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

Abstract

Real-time ESR observations have been made on the aqueous solutions of di-tert-butyl nitroxide and 2,2,6,6-tetramethylpiperidine-1-oxyl-4-ol flowing in a quartz column of 0.81 mm ⌀ packed with well-dried MCM-41. In both systems, the ESR spectrum is composed of two signals, the major signal (>98% of the total) is a very broad one assigned to the radical in the nanochannel of MCM-41, and the minor signal is a sharp one due to the radical in the bulk space between the MCM-41 particles. Although the spin probes are highly condensed deep in the cylindrical nanospace of MCM-41, they are transported downstream in a rather short time. The analysis of these observations led us to conclude that the aqueous solution is transported through the nanochannel of MCM-41 at a small rate but still much larger than that predicted by the conventional law. The same type of experiment was made with ethanol solutions of the same spin probes, whose ESR spectra also show different shapes in the two spaces at a high concentration but not a distinct adsorption. In this case, the transportation must be smooth for both components, since the time profile of the ESR signal for the flow in the MCM-41-packed column is almost homologous with that observed in the open column. Since the translational diffusion of the individual molecules is quenched in the nanochannel, as being reported in earlier studies, the solute as well as the solvent molecules should move collectively through the nanochannel. The present technique to study the fluid flow in the nanospaces may be called “spin probe nano flowmetry”.