Contribution of Different Molecules and Moieties to the Surface Tension in Aqueous Surfactant Solutions

Amphiphilic surfactants are changing the surface tension of solutions by adsorbing at their surfaces. In general, however, little is known about the actual distribution of the surface tension across the interface, as well as about the extent of the contribution of different moieties to the surface tension. Here, we consider the liquid vapor interface of the solutions of five different amphiphilic molecules, representative of anionic, cationic and nonionic (alcoholic) surfactants. We investigate, by means of molecular dynamics simulations, the contribution of various chemical species and moieties to the surface tension distribution in these aqueous solutions at various surface coverages. We find that the headgroups of alcoholic surfactants give a negligible contribution to the surface tension. The opposite is true for ionic surfactants, whose effect depends on their hardness within the Hofmeister series, even though there is a large compensation between ions and counterions. In addition, we find that water molecules contribute negatively to the surface tension when they are hydrating the ionic headgroups and counterions, instead of being exposed to the vapor phase.