Computer simulation of liquid-vapor coexistence of confined quantum fluids

Abstract

The liquid-vapor coexistence (LV) of bulk and confined quantum fluids has been studied by Monte Carlo computer simulation for particles interacting via a semiclassical effective pair potential Veff(r) = VLJ + VQ, where VLJ is the Lennard-Jones 12-6 potential (LJ) and VQ is the first-order Wigner-Kirkwood (WK-1) quantum potential, that depends on β = 1∕kT and de Boer's quantumness parameter Λ=h/σ√mε, where k and h are the Boltzmann's and Planck's constants, respectively, m is the particle's mass, T is the temperature of the system, and σ and ε are the LJ potential parameters. The non-conformal properties of the system of particles interacting via the effective pair potential Veff(r) are due to Λ, since the LV phase diagram is modified by varying Λ. We found that the WK-1 system gives an accurate description of the LV coexistence for bulk phases of several quantum fluids, obtained by the Gibbs Ensemble Monte Carlo method (GEMC). Confinement effects were introduced using the Canonical Ensemble (NVT) to simulate quantum fluids contained within parallel hard walls separated by a distance Lp, within the range 2σ ≤ Lp ≤ 6σ. The critical temperature of the system is reduced by decreasing Lp and increasing Λ, and the liquid-vapor transition is not longer observed for Lp∕σ < 2, in contrast to what has been observed for the classical system.

Equation of state and force fields for Feynman–Hibbs-corrected Mie fluids. I. Application to pure helium, neon, hydrogen, and deuterium

“…This is a well-established approach for classical Mie fluids. [66][67][68] Previous work 13 has suggested that the LJ-FH1 potential yields an accurate representation of the VLE envelope of neon, normal hydrogen, and helium. By performing GEMC simulations with the same potentials, we were able to reproduce their simulation results for neon and hydrogen, but obtained different results for helium.…”

“…Theory and simulation studies based on these effective potentials have established them as successful in accounting for quantum corrections. 8,10,11,13,32,33 The complete expression for the quantum-corrected Mie-FH2 potential between particles of type i and j is…”

“…To increase the accuracy, we equilibrated each simulation with 10 4 cycles and subsequently sampled averages during a production run of 10 6 cycles. Following previous work, 13,51,52 the critical temperature was estimated from the simulations using the renormalization group scaling laws, and the critical pressure was estimated by extrapolating a least-square fit of the Antoine equation ln P = A + B/T.…”

“…60 This is particularly convenient if one wants to study confined fluids. 13 We have regressed the potential parameters for neon, helium, orthohydrogen, parahydrogen, normal-hydrogen, and deuterium, and refer to Sec. III C for details on the regression procedure.…”

“…This simulation approach has since been used to study phenomena such as adsorption of hydrogen in porous materials, 8,9 quantum clusters, 10,11 helium at low temperatures, 12 and quantum fluids under confinement. 13 Accurate thermodynamic property predictions of fluids with quantum effects are needed in a range of applications, for example to develop better hydrogen liquefaction processes. 14 At present, the most accurate representation of the thermodynamic properties of pure helium, neon, hydrogen, and deuterium is given by multiparameter Equations of State (EoSs).…”