Effect of Lewis acid-base complexes between CO₂ and alkanols on phase behavior at high pressure

Due to the electropositivity of the carbon atom in CO₂ which results from its lower electron density compared to that of the oxygen atom, CO₂ molecules can act either as proton acceptor or electron acceptor. The specific intermolecular interaction (i.e., Lewis acid-base interaction) between CO₂ and organic chemicals with electron donor groups exists, and this phenomenon is supported by many experimental studies using spectrometry and ab initio studies. CO₂/alcohol solvent systems under high-pressure conditions are important for supercritical extraction, dehydration of alcohols, and extraction of natural products. Furthermore, the study of the complex between CO₂ and alcohols due to the Lewis acid-base interaction is crucial for understanding the solvation process under high-pressure conditions, and for the development of thermodynamic models. In this study, the Lewis acid-base interaction between alcohols and CO₂ is treated as a hydrogen bonding interaction, and according to the different association schemes for CO₂, the effect on the phase behavior of the CO₂/alcohol systems at high pressure was investigated using the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) model. Taking into consideration the self-association of CO₂ yields a more satisfactory performance in the high-pressure region than it would without, and increasing association sites of CO₂ do not lead to a better performance. In addition, the cross-association energy (i.e., Lewis acid-base interaction energy) parameter is estimated by directly regressing the experimental data over the entire temperature and pressure range, and the calculated results show an improved performance even at much higher pressures. It was found that strong cross-association interactions lead to higher cross-association in both liquid and vapor phases, which contributes to the improved phase behavior at high-pressures.