Monomeric vs. polymeric C18
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For the octadecylsilane stationary phase, SiliCycle proposes two different bonding types: the monomeric and the polymeric functionalized C18.
Silica surface possesses active silanols (Si-OH), which permits the modification of the surface chemistry by grafting silane moieties. This property permits the control of the surface polarity useful in separation techniques. In HPLC, two types of silane are grafted over the surface: chlorodimethylsilane or trichlorosilane.
Monomeric functionalization (Mono)
By grafting chlorodimethylsilane, only one bond can be formed with the silica surface. This type of grafting is called monomeric. The dimethyl groups help to protect the surface by steric hindrance, which also prevent from reaching the highest possible silane density. The residual silanol groups are inhibited by the grafting of a small molecule, trimethylsilane Si(CH3)3.
This small reagent is called a capping agent, and this technique is called endcapping. Most of our modified silicas are available endcapped or non-endcapped. Even after endcapping, a small portion of the initial silanols are still present, unable to react due to steric hindrance and hence isolated from the mobile phase and analytes present. This product presents a very high stability, batch-to-batch reproducibility and good hydrophobic properties. The fact that the silane possesses only one bond with the surface makes this phase less stable at low pH, which may lead to silane hydrolysis and consequently leaching. For low pH, the polymeric phase is preferred.
Polymeric functionalization (Poly)
By grafting a trichlorosilane, it is possible to form multiple bonds in three space directions with the surface and also between silane molecules. This grafting method is called polymeric functionalization. The silica surface is more hydrophobic, has greater stability in strong acidic condition (pH 2, 3) and has a longer lifetime.