What is Endcapping?

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When we functionalize our silica it is impossible to react with all available silanol groups (free OH groups on the silica surface) so we "endcap" the residual ones with a capping agent such as trimethylsilylchloride to form Si-O-Si(CH₃)₃.

There are two main benefits of end capping:

• 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 chromatography this polar interaction is the basis for the separation, but for functionalized silica that is used as 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, the end-capping prevents the surface from being attacked and destroyed. The figure below represents what the surface structure looks like.