Low Temperature Properties of Acetonitrile Confined in MCM-41
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...
Saved in:
| Published in: | The journal of physical chemistry. B Vol. 109; no. 49; pp. 23162 - 23169 |
|---|---|
| Main Authors: | , , , , , , , |
| Format: | Journal Article |
| Language: | English |
| Published: |
United States
American Chemical Society
15-12-2005
|
| Online Access: | Get full text |
| Tags: |
Add Tag
No Tags, Be the first to tag this record!
|
| Summary: | 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., Δγl/ 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. |
|---|---|
| Bibliography: | ark:/67375/TPS-289K54JN-3 istex:DE195EB4B77D4093CAF50C7D44D8CF87521600E3 ObjectType-Article-1 SourceType-Scholarly Journals-1 ObjectType-Feature-2 content type line 23 |
| ISSN: | 1520-6106 1520-5207 |
| DOI: | 10.1021/jp052476g |