Simple and complex micelles in amphiphilic mixtures: a coarse-grained mean-field study

Abstract

Binary mixtures of amphiphiles in solution can self-assemble into a wide range of structures when the two species individually form aggregates of different curvatures. In this paper, we focus on small, spherically-symmetric aggregates in a solution of sphere-forming amphiphile mixed with a smaller amount of lamella-forming amphiphile. Using a coarse-grained mean-field model (self-consistent field theory, or SCFT), we scan the parameter space of this system and find a range of morphologies as the interaction strength, architecture and mixing ratio of the amphiphiles are varied. When the two species are quite similar in architecture, or when only a small amount of lamella-former is added, we find simple spherical micelles with cores formed from a mixture of the hydrophobic blocks of the two amphiphiles. For more strongly mismatched amphiphiles and higher lamella-former concentrations, we instead find small vesicles and more complex micelles. In these latter structures, the lamella-forming species is encapsulated by the sphere-forming one. For certain interaction strengths and lamella-former architectures, the amount of lamella-forming copolymer encapsulated may be large, and the implications of this for the solubilization of hydrophobic chemicals are considered. The mechanisms behind the formation of the above structures are discussed, with a particular emphasis on the sorting of amphiphiles according to their preferred curvature.