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

Quantifying the Counterion-Specific Effect on Surfactant Adsorption Using Modeling, Simulation, and Experiments

“…Several previous simulation results show exactly the opposite trend of counterion affinity compared to our results: small ions such as Li + not only get closer to the adsorption layer but also have higher concentrations than larger ions. − These results are in direct contradiction to our experimental and modeling results. This error has previously been attributed to the lack of a hydration shell around the small ions .…”

“…For medium to low concentration, SFG can be useful, and there are many SFG studies regarding ion-specific effects. − It is also important to combine the experimental results with simulation to better understand microscopic interactions. Although there are many MD studies of ionic surfactants, − these simulations can be problematic as the LJ and charge parameters are not yet well justified. Parameterization is often carried out based on the aggregated state, − which may not capture the surface properties of the surfactant.…”

“…We have not found any MD studies that reproduce the experimentally observed order of counterion affinity , for SDS. In fact, several previous simulations produced exactly the opposite ordering, where the Li + ion forms the closest contact with the surfactant headgroup. − In this work, we avoid this problem by scaling down the headgroup–ion interaction and the charge for ions, as suggested by the literature. − …”

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

“…Although MD is a useful tool for investigating the surfactant adsorption, its predictions need to be compared with experiments and modeling. Many previous simulations show that small ions such as Li + have higher concentrations close to the surfactant headgroup than larger ions, − which is exactly the opposite trend in experimental observations. ,, We believe that the attraction between the surfactant headgroup and the counterions might be overestimated if default Lennard-Jones parameters are used. We reduce this attraction according to the literature, − and reasonable surfactant/counterion interaction is achieved, which agrees with the experimental results.…”

“…However, model predictions of the surface excess and surface potential are still not in quantitative agreement with experimental results. This is probably because the adsorption layers assumed by adsorption models differ significantly from the actual structures at the air−water interface. , Molecular dynamics (MD) simulation results generally agree well with the experiment, especially for the density distribution of surfactant headgroups and alkyl chains. − However, MD cannot address the counterion distribution effectively and can predict the opposite trends in experiments and modeling for the counterion affinity with the surfactant headgroup. − Therefore, our understanding of surfactant adsorption is far from being complete.…”