The role of general and hydrogen-bonding interactions in the solvation processes of organic compounds by supercritical CO2/n-alcohol mixtures

UV/visible absorption spectroscopy was applied to measure the microscopic solvent dipolarity/polarizability, pi*, and hydrogen-bond-acceptor basicity, beta, parameters, of supercritical CO2/n-alcohol mixtures. Experiments were conducted at 45 degreesC by varying the pressure between 86 and 230 bar, the type of n-alcohol from methanol to n-hexanol, and the n-alcohol mole fraction, x(2), from 0 to 0.05. The effect of our experimental conditions on maximum Deltapi* changes can be represented in the following order: system pressure (Deltapi* approximate to 0.4) > n-alcohol mole fraction (Deltapi* approximate to 0.3) > type of n-alcohol (Deltapi* approximate to 0.1). However, the corresponding order for Deltabeta changes was as follows: n-alcohol mole fraction (Deltabeta approximate to 0.6) > type of n-alcohol (Deltabeta approximate to 0.3) > system pressure (Deltabeta approximate to -0.1). Under our experimental conditions, the pi* parameters of the mixtures were below the corresponding value for liquid cyclohexane, whereas beta parameters at x(2) = 0.05 approached the values for liquid alcohols under ambient conditions, Because the solvent strength for CO2/methanol mixtures can be varied over the widest range by changing the methanol mole fraction and system pressure, methanol is the best cosolvent among the n-alcohols studied here.