Demonstration Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
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Liquid Crystal Demonstration of Binary Phase Behavior for the Classroom Marissa E. Tousley* Department of Chemical Engineering, Rose-Hulman Institute of Technology, Terre Haute, Indiana 47803, United States
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S Supporting Information *
ABSTRACT: 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. KEYWORDS: Upper-Division Undergraduate, Chemical Engineering, Physical Chemistry, Materials Science, Misconceptions/Discrepant Events, Hands-On Learning/Manipulatives, Nanotechnology, Phases/Phase Transitions/Phase Diagrams
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INTRODUCTION Binary phase diagrams are introduced to undergraduate students in materials science and physical chemistry courses. These diagrams are crucial tools for describing the complex phase behavior of technologically significant materials and for illustrating the inter-relationship between material structure, properties, and processing. Students often struggle to accurately interpret binary phase diagrams due to misunderstandings of fundamental concepts, such as solubility and saturation, the nature of different solid phases (e.g., α and β in a simple binary eutectic phase diagram), and the difference between phase fraction and composition.1−3 To address some of these challenges, educators have developed laboratory experiments and demonstrations that allow students to explore binary solid−liquid phase behavior,4−7 partial miscibility in binary liquid systems,8 and phase transitions in single-component thermotropic liquid crystalline systems.9−12 These experiments and demonstrations can be difficult to conduct in a traditional classroom environment, however, due to the nature of the equipment and chemicals required. Here, a demonstration is described using lyotropic liquid crystals, which have been largely unexplored as an educational tool,13 that enables students to observe binary phase formation and transformations directly in the classroom. Liquid crystals, which are fluids that exhibit long-range order, are a technologically important class of material in both applied © XXXX American Chemical Society and Division of Chemical Education, Inc.
and fundamental settings. There are two common types: thermotropic and lyotropic. Liquid crystal displays and liquid crystal thermometers contain thermotropic liquid crystals: liquid crystals that can be formed by a single component (i.e., one type of molecule) and exhibit temperature-dependent phase behavior.14 Cell membranes, soaps and detergents mixed with water, and some foods contain lyotropic liquid crystals (LLCs): liquid crystals that are formed through the dissolution and self-assembly of amphiphilic molecules in a solvent.14,15 Self-assembly of molecular aggregates occurs due to hydrophobic/hydrophilic interactions. These aggregates may exist in a variety of different geometries or phases, as shown in Figure 1, depending on composition and temperature.16 LLCs are often formed in binary water−surfactant systems. LLC phases are analogous to the ordered phases observed in metals and ceramics. Due to their low temperature phase transitions (often
