Solute-stationary phase interactions

The second type of interaction, displacement interaction, is depicted in Figure 10. This type of interaction occurs when a strongly polar solute, such as an alcohol, can interact directly with the strongly polar silanol group and displaces the adsorbed solvent layer. Depending on the strength of the interaction between the solute molecules and the silica gel, it may displace the more weakly adsorbed solvent and interact directly with the silica gel but interact with the other solvent layer by sorption. Alternatively, if solute-stationary phase interactions are sufficiently strong, then the solute may displace both solvents and interact directly with the stationary phase surface. 

Scott and Kucera [4] carried out some experiments that were designed to confirm that the two types of solute/stationary phase interaction, sorption and displacement, did, in fact, occur in chromatographic systems. They dispersed about 10 g of silica gel in a solvent mixture made up of 0.35 %w/v of ethyl acetate in n-heptane. It is seen from the adsorption isotherms shown in Figure 8 that at an ethyl acetate concentration of 0.35%w/v more than 95% of the first layer of ethyl acetate has been formed on the silica gel. In addition, at this solvent composition, very little of the second layer was formed. Consequently, this concentration was chosen to ensure that if significant amounts of ethyl acetate were displaced by the solute, it would be derived from the first layer on the silica and not the less strongly held second layer. 

Apart from enabling rapid prediction of solute retention, the Soczewinski equation allows a moleeular-level scrutiny of the solute — stationary phase interactions. The numeiieal value of the parameter n from Equation 2.14, which is at least approximately equal to unity (n 1), gives evidence of the one-point attachment of the solute moleeule to the stationary phase surface. The numeiieal values of n higher than unity prove that in a given chromatographic system, solute molecules interact with the stationary phase in more than one point (the so-ealled multipoint attachment). 

In order to avoid tedious procedures required to prepare packed CEC columns, some groups are studying the use of empty capillaries. Since solute-stationary phase interactions are key to the CEC process, appropriate moieties must be bound to the capillary wall. However, the wall surface available for reaction is severely limited. For example, a 100 pm i.d. capillary only has a surface area of 3xl0 4m2 per meter of length, with a density of functional sites of approximately 3.1 xlO18 sites/m2, which equals 0.5 pmol sites/m2. Moreover, surface modification cannot involve all of the accessible silanol groups, since some must remain to support the EOF. As a result, the use of bare capillaries in CEC has been less successful. 

Such precise control of porous properties is expected to be very useful in the design of specialized CEC columns for separation in modes other than reversed-phase. For example, size exclusion chromatography (SEC) is an isocratic separation method that relies on differences in the hydrodynamic volumes of the analytes. Because all solute-stationary phase interactions must be avoided in SEC, solvents such as pure tetrahydrofuran are often used as the mobile phase for the analysis of synthetic polymers, since they dissolve a wide range of structures and minimize interactions with the chromatographic medium. Despite the reported use of entirely non-aqueous eluents in both electrophoresis and CEC [65], no appreciable flow through the methacrylate-based monoliths was observed using pure tetrahydrofuran as the mobile phase. However, a mixture of 2% water and tetrahydrofuran was found to substan-... 

Separation of solutes injected into the system arises from differential retention of the solutes by the stationary phase. The net retention of a particular solute depends upon all the solute-solute, solute-mobile phase, solute-stationary phase and stationary phase-mobile phase interactions that contribute to retention. The t3q3es of solute-stationary phase interactions involved in chromatographic retention include hydrogen bonding, van der Waal s forces, electrostatic forces or hydrophobic forces. 

Fig. 1 Solute-micelle and solute-stationary phase interactions in hybrid micellar mobile phases (see text for meaning of equilibrium constants).

XVII. The Characterisation of 24 Gas-Liquid Chromatographic Stationary Phases Studied by Poole and Co-Workers, Including Molten Salts, and Evaluation of Solute-Stationary Phase Interactions. J.Chromat., 587, 229-236. 

Anti-Langmuir type isotherms are more common in partition systems where solute-stationary phase interactions are relatively weak compared with solute-solute interactions or where column overload occurs as a result of large sample sizes. In this case, analyte molecules already sorbed to the stationary phase facilitate sorption of additional analyte. Thus, at increasing analyte concentration the distribution constant for the sorption of the analyte by the stationary phase increases due to increased sorption of analyte molecules by those analyte molecules already sorbed by the stationary phase. The resulting peak has a diffuse front and a sharp tail, and is described as a fronting peak. 

Separations are possible in gas chromatography if the solutes differ in their vapor pressure and/or intensity of solute-stationary phase interactions. As a minimum requirement the sample, or some convenient derivative of it, must be thermally stable at the temperature required for vaporization. The fundamental limit for sample suitability is established by the thermal stability of the sample and system suitability by the thermal stability of column materials. In contemporary practice an upper temperature limit of about 400°C and a sample molecular weight less than 1000 is indicated, although higher temperatures have been used and higher molecular weight samples have been separated in a few instances. 

M.J. Medina Hem dez and M.C. Garda Alvarez-Coque, Solute-Mobile Phase and Solute-Stationary Phase Interactions inMLC, Analyst, 117 831 (1992). 

II. I. Solute-Micelle and Solute-Stationary Phase Interactions... 

The ability of micelles to solubilize and to interact selectively with solute molecules is believed to be the basis of separation in MLC. However, surfactant molecules readily adsorb on bonded stationary phases, such as C18 or C8, and the modifications produced can have profound implications with regard to retention and selectivity. Solute-stationary phase interactions in MLC are thus very important. Perhaps some of the reported differences in selectivity, between MLC and RPLC with aqueous-organic mobile phases, are due in some measure to the modification of the stationary phase by adsorbed surfactant. 

Although SDS adsorption enhances the selectivity of the stationary phase toward the vanillin compounds, SDS micelle-solute interactions also contribute to the selectivity of this separation. For example, SDS micelles interact more strongly with vanillin than with isovanillin, as evidenced by the greater K m binding constant for vanillin, and this interaction is responsible, at least in part, for the baseline resolution of these two compounds. Nevertheless, the successful separation of the vanillin compounds with the 0.02 M SDS mobile phase is primarily due to solute-stationary phase interactions, which is also the reason why the separation of the vanillin test mixture is more favorable at lower SDS concentrations (see Fig. 7.7). 

The first uses of micellar phases by Armstrong were done in GPC and thin layer chromatography (TLC). This was described in Chapter 3. TLCwas a useful tool for the determination of solute partition data in micellar systems. The micellar partition coefficient, Pv, the solute-stationary phase interaction coefficient, P s, and the micellar binding constant, Kd, could be obtained from the solute-Rf parameters with an equation very similar to eq. 13.5 [22]. A number of solutes were separated by micellar TLC such as phenols and dyes [22], indicators, caffeine, biphenyl, naphthol and benzamide [23], PAHs and amino acids [24], vitamins [25] or fluorescein derivatives [26]. The low cost, low toxicity, peculiar selectivity and ease of... 

The most difficult factor to assess is the ability of a phase to effect the desired separation. From this perspective, the selection of a stationary phase and column is a daunting prospect. In theory, the selection is based upon maximizing the difference in selectivity between the solutes toward the phase. The separation is increased by exploiting solute-stationary phase interactions that retard the progress of some solutes relative to others so as to increase their retentions. The types of interactions to consider are ... 

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