Molecular mechanism of capillary condensation of acetonitrile vapor on MCM-41 with the aid of a time-correlation function analysis of IR spectroscopy

The adsorption isotherm and IR spectra of acetonitrile adsorbed on MCM-41 (pore-width=3.2 nm) were measured at 303 K. The adsorption isotherm had a sharp jump at P/P0=0.3 without adsorption hysteresis. The CN stretching ν2-band of adsorbed acetonitrile had two-component α- and β-bands at 2263 and 2254 cm−1, respectively, assigned to hydrogen-bonded molecules on surface hydroxyls of MCM-41 and physisorbed molecules in mesopores whose walls are coated with hydrogen-bonded molecules, respectively. The β-band was analyzed with a time correlation function, giving a reorientation time τ. The τ value of the adsorbed molecule before capillary condensation was smaller than that of the bulk liquid, indicating a gas-like state. After capillary condensation, τ was the same as for the bulk liquid.