Cosolvent effects on the diffusions of 1,3-dichlorobenzene, l-carvone, geraniol and 3-fluorophenol in supercritical carbon dioxide

In order to get an insight into solute–cosolvent interactions and to predict the cosolvent effects on diffusions, several cosolvents and solutes with different size, hydrogen-bond donor (HBD) acidity, and hydrogen-bond acceptor (HBA) basicity were chosen, and the molecular diffusion coefficients of the solutes in pure and modified supercritical carbon dioxide were measured by the Taylor–Aris chromatographic peak broadening method. Cosolvent effect parameter, which is defined as the ratio of diffusion coefficient with and without cosolvent, respectively, is introduced to quantitatively compare the strength of different types of interaction between solute and cosolvent, especially hydrogen bonding. For cosolvent without hydrogen-bond ability, the cosolvent effect parameter decreases with the increase of molecular volume and weight of the solutes, in that the strength of dispersion force with cosolvent increases with their size. For cosolvent with only HBA basicity, the solute with higher HBD acidity has smaller cosolvent effect parameter, due to the fact that the strength of hydrogen-bond interaction between the solute and cosolvent only depends on and increases with the HBD acidity of the solute. For amphiprotic cosolvent, the solute with higher HBD acidity has smaller cosolvent effect parameter, it can be inferred from which that HBD acidity rather than HBA basicity of the solute makes a major contribution to the hydrogen-bond interaction between solute and cosolvent. These results are consistent with the interpretation by the Kamlet–Taft solvatochromic parameters (HBD acidity and HBA basicity). Moreover, a nonlinear decrease in diffusion coefficient with the increase of cosolvent concentration is observed, which is typical of the behavior of the system with strong interaction.

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• The molecular diffusion coefficients of 1,3-dichlorobenzene, 3-fluorophenol, geraniol, and l-carvone in supercritical carbon dioxide and CO2 with n-hexane, tetrahydrofuran, methanol, isopropanol, and acetonitrile as cosolvents were measured by the Taylor–Aris chromatographic peak broadening method.
• Cosolvent effect parameter was introduced to quantitatively compare the strength of interaction between solute and cosolvent.
• For cosolvent without hydrogen-bond ability, the cosolvent effect parameter decreases with the increase of molecular volume and mass of the solutes.
• For cosolvent with only HBA basicity, the solute with higher HBD acidity has smaller cosolvent effect parameter.
• For amphiprotic cosolvent, the solute with higher HBD acidity has smaller cosolvent effect parameter, implying HBD acidity rather than HBA basicity of the solute makes a major contribution to the hydrogen-bond interaction between solute and cosolvent.