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 mm) were measured at 303 K. The adsorption isoth erm had a sharp jump at P/P-0 = 0.3 without adsorption hysteresis. The CN stretching nu(2)-band of adsorbed acetonitrile had two-component a lpha- and beta-bands at 2263 and 2254 cm(-1), respectively, assigned t o hydrogen-bonded molecules on surface hydroxyls of MCM-41 and physiso rbed molecules in mesopores whose walls are coated with hydrogen-bonde d molecules, respectively. The beta-band was analyzed with a time corr elation function, giving a reorientation time tau. The tau value of th e adsorbed molecule before capillary condensation was smaller than tha t of the bulk liquid, indicating a gas-like state. After capillary con densation, tau was the same as for the bulk liquid. (C) 1998 Elsevier Science B.V. All rights reserved.