Theoretical equations of state for a charged fluid.

In this article, we present a molecular thermodynamic study of a system of N particles contained within a volume V and interacting via a hard-core pair potential with an attractive interaction according to the Wolf model for charged systems. This variable-range potential is characterized by three parameters: the repulsive hard-core diameter σ, the energy-well depth ϵ, and the inverse range α; a fourth parameter of the model is a cut-off distance xc that depends on α according to the relation xc = 2/α. Two equations of state (EOSs) are presented and derived from thermodynamic perturbation theory and Monte Carlo (MC) simulation data. The first EOS is given by the standard Zwanzig's high-temperature expansion of the Helmholtz free energy, where the first three perturbation terms a1, a2, and a3 were obtained from MC simulations in the canonical ensemble (NVT) and parameterized as functions of α and the reduced density of particles ρ* = Nσ3/V. The second EOS was obtained from the discrete perturbation theory applied to a discrete representation of the Wolf potential. Results for pressures, internal energies, and isochoric heat capacities are compared to the MC computer simulation data of the Wolf system, including vapor-liquid coexistence curves, for different values of α. Overall, both EOSs give a very good representation of the thermodynamic properties of the Wolf fluid when 0.3 ≤ α ≤ 1.0 and 0.05 ≤ ρ* ≤ 0.8. Since the Yukawa fluid can reproduce information of screened ionic interactions, we discuss the equivalence between the Wolf and Yukawa fluids in the context of equivalent systems in liquid theory.

Water phase transitions from the perspective of hydrogen-bond network analysis.

We analyze the water vapour-liquid and solid-liquid phase transitions from the perspective of hydrogen bond networks. Using molecular dynamics simulation data for the TIP4P/2005 and TIP4P/ice water models, we built hydrogen bond networks in the neighbourhood of the transitions. We studied the behaviour of some topological network properties: the average degree, clustering coefficient, and average path length. We found that these properties exhibit a discontinuity while approaching a phase transition region, similar to those that appear for some thermodynamic properties in the same region. This approach can be extended to characterize other water phase transitions. Besides, it can also be applied to study the phase transitions of other hydrogen-bonded substances or to other scenarios whose relevant "interaction" could be identified together with a "proper criterion" defined in an analogous way as in the case of hydrogen bonded systems.

Discrete perturbation theory for continuous soft-core potential fluids.

In this work, we present an equation of state for an interesting soft-core continuous potential [G. Franzese, J. Mol. Liq. 136, 267 (2007)] which has been successfully used to model the behavior of single component fluids that show some water-type anomalies. This equation has been obtained using discrete perturbation theory. It is an analytical expression given in terms of density, temperature, and the set of parameters that characterize the intermolecular interaction. Theoretical results for the vapor-liquid phase diagram and for supercritical pressures are compared with previous and new simulation data and a good agreement is found. This work also clarifies discrepancies between previous Monte Carlo and molecular dynamics simulation results for this potential.

Multiply associating electrolytes in the binding mean spherical approximation: thermodynamic properties and speciation.

Ionic solutions exhibiting multiple association are described within the binding mean spherical approximation (BiMSA). This model is based on the Wertheim formalism, in the framework of the primitive model at the McMillan-Mayer level. The cation and the anion form the various complexes according to stepwise complexation-equilibria. Analytic expressions for the Helmholtz energy, the internal energy, the speciation, and for the osmotic and activity coefficients are given considering a binary solution with an arbitrary number of association sites on one type of ion (polyion) and one site on the ions of opposite sign (counterions). As an alternative, mean field expressions, as developed in SAFT-type theories, are also presented. The result obtained from the latter approximate method exhibits a reasonable agreement with those from BiMSA for the speciation, and a remarkable one for the osmotic coefficient.

Discrete perturbation theory for the hard-core attractive and repulsive Yukawa potentials.

In this work we apply the discrete perturbation theory [A. L. Benavides and A. Gil-Villegas, Mol. Phys. 97, 1225 (1999)] to obtain an equation of state for the case of two continuous potentials: the hard-core attractive Yukawa potential and the hard-core repulsive Yukawa potential. The main advantage of the presented equation of state is that it is an explicit analytical expression in the parameters that characterize the intermolecular interactions. With a suitable choice of their inverse screening length parameter one can model the behavior of different systems. This feature allows us to make a systematic study of the effect of the variation in the parameters on the thermodynamic properties of this system. We analyze single phase properties at different conditions of density and temperature, and vapor-liquid phase diagrams for several values of the reduced inverse screening length parameter within the interval kappa( *)=0.1-5.0. The theoretical predictions are compared with available and new Monte Carlo simulation data. Good agreement is found for most of the cases and better predictions are found for the long-range ones. The Yukawa potential is an example of a family of hard-core plus a tail (attractive or repulsive) function that asymptotically goes to zero as the separations between particles increase. We would expect that similar results could be found for other potentials with these characteristics.