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Colors in Liquid Crystals

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George Lisensky and Elizabeth Boatman

Department of Chemistry, Beloit College, Beloit, WI 53511-5595

J. Chem. Educ., 2005, 82 (9), p 1360A

DOI: 10.1021/ed082p1360A

Publication Date (Web): September 1, 2005

Abstract

Liquid crystals have a phase in between liquid and solid: the molecules can flow and drip but remain somewhat organized. The cholesteric liquid crystals prepared in this Activity use mixtures of molecules related to cholesterol that align in layers. Stacks of layers are rotated with respect to one another similar to DNA, spiral staircases, or screw threads. The rotation between layers increases with temperature. A color will be reflected when the pitch, the distance between layers that have the same orientation, is approximately equal to the colors wavelength of light. This change in pitch causes the color changes we see when we apply pressure to or heat or cool cholesteric liquid crystals. This Activity is suitable for exploring relationships between color, wavelength, reflection, and transmission and illustrates how temperature changes the liquid crystal's Bragg reflection wavelength. This Activity can also be used to explore the relationship between melting point and crystal packing. Because one component contains a long chain cis-alkene connected to the cholesterol molecule, packing efficiency and melting point increase as the relative amount of this component within the mixture is decreased.

Keywords (Audience):

High School / Introductory Chemistry

Keywords (Domain):

Physical Chemistry

Keywords (Feature):

JCE Classroom Activity

Keywords (Pedagogy):

Hands-On Learning / Manipulatives

Keywords (Subject):

Fatty Acids

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