Stability of aqueous silica nanoparticle dispersions

In this study, we present quantification methods for nanoparticle stability analysis using non-intrusive analytical techniques: attenuated total reflectance, Fourier transform infrared (ATR-FTIR) spectroscopy, ultraviolet-visible (UV-vis) spectrophotometer, zeta potential analyses, and dynamic light scattering (DLS). We use these techniques to study the stability of silica nanoparticle dispersions and the effects of pH, temperature, and electrolytes that would be encountered in oil field brines in a reservoir. Spectral analysis of the Si-O bond at wavenumber of 1110 cm(-1) with the ATR-FTIR indicates a structural change on the surface of silica particles as the dispersion pH changes, which agrees with zeta potential measurements. We define a critical salt concentration (CSC) for different salts, NaCl, CaCl2, BaCl2, and MgCl2, above which the silica dispersion becomes unstable. Three distinct stages of aggregation occur in the presence of salt: clear dispersed, turbid, and separated phases. Divalent cations Mg2+, Ca2+, and Ba2+ are more effective in destabilizing silica nanoparticle dispersion than the monovalent cation Na+. The CSC for Na+ is about 100 times more than for Ca2+, Ba2+, and Mg2+. Among the divalent cations studied, Mg2+ is the most effective in destabilizing the silica particles. The CSC is independent of silica concentration, and lowers at high temperature.