Liquid Crystal Demonstration of Binary Phase Behavior for the Classroom

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.

Constructing a Low-Cost Polarized Optical Microscope for Undergraduate Material-Characterization Studies

“…The motivation behind this work is to reduce the financial requisite for undergraduate laboratories to integrate polarized optical microscopy into their repertoires. This low-cost setup could be a great supplement to a liquid-crystal modules such as those written by Tousley, Hartley, and Liberko . Exposure of the students to POM could possibly invigorate interest in one of the many fields that use this piece of equipment as an integral part of research.…”

Electrochemistry under Microscope: Observing the Diffusion Layer and Measuring Diffusion Coefficient

“…Considering these factors, the expansion of the diffusion layer is proportional to the square root of time ( t ) and the square root of time diffusion coefficient ( D ). The following equation presents a handy rule of thumb to calculate the thickness of the diffusion layer quickly. , normalΔ̅ = 2 italicD italict where Δ̅ is the thickness of the diffusion layer in cm, D is the diffusion coefficient in cm 2 s –1 , and t is time in s. Assuming D equals 5.0 × 10 –6 cm 2 s –1 after 10 s of electrode reaction Δ̅ becomes 0.01 cm, barely visible by the naked eye. − However, using an optical microscope not only makes it possible to see the diffusion layer but also provides a means to measure the rate of expansion over time, which is proportional to the square root of D . The other requirement of making the diffusion layer visible and measuring its thickness (Δ̅) is to design a colorful electrochemical reaction.…”