Silanophilicity/hydrophobicity

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... 

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. 

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). 

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. 

To deconvolve the silanophilic effect from the electrostatic repulsion, a nonsilica-based stationary phase may be suitable in research work. On a polystyrene-divinylbenzene reversed phase column, an ethylammonium formate RTIL was not able to produce effective ion-pairing interactions with acidic and basic model compounds, and baseline resolution was only obtained in the presence of classical IPRs (tetrabutylammonium and dodecylsulfate ions, respectively). However, the RTIL was able to mimic the methanol role [123,126]. In summary, IL cations reduce positively charged analyte retention since they (1) screen free silanols and (2) electrify the stationary phase with a positive surface charge that is repulsive for cationic analytes. The hydrophobic character of IL anions is responsible for possible analyte retention increases via ion-pairing. 

If NaCl is replaced by the butyl-3-methyl imidazolium (BMIM) chloride IL, a 30% decrease in retention factor associated with a remarkable peak shape improvement is observed. In this case, the IL cation adsorbs on the C18 stationary phase more than Cl", thereby preventing detrimental attractive silanophilic interaction of the cationic additive. Charge-charge repulsion occurs, the retention factor is lower, and the peak shape is better. The analyte cation is largely retained by hydrophobic fast interactions. When BMIM BF4 IL replaces NaCl, both the cation and anion of the IL adsorb on the C18 surface and all the interactions cited above take place simultaneously and contradict each other. Global retention depends on the extent to which one interaction is stronger than the other [124],... 

Ionic liquids (ILs), introduced to suppress deleterious effects of silanophilic interactions, soon proved to be extraordinary IPRs (see Chapter 7.4 for details). An ILs is an equimolar mixture of cationic and anionic hydrophobic ions, each one able to adsorb onto the stationary phase. The synergistic contributions of both cationic and anionic components generates the unique properties of ILs. 

In RP-TLC, silica gel plates impregnated with a strong hydrophobic agent (paraffin oil or silicone oil, usually 5%) have been extensively used in the past as nonpolar stationary phases. Nowadays, plates covered with octa-decyl-silanized (ODS) silica gel are available. In this material, the silanol groups are etherified with alkyls containing 8 (Cg) or 18 (Cig) carbon atoms. The low wettability of HPTLC plates coated with highly etherified silica gel poses limitations in the water content of the mobile phase. This problem is circumvented by the use of RP-Cig plates with 50% etherification. However, the presence of free silanol groups may lead to undesirable silanophilic interactions, especially with low water content in the mobile phase. 

Residual silanol activity can have a beneficial effect on the separation of neutral polar compounds by supplementing hydrophobic interactions. It should not be constmed that columns with high silanophilicity are undesirable for all chromatographic separations, just mainly for the separation of strong bases. They are the preferred choice for some separations. 

Maximum hydrolytic stability of silica-based reversed-phase packings is obtained at pH values around 4. At pH 2 and pH 8 one may encounter a faster aging of the column, visible through decreased hydrophobic interaction and increased silanophilic interaction. However, the actual rate depends on many different parameters, including the choice of buffer, temperature, and the organic modifier. Columns tend to be mote stable in acetonitrile-based mobile phases than in methanol-based mobile phases, and recent results indicate that dtrate or Tris buffers are preferred over phosphate buffers (32). 

For the user of HPLC columns, the most important features are the chromatographic selectivities of the different packings. In the following, we will categorize reversed-phase packings using measures of their hydrophobicity, silanophilic interaction at pH 7, hydrogen-bond formation, and their steric selectivity. 

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