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.

Abstract: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 mol/m 2 (corresponding to unsaturated adsorption layer), and 4 mol/m 2 (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.

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