Physical properties of bulk water are extremely complex, be it liquid or solid, due to specific features of the H2O molecule, such as large electric dipole moments, highly-directional H-bonds or relatively small moment of inertia. Novel properties of water arise when separate H2O molecules or molecular groups are confined within micro- or nano-sized spaces. Since these kinds of objects are widespread in various technological, geological, meteorological, biological, etc. systems, there is an urgent need to study their physical properties. However, along with a large number of theoretical studies and computer simulations, the respective experimental observations are rather poor and often controversial.
We carried out a series of experiments using objects that are convenient and thereby model for this type of research. These are dielectrics whose crystal lattice contains periodically arranged nano-sized cages where distinct water molecules enter during crystal growth. The H2O molecules are weakly coupled (via Van der Waals forces) to the lattice that serves as a matrix. The matrix holds the molecules at a distance 5-10 Å that is large enough to suppress the H-bonding but not too large to keep effective the electric dipole-dipole interaction. The resulting water molecular network can be considered to form a state that is intermediate between that of liquid water (H-bonded H2O) and water vapor (quasi-free H2O molecules).
Using radio-frequency, terahertz, and infrared spectroscopy, we have studied the energetics of water molecular networks hosted by dielectric matrices of hexagonal beryl and orthorhombic cordierite. We discover an incipient ferroelectric phase and signatures of quantum critical behavior within the dipole-dipole interacting H2O molecules in beryl and fingerprints of relaxor response of water subsystem in cordierite. We observe specific librational and translational excitations of separate confined H2O molecules. At infrared frequencies, combined excitations arising from the coupling between H2O intramolecular and lower-frequency modes are seen. Isotope effects caused by replacement of H2O with heavy water are also investigated. To analyze the observed effects, computer simulations within the density-functional-theory approach are performed.