Phase behavior of solvent-nematogen mixtures.

Liquid mixtures with a nematogen can undergo both fluid phase separation and a transition from an isotropic to a nematic state. These phase transitions can couple and lead to phase behavior distinct from simple liquid mixtures or pure liquid crystals. We measured the phase behavior of mixtures of a nematogen (5CB) with simple liquid solvents (squalane and/or squalene). We observed two distinct kinds of binary phase diagrams: with and without a region of isotropic-isotropic coexistence. Varying the ratio of squalene to squalane, we continuously tuned the phase boundaries of the apparent binary system and revealed a region of three-phase coexistence. A mean-field model combining classical models of liquid mixing and nematic ordering quantitatively describes both binary and ternary phase behavior. This simple model predicts a range of topologically complex ternary phase diagrams and extends naturally to systems with more components.