Molecular structure of ionic surfactant solution surface and effects of counter-ion therein—a joint investigation by simulation and experiment

Abstract

The main goal of the current paper is to investigate the surface structure of ionic surfactant solutions at molecular level and the influences of the counter-ion in this course. The used methods are molecular dynamics simulation and neutral impact collision ion scattering spectroscopy. The surfactants are sodium dodecyl sulfate and cesium dodecyl sulfate. With Material Studio (MS), the concentration-depth profiles of all the components in the surface layer have been simulated. With those profiles, the surface pictures of the ionic surfactant solutions are mapped out in angstrom scale and the orientation of surface active dodecyl sulfate ion at the surface dependent on its surface density is derived. Simultaneously, neutral impact collision ion scattering spectroscopy is employed to investigate the same systems experimentally. The results indicate that the surface structure is profoundly influenced by surface density of the surfactant and specificity of the counter-ion; however, the orientation of the dodecyl sulfate ion on the surface is only subjected to its surface density but hardly affected by the properties of its counter-ion. The comparison between those simulated and experimental results reveals reasonable agreements, indicating the reliability of this investigation as well as the reliability of the simulation strategy applied in these systems.