Determination of log P for pressurized carbon dioxide and its characterization as a medium for enzyme reaction

The log P value of pressurized CO2 at 50°C was determined from the solubility of 1-octanol in CO2 and compared with other solvent parameters such as permittivity, ϵ, and polarity parameter, ET(30). The log P indicated that pressurized CO2 is rather hydrophilic although it seems hydrophobic being judged from ϵr and ET(30). With a change in pressure from 3 to 11.8 MPa, the log P changed from 0.9 to 2.0 while ϵr and ET(30) changed only slightly. The log P was linearly correlated to the logarithm of the solubility of water among organic solvents. Pressurized CO2 was located close to the linear correlation line among the solvents at high pressure (>11 MPa) but its location deviated to the hydrophilic side with a decrease in pressure. Lipase-catalyzed esterification of stearic acid with ethanol and hydrolysis of ethyl stearate were carried out in pressurized CO2, benzene (log P = 2.0), and n-hexane (log P = 3.5). In spite of the lowest log P value for CO2, the reaction rate in CO2 was the highest among solvents tested in pressure range over 10 MPa. The reaction rate was strongly dependent on pressure of CO2.