Liquid-vapor phase diagram and surface properties in oppositely charged colloids represented by a mixture of attractive and repulsive Yukawa potentials

Abstract

The liquid-vapor phase diagrams of equal size diameter σ binary mixtures of screened potentials have been reported for several ranges of interaction using Monte Carlo simulation methods [J. B. Caballero, A. M. Puertas, A. Ferńandez-Barbero, F. J. de las Nieves, J. M. Romero-Enrique, and L. F. Rull, J. Chem. Phys. 124, 054909 (2006); A. Fortini, A.-P. Hynninen, and M. Dijkstra, J. Chem. Phys. 125, 094502 (2006)]. Both works report controversial results about the stability of the phase diagram with the inverse Debye screening length κ. Caballero found stability for values of κσ up to 20 while Fortini reported stability for κσ up to 20 while Fortini reported stability for κσ ≤ 4. In this work a spinodal decomposition process where the liquid and vapor phases coexist through an interface in a slab geometry is used to obtain the phase equilibrium and surface properties using a discontinuous molecular dynamics simulations for mixtures of equal size particles carrying opposite charge and interacting with a mixture of attractive and repulsive Yukawa potentials at different values of κσ. An crude estimation of the triple point temperatures is also reported. The isothermal-isobaric method was also used to determine the phase stability using one phase simulations. We found that liquid-vapor coexistence is stable for values of κσ > 20 and that the critical temperatures have a maximum value at around κσ = 10, in agreement with Caballero et al. calculations. There also exists a controversy about the liquid-vapor envelope stability of the pure component attractive Yukawa model which is also discussed in the text. In addition, details about the equivalence between continuous and discontinuous molecular dynamics simulations are given, in the Appendix, for Yukawa and Lennard-Jones potentials.

Fluid-solid coexistence from two-phase simulations: Binary colloidal mixtures and square well systems

“…A direct comparison between two-phase simulations and free energy calculations allows showing that the results for the fluid-solid equilibrium from both methods are in excellent agreement. The phase diagram reported by Chapela et al 20 and Hynninen et al 16 is used as a guide to choose the total density and temperature for the vapor-liquid and fluid-solid using the two-phase simulations method.…”

“…In those calculations, there was not an evidence of a low density fluid-solid transition, in contrast with results reported by Fortini et al 14 To the best of our knowledge, the stability of the high density fluid-solid transition obtained from NPT simulations has not been discussed for short ranged potentials. According to NPT results at T * = 0.18, obtained by Chapela et al 20 for the binary mixture of colloids with κσ = 6, the fluid density where the fluid-solid transition occurs is 0.8 while the value reported by Fortini et al is around 0.1. The differences, as it is shown in this work, are because the solid obtained using NPT simulations do not have any crystalline structure.…”

“…In a previous work, Chapela et al 20 reviewed several of the reported results for fluids interacting with the Yukawa model. They performed NVT (constant number of particles, volume, and temperature) simulations using continuous molecular dynamics and discontinuous molecular dynamics (DMD) to determine the vapor-liquid phase equilibrium and surface tension for the one component attractive Yukawa fluid with values of κσ from 1.8 to 15 and also for binary mixtures of colloidal particles with interaction parameters κσ from 1.8 to 20.…”

“…There was not found a signal of metastability. In the same work, Chapela et al 20 performed NPT simulations (constant number of particle, pressure, and temperature) for binary soft Yukawa mixtures for a short ranged interaction model using κσ = 6 in order to analyze the vapor-solid, vapor-liquid, and fluid-solid transitions. From the compression of one phase systems using NPT simulations at a given temperature, the vapor-liquid transition was found, as expected, at densities slightly higher compared to the equilibrium conditions obtained from slab simulations using the NVT ensemble.…”

Room temperature ionic liquids: A simple model. Effect of chain length and size of intermolecular potential on critical temperature

“…The critical point and the orthobaric curve are also obtainable with the same series, as it has been done before for many different systems. [42][43][44][45][46] The study of simplified models is useful to understand the basic principles that govern the behavior of complex systems like RTIL. The explanation of the lower melting point of RTIL using simple models has received some attention in the past.…”