Solubility of fused silica in sub- and supercritical water: Estimation from a thermodynamic model

Near- and supercritical water appears to be a useful green agent for surface treatment of fused silica-based devices to be used in separation methods of analytical chemistry. To assist the development of the devices, a simple model has been developed to estimate the aqueous solubility of fused (amorphous) silica as a function of temperature and pressure. The central assumption of the model is that, at the particular temperature and pressure, the aqueous species formed by dissolution of SiO2 are the same regardless of whether they resulted from dissolution of quartz or from dissolution of fused silica. Conceptually, the model combines a correlation of the aqueous solubility of quartz with a thermochemical cycle to estimate the ratio of the fugacity of fused silica to the fugacity of quartz. The existing experimental data on aqueous solubility of fused (amorphous) silica are used to build up the model, and they are reproduced by the model with an average relative deviation of 2.9%. Depending on the temperature and pressure, the “enrichment factor” of the aqueous solubility of fused silica over that of quartz is predicted to range from 2 to 18.