Observation of strong synergy in the interfacial water response of binary ionic and non-ionic surfactant mixtures

We have used heterodyne detected vibrational sum-frequency generation spectroscopy (HDVSFG) to probe the interfacial structure of binary mixtures of sodium dodecyl sulfate (SDS) and hexaethylene glycol monododecyl ether (C 12 E 6 ) surfactants. Our results show that in the presence of C 12 E 6 at CMC (70 µM) the effect of SDS on the orientation of interfacial water molecules is strongly enhanced. Molecular dynamics simulations show that the adsorption free energy of DS − ions to a water surface covered with C 12 E 6 is significantly more attractive by ~ 10 k B T compared to the adsorption energy of DS − to the surface of pure water. The simulations also show that the adsorption of DS − to a water surface covered with C 12 E 6 induces a strong restructuring and enhanced orientation of the near-surface water layers, explaining the HDVSFG results.