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The Condensation Process of Alcohol Molecules in the Nanochannel of MCM-41:  A Spin-Probe ESR Study

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journal contribution
posted on 24.01.2008, 00:00 by Masaharu Okazaki, Shinpei Iwamoto, Yoshimi Sueishi, Kazumi Toriyama
The solution of di-tert-butylnitroxide (DTBN, a spin probe) in a low molecular weight alcohol was condensed in the MCM-41 nanochannel with changing its volume successively. When the solution volume is much smaller than that of nanochannel, the broad ESR spectrum of immobilized DTBN is observed. The spectrum becomes sharp abruptly when the solution volume exceeds a critical value that is necessary to cover the whole surface of the nanochannel as the monomolecular layer. With a further increase in the solution volume, little change is observed in the line width. From these observations a model is derived for the physicochemical state of alcohol:  (1) When the dose is below the critical value, alcohol molecules are adsorbed on the surface sporadically as isolated molecules or as clusters and have strong interactions with the silica wall. (2) At the critical value the alcohol molecules form the monomolecular layer on the surface. They interact with each other by forming a network structure and become free from the nanochannel surface. (3) The DTBN molecule behaves as the solute of alcohol and the ESR spectrum reflects well the physicochemical state of alcohol. To support the above model, the oxygen gas effect on the ESR spectrum was observed and a geographical consideration for the alcohol molecules in the nanochannel is made. The collective molecular flow of alcohols through the nanochannel, which was reported in a previous paper, is discussed with the obtained model.