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

Abstract

Infinite dilution binary diffusion coefficients D 12 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 D 12 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 D 12 values significantly. Moreover, the differences in the values between free fatty acid and the ester forms were also small, whereas the D 12 values increased in order of acid to ethyl ester to methyl ester. The validities of the predictive correlations we had proposed such as the D 12/ T–CO 2 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 CO 2 density for each compound, and the solute partial molar volumes were determined from the k–CO 2 density correlation.