Silanophilic interaction

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. 

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). 

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. 

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. 

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. 

Another experimental deviation from equations (5-1) or (5-2) was determined to be due to the localization of solvent molecules onto the adsorption sites of stationary phase resulting from silanophilic interactions. When the... 

S.4 Concentration of Buffers. A buffer concentration in the range of 10 to 50 mM is adequate for most re versed-phase applications. However, sometimes the concentration of the buffer does lead to improvement of peak shape, presumably because the cation of the buffer suppresses silanophilic interactions of the protonated base with accessible ionized residual silanols. 

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. 

ODS silica gel is, in most cases, the filling material in HPLC columns. Because the columns in HPLC are not disposable, one should take into account the pH limitations of this material (i.e., outside the pH range 2-7.5). The second problem is associated with the presence of free silanol groups, which may be responsible for silanophilic interactions, as already mentioned. " Nowadays, end-capped BDS or ABZ columns are available, which are treated with secondary silanization using small alkyls or zwitterionic fragments to bind the free silanol groups, thus suppressing their contribution to... 

Octadecyl-polyvinylalcohol copolymer gel, ODP, offers an alternative as a nonpolar stationary phase in HPLC. With this material, no silanophilic interactions take place and there are no pH limitations. Drawbacks of ODP columns are the large retention times observed and the longer equilibration time required. 

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... 

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