Transfer of non-ionic surfactants across the water-oil interface: A molecular dynamics study

Compounds able to adsorb at the interface of two immiscible liquids are of high importance for numerous chemical, physical and biological processes. Deeper understanding of the interfacial phenomena in such systems can contribute significantly to the rational practical application of small amphiphiles in electrochemistry, extraction, stabilization of emulsions, and drug design. Particularly interesting are the oil-water formulations, where the aqueous and the hydrophobic phases tend to express different molecular structure and characteristics under identical conditions. Surfactant behaviour in such environments is known to differ distinctly from the one observed in bulk water or oil or at the gas-liquid interface. It has been demonstrated that the free energy of adsorption and transfer of amphiphiles across water-oil interfaces can be decomposed into specific contributions of molecular fragments. The models developed so far do not provide information about the specifics of the functional groups contributions. To elaborate further on this topic, classical molecular dynamics simulations, combined with umbrella sampling calculations and weighted histograms method analysis are applied to reconstruct the free energy profile for several water-oil-amphiphile models along a relocation coordinate normal to the interface. Normal pentane, hexane, and heptane are chosen as models of the hydrophobic liquids and three short-chained normal alcohols as low-molecular-weight amphiphiles. Gibbs energies of transfer and adsorption, as well as the average contributions per unit of elongation of the alkyl chain, are estimated and compared to experimental data and empirical results. The experimental trends of all quantities are reproduced correctly. Some quantitative differences are discussed. A detailed analysis of the free energy change vs. position of alcohol molecules shows that the transfer process can be decomposed into several stages. Such more precise stratification of the interfacial region is employed to determine and rationalize the free energy contributions of the separate alkanol functional groups. (C) 2016 Elsevier B.V. All rights reserved.