INFLUENCE OF SOLUTE SIZE AND THE NONPOLAR INTERACTION TERM ON THE SELECTION OF TEST SOLUTES FOR THE CLASSIFICATION OF STATIONARY PHASE SELECTIVITY IN GAS-CHROMATOGRAPHY

A new parameter DELTA-G(S)INT (X) is introduced for the determination of the capacity of a stationary phase to enter into orientation and hydrogen-bonding interactions. DELTA-G(S)INT (X) is defined as the component of the free energy of solution that is equivalent to the solute-solvent interactions of solute (X) that exceeds those interactions typified by an n-alkane of identical volume in solvent S reduced by the identical interactions of solute (X) in a hydrocarbon solvent (squalane). DELTA-G(S)INT (X) provides a quantitative scale of orientation and proton donor-acceptor interactions that is independent of solute size and non-polar solute-solvent interactions. Multivariate analysis of DELTA-G(S)INT (X) values for 21 test solutes on 20 stationary phases was used to identify acceptable test solutes for characterizing specific intermolecular interactions and to classify the stationary phases based on their capacity for these interactions. Several highly correlated solutes were identified as suitable for determining orientation and solvent proton acceptor capacity of which nitrobenzene and n-octanol, respectively, are recommended as the preferred solutes. Benzene and dioxane were identified as the most favorable test solutes for solvent proton donor capacity within the data set. It seems likely that a more appropriate solute could be found for this interaction. The classification of stationary phases by DELTA-G(S)INT (X) is informative and shows a logical clustering of phases of similar type and a linearly related change in properties for the homologous series of poly(methylphenylsiloxane) solvents as the mole percentage of phenyl groups is increased.