Department of Chemistry, Faculty of Science, Okayama University of Science, 1-1 Ridaicho, Okayama 700-0005, Japan.
J Chem Phys. 2013 May 28;138(20):204714. doi: 10.1063/1.4807593.
The phase properties of water confined in mesoporous silica MCM-41 were investigated over a temperature range of 100-298 K as a function of pore size by specific heat capacity and inelastic neutron scattering (INS) measurements. The water content of the samples was carefully controlled to ensure the capillary filled state and no overloading of water. The values of heat capacity of the pore water are higher than those of bulk ice and liquid water over the whole temperature range measured. The contribution of water in the inner part of pores (abbreviated as the internal water) was elucidated by using the heat capacity data of monolayer water measured. The entropy of the internal water was then estimated from integration of the heat capacity of the internal water. The entropy values of the internal water increase by confinement in the pores of MCM-41 in both liquid and frozen regions, indicating an increase in the deformation of the structure and∕or a change in the dynamics in both regions. The INS spectra show the density of states for the librational motion of water frozen at 50 K, suggesting that the confined water is similar to amorphous ice rather than to crystalline ice. When the sample is warmed to melt, the band edge of the librational motion for water frozen in large pores (diameter of 3.6 nm) shifts to a lower energy side, indicating the weakening of intermolecular hydrogen bonds. For water in small pores (2.1 nm), on the contrary, the librational band shifts slightly to a higher energy side, suggesting the low density liquid to high density liquid transition (L-L transition) at 225-250 K. A plausible mechanism of the L-L transition of water in confinement is proposed in terms of incomplete growth of homogeneous nucleation of ice due to an interfacial free energy effect to inhibit crystallization of water confined in small pores.
介孔硅 MCM-41 中受限水的相态在 100-298 K 的温度范围内通过比热和非弹性中子散射 (INS) 测量作为孔径的函数进行了研究。样品中的含水量经过仔细控制，以确保处于毛细填充状态且不会过度加载水。在整个测量温度范围内，孔中水的热容值均高于体相冰和液态水的值。通过测量单层水的热容数据阐明了孔内水（简称内部水）的贡献。然后通过对内部水的热容积分来估算内部水的熵。在液体和冻结区域中，由于在 MCM-41 孔中的受限，内部水的熵值增加，表明结构的变形增加和/或两种区域中的动力学发生变化。INS 光谱显示在 50 K 冻结的水的旋转运动的态密度，表明受限水类似于无定形冰而不是结晶冰。当样品升温至融化时，大孔（直径 3.6nm）中冻结水的旋转运动的带边缘向较低能量侧移动，表明分子间氢键减弱。对于小孔（2.1nm）中的水，相反，旋转带略微向更高能量侧移动，表明在 225-250K 时从低密度液体到高密度液体的转变（L-L 转变）。提出了一个关于受限水中 L-L 转变的合理机制，即由于界面自由能效应抑制了小孔中受限水的结晶，导致冰的均匀成核不完全生长。
