Solvophobic-silanophilic interactions

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 main reason for chromatographic retention on reversed phases is solvophobic interaction, but under certain conditions silanophilic interaction and additional effects contribute to, or even govern, the chromatographic process. A concise review of physicochemical phenomena contributing to retention in RPC has been recently given by Horvath54). 

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). 

Recently, Horvath and co-workers (B9, N1) introduced the concept of a dual binding mechanism to explain the atypical behavior of some solutes under reversed-phase conditions. In addition to solvophobic forces, it is possible for solutes to interact with the free surface silanols of the silica-based hydrocarbonaceous packing material. The term silanophilic interaction has been introduced to denote a reversible binding mechanism between solute molecules and silanol groups. 

On the other hand solvophobic theory attributes stationary phase effects to solute-ligand interactions in the gas phase and is unable to account for silanophilic interactions... 

In addition to solvophobic and silanophilic interactions other stationary phase effects, including steric recognition and pi-pi interactions between solutes and stationary phase, have been demonstrated. Thus, the specific nature of the stationary phase and solutes in-... 

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