Thermodynamic, dynamic and structural properties of liquid and supercritical matter

Abstract

Among three basic states of matter (solid, liquid, gas), liquids are least understood from the theoretical point of view. The perceived diffculty is that interactions in a liquid are both strong and system-specific, implying that the energy strongly depends on the liquid type and that, therefore, liquid energy can not be calculated in general form. In this thesis, a phonon theory of liquids is developed where this problem is avoided. Central to the thesis is a development an alternative to calculating liquid energy and heat capacity. The proposed phonon theory of liquids covers both classical and quantum regimes and accounts for the contribution of anharmonicity and thermal expansion to liquid energy and heat capacity. Within the framework of the phonon theory of liquids a good agreement of calculated and experimental heat capacity of liquids, including helium, noble, metallic, molecular and hydrogen-bonded network liquids in a wide range of temperature and pressure is demonstrated. It was also found that in the very wide pressure range 5 MPa-500 MPa liquid helium near melting temperature is both solid-like and quantum. The thermodynamic properties of the supercritical state are studied, which lead to discovery that specific heat shows a crossover between two different dynamic regimes of the low-temperature rigid liquid and high-temperature non-rigid supercritical fluid. A theory of heat capacity above the crossover is formulated, and good agreement between calculated and experimental data for rare-gas supercritical liquids is obtained. The relationship between power exponents of heat capacity and viscosity in the supercritical region is derived. The thermodynamic properties are explained by the temperature behaviour of the maximal length of the longitudinal phonons that can exist in the supercritical system and that is not sensitive to system details. We also introduce a new idea that enables a unified description of all three states of matter. A generic form of an interacting phonon Hamiltonian with ground state configurations minimising the potential is introduced. Symmetry breaking leads to emergence of energy gaps of shear excitations as a consequence of the Goldstone theorem, and readily results in the emergence of energy spectra of solid, liquid and gas phases.