Near-infrared spectroscopic investigation of water in supercritical CO2 and the effect of CaCl2

Near-infrared (NIR) spectroscopy was used to investigate the dissolution and chemical interaction of water dissolved into supercritical carbon dioxide (scCO2) and the influence of CaCl2 in the co-existing aqueous phase at four temperatures: 40, 50, 75 and 100 °C at 90 atm. Consistent with the trend of the vapor pressure of water, the solubility of pure water in scCO2 increased from 40 °C (0.32 mol%) to 100 °C (1.61 mol%). The presence of CaCl2 negatively affects the solubility of water in scCO2. At a given temperature and pressure the solubility of water decreased as the concentration of CaCl2 in the aqueous phase increased, following the trend of the activity of water. At 40 °C, the water concentration in scCO2 in contact with saturated CaCl2 aqueous solution was only 0.16 mol%, a drop of 50% whereas that at 100 °C was 1.12 mol%, a drop of over 30%, as compared to pure water under otherwise the same conditions. Analysis of the spectral profiles suggested that water dissolved into scCO2 exists in the monomeric form under the evaluated temperature and pressure conditions, for both neat water and CaCl2 solutions. However, its rotational degrees of freedom decrease at lower temperatures due to higher fluid densities, leading to formation of weak H2O:CO2 Lewis acid–base complexes. Similarly, the nearly invariant spectral profiles of dissolved water in the presence and absence of saturated CaCl2 under the same experimental conditions was taken as evidence that CaCl2 dissolution in scCO2 was limited as the dissolved Ca2+/CaCl2 would likely be highly hydrated and would alter the overall spectra of waters in the scCO2 phase.