Low temperature properties of acetonitrile confined in MCM-41

Shigeharu Kittaka, Takafumi Iwashita, Akihiro Serizawa, Miki Kranishi, Shuichi Takahara, Yasushige Kuroda, Toshinori Mori, Toshio Yamaguchi

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39 Citations (Scopus)

Abstract

The effect of confinement on the phase changes and dynamics of acetonitrile in mesoporous MCM-41 was studied by use of adsorption, FT-IR, DSC, and quasi-elastic neutron scattering (QENS) measurements. Acetonitrile molecules in a monolayer interact strongly with surface hydroxyls to be registered and perturb the triple bond in the C≡N group. Adsorbed molecules above the monolayer through to the central part of the cylindrical pores are capillary condensed molecules (cc-acetonitrile), but they do not show the hysteresis loop in adsorption-desorption isotherms, i.e., second order capillary condensation. FT-IR measurements indicated that the condensed phase is very similar to the bulk liquid. The cc-acetonitrile freezes at temperatures that depend on the pore size of the MCM-41 down to 29.1 Å (C14), below which it is not frozen. In addition, phase changes between α-type and β-type acetonitriles were observed below the melting points. Application of the Gibbs-Thomson equation, assuming the unfrozen layer thickness to be 0.7 nm, gave the interface free energy differences between the interfaces, i.e., Δγ1/α = 22.4 mJ m-2 for the liquid/pore surface (ps) and α-type/ps, and Δγα/β = 3.17 mJ m-2 for α-type/ps and β-type/ps, respectively. QENS experiments substantiate the differing behaviors of monolayer acetonitrile and cc-acetonitrile. The monolayer acetonitrile molecules are anchored so as not to translate. The two Lorentzian analysis of QENS spectra for cc-acetonitriles showed translational motion but markedly slowed. However, the activation energy for cc-acetonitrile in MCM-41 (C18) is 7.0 kJ mol -1 compared to the bulk value of 12.7 kJ mol-1. The relaxation times for tumbling rotational diffusion of cc-acetonitrile are similar to bulk values.

Original languageEnglish
Pages (from-to)23162-23169
Number of pages8
JournalJournal of Physical Chemistry B
Volume109
Issue number49
DOIs
Publication statusPublished - Dec 15 2005

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Surfaces, Coatings and Films
  • Materials Chemistry

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