Specific effects of monovalent counterions on the structural and interfacial properties of dodecyl sulfate monolayers

A series of molecular dynamics simulations have been conducted in order to study the specific ion effects of Li⁺, Na⁺, Cs⁺ and NH₄⁺ cations on dodecyl sulfate (DS^-) monolayers. Varying the counterion had no appreciable effect on the structure of the surfactant molecules within the different monolayers. However, the different counterions have a significant effect on the interfacial properties of the monolayer. In particular, we have investigated to what extent each of the counterions is dehydrated when interacting with the DS^- headgroup, the specific interactions between the counterions and the headgroup and the salt bridging of the headgroups caused by each counterion. The NH₄⁺ ions are found to directly compete with water molecules to form hydrogen bonds with the DS^- headgroup and as a result the ammonium dodecyl sulfate monolayer is the least hydrated of any of those studied. The Cs⁺ ions are strongly bound to the headgroup and weakly hydrated, such that they would prefer to displace water in the DS^- hydration shell to interact with the headgroups. In the case of the Li⁺ ions, they interact almost as strongly with the DS^- headgroups as the Na⁺ ions, but are generally less hydrated than the Na⁺ ions and consequently the lithium dodecyl sulfate monolayers are less hydrated than the sodium dodecyl sulfate monolayers. Therefore, by changing the counterion, one can modify the interfacial properties of the surfactant monolayer, and thus affect their ability to encapsulate poorly water soluble drug molecules, which we discuss further in the manuscript.