Sum frequency generation (SFG) study of the pH-dependent water structure on a fused quartz surface modified by an octadecyltrichlorosilane (OTS) monolayer

The interface-sensitive spectroscopic method, sum frequency generation (SFG), has been used to investigate the interfacial water structure on a fused quartz surface modified by an octadecyltrichlorosilane (OTS) self-assembled monolayer in phosphate buffered solutions at various pHs. The experimental results demonstrate that the water molecules at the quartz/OTS surface flip while the water molecules at the OTS surface maintain their orientation when the solution pH is changed from neutral to acidic. The results show that most of the silanol groups still exist on the fused quartz surface even after a silane coupling reaction of OTS under the reported experimental conditions.