Dynamics of water and aqueous solutions in geometrical confinement

• Main Author: Matthias, Sattig
• Format: Others
• Language: en
• Published: 2016
• Online Access: https://tuprints.ulb.tu-darmstadt.de/5605/7/Thesis.pdf; Matthias, Sattig <http://tuprints.ulb.tu-darmstadt.de/view/person/Matthias=3ASattig=3A=3A.html> (2016): Dynamics of water and aqueous solutions in geometrical confinement.Darmstadt, Technische Universität Darmstadt, [Ph.D. Thesis]

Water is one of the most vital substances for life, science, and technology. In many situations, water is confined to very narrow geometries, for example, in living cells it is severely confined in between biomolecules. The peculiarities of such systems are not yet understood and have drawn a lot of attention in current research. Additionally, the anomalous behavior of water in the bulk, e.g. the density anomaly, is not yet explained. The most common theories aiming to rationalize the behavior of water base on the assumption of a liquid-liquid phase transition at very low temperatures. Direct observation of water at these temperatures is impossible due to crystallization. In water confined in narrow geometries or in aqueous mixtures freezing is suppressed and observation of liquid water at very low temperatures is possible, what can provide valuable in- formation about the nature of water and the interactions in biologically relevant systems. The aim of this thesis is to characterize the dynamical behavior of water and aqueous mixtures in the regularly structured mesoporous silica MCM-41 over a large temperature range. For this purpose, 2H NMR methods are used, which can provide information about time scales and geometry of the motional mechanism. These capabilities render 2H NMR a valuable method to investigate supercooled liquids in confinement. The research in this work shows that current theories on water in confinement are incomplete. A dynamic crossover is found near the suggested liquid-liquid phase transition temperature. It is accompanied by the emergence of a second dynamically distinguishable water species, suggesting that the observed transition is not caused by a liquid-liquid phase transition but rather by a solidification of the pore internal water. The residual liquid resides at the pore walls and shows the characteristic behavior of a β-process. This process is found in many systems where water is close to an interface and shows several universal features. One is an additional mild crossover at ca. 185 K that may be related to a glass transition. A new method is introduced to measure the temperature dependence of the corresponding α-process and a novel model of water in confinement is proposed in order to explain the present findings. In comparison to water, the glass former glycerol does not show such drastic confinement effects in MCM-41. On reduction of the confinement size, the glycerol molecules merely show a slight acceleration of dynamics. The weak influence of the confinement on glycerol shows that a generalization of the pro posed interpretation model for water is not applicable to other simple liquids. Bulk and confined aqueous mixtures have been investigated in this work in a broad temperature range and their dynamics were characterized. The added alcohols in the mixtures are structurally similar and vary mainly in their hydrogen bonding capabilities. It was found that the phase behavior of the mixtures strongly depends on the interactions between the constituents. In the MCM-41 confinement, phase separation happens in mixtures where water clusters are prefered to spatially extended H-bond network of both water and alcohol molecules. The results indicate that water clusters in the pore center rather than at the interface. Depending on the pore size and the size of the solvent, the water cluster may reach a critical size for crystallization. Crystallization was not found in previous studies of similar mixtures in smaller confinements, demonstrating the importance of the pore size and the specific interactions for investigations of dynamics of water and aqueous mixtures.