Interaction model, retention

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. 

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). 

V) Solute-solvent interactions in the mobile phase are solely responsible for solute retention as in practice the surface of the a orbent is assumed to be completely covered by a modulator. This smvent interaction model was introduced by Scott and Kucera (79-5/)<... 

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 ... 

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-... 

The first attempts of Bidlingmeyer and co-workers [15,16] to formulate an ion interaction model quantitatively [21-23] did not provide a rigorous description of the system. Stranahan and Deming [22] accounted for electrostatic effects via a simplified activity coefficient in the stationary phase. An interfadal tension decrease with increasing IPR concentration was considered responsible for the appearance of maxima in the plot of retention factor, k versus IPR concentration, but experimental results were at odds with known surfactant chemistry. 

In a stndy of retention of aromatic carboxylic acids under IPC conditions, linear free energy relationships were observed between the capacity factors and the extraction eqnilibrinm constants of benzoic acid and naphthalene carboxylic acid. The capacity factor of benzene polycarboxylic acids was directly related to then-association constants and qnatemary ammonium ions calculated on the basis of an electrostatic interaction model [27,28],... 

The present model has so f2ir assumed that interactions between molecules of solvent or solute in either phase can be ignored. Now we will examine the effects of these interactions on retention in various LSC systems. Equation (4) for the retention of a solute X in a mobile phase M recognizes intermolecular interactions in the mobile phase (n), but assumes that adsorbed-phase free energies ( ia) are not a function of intermolecular interactions within the adsorbed phase. We can recognize these adsorbed-phase intermolecular interactions by adding an energy term Eja" to Eq. (4) for each adsorbed species i ... 

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... 

Finally, in 1979, Bidlingmeyer et al. [13,14] introduced a third model which they termed the ion interaction model. It is based on conductivity measurements, the results of which rule out the formation of ion pairs in the mobile phase. This retention model, also used by Pohl [15] to interpret the retention mechanism on a MPIC phase, neither presupposes the formation of ion pairs nor is it based on classical ion-exchange chromatography. 

Theoretically, a linear correlation between S and log kw is expected for any system where a single solute interaction dominates retention. Members of a homologous series, for example, have similar polarity and differ only in size and should conform to the linear model. For compounds that differ significantly in the type and capacity for polar interactions only poor correlations are generally observed. 

The partition and displacement model considers retention to result from a two step process. The first involves formation of a mixed stationary phase by intercalation of solvent molecules from the mobile phase. The composition of the solvents in the stationary phase is established according to thermodynamic equilibrium and is usually different to the bulk mobile phase composition. Competitive sorption of solvents is modeled as a displacement process and is complete before the solute is introduced into the two-phase system. Solute retention is then modeled as a partition process between the solvent modified stationary phase and the mobile phase by taking into account all solute-solvent interactions in both phases. The phenomenological model of solvent effects attempts to model retention as a combination of solute-solvent interactions (the solvation effect) and solvent-solvent interactions (the general medium... 

The manipulation facilities of a modeling environment comprise the collection of resources that expedite user interaction with the model. Here model entry, model modification, and model retention and retrieval are examples of what we have in mind. In addition, provision of support to compare the utility and viability of alternate structural organizations of a model is needed. From the user interface perspective this aspect of modeling software development is extremely challenging on the one hand, easy and efficient access to model manipulation needs to be provided, and, on the other hand, a layer of protection is required to ensure that each modeling step the user takes is not in conflict with either the software rules or past modeling steps. 

In certain cases, a values of 30 or more have been found, which then correspond to A(AG) values in the range of 2 kcal/mol (8.4 kJ/mol). Generally, such values are obtained owing to very low retention of the first enantiomer eluted. This means that a very enantioselective sorption process is operating in the column, i.e., one of the enantiomers is virtually unbound by the CSP for steric reasons. Such phenomena are not easily explained by the three-point interaction model, but rather indicate the operation of a sort of chiral steric exclusion mechanism, more in line with a steric fit concept involving only one binding interaction. ... 

To better distinguish the contributions of polar interactions to retention, the LEER model was transformed into the so-called hydrophobic subtraction model (HSM) for RPLC, where the hydrophobic contribution to retention is compensated for by relating the solute retention to a standard nonpolar reference compound. This approach was applied to characterize more than 300 stationary phases for RPLC, including silica gel supports with bonded alkyl-, cyanopropyl-, phenylalkyl-, and fluoro-substituted stationary phases and columns with embedded or end-capping polar groups. The QSRR models can be used to characterize and compare the suitabihty of columns not only for reversed-phase, but also for NP and HILIC systems. 

Fig. 1 Schematic of (A) the ion-pair model (B) the dynamic ion-exchange model and (C) the ion-interaction model for the retention of anionic solutes in the presence of a lipophilic cationic HR. The solute and the HR are labeled on the diagram. The large hatched box represents the lipophilic stationary phase, the black circle with the negative charge represents the counter-anion of the HR, whilst the white circle with the positive charge represents the counter cation of the solute.

Predictions of solvent elution strength e° and the retention parameter R/made with the help of Eqs. 54-56 cannot be regarded as error-free. The observed differences between the experimental and calculated e° and Revalues are in the first instance due to the simplicity of the assumed intermolecular interactions model in systems composed of solute, solvent, and mobile phase (see Eqs. 46, 46a, and 47). In fact, the model discussed fully ignores self-association of solute and solvent, as well as mixed intermolecular interactions simultaneously engaging the solute and the mobile phase. For the aforementioned reason the most successful optimization of the mobile phase can be attained for these solutes and solvents that are practically unable to interact intermolecularly (such as hydrocarbons). Still, the importance of Snyder s approach is undeniable as an easy-to-apply strategy for multi-component mobile-phase optimization. 

It is important to realize that many important processes, such as retention times in a given chromatographic column, are not just a simple aspect of a molecule. These are actually statistical averages of all possible interactions of that molecule and another. These sorts of processes can only be modeled on a molecular level by obtaining many results and then using a statistical distribution of those results. In some cases, group additivities or QSPR methods may be substituted. 

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