Surface-specific vibrational spectroscopy of the water/silica interface: screening and interference.

Surface-specific vibrational sum-frequency generation spectroscopy (V-SFG) is frequently used to obtain information about the molecular structure at charged interfaces. Here, we provide experimental evidence that not only screening of surface charges but also interference limits the extent to which V-SFG probes interfacial water at sub-mM salt concentrations. As a consequence, V-SFG yields information about the ∼single monolayer interfacial region not only at very high ionic strength, where the surface charge is effectively screened, but also for pure water due to the particularly large screening length at this low ionic strength. At these low ionic strengths, the large screening lengths cause destructive interference between contributions in the surface region. A recently proposed theoretical framework near-quantitatively describes our experimental findings by considering only interference and screening. However, a comparison between NaCl and LiCl reveals ion specific effects in the screening efficiency of different electrolytes. Independent of electrolyte, the hydrogen bonding strength of water right at the interface is enhanced at high electrolyte concentrations.