Computer simulations of surfactant mixtures at the liquid/liquid interface

We performed molecular dynamics simulations on monolayers of surfactant mixtures at the water/carbon tetrachloride interface. The Mixture was composed of sodium dodecyl sulfate (SDS) anionic surfactants and molecules that were chosen to be identical to SDS except that the charges were different in order to make them nonionic. Simulations were conducted for different concentrations of the monolayer mixture; i.e., at a fixed surface area we kept the number of SDS molecules constant while the number of the nonionic molecules was increased. Simulations of different SDS/nonionic mixtures show that the location of the surfactants in the interface is determined by the interaction and the charge distribution of the molecules. We also observe that the structure of the two different surfactants in the monolayer is different and is affected by the concentration of the mixture. For a particular SDS/nonionic mixture, the results indicate that the tails of the SDS molecules are less ordered than the tails of the nonionic molecules. We also show that the length of the SDS chains is shorter than that of the nonionic chains in the same mixture. To investigate the behavior of the SDS molecules in one component monolayer and in the mixture, we also performed simulations of monolayers composed of just SDS molecules at the same surface coverage of the mixtures. The results show that the SDS tails are slightly more disordered in the mixture than in the single monolayer. Finally, the electrostatic potential difference across the interface was also investigated in the mixtures and we find that it increases or decreases depending on the interactions (charge distribution) of the SDS and nonionic molecules in the mixture.