Molecular Signature of an Unexpected Hydrophobization of Silica Surfaces in Lead Nitrate Solutions as Detected by Sum Frequency Generation Spectroscopy

Abstract

Responsive silica surfaces empower their smart applications for catalysis, mass transfer, and self-cleaning. Here, we reveal the molecular signature of an unexpected hydrophobization of silica surfaces as their pH response in a lead nitrate solution. Strikingly, the pH-dependent sum frequency generation (SFG) spectra of silica surfaces show two profound minima of the intensity at pH = 7.2 and 10.2, respectively. Our in-depth analysis of SFG spectra using the maximum entropy method show emerging vibrational bands at >3600 cm–1 for the surface-bound water at this pH range, which are similar to the stretching modes of the dangling OH at the air–water surface. The observed minima in SFG intensity and the occurrence of the “hydrophobic-like” stretching modes of interfacial water provide a molecular fingerprint of the hydrophobicity of the silica surfaces, which is confirmed by contact angle measurements. These step-change findings help better deal with silica-related processes in nature and technologies.