Interfacial structures of water at polyvinyl alcohol (PVA) and poly(2-acrylamido-2-methypropane) sulfonic acid sodium salt (PNaAMPS)/quartz interfaces were investigated by sum frequency generation (SFG) spectroscopy. Two broad peaks were observed in OH stretching region at 3200 and 3400 cm(-1), corresponding to the symmetric OH stretching of tetrahedrally coordinated, i.e., strongly hydrogen bonded "ice-like" water, and the asymmetric OH stretching of water in a more random arrangement, i.e., weakly hydrogen bonded "liquid-like" water, respectively, in both cases. The "liquid-like" water became dominant when the PVA gel was pressed against the quartz surface. The relative intensity of the SFG signal due to the "liquid-like" water to that due to the "ice-like water" at the quartz surface modified with a self-assembled monolayer of aminopropyltrimethoxysilane (APS) became higher when the negatively charged PNaMPS gel was contacted to the APS modified quartz surface in a solution of pH = 12, where the surface was negatively charged and electrostatic repulsive interaction and low friction were present between the PNaMPS gel and the APS modified surface. It, however, did not change in a solution of pH = 2, where the surface was positively charged and electrostatic attractive interaction and very high friction were present between the PNaMPS gel and the APS modified surface. These results suggest the important role of water structure for small friction at the polymer gel/solid interface.

Humidity-dependent water structure on the Nafion thin film surface was determined at room temperature by using sum frequency generation spectroscopy (SFG). When the Nafion thin film was exposed to a low relative humidity (RH) environment, a peak at 3720 cm -1 due to the "dangling OH" or "free OH" of water molecules and a peak centered around 3600 cm -1, which was assigned to be due to water molecules interacting with sulfonate groups of the Nafion surface, were observed. The intensities of these peaks increased as RH was increased. A broad peak centered around 3200-3300 cm -1 due to water adsorbed on hydrophobic, i.e., fluorocarbon, sites of Nafion was observed when the Nafion thin film was exposed to a high RH environment (RH > 60%). The behavior of surface water is compared with that of water in bulk Nafion.