Paul D. I. Fletcher

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.

Aspects of aqueous foam stability in the presence of hydrocarbon oils and solid particles

Abstract The control of foaming industrially is an important process, and in the case of aqueous foams is often achieved by addition of a dispersion of small hydrophobic solid particles in a mineral oil. Our work is concerned with the acquisition of reliable data for well-defined, chemically pure systems in order to evaluate in a convincing way some of the possible mechanisms of foam breakdown which have been presented in the literature. We present here some of our findings on the destabilisation of foams and single films by dispersed hydrocarbon oil droplets, by spherical hydrophobic particles and (in the case of single films) cylindrical hydrophobic rods, and by solids and oils in combination. In our work on oils we have focused on the process of oil drop entry into the air-aqueous solution interface, which is a prerequisite for foam and film breaking. The feasibility of entry is related to the various interfacial tensions in the system, usually combined in the form of the entry coefficient. We discuss some of the problems in the determination and use of entry coefficients in predicting whether entry should or should not occur. Where entry is possible, the rate of drop entry is an important factor in the consideration of foam breakdown. We report on the residence times of single oil drops placed under the air-solution interface and relate these to the lifetimes of films and foams in the presence of dispersed entering oil drops. In the description of the effects of solid particles on foam breakdown the parameter of central importance is the contact angle which the air-solution interface makes with the solid surface. The effectiveness of cylindrical hydrophobic rods in rupturing single soap films rises dramatically when the contact angle approaches 90°. On the other hand, when spherical hydrophobic particles are added to a foam we find that foams can be stabilised, and indeed maximum stability has been observed for a contact angle of 90°. For slightly higher angles however, foam stability falls drastically. In connection with planned work on the effects of solids and oils together on foam stability we discuss ways of modifying contact angles of oil-solution interfaces with solid surfaces.

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.

Microwave heating of heterogeneously catalysed Suzuki reactions in a micro reactor

The Suzuki cross-coupling reaction of aryl halides with phenylboronic acid to form biaryls has been used to illustrate the development of a microwave based technique capable of delivering heat locally to a heterogeneous Pd-supported catalyst located within a micro reactor device. A 10-15 nm gold film patch, located on the outside surface of the base of a glass micro reactor, was found to efficiently assist in the heating of the catalyst when irradiated with 5-7 W of microwave power at 2.45 GHz. Using a hydrodynamically pumped system, reactant-catalyst contact times of less than 60 s were found to give conversions for different substrates which were in the range 50-99%. Two methods of loading catalysts into the micro reactor were investigated which required either 1.5 or 6 mg of material.

The partitioning of solutes between water-in-oil microemulsions and conjugate aqueous phases

The partitioning of chymotrypsin, pepsin and lysozyme between water-in-oil microemulsion phases stabilised by the anionic surfactant sodium bis-2-ethylhexyl sulphosuccinate (AOT) and conjugate aqueous phases containing NaCl has been measured. For chymotrypsin and pepsin the extent of partitioning as a function of AOT concentration may be described in terms of a partition coefficient between the microemulsion-dispersed water and the bulk water phase. For aqueous phases containing low salt concentrations in equilibrium with microemulsion phases of relatively large droplet sizes, the value of the partition coefficient is sensitive to the overall protein charge. High values of the partition coefficient are observed for positively charged proteins owing to attractive electrostatic interactions with the negatively charged surfactant film. At high salt concentrations, corresponding to small microemulsion droplets, the partitioning is less sensitive to protein charge. Proteins have a higher affinity for the microemulsion-dispersed water than for bulk water even when the overall protein charge is negative. Lysozyme forms a solid-like phase with AOT which dissolves in the oil when sufficient AOT is present for monolayer coverage of the lysozyme surface. Similar partitioning studies with an enzyme substrate provides useful information for the interpretation of enzyme kinetics in microemulsions.

Evaporation rates of pure liquids measured using a gravimetric technique

We describe a gravimetric method for the determination of evaporation rates. The liquid sample is held in a partially filled, cylindrical open-topped tube within a vertically flowing gas stream. A simple model appropriate to this geometry is found to account for the variation of rate with liquid height within the sample tube and gas flow rate. Evaporation rates for a range of pure liquids with vapour pressures ranging from 0.1 to 500 Torr were determined and showed reasonable agreement with theoretical values estimated using literature values of the vapour pressures and vapour diffusion coefficients in air.

The resolution of water-in-crude oil emulsions by the addition of low molar mass demulsifiers

Abstract We have measured the rates of demulsification of water-in-crude oil emulsions using a series of octylphenyl-polyethoxylates and sodium bis(2-ethylhexyl)sulfosuccinate (AOT) as demulsifiers. The hydrophilic—lipophilic balance (HLB) of the surfactant systems was varied systematically by changing the number of ethoxy groups in the case of the nonionic surfactants and by changing the concentration of added NaCl in the case of AOT. For all surfactants and conditions, the demulsification rate increased with surfactant concentration up to the onset of surfactant aggregation in the oil, the water, or a third, surfactant-rich phase. The highest rate reached is estimated to be close to the diffusion controlled value. Higher concentrations of the surfactants produced either a decrease in demulsification rate (“overdosing”) or a slight increase to a plateau value of the rate. Overdosing was observed for solutions where the surfactants formed aggregates in the oil phase (“hydrophobic surfactants”). “Hydrophilic surfactants” (for which aggregation occurred in the water phase) showed overdosing behavior only for cases where a high viscosity aqueous phase was produced. Higher concentrations of hydrophilic surfactants which gave low viscosity aqueous phases on demulsification gave a plateau value in the demulsification rate.

Structural study of aerosol-OT-stabilised microemulsions of glycerol dispersed in n-heptane

Thermodynamically stable Aerosol-OT-stabilised dispersions of glycerol in n-heptane (microemulsions) have been studied using dynamic light scattering and viscometry. Up to five moles of glycerol can be dispersed per mole of aerosol-OT in n-heptane. The resulting solutions consist of discrete spherical droplets of glycerol stabilised by the surfactant. Droplet size is independent of temperature and depends primarily on the mole ratio (R) of glycerol to AOT according to hydrodynamic radius/nm = 1.7(±0.2)+0.88(±0.15)R. The apparent interfacial area occupied per AOT molecule is ca. 20% less in the glycerol dispersion than in the corresponding water dispersion. Attractive interactions between the droplets increase as the microemulsion phase stability limit is approached.

Microcontact printing of DNA-surfactant arrays on solid substrates

We have designed a novel method for fabrication of DNA arrays based on microcontact printing of DNA-surfactants on solid substrates. DNA-surfactants were prepared by covalent attachment of a hydrophobic anchoring group to the (3′- or 5′-) end of DNA oligonucleotides. This anchoring group allows DNA-strands to be immobilised on hydrophobic surfaces by hydrophobic interactions. The microcontact printing method was adapted for aqueous “inks” containing DNA-surfactants. Special attention was paid to the wetting properties of the ink with respect to the stamp and the solid substrates. The method allows for efficient attachment of DNA strands to solid surfaces and hybridisation with complementary DNA strands. This new technology could be utilised for rapid preparation of DNA-assays and genetic biochips.

Interaction of enzymes with surfactants in aqueous solution and in water-in-oil microemulsions

Kinetic studies have been carried out on hydrolysis reactions catalysed by a variety of lipases and α-chymotrypsin in aqueous solution in the presence of surfactants in order to provide information on the role of the surfactant in similar processes in microemulsions. The main conclusions concerning the influence of surfactants on the activity of lipase and α-chymotrypsin are as follows. For ex-Candida and Chromobacterium viscosum lipase, the kinetic behaviour of the enzyme in the hydrolysis of p-nitrophenylbutyrate in the presence of surfactants was dependent on the source of lipase. Most experiments were carried out with an unfractionated sample of C. viscosum-lipase. Both competitive and non-competitive inhibition was detected, the former with sodium dodecylsulphate (SDS) and cetyltrimethylammonium bromide (CTAB) and the latter with aerosol-OT (AOT). In all cases inhibition was reversible. In addition, lipases are resistant to slow denaturation by surfactants over periods of several hours. For lipases, there is apparently no correlation between the behaviour of the enzyme in aqueous solutions of surfactant and the behaviour in water-in-oil microemulsions stabilised by AOT and CTAB.In contrast, for the hydrolysis of N-glutaryl-L-phenylalanine-p-nitroanilide (GPNA) by α-chymotrypsin in aqueous solution and in microemulsions, we find both inhibiting and denaturing interactions. AOT is a competitive inhibitor for α-chymotrypsin in aqueous solution but enzyme stability is unaffected. The same behaviour is observed in AOT-stabilised microemulsions. The turnover number (kcat) is essentially unchanged in aqueous solution and microemulsions, but the Michaelis constant (KM), expressed as an aqueous concentration, is increased in AOT microemulsions by a factor of ca. 100 for the hydrolysis of GPNA. For C12E6 and SDS we find both inhibition and denaturation interactions with α-chymotrypsin in water. The stability of α-chymotrypsin is decreased in the presence of SDS and C12E6 in water and in microemulsions as compared with systems containing AOT. CTAB does not inhibit α-chymotrypsin (in terms of an increased KM), but CTAB is an effective denaturing agent for α-chymotrypsin. However, the denaturation reaction does not occur in the presence of substrate.The various surfactants form stable inclusion complexes with α-cyclodextrin (α-CD). Hence α-CD is an effective scavenger for the surfactant in solution. On addition of α-CD, the enzyme activity can be restored to that found in the absence of surfactant.