Thermodynamics and collective modes in hydrogen-bonded fluids.

The thermodynamics of liquids and supercritical fluids is notorious for eluding a general theory, as can be done for crystalline solids on the basis of phonons and crystal symmetry. The extension of solid state notions, such as configurational entropy and phonons, to the liquid state remains an intriguing but challenging topic. This is particularly true for liquids, such as water, whose many structural anomalies give it unique properties. Here, for simple fluids, we specify the thermodynamics across the liquid, supercritical, and gaseous states using the spectrum of propagating phonons, thereby determining the non-ideal entropy of the fluid using a single parameter arising from this phonon spectrum. This identifies a marked distinction between these "simple" fluids and hydrogen bonded fluids whose non-ideal entropy cannot be determined by the phonon spectrum alone. We relate this phonon theory of thermodynamics to the previously observed excess entropy scaling in liquids and how the phonon spectrum creates corresponding states across the fluid phase diagram. Although these phenomena are closely related, there remain some differences, in practice, between excess entropy scaling and the similar scaling seen due to phonon thermodynamics. These results provide important theoretical understanding to supercritical fluids, whose properties are still poorly understood despite widespread deployment in environmental and energy applications.