Exchange between alkylammonium and sodium ions at the surface of dodecylsulfate micelles

Abstract

The ion exchange between sodium and a series of alkylammonium cations of the general formula R 4− Y NH Y + (with R = methyl, ethyl, propyl, butyl; Y = 0 to 4), at the surface of dodecylsulfate micelles, was studied using fluorescence techniques. The selectivity coefficients obtained indicate that all of the organic counterions considered bind more strongly to the micelles than do either sodium or ammonium ions. Replacement of hydrogen by methyl groups ( Y = 1 to 3) in the ammonium ion increases the binding strength, the free energy increment for transfer of the counterion from water to the micelle, Δμ O, being −300 ± 30 cal/mole per methyl group. For alkylammonium ions, a Δμ O for transfer of −570 ± 50 cal/mole per methylene group, irrespective of the number of alkyl chains, was found. This indicates that the binding strength of alkylammonium ions is dominated by their hydrophobicity, independent of the spatial distribution of the alkyl groups attached to nitrogen. The possible use of these results to decide between “open” or “structured” micellar models is discussed. Tetraalkyl ions show considerably lower binding affinities than might be expected from the total hydrophobicity of their chains. This result is interpreted in terms of an unfavorable location of the fourth alkyl chain.