Surface Activity of Soft Polymer Colloids.

We investigate the behavior of polymer colloids at the interface between two immiscible liquids using molecular dynamics simulations. We study several colloid morphologies with various degrees of amphiphilicity, that is, purely solvophobic homogeneous and Janus particles and amphiphilic Janus and core-shell particles. Regardless of the specific morphology, the polymer colloids irreversibly anchor at the liquid-liquid interface, accompanied by a marked reduction of the interfacial tension, γ. Purely solvophobic particles lower γ because they reduce the interfacial area shared by the two immiscible liquids, whereas amphiphilic colloids have an additional enthalpic contribution. At the liquid-liquid interface, the solvophobic particles deform into oblate ellipsoids to maximize the occluded area at the interface. In contrast, amphiphilic Janus colloids orient their solvophobic/solvophilic parts toward the preferred liquids and form a prolate particle shape. The amphiphilic core-shell particles undergo a morphological transition to a prolate Janus-like structure as they anchor at the interface. We rationalize the deformation of the polymer colloids by considering a simple model system of spheroidal particles pinned at the liquid-liquid interface. We systematically compute the interfacial free energy for the various colloids as a function of their asphericity and find excellent qualitative agreement with the simulation findings. Our results show that solvophobic homoparticles can be almost as efficient surface-active agents as amphiphilic Janus colloids.