Effect of droplet deformation on the interactions in microemulsions

The deformability of droplets is accounted for as a new effect in the statistical mechanics of microemulsions. It is assumed that a plane-parallel film is formed between two colliding microemulsion droplets. The collision is accompanied by deformation of the droplets and by an increase of their interfacial area. The increased interfacial energy gives rise to an effective soft repulsion between the two droplets. In addition, the deformed droplets, having the shape of spherical segments, exhibit considerably greater van der Waals attraction than two spheres of the same volume, separated at the same surface-to-surface distance. The superposition of these two effects leads to the appearance of a potential well in the energy of droplet—droplet interaction. The latter depends on the droplet interfacial tension, the Gibbs elasticity, the Hamaker constant, and the thickness of the liquid film separating the droplets. The soft repulsion and the increased attraction predicted by this model lead to pronouncedly lower values of the calculated second virial coefficient in comparison with the hard sphere model of microemulsions. The deformable droplet model proposed in this paper allows the explanation of experimental findings such as the abnormally low measured values of the second virial and diffusion coefficients as well as the dimer formation in microemulsions.