REVISIONIST LOOK AT SOLVOPHOBIC DRIVING FORCES IN REVERSED-PHASE LIQUID-CHROMATOGRAPHY .3. COMPARISON OF THE BEHAVIOR OF NONPOLAR AND POLARSOLUTES

Recently we showed that in reversed-phase liquid chromatography (RPLC) , solute methylene units are embedded in the bonded alkyl chains of th e stationary phase. That is, the ''partition mechanism'', not the ''ad sorption mechanism'', controls retention of methylene groups. We have also shown that the net interactions of the nonpolar groups of a solut e with a nonpolar stationary phase contribute more to the overall free energy of transfer than does the solvophobic interaction in the mobil e phase. The present work explores the behavior of several polar funct ional groups (Cl, OCH3, CHO, NO2, CN, and COOCH3) in RPLC. In contrast to the behavior of methylene groups, the net interactions of more pol ar functional groups with the mobile phase are larger than are their n et interactions with the stationary phase. Further, the data indicate that the free energy of transfer of a polar functional group in bonded phase RPLC is quite different from the free energy of transfer betwee n the mobile phase and a bulk nonpolar liquid used to emulate the bond ed phase. This implies that either sorbed mobile phase significantly i nfluences the retention properties of polar groups in the bonded phase , or that the solute functional groups reside at the interface between the mobile and stationary phases.