Displacement model adsorption

According to the stoichiometric displacement model, the equilibrium constant for peptide adsorption with the solvated nonpolar ligands can be expressed as follows ... 

Retention on these supports is adaquetely described by the adsorption displacement model. Nevertheless, the adsorption sites are delocalized due to the flexible moiety of the ligand, and secondary solvent effects play a significant role. The cyano phase behaves much like a deactivated silica toward nonpolar and moderately polar solutes and solvents. Cyano propyl columns appear to have basic tendencies in chloroform and acidic tendencies in methyl tertiobutyl ether (MTBE)... 

Models for the interactions of solutes in adsorption chromatography have been discussed extensively in the literature [7-9]. Only the interactions with silica and alumina will be considered here. However, various modifications to the models for the previous two adsorbents have been applied to modern high-performance columns (e.g., amino-silica and cyano-silica). The interactions in adsorption chromatography can be very complex. The model that has emerged which describes many of the interactions is the displacement model developed by Snyder [1,3,4,7,8], Generally, retention is assumed to occur by a displacement process. For ex-... 

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 semiempirical model of adsorption chromatography, analogous to that of Martin and Synge, was established only in the late 1960s by Snyder [3] and Soczewinski [4] independently, and it is often referred to as the displacement model of solute retention. The crucial assumption of this model is that the mechanism of retention consists in competition among the solute and solvent molecules for the active sites of the adsorbent and, hence, in a virtually... 

Retention mechanisms of adsorption chromatography have been extensively studied. There are two popular models for this process. The displacement model, originally proposed by Snyder, treats the distribution of solute between a surface phase, usually assumed to be a monolayer, and a mobile phase as a result of a competitive solute and solvent adsorption. A treatment of this model, including the significance of predictions of solvent strength and selectivity in terms of mobile-phase optimization strategies, has been published by Snyder (81). 

Figure 8 Schematic illustration of four different adsorption/displacement models proposed by Wahlgren and Amebrant [63] for protein and surfactant adsorption to solid surfaces. The three diagrams for each model show protein adsorption, surfactant addition, and state after rinsing. Figure A represents the case where surfactant binds to the protein and the protein-surfactant complex desorbs. Figure B represents protein displacement by the surfactant. Figure C represents reversible adsorption of the surfactant by the protein. Figure D represents reversible adsorption by the surfactant resulting in partial desorption of the protein. The figures relate to a hydrophilic surface at a hydrophobic surface the orientation of the surfactant molecules with respect to the surface will be different. (Reproduced from [63] with permission from Academic Press.)...

The principal limitation of Wright s static displacement model is that it does not consider the accumulation of deformations due to the passage of a number of waves. This problem has been approached by Schapery and Dunlap (1978), modeling the soil as a linearly viscoelastic material. Their analysis also included the effect of energy adsorption of the seafloor on the wave characteristics. 

The displacement model elaborated by Snyder in the 1960s to 1980s is well suited to explain retention in NPLC. On the basis of this model, chromatographic system is described quantitatively with various equations. According to the displacement model there is a competition between an analyte X and a mobile phase M for adsorption on the stationary-phase (adsorbent) active sites ... 

The importance of three-dimensional structure to chromatographic behavior is reflected in the nonmechanistic model, the stoichiometric displacement model (SDM). The central hypothesis of the SDM is that the displacement of a solute from a surface is accompanied by the adsorption of a stoichiometric amount of displacing agent. The process may be described by the equilibrium expression ... 

Consequences of the Snyder and Soczewinski model are manifold, and their praetieal importance is very signifieant. The most speetaeular conclusions of this model are (1) a possibility to quantify adsorbents ehromatographic activity and (2) a possibility to dehne and quantify chromatographic polarity of solvents (known as the solvents elution strength). These two conclusions could only be drawn on the assumption as to the displacement mechanism of solute retention. An obvious necessity was to quantify the effect of displacement, which resulted in the following relationship for the thermodynamic equilibrium constant of adsorption, K,, in the case of an active chromatographic adsorbent and of the monocomponent eluent ... 

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