Softness dependence of the anomalies for the continuous shouldered well potential

Abstract

By molecular dynamic simulations we study a system of particles interacting through a continuous isotropic pairwise core-softened potential consisting of a repulsive shoulder and an attractive well. The model displays a phase diagram with three fluid phases, a gas-liquid critical point, a liquid-liquid critical point, and anomalies in density, diffusion and structure. The hierarchy of the anomalies is the same as for water. Here we study in a systematic way the effect on the anomalies of varying the softness of the potential. We find that, making the soft-core steeper and more penetrable, the regions of density and diffusion anomalies contract in the T − ρ plane, while the region of structural anomaly is weakly affected. Therefore, a liquid can have anomalous structural behavior without having density or diffusion anomalies. We show that, by considering as effective distances those corresponding to the maxima of the first two peaks of the radial distribution function g(r) in the high-density liquid, we can generalize to continuous two-scales potentials a criterion for the occurrence of the anomalies of density and diffusion, originally proposed for discontinuous potentials. However, we observe that the knowledge of the structural behavior within the first two coordination shells of the liquid is not enough to establish, in general, the occurrence of the anomalies. By introducing the density derivative of the the cumulative order integral of the excess entropy, measuring shell by shell the amount of order in the liquid, we show that the anomalous behavior is regulated by the structural order at distances as large as the fourth coordination shell. By comparing the results for different softness of the potential, we conclude that the disappearing of the density and diffusion anomalies for the steeper potentials is due to a more structured short-range order. All these results increase our understanding on how, knowing the interaction potential, we can evaluate the possible presence of anomalies for a liquid.

Effects of confinement between attractive and repulsive walls on the thermodynamics of an anomalous fluid

“…Also in confined water clustering, due to hydrogen-bond correlation, has been considered as the cause for increased mobility [67]. In our case layering [31] and long-range correlations and structural changes [30] can be the origin of correlated aggregates and collective modes responsible for the moderate increase of mobility.…”

“…The distance is large enough to have homogeneous fluid inside the pore [31] for a range of T and ρ comparable to that investigated for the bulk case in previous works [28][29][30]. The attractive wall is made of CSW particles quenched in a triangular lattice with constant d = a and centered at z attr = 0.…”

“…It is suitable for describing globular proteins in solution [37], methanol hydroxyl groups [38,39], liquid metals [40], water-like liquids [29] and can properly describe the hydrophobic effect of water as a solvent [41]. In particular, the CSW model reproduces density, diffusion, and structure anomalies following the water hierarchy and displays a gas-liquid and a liquid-liquid phase transition (LLPT), both ending in critical points [29,30].…”

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

“…Recently it has been proposed that a CS potential exhibits density anomaly if the two length scales identified with the softened region would be accessible 22,24,48 what can be understood as follows. The radial distribution, g(r) of a CS potential has peaks at g(r 1 ) and g(r 2 ) where r 1 and r 2 > r 1 are the two length scales of the CS potential 1,20 .…”

Correctness of certain integral equation theories for core-softened fluids

“…12 CSW consists of five adjustable parameters that allow it to assume several quite distinct forms, ranging from deep double walls to repulsive ramp-like potentials. In our work we used original parameters as proposed by Franzese et al [13][14][15] Due to the presence of strongly repulsive wall, mildly repulsive shoulder and attractive well, CSW is rather difficult to treat with IE. As we show later, some standard procedures and closures fail or yield erroneous results.…”