Krichevskii parameters of heavy n-alkanes in carbon dioxide: comparison of the results from solubility measurements and from supercritical fluid chromatography

Thermodynamic properties of dilute binary mixtures near the solvent’s critical point are largely governed by the Krichevskii parameter [J. Supercrit. Fluids 4 (1991) 215]. In binary mixtures of components of comparable volatilities, the Krichevskii parameter can be determined from exact relationships connecting it to the initial slopes of the critical line. In highly asymmetric mixtures such as those encountered in supercritical fluid extraction, the exact procedure is usually not feasible, and the Krichevskii parameter has to be estimated from other properties employing a suitable thermodynamic model. In this contribution, the Krichevskii parameters of C21 to C40 n-alkanes in carbon dioxide are derived from retention data of the n-alkanes in supercritical fluid chromatography (SFC), and the results are compared to the literature values obtained from solubility data. The results from SFC suggest that, to a first approximation, the Krichevskii parameter of an n-alkane in CO2 is a linear function of the alkane carbon number.