What is Endcapping?

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When functionalizing silica gels, it is impossible to react with all available silanol groups (free OH groups on the silica’s surface), thus a second grafting step is done. This is generally done with a small capping agent with low steric hindrance (such as TMS, trimethylsilylchloride, for example). This second grafting step is called endcapping and is done to reduce silanol activity.

There are two main benefits of endcapping:

• First, it renders the silica non-acidic and non-polar. Bare silica gel for chromatography with all its free silanols is polar and somewhat acidic (pKa ~5). In normal chromatography, this polar interaction is the basis for the separation. However, for functionalized silica that is used as a reversed-phase, a scavenger or a reagent, the only interaction that should occur is the one with the grafted function and not a non-specific binding with the surface.
• The second reason is that when functionalized silica is used in harsh conditions (for example extreme pH), endcapping helps prevent the surface from being attacked and hydrolized. Figure 1 represents what the surface structure looks like.

Figure 1: Surface structure of functionalized silica gel. X = grafted function