Silica support residual silanol

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. 

In view of the foregoing discussion of the properties of hydrocarbonaceous bonded phases on silica support it is readily appreciated that well-prepared stationary phases presently used in RPC approach the ideal with the exception of their relatively poor stability in contact with aqueous eluents, particularly at high pH, and the fact that surface silanol groups cannot be completely eliminated. The latter may interact with polar solutes, particularly when the dielectric constant of the eluent is rrlHlively low. Neveritieirss, residual surface silanolii can be masked bs alkylamines in the eluent with the result that peak tailing, when it is due to... 

Unreacted chloro groups can be hydrolyzed to hydroxyl groups and then be reacted again with trimethylchlorosilane to eliminate as many hydroxyl groups as possible. This last silanization step is referred to as end capping-, it also removes most of the previously unreacted, residual silanol groups on the surface of the silica where the larger ODS was not able to penetrate. Supports that have been end capped are usually found to have different selectivities from those that are not as fully reacted. 

The number and nature of unreacted surface silanols affects the character of a stationary phase. Initially free, geminol or associated silanols are minimized through a process known as endcapping, which bonds various species to the residual silanols. Hydrophilic endcaps or bulky steric endcaps that separate the hydrocarbon chains and prevent analyte interaction with the silica surface can be used. If residual silanols are left unreacted (and some always are), the analyte will be separated based on a combination of interactions with both the reverse-phase support and the highly polar silanol groups. Increased retention, changes in elution order, and tailing will result for basic compounds. 

Silica (Si02) is the dominant support material, with excellent physical and chromatographic performance.1,5 Columns packed with unbonded silica are rarely used for analytical purposes due to the strong adsorptive characteristics. Silanol groups (Si-OH) found on silica surfaces are typically bonded with monochlorosilanes to create a hydrophobic liquid-like stationary phase for reversed-phase applications.1,12 Unreacted or residual silanols remaining after the bonding step are further reacted with a smaller silane (end-capped) to reduce the number of these adsorptive sites (Figure 3.4). One limitation of... 

Figure 3.12. Diagram illustrating the silica surface of unbonded and bonded Waters XTerra hybrid particles. Note that XTerra contains 50% less residual silanols by nature of its hybrid support chemistry. Diagram courtesy of Waters Corporation.

RPLC with chemically bonded C18, C8, or other alkyl-bonded phases. The reason is the activity of polar residual silanol groups that remain on the support surface after incomplete reaction of silica gel with organosilanes. Thus, such a stationary phase may behave as a deactivated polar adsorbent in nonpolar or weakly polar organic solvents. 

With regard to the silica-based RP stationary phase, unreacted residual silanol groups may play a significant role in IIC because it was shown that they are ion-exchange sites not only for analyte cations but also for alkylammonium HR. The higher retentions that were noticed for the silica-based stationary phase if compared to end-capped or polymer-based packings supports this. 

The mobile phase plays a number of interactive roles in a chromatographic separation. First and foremost, it is in dynamic equilibrium with the stationary phase. For base silica (and other unmodified supports), there exists an adsorbed layer of mobile phase components, not necessarily identical to the mobile phase composition. This is why water deactivates a silica column it forms strong hydrogen bonds with the residual silanol groups (the active adsorption sites on the surface) and renders them unavailable for adsorptive interactions with the solute. 

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