Soret effect on the diffusion of CO2 in aqueous solution under high-pressure

Diffusive dissolution processes of CO2 into aqueous solution subjected to temperature gradients were investigated under 20 and 30 MPa in a silica capillary tube. The temperature and concentration profiles were measured simultaneously by quantitative Raman spectroscopy. Thermodynamic CO2 saturated concentration profiles along the temperature gradients in horizontal direction at steady state were observed, which indicates that the thermodynamic contribution is dominant and kinetic contribution can be neglected. Based on the assumption of thermodynamic saturated concentration profile at steady state, Soret coefficients (the ratio of the thermal and mass diffusion coefficients, ST) of dissolved CO2 were calculated at P-T ranges from 0.1 to 50 MPa and from 273.15 to 623.15 K. At constant pressure, ST shows a “V” shaped non-monotonic function of temperature with a minimum at T0. The temperature T0 (at which ST reaches its minimum value) increases with increasing pressure. ST generally decreases with increasing pressure, and could change sign from positive to negative in the temperature ranges around T0 at pressures higher than 7.5 MPa. Soret coefficients are discontinuous near the critical point of CO2.