Effect of Surfactant Architecture on Micelle Formation and Interfacial Tension from iSAFT Molecular Density Functional Theory

Surfactants reduce the interfacial tension making them an important additive in a number of industrial and commercial applications from enhanced oil recovery to personal care products (e.g., shampoo, detergents, etc).  Amphiphilic surfactants, consisting of hydrophilic components and hydrophobic components, have been studied experimentally and theoretically.  Due to the distinct interactions between these two components and its surrounding molecules, surfactants can self-assemble to form interesting microstructures, i.e. micelles, monolayer, etc, in the bulk phase as well as at the interface.  For enhanced oil recovery, surfactants of different molecular architecture as well as formulations are studied experimentally to create the middle phase micro-emulsion phase that produces ultra-low interfacial tension.  The trial and error experiments needed to produce the microemulsion phase can be minimized by developing a theory that provides insight into the system behavior.

A density functional theory, also known as interfacial Statistical Associating Fluid Theory (iSAFT), has been developed and utilized to study complex inhomogeneous fluids.  In comparison with molecular simulation, the iSAFT free energy functionals accurately predict molecular scale structuring while including the effects of hydrogen bonding and molecular size and shape.  In the current work, iSAFT is applied to study micelle formation of surfactants. The dependence of micelle structure on the surfactant architecture and formulation is obtained.  Further, the elements necessary to produce ultra-low interfacial tension are investigated.

Figure 1. Density Profile of Micelle in Water