Surface structure of aqueous ionic surfactant solutions and effects of solvent therein-a computer simulation study

The current paper reports a simulation study on the molecular structure of aqueous solutions of anionic surfactant under the environment of Material Studio 6.0. From the simulated configurations, the concentration-depth profiles of each constituent in the surface layer have been analyzed. With that, the structure of electrical double layer and molecular orientation of surface active dodecyl sulfate ion dependent on its surface density have been derived straightforward. Thereby, the surface pictures of the investigated solutions are mapped out in angstrom scale and become visible. The results demonstrate that the structure of electrical double layer is heavily dependent on the specific counter-ions. However, this dependence is opposite to its experimental counterpart and reversed to that found in the formamide solution. This contradictory finding can be explained by the salvation effect of the counter-ions. Additionally, the orientation of dodecyl sulfate ion is found only subjected to its surface density but hardly influenced by its counter-ion specificity, which keeps coincident to the past investigation on formamide solution.