Chromatography Forum

Dear all ,
Why is no other form used of carbon chain expect C18 and C8 with silica ?
If anyone have answer please tell me.
Thank you for reading.
Neha.

Supposedly, C18 was used early on because octadecyltrichlorosilane was commercially used for silanizing glass surfaces, and therefore was available and cheap. Making a one-carbon change in the chain has little impact on a C18 because the effect of chain length is approximately "logarithmic"; you more-or-less have to cut the chain in half or double it. The second most-common chain length is C8, followed by C4.

All of those alkyl phases have "similar" selectivity. Significant changes in selectivity are easier to make by changing the functionality (cyano, phenyl, diol, etc. etc.), not to mention things like the pore size and surface chemistry of the silica, surface coverage, etc.

I'm guessing, too, that C18 must have appealed because it's so easy to get ready-made C18 chains from nature (stearic acid in fats).

Other chain lengths are also availabel like C30! In general every chain length is possible. It is just a matter of the availability of raw materials and the price! C1, C4, C8,C18 and C30 are standard. But whould make a C5 or C7 sence?