Mobile phase solvation processes

It is interesting to note that the ionic mobilities lie in the order K > Na > Li, whilst the true ionic radii are in the inverse order (Bom, Zeit. f. Physik, I. 221, 1920). It is probable that the hydrated ions are always present in the surface but not necessarily in equal amounts, and that as the difference in concentration between surface and bulk phases increases this is accompanied by a simultaneous increase in the steepness of the concentration gradient from surface to bulk phase, a process which may be associated with the removal of water of solvation from around the ions. 

The method of linear solvation energy (LSER), based on the Kamlet-Taft multiparameter scale (10) has been successfully exploited to study retention in LC. The LSER approach, when applied to phase-transfer processes, correlates a general solute property (SP), such as logarithmic capacity factor, with parameters of the solute and both the mobile and stationary phases ... 

If the mobile phase contains a less polar solvent such as butanol, then one could expect interaction between the nonpolar reversed phase and nonpolar regions of the solvent molecules. Such a solvation process may be important in determining the conformation of the nonpolar bristles of a reversed phase in the presence of an aqueous mobile phase. Figure 10 shows an artist s visualization of the solvation processes and illustrates how the surface chemistry of the reversed phase can be significantly affected by solvent interactions. Such a concept is consistent with the data of Scott and Kucera (55), who demonstrated that a reversed phase can adsorb a monomolecular layer of either polar or nonpolar solvent molecules. The scheme is also consistent with the observation that many pro-... 

The analyte nature and its appearance (e.g., ionization state) in the mobile phase are also factors that affect the retention mechanism. Eluent pH influences the analyte ionization equilibrium. Eluent type, composition, and presence of counterions affect the analyte solvation. These equilibria are also secondary processes that influence the analyte retention and selectivity and are of primary concern in the development of the separation methods for most pharmaceutical compounds. 

Because different forms of analyte usually show different affinity to the stationary phase, secondary equilibria in HPLC column (ionization, solvation, etc.) can have a significant effect on the analyte retention and the peak symmetry. HPLC is a dynamic process, and the kinetics of the secondary equilibria may have an impact on apparent peak efficiency if its kinetics is comparable with the speed of the chromatographic analyte distribution process (kinetics of primary equilibria). The effect of pH of the mobile phase can drive the analyte equilibrium to either extreme (neutral or ionized) for a specific analyte. Concentration and the type of organic modifier affect the overall mobile phase pH and also influence the ionization constants of all ionogenic species dissolved in the mobile phase. 

In reversed-phase HPLC with water/organic eluents, ionic interactions always play an important role in regard to analyte retention, solvation, ionic equilibria, and other processes. To some extent, chromatographic effects and practical use of ionic interactions have been discussed in the previous sections of this chapter. In this section the influence of the ionic additives in the mobile phase on the retention of ionic or ionizable analytes will be discussed. 

As was shown above, the chaotropic effect is related to the influence of the counteranion of the acidic modifier on the analyte solvation and is independent on the mobile-phase pH, provided that complete protonation of the basic analyte is achieved. Analyte interaction with a counteranion causes a disruption of the analyte solvation shell, thus affecting its hydrophobicity. Increase of the analyte hydrophobicity results in a corresponding increase of retention. This process shows a saturation limit, when counteranion concentration is high enough to effectively disrupt the solvation of all analyte molecules. A further increase of counteranion concentration does not produce any noticeable effect on the analyte retention. 

Size-exclusion chromatography (SEC) differs from the other methods in that the separation is based on physical sieving processes and not on chemical phenomena. The stationary phase is chemically inert and there is selective diffusion of solute molecules into and out of the mobile phase-filled pores in a three-dimensional network which may be a gel or a porous inorganic solid. The degree of retention is dependant on the size of the solvated solute molecule relative to the size of the pore. Smaller molecules will permeate the smaller pores, intermediate-sized molecules will permeate some pores and... 

Perhaps the worst problem of gradient elution separations is the need to reequilibrate the column with the initial solvent before a second sample can be run. An often-quoted rule of thumb is that up to 20 column volumes of the initial solvent may be necessary for this reequilibration process. The best test of reequilibration is the elution time of a weakly retained solute. These solutes will be greatly affected by an incompletely equilibrated stationary phase, and the retention time will vary. Cole and Dorsey have described a simple and convenient method for the reduction of column reequilibration time following gradient elution reversed-phase chromatography (119). Their method utilizes the addition of a constant 3% 1-propanol to the mobile phase throughout the solvent gradient to provide consistent solvation of the stationary phase. They noted reductions in reequilibration times of up to 78% ... 

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... 

Retention in SFC is a complex function of temperature, pressure, density and solvent modifier concentration and a more complete understanding of these phenomena should directly benefit the development of SFC. The dynamic processes of stationary phase solvation and solute-solvent interactions in the stationary and mobile phases, respectively, impact on solute retention in SFC. The study of the retention process necessitates a multi-dimensional approach to understand the basic physicochemical processes underlying solute retention in SFC. The discussion in this chapter outlines three interrelated areas of study, probing specific areas of solute... 

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