Quenching of Collision between the Solute Molecules in the Nanochannel of MCM-41: A Spin-Probe ESR Study on the Alcoholic Solutions

ESR spectra were observed for the alcoholic solutions of spin probes at a very high concentration, 30 mM, both in the clear solution and encapsulated in the nanochannel of MCM-41. At a high temperature, the clear solution shows a very broad ESR spectrum, whose line width is mainly due to the spin exchange induced by collision between the solute spin probes. On the other hand, the solution encapsulated in the nanochannel of MCM-41 shows a much sharper spectrum. This sharp line width indicates that collision between the solute molecules is quenched in the nanochannel. The effect is more prominent for the MCM-41 system with smaller channel diameter. In the suspension of MCM-41, the ESR spectrum is composed of the two parts, the sharp one from the solution in the nanochannel and the broad one from the bulk phase. Thus, the quenching of collision in the nanochannel is independent of the coexistence of the solution out of the nanochannel. When the system is cooled down at a temperature 10-20 degree above the melting point, the ESR spectrum of the solution in the nanochannel changes to the glassy signal. Upon warming the system, the spectrum is sharpened gradually and the glassy pattern disappears at around 293 K. These observations have been explained with a model that the solvent molecules form a liquid-crystalline like structure in the nanochannel with hydrogen bonding which prevents the solute from making translational diffusion in the nanochannel. The unexpected rapid flow of the alcoholic solution through the nanochannel and a large cage effect in a photoreaction of the solute in it, both of which were reported by the present authors, are consistent with this model.