Effect of film thickness on the retention index of nitro and cyano compounds. Interpretation of the logarithmic equation

Abstract

Recently, a minimum was observed on the temperature dependence of retention indices in case of polar solutes chromatographed on apolar stationary phases. Physical significance is attributed to the equation constants describing the minimum. The equation constants are explained using a thermodynamic and a kinetic approach. Statistical properties of the models for different film thicknesses are compared. Chemical potential for one methylene unit is calculated in different ways for various film thicknesses and compared with literature values. The activation enthalpy attributed to the solvation process decreases as the carbon atom number (molecular mass) of the compounds decreases. The activation enthalpies for cyanoalkanes are always higher than those for the respective nitroalkanes. The interpretation of the equation constants does not change if the film thickness of the stationary phase increases (it is varied). This fact eliminates adsorption on the surface of the column wall from the possible causes responsible for the minimum on the retention index versus temperature curve.