Ultraviolet (UV) Raman Spectroscopy Study of the Soret Effect in High-Pressure CO 2 –Water Solutions

Spatially resolved deep-ultraviolet (UV) Raman spectroscopy was applied to solutions of CO₂ and H₂O or D₂O subject to a temperature gradient in a thermally regulated high-pressure concentric-tube Raman cell in an attempt to measure a Soret effect in the vicinity of the critical point of CO₂. Although Raman spectra of solutions of CO₂ dissolved in D₂O, at 10 MPa and temperatures near the critical point of CO₂, had adequate signal-to-noise and spatial resolution to observe a Soret effect with a Soret coefficient with magnitude |S_T| > 0.03, no evidence for an effect of this size was obtained for applied temperature gradients up to 19 °C. In contrast, the concentration of CO₂ dissolved in H₂O was shown to vary significantly across the temperature gradient when excess CO₂ was present, but the results could be explained simply by the variation in CO₂ solubility over the temperature range and not by kinetic factors. For mixtures of D₂O dissolved in scCO₂ at 10 MPa and temperatures close to the critical point of CO₂, the Raman peaks for D₂O were too weak to measure with confidence even at the limit of D₂O solubility.