Quantitative structure-retention relationships VI. Thermodynamics of Kovats retention index boiling point correlations for alkylbenzenes in gas chromatography

Empirical equations known from literature have been interpreted thermodynam ically and their performance was checked for alkylbenzenes and for stationa ry phases of low and medium polarity. These equations relate Kovats retenti on index (I) and boiling point (T-B) in a linear and a reciprocal way: {I = AT(B) + B and I = C/T-B + D}. It was possible to attribute the proper phys ical meaning to the equation constants: A, B, C, and D with the help of som e assumptions [i.e., (i) Trouton's rule; (ii) the transfer of molecules bet ween the Liquid and mobile phases was assumed to be similar to, although no t identical with, vaporization of any given pure liquid]. The thermodynamic meaning of the equation constants involves the average heat of vaporizatio n for the congener set of solutes (<(Delta H)over bar>(vap)), the standard chemical potential of partitioning of one methylene group of n-alkane (Delt a mu(p(-CH2-))), the Trouton's constant (85 J mol(-1) K-1), and the column temperature (T). By using the residual analysis we have found a considerabl e curvature in the residual plots, both for the linear and the reciprocal m odel. Despite of the small bias present in the parameters the validity of t his thermodynamic model of solute retention was confirmed by a reasonably g ood agreement between the theoretical and the experimental values for the e quation constants. Beside attributing a physical meaning to the Kovats rete ntion index-boiling point correlations, the other descriptive equations wer e further developed. A statistical analysis has been made to compare the de scription by all models discussed. High correlation coefficients, values of Fischer's statistic and acceptable residual error indicate the goodness of fit. The quadratic model provides the best description, whereas the linear equation is better than the reciprocal. The relationships discussed are ap plicable to all congener solutes of low or medium polarity analyzed on stat ionary phases of low or moderate polarity, and the Trouton's rule is applic able to all the enumerated cases. (C) 1999 Elsevier Science B.V. All rights reserved.