Quantitative structure–retention relationships: VI. Thermodynamics of Kováts retention index–boiling point correlations for alkylbenzenes in gas chromatography

Empirical equations known from literature have been interpreted thermodynamically and their performance was checked for alkylbenzenes and for stationary phases of low and medium polarity. These equations relate Kováts retention index (I) and boiling point (TB) in a linear and a reciprocal way: {I=ATB+B and I=C/TB+D}. It was possible to attribute the proper physical meaning to the equation constants: A, B, C, and D with the help of some assumptions [i.e., (i) Troutonʹs rule; (ii) the transfer of molecules between the liquid and mobile phases was assumed to be similar to, although not identical with, vaporization of any given pure liquid]. The thermodynamic meaning of the equation constants involves the average heat of vaporization for the congener set of solutes (ΔHvap), the standard chemical potential of partitioning of one methylene group of n-alkane (Δμp(–CH2–)), the Troutonʹs constant (85 J mol−1 K−1), and the column temperature (T). By using the residual analysis we have found a considerable curvature in the residual plots, both for the linear and the reciprocal model. Despite of the small bias present in the parameters the validity of this thermodynamic model of solute retention was confirmed by a reasonably good agreement between the theoretical and the experimental values for the equation constants. Beside attributing a physical meaning to the Kováts retention index–boiling point correlations, the other descriptive equations were further developed. A statistical analysis has been made to compare the description by all models discussed. High correlation coefficients, values of Fischerʹs 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 applicable to all congener solutes of low or medium polarity analyzed on stationary phases of low or moderate polarity, and the Troutonʹs rule is applicable to all the enumerated cases.