Computer simulations of catanionic surfactants adsorbed at air/water interfaces. II. Full coverage

We extend our previous molecular dynamics experiments [Rodriguez et al., J. Phys. Chem. B 109, 24427 (2005)] to the analysis of the adsorption of catanionic surfactants at water/air interfaces, at a surfactant coverage close to that of the saturated monolayer: 30.3 A(2) per headgroup. The mixture of surfactants investigated corresponds to equal amounts of dodecytrimethylammonium (DTA) and dodecylsulfate (DS). The structure of the interface is analyzed in terms of the local densities and orientational correlations of all relevant interfacial species. In accordance with experimental evidence, the DTA headgroups penetrate deeper into the aqueous substrate than the DS ones, although the average positions of all headgroups, with respect to the interface, lie in positions somewhat more external than the ones observed at lower coverages. Average tail tilts are close to 45 degrees. The characteristics of the headgroup-water substrate correlations are also analyzed using a tessellation procedure of the interface. The density and polarization responses of the interfacial domains closest to the DS headgroups are enhanced, compared to those adjacent to the DTA detergents. Dynamical aspects related to the diffusion and to the orientational correlations of different water layers in close contact with the surfactant are also investigated.