Binary diffusion coefficients and retention factors for γ-linolenic acid and its methyl and ethyl esters in supercritical carbon dioxide

Infinite dilution binary diffusion coefficients D12 and retention factors k for γ-linolenic acid were measured by chromatographic impulse response technique in supercritical carbon dioxide over the ranges of temperature from 308 to 343 K and pressure from 9 to 30 MPa, together with those for the methyl and ethyl esters at 313 and 343 K and pressures from 8 to 16 MPa. In comparison with literature values for C18 free fatty acids and the esters, the differences in D12 data were almost indiscernible for each isomer group: α- and γ-linolenic acid, oleic and elaidic acids, oleic and elaidic acid methyl esters, and their ethyl esters. The number of carbon double bonds did not affect the D12 values significantly. Moreover, the differences in the values between free fatty acid and the ester forms were also small, whereas the D12 values increased in order of acid to ethyl ester to methyl ester. The validities of the predictive correlations we had proposed such as the D12/T–CO2 viscosity and the Schmidt number correlation were verified. The retention factor k values for the acid form were much higher than those for the methyl and ethyl esters. The k values were correlated with temperature and CO2 density for each compound, and the solute partial molar volumes were determined from the k–CO2 density correlation.