Molecular Thermodynamics for Branched Networks and Bicontinuous Phases in Water+Oil+Surfactant Microemulsions

Predicting the macroscopic behavior of microemulsions is important for fabrication of cosmetics, food and pharmaceuticals, production of oil, etc. Macroscopic properties of microemulsion systems strongly depend on their mesoscale structure. The phenomenological approach of Helfrich and Safran leads to the well-known state diagram of a microemulsion delineating the zones of stable droplets that have spherical or cylindrical shape, the lamellar structures, and the emulsification failure, all in terms of the elasticity moduli and the spontaneous curvature of the surfactant film.

In this work we complement this state diagram of a microemulsion by locating the stability zones for the mesoscale structures that have elements of a toroidal shape: perforated bilayers, sponge phases, and branched wormlike aggregates. We discuss the relation between the surfactant’s molecular properties and the elasticity constants that determine the behavior of the microemulsion using simple mean field theories. We discuss the mechanism of stabilization of perforations and branches between the aggregates and the effect of surfactant structure: the length and flexibility of the hydrocarbon tail, the size and charge of the head. For classical ionic surfactants and diblock copolymers containing weak polyelectrolyte subchain, these theories help to describe the response of the microemulsion to a number of important factors, including the acidity and salinity of the medium and the specific chemistry of added salt.

We thank the Russian Science Foundation (project # 16-13-10042) for financial support.