Water in Confined Space: Study of Density Anomalies and Phase Transitions

使用親水及疏水性奈米孔洞材料將水限制於狹小空間中以避免均質成核的發生，進而研究水在傳統實驗方法上無法探知的極低溫過冷狀態下，其密度特異性質及固－液相變化。由於孔洞材料和被限制水的對比會影響散射強度，藉於觀察布拉格峰的強度變化，即可實際測量被限制在MCM-41-S及CMK-1材料中過冷水於常壓下的密度。在親水材料中，水密度的最大值及最小值皆被觀察到；然而在疏水材料中，水密度的最大值被發現位移至較低的溫度，且最小值沒有出現在實驗的溫度範圍中。由密度對溫度微分計算出的熱膨脹係數也在兩類材料中表現出截然不同的形態。這些數據在熱力學與動力學上所可能代表的意義相當重大，亦可藉此檢驗目前對水特異性質所建立的理論模型與計算結果。 另一部份研究兩種不同晶型的冰在不同溫度下生成於一系列的疏水孔洞材料。本研究證實立方晶型的冰生成於孔洞之中，其熔點遵守吉布斯－湯姆森等式。 本論文的結果支持水可能擁有液態－液態相變的假設，亦提供了對於水在狹小空間中性質瞭解與可能應用的發展方向。Hydrophilic and hydrophobic nano porous materials are utilized to confine water and bypass the homogeneous nucleation. The density anomalies and solid/liquid phase transitions of water at traditionally inaccessible region of phase diagram are studied. The densities versus temperature at ambient pressure of deeply supercooled H2O confined in MCM-41-S and CMK-1 are obtained experimentally by monitoring the Bragg peaks intensity change, which is related to the contrast between matrices and the confined water. Both density maximum and minimum are observed in hydrophilic confinement, however, a delayed to lower temperature density maximum and absence of minimum are shown in hydrophobic system. The deduced thermal expansion coefficients are also revealed different profiles. The thermodynamic and dynamic implications of the maximum in thermal expansion coefficient are critical to scrutinize available theoretical and computational models of water. In the other part of study, ice Ih and Ic are discovered in a series of hydrophobic porous materials with the pore sizes ranging from 1.9 - 3.8 nm at different temperatures by DSC, XRD, and SAXS. Ice Ic is confirmed to be inside the pores, and obey the Gibbs-Thomson equation. The results in this dissertation may support the hypothetical liquid-liquid phase transition and shed the road to applications and understandings of water in small spaces