During the reaction of trifunctional alkylsilane, one or two linkagea are formed per silane molecule leading to a stoichiometric factor. How we can estimate this factor from 29Si solid state NMR? and to calculate the surface coverage, do we just need to multiply the value calculated with Berendsen–de Galan’s equation (used for monomeric functionalization) by this factor?
Thank you
You need to specify what type of silica you are using. I assume you are either using fumed silica or precipitated silica with very high surface area of above 100 m2/g. Otherwise, it would be very difficult to detect the silicon resonance of the coupling agent on the silica surface using solid-state 29Si NMR. Assuming that you can obtain reasonable signal quality, you will then be able to distinguish the interfacial bond between the silane and silica surface as the silica Si is surrounded by four oxygen atoms which gives different resonance frequency from the silane Si which is connected with one carbon. Another signal you can distinguish is the silane-silane bonds which is the bond between two silicon atoms both of which has one carbon connected. Thus, you will see three silicon resonces, silica, silica-silane, silane-silane bonds. You might see the last type, the central silicon from the silane-silane-silane bonds, but the concentration of this is rather low and Si NMR signal is quite noisy, thus it is difficult to detect.
If you want to study the surface coverage, the clearest point you might be able to detect is the first monomolecular equivalent coverage of the silane. If you prepare multiple samples from different silane concentration of the treating solution, the silane-silane bond concentration will increase suddenly around the monomolecular equivalent coverage, thus you will be able to identify monomolecular coverage. If you are lucky to see the silane-silane-silane bonds, this resonance should start increasing quickly at this coverage.
In case you have FT-IR spectrometer, you can examine the same sample and see the disappearance of the very sharp 3647 cm-1 band which is due to the OH stretching mode of the surface SiOH group. Upon monomolecular coverage, this band will completely disappear. But make sure that you do not use KBr pellet technique. You should use either wafer sample or diffuse reflectance technique.
Thank you for your answer Hatsuo Ishida , I am using non porous particles of about 320 nm particle size and their surface area is 15 m2/g.
Will I have two different signals for a silane bound to the silica with one bond and the one bound with two? and is the ratio of the area of these two signals will allow me to calculate the stoichiometric factor I am looking for? (assuming that their signal is high enough)
My second question will 13C NMR help me distinguish between the first carbon of a silane linked to silica by one bond and the one of a silane linked by two bonds (or eventually three) ?
From my opinion this might be quite challenging. I suggest using a second technique to verify the results (if any detectable Signal is obtainable). E.g. mass loss (TGA,…)
What technique did you use to fabricate the surface?
If you check our publications you might see that we performed different kinetic Analysis (Raman) and characterization of siloxane covered surface (inter alia with surface sensitive IR-ERAS).
The modification of the surface of silica particles was done in toluene using octadecyltrichlorosilane (reaction overnight under agitation with pyridine as a catalyst) Michael Schmitt
Did you check using a IR? How do you control the necessary water content (uncontrolled adsorbed water of the silica particles?)?
To Soumia's question, if you obtain high enough signal-to-soise ratio spectrum, you should be able to distinguish one and two bonds to silica surface and calculate whatever you want. Surface coverage calculation needs to assume the silane orientation on the surface to project the surface area occupied by one silane molecule.
I doubt that if you can distinguish those different silanes using 13 C NMR.
To Michael, the signal you need to look for is the surface free SiOH (OH stretching) band at 3750 cm-1. This is an extremely sharp band and adsorbed water does not show any band near this frequency. Adsorbed water will show around 3400 cm-1.
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