Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography III. Comparison of the behavior of nonpolar and polar solutes

Recently we showed that in reversed-phase liquid chromatography (RPLC), solute methylene units are embedded in the bonded alkyl chains of the stationary phase. That is, the "partition mechanism", not the "adsorption mechanism", controls retention of methylene groups. We have also shown that the net interactions of the nonpolar groups of a solute with a nonpolar stationary phase contribute more to the overall free energy of transfer than does the solvophobic interaction in the mobile phase. The present work explores the behavior of several polar functional groups (Cl, OCH₃, CHO, NO₂, CN, and COOCH₃) in RPLC. In contrast to the behavior of methylene groups, the net interactions of more polar functional groups with the mobile phase are larger than are their net 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 between the mobile phase and a bulk nonpolar liquid used to emulate the bonded phase. This implies that either sorbed mobile phase significantly influences 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.