We report measurements of the diffusion coefficients of CO2 and N2 in pure water at temperatures between (298.15 and 423.15) K and pressures between (15 and 45) MPa. The measurements were made by the Taylor dispersion method and have a standard relative uncertainty of 2.3 %. The results were found to be essentially independent of pressure over the range investigated and a simple relation, based on the Stokes–Einstein equation, is proposed to correlate the experimental data. Some experimental difficulties arising in the measurement of the diffusivities of slightly soluble acid-gas solutes such as CO2 in water are also discussed.

We report experimental measurements of the mutual diffusion coefficients in binary systems comprising CO2 + liquid hydrocarbon measured at temperatures between (298.15 and 423.15) K and at pressures up to 69 MPa. The hydrocarbons studied were the six normal alkanes hexane, heptane, octane, decane, dodecane and hexadecane, one branched alkane, 2,6,10,15,19,23-hexamethyltetracosane (squalane), and methylbenzene (toluene). The measurements were performed by the Taylor Dispersion method at effectively infinite dilution of CO2 in the alkane, and the results have a typical standard relative uncertainty of 2.6 %. Pressure was found to have a major impact, reducing the diffusion coefficient at a given temperature by up to 55 % over the range of pressures investigated. A correlation based on the Stokes-Einstein model was investigated in which the effective hydrodynamic radius of CO2 was approximated by a linear function of the reduced molar volume of the solvent. This represented the data for the normal alkanes only with an average absolute relative deviation (AAD) of 5 %. A new universal correlation, based on the rough-hard-sphere theory, was also developed which was able to correlate all the experimental data as a function of reduced molar volume with an AAD of 2.5 %.2

Tracer diffusion coefficients of CO 2 in several brines were measured by 13 C 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), CaCl 2 (aq), Na 2 SO 4 (aq), and a mixed brine prepared from seven salts: NaCl, CaCl 2 , MgCl 2 , KCl, Na 2 SO 4 , SrCl 2 , NaHCO 3 . 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 CO 2 molecule in aqueous solution is taken to be 168 pm, as determined in an earlier study of the (CO 2 + H 2 O) 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 CO 2 in aqueous electrolyte solutions, based on knowledge of the brine viscosity and the hydrodynamic radius of CO 2 in pure water at the same temperature.

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