Unreacted silanol groups

V) If the C-18 bonded phase contained unreacted silanol groups, how would the retention of the solutes be affected by end-capping the stationary phase7... 

The hydrolysis rate of organosilanes is a strong function of the size of the alkyl group and steric hindrance. Actually, however, these reactions never result in the formation of pure silica oxides, and it is precisely the fact that m A 0 and that a large number of unreacted silanol groups exist at the material s surface that gives rise to the impressive variety of chemical applications of doped silica xerogels.2... 

The relationship of the selectivity to the polarity of the analytes can be understood from the differences in the retention factors of homologous alkanols (Figure 3.12). The polar alkanols are relatively more retained on the non-endcapped bonded phases (LOC-ODS-NE and HIC-ODS-NE) because smaller-size alkanols can reach the unreacted silanol groups on the surface of silica gels. 

Residual Silanol-Solute. Even after bonding hydrocarbon chains to the silica, some residual unreacted silanol groups remain. These polar groups can interact with polar functional groups of solutes, i.e., normal phase attraction. The precise role they play in retention and selectivity is not well understood, but the different selectivities exhibited by various packings probably result at least in part from their different amounts of unreacted silanol groups. 

In the real world, the interactions with the stationary phase contribute more or less to the retention. Theoretically, alkylsilica stationary phases are similar to liquid alkanes immobilised on a solid support, but the bonded alkyl chains differ from the free molecules of liquid hydrocarbons by limited mobility, which may affect the retention. Further, the retention behaviour can be more or less complicated by specific polar interactions of unreacted silanol groups in silica-based bonded phases, especially with basic solutes. Finally, organic solvents used as the components of the mobile phases in reversed-phase systems can be preferentially adsorbed by the stationary phase and modify its properties 159,60]. 

However, more interesting from an application point of view are silylation reactions which introduce new functions into the materials. These can be created either directly or in subsequent further steps after silylation. Most simple is the direct conversion of the silica to a basic material by reaction, for instance, with 3-aminopropyltriethoxysilane [17]. Also two-step processes have been employed to synthesize basic materials, where first chloropropyl groups are anchored to the surface with subsequent conversion of the chloro group into an amine. In order to remove the residual, unreacted silanol groups, a second silylation with hexamethyldisilazane can be used. Such materials were found to be reasonably active in different base-catalyzed reactions, such as Knoevenagel condensations and Michael additions. A survey of the catalyzed reactions and the types of modification used can be found in Ref. [5]. 

Surface coverage by silanization is limited to 4 pmol/m or less because of steric effects, The unre-acied SiOH groups, unfortunately, impart an undesirable polarity to the surface, which may lead to tailing of chromatographic peaks, particularly for basic solutes. To lessen this cffcci. siloxane packings are frequently capped by further reaction with chlorotrtmethylsilane that, because of its smaller size, can bond some of the unreacted silanol groups. 

In practice, dry silica is refluxed for several hours with a solution of the organosilane (RS1X3) in toluene or carbon tetrachloride under a dry inert gas atmosphere (Little et al., 1979a). The silica is recovered by filtration, washed (when X = Cl it is hydrolysed with water) and then capped to mask all unreacted silanol groups with trimethylchlo-... 

---