Abstract
The study of solvation structures formed by para-hydroxybenzoic acid (p-HBA) and its methyl (methylparaben, MP) and propyl (propylparaben, PP) esters in methanol-modified (0.03mol fraction of methanol) supercritical carbon dioxide at the density of 0.772g/cm3 and the temperature of 328K has been made using classical molecular dynamics. It is shown that the solvation occurs through the formation of stable hydrogen bonds between the solutes and the cosolvent. Changes in solvation mechanism depending on the alkyl substituent in the sequence of p-HBA – MP – PP are considered. For p-HBA a hydrogen bond via the carboxyl hydrogen is typical, and the probability of the formation of hydrogen bonds by the hydroxyl hydrogen is much smaller. In the case of MP and PP when carboxyl hydrogen is substituted by alkyl group, the hydrogen bonds via the hydroxyl hydrogen have a much greater probability of formation and existence duration. Besides, MP and PP, unlike p-HBA, have a high probability of forming hydrogen bonds with methanol as proton acceptor via carbonyl oxygen. This feature can provide an advantage of protic compounds such as low molecular weight alcohols, by selection of cosolvent to improve parabens solubility in supercritical CO2.
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