Enthalpies of Solution and Excess Enthalpies of Oxo Compounds by Capillary Gas Chromatography

The temperature dependence of retention data is measured for 5 n-alkanes and 35 aliphatic ketones and aldehydes on capillary columns using 4 bonded stationary phases: dimethyl polysiloxane (HP-1), phenyl methyl polysiloxane (HP-50), trifluoropropyl methyl polysiloxane (DB-210), and polyethylene glycol (HP-Innowax). A novel equation is deduced from the expressions describing the thermodynamic relations of the specific retention volume. The enthalpy of solution, ØsolH°, is determined from the fits of In (k/T) versus 1/T, where k is the retention factor and T is the column temperature. The excess enthalpy HE is determined from the enthalpy of vaporization in the middle of the temperature range (the enthalpy of vaporization is available from the literature) and from ØsolH°. The ØsolH° values of the n-alkanes decrease with increasing column polarity on the McReynolds scale, whereas those of the alkanals and ketones tend to increase. The values of HE show that, with increasing polarity of the stationary phase, the stronger interaction with the C=0 group is cancelled out by a similar decrease in interaction with the apolar part of the solute. With increasing molecular size of the n-alkanals, the solute—stationary phase interaction is increasingly less able to compensate the energy necessary for hole formation in the stationary phase.