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

Abstract

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.

Transportation of Aqueous and Alcoholic Solutions through the Nanochannel of MCM-41: A Spin Probe Electron Spin Resonance Study

“…The unequal heights of the three major lines of spectra c and d have been assigned to the restricted motion of these radicals in the liquid-crystalline-like water in the nanochannel of MCM-41 (Supporting Information, Figure 3S) . Since little effect of spin exchange is observed on the spectrum even at these high concentrations, translational diffusion of the individual free radicals (spin probes) must be quenched strictly. , The rather sharp spectrum e is that of an aqueous solution of TEMPOCA anion under the same experimental situation. Although the broad pattern is also overlapped, its intensity is much less than those in the two above systems.…”

“…SiOH groups on the surface of MCM-41 are randomly distributed in a medium surface density; 31 thus, they do not strongly disturb the networking of the solvent molecules with the H-bonding. 12 Under these conditions, the collective diffusion and collective flow model work more efficiently when the length of the nanochannel becomes shorter. When we remember the plug flow near the inlet of a pipe in the bulk dimension, the mechanism of the collective molecular flow can be understood easily.…”

“…The slow diffusion of the latter is due to the intensified intermolecular H-bonding in the nanochannel. As for the chemical reactions in the nanochannel, a large cage effect has been reported for the photochemical reaction of a ketone in an alcohol flowing in the column packed with MCM-41 and has been interpreted with a model, in which the reaction intermediate radicals are not allowed to diffuse away since the molecules move collectively in the nanochannel. − In previous works, we have also shown that upon condensation in the nanochannel the alcohol molecules form a wide hydrogen bond network after the completion of the first condensation molecular layer on the surface, and thus, the interaction with the silica wall is then weakened . At the same time the solute molecules, DTBN, for example, get a considerable rotational mobility in the nanochannel, but the translational diffusion is left severely inhibited. , Therefore, we have concluded that the alcohol molecules form a liquid-crystalline-like structure in the nanochannel and flow with slipping on the surface of the nanochannel collectively under much reduced shear .…”

“…[9][10][11] In previous works, we have also shown that upon condensation in the nanochannel the alcohol molecules form a wide hydrogen bond network after the completion of the first condensation molecular layer on the surface, and thus, the interaction with the silica wall is then weakened. 12 At the same time the solute molecules, DTBN, for example, get a considerable rotational mobility in the nanochannel, but the translational diffusion is left severely inhibited. 13,14 Therefore, we have concluded that the alcohol molecules form a liquid-crystalline-like structure in the nanochannel and flow with slipping on the surface of the nanochannel collectively under much reduced shear.…”

“…The integrated spectra of these spectra are given as (c). The thin part of the duplex spectra at the bottom of (c) is the ESR spectrum observed for the DTBN solution condensed in the nanochannel of MCM-41 12. (B) ESR intensities as functions of the flow volume determined by the same procedures described above (cf.…”