Analysis of Solvent–Solvent Interactions in Mixed Isosteric Solvents by Inverse Gas Chromatography

The Flory–Huggins interaction parameter, χ23, characterizing the interaction between solvents in a mixed stationary phase has been calculated using inverse gas chromatography (IGC). The χ23 parameters for four mixed solvent systems formed by mixing a non-polar branched alkane, 19, 24-dioctadecyldotetracontane (C78), with four different polar solvents are analysed as a function of temperature. Two of the polar solvents are formed by replacing one of the –CH3 groups of C78 by –OH (POH) group and –CN (PCN) group and the other two polar solvents are formed by replacing all the four –CH3 groups of C78 by four –CF3 (TTF) groups and four –OCH3 (TMO) groups. The five solvents have similar structures and nearly the same molar volume and in mixtures, they form regular solutions. For the mixture, C78 + POH, χ23 was calculated at two compositions and for the remaining three mixtures χ23 was calculated at equimolar composition. Eight solute probes representing all possible chemical structures and polarity were used under conditions approaching infinite dilution. The effects of temperature, concentration of the mixed solvent and probe solute on χ23 are presented and discussed. The probe-independent χ23 values were also calculated by linear regression analysis using the retention data of all the solute probes. In all the mixed solvents considered here the χ23 values are probe-dependent and decrease with increase of temperature.