The Intramolecular Pressure and the Extension of the Critical Point’s Influence Zone on the Order Parameter

Abstract

The critical point affects the coexistence behavior of the vapor-liquid equilibrium densities. The length of the critical influence zone is under debate because for some properties, like shear viscosity, the extension is only a few degrees, while for others, such as the density order parameter, the critical influence zone covers up to hundreds of degrees below the critical temperature. Here we show that, for ethane, the experimental critical influence zone covers a wide zone of tens of degrees (below the critical temperature) down to a transition temperature, at which the apparent critical influence zone vanishes, and the transition temperature can be predicted through a pressure analysis of the coexisting bulk liquid phase, using a simple molecular potential. The liquid phases within the apparent critical influence zone show low densities, making them behave internally like their corresponding vapor phases. Therefore, Molecular Dynamics simulations reveal that the experimentally observed wide extension of the critical influence zone is the result of a vapor-like effect due to low bulk liquid phase densities.

Interfacial properties of the ionic liquid [bmim][triflate] over a wide range of temperatures

“…The pressure proles were calculated with the tension tensor that was used to calculate the pressure proles of Harasima, 26 implemented in LAMMPS, 27 where contributions to the proles are distributed only in the two slabs that originate the interactions and not uniformly throughout all the slabs between those two slabs. 28,29 The previous procedure were applied to all intraand intermolecular interactions. The normal pressure prole obtained from this denition was not uniform along the interface, which was thought to make no physical sense, even though it is not experimentally feasible to validate the measurements of the components of the pressure at the interface.…”

“…[30][31][32][33] As contributions to the Harasima 26 pressure proles are specically located, regions that contain inhomogeneities in their density proles are highlighted in the pressure proles. g was calculated through its mechanical denition: 29,34 g…”

“…35,36 These are highlighted in Fig. 1a as shaded areas above the adjustment to the next equation, which commonly describes simple systems with a single component or binary and ideal systems where there are no molecules in the vapor phase and where the density at the interface decays monotonically: 20,29,34 rðzÞ ¼…”

“…Thus, the intermolecular pressures are positive (repulsive), unlike what occurs in a simpler uid, such as ethane, where the intermolecular pressures are negative (attractive) in all directions. 29 For a simple uid such as ethane, which is modeled as two exibly-bonded Lennard-Jones sites, it can be concluded that the intermolecular separations are, on average, longer (attractive) than the separation corresponding to the minimum energy of the intermolecular potential (Lennard-Jones alone); however, the ions of the ionic liquid [bmim][triate] are more complex because they also include coulombic interactions, and each interaction site has different parameters. Therefore, it is complicated to infer an average trend of how the intermolecular separations of the different sites in the interface produce this unexpected behavior.…”

Cation folding and the thermal stability limit of the ionic liquid [BMIM+][BF4−] under total vacuum

“…In contrast, at temperatures below the thermal stability temperature P n transforms from a region of two peaks with opposite signs at the interface to a region with intermediate negative values at the limiting temperature. The two-peak zone at temperatures below the thermal limit is associated with zones of different cohesivity-an outermost, external interfacial zone (negative peak), where the bulk liquid zone strongly attracts chains of cations in ionic liquids, 56 chains in alkanes and polymers, 45 or atoms in atomic systems. 43,44 Interactions between the outermost cohesive zone and the bulk liquid create an intermediate crunched zone, which manifests as the positive peak.…”