R. Nasuto and J. K. Różyło
Acta Chromatogr. 4, 50 (1995)
In their paper the authors presented a method for modifying silica with vinyltri(2- methoxyethoxy)silane. They used 10 µm silica of unspecified origin and specific surface area. According to the authors the modification blocked virtually all the silanol groups on the surface. I make this comment because I believe the authors are wrong in their conclusion.
Silanization of the silica surface with trifunctional reagents is possible in several ways:
Di- and trifunctional silanes form polymeric species on the surface by a formation of Si-O-Si- bonds. Thus the concentration of the monomers on the surface cannot be used for determination of the amount of blocked silanols, because. Steric hindrance would hardly allow end-capping of all the secondary Si-OH groups formed by the hydrolysis of unreacted 2- ethoxymethoxy groups at mono- or bifunctionally reacted modifier. So far no one has proved that it is possible. Scheme II is unrealistic because steric conditions do not enable complete removal of surface hydroxyls.
There is general agreement in the literature that the total concentration of silanols on a silica surface is approximately 8-9 µmol m-2, of which only ca 50% can react with silane modifiers. In view of their data it is clear that authors observe polymerization of the reagent despite their efforts to dry it.
There are many obvious errors in the paper. Instead of using µmol m-2 authors use µmol g-1 or even µmol m-1 when discussing surface concentration. They also use the misleading term "„methylvinyl group"” whereas there are vinyl and trimethylsilyl groups on the modified surface. In the `‘Results and Discussion’' section the authors claim that all silanols are blocked with >Si-CH=CH2 groups (p.52). On the next page they claim that all the secondary - OH groups are end-capped with trimethylsilyl groups whereas 1.7 µmol m-2 is consumed for blockage of the remaining (?) surface silanols. That is only speculation. If there are no silanols on the surface the authors should not detect any silanophilic interactions in HPLC. They have, however, chosen test mixtures which can hardly detect any interactions with silanols. Properly chosen compounds should have particular affinity for silanols, e.g. cryptands [1, 2] or pyridine or 2, 6-dimethylpyridine .There are, moreover, two examples of total blockade of silanols in chromatography [2, 4] and both led to deterioration of separation.
The number of silanols reacted can only be estimated from the quantity of modifier if a monofunctional silane is used. The quantification of the silanols on a silica surface is a very complex problem per se. It is difficult to react all the silanols even with very reactive compounds such as Grignard reagents or methyllithium , dimethylzinc [6, 7] or organoboron compounds [5, 8].The best method of determination of silanol concentration is isotopic exchange. This is why all conclusions concerning estimation of unreacted silanols should be well documented and well supported with hard data and not by speculation.
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