Intensity-Calibrated Variable-Angle Surface Spectroscopy Provides the Electrostatic Potential at the Interface between Bonded and Diffuse Regions of the Electrical Double Layer

Second-order nonlinear optical spectroscopy is capable of separating contributions of water molecules bonded to a charged surface from those in the diffuse portion of the electrical double layer on account of the phase-matching considerations that dictate how signals are accumulated over a finite distance. One of the most characteristic features of this interfacial water structure is the electrical potential at the distance from the surface that separates these two regions. Here we describe a scheme for calibrating the absolute second-order susceptibility of the silica-aqueous interface and thereby provide the first account of intensity-based on-resonance sum-frequency generation to measure the potential at the interface between the bonded and diffuse water structures at low ionic strength. Our finding that the magnitude of the potential is slightly greater than the ζ-potential suggests that the structural influence of the surface silanol groups does not extend quite to the slipping plane.