Authors: Z.S. Nickolov, D.W. Britt and J.D. Miller
Affilation: Drexel University, Drexel Nanotechnology Institute, United States
Pages: 32 - 35
Keywords: sum-frequency spectroscopy, Langmuir films, interfacial water structure
Sum frequency spectroscopy (SFS) in the OH stretching region was employed to obtain structural information about the interfacial water structure underneath Langmuir monolayers on H2O subphase. The films were composed by the model lipid dioctadecyldimethylammonium bromide (DOMA or DODAB), the surfactants methyl stearate (SME) and stearic acid (STA), and their mixtures. The results demonstrated that the uncompensated head group charge of DODAB at the interface is the reason for structuring of interfacial water close to the mixed DODAB-methyl stearate monomolecular films. To get insight into this charge-induced effect SFS studies of the structure of the stearic acid monolayer _ water interface as a function of pH were performed. At low pH values the stearic acid monolayer is neutral and the water structure close to it is mostly disordered, while at high pH values it is negatively charged and interfacial water structure is strongly promoted by the surface electric field.