Condensation process of alcohol molecules on mesoporous silica MCM-41 and SBA-15 and fumed silica: a spin-probe ESR study

A few alcoholic solutions of di- tert -butyl nitroxide (DTBN), a spin probe, at a high concentration were condensed on several silica materials, such as MCM-41, two types of SBA-15, and fumed silica, at various amounts in vacuum. At a very low solution dose the electron spin resonance (ESR) spectrum...

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Bibliographic Details
Published in:Applied magnetic resonance 2009-04, Vol.35 (3), p.363-378
Main Authors: Okazaki, Masaharu, Seelan, S., Toriyama, K.
Format: Article
Language:English
Subjects:
Alcohols
Atoms and Molecules in Strong Fields
Electron paramagnetic resonance
Electron spin
Laser Matter Interaction
Molecular flow
Nanochannels
Organic Chemistry
Physical Chemistry
Physics
Physics and Astronomy
Silica fume
Solid State Physics
Spectroscopy/Spectrometry
Spin resonance
Tumbling motion
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Summary:A few alcoholic solutions of di- tert -butyl nitroxide (DTBN), a spin probe, at a high concentration were condensed on several silica materials, such as MCM-41, two types of SBA-15, and fumed silica, at various amounts in vacuum. At a very low solution dose the electron spin resonance (ESR) spectrum is that of an immobilized nitroxide radical. With increasing solution dose, the spectrum is gradually sharpened and a well-separated three-line spectrum is observed at the dose that is estimated to fill the surface with a monomolecular layer. Thus, the DTBN molecule can make rapid tumbling motion on this solvent layer. With a further increase in the solution dose the ESR spectrum is modified in different ways from system to system: the line width increases approximately linearly with respect to the solution dose for the SBA-15 and fumed silica systems, but it remains almost constant for the MCM-41 system until the solution dose exceeds the total volume of a nanochannel. The line width increase with respect to the solution dose is small for the SBA-15 system but large for the fumed silica system. These results have been interpreted geometrically with the structures of these silica materials and a condensation model for the alcohols on these surfaces. In relation to the present results, a model of the collective molecular flow of the alcohol solutions through the nanochannel of MCM-41 is given.
ISSN:0937-9347
1613-7507
DOI:10.1007/s00723-009-0168-2