High pressure NMR study of carbon dioxide and supercritical carbon dioxide-n-hexadecane mixtures

[摘要] For the first time, an NMR spectrometer equipped with a high pressure probe has been interfaced with a Vapor-Liquid-Equilibrium apparatus to study solvent-solute interactions in supercritical fluids. The following three paragraphs detail the sequential progress of the work where we focused on the pure solvent, the pure solvent near its critical point and then the mixtures.Spin-lattice relaxation time T$sb1$ and self-diffusion coefficient D in $msp{13}COsb2$ have been measured from 0 to 75$spcirc$C at pressures up to 500 bar. The governing relaxation mechanism in this range is shown to be spin-rotation relaxation. For both T$sb1$ and D data, a kinetic theory based model describes well the low density values, whereas a hydrodynamics based model works adequately at high densities. Using recent molecular dynamics calculations, we found that the smooth hard-sphere theory predicts surprisingly well the self-diffusion of CO$sb2$ at densities above critical.T$sb1$ and D have then been measured near the critical point of CO$sb2$. T$sb1$ values are unprecedented and this is the third D determination in the critical region. The two previous D determinations are in serious disagreement. One reported the presence of a strong critical anomaly whereas the other observed that D behaves normally in the critical region. No critical anomaly was found for either T$sb1$ or D.Finally, T$sb1$ and D of $msp{13}COsb2$ and $m Csb{16}sp1Hsb{34}$ have been measured on relevant isotherms from atmospheric pressure up to the critical point of the mixture in the coexisting vapor and liquid phases. The data have been correlated with phase compositions determined earlier in this laboratory and with calculated viscosities. $msp{13}CTsb1$ data indicate that the reorientational correlation time of the CO$sb2$ molecule is constant in the liquid phase up to the critical region. $msp1HTsb1$ data are in qualitative agreement with theory. The ratios of the self-diffusion coefficients in each phase is closely related to the ratios of the partial molar volumes, in agreement with arguments from statistical mechanics and irreversible thermodynamics. Calculated mutual diffusivities are found to be in excellent agreement with available experimental diffusivities.

[发布机构] Rice University

[效力级别] engineering