Nature of Fluids in the Hypercritical Region

Abstract

IN the opinion of Andrews¹ the question of the appropriate nomenclature to be applied to a fluid at temperatures and pressures above the critical point “does not admit … of a positive answer. . . . Carbonic acid at 35.5°, and under 108 atmospheres pressure, stands nearly midway between the gas and the liquid ; and we have no valid grounds for assigning it to the one form of matter any more than to the other”. The absence of any sharp visible transition when a substance is taken from the gas state below the critical temperature to the liquid state, also below the critical temperature, by a route which does not enter the two-phase region of the p,v diagram (that is, cross the two-phase line on the T,p diagram), gave rise to the concept of the ‘continuity of gaseous and liquid states’ ; and this has, perhaps, discouraged inquiry into the structural changes which must occur on passing from a dilute gas to liquid. Suggestions concerning the distinction between gaseous and liquid states of hypercritical fluids were made by Thiessen² in 1897, and later by Eucken³ and by Plank⁴. In recent years, interest in this subject has revived, and several suggestions have been made^5–8 that the hypercritical region of the T,p diagram should be divided into areas corresponding to ‘gas-like’ and ‘liquid-like’ states of matter by what Bernal⁶ proposes should be called the ‘hypercritical line’. The present communication discusses the criteria which have been suggested for making this subdivision.