Abstract
The critical micelle concentration (CMC) of sodium dodecyl sulfate in water at 25°C was measured as a function of the concentration of the added alcohols (C 2C 4) including all isomers. On addition of alcohols, the CMC decreases. The CMC depression becomes more marked, the stronger the hydrophobicity of the added alcohol. As a measure of the hydrophobicity of the alcohols, the free energy of transfer of an alcohol molecule from pure liquid to the extremely diluted aqueous solution was employed. The logarithm of the CMC-decreasing effect vs. this free-energy plot gives a line which expresses the CMC-decreasing effect in terms of a limiting slope of a log (CMC) vs. mole fraction of the added alcohol plot. In order to explain the above phenomena, the factors that affect the CMC decrease may be considered as reduction of the free energy of a micelle due to the diluted surface charge density on a micelle, the hydrophobic interaction to the hydrocarbon exposed to water, and the entropy of mixing of a mixed micelle on addition of alcohol, and they contribute in this order.
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