pka of silica type B

[sofiane]

Hello,
Silica exist in different type A, B and C. The difference between A and B is the purity, type B is more pure and contains less metals that deprotonate silanols. However type A is less pure and has higher acidity (pka 4-5). Literature indicates that pka of type B is higher but there isn't a value. I found in this article (J. Chem. Theory Compt. 8 (2012) 1037 https://pubs.acs.org/doi/abs/10.1021/ct2007154) a value of 8.5.
What do you think?
Thank you

[Andy Alpert]

The article that you linked to is using model building to calculate the pKa of silica; a deductive approach to science. Empirical observations then tell you how much of the model actually applies to the world. With that in mind, I note that any silica exhibits at least some degree of negative charge at a pH > 4, and frequently a considerable amount of it. This is true even of BEH-type silica-silicon hydride hybrid materials, albeit to a lesser extent than with pure silica.

[sofiane]

Thank you Mr Alpert.
I try to study the retention of a compounds in HILIC mode, it has 2 negative charge that depends of pH and a fixed positive charge.
Can I send you my interpretation in your private mail.
Thank you

[Andy Alpert]

Sure. aalpert@polylc.com