September 26, 2023, 12:52:16 PM
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Topic: Silanol behavior on chromatographic phases (pH, solvent,...)  (Read 660 times)

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Offline HMDS

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A couple of things I'd like some clearing up on regarding the behavior of (residual) silanols on chromatographic silica phases in different environments.

Let's assume a hypothetical silica phase which has one unique type of silanols with a pka well below 7 (e.g. pKa=3). Imaging bringing this phase into contact with ~neutral water (pH 6-7). A large portion of silanols will deprotonate towards an equilibrium Si-OH <=>> Si-O-

What would happen to those silanols if A)the water would be removed completely from the surface through gradual evaporation? Will the silanols be gradually reprotonate again as the water layer decreases? And to what extent? Or can there exist a negatively charged silica upon drying?

B) what if the water-phase on such a silica column was gradually displaced with acetonitrile (ACN)? pKa of silanols in pure ACN is very high (~30-40) as the protons aren't stabilized properly in the solvent, so I would assume it to be unlikely that the silanols stay deprotonated? If so, what's providing the protons? The water itself as it is displaced? Is there a 'permanent' water layer effect to consider? I.e., can ACN completely displace the water from the silica surface, or are there are always a few monolayers of water remaining where an equilibrium in silanols (protonated/deprotonated) can instigate? And what if the surface contained a hydrophobic C18 chemistry, with a few residual silanols?

Lastly, considering the pH of the mobile phases. Imagine formic acid (FA) is used in both mobile phase solvents; water and acetonitrile. If we start in water at a certain acidic pH (e.g., 3) where the silanols are partially protonated/deprotonated and the formic acid as well, being a weak acid. As the ACN fraction increases during a gradient separation, I would assume the medium effect of ACN influence the dissociation of FA causing the equilibrium to shift to the neutral acid. I'm assuming this would increase the pH as there are less protons in the mobile phase. What happens in such a regime to surface silanols? Do they also prefer to stay protonated as the ACN cannot stabilize their protons sufficiently? Or does the increasing pH (from the less dissociating FA) in fact trigger the silanols to deprotonate? If the latter, what stabilizes the protons? Is there again a 'permanent' water layer to consider?

Thanks in advance  :)

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