Water and other tetrahedral liquids: order, anomalies and solvation

Abstract

In order to understand the common features of tetrahedral liquids with water-like anomalies, the relationship between local order and anomalies has been studied using molecular dynamics simulations for three categories of such liquids: (a) atomistic rigid-body models for water (TIP4P, TIP4P/2005, mTIP3P, SPC/E), (b) ionic melts, BeF(2) (TRIM model) and SiO(2) (BKS potential) and (c) Stillinger-Weber liquids parametrized to model water (mW) and silicon. Rigid-body, atomistic models for water and the Stillinger-Weber liquids show a strong correlation between tetrahedral and pair correlation order and the temperature for the onset of the density anomaly is close to the melting temperature. In contrast, the ionic melts show weaker and more variable degrees of correlation between tetrahedral and pair correlation metrics, and the onset temperature for the density anomaly is more than twice the melting temperature. In the case of water, the relationship between water-like anomalies and solvation is studied by examining the hydration of spherical solutes (Na(+), Cl(-), Ar) in water models with different temperature regimes of anomalies (SPC/E, TIP4P and mTIP3P). For both ionic and nonpolar solutes, the local structure and energy of water molecules is essentially the same as in bulk water beyond the second-neighbour shell. The local order and binding energy of water molecules are not perturbed by the presence of a hydrophobic solute. In the case of ionic solutes, the perturbation is largely localized within the first hydration shell. The binding energies for the ions are strongly dependent on the water models and clearly indicate that the geometry of the partial charge distributions, and the associated multipole moments, play an important role. However the anomalous behaviour of the water network has been found to be unimportant for polar solvation.

Relationship between structural order and water-like anomalies in metastable liquid silicon: Ab initio molecular dynamics

“…With the decrease of temperature, the correlation becomes weaker and weaker. These results are different from those observed in SW Si252627, in which q tetra and t trans are perfectly correlated in the structural anomaly region. This degree of independence renders it impossible to use the isotaxis line to quantify the degree of structural order needed for the water-like anomalies to occur.…”

“…is similar to SPC/E water. The systematic comparison for different model potentials of water (mTIP3P, TIP4P, TIP4P/2005, TIP5P, SPC/E), ionic liquids (SiO 2 and BeF 2 ), and liquids characterised by the Stillinger-Weber (SW) potential (including Si)24252627 suggests that a strong correlation between q tetra and t trans exists only in rigid-body model potentials for water and SW liquids (in a limited range of tetrahedrality strength), but not in ionic melts. In addition, to obtain a more general picture of the origins of these anomalies, the excess entropy has also been attempted to understand these anomalies and has been tested to be able to predict the regions of anomalies in the phase diagram2223242526272829.…”

The interactions between thermodynamic anomalies

“…(9) and (10) or Eq. (11)] define this as the first order. On the other hand, once we expand the full expression for the compressibility anomalies [Eq.…”

“…Besides water, the next best studied system in terms of the thermodynamical anomalies is silicon. 11,29,[31][32][33][34][35] Since thermodynamic anomalies often appear in metastable regions of phase space, simulations are often used to probe these regions as they may not be easily accessible to experimental techniques. In the case of silicon, simulations are often performed using the Stillinger-Weber (SW) potential.…”

“…36 The SW potential contains a two body Lennard-Jones-like term and a three body term that biases the local configuration toward a certain ideal bonding angle. This potential has been thoroughly studied in terms of potential anomalous behavior for silicon 11,28,29,[31][32][33][34][35]37 and for alternative parameterizations to model related systems. 36,[38][39][40] The flexibility to simulate a wide range of anomalous systems by changing the relative strength of two and three body terms has made the SW potential a key model for studying this behavior although soft-core potentials have also been successfully employed.…”