Linking Surface Potential and Deprotonation in Nanoporous Silica: Second Harmonic Generation and Acid/Base Titration

The adsorption of ions at the oxide/water interface is important for a variety of technological and environmental applications. Understanding ion adsorption is complicated, even for smooth planar surfaces, because it is the result of both chemical and electrostatic effects. Adsorption on rough or porous phases further requires description of the influence of pore geometry. In this work we systematically study ion adsorption and charge development in nanoporous silica. We do so using conventional titration techniques and a noninvasive, all optical method of measuring particle surface potential: second harmonic generation. We find that at pH greater than approximate to 6.5 surface proton loss is completely compensated by counterion adsorption, leading to little detectable gain in surface charge, and that surface deprotonation above this threshold pH is dominated by internal surface sites. Such a threshold pH, above which acid/base or ion titration curves of porous mineral phases are dominated by adsorption to internal surfaces and surface charge is relatively constant, is expected to be a general feature of the porous oxide/water interface and its explicit consideration important in describing the adsorption of large ions on porous mineral phases and the stability of aqueous suspensions of porous mineral particles. Our results also suggest that the surface of nanoporous silica, synthesized using the method of Stober, does not polymerize in aqueous solution.