Immersion depth of surfactants at the free water surface: a computer simulation and ITIM analysis study

The adsorption layer of five different surfactants, namely, pentanol, octanol, dodecanol, dodecyl trimethyl ammonium chloride, and sodium dodecyl sulfate, has been analyzed on the basis of molecular dynamics simulation results at two surface densities, namely, 1 and 4 μmol/m(2). The analyses have primarily focused on the question of how deeply, in terms of atomistic layers, the different surfactant molecules are immersed into the aqueous phase. The orientation and conformation of the surfactant molecules have also been analyzed. The obtained results reveal a clear difference between the immersion behavior of the alcoholic and ionic surfactants. Thus, alcoholic surfactants are found to be located right at the water surface, their apolar tails not being considerably immersed into the aqueous phase and the alcoholic headgroups being preferentially located in the surface layer of water. Ionic surfactants are immersed several layers deep into the aqueous phase, with headgroup atoms reaching the sixth-eighth and tail carbon atoms reaching the third-fourth subsurface layer in several cases. The observed difference is related, on the one hand, to the ability of the alcoholic surfactants of substituting surface water molecules in their lateral hydrogen bonding network at the water surface and that of their apolar tails for replacing dangling hydrogens and, on the other hand, to the energetic gain of the ionic headgroups if they are fully hydrated rather than being in contact with hydrocarbon tail groups.