The Selective Binding of Carboxylate Ions at Cationic Surfactant Solution/Air Interfaces

Abstract

In this study the binding of a homologous series of single chainn-alkyl carboxylates (formate through octanoate) to the tetradecyltrimethylammonium surfactant cation has been investigated using ion flotation. The binding of these ions to the interface is mainly due to their hydrophobicity. In this series, methylenes are added sequentially and the effect of the increase in the number of methylene groups on binding is determined. The selective binding of one carboxylate over the other is quantified in terms of a selectivity coefficient and the standard free energy of exchange. The results show that there is complete transfer of the hydrophobic tail from the aqueous phase to the interface in the case of the longer chain carboxylates, beginning with pentanoate, but that shorter chained compounds are largely electrostatically bound.