Diffusion Coefficient In Pure And Modified Supercritical CO₂and Their Structure-property Relationship

Supercritical carbon dioxide (scCO2) is widely used in chemical industry because scCO2has many superior properties involving accessible critical condition, a stronger solvent power, essentially nontoxic, higher diffusivities. Diffusion coefficient is useful in design, scale-up and optimization of reactors and separators for supercritical fluid extraction or chromatography. Therefore, in this study, infinite dilution diffusion coefficients of a series of solutes in pure and cosolvent modified CO2were measured, and the effects of physicochemical properties of fluid, properties and structure of solutes, the types of cosolvent, intermolecular interaction on the diffusion coefficients in supercritical fluid were investigated systematically by means of a combination of experimental measurement and quantum chemical method from molecular level. On the basis of the above, Linear solvation energy relationship (LSER) model of diffusion coefficients was developed.The molecular diffusion coefficients of L-menthone and L-carvone in scCO2and scCO2containing5and10mol%ethanol as a modifier were measured by the Taylor dispersion method over the ranges of temperature from308.15to338.15K and pressure from15to30MPa. It was found that the diffusion coefficients increased almost linearly with temperature and decreased with the solvent pressure, density, viscosity and ethanol mole fraction. Moreover, the correlation relationships between diffusion coefficients and the temperature, pressure, viscosity, and density, which were valid in binary systems were also suitable for ternary systems of carbon dioxide containing modifier. Of several models used to predict experimental data in pure carbon dioxide, the two models of Funazukuri-Ishiwata-Wakao and He-Yu-1998were the best with the AAD less than3.2%. Furthermore, the modified Stokes-Einstein (SE) models overestimated the diffusion coefficients in ethanol modified scCO2with the AAD values increasing with the percentage of ethanol. which were probably due to the increase of the volume of solvation sphere as a true diffusion unit with the percentage of ethanol. Moreover, the free volume model of Dymond is good for predicting the experimental data in pure carbon dioxide and ethanol modified scCO2with the AAD values less than3.2%. The clustering of cosolvent molecules around the solute driven by van der Waals’ forces and hydrogen-bond interaction between solute and cosolvent resulted in solute cluster with increased effective size. Such solute cluster was considered as a true diffusion unit, leading to slower solute diffusion in ternary systems than in pure CO2. Cosolvent effect parameter, which was defined as the ratio of diffusion coefficient with and without cosolvent, respectively, was introduced to 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 decreased 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 had smaller cosolvent effect parameter, due to the fact that the strength of hydrogen-bond interaction between the solute and cosolvent only depended on and increased with the HBD acidity of the solute. For amphiprotic cosolvent, the solute with higher HBD acidity had smaller cosolvent effect parameter, it could be inferred from which that HBD acidity rather than HBA basicity of the solute made a major contribution to the hydrogen-bond interaction between solute and cosolvent. The cosolvent effect parameter increased with pressure, because solute-cosolvent interaction decreased with pressure. Moreover, a nonlinear decrease in diffusion coefficient with the increase of cosolvent concentration was observed, which was typical of the behavior of the system with strong interaction.The diffusion coefficients of non-H-bonded solutes in pure and modified supercritical carbon dioxide depended largely on molecular volume, and the para-isomer with more linear structure diffused more quickly. The large value of cosolvent effect parameter for the H-bonded solutes with more and longer-chain substituent group was probably due to the steric hindrance effect provided by substituent group in the ortho position of functional group with hydrogen-bond ability. For Di-H-bonded solute, the diffusion coefficients of ortho-isomer were larger than that of meta-isomer, which could probably be attributed to the fact that intramolecular H-bonding weakened the hydrogen-bond interaction between solute and CO2as well as between solute and cosolvent.Quantum chemical methods were employed to investigate the interaction mechanism between cosolvent (tetrahydrofuran, methanol) and solute, which can help to get insight into the effect of intermolecular interaction on diffusion coefficients. It was found that the strength of hydrogen-bond interaction between the solute and tetrahydrofuran, which had only HBA basicity, only depended on the HBD acidity of the solute. Compared to geraniol, the3-fluorophenol with higher HBD acidity formed stronger hydrogen-bond interaction with tetrahydrofuran. Since both1.3-dichlorobenzene and L-carvone had no the HBD acidity, only weak van der Waals’forces could be formed. L-carvone with larger molecular volume and higher polarity formed stronger van der Waals’ forces with tetrahydrofuran. Amphiprotic Methanol could form two kinds of hydrogen-bond complexes with amphiprotic3-fluorophenol and geraniol. Relative to L-carvone, the amphiprotic3-fluorophenol and geraniol could form stronger hydrogen-bond interaction with methanol, While, the1.3-dichlorobenzene formed the weakest van der Waals’forces with methanol. The above results from quantum chemical calculation could effectively explain why the cosolvent effect parameter in CO2containing tetrahydrofuran and containing methanol increased in the following order:3-fluorophenol<geraniol<L-carvon<1.3-dichlorobenzene.Based on the diffusion coefficients and cosolvent effect parameters of23training solutes in scCO2and CO2containing10mol%cosolvent (tetrahydrofuran, methanol). the Linear solvation energy relationship (LSER) method was used to develop predictive model. These models for the above three system were validated by making predictions for12test solutes and had good prediction accuracy with the AAD values less than2.45%,3.17%and3.40%, respectively. The coefficients of these models shown that the diffusion coefficients in CO2were strongly dependent on the dispersion force between solute and CO2. and the cosolvent effect parameters in CO2containing10mol%cosolvent (tetrahydrofuran. methanol) were mainly affected by the hydrogen-bond interaction between cosolvent and solute which served as proton donor.