We performed molecular dynamics simulations on sodium dodecyl sulfate (SDS) monolayer at the water/ carbon tetrachloride and water/vapor interfaces. We observed that the tails are slightly less ordered when SDS is at the water/vapor interface. Also, at the water/carbon tetrachloride interface the amphiphilic molecule is less inclined with respect to the surface normal compared to the one at the water/vapor interface. We also carried out investigations of the electrostatic properties and surface tension of the water/carbon tetrachloride interface in the presence of the SDS monolayer with and without the dipole correction term in the Ewald sum. We observed that an inclusion of the correction term makes a difference in quantitative results.

A new strategy to develop force fields for molecular fluids is presented. The intermolecular parameters are fitted to reproduce experimental values of target properties at ambient conditions and also the critical temperature. The partial charges are chosen to match the dielectric constant. The Lennard-Jones parameters, εii and σii, are fitted to reproduce the surface tension at the vapor-liquid interface and the liquid density, respectively. The choice of those properties allows obtaining systematically the final parameters using a small number of simulations. It is shown that the use of surface tension as a target property is better than the choice of heat of vaporization. The method is applied to molecules, from all atoms to a coarse-grained level, such as pyridine, dichloromethane, methanol, and 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIM-BF4) at different temperatures and pressures. The heat of vaporization, radial distribution functions, and self-diffusion coeficient are also calculated.

Molecular dynamics simulations of sodium dodecyl sulfate (SDS) molecules on a graphite surface are presented. The simulations were conducted at low and high surface coverage to study aggregation at the water/graphite interface. Results showed that at low surface coverage, the SDS molecules form hemicylindrical aggregates, in agreement with AFM experiments, whereas at high surface coverage, the surfactants form full cylinders. The latter aggregates have not been reported in systems of SDS on hydrophobic substrates, such as graphite. The unexpected results are explained in terms of a water layer adsorbed at the solid surface which was the responsible for the formation of these aggregates. Moreover, the SDS tails in the full cylindrical configuration became straighter than those of the hemicylindrical aggregate. Hydrogen bond formation between water and surfactant head groups was also studied, and it was found that they did not depend on the surfactant concentration.

We performed molecular dynamics computer simulations on DLPC and DPPC phospholipid monolayers at air/water and water/carbon tetrachloride interfaces. In our simulations the area per phospholipid molecule was 55 Å 2 but the temperature was high enough, so that monolayers were in a liquid crystalline phase. The results of the simulations show that at this area per phospholipid molecule, carbon tetrachloride molecules do not penetrate extensively into the phospholipid tails. We also obtained information about the headgroup orientation, tail lengths, tail tilts, and S CD order parameters for the tails. The S CD order parameters indicate that phospholipids are more ordered at water/carbon tetrachloride interface than at water/air 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.