Solvophobic theory hydrophobic interaction

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... 

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. 

The theoretical treatment of the hydrophobic effect is limited to pure aqueous systems. To describe chromatographic separations in RPC Horvath and Melander developed the solvophobic theory [47]. In this theory, no special assumptions are made about the properties of solute and solvent, and besides hydrophobic interaction electrostatic and other specific interactions are included. The theory has been valuable to describe the retention of nonpolar [48], polar [49], and ionizable [50] solutes in RPC. The modulation of selectivity via secondary equilibria (variation of pH, ion pair formation [51]) can also be described. On the other hand, it is not a problem to find examples of dispersive interactions in literature, e.g., separation of carotinoids with a long chain (C30) RP gives a higher selectivity compared to standard RP C18 cyclohexanols are preferentially retarded on cyclohexyl-bonded phases compared to phases with linear-bonded alkyl groups. 

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... 

Meliander, W. R., Corradini, D., and Horvath, C. (1984). Salt-medated retention of proteins in hydrophobic-interaction chromatography. Application of solvophobic theory. J. Chromatogr. 317, 67-85. 

This proposed mechanism for protein separations is supported by the recent theoretical studies of Horvath ef al. (29) and Horvath and Melander (28). In these studies, the hydrophobic effect in aqueous-organic systems (termed the solvophobic theory) was used to predict the retention of peptides on a nonpolar column. These authors found that the dominant interactions were between the mobile and stationary phases and between the mobile phase and the sample molecules. The driving force in both interactions was the shielding of a nonpolar region of either the column or sample molecule from the polar aqueous phase. 

HP-RPC separates compounds according to their relative nonpolarity or hydrophobicity. In RPC, the polarity of the stationary and mobile phase is to the reverse of that used in NPC. HP-RPC is performed on porous or nonporous stationary phases with immobilized nonpolar polymers (i.e., -alkylsilicas) or nonpolymer polymers (i.e., microparticulate polystyrenes). The most commonly accepted retention mechanism in RPC is based on the solvophobic theory, which describes the hydrophobic interaction between the nonpolar surface regions of the analytes and the nonpolar ligands/surfaces of the stationary phase.15 16... 

Figure 4,14. Diagram of the thermodynamic cycle used to explain retention in reversed-phase chromatography by solvophobic theory. Na = Avogadro number, AA = reduction of hydrophobic surface area due to the adsorption of the analyte onto the bonded ligand, y = surface tension, = energy correction parameter for the curvature of the cavity, V = molar volume, R = gas constant, T = temperature (K), Pq = atmospheric pressure, AGydw.s.i a complex function of the ionization potential and the Clausius-Moscotti functions of the solute and mobile phase. Subscripts i = ith component (solute or solvent), S = solute, L = bonded phase ligand, SL = solute-ligand complex, R = transfer of analyte from the mobile to the stationary phase (retention), CAV = cavity formation, VDW = van der Waals interactions, ES = electrostatic interactions.

Ben-Naim (1972b, c) has examined hydrophobic association using statistical mechanical theories of the liquid state, e.g. the Percus-Yevick equations. He has also examined quantitative aspects of solvophobic interactions between solutes using solubility data for ethane and methane. The changes in thermodynamic parameters can be calculated when two methane molecules approach to a separation of, 1-533 x 10-8 cm, the C—C distance in ethane, and the solvophobic quantities 8SI/i, s 2 and 8SiS2 can be calculated. In water (solvophobic = hydrophobic) 5si/i is more negative than in other solvents and decreases as the temperature rises both 8s iH%... 

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