Thermodynamic properties of water under confinement

In this context the viscous properties of water and aqueous electrolytes as well as the frequency dependent (1 Hz to 1 MHz) electrical polarizability in mesoporous materials as a function of pore diameter, pore wall chemistry (hydrophilic, hydrophobic silicon and silicate glasses, PMOs and titanium dioxide) and temperature will be investigated. Within the research group these results will be related to the microscopic structure and dynamics (self-diffusion and molecular relaxation kinetics) of water and aqueous solutions in mesoporous media.

In addition, all essential phase equilibria of water and aqueous electrolyte solutions are significantly altered compared to bulk electrolyte systems in mesopores. Well known and investigated are both the vapour pressure reduction (capillary condensation) and the melting point reduction in mesoporous materials. However, the additional influence of dissolved electrolytes on such equilibria is little known.

Subproject 3: Water and aqueous electrolytes in mesoporous solids: phase behaviour, hydrodynamic transport and dielectric behaviour

Principal Investigator: Patrick Huber / TUHH

Far less is known about the influence of restricted geometries on water and aqueous solutions, especially with regard to transport properties (shear viscosity) and dielectric properties, than, for example, about simple fluids such as linear hydrocarbons.

For water, in particular, a significantly reduced static dielectric constant for water and aqueous electrolytes is predicted theoretically compared to the volume material, and also the shear viscosity of water, which is directly linked to the microscopic self-diffusion dynamics via the Stokes-Einstein relationship, is often significantly modified in nanopores compared to the volume behaviour. In particular, the adsorption of water molecules on the pore wall can lead to boundary layers with a viscosity that is higher than the volume, so that the hydraulic flow rates in nanopores can be significantly reduced compared to the predictions of macroscopic hydrodynamics.

As preliminary work and complementary experiments for the investigations of the other cooperation partners on microscopic dynamics (subprojects 4 and 5), the hydrodynamics and dielectric behaviour of water on the rather macroscopic level will be investigated in laboratory experiments in this subproject. For this purpose, hydraulic flow measurements as well as gravimetric and laser-interferometric capillary rise experiments, as a function of pore diameter and pore wall chemistry, will be carried out, which allows to determine the viscous properties in the pore space in comparison to the volume properties of water.

Dielectric spectroscopy allows to correlate characteristic relaxation modes in water with mesoscopic structural dynamics, which provides the deduction of the phase behaviour (solid-liquid phase transition, glass transition) of water and aqueous solutions in porous media.

Subproject 6: Phase equilibria of aqueous electrolyte systems in mesopores

Principal Investigator: Michael Steiger / UHH

In addition to the limiting influence of the pore walls (the "size effect"), the interactions of the solvent water with the pore walls and thus the chemical properties of the pore walls most probably also play a significant role in the shift of phase equilibria in mesopores. These interactions not only influence the properties of the water molecules in the immediate proximity of the wall (up to approx. 0.6 nm), but probably also the water structure in the core area of a cylindrical pore.

Within the scope of the project described here, a systematic experimental investigation of different phase equilibria in mesoporous materials is to be carried out. In preliminary studies, first investigations were carried out on the water absorption behaviour of NaCl crystals, the melting points of salt hydrates and the freezing and melting behaviour of aqueous electrolyte solutions. It could be shown that the combined influence of salt and confinement shifts the stability range of liquid water in favour of the liquid phase not only with regard to the melting point of ice but also with regard to the equilibrium between water vapour and solution.

In the project different properties of the electrolyte systems are systematically investigated, varying the pore size and pore geometry of the mesoporous materials used. The influence of the pore wall properties on the phase equilibria is investigated in close cooperation with the project partners. The main methods used in the investigations are water vapour sorption, calorimetry and Raman spectroscopy.

Finally, specific investigations on the pressure build-up by crystals growing in pores (so-called crystallization pressure) will be carried out and, based on an ion interaction model, a thermodynamic model will be used to investigate the various influences.