Low temperature properties of acetonitrile confined in MCM-41.
Low temperature properties of acetonitrile confined in MCM-41.
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DOI：
10.1021/jp052476g
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发表时间：
2005-11
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影响因子：
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通讯作者：
S. Kittaka;T. Iwashita;A. Serizawa;Miki Kranishi;S. Takahara;Y. Kuroda;Toshinori Mori;T. Yamaguchi
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文献类型：
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作者：
S. Kittaka;T. Iwashita;A. Serizawa;Miki Kranishi;S. Takahara;Y. Kuroda;Toshinori Mori;T. Yamaguchi
采用吸附、红外光谱、差示扫描量热法和准弹性中子散射等方法研究了乙腈在介孔MCM-41中的相变和动力学。乙腈分子在一个单层强烈相互作用的表面羟基注册和扰动的三键在C[三键]N组。吸附在单层以上的分子通过圆柱形孔的中心部分是毛细管凝聚分子（α-乙腈），但它们在吸附-脱附等温线中不显示滞后回线，即，二级毛细凝聚FT-IR测量表明，凝聚相是非常相似的本体液体。α-乙腈在取决于MCM-41的孔径的温度下冷冻，低至29.1 A（C14），低于该温度时不冷冻。此外，在熔点以下观察到α型和β型乙腈之间的相变。应用Gibbs-Thomson方程，假设未冻结层厚度为0.7 nm，给出界面之间的界面自由能差，即，对于液体/孔表面（ps）和α-型/ps，Δ γ（1/α）= 22.4mJ m（-2），并且对于α-型/ps和β-型/ps，Δ γ（α/β）= 3.17mJ m（-2）。QENS实验证实了单层乙腈和α-乙腈的不同行为。单层乙腈分子被锚定以不平移。α-乙腈的QENS光谱的两个洛伦兹分析显示平移运动，但明显减慢。然而，在MCM-41（C18）中α-乙腈的活化能为7.0 kJ mol（-1），相比之下，本体值为12.7 kJ mol（-1）。α-乙腈翻滚旋转扩散的弛豫时间与体积值相似。
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[triple bond]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 A (C14), below which it is not frozen. In addition, phase changes between alpha-type and beta-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., Deltagamma(l/alpha) = 22.4 mJ m(-2) for the liquid/pore surface (ps) and alpha-type/ps, and Deltagamma(alpha/beta) = 3.17 mJ m(-2) for alpha-type/ps and beta-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.