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 (C12E6) surfactants. Our results show that in the presence of C12E6 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 C12E6 is significantly more attractive by ~ 10 kBT 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 C12E6 induces a strong restructuring and enhanced orientation of the near-surface water layers, explaining the HDVSFG results.