The phonon theory of liquid thermodynamics

TL;DR

This paper introduces a phonon-based theoretical framework for understanding and calculating the heat capacity of liquids, successfully matching experimental data across various liquid types and conditions.

Contribution

It develops a unified phonon theory of liquids that overcomes previous difficulties related to strong and system-specific interactions.

Findings

Good agreement with experimental heat capacities for 21 liquids

Applicable across classical and quantum regimes

Works for noble, metallic, molecular, and hydrogen-bonded liquids

Abstract

Heat capacity of matter is considered to be its most important property because it holds information about system's degrees of freedom as well as the regime in which the system operates, classical or quantum. Heat capacity is well understood in gases and solids but not in the third state of matter, liquids, and is not discussed in physics textbooks as a result. The perceived difficulty 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. Here, we develop a phonon theory of liquids where this problem is avoided. The theory covers both classical and quantum regimes. We demonstrate good agreement of calculated and experimental heat capacity of 21 liquids, including noble, metallic, molecular and hydrogen-bonded network liquids in a wide range of temperature and pressure.