Adsorption chromatography solvent interaction model

The retention behavior of solutes in adsorption" chromatography can be described either by the "competition" model or by the "solute-solvent interaction" model depending on the eluent composition. It appears that both mechanisms are operative but their importance depends on the composition of the eluent mixture 84). 

The subscripts n and a in the above equation represent a molecule in a nonsorbed and adsorbed phase, respectively. In other words, retention in adsorption chromatography involves a competition between sample and solvent molecules for sites on the adsorbent surface. A variety of interaction energies are involved, and the various energy terms have been described in the literature [7,8], One fundamental equation that has been derived from the displacement model is... 

The retention mechanism in the normal phase is often referred to as adsorption chromatography. It is described as the competition between analyte molecules and mobile-phase molecules on the surface of the stationary phase. It is assumed that the adsorbing analyte displaces an approximate equivalent amount of the adsorbed solvent molecules from the monolayer on the surface of the packing throughout the retention process [18]. The solvent molecules that cover the surface of the adsorbent may or may not interact with the adsorption sites, depending on the properties of the solvent. This retention model, proposed by Snyder, was originally used to describe retention with silica and alumnina adsorbents, but several other studies have shown that this model may also be used for polar bonded phases, such as diol, cyano, and amino bonded silica [10,19]. 

The retention model in adsorption chromatography developed by Snyder and Soczewinski is based on the assumption that there is flat adsorption in a monomolecular layer on a homogeneous adsorption surface. The adsorption is understood as a competition phenomenon between the molecules of the solute and the solvent on the adsorbent surface, so that the retention of a sample molecule requires the displacement of one or more previously adsorbed polar solvent molecules. Later, the model was corrected for adsorption on a heterogeneous surface of adsorbent. To first approximation, the solute-solvent interactions in the mobile and stationary phases are assumed to compensate each other and possible liquid-liquid partition effects are neglected. In this case, the retention in a mixed binary mobile phase comprising a nonpolar solvent, A (usually an aliphatic hydrocarbon), and a polar solvent, B, can be described by eqn [1] ... 

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