Hydrocarbonaceous ligates

The thermodynamics of solute interaction with nonpolar ligates of the stationary phase will be treated later in this chapter within the framework of the solvophobic theory 107-108). According to this theoretical approach the equilibrium constant for the reversible binding of a given eluite to the hydrocarbonaceous ligates at fixed eluent properties and temperature can be approximated by the relationship... 

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. 

In the case of RP-HPLC and HP-HIC of polypeptides and proteins, three situations can be contemplated29,30,237,258 for the interaction of a polypeptide or protein with an hydrocarbonaceous ligate, whereby the change in heat capacity A C, - of the system (i) is zero and remains invariant with regard to temperature (the isothermic scenario), (ii) is not zero and is linearly... 

Evaluations of the interaction of P, with the hydrocarbonaceous ligates in terms of Eqs. (74)—(81), however, do not represent complete descriptions of the binding process, because the condition(s) under which the thermal energy of the system is increased has not been specified. If the heat capacity of the system is increased under experimental conditions where the pressure P does not change significantly, then Eq. (80) can be simplified and the specific heat capacity can be redefined as follows ... 

Horvath et a . (B9, NI) point out that, in addition to the hydrocarbonaceous ligates, silanol groups at the alkyl-silica stationary phase surface are accessible to solute molecules. The eluite can therefore independently bind in two different ways to the surface of a given stationary... 

In the case of the RPC and HIC of peptides and proteins, three situations have been documented [16,20,146,171,342] for the interaction of these biomolecules with hydrocarbonaceous ligates, whereby the change in heat capacity, AC", of the system is (1) zero and remains invariant with regard to T (the isothermic scenario), (2) not zero but is linearly dependent on T (the homo-thermic scenario) or (3) not zero and shows a nonlinear dependence on T (the heterothermic scenario), In the bulk state, the heat capacity, C°, of a substance is usually defined as the quantity of heat necessary to raise the temperature of a unit mass of the substance by one kelvin. In the case of RPC or HIC, the change in heat capacity, AC ], for a peptide (or protein) interaction with a nonpolar ligate system as a function of T can be expressed according to the KirchhofF relationships [343,344] as... 

As discussed by Minors in the first volume of this series (57), modern liipiid chromnlography employs bonded stationary phases not only with hydrocarbonaceous, but also with polar ligates, me bonded phases of intermediate polarity (7/5) can be used with either polar or nonpolar mobile phases. With a sufficiently polar eluent the technique falls into the category of RPC as this chromatographic method by defiiiition employs a mobile phase more polar than the stationary phase so that retention increases with decreasing polarity of eluites having similar molecular dimensions. 

The two obvious ways for classification are based on the iilica gel support and the chemical nature of the hydrocarbonaceous liga. The names of commercial stationary phases usually refiect such a d tinction and contain the designations of both the silica and the ligate, e.. Spherisorb ODS HtuI I.K hrosiirb RP8, where Spherisorb and LK hroj rb are trade... 

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