Phase behavior of a fluid with a double Gaussian potential displaying waterlike features

Abstract

Pair potentials that are bounded at the origin provide an accurate description of the effective interaction for many systems of dissolved soft macromolecules (e.g., flexible dendrimers). Using numerical free-energy calculations, we reconstruct the equilibrium phase diagram of a system of particles interacting through a potential that brings together a Gaussian repulsion with a much weaker Gaussian attraction, close to the thermodynamic stability threshold. Compared to the purely repulsive model, only the reentrant branch of the melting line survives, since for lower densities solidification is overridden by liquid-vapor separation. As a result, the phase diagram of the system recalls that of water up to moderate (i.e., a few tens of MPa) pressures. Upon superimposing a suitable hard core on the double-Gaussian potential, a further transition to a more compact solid phase is induced at high pressure, which might be regarded as the analog of the ice I-to-ice III transition in water.

Phase behavior near and beyond the thermodynamic stability threshold

“…Equation ( 6) is checked below against the computed BL. Far below η c , the DGM exhibits a liquid-vapor transition for very low ρ and T [10], with a bell-shaped coexistence line whose height and width increase with η [11]. Getting closer to η c , the binodal line undergoes a remarkable modification for low T , where it widens substantially (Fig.…”

“…To be specific, we inquired into the behavior of particles interacting through a Gaussian pair repulsion augmented with a weaker Gaussian attraction at larger distances [10,11]:…”

Entropy Multiparticle Correlation Expansion for a Crystal

“…To estimate d 3 r 1 ρ 2 1 we employ the one-body density in (21), which is sufficiently generic for our purposes:…”

“…1: We show a comparison between g(r) for a fcc crystal of hard spheres (η = 0.600) and the g 0 (r) function given in Eq. (39), where the value of α (95) has been chosen such that the Tarazona ansatz (21) fits at best the one-body density drawn from simulation.…”

“…The change of sign of the RMPE close to freezing indicates that fluid particles, which at high enough densities are forced by more stringent packing constraints, start exploring, this time in a cooperative way, a different structural condition on a local scale, preluding to crystallization on a global scale. Since the original observation in [ 13 ], a clear correspondence between the RMPE zero and the ultimate threshold for spatial homogeneity in the system has been found in many simple and complex fluids [ 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 ], thereby leading to the belief that the vanishing of the RMPE is a signature of an impending structural or thermodynamic transition of the system from a less ordered to a more spatially organized condition (freezing is just an example of many). Albeit empirical, this entropic criterion is a valid alternative to the far more demanding exact free-energy methods when a rough estimate of the transition point is deemed sufficient.…”