Structural dynamics of water under confinement

The dynamics of the complex and rapidly changing network of hydrogen bonds between water molecules is inextricably linked to its micro- and macroscopic properties. At interfaces, the dynamic properties are strongly influenced by the interactions between the water molecules in the interface and the constituents of the latter. This leads to a changed physical and chemical behaviour of aqueous solutions on surfaces and especially in nanoscopic and mesoporous solid state structures.

Therefore, the molecular structure of water and salt solutions in nanopores is to be determined (X-ray diffraction/pair distribution function) and also spatial (structural) reorganizations (using XPCS, X-ray photon correlation spectroscopy) are to be characterized. In addition, XPCS can be used to compare the microscopic local dynamics of intensity correlations and self-diffusion using dynamic structural factors. In addition, systematic studies of water dynamics and the dynamics of chemical reactions as a function of pore dimensions and interface properties (femtosecond 2D-IR spectroscopy) are performed.

Subproject 4: X-ray scattering experiments - structure and dynamics on molecular length scale

Principal Investigator: Felix Lehmkühler / DESY

In this subproject the structure and dynamics of water and the aqueous solutions, which were presented and synthesized in subprojects 1-3, are investigated in nanopores using modern X-ray scattering methods. For this purpose, experiments are planned both at DESY's synchrotron radiation source PETRA III and the European XFEL, as well as at latest-generation sources such as ESRF-EBS (European Synchrotron Radiation Facility - Extremely Brilliant Source; Grenoble).

Initially, the molecular structure of water and salt solutions in nanopores will be determined by means of X-ray diffraction measurements in so-called total scattering experiments. In these experiments the pair-distribution function (PDF) of the water molecules will be determined, in particular the oxygen-oxygen correlation. Since the PDF describes the distribution of the distances between water molecules, it is particularly sensitive to the local structure of the liquids in the nanopores. Pair distribution functions will be performed for the different nanoporous materials discussed and characterized in subprojects 1-3. Special attention will be paid to the influence of the different functionalizations of the pores on the structure of the liquids.

The results on the structure of water and salt solutions in the nanopores will be complemented by analyses of the self-diffusion dynamics of water using X-ray photon correlation spectroscopy (XPCS). On the one hand, the macroscopic viscosity will be related to the microscopic self-diffusion (via the dynamic structure factor), but also the dielectric properties will be attributed to microscopic structural reorganizations. In particular, temperature-dependent dielectric spectroscopy with temperature-dependent XPCS measurements on pore condensed water will be performed.

The results on the structure of water and salt solutions in the nanopores will be complemented by measurements of the dynamics using X-ray photon correlation spectroscopy (XPCS).

Subproject 5: Structural dynamics and chemical exchange of water and solutes in mesoporous solids

Principal Investigator: Nils Huse / UHH

At the microscopic level, the structural dynamics of molecules on ultrashort (<1ps) time scales are accessible by laser-based spectroscopic methods. Here the relationship between structure and chemically highly specific local vibrational excitations is exploited: High-frequency oscillations, which are essentially limited to two to three atoms of chemically functional groups, provide local time-resolved information about changes in bond strength, chemical exchange, and energy flow due to their characteristic frequency and sensitive change of the absorption line shape. This is especially true for the OH stretching vibration of water molecules, which has an often monotonic relationship between its vibration frequency and the strength of hydrogen bonds and electrostatic interactions and has therefore been used as a structural spectroscopic probe for more than one hundred years.

In this subproject, the dynamics of water and substances dissolved therein such as small molecules and molecular ions in mesoporous solids (in particular silicas, organosilicas and transition metal oxides) will be investigated using femtosecond 2D-IR spectroscopy to systematically study the water dynamics and the dynamics of chemical reactions as a function of pore dimensions and interface properties. In particular, the periodic variation of the interfaces as quasi nanoscopic superlattices on the crystalline arrangements of the solids offers a chemical variability that raises many questions regarding water structuring and dynamics at the nanometer level. At interfaces a slowed down water dynamics can be observed as the hydrogen bridge network is limited and the available density of states of the water molecules is reduced. Local interactions have an additional influence on the dynamics and chemical exchange, depending on whether the boundary layer has hydrophobic or hydrophilic character or can act as donor or acceptor for hydrogen bonds. By periodic surface modification with periods on the nanometer scale, artificial water superlattices will also be studied, e.g. the exchange between domains. This allows fundamental and important insights into the nature of water-water interfaces.