Chain length effects on complex formation in solutions of sodium alkanoates and tetradecyl trimethyl ammonium bromide

An electroneutral two-dimensional solution model has been used to describe the impact of the chain length on complex formation in mixed ionic surfactant solutions, in particular for mixtures of anionic sodium alkanoates (CH3(CH2)nCOONa) with various chain lengths and cationic tetradecyl trimethyl ammonium bromide (C14TAB). The equilibrium surface tensions of the homologous series of alkanoates are studied using plate tensiometry. The experimental data of the pure alkanoates, the resulting 1:1 complexes and the isotherms of the mixtures are analyzed on the basis of the Frumkin model. It turns out that the formation of complexes depends on the chain length of the alkanoate as reflected in the adsorption constant b and that the adsorption behavior is dominated by the presence of 1:1 complexes in the mixtures.