TOWARDS AN EXPERIMENTAL DETERMINATION OF THE INTERFACIAL POTENTIAL AT THE COLLOIDAL SILICA WATER INTERFACE USING SECOND HARMONIC GENERATION

The mineral surface potential plays a significant role in the quantitative understanding of mineral dissolution, contaminant transport and mineral colloid stability yet generally is not directly accessible experimentally. In particular for most mineral surfaces of interest there are thought to be (at least) two relevant surface potentials: that of the mineral surface (Φ₀) and, some distance into the aqueous phase, a diffuse layer potential (Φ_D). In the past determination of surface potential has often been surmounted by a combination of measurements of electrophoretic mobility and conductivity yet relating these measurements to Φ_D or Φ₀ requires employing a model with difficult to constrain parameters.

Second Harmonic Generation (SHG) is a second order nonlinear optical process, driven by an intense laser source, which is sensitive to interfacial electrical fields. For this reason the variation in the intensity of measured SHG (I_SHG) with changes in pH (for systems where H+ is the sole potential determining ion) and salt concentration has previously been used to describe the surface potential of a variety of noncrystalline organic materials in water, as well as to determine the pH_pzc of several mineral/water systems. Here we measure the I_SHG for the colloid silica/water system and use the results, in tandem with acid/base titration, to quantify the mineral surface potential as a function of pH.