New data on the solubility of amorphous silica in organic compound-water solutions and a new model for the thermodynamic behavior of aqueous silica in aqueous complex solutions

Experimental solubilities of amorphous silica in several aqueous electrolyte solutions and in aqueous solutions of organic compounds, and theoretical considerations concerning cavity formation, electrostriction collapse, ion solvation, and long- and short-range interaction of the solvated ions with one another⁽¹⁾ permit the calculation of the partial excess free energies and the activity coefficients of aqueous silica. It is shown that, in the case of non-dissociated aqueous organic solutions, the variation of log m (SiO₂) with the reciprocal of the dielectric constant of the solution is described by a single linear equation independent of the nature of the organic compound. For aqueous electrolyte solutions, a specific linear relationship between log m (SiO₂) and the reciprocal of the dielectric constant occurs for each electrolyte. The success of the equation in reproducing the experimental solubilities of amorphous silica in aqueous solutions of electrolytes and organic compounds supports previous evidence indicating a polar charge distribution in the solvated SiO₂ molecule. Our data permit the calculation of the effective local charge of dissolved SiO₂ molecules and of the short-range interaction parameters between SiO₂ and various ions. The proposed equation of state can be used to calculate the affinity of reactions among SiO₂ minerals and complex aqueous solutions.