The effect of monovalent counterions on the struc - tural and interfacial properties of dodecyl sulfate monolayers † Daniel

A series of molecular dynamics simulations have been conducted in order to study the structural and interfacial properties of dodecyl sulfate (DS−) monolayers with Li+, Na+, Cs+ and NH4 cations. Varying the counterion has no significant effect on the structural properties of the surfactant molecules within the different monolayers. However, the different counterions have a significant effect on the interfacial properties of the monolayer. The NH4 ions are the most strongly bound to the headgroup, the least hydrated at the interface, directly compete with the hydrating water molecules for hydrogen bonds with the headgroup and more frequently interact with more than one headgroup. 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 headgroup. Also, the Cs+ ions frequently interact with more than one headgroup. 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 therefore they bring less water to the monolayer interface than the Na+ ions. Therefore, by changing the counterion, one can modify the interfacial properties of the aggregates, and therefore effect their ability to encapsulate drug molecules, which we discuss in further greater detail.