Liquid Crystal Demonstration of Binary Phase Behavior for the Classroom

A demonstration showing binary phase formation and transformation, two concepts relevant to understanding binary phase diagrams, is presented using lyotropic liquid crystals. An optical microscope, in cross-polarized illumination, is used to observe phase formation as a function of composition at a surfactant water interface. Variations in optical texture as a function of position with respect to the interface, and hence composition, reveal distinct lyotropic liquid crystal phases. This phenomenon is illustrated using two different binary surfactant-water systems: one exhibiting simple phase behavior (three distinct phases through the compositional space) and one exhibiting complex phase behavior (six distinct phases through the compositional space). The change in phase fraction during a temperature-induced phase transition and the effect of cooling rate on microstructure development are also demonstrated by examining the microstructure, and changes in the microstructure, of a fixed-composition lyotropic liquid crystalline sample using polarized optical microscopy. This demonstration, which can be performed directly in the classroom, aims to provide students with tangible context for concepts related to binary phase diagrams, which are commonly introduced in introductory materials science and physical chemistry courses.