Monofunctional silanes

Di- and trifimctional silanes are also used for the preparation of alkyl-modified silicas to be employed as chromatographic sorbents. Under anhydrous reaction conditions, sorbents prepared from these silanes have similar surface coverages and chromatographic behavior to monomeric stationary phases prepared with monofunctional silanes. 

Si(OC2H5). In the two-step process of hydrolysis of the silane to the silanol species and the condensation (silanols to siloxane), the reaction-determining step is the rate of hydrolysis [54]. However, for polyfunctional silanes, hydrolysis and condensation overlap. Under defined reaction conditions, the reaction between the surface hydroxyl groups of the sihca and the silane compound follows a distinct stochiometry, which can be expressed by the number of surface hydroxyl groups that react with the organosilane. For monofunctional silanes this ratio is unity. 

The nomenclature of the RP is not consequent. The RP most often used contains octyl (RP C8) or octadecyl (RP C18) groups. There is no differentiation even when two methyl groups are introduced additionally with the silane (as with monofunctional silanes) or only one (difunctional) or none (trifunctional silane). Some manufacturer use silanes with bulky side groups (e.g., isopropyl groups) to improve the hydrolytic stability of the bonded phases, but here also, only the longest alkyl group is used in nomenclature. RP C8 and RP C18 are the work horses in HPLC. Shorter chains (RP4) are used in protein separations, and special selectivity can be obtained with bonded phenyl, cyano, amino or fluoro groups. 

Difunctional and monofunctional silanes condense as well, but the equilibrium constant is much smaller. The position of the equilibrium may favor solution stability relative to trifunctional materials but implies less effective coupling in wet environments. 

Cohydrolysis with Monofunctional Compounds. The effective functionality can be reduced to two (or less, if desired) by cohydrolysis of a trifunctional and a monofunctional silane. This method was used by Andrianov, et al. (5) to prepare linear cohydrolysis products, according to the equation ... 

The term polymeric phases arises from the fact that trifunctional reagents may just as well react with each other as with the silica surface under the influence of (inevitably present) traces of water. Hence, the resulting material is not necessarily a well-defined monomolecular layer. Moreover, for every silanol group that disappears during the reaction, two new ones are potentially formed once the product is brought in contact with water. Many of these newly formed silanol groups can subsequently be removed by reaction with a small monofunctional silane (e.g. trimethylchlorosilane, TMCS). Also,... 

During manufacture such materials may be reacted with a small monofunctional silane (e.g. trimethylchlorosilane) to reduce further the number... 

Monofunctional silane reagents yield efficient stationary phases with flexible furlike or brushlike structure of the chains bonded on the silica surface. When bifunctional or trifunctional silanes are used for modification, Cl or alkoxy groups are introduced into the stationary phase, which are subject to hydrolysis and react with excess molecules of reagents to form a polymerized spongelike bonded phase structure. Stationary phases prepared in that way usually show stronger retention but lower separation efficiency (plate number) than mono-merically bonded stationary phases. 

Depending on the surface modification reversed phase silicas can be grouped into (a) monomeric reversed phase silicas chemically modified with monofunctional silanes and (b) polymeric reversed phase silicas with a polymeric layer made by surface reaction with trifunctional silanes. 

Capillary gas chromatographic methods are described for the analysis of chemically bonded reversed-phase HPLC stationary phase materials. Procedures for acid digestion of the packing material producing stable and volatile derivatives easily analyzed by gas chromatography are compared for precision and accuracy. The methods were used to analyze a variety of commercial phases, possessing widespread monofunctional silane chemistries and bonding consistencies. 

In the previous paragraph we mentioned di- and trifunctional silanes, but we discussed only the surfiaoe reaction with monofunctional silanes. When a... 

While the surface coverage obtainable with monofunctional silanes is limited by steric hindrance, the reaction of multifunctional silanes can be carried out under conditions that result in a significantly higher content of bonded phase than is possible with monofunctional silanes. The addition of water to the reaction results in the formation of additional silanols, which then can react further and add more bonded phase. Traditionally, phases with a... 

The majority of the commercially available bonded phases are today based on monofunctional silanes. The main reason for this is the increased reproducibility of the preparation. With di- and trifunctional silanes, even small amounts of water present in the reagents, either on the surface of the silica or in the reaction solvent, can increase the amount of bonded phase attached to the surface substantially. This problem does not exist with monofunctional silanes. Also, hindered mass transfer has been observed when multifunctional silanes were reacted with small-pore silicas to a high coverage level. The latter issue should, however, not be a fundamental problem, since the pore size can be matched to the coverage level to avoid this phenomenon. 

Several different versions are commercially available. They may be based on a mono-, di-, or trifimctional silane. From the standpoint of normal-phase diromatography, there is no principal advantage of one type over the other, excq>t that tte r odudl ty of a bonded phase based on a monofunctional silane is easier to control... 

The amount of ligands that can be bonded to the surface is limited by the steric hindrance of the head group that attaches the ligand to the surface. With di- or trifunctional silanes, higher surface coverages can be achieved, but the reaction is sensitive to the water content in the reaction medium. Water causes the hydrolysis of additional functional groups, which, in turn, results in the attachment of additional silane to the surface. This process is more difficult to control than the process involving monofunctional silanes. This is the reason why the majority of bonded phases available today are based on monofunctional silanes rather than pol unctional silanes. 

The reproducibility of the surface modification of a bonded phase depends first on the choke cff the rilane. Di- and trifiincUonal silanes can polymerize, and their reaction with the silica surface is less controllable than that of monofunctional silanes. Thus packings based on monofunctioiutl silanes are intrinsically more reproducible. 

Then, they considered fluorinated hybrid copolymers (cf Scheme 20). These copolymers were prepared by condensation of hybrid bis-silanol monomers and dichloro or diacetamido silanes, in the presence of a monofunctional silane as the chain stopper, according to the following scheme ... 

It is desirable to start with a surface that has been mildly hydroxylated for surface modification, as opposed to the stoichiometric surface shown in Fig. 1, so that there are sufficient reactive sites for silanization. If the redox chemistry of an attached molecule is to be optimized, it is also desirable to work with monofunctional silanes. In this case, only one... 

Another important development in optimizing cereal protein RP-HPLC was reported by Marchylo et al. [56]. Sterically protected wide-pore monofunctional-silane bonded Cg and CN columns (Zorbax RX-300) gave better resolution and reproducibility of gliadins and glutenin subunits than did conventional silica-based columns and were more stable. A typical separation is shown in Fig. 1. 

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