| Abstract
| - The effect of confinement on the phase changes and dynamics of acetonitrile in mesoporous MCM-41 wasstudied 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 perturbthe triple bond in the C⋮N group. Adsorbed molecules above the monolayer through to the central part ofthe cylindrical pores are capillary condensed molecules (cc-acetonitrile), but they do not show the hysteresisloop in adsorption−desorption isotherms, i.e., second order capillary condensation. FT-IR measurementsindicated that the condensed phase is very similar to the bulk liquid. The cc-acetonitrile freezes at temperaturesthat 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. Applicationof the Gibbs−Thomson equation, assuming the unfrozen layer thickness to be 0.7 nm, gave the interface freeenergy differences between the interfaces, i.e., Δγl/α = 22.4 mJ m-2 for the liquid/pore surface (ps) andα-type/ps, and Δγα/β = 3.17 mJ m-2 for α-type/ps and β-type/ps, respectively. QENS experiments substantiatethe differing behaviors of monolayer acetonitrile and cc-acetonitrile. The monolayer acetonitrile moleculesare anchored so as not to translate. The two Lorentzian analysis of QENS spectra for cc-acetonitriles showedtranslational 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 rotationaldiffusion of cc-acetonitrile are similar to bulk values.
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