Solvation molar enthalpies and heat capacities of n-alkanes and n-alkylbenzenes on stationary phases of wide-ranging polarity

摘要

A comparison of the most usual gas chromatographic methods for the calculation of partial molar enthalpies of solvation (Δ_(sol)H~o) has been carried out. Those methods based on the fitting of lnV_g or ln(k/T) vs. 1/T and ln(k/T) vs. (1/T and the temperature arrangement, T_a) are the most adequate ones for obtaining Δ_(sol)H~o values. However, the latter is the only reliable option for Δ_(sol)H~o estimation when commercial WCOT capillary columns are used, since in this case the estimation of some variables involved in the V_g determination is less accurate or even impossible. Consequently, in this paper, Δ_(sol)H~o obtained from ln(k/T) vs. (1/T+T_a) fitting at 373.15 and 298.15K for n-alkanes and n-alkylbenzenes on 12 commercial capillary columns coated with stationary phases covering the 203-3608 McReynolds polarity range are reported. Moreover, molar heat capacities of solvation at constant pressure (ΔsolCpo) have also been calculated using this method. A clear influence on Δ_(sol)H~o of the type and content of the substitution group in the stationary phase was observed. In addition, a linear relationship of ΔsolCpo with the van der Waals volume of the n-alkanes and the temperature gradient of density of the stationary phase was found. The effect of the size of the hydrocarbon on both thermodynamic variables was also investigated.