Solute retention adsorption mechanism

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 ... 

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). 

With binary and ternary supercritical mixtures as chromatographic mobile phases, solute retention mechanisms are unclear. Polar modifiers produce a nonlinear relationship between the log of solute partition ratios (k ) and the percentage of modifier in the mobile phase. The only form of liquid chromatography (LC) that produces non-linear retention is liquid-solid adsorption chromatography (LSC) where the retention of solutes follows the adsorption isotherm of the polar modifier (6). Recent measurements confirm that extensive adsorption of both carbon dioxide (7,8) and methanol (8,9) occurs from supercritical methanol/carbon dioxide mixtures. Although extensive adsorption of mobile phase components clearly occurs, a classic adsorption mechanism does not appear to describe chromatographic behavior of polar solutes in packed column SFC. 

Conventional filters, such as a coffee filter, termed depth filters , consist of a network of fibers and retain solute molecules through a stochastic adsorption mechanism. In contrast, most membranes for the retention of biocatalysts feature holes or pores with a comparatively narrow pore size distribution and separate exclusively on the basis of size or shape of the solute such membranes are termed membrane filters . Only membrane filters are approved by the FDA for sterilization in connection with processes applied to pharmaceuticals. Table 5.3 lists advantages and disadvantages of depth and membrane filters. 

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... 

The exact mechanism(s) of solute retention in reversed-phase high-performance liquid chromatography (RPLC) is not presently well understood. The lack of a clear understanding of the mechanics of solute retention has led to a myriad of proposals, including the following partition (K21, L6, S16) adsorption (C9, CIO, H3, H15, H16, K13, L3, T2, U2) dispersive interaction (K2) solubility in the mobile phase (L7) solvophobic effects (H26, K6, M5) combined solvophobic and silanophilic interaction (B9, M12, Nl) and a mechanism based upon compulsary absorption (B5). 

Bonded stationary phases for NPC are becoming increasingly popular in recent years owing to their virtues of faster column equilibration and being less prone to contamination by water. The use of iso-hydric (same water concentration) solvents is not needed to obtain reproducible results. However, predicting solute retention on bonded stationary phases is more difficult than when silica is used. This is largely because of the complexity of associations possible between solvent molecules and the chemically and physically heterogeneous bonded phase surface. Several models of retention on bonded phases have been advocated, but their validity, particularly when mixed solvent systems are used as mobile phase, can be questioned. The most commonly accepted retention mechanism is Snyder s model, which assumes the competitive adsorption between solutes and solvent molecules on active sites... 

In normal-phase chromatography, polar stationary phases are employed and solutes become less retained as the polarity of the mobile-phase system increases. Retention in normal-phase chromatography is predominately based upon an adsorption mechanism. Planar surface interactions determine successful use of NPC in separation of isomers. The nonaqueous mobile-phase system used in NPC has found numerous applications for extremely hydrophobic molecules, analytes prone to hydrolysis, carbohydrates, and sat-urated/unsaturated compounds. In the future, with the advent of new stationary phases being developed, one should expect to see increasingly more interesting applications in the pharmaceutical industry. 

A() and m are experimental constants, Ao being the retention factor in pure strong solvent. Eq. (1.15) can be derived also on the basis of molecular statistical-mechanical theory of adsorption chromatography [,131. Eq. (1.15) applies in systems where the solute retention is very high in the pure non-polar solvent. If this is not the case, another retention equation was derived from the original Snyder model [34,351 ... 

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... 

There is considerable discussion in the literature regarding the adsorption mechanism of ions from aqueous solutions onto RPLC stationary phases [87-90]. It has been shown that, under certain conditions, organic ions are adsorbed as ion pairs [87,89,91], and that, under other conditions, they may be adsorbed as separate ions. In this case, the model derived by StMUberg [92] may be useful. In his theory of the retention mechanism in ion-pair chromatography, StMilberg focused on the derivation of the isotherm of the amphiphilic compoimd, that is, the counter-ion used in this technique to adjust the retention factors of the sample components and their separation factors e.g., the cation tetrabutulammonium). The counter-ion (Br, Cl , H2PO4 ) may not be strongly associated with the cation in a mobile phase that is a mere aqueous buffer. Other cations, rmder other experimental conditions may adsorb as true ion pairs, in which case the isotherm behavior is quite different. 

Polymer retention includes adsorption, mechanical trapping, and hydrodynamic retention. These different mechanisms were discussed by Willhite and Dominguez (1977). Mechanical entrapment and hydrodynamic retention are related and occur only in flow-through porous media. They play no part in free powder/ bulk solution experiments. Retention by mechanical entrapment is viewed as occurring when larger polymer molecules become lodged in narrow flow channels (Willhite and Dominguez, 1977). 

However, in normal phase adsorption systems (or liquid-solid chromatography) the interaction of the mobile phase solvent with the solute is often less Important than the competing Interactions of the mobile phase solvent and the solute with the stationary phase adsorption sites. Solute retention is based upon a displacement mechanism. Multicomponent mobile phases and their combination to optimize separations in liquid-solid chromatography have been studied in detail (31-35). Here, solvents are classified as to their interaction with the adsorption surface (Reference 32, in particular) ... 

There are certain conditions that must be fulfilled if Eqs. (2.2), (2.3) and (2.4) are to be used to calculate partition coefficients. The basic assumption is that the individual retention mechanisms are independent and additive. This will be true for conditions where the infinite dilution and zero surface coverage approximations apply or, alternatively, at a constant concentration with respect to the ratio of sample size to amount of liquid phase. The infinite dilution and zero surface coverage approximations will apply to small samples where the linearity of the various adsorption and partition isotherms is unperturbed and solute-solute interactions are negligible. The constancy of the solute retention volume with variation of the sample size for low sample amounts and the propagation of symmetrical peaks is a reasonable indication that the above conditions have been met. For asymmetric peaks, however, the constant concentration method must be employed if reliable gas-liquid partition coefficients are to be obtained [191]. It is difficult to state absolutely the conditions for which contributions to retention from the structured liquid phase layer can be neglected. This will occur for some minimum phase loading that depends on the support surface area, the liquid phase... 

The adsorption and partition mechanisms of solute retention are the two most universal mechanisms of... 

It is widely recognized that during polymer solution flow in porous media, a portion of the polymer is retained [1-11]. It is evident from the cited papers that both the physical adsorption of polymer onto solid surfaces and the polymer retention by mechanical entrapment play a role in the total polymer retention. The question which of these mechanisms of polymer retention is more important has remained unanswered. 

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