Surface charge determines the particle interactions in colloids, or suspensions of microscopic particles in solution, which exist in environmental systems (e.g. groundwater) and in manufactured goods (e.g. paint). For example, particles with similar surface charge repel each other and remain suspended in solution while neutral colloidal particles aggregate and can precipitate. Surface charge also governs interactions between particles and molecules in solution, which is important in understanding contaminant propagation through groundwater, since particles may have stronger interactions with hydrophobic organic contaminants than particles with high surface charge. These interactions affect the transport of organic contaminants through ecosystems.

In order to develop a broader knowledge of the surface charge of particles, previous studies used a number of techniques, such as potentiomeric titration and zeta potential, to determine the surface charge of particles indirectly. Recently, second harmonic generation (SHG), a nonlinear optical technique, served as a direct, noninvasive probe of surface potential, which theoretical models relate to surface charge.

In this study, surface charge measurements of the silica-aqueous interface were made using a combination of potentiomeric titration and in situ SHG measurements. The comparison of surface charge measurements made by titration and SHG suggests that each technique quantifies different aspects of the surface charge density. Our measurements suggest that a deprotonation of surface silanol groups cannot account for silica's changes in surface charge density as a function of ionic strength. We will present interpretations, using other descriptions of surface charge such as the Triple Layer and Helmholtz models, to explain the data.