Crossover between liquid-like and gas-like behaviour in CH4 at 400 K

TL;DR

This study provides experimental evidence of a crossover between liquid-like and gas-like states in methane at high temperature, confirming the existence of the Frenkel line through Raman spectroscopy and X-ray diffraction.

Contribution

It demonstrates the reversible liquid-gas crossover in methane at 400 K, supporting the Frenkel line hypothesis with experimental data.

Findings

Crossover observed up to 397 K, 2.1 times the critical temperature

Vibrational frequency increases linearly with pressure in liquid-like state

Vibrational frequency decreases with pressure in gas-like state

Abstract

We report experimental evidence for a crossover between a liquid-like state and a gas-like state in fluid methane (CH4). This crossover is observed in all of our experiments, up to 397 K temperature; 2.1 times the critical temperature of methane. The crossover has been characterized with both Raman spectroscopy and X-ray diffraction in a number of separate experiments, and confirmed to be reversible. We associate this crossover with the Frenkel line - a recently hypothesized crossover in dynamic properties of fluids extending to arbitrarily high pressure and temperature, dividing the phase diagram into separate regions where the fluid possesses liquid-like and gas-like properties. On the liquid-like side the Raman-active vibration increases in frequency linearly as pressure is increased, as expected due to the repulsive interaction between adjacent molecules. On the gas-like side this competes with the attractive Van der Waals potential leading the vibration frequency to decrease as pressure is increased.