Exclusion from Hexagonal Mesophase Surfactant Domains Drives End-to-End Enchainment of Rod-Like Particles

Abstract

Anisotropic rod-like particles assemble end-to-end when the surfactant/water matrix in which they are dispersed is cooled from the isotropic to the lyotropic hexagonal phase. We demonstrate the formation of such end-to-end assemblies for gold nanorods, which are tens of nanometers in size, as well as for micrometer-sized ellipsoidal polystyrene particles. In both cases, the particles are well-dispersed in the low-viscosity surfactant/water phase above the isotropic-H1 transition temperature. On cooling into the H1 phase, mesophase domains form and the particles are expelled to the isotropic phase. As the H1 domains grow and finally impinge, the particles are localized at the domain boundaries where they reorient and assemble end-to-end. Remarkably, we observe the formation of end-to-end assemblies of gold nanorods even for volume fractions as low as 2 × 10(-6) in the initially dispersed state. The extent of particle "enchainment" increases with the particle concentration and with the aspect ratio of the particles.