Diffusion Coefficients of Carbon Dioxide in Brines Measured Using 13C Pulsed-Field Gradient Nuclear Magnetic Resonance

Tracer diffusion coefficients of CO2 in several brines were measured by C-13 pulsed-field gradient NMR at a temperature of 298 K and at salt molalities of up to 5 mol.kg(-1). The brines studied were NaCl(aq), CaCl2(aq), Na2SO4(aq), and a mixed brine prepared from seven salts: NaCl, CaCl2, MgCl2, KCl, Na2SO4, SrCl2, NaHCO3. The experimental results are compared with the predictions of a modified Stokes-Einstein relation in which the Stokes-Einstein number is 4 and the hydrodynamic radius of the CO2 molecule in aqueous solution is taken to be 168 pm, as determined in an earlier study of the (CO2 + H2O) binary system at the same temperature (Cadogan et al. J. Chem. Eng. Data 2014, 59, 519-525). This comparison shows agreement to within the experimental uncertainty, independent of salt type and molality. We conclude that the modified Stokes-Einstein relation provides a reliable means of estimating the tracer diffusion coefficient of CO2 in aqueous electrolyte solutions, based on knowledge of the brine viscosity and the hydrodynamic radius of CO2 in pure water at the same temperature.