Solvent-interaction model

The competition model and solvent interaction model were at one time heatedly debated but current thinking maintains that under defined r iitions the two theories are equivalent, however, it is impossible to distinguish between then on the basis of experimental retention data alone [231,249]. Based on the measurement of solute and solvent activity coefficients it was concluded that both models operate alternately. At higher solvent B concentrations, the competition effect diminishes, since under these conditions the solute molecule can enter the Interfacial layer without displacing solvent molecules. The competition model, in its expanded form, is more general, and can be used to derive the principal results of the solvent interaction model as a special case. In essence, it seems that the end result is the same, only the tenet that surface adsorption or solvent association are the dominant retention interactions remain at variance. 

Rettig W. (1982) Application of Simplified Microstructural Solvent Interaction Model to the Solvatochromism of Twisted Intramolecular Charge Transfer (TICT) States, /. Mol. Struct. 8, 303-327. 

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). 

Two models have been developed to describe the adsorption process. The first model, known as the competition model, assumes that the entire surface of the stationary phase is covered by mobile phase molecules and that adsorption occurs as a result of competition for the adsorption sites between the solute molecule and the mobile-phase molecules.1 The solvent interaction model, on the other hand, suggests that a bilayer of solvent molecules is formed around the stationary phase particles, which depends on the concentration of polar solvent in the mobile phase. In the latter model, retention results from interaction of the solute molecule with the secondary layer of adsorbed mobile phase molecules.2 Mechanisms of solute retention are illustrated in Figure 2.1.3... 

Figure 2.1 (a) Competition and (b) solvent interaction models of solute retention in normal-... 

This section aims to show how the LFER approach compares to other property calculation methods. Biological, chemical, and physical responses originate from interactions between two or more molecules. Many of these interactions can be looked at as involving a solute molecule surrounded by solvent molecules. The successful application of solute-solvent interaction models to many such properties has been well documented. Examples of these properties include solubility, partition coefficients, rate constants, and biological activities, such as equilibrium binding constants, effective doses, and toxicities, as well as other topics of interest in medicinal chemistry. 

In an ion-solvent interaction model (Fig. P2.1), solvated coordinated water has two sites capable of forming hydrogen bonds with water molecules in the SB region. Are these two sites identical in bonding For nonsolvated coordinated water, there are three sites for hydrogen bonds. Are these three identical Why (Xu)... 

Two models have been proposed to describe the process of retention in liquid chromatography (Figure 3.3), the solvent-interaction model (Scott and Kucera, 1979) and the solvent-competition model (Snyder, 1968 and 1983). Both these models assume the existence of a monolayer or multiple layers of strong mobile-phase molecules adsorbed onto the surface of the stationary phase. In the solvent-partition model the analyte is partitioned between the mobile phase and the layer of solvent adsorbed onto the stationary-phase surface. In the solvent-competition model, the analyte competes with the strong mobile-phase molecules for active sites on the stationary phase. The two models are essentially equivalent because both assume that interactions between the analyte and the stationary phase remain constant and that retention is determined by the composition of the mobile phase. Furthermore, elutropic series, which rank solvents and mobile-phase modifiers according to their affinities for stationary phases (e.g. Table 3.1), have been developed on the basis of experimental observations, which cannot distinguish the two models of retention. 

Both the solvent-interaction model (Scott and Kucera, 1979) and the solvent-competition model (Snyder, 1968, 1983) have been used to describe the effects of mobile-phase composition on retention in normal-phase liquid chromatography. The solvent interaction model on the one hand provides a convenient mathematical model for describing the relationship between retention and mobile phase composition. The solvent competition model on the other hand provides a more complete, quantitative description of the relative strengths of adsorbents and organic solvents used in normal-phase chromatography. 

Solvent interaction model for normal-phase liquid chromatography. The solvent-interaction model of Scott and co-workers (Scott and Kucera, 1979) assumes that the analyte partitions between the bulk mobile phase and a layer of solvent absorbed onto the stationary phase. The quantitative description of the relationship between retention and the composition of the mobile phase in the solvent-interaction model requires the definition of the void volume corrected retention volume (V), which is related to the retention volume (F ) and the void volume (Fq) by... 

Solvent competition model for normal-phase liquid chromatography. Like the solvent-interaction model, the solvent-competition model assumes that the stationary phase is covered with a monolayer of molecules of the strongest component of the mobile phase. This model also assumes that the concentration of analyte in the stationary phase is small compared with the concentration of solvent molecules and that solute-solvent interactions in the mobile phase are cancelled by identical interactions in the stationary phase. The competition between the analyte molecules, x, and the mobile phase molecules. A, for the active site or sites on the stationary phase is given by... 

Fundamental principles governing the use of solvents such as chermcal stractine, molecular design, basic physical and chemical properties, as well as classification of inter-molecular solute/solvent interactions, modeling of solvent effects, and solvent influence... 

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