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Rotational Mobility in a Crystal Studied by Dielectric Relaxation Spectroscopy. An Experiment for the Physical Chemistry Laboratory

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Madalena S. C. Dionísio

Departamento de Química, Centro de Química Fina e Biotecnologia, Faculdade de Ciencias e Tecnologia, Universidade Nova de Lisboa, 2825-114 Lisboa, Portugal

Hermínio P. Diogo

Centro de Química Estrutural, Complexo I, IST, 1049-001 Lisboa, Portugal

J. P. S. Farinha and Joaquim J. Moura-Ramos

Centro de Química-Física Molecular, Complexo I, IST, 1049-001 Lisboa, Portugal

J. Chem. Educ., 2005, 82 (9), p 1355

DOI: 10.1021/ed082p1355

Publication Date (Web): September 1, 2005

Section:

History, Education, and Documentation

Abstract

In this article we present a laboratory experiment for an undergraduate physical chemistry course. The purpose of this experiment is the study of molecular mobility in a crystal using the technique of dielectric relaxation spectroscopy. The experiment illustrates important physical chemistry concepts. The background of the experimental technique deals with the concepts of orientational and induced polarization and frequency-dependent relative permittivity (or dielectric constant). The kinetic concepts of temperature-dependent relaxation time, activation energy, and activation entropy are involved in the concept of molecular mobility. Finally, the proposed laboratory experiment can be used in the undergraduate laboratory to illustrate a wide variety of topics with pedagogical interest: mesophases (plastic crystals and liquid crystals); motions in orientationally disordered crystals; phase transitions in solids; orientational glasses and orientational glass transition; and so forth.