Structure of the adsorption layer of various ionic and non-ionic surfactants at the free water surface, as seen from computer simulation and ITIM analysis

Molecular dynamics simulations of the adsorption layer of five different surfactant molecules, namely pentanol, octanol, dodecanol, dodecyl trimethyl ammonium chloride, and sodium dodecyl sulphate have been performed at the free surface of water at two different surface densities, namely 1 μmol/m2 (corresponding to unsaturated adsorption layer), and 4 μmol/m2 (corresponding to saturated adsorption layer), on the canonical ensemble at room temperature. The surfactants have been chosen in such a way that the effect of their headgroup charge as well as alkyl tail length on the properties of the adsorption layer can be separately investigated. The results are analysed in terms of the molecular level structure of the adsorption layer; organisation of the different groups and molecules along the macroscopic surface normal axis as well as conformation and orientation of the apolar tail is investigated in detail. In addition, the roughness of the surface of the aqueous phase is also analysed, using the ITIM method for accurately locating the real, capillary wave corrugated surface of the aqueous phase.