Soczewinski-Snyder model

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... 

To illustrate the general elution problem and its solution, let us consider the following situation. Consider a 20-component mixture with capacity factors k of the components forming a geometrical progression and exponentially dependent on the modifier concentration (molar or volume fraction c), in accordance with the Snyder-Soczewinski model of adsorption [2]. The log k versus log c plots of the 20 solutes are given in Fig. 1, which has a parallel Rf axis subordinated to the right-hand-side log k axis. It can be seen that no isocratic eluent can separate all the components. A pure modifier [c = 1.0 (100%)] separates well solutes 1-7, and the less polar solutes are accumulated near the solvent front for c = 0.1 (10%), solutes 8-14 are... 

As can be easily deduced, quantification of sorbent activity cannot be done in the absolute, but in relative values only. The most complete approach to this problem was derived from the Snyder-Soczewinski model of adsorption chromatography, and it will be briefly discussed here. 

Quantification of solvent elution strength is based on the Snyder-Soczewinski model of adsorption chromatography. A possibility of appropriate quantification is offered by Eq. 47. For the sorbent activity coefficient a = 1, Eq. 47 can be rewritten in the following form ... 

From the general framework of the Snyder and Soczewinski model of the linear adsorption TLC, two very simple relationships were derived, which proved extremely useful for rapid prediction of solute retention in the thin-layer chromatographic systems employing binary mobile phases. One of them (known as the Soczewinski equation) proved successful in the case of the adsorption and the normal phase TLC modes. Another (known as the Snyder equation) proved similarly successful in the case of the reversed-phase TLC mode. 

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 ... 

A single solvent only rarely provides suitable separation selectivity and retention in normal-phase systems, which should be adjusted by selecting an appropriate composition of a two- or a multi-component mobile phase. The dependence of retention on the composition of the mobile phase can be described using theoretical models of adsorption. With some simplification, both the Snyder and the Soczewinski models lead to identical equation describing the retention (retention factor. A) as a function of the concentration of the stronger (more polar) solvent, (p. in binary mobile phases comprised of two solvents of different polarities [,121 ... 

Simplified Relationships Derived from the Snyder and Soczewinski Model... 

Snyder s model provides a good understanding of separations on alumina as the adsorbent and is fairly good at explaining separations on silica gel using weak solvents. An almost similar model developed by Soczewinski (Soczewinski, 1969) is more suitable for separations on silica gel using strong eluents. 

SIMPLIFIED RELATIONSHIPS DERIVED FROM THE SNYDER AND SOCZEWINSKI MODEL... 

Apart from the most widely utilized Snyder and Soczewinski semiempirical model of linear TEC, several other physicochemically grounded approaches to the same question exist as well [12]. Also, a choice of the empirical rules in mathematical... 

Snyder and Soczewinski created and published, at the same time, another model called the S-S model describing the adsorption chromatographic process [19,61]. This model takes into account the role of the mobile phase in the chromatographic separation of the mixture. It assumes that in the chromatographic system the whole surface of the adsorbent is covered by a monolayer of adsorbed molecules of the mobile phase and of the solute and that the molecules of the mobile phase components occupy sites of identical size. It is supposed that under chromatographic process conditions the solute concentrations are very low, and the adsorption layer consists mainly of molecules of the mobile phase solvents. According to the S-S model, intermolecular interactions are reduced in the mobile phase but only for the... 

When a limited range of organic modifier proportions within the mobile phase is considered, the retention changes in RPLC with mobile phase composition are generally adequately described by the linear Soczewinski-Snyder model (Eq. 4) adapted to both isocratic and gradient mode [5]. 

The solute competes with eluent molecules for the ac ve adsorption sites on the surface of the stationary phase. Interactions tetween solute and solvent molecules in the liquid phase are cancelled by milar interactions in the adsorbed phase. This model has been introdu d by Snyder (2) and by Soczewinski (77, 78) and is called the competition model."... 

Yet another adsorption-based retention model similar to that of Snyder was proposed by Soczewinski [6] to describe the retention in NPC. It assumes that retention in NPC is the product of competitive adsorption between solute and solvent molecules for active sites on the stationary phase surface. The stationary-phase surface consists of a layer of solute and/or solvent molecules, but, unhke the former, the latter model assumes an energetically heterogeneous surface where adsorption occurs entirely at the high-energy active sites, leading to discrete, one-to-one complexes of the form... 

HPLC retention data for QSRR analysis are usually obtained by measuring log at several eluent compositions (isocratic conditions) and then extrapolating the dependence of log on a binary eluent composition to a fixed mobile phase composition, common for all the analytes studied, based on the Soczewinski-Snyder model ... 

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