Thermodynamics of cationic and anionic surfactant interaction.

The interaction between positively and negatively charged linear surfactants is an interesting system for the understanding of the fundamental interplay of hydrophobic and ionic forces in lipid membranes and proteins. We used isothermal titration calorimetry to dissect the Gibbs free energies, enthalpies, entropies, and heat capacities of interaction into hydrophobic and ionic contributions for alkylamine interaction with alkyl sulfates and alkane sulfonates. Dependence on aliphatic chain length, surfactant concentration, temperature, and ionic strength provided a detailed thermodynamic description of this interaction. Reactions of surfactants with tails longer than approximately 10 carbon atoms were primarily driven by enthalpy changes arising from solid-phase interactions between aliphatic tails. Entropic contributions were small relative to enthalpic ones. Contributions of methylene groups were additive. The binding reaction can yield a solid or liquid complex, depending on temperature. Thermodynamic dissection yielded the parameters of the phase transition.