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.

Surface Potential Explained: A Surfactant Adsorption Model Incorporating Realistic Layer Thickness

“…28,30 However, this model is still limited because it artificially divides surfactant ions into two layers, which contradicts the continuous change of concentration determined by experimental techniques. 4,31 Also, this model focuses on surface excess only and does not address the surface potential issue explicitly.…”

“…A similar idea of surfactant immersion was proposed by Ivanov et al The immersion of ionic surfactants was found to enhance the efficiency of surfactant adsorption, and the two-layer model successfully explained the reason for the unexpected decreases of sodium dodecyl sulfate (SDS) adsorption at the oil–water interface. Potentially, this model may be used to explain several strange phenomena of soluble surfactants at the free interface and thin films. , However, this model is still limited because it artificially divides surfactant ions into two layers, which contradicts the continuous change of concentration determined by experimental techniques. , Also, this model focuses on surface excess only and does not address the surface potential issue explicitly.…”

Effect of Heterocyclic Atoms (O, N, S) on surfactant solutions investigated by molecular dynamics simulation

“…All simulation processes were calculated by using the Materials Studio software package [19] . Firstly, the Visualizer module was used to build the oil phase, water phase and surfactant molecular models.…”

From Surface Tension to Molecular Distribution: Modeling Surfactant Adsorption at the Air–Water Interface

“…Soluble surfactants, especially ionic surfactants such as SDS, form thick adsorption layers at the air–water interface which cannot be sufficiently represented by infinitely thin layers assumed by adsorption models. The existence of a thick adsorption layer was proven by many experimental and simulation results such as NR, ,− X-ray reflectivity, − neutral impact collision ion scattering spectroscopy (NICISS), − SFG, ,, and MD. − Because the adsorption layer is a few nanometers thick, there could be a surfactant-depleting region or under-monolayer adsorption underneath the first adsorption layer. , Although those changes in surfactant concentration cannot be detected by measurement techniques because of their limited probing depth, they still contribute to the surface tension drop . While the surface excess derived from the surface tension represents the whole system, the surface excess determined from RT and NR may come from only the first adsorption layer, so their values should be different.…”