Károly Hébergera,* Teresa Kowalskab and Miklós Görgényic
a Institute of Chemistry, Chemical Research Center, Hungarian
Academy of Sciences H-1525 Budapest, P. O. Box 17, Hungary, E-mail: heberger@chemres.hu;
Fax: +36 1 325 75 54; Phone: +36 1 327 79 00
bInstitute of Chemistry, Silesian University, 9 Szkolna
Street, 40-006 Katowice, Poland
c Institute of Physical Chemistry, University of Szeged,
H-6701 Szeged, P. O. Box 105, Hungary
Temperature dependence of the Kováts retention index (I) was measured for some aliphatic ketones and aldehydes on a dimethylpolysiloxane (HP-1) stationary phase [1-4]. An attractive non-linearity (minimum) was observed for the I vs. isothermal column temperature relationships. A novel, linear model was derived, providing physical significance for the equation constants (A, B, C) linked with the 1/T and lnT terms:
I = A + B / T + C lnT
This model was compared with the best literature equation, the so-called Antoine (type) equation:
I = a + b / (T + c)
where a, b, c are purely empirical constants.
This model enables determination of the activation enthalpy of solution
and of the Gibbs free energy of partitioning for one methylene unit (DGCH2).
The model is founded on the transition state theory (TST), i.e. on Eyring
equation. A detailed statistical analysis clearly shows superiority of
the extended model (i.e. of this containing the logarithm of the temperature
(lnT) term) over the earlier established Antoine-type reciprocal
equation. The correctness of the new model is supported by the thermodynamically
sound quantities, plausible explanation of the solution process, and the
tendencies observed. Several interesting trends are observed, and namely:
(i) with the increasing molecular weights of oxo compounds, the respective
activation enthalpies of solution increase (cf. hole-forming); (ii) ketones
need higher absolute value of activation enthalpy than aldehydes; and (iii)
branching of the carbon chain lowers the absolute value of activation enthalpy
needed. The activation enthalpy is unambiguously bounded to polarity of
solutes. As this activation process is exothermic it can be due to adsorption.
The effect may be called “polar” adsorption.
References
[1] Miklós Görgényi, Károly Héberger, Enthalpies and Excess Enthalpies of Solution of Carbonyl Compounds by Capillary Gas Chromatography, J. Chromatogr. Sci. 37, 11-16 (1999).
[2] Károly Héberger, Miklós Görgényi, Michael Sjöström, Partial Least Squares Modeling of Retention Data of Oxo Compounds in Gas Chromatography, Chromatographia, 51, 595-600 (2000).
[3] Tamás Körtvélyesi, Miklós Görgényi, Károly Héberger, Correlation between retention indices and quantum-chemical descriptors of ketones and aldehydes on stationary phases of different polarity, Anal. Chim. Acta428, 73-82 (2001).
[4] Károly Héberger, Miklós Görgényi, Estimation of Molar Heat Capacities in Solution from Gas Chromatographic Data, J. Chromatogr. Sci. 38, 113-120 (2001).