Adsorption Equilibria of cis-5,8,11,14,17-Eicosapentaenoic Acid Ethyl Ester and cis-4,7,10,13,16,19-Docosahexaenoic Acid Ethyl Ester on C18-Bonded Silica from Supercritical Carbon Dioxide

The adsorption equilibrium of cis-5,8,11,14,17-eicosapentaenoic acid ethyl ester (EPA-EE) and cis-4,7,10,13,16,19-docosahexaenoic acid ethyl ester (DHA-EE) on C18-bonded silica from supercritical carbon dioxide was studied. The adsorption isotherms at (318.15, 328.15, and 338.15) K under pressures corresponding to carbon dioxide densities of (0.497, 0.618, and 0.723) g·mL−1 were determined by a static method. The isotherms were found to be of type II and could be well fitted by the BET equation. The monolayer adsorption amount of EPA-EE was (0.039 to 0.048) mmol·g−1, and that of DHA-EE was (0.040 to 0.063) mmol·g−1. At a given equilibrium molecular fraction of EPA-EE or DHA-EE in the bulk phase, the adsorption amount decreases slightly with temperature but decreases significantly with carbon dioxide density. Under the same conditions, the adsorption amount of DHA-EE is greater than that of EPA-EE. The adsorption isotherms of both EPA-EE and DHA-EE at different temperatures and carbon dioxide densities coincide to a general characteristic curve when they are plotted as a fractional monolayer coverage (q/q m) vs reduced concentration (y/y 0). The isochoric heats of adsorption of EPA-EE and DHA-EE were in the range of (10 to 36) kJ·mol−1.