Silanophilicity

Column Packing Hydrophobicity Index(a) Silanophilic Index(b) ... 

The silanophilic character of 16 reversed-phase high-performance liquid chromatographic columns was evaluated with dimethyl diphenycyclam, a cyclic tetraza macrocycle [101]. The method is rapid, does not require the removal of packing material, and uses a water-miscible solvent. The results demonstrate two points first, cyclic tetraza macrocycles offer substantial benefits over currently used silanophilic agents second, the method can easily differentiate the performance of various columns in terms of their relative hydrophobic and silanophilic contributions to absolute retention. 

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. 

One of the earlier tests [58] contains as hydrophobic samples toluene and ethyl benzene, as a weakly acidic component phenol, and weakly basic analytes like aniline and the isomeric toluid-ines. Chemometric analysis showed the proper selection of the analytes for characterization, with the surprising result that /V,/V-dimethyl aniline is not a signihcant analyte in characterization for silanophilic interactions [59]. As the mobile phase, a mixture of 49 Vol.% methanol with 51 Vol.% water has been used. In the beginning, an unbuffered mobile phase has been used because silanophilic interactions can be blocked by buffer constituents. For better reproducibility and transfer-ability, a 10 mM phosphate buffer of pH 7 is recommended. The comparison of RP columns for hydrophobic interaction by this test procedure is shown in Figure 2.8. The k value of toluene... 

FIGURE 2.20 Characterization of RP columns for polar and silanophilic activity. Silanophilic activity asymmetry of p-ethyl aniline. Conditions as in Figure 2.8. Polar activity relative retention of hutylparabene and dipropylphthalate. Conditions as in Figure 2.19. 

With some stationary phases at low pH values (<4) benzyl amine as benzyl ammonium ion can be excluded by a Donnan potential from the pores, when positive charges are present at the surface. These could have stemmed from the manufacturing process or could have been introduced on purpose to shield amines from interacting with silanols. With an increasing pH, the Donnan exclusion decreases and at pH >5 benzyl amine is retarded increasingly. An example of this effect with a modern RP with low silanophilic properties is demonstrated in Figure 2.22, where the elution peaks of benzyl amine are presented as a function of pH. With these stationary phases, basic analytes cannot be separated at low pH values. 

In comparing the various test procedures, there is always a good agreement found for hydrophobic retention and selectivity as well as for shape selectivity. However, the characterization of silanophilic interaction is still a matter of discussion. In part, the differences are due to the selection of the basic analyte. Therefore, the outcome of every test is different. It has been shown, that the peak asymmetry—used for detection of silanophilic interactions—does not correlate to the pA" value of the basic test solute [64]. A closer look at these data leads to the assumption, that the differences are related to the structure of the basic solute, irrespective of whether a primary, secondary, or a tertiary amine is used. The presence of NH bonds seems to be more important in stationary-phase differentiation than the basicity expressed by the pA value. For comparable test procedures for silanophilic interactions further studies seem to be required. 

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). 

Fig. 9 Hexagon visualizing the desired properties of a C18-bonded silica column efficiency (1), symmetry (2), capacity (3), hydrophobicity (4), steric selectivity (5), metal shielding (5), and silanophilic properties (6). (Reproduced with permission from Merck, Darmstadt, Germany.)...

Toluene, an apolar solute, seems to have a higher affinity for the CTAB covered phases than for the SDS covered phases. If we consider the two processes of surfactant adsorption (Figure 1), the silanophilic process is important in CTAB adsorption given the great affinity of quaternary ammonium for surface-sllanols (11). As a result, the stationary phase becomes more hydrophobic with CTAB than with SDS, which could explain the magnitude of the Kg values for toluene. Caffeine is a polar solute, its Kg values on the more hydrophobic stationary phases (MOS and ODS Hypersil) were weak and much lower than the respective toluene-Kg values. The caffeine Kg values were greater on SDS covered stationary phases than on CTAB covered ones. The explanation in the case of toluene holds true, that is to say caffeine has more affinity for the more polar SDS covered stationary phases (9.6 on C8, for example) than for the same phase but CTAB covered (1.8 on C8 with CTAB) (Table VI). 

Silica-Based Stationary Phases and Silanophilic Interactions... 

In liquid chromatography and electrophoretic methods, ILs are mostly used in diluted form in aqueous solutions. If its concentration is lowered to the millimolar range, an IL may be used as a mobile phase ionic additive. Their breakthrough for use in RP-HPLC was due to their ability to suppress deleterious effects of silanophilic interactions that represent the main drawback of silica-based stationary phases they also exhibit many other favorable physical attributes [116]. 

The influence of the nature of the IL anion confirms the importance of ion-pairing for fine tuning retention. The lower the hydrophobicity and ion-pairing ability of the IL anions, the lower the retention. Ion pairing of the cationic analyte and the IL anion decreases silanophilic interactions and this in turn results in better peak shapes, and may eventually increase analyte retention. Actually, even if most research related the retention decrease upon IL addition in the mobile phase to decreased silanophilic interactions, it must be noted that positively charged analytes and IL cations also undergo repulsive electrostatic interactions. 

---