Exclusion from hexagonal mesophase surfactant domains drives end-to-end enchainment of rod-like particles

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.