Correlation between reversed-phase retention and solute molecular surface type and area : III. The influence of polar and solvent-induced polar groups

The introduction of functional groups into n-alkanes causes changes in retention behaviour. As a general rule, in reversed-phase systems an increase in solute polarity lowers retention, due to an increased solute-eluent interaction. The overall retention loss may be caused by (a) direct interaction of the substituent with the eluent (C  N, OH, O), (b) build-up of polar centres at the CH 2X bonds accompanied by solvation of the solute, (c) influence of the substituent on the retention contribution of the alkyl residues. The retention index increment of a methylene group which is 100 in n-alkanes is lowered to 86.7 in ethers. The molar free energies of sorption are negative for CH 3, CH 2, Br and chalcogen atoms, positive for OH, CN, O and S. In thioethers and thiols, however, the S atoms themselves seem to be solvophobic, exhibiting negative free energies. In these cases the positive (or zero) values found are due to the strongly positive Δ G of solvated polarized bonds. This polarization is about twice as strong in CH 2S bonds as in CH 2Se or CH 2Te bonds.