The Frenkel Line: a direct experimental evidence for the new thermodynamic boundary

Abstract

Supercritical fluids play a significant role in elucidating fundamental aspects of liquid matter under extreme conditions. They have been extensively studied at pressures and temperatures relevant to various industrial applications. However, much less is known about the structural behaviour of supercritical fluids and no structural crossovers have been observed in static compression experiments in any temperature and pressure ranges beyond the critical point. The structure of supercritical state is currently perceived to be uniform everywhere on the pressure-temperature phase diagram, and to change only in a monotonic way even moving around the critical point, not only along isotherms or isobars. Conversely, we observe structural crossovers for the first time in a deeply supercritical sample through diffraction measurements in a diamond anvil cell and discover a new thermodynamic boundary on the pressure-temperature diagram. We explain the existence of these crossovers in the framework of the phonon theory of liquids using molecular dynamics simulations. The obtained results are of prime importance since they imply a global reconsideration of the mere essence of the supercritical phase. Furthermore, this discovery may pave the way to new unexpected applications and to the exploration of exotic behaviour of confined fluids relevant to geo- and planetary sciences.

Structural behavior of supercritical fluids under confinement

“…We use a cut-off of 20Å (used previously for Ar in supercritical regime [15]) and shift the potential to make the potential and force continuous at the cut-off. In order to shift the potential to zero at the cut-off we have added ∆ = -9.94 × 10 −7 ev to the potential, which is numerically, too small to affect the critical point significantly.…”

“…Structural and thermodynamic properties, associated with Frenkel line crossover, have been deduced from MD simulations in terms of RDF and specific heat capacity [14]. D.Bolmatov et al experimentally proved the presence of thermodynamic boundary associated with Frenkel line from a diffraction experiment on supercritical Argon in a diamond anvil cell (DAC) [15]. Extensive investigations have been done on the dynamic crossover of supercritical phases of water [10,16], Iron [17], CO 2 [10,18,19], Argon [13,15], CH 4 [10] etc.…”

“…D.Bolmatov et al experimentally proved the presence of thermodynamic boundary associated with Frenkel line from a diffraction experiment on supercritical Argon in a diamond anvil cell (DAC) [15]. Extensive investigations have been done on the dynamic crossover of supercritical phases of water [10,16], Iron [17], CO 2 [10,18,19], Argon [13,15], CH 4 [10] etc. In a recent review article, J.M.Stubbs covers a wide range of molecular simulation studies of supercritical fluids (SCF) [20].…”

Experimental evidence of the Frenkel line in supercritical neon

“…As a result, no structural crossover was detected. Another study of argon [25] addressed the structure at supercritical conditions. Similarly to the previous work [24], the pressure and temperature were below the FL.…”

Density of states and dynamical crossover in a dense fluid revealed by exponential mode analysis of the velocity autocorrelation function

“…In this context it is worth to mention recent studies [25][26][27][28] that have discussed the existence of a transition between a low-density gaslike and a high-density liquidlike dynamical regime of a supercritical fluid. This crossover was identified by Brazhkin and coworkers [29] with the crossing of the so-called Frenkel line, which has been eventually connected directly to the onset of a nonmonotonic time dependence of the VAF [30].…”