Using molecular dynamics simulations and integral equations (Rogers-Young, Percus-Yevick and hypernetted chain closures) we investigate the thermodynamic of particles interacting with continuous core-softened intermolecular potential. Dynamic properties are also analyzed by the simulations. We show that, for a chosen shape of the potential, the density, at constant pressure, has a maximum for a certain temperature. The line of temperatures of maximum density (TMD) was determined in the pressure-temperature phase diagram. Similarly the diffusion constant at a constant temperature, D, has a maximum at a density ρ max and a minimum at a density ρ min < ρ max .In the pressure-temperature phase-diagram the line of extrema in diffusivity is outside of TMD line. Although in this interparticle potential lacks directionality, this is the same behavior observed in SPC/E water.

Using molecular dynamics simulations we investigate the structure of a system of particles interacting through a continuous core-softened interparticle potential. We found for the translational order parameter t a local maximum at a density rho(t-max) and a local minimum at rho(t-min)>rho(t-max). Between rho(t-max) and rho(t-min), the t parameter anomalously decreases upon increasing pressure. For the orientational order parameter Q(6) a maximum was observed at a density rho(t-max)

We present the results of molecular dynamics simulations of the extended simple point charge model of water to investigate the thermodynamic and dynamic properties of stretched and supercooled water. We locate the liquid-gas spinodal, and confirm that the spinodal pressure increases monotonically with T, supporting thermodynamic scenarios for the phase behavior of supercooled water involving a ''non-reentrant'' spinodal. The dynamics at negative pressure show a minimum in the diffusion constant D when the density is decreased at constant temperature, complementary to the known maximum of D at higher pressures. We locate the loci of minima of D relative to the spinodal, showing that the locus is inside the thermodynamically metastable regions of the phase diagram. These dynamical results reflect the initial enhancement and subsequent breakdown of the tetrahedral structure and of the hydrogen bond network as the density decreases.

We investigate by molecular dynamics simulations a continuous isotropic core-softened potential with attractive well in three dimensions, introduced by Franzese [J. Mol. Liq. 136, 267 (2007)], that displays liquid-liquid coexistence with a critical point and waterlike density anomaly. Besides the thermodynamic anomalies, here we find diffusion and structural anomalies. The anomalies, not observed in the discrete version of this model, occur with the same hierarchy that characterizes water. We discuss the differences in the anomalous behavior of the continuous and the discrete model in the framework of the excess entropy, calculated within the pair correlation approximation.

A simple theory of the fluid state of a charged colloidal suspension is proposed. The full free energy of a polyelectrolyte solution is calculated. It is found that the counterions condense onto the polyions forming clusters composed of one polyion and n counterions. The distribution of cluster sizes is determined explicitly. In agreement with the current experimental and Monte Carlo results, no liquid-gas phase separation was encountered.PACS numbers: 05.70. Ce; 61.20.Qg; 61.25.Hq The thermodynamic properties of systems in which the predominant interactions are due to the long-ranged Coulomb potential still remain largely not understood in spite of the tremendous effort that has been invested over the span of this century. Nevertheless, it would be unfair to say that no great progress has been done. The pioneering work of Debye and Hückel [1] has lead to our understanding of dilute electrolyte solutions. The subsequent improvements by Bjerrum extended the validity of the limiting laws to larger densities [2]. These developments were followed by the introduction of powerful integral equations and by the computational methods such as Monte Carlo (MC) simulations [3]. Surprisingly, the theoretically obtained coexistence curve [4], that is in closest agreement with MC, is based on the fundamental ideas advanced by Debye, Hückel and Bjerrum more than 70 years ago [1,2]. The simplicity and the transparency of the ideas forming the basis of the Debye-Hückel-Bjerrum (DHBj) theory makes it easy to apply to other coulombic systems [5].The charged colloidal suspensions present a severe challenge to any statistical mechanics theory. The asymmetry between the charge on a polyion and a counterion, which can be as high as 10000:1, makes the usual integral equations of the liquid-state theory impossible to solve. For low charge asymmetry, less than 20:1, it was found that there exists a region in the temperature density plane where the Hyperneted Chain equation (HNC) ceases to have solutions [6]. This could be interpreted as a region of instability, in which the sample phase separates into the coexisting liquid and gas. It is, however, still unknown to what extent the break down in HNC equation can be attributed to the underlying phase separation, since the region of instability of HNC does not coincide with the true spinodal line [7]. Furthermore, the extensive experimental and simulation search for this gasliquid transition for polyelectrolytes has, so far, proven to be futile [8].At high volume fractions the strongly charged polyions form a lattice (bcc or fcc). This "solid state" provides us with a major simplification in that each polyion can be studied individually inclosed in its own Wigner-Seitz cell and surrounded by its own counterions [9]. Unfortunately, once the lattice melts the cell picture is no longer valid [10]. As is usual, the liquid state is significantly more complex than the solid state.In this letter we shall attempt to construct a theory for the fluid state of a polyelectrolyte solution. We shall wor...

A theory is presented for the effective charge of colloidal particles in suspensions containing multivalent counterions. It is shown that if colloids are sufficiently strongly charged, the number of condensed multivalent counterion can exceed the bare colloidal charge leading to charge reversal. Charge renormalization in suspensions with multivalent counterions depends on a subtle interplay between the solvation energies of the multivalent counterions in the bulk and near the colloidal surface. We find that the effective charge is not a monotonically decreasing function of the multivalent salt concentration. Furthermore, contrary to the previous theories, it is found that except at very low concentrations, monovalent salt hinders the charge reversal. This conclusion is in agreement with the recent experiments and simulations.

We explore the effects of counterion condensation on fluid-fluid phase separation in charged colloidal suspensions. It is found that formation of double layers around the colloidal particles stabilizes suspensions against phase separation. Addition of salt, however, produces an instability which, in principle, can lead to a fluid-fluid separation. The instability, however, is so weak that it should be impossible to observe a fully equilibrated coexistence experimentally.

Simulating coarse-grained models of charged soft-condensed matter systems in presence of dielectric discontinuities between different media requires an efficient calculation of polarization effects. This is almost always the case if implicit solvent models are used near interfaces or large macromolecules. We present a fast and accurate method (ICC( small star, filled)) that allows to simulate the presence of an arbitrary number of interfaces of arbitrary shape, each characterized by a different dielectric permittivity in one-, two-, and three-dimensional periodic boundary conditions. The scaling behavior and accuracy of the underlying electrostatic algorithms allow to choose the most appropriate scheme for the system under investigation in terms of precision and computational speed. Due to these characteristics the method is particularly suited to include nonplanar dielectric boundaries in coarse-grained molecular dynamics simulations.