Interaction of testosterone-based compounds with dodecyl sulphate monolayers at the air–water interface

A series of atomistic molecular dynamics simulations were performed for investigating the interactions between three different testosterone-based compounds (testosterone (T), testosterone propionate (TP) and testosterone enanthate (TE)) and sodium dodecyl sulphate (SDS) and ammonium dodecyl sulphate (ADS) monolayers, which vary only in the sodium or ammonium counterions used to neutralise the sulphate headgroup. These simulations were used to investigate how the structural and interfacial properties of the monolayer were affected by changing the number of drug molecules present per monolayer, and the chemical nature of the surfactant counterions and the testosterone-based compounds. Our results show that the structure of the interfacial water layer is affected by the change of the counterion but not the chemistry of the drug molecules. As a result of the difference in their chemical structure, the T, TP and TE drug molecules prefer different locations and orientations within the monolayers. Finally, we observed that the hydration of the drug molecules encapsulated within the ADS monolayers is significantly less than when they are encapsulated within the SDS monolayers. Understanding the role that the counterion and the chemistry of the drug molecules play in these systems provides us with a detailed description of the interactions that cause ADS micelles to encapsulate significantly less drug molecules than SDS micelles, which we have recently observed experimentally.