Colin F. PooleUSA

The limited capability of models to describe retention for silica gel layers arises from difficulties in separating out the simultaneous contributions of solute and solvent adsorption interactions as well as limitations of the descriptors employed by current models to represent interactions at the adsorbent surface. The heterogeneous energy distribution of adsorption sites results in site-specific interactions and the steric requirements associated with immobile sites for solute adsorption at the adsorbent surface are not easily handled for varied compounds. The separate solute and solvent adsorption parameters of the competition model can be modeled with reasonable success but are difficult to reconcile with their stated properties. The reduction in the contribution of site-specific and steric interactions for RPTLC separations and the formation of a more extensive solvated interphase region provides a more favorable fit to the solvation parameter model. These models emphasize the importance of water in the mobile phase and the general contributions of mobile phase interactions on the retention mechanism. This allows system maps and correlation diagrams based on contribution of individual intermolecular interactions to the retention mechanism to be used for the comparison of selectivity for different layers with the same mobile phase composition or different mobile phase compositions for the same layer.