Size-dependent dissociation of surface hydroxyl groups of silica in aqueous solution

Abstract

The size-dependence of surface hydroxyl dissociation of solid nanoparticles dispersed in aqueous solutions is a basic colloidal interface phenomenon, but there is no model to describe this phenomenon. In this work, nine amorphous silica samples with different radii (r, 6–196 nm) were chosen as model particles, and their effective surface hydroxyl density (Nseff) and dissociation equilibrium constants (Ka) were determined using acid-base titration, showing an obvious size-dependence. With the decrease of r, the Nseff (mol/m2) and pKa all decrease. The decrease of pKa indicates an enhanced dissociation tendency of surface hydroxyls. Two size-dependent models relating Nseff and Ka separately with r were developed, predicting that both lnNseff and pKa decrease linearly with an increase in 1/r. The model equations can well describe the size-dependent data of Nseff and Ka of silica particles. The characteristic parameters Nseff, apparent pKa, and intrinsic pKa corresponding to flat surfaces (r → ∞) for silica particles are estimated to be 9.12 μmol/m2 (or 5.49 sites/nm2), 9.85, and 8.99, respectively. The size-effect on Nseff and Ka becomes obvious when r < 40 nm. This work deepens the understanding of the size-dependent solid/liquid interface phenomena.