octan-2-ol was observed for all three lipases with Chromobacterium viscosum (CV) lipase yielding product with an enantiomeric excess of 92%. Repeated usage of a CV lipase-containing MBG resulted in a visually unchanged gel whose activity was 75% of the initial value after 30 days. The sectioned MBGs were well suited for use in column flow reactors and were also found to be effective esterification catalysts at temperatures as low as -20 degrees C.

Small-angle neutron-scattering study of microemulsions stabilised by aerosol-OT. Part 1.—Solvent and concentration variation

Small-angle neutron-scattering (SANS) measurements have been made for a series of aerosol-OT (AOT)-stabilised water-in-oil microemulsions. The intensity pattern has been used to extract a value for the radius of the water core, rw, using D2O to provide the required contrast profile. In heptane the radii are found to follow an approximately linear relationship with respect to the [D2O]/[AOT] concentration ratio, R. At 20 °C, and R= 20, the structure of the water-droplet system is dependent on the hydrocarbon chain length of the oil medium. The experimental SANS patterns show increasing discrepancies with a fitted function based on monodisperse spheres as the length of the alkane chain is increased from n-heptane to n-dodecane. This effect is attributed to polydispersity and indicates that the droplet-size distribution within these microemulsion systems is much larger than had previously been thought.

Structural studies of microemulsions stabilised by aerosol-OT

Abstract In this article we review structural studies of AOT-stabilised water-in-oil microemulsions. Nanometer-sized spherical droplets of water coated by a curved monolayer of surfactant are preferred over a wide range of pressure and temperature conditions, as well as in very different physical environments, for example a supercritical or a solid alkane. Within the one-phase region variation of pressure and temperature essentially serves to fine-tune the strength of the attractive interactions between droplets.

Thermodynamic and kinetic stability. Properties of micelles and vesicles formed by the decanoic acid/decanoate system

The dialysis of micellar and vesicular systems, through a cellulose acetate membrane permeable only to monomer surfactant, has been investigated for the system sodium decanoate/decanoic acid as a function of pH, in pH regions where micelles (pH 9) or vesicles (pH 7.5) are present. The results provide data that are helpful in establishing the thermodynamic state of the systems and the interplay of the relevant equilibrium and kinetic considerations. Additionally, the kinetics of spontaneous formation and destruction of vesicles when subjected to a pH-jump perturbation have been studied. Changes are rapid, taking place over a time scale of a few seconds.