Liquid-liquid phase transitions for soft-core attractive potentials

Abstract

Using event-driven molecular dynamics simulations, we study a three-dimensional one-component system of spherical particles interacting via a discontinuous potential combining a repulsive square soft core and an attractive square well. In the case of a narrow attractive well, it has been shown that this potential has two metastable gas-liquid critical points. Here we systematically investigate how the changes of the parameters of this potential affect the phase diagram of the system. We find a broad range of potential parameters for which the system has both a gas-liquid critical point C 1 and a liquid-liquid critical point C 2 . For the liquid-gas critical point we find that the derivatives of the critical temperature and pressure, with respect to the parameters of the potential, have the same signs: they are positive for increasing width of the attractive well and negative for increasing width and repulsive energy of the soft core. This result resembles the behavior of the liquid-gas critical point for standard liquids. In contrast, for the liquid-liquid critical point the critical pressure decreases as the critical temperature increases. As a consequence, the liquid-liquid critical point exists at positive pressures only in a finite range of parameters. We present a modified van der Waals equation which qualitatively reproduces the behavior of both critical points within some range of parameters, and gives us insight on the mechanisms ruling the dependence of the two critical points on the potential's parameters. The soft-core potential studied here resembles model potentials used for colloids, proteins, and potentials that have been related to liquid metals, raising an interesting possibility that a liquid-liquid phase transition may be present in some systems where it has not yet been observed.

Thermodynamic and dynamic anomalies for a three-dimensional isotropic core-softened potential

“…In two dimensions, such potentials have density and diffusion anomalies and in some cases a second critical point [22,27,28,29]. In three dimensions, these potentials do not have dynamic and thermodynamic anomalies but possess a second [30] and sometimes a third [25] critical point, accessible by simulations in the region predicted by the hypernetted chain integral equation [23,26,31].…”

“…In the first case, the potential consists of a hard core, a square repulsive shoulder and, in some cases, an attractive square well [21,22,23,24,25,26,27,28,29,30,31,34]. In two dimensions, such potentials have density and diffusion anomalies and in some cases a second critical point [22,27,28,29].…”

“…They possess a repulsive core that exhibits a region of softening where the slope changes dramatically. This region can be a shoulder or a ramp [21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36].…”

Relation between occupation in the first coordination shells and Widom line in core-softened potentials

“…Even with this limitation, this hypothesis has been supported by indirect experimental results 18, 33,46 . The presence of two liquid phase and of second critical point is also observed in certain CS potentials [6][7][8][9][10][11][12][13][14][15][16][17][18][22][23][24][25][26][27] .…”

Crystal-clear transition

“…One is to introduce multiple scales for the isotropic intermolecular interactions, which favour distinct high-and low-density configurations 4 . Another approach uses the idea that directional bonds combined with soft-core interactions enable low-density networks that can potentially interpenetrate, yielding a distinct high-density, interconnected network state 5 .…”