Observation of Strong Synergy in the Interfacial Water Response of Binary Ionic and Nonionic Surfactant Mixtures.

Interfacial vibrational footprints of the binary mixture of sodium dodecyl sulfate (SDS) and hexaethylene glycol monododecyl ether (C 12 E 6 ) were probed using heterodyne detected vibrational sum frequency generation (HDVSFG). 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 enhanced 10 times compared to just pure surfactants. The experimental results contest the traditional Langmuir adsorption model predictions. This is also evidenced by our molecular dynamics simulations that show a remarkable restructuring and enhanced orientation of the interfacial water molecules upon DS - adsorption to the C 12 E 6 surface. The simulations show that the adsorption free energy of DS - ions to a water surface covered with C 12 E 6 is an enthalpy-driven process and more attractive by ∼10 k B T compared to the adsorption energy of DS - to the surface of pure water.