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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 12 Issue 2 (2007) pp 80-84

The Effect of Hydrocarbon Chain Length on the Critical Micelle Concentration of Cationic Surfactants: An Undergraduate Physical Chemistry Experiment

Benjamin Peterson and Charles J. Marzzacco*

Department of Physical Science, Rhode Island College, Providence RI 02906, Department of Chemistry, Florida Institute of Technology, Melbourne, FL 32901,
Received June 23, 2006. Accepted January 11, 2007.

Published online: 1 April 2007

Abstract. A physical chemistry experiment is presented that examines the effect of hydrocarbon chain length on the critical micelle concentration of three alkyltrimethylammonium bromide compounds. The surfactants studied are dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB) and cetyltrimethylammonium bromide (CTAB). The three substances are investigated by conductivity and fluorescence probe methods. The fluorescence probe method is based on changes in the fluorescence quenching of phenanthridine or 5,6-benzoquinoline as the concentration of the surfactant exceeds the CMC. It is shown that the phenanthridine and 5,6-benzoquinoline molecules associate with the cationic micelles where their fluorescence is strongly quenched by the Br counter ions on the surface of the micelles. Fluorescence quenching results for the probe lucigenin (N,N-dimethyl-9,9-bisacridinium nitrate), which does not associate with the cationic micelles are also presented. The CMC values obtained by the conductivity and fluorescent probe methods exhibit excellent agreement with each other and also agree well with reported literature values. The results verify that the CMC decreases with increasing hydrocarbon chain length. The use of these methods to determine the fractional ionization of the micelle will also be discussed.

Key Words: Laboratories and Demonstrations; physical chemistry, micelles, laboratory instruction, fluorescence spectroscopy

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Article in PDF format (171 KB) HTML format

Supporting Materials:

A student laboratory handout and instructor notes are available in a Zip file (306 KB).

Issue date: April 1, 2007

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