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 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.