Silanophilic retention

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. 

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. 

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). 

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],... 

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). 

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. 

The cationic micellar eluents of ChTAB gave better linear correlations for k vs. log Pow than the anionic surfactant SDS [16]. This might indicate that the cationic micellar systems have a similar hydrophilic/lipophilic balance to octanol-water, and/or the polar interactions betw n the solutes and the cationic head group better resemble those in octanol-water. For the chromatographic correlations, one should also note that the cationic surfactants adsorbed on the stationary phase can better shield the residual silanol groups on the silica surface, than SDS. Consequently, the reduction (or elimination) of the silanophilic interactions would increase the correlations between RPLC retention and log P w ... 

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