Surface solvophobic

In section 8.2 we described the solvophobic effect, which theory leads us to expect is related to the solvent surface tension. Abraham et al. have developed a different measure of solvophobicity by relating the transfer free energy W)... 

Retention in HIC can be described in terms of the solvophobic theory, in which the change in free energy on protein binding to the stationary phase with the salt concentration in the mobile phase is determined mainly by the contact surface area between the protein and stationary phase and the nature of the salt as measured by its propensity to increase the surface tension of aqueous solutions [331,333-338]. In simple terms the solvopbobic theory predicts that the log u ithn of the capacity factor should be linearly dependent on the surface tension of the mobile phase, which in turn, is a llne2u function of the salt concentration. At sufficiently high salt concentration the electrostatic contribution to retention can be considered constant, and in the absence of specific salt-protein interactions, log k should depend linearly on salt concentration as described by equation (4.21)... 

Separations in hydrophobic interaction chromatography have been modeled as a function of the ionic strength of the buffer and of the hydrophobicity of the column, and tested using the elution of lysozyme and ovalbumin from octyl-, butyl- and phenyl-Sepharose phases.2 The theoretical framework used preferential interaction analysis, a theory competitive to solvophobic theory. Solvophobic theory views protein-surface interaction as a two-step process. In this model, the protein appears in a cavity in the water formed above the adsorption site and then adsorbs to the phase, with the free energy change... 

Entropy-related adsorption, denoted hydrophobic sorption (or solvophobic interaction) is the partitioning of nonpolar organics out of the polar aqueous phase onto hydrophobic surfaces. Fig. 5.6 shows a schematic model of forces that contribute to the sorption of hydrophobic organics, relevant to the subsurface environment. 

The solvophobic theory has been successfully applied to treat the effect of solute ionizatiqn as well as the effect of salts on the retention of both neutral and ionized species. There is ample experimental evidence that retention of a spfeies decreases upon ionization according to the theoretical prediction. Addition of salts to aqueous eluents increases surface tension and consequently. the retention of neutral eluites on nonpolar stationary phases. With ioni d solutes, however, the solvophobic theory predicts a minimum at low ionic strength in plots of retention versus ionic strength and this phenomenon has also been experimentally demonstrated. 

Factors that influence the retentive powers and selectivity of such bonded phases include the surface concentrations of hydrodartenaceous ligates and free silanol groups. The thermodynamic aspectitm solute interactions with the hydrocarbonaceous ligates at the surface, which are hydrophobic interactions in the case of aqueous eluents, are discussed later in this chapter within the framework of the solvophobic theory. In practice, however, solute interactions with surface silanol which may be termed silanophilic interactions can also contribute ]to retention (71, 75, 93), particularly in the case of amino compounds. Consequently the retention mechanism may be different from that which would be ol served with an ideal nonpolar phase. Therefore, increasing attention is paid to the estimation of the concentration of accessible sianols and to their elimination from the surface of bonded phases. 

If the retention enthalpies of the two sites differ, curvature may be observed in the plots. Moreover, if the enthalpies are opposite in sign, a minimum will occur in the van t Hoff plot at a temperature where the ratio of the retention foctors for the two mechanisms equals the absolute value of the reciprocal of the ratio of the corresponding enthalpies. Most frequently, however, less dramatic curvature would be expected. Such behavior may be anticipated in the RPC of amines with- arge nonpolar moieties which could be retained by silmiophilic interactions with surface silanols and by solvophobic interactions with nonpolar ligates of a reversed phase with low surface coverage. Recently an lihalysis of this behavior has been reported 93). 

The solvophobic theory could be extended to the treatment of other special effects such as hydrogen bonding between eluite and species. present in the eluent. The predictive power of the theory may be improved by such extension. In addition a rigorous theory for treatment eluite interaction with surface silanols would be needed. 

All of these models predict that the hydrophobic effect provides a significant driving force for the exclusion of even highly polar, charged peptides from an aqueous environment to the nonpolar environment of the RPC sorbent. According to the solvophobic model, in order to place a peptide into a mobile phase, a cavity of the same molecular dimensions must first be created. The energy required to create this cavity is related to the cohesive energy density or the surface tension of the mobile phase. Conceptually, each solvent-accessible unit... 

Changes in the magnitude of In k with solvent concentration in peptide-ligand interactions in RPC involving hydrophobic effects can also be related in solvophobic terms to the chemical potential, Ap°, for the process, or to the surface tension, y, of the mobile phase through the following expressions ... 

Two main theories, the so-called solvophobic and partitioning theories, have been developed to explain the separation mechanism on chemically bonded, non-polar phases, as illustrated in Figure 2.4. In the solvophobic theory the stationary phase is thought to behave more like a solid than a liquid, and retention is considered to be related primarily to hydrophobic interactions between the solutes and the mobile phase14-16 (solvophobic effects). Because of the solvophobic effects, the solute binds to the surface of the stationary phase, thereby reducing the surface area of analyte exposed to the mobile phase. Adsorption increases as the surface tension of the mobile phase increases.17 Hence, solutes are retained more as a result... 

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