Computer Simulations of Catanionic Surfactants Adsorbed \\ at Air/Water Interfaces. II. Full coverage

CLAVERO, E; RODRIGUEZ, J; LARIA, D

JOURNAL OF CHEMICAL PHYSICS

American Institure of Physics

Año: 2007 vol. 127 p. 124704 - 124711

0021-9606

We extend our previous molecular dynamics experiments[Rodriguez {it et al.} J. Phys. Chem. B {f 109}, 24427, (2005)]to the analysis of the adsorption of catanionic surfactants at water/airinterfaces, at a surfactant coverageclose to that for the saturated monolayer: 30.3 AA$^2$ per headgroup.The mixture of surfactants investigated corresponds to equal amountsof dodecytrimetylammonium (DTA) and dodecylsulphate (DS).The structure of the interface is analyzed in terms of thelocal densities and orientational correlations of all relevant interfacialspecies.  In accordance with experimental evidence, DTA headgroupspenetrate deeper into the aqueous substrate than DS ones, althoughthe 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 $sim 45^o$.The characteristics of the headgroup-water substrate correlations are alsoanalyzed using a tessellation procedure of the interface. The density andpolarization responses of the interfacial domains closest to DS headgroups areenhanced, compared to those adjacent to DTA detergents. Dynamical aspectsrelated to the diffusion and the orientational dynamics of different water layers inclose contact with the surfactant are also investigated.