Thermodynamics of solid methanes: retrospective and prospective

Abstract

Methane has turned out to be very much more complex than hydrogen. There are more nuclear-spin species, more structures, and more possible combinations of more complex phase diagrams. There is some convenience gained from methane as it bridges the quantum-to-classical effects. Heavy methane CD 4 can be treated classically and light methane CH 4 exhibits quantum effects, which must be handled using quantum statistics. Since the methane problem is more complex, there are more chances for unusual or unexpected effects to occur and therefore it has attracted intrinsic interest. Frequently, a single type of experiment leaves us with incomplete answers. The methane problem would never have been solved to the degree it has if a single type of experiment had been used. In achieving the progress to date, it has been urgent that information from various experiments be combined, and theories tested, revised, and new experiments done. Thermodynamic properties must be exploited as one of the important experimental tools, since thermodynamic properties are sensitive to what is happening within the system. The area of thermodynamics is not an end in itself. All the mysteries enshrouding the methane solids have yet to be disentangled, but substantial progress has been made since James and Keenan first attempted an explanation. But then—Nature does not reveal its secrets easily.