Studies on the critical crystallite size of ice and glassy states in mesoporous confinement

当水凝结在细小的孔隙中时，它会在低于散装水的温度下冻结和熔化。用经典的Gibbs-Thomson关系解释了孔径对这一现象的影响。本工作通过介孔二氧化硅、MCM-41和SBA-15介孔分子筛的DSC测量详细地研究了这一现象，并提出了该关系的适用范围。此外，热力学分析还发现了界面自由能的变化，表明在230 K左右的过冷水中存在相变，也证实了水分子不会在2.1 nm以下的孔隙中冻结为结晶冰，即它们以玻璃冰的形式冻结。利用中子自旋回波测量研究了重水在MCM-41(d=2.1 nm)孔内的动力学性质(平移运动)。结果表明，水分子在200K以下是运动的，KWW函数分析的平动运动弛豫时间在229K附近改变了温度依赖模式，高于229K服从Vogel Fulcher Tamman关系，低于229K服从Arrhenius关系。这一事实表明，细孔中的过冷水代表了一种液-液转变。这种转变仅仅是通过诺曼陆地温度区域的分子动力学模拟提出的，并没有通过大量水的实验来实现。在2.4MCM-41介孔中的乙腈-水混合物中，冰晶的临界尺寸估计为2.2 nm

When the water is condensed in fine pores, it is frozen and molten at the temperatures lower than those of bulk water. Effect of pore size on this phenomenon has been explained by classical Gibbs-Thomson relation. Present work has investigated this phenomena in detail through DSC measurements by use of mesoporous silica, MCM-41 and SBA-15, which have well-regulated pore sizes, and suggested the limit of applicability of the relation. Furthermore, thermodynamic analysis of the heat of melting presented an interfacial free energy change indicating a phase change in the supercooled water around at 230 K. It was also confirmed that water molecules does not freeze to crystalline ice in the pores below 2.1 nm, that is, they are frozen as glassy ice.Dynamic property (translational motion) of heavy water confined in the pore of MCM-41 (d=2.1 nm) was investigated by using neutron spin echo measurement. It was found that water molecules are mobile at temperatures down to 200 K. Relaxation times of translational motion analyzed by KWW function changed the mode of temperature dependence around at 229 K, above which it obeys the Vogel Fulcher Tamman relation and below which the Arrhenius relation. This fact suggests that supercooled water in fine pores represents a liquid-liquid transition. The transition has been suggested just by molecular dynamic simulation in the noman's land region of temperatures and has not been realized by experiments with bulk water. And thus, we can conclude that one can prepare the noman's land in water in confinement.Critical minimum size of ice crystallite was estimated to be 2.2 nm in the acetonitrile-water mixture confined in the MCM-41 mesopore of 2.4