SOLVOPHOBIC THEORY AND NORMALIZED FREE-ENERGIES OF NONPOLAR SUBSTANCES IN REVERSED-PHASE CHROMATOGRAPHY

A simplified version of the solvophobic theory is employed to reexamin e a large set of retention data with nonpolar and weakly polar eluites in reversed phase chromatography (RPC) to test certain predictions by the theory and to clarify further the roles of the mobile and the sta tionary phase in the retention process. The free energy of retention i n RPC is expressed in terms of the nonpolar surface area of the eluite , the pertinent interfacial tensions, and the energetics of eluite-sta tionary phase interactions in the gas phase. Within this framework cha nges in retention free energy per unit water accessible nonpolar surfa ce area are evaluated in the entire range of the organic modifier conc entration by using thermodynamic data for the retention of nonpolar el uites on alkyl silica bonded phase in gas chromatography and for the t ransfer of nonpolar solutes from the hydroorganic solvent to the gas p hase. The normalized retention free energy parameters thus obtained ac cording to the theory are in excellent agreement with those evaluated from experimental retention data using methanol, acetonitrile, tetrahy drofuran, and 2-propanol as organic modifiers in RPC. Furthermore, the se parameters are found to be the same from column to column for a par ticular stationary phase and therefore characterize the RPC retention process. Analysis of experimental retention data obtained with various bonded phases indicates that the stationary phase chain length has a much smaller effect on the changes in selectivity of nonpolar aliphati c eluites than the mobile phase concentration. The results demonstrate the usefulness of the solvophobic theory for the evaluation of physic ochemical parameters associated with retention of hydrocarbonaceous el uites in RPC and confirm the dominant role of the mobile phase in gove rning the retention and selectivity changes in RPC of nonpolar eluites .