Thermodynamic properties of H4SiO4 in the ideal gas state as evaluated from experimental data

Solid phases of silicon dioxide react with water vapor with the formation of hydroxides and oxyhydroxides of silica. Recent transpiration and mass-spectrometric studies convincingly demonstrate that H4SiO4 is the predominant form of silica in vapor phase at water pressure in excess of 10−2 MPa. Available literature transpiration and solubility data for the reactions of solid SiO2 phases and low-density water, extending from 424 to 1661 K, are employed for the determination of ΔfG0ΔfG0, ΔfH0ΔfH0 and S0 of H4SiO4 in the ideal gas state at 298.15 K, 0.1 MPa. In total, there are 102 data points from seven literature sources. The resulting values of the thermodynamic functions of H4SiO4(g) are: ΔfG0ΔfG0 = −1238.51 ± 3.0 kJ mol−1, ΔfH0ΔfH0 = −1340.68 ± 3.5 kJ mol−1 and S0 = 347.78 ± 6.2 J K−1 mol−1. These values agree quantitatively with one set of ab initio calculations. The relatively large uncertainties are mainly due to conflicting Cp0 data for H4SiO4(g) from various sources, and new determinations of Cp0 would be helpful. The thermodynamic properties of this species, H4SiO4(g), are necessary for realistic modeling of silica transport in a low-density water phase. Applications of this analysis may include the processes of silicates condensation in the primordial solar nebula, the precipitation of silica in steam-rich geothermal systems and the corrosion of SiO2-containing alloys and ceramics in moist environments.