Interdroplet Interactions Research Articles

Solubilization and phase equilibria of water-in-oil microemulsions: I. Effects of spontaneous curvature and elasticity of interfacial films

The solubilization and phase equilibria of w/o microemulsions have been shown to be dependent on two phenomenological parameters, namely the spontaneous curvature and elasticity of the interfacial film, when interfacial tension is very low. The spontaneous curvature of an interface is basically determined by the geometric packing of surfactant and cosurfactant molecules at the interface, whereas the interfacial elasticity is related to the energy required to bend the interface. The droplet size and solubilization of microemulsions is mainly determined by the radius of spontaneous curvature, and is further influenced by interfacial elasticity and interdroplet interactions. A w/o microemulsion with a highly curved and relatively rigid interfacial film can exist in equilibrium with excess water at the solubilization limit due to the interfacial bending stress. Increasing the natural radius and fluidity of the interface can increase the droplet size and hence the solubilization in the microemulsion. On the other hand, a w/o microemulsion with a highly fluid interfacial film can exist in equilibrium with an excess oil phase containing a low density of microemulsion droplets due to attractive interdroplet interaction. Increasing the interfacial rigidity and decreasing the natural radius in this case can increase water solubilization in the microemulsion by retarding the phase separation process. Thus, a maximum water solubilization in a w/o microemulsion can be obtained by minimizing both the interfacial bending stress of rigid interfaces and the attractive interdroplet interaction of fluid interfaces at an optimal interfacial curvature and elasticity. The study of phase equilibria of microemulsions can serve as a simple method to evaluate the property of the interface and provide phenomenological guidance for the formulation of microemulsions with maximum solubilization capacity.

Solubilization and phase equilibria of water-in-oil microemulsions: II. Effects of alcohols, oils, and salinity on single-chain surfactant systems

The solubilization and phase equilibria of water-in-oil (w/o) microemulsions have been studied to elucidate how the molecular structure of various components of microemulsions and salinity influence two interfacial parameters of the system, namely the spontaneous curvature and elasticity of interfacial films. Most solubilization and phase equilibria data presented are explained by the change of these two parameters. It is indicated that oil can influence the property of an interfacial film by a solvation effect. Oil molecules with small molecular volume or high aromaticity produce a strong solvation effect and consequently lead to a greater penetration of oil molecules into the surfactant chain layer, thus increasing the rigidity and curvature of the interface. The addition of alcohol exhibits strong effects on the elasticity and spontaneous curvature of an interface. Decreasing alcohol chain length can increase the fluidity of the interfacial film and hence increase the attractive interdroplet interaction. On the other hand, increasing alcohol chain length often increases the rigidity and curvature of an interface. For alcohols which increase the interfacial fluidity, increasing alcohol partitioning at the interface will increase the fluidity and natural radius of the interface. The addition of electrolytes results in tighter molecular packing at the interface and a decrease in the natural radius and interfacial fluidity. There often exists an optimal salinity for a given w/o microemulsion system at which a maximum brine solubilization occurs. The maximum solubilization is interpreted as a result of counteracting effects of attractive interdroplet interaction of fluid interfaces and the bending stress of rigid interfaces. As a result of this study, phenomenological guidelines for the formulation of microemulsions have been proposed. It is inferred that the addition of a small amount of alcohol at optimal salinity together with the condition of chain length compatibility can result in the largest possible brine solubilization in a given w/o microemulsion.

Studies on the clustering Behaviour of Water-Aerosol OT-Isooctane Microemulsions from their Low Frequency Permittivity Data

The low frequency limiting permittivity of water-Aerosol OT-isooctane microemulsions containing additives like cholesterol and n-propanol have been measured over a temperature range 15-45°C. The concentration dependent polarizability data were always found to be equal to or higher than those calculated from the Clausius-Mossotti relation. The excess polarizability of the system has been explained in terms of a parameter which has been found to be dependent on the water permeability of the surfactant monolayer of the droplets. It has been concluded that the monolayer permeability allows the droplets to form clusters through a strong interdroplet interaction.

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.

Solubilization and interaction of α-tocopherol in water-aerosol OT-isooctane systems

Solubilization and interaction of α-tocopherol into bis(2-ethylhexyl)sulphosuc cinate sodium salt microemulsion systems have been studied by temperature dependent phase transition, viscosity and nuclear magnetic resonance studies. Tocopherol being an amphiphilic molecule dissolves into the interfacial surfactant monolayer of the microemul sion droplets. The dissolution leads to an enhancement of the rigidity of the surfactant monolayer as studied by the increase in mixing and phase transition temperatures of the microemulsion droplets. Solubilization of tocopherol into microemulsion droplets causes an increase in the effective size of the droplet and as a consequence, the inter-droplet interactions are also increased. The water binding capacity of the surfactant (bis(2-ethylhexyl)sulphosuccinate sodium salt) is reduced due to solubilization of tocopherol as is evidenced from the downfield shifts of water proton magnetic resonances. In the presence of the dissolved electrolytes into the aqueous core, tocopherol is squeezed out of the microemulsion droplets increasing the membrane fluidity and permeability.

Rheological Behavior of Soya Oil‐Water Emulsions: Dependence upon Oil Concentration

ABSTRACTStress‐strain measurements have been performed in the linear region for soya oil‐water emulsions stabilized with xanthan gum for various oil concentrations and for xanthan gum solutions for various xanthan gum concentrations. The big increase observed in the shear modulus for the emulsions (as compared to the solutions) is primarily due to the interaction between xanthan gum molecules and emulsion droplets. The results obtained are in broad agreement with the existing theories for concentrated dispersions and tend to show that the interdroplet interaction is similar to that expected for ‘soft’ deformabe spheres. The results for xanthan gum solutions can be explained if the xanthan gum solutions are not true solutions, but some form of dispersions.