What is the difference between a monomeric and polymeric C18 bonded phase?
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Chemically bonded alkyl phases can be prepared through various reactions. Therefore, most commercial reversed phase HPLC packing materials today are prepared from alkylsilane reagents, which result in Si-O-Si-alkyl linkages, and are stable in aqueous/organic mobile phases from pH 2 to 7.
When a bonded phase is prepared from a monofunctional alkylsilane reagent, the reagent can only form a single bond with a surface silanol group. The resulting bonded phase is called a monomeric bonded phase, resembling a brush type phase, in which each of the bristles represents the same chemical entity.
When using a di- or trifunctional alkylsilane reagent, the reagent can potentially attach to the silica surface with one, two or three bonds. When the reaction is performed in the presence of water, the reagent molecules can also react with other reagent molecules. The resulting structure is called a polymeric bonded phase. A polymeric bonded C18 phase resembles a three-dimensional network rather than a brush-type monomeric bonded phase.