Effect of polymer adsorption on the water structure at the quartz/water interface studied by optical sum frequency generation

The effect of polymers weakly adsorbed on a quartz surface on the structure of the interfacial water molecules was investigated by optical sum frequency (SF) spectroscopy. As polymers, poly(ethylene glycol) (PEG), poly(propylene glycol) (PPG), and two types of tri-block copolymer of Pluronic L64 and 17R-4 were used. SF intensity spectra of OH stretching mode of water molecules at the interface between a quartz substrate and aqueous solutions of the polymers were measured. The total SF intensity of the interfacial water of the L64 aqueous solution was smaller than those of other solutions. This result indicates that the L64 aqueous solution has smaller number of oriented interfacial water molecules. It is suggested that the rapid motion of hydrophilic parts of the adsorbed L64 disturbs the average orientation of the interfacial water molecules. On the other hand, the SF intensity from the interfacial water molecules of aqueous solutions with high ion strength did not depend on the species of the polymers in the solutions. The latter result suggests that the hydration of ions determines the structure of the interfacial water molecules.