(183a) Entropic Compartmentalization in Open-Structure Colloidal Clathrates with Rotating Guest Particles

Clathrate crystals consisting of polyhedral cages are usually found in gas hydrate or intermetallic systems where the size of particles forming the structures is on atomic or molecular length scales. We report that hard colloidal particles can self-assemble entropically into colloidal clathrate crystals with open cages containing single or multiple guest particle whose rotation stabilizes the crystal. Monte Carlo simulations show that a single component system of hard truncated triangular bipyramids (TBPs) self-assembles into five different open colloidal clathrates with rotating guests depending on the truncation of the TBP. Truncation of the TBP creates a cavity at the center of the clathrate cage-like motifs, and the cavity is occupied by guest particles when the size of the cavity reaches a comparable size to the guest. We show that crystallization of the clathrate structure occurs via entropy compartmentalization, where the entropy of the system splits into host and guest particles differently. Four different types of guest dynamics are identified (free rotation, rotation around an axis, quantized rotation and rattling) depending on the guest/cavity size ratio and the prolateness of the cavity. When the TBPs are mixed with another type of polyhedron, the binary system forms a colloidal clathrate in which the guest is encapsulated by the host TBPs.