Mobile phase displacement model

In its simplest form the competition model assumes the entire adsorbent surface is covered by a monolayer of solute and mobile phase molecules. Under normal chromatographic conditions, the concentration of sample molecules will be small and the adsorbed monolayer will consist mainly of mobile phase molecules. Retention of a solute molecule occurs by displacing a roughly equivalent volume of mobile phase molecules from the monolayer to make the surface accessible to the adsorbed solute aiolecule. For elution of the solute to occur -the above process must be reversible, and can be represented by the equilibrium depicted by equation (4.6)... 

According to this theoretical treatment, the slope of the plots of In k versus the solvent concentration, [3]m, can be employed to derive the contact area associated with the peptide-nonpolar ligand interaction. The retention and elution of a peptide in RPC can then be treated as a series of microequilibriums between the different components of the system, as represented by eq 6. The stoichiometric solvent displacement model addresses a set of considerations analogous to that of the preferential interaction model, but from a different empirical perspective. Thus, the affinity of the organic solvent for the free peptide P, in the mobile phase can be represented as follows ... 

From the Snyder-Soczewinski model (12, 13), the entire adsorbent surface is covered by an adsorbate monolayer that consists of mobile phase. Retention is assumed to occur as a displacement process in which an adsorbing solute molecule X displaces some number n of previously adsorbed mobile-phase molecules S... 

Snyder [350] has given an early description and interpretation of the behaviour of LSC systems. He explained retention on the basis of the so-called competition model . In this model it is assumed that the solid surface is covered with mobile phase molecules and that solute molecules will have to compete with the molecules in this adsorbed layer to (temporarily) occupy an adsorption site. Solvents which show a strong adsorption to the surface are hard to displace and hence are strong solvents , which give rise to low retention times. On the other hand, solvents that show weak interactions with the stationary surface can easily be replaced and act as weak solvents . Clearly, it is the difference between the affinity of the mobile phase and that of the solute for the stationary phase that determines retention in LSC according to the competition model. 

The steric mass action (SMA) model has been shown to successfully predict nonlinear, multicomponent behavior in ion-exchange systems over a range of mobile phase salt concentrations.71-75 It has also been widely employed as a methods development tool for displacement separations.42,45,50 In this section, we will describe several graphical techniques derived this theory which can facilitate methods development in ion-exchange displacement systems. 

The various aspects of displacement and localization are now well understood, and predictions of their effects on retention in LSC can be made with some confidence. Hydrogen bonding between solute and solvent molecules requires further investigation, and it is likely that such studies will contribute to our understanding of hydrogen bonding in solution as well. On the basis of the present model it should prove possible to systematically explore new stationary phase compositions for unique separation potential. However, this subject falls outside the area of mobile-phase effects per se, and will be reserved for another time. 

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]. 

In contrast, the Scott-Kucera model considers a solvent system composed of an apolar solvent A and a polar solvent B (Scott and Kucera, 1975). When this mixture is pumped through a column, a monolayer of the most polar solvent B is formed by adsorption of B on the adsorbent. Sample molecules are adsorbed on this monolayer instead of on the adsorbent surface. In other words, there is no displacement of adsorbed solvent molecules, and interaction between the molecules of the monolayer and the sample molecules determines the retention of the component. This theory has been adapted by saying that the model is only valid for medium polar mobile phases and solutes with a polarity lower than the most polar solvent in the eluent. These medium polar solvents are called hydrogen-bonding solvents (esters, ethers, ketones). A monolayer of these solvents behaves as a hydrogen-bonding phase. Inter-... 

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