Kinetics of silica oligomerization and nanocolloid formation as a function of pH and ionic strength at 25°C

Rate laws reported for the oligomerization of silica in natural environments are often contradictory, and the kinetics of monosilicic acid condensation are poorly understood. Here we present rate expressions that systematically describe the initial oligomerization of silica in terms of concentration of initial silica, ionic strength, and pH for a natural brine solution. The oligomerization of silica in dilute aqueous solutions was examined in solutions with ionic strengths of 0.01 and 0.24 molal, from pH 3 to 11, and with initial silica concentrations of 4.2, 12.5, and 20.8 millimolal (250, 750, and 1250 ppm SiO₂ respectively). The decrease in concentration of molybdate-reactive silica was monitored over time to determine the extent of oligomerization. This decrease in concentration of molybdate-reactive silica is accompanied by the appearance of a transient population of nanocolloidal particles with diameter ∼3 nm, as determined by atomic force microscopy (AFM). The oligomerization rate increases as pH approaches near neutral and as ionic strength increases. Early in the reaction where the concentration of molybdate-reactive silica, [SiO₂]_n≤3, is assumed to equal the concentration of monosilicic acid, [H₄SiO₄], the rate of change of monosilicic acid as a function of time, R, shows a fourth-order dependence: R = k₄[H₄SiO₄]⁴ Furthermore, the log of the rate constant (k₄, millimolal^-3s^-1) varies linearly with pH according to log k₄ = m pH + log k_o, where k_o is the rate constant at pH 0 and m is an empiric constant. The value of m is positive below pH 7 and negative above pH 7. The observed fourth-order rate dependence with respect to [H₄SiO₄] is consistent with formation of a critical nucleus with four silica groups during the oligomerization of silica into metastable nanocolloidal silica. The sensitivity of the reaction rate constant, k₄, to ionic strength and to silica concentration as a function of pH suggests that the critical species (in our model, a cyclic tetramer) chemically behaves as a bulk surface. In the brine solution employed in this study at 25°C, nanocolloids are relatively stable at low pH and at low ionic strength, and thus such colloids would be expected to occur in natural solutions even as they approach steady-state equilibrium with amorphous silica. The form of the log rate (oligomerization) vs. pH curve is roughly the inverse of the form of the log rate (dissolution)-pH curve for feldspar dissolution, and therefore may also be indicative of the importance of silica condensation reactions during feldspar dissolution.