Silanols accessibility

All these effects are strongly dependent on the surface concentration of bonded ligands (bonding density), and despite the presence of a significant amount of unreacted silanols on the sihca surface, only the silanols accessible for analytes should be considered. 

An important difference between Protein-Pak columns and other size exclusion columns is the silica backbone of the Protein-Pak columns. Because the silica structure is unaffected by the solvent, these columns do not swell or shrink as a function of the solvent. This is a general advantage compared to other size exclusion columns. However, silica-based columns can only be used up to pH 8, which limits their applicability. Also, surface silanols are accessible for interaction with the analytes, but this phenomenon has been minimized by proper derivatization techniques. Generally, a small amount of salt in the mobile phase eliminates interaction with silanols. 

Reversed-phase PLC precoated plates are based on silica gel matrices with chemical modifications in such a manner that the accessible polar, hydrophilic silanol groups at the silica gel surface are replaced by nonpolar, hydrophobic alkyl chains via silicon-carbon bonds. For preparative purposes, up to now only PLC precoated RP plates with C-18 modification are available. This abbreviation is often also designated as RP-18, meaning that an octadecyl alkyl chain is chemically bonded to the silica gel surface. 

To get further insight into the reactivity of the metallo-silanols, condensation with diverse chlorosilanes has been studied. It offers easy and general access to the ferrio-disiloxanes 15a-d via interaction of 2a or its lithiation product 13 with alkyl-, aryl- or metallo-chlorosilanes in the presence of NEt3 (Eq.(3)). 

In the range of concentrations observed (from 0.06 to 2.5 pg/ml), the amount of GFP adsorbed on SBA-15 silica material increases linearly increasing the concentration of the GFP solution (Fig. 2), while the amount of GFP adsorbed on Aerosil flattens towards a plateau around 2.5 pg/ml (Fig. 2, inset). All silanol groups accessible on the surface of Aerosil interact with the protein that occupies all the available space, and no more protein can be loaded on the amorphous nanoparticles. 

In order to avoid tedious procedures required to prepare packed CEC columns, some groups are studying the use of empty capillaries. Since solute-stationary phase interactions are key to the CEC process, appropriate moieties must be bound to the capillary wall. However, the wall surface available for reaction is severely limited. For example, a 100 pm i.d. capillary only has a surface area of 3xl0 4m2 per meter of length, with a density of functional sites of approximately 3.1 xlO18 sites/m2, which equals 0.5 pmol sites/m2. Moreover, surface modification cannot involve all of the accessible silanol groups, since some must remain to support the EOF. As a result, the use of bare capillaries in CEC has been less successful. 

Another approach to preparing a stable reversed phase with fewer residual silanols is the use of polyfunctional silanes of the type R2SiX2. These react to form a polymeric stationary phase that shields the siloxane bonds and restricts access to residual silanols. Polymer phases have higher carbon loads and are typically more retentive than monomeric phases. However, they are more difficult to synthesize reproducibly and may exhibit batch-to-batch variability in their properties. They also exhibit poorer mass transfer kinetics and so provide poorer efficiency than monomeric phases. 

Another approach to a reduction in silanol activity has been the use of mixed silanes. This was first practised in the design of the YMC Basic packing, but a recent and more rigorous application of this idea is the Luna packing. The surface is derivatized with a mixture of C13 and Cg silanes. This improves the surface coverage that can be achieved, and reduces the access to surface silanols. The result of this procedure is an improvement in the peak shape of basic analytes (see Table 1). 

However, as was shown by Stbber (219), the silanol groups on a silica surface do not react quantitatively. Apparently, the trimethylsilyl group is too large and will block access to neighboring silanol groups. About 40% of the silanol groups on Aerosil formed trimethylsiloxane... 

The surface of the silica is covered with weakly acidic silanol groups and it is assumed that the pK, value is in the range of 5 to 7. The maximum surface concentration of silanol functions is about 8 /imol/m. As the binding of ihe organic funclioitK lakes place via Ihe accessible silanol groups it is desirable to maximize their surface concentralion and to pre... 

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. 

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