Macroscopic Properties of Emulsions and Microemulsions

Department of Chemistry, 1393 Brown Building, Purdue University, 

Emulsions and microemulsions are dispersions of liquid in liquid. Therefore, an important feature is their interfacial fluidity and deformability, which distinguishes them from suspensions of solid particles. The stability of the latter is usually treated in the framework of the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory, which accounts for the electrostatic and van der Waals interactions between solid particles. During recent years it was shown that other types of interparticle forces may often play an important role for the stability of dispersions—hydrodynamic interactions, hydration and hydrophobic forces, oscillatory structure forces, etc. - It was proven both experimentally and theoretically that steric and depletion interactions may sometimes be a decisive factor for the dispersion stability. 

Hamaker. The contribution of the surface extension energy and/or bending elasticity to the pair interaction potential is also included. The extension of the drop surface upon the deformation corresponds to a soft interdroplet repulsion. All the remaining possible interactions (electrostatic, steric, depletion, etc.) can usually be treated in the framework of Deqaguin s approximation, which allows one to account for the two contributions of the total interaction energy (i) across the flat film and (ii) between the spherical surfaces surrounding the fllm. Combined with relevant expressions for the hydrodynamic interactions, this approach could be used for studying the coalescence of Brownian emulsion and microemulsion droplets.  

The proper account of the droplet deformability is particularly important for systems with low interfacial tension. Microemulsions are a typical example as well as vesicles. Larger emulsion droplets, on the other hand, are prone to deformation even at relatively higher (compared to microemulsions) interfacial tensions because of their lower capillary pressure. Even small deformations are sufficient to introduce a remarkable change in the pair interdroplet energy. The impact of these changes on the macroscopic properties (e.g. the osmotic pressure) of emulsions and microemulsions could be great and is by no means to be neglected. 

This chapter summarises the recent effbrts - - to define and calculate the pair energy of interaction between two droplets, including the effect of their deformation. It also shows how the deformability contributes to the macroscopic properties of the systems using integral equation methods and Brownian dynamics simulations. 

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This section presents a theoretical study of more concentrated deformable emulsions and microemulsions where higher order interactions become important. The purpose is to relate the microseopic droplet deformability to the structure of such systems and further to their macroscopic (thermodynamic) properties. The radial distribution function and static structure factor are calculated utilising an integral equation approach in an appropriate closure approximation. This method allows us to obtain the virial equation of state as well. A semi-empirical equation of state, based on modifying the Camahan-Starling expression, as well as comparison with Brownian dynamics simulations are also presented. 

Perhaps the most striking property of a microemulsion in equilibrium with an excess phase is the very low interfacial tension between the macroscopic phases. In the case where the microemulsion coexists simultaneously with a water-rich and an oil-rich excess phase, the interfacial tension between the latter two phases becomes ultra-low [70,71 ]. This striking phenomenon is related to the formation and properties of the amphiphilic film within the microemulsion. Within this internal amphiphilic film the surfactant molecules optimise the area occupied until lateral interaction and screening of the direct water-oil contact is minimised [2, 42, 72]. Needless to say that low interfacial tensions play a major role in the use of micro emulsions in technical applications [73] as, e.g. in enhanced oil recovery (see Section 10.2 in Chapter 10) and washing processes (see Section 10.3 in Chapter 10). Suitable methods to measure interfacial tensions as low as 10 3 mN m 1 are the sessile or pendent drop technique [74]. Ultra-low interfacial tensions (as low as 10 r> mN m-1) can be determined with the surface light scattering [75] and the spinning drop technique [76]. 

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