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