RESUMO

Liquid-vapor coexistence is calculated via molecular dynamics for a variety of parallelepiped shaped molecules. Models are constructed as an array of tangential hard spheres interacting with an attractive square-well potential. Each shape is formed by varying the number of spheres in their three sides. The initial density of the system is chosen close to the critical density of a SW fluid to obtain an equilibrated liquid-vapor coexistence curve by the process of spinodal decomposition. A pattern that relates the geometry of the molecular models and the existence or non-existence of a liquid-vapor orthobaric curve is shown.

RESUMO

Integral equations of uniform fluids have been considered unable to predict any characteristic feature of the fluid-solid phase transition, including the shoulder that arises in the second peak of the fluid-phase radial distribution function, RDF, of hard-core systems obtained by computer simulations, at fluid densities very close to the structural two-step phase transition. This reasoning is based on the results of traditional integral approximations, like Percus-Yevick, PY, which does not show such a shoulder in hard-core systems, neither in two nor three dimensions. In this work, we present results of three Ansätze, based on the PY theory, that were proposed to remedy the lack of PY analytical solutions in two dimensions. This comparative study shows that one of those Ansätze does develop a shoulder in the second peak of the RDF at densities very close to the phase transition, qualitatively describing this feature. Since the shoulder grows into a peak at still higher densities, this integral equation approach predicts the appearance of an orientational order characteristic of the hexatic phase in a continuous fluid-hexatic phase transition.

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Molecular dynamics simulations are performed to clarify the reasons for the disagreement found in a previous publication [G. A. Chapela, F. del Río, and J. Alejandre, J. Chem. Phys. 138(5), 054507 (2013)] regarding the metastability of liquid-vapor coexistence on equimolar charged binary mixtures of fluids interacting with a soft Yukawa potential with κσ = 6. The fluid-solid separation obtained with the two-phase simulation method is found to be in agreement with previous works based on free energy calculations [A. Fortini, A.-P. Hynninen, and M. Dijkstra, J. Chem. Phys. 125, 094502 (2006)] only when the CsCl structure of the solid is used. It is shown that when pressure is increased at constant temperature, the solids are amorphous having different structures, densities, and the diagonal components of the pressure tensor are not equal. A stable low density fluid-solid phase separation is not observed for temperatures above the liquid-vapor critical point. In addition, Monte Carlo and discontinuous molecular dynamics simulations are performed on the square well model of range 1.15σ. A stable fluid-solid transition is observed above the vapor-liquid critical temperature only when the solid has a face centered cubic crystalline structure.

Assuntos

RESUMO

A vibrating version of patchy particles in two dimensions is introduced to study self-assembly of kagome lattices, disordered networks of looping structures, and linear arrays. Discontinuous molecular dynamics simulations in the canonical ensemble are used to characterize the molecular architectures and thermodynamic conditions that result in each of those morphologies, as well as the time evolution of lattice formation. Several versions of the new model are tested and analysed in terms of their ability to produce kagome lattices. Due to molecular flexibility, particles with just attractive sites adopt a polarized-like configuration and assemble into linear arrays. Particles with additional repulsive sites are able to form kagome lattices, but at low temperature connect as entangled webs. Abundance of hexagonal motifs, required for the kagome lattice, is promoted even for very small repulsive sites but hindered when the attractive range is large. Differences in behavior between the new flexible model and previous ones based on rigid bodies offer opportunities to test and develop theories about the relative stability, kinetics of formation and mechanical response of the observed morphologies.

RESUMO

The phase behavior of a two-dimensional square-well model of width 1.5σ, with emphasis on the low-temperature and/or high-density region, is studied using Monte Carlo simulation in the canonical and isothermal-isobaric ensembles, and discontinuous molecular-dynamics simulation in the canonical ensemble. Several properties, such as equations of state, Binder cumulant, order parameters, and correlation functions, were computed. Numerical evidence for vapor, liquid, hexatic, and triangular solid is given, and, in addition, a non-compact solid with square-lattice symmetry is obtained. The global phase diagram is traced out in detail (or sketched approximately whenever only inaccurate information could be obtained). The solid region of the phase diagram is explained using a simple mean-field model.

RESUMO

Liquid-vapor coexistence and interfacial properties of short lineal rigid vibrating chains with three tangent monomers in two and three dimensions are calculated. The effect of the range and position of a long ranged square well attractive potential is studied. Orthobaric densities, vapor pressures, surface tensions, and interfacial widths are reported. Two types of molecules are studied. Chains of three tangent hard sphere monomers and chains of three and five tangent hard sphere monomers interacting with a square well attractive potential with λ(∗) = λ∕σ = 1.5 in units of the hard core diameter σ. The results are reported in two and three dimensions. For both types of chains, a long ranged square well attractive potential is located at various positions in the chain to investigate its effect in the properties of the corresponding systems. Results for hard sphere chains are presented for a series of different sizes of λ(∗) between 2.5 and 5. For square well chains the position in the chain of the long ranged potential has no influence in the coexistence and interfacial properties. Critical temperatures increase monotonically with respect to λ(∗) and critical densities decrease systematically for both types of chains. When the long ranged potential is located in the middle monomer of the hard sphere chains no critical point is found for λ(∗) < 2.4. No critical point is found when the long ranged potential is located in one of the extremes of the hard sphere chains.

Assuntos

RESUMO

The effect of flexibility on liquid-vapor and interfacial properties of tangent linear vibrating square well chains is studied. Surface tension, orthobaric densities, vapor pressures, and interfacial thicknesses are reported and analyzed using corresponding states principles. Discontinuous molecular dynamics simulations in two and three dimensions are performed on rigid tangent linear vibrating square well chains of different lengths. In the case of two dimensions, simulation results of completely flexible tangent linear vibrating square well chains are also reported. Properties are calculated for chains of 2-12 monomers. Rigidity is controlled by trapping the first and last monomer in the chain in a vibrating well at half of the distance of the whole chain. Critical property values are reported as obtained from orthobaric densities, surface tensions, and vapor pressures. For the fully flexible chains, the critical temperatures increase with chain length but the effect saturates. In contrast, the critical temperatures increase for the rigid chains until no more critical point is found.

Assuntos

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A four-site rigid water model is presented, whose parameters are fitted to reproduce the experimental static dielectric constant at 298 K, the maximum density of liquid water and the equation of state at low pressures. The model has a positive charge on each of the three atomic nuclei and a negative charge located at the bisector of the HOH bending angle. This charge distribution allows increasing the molecular dipole moment relative to four-site models with only three charges and improves the liquid dielectric constant at different temperatures. Several other properties of the liquid and of ice Ih resulting from numerical simulations with the model are in good agreement with experimental values over a wide range of temperatures and pressures. Moreover, the model yields the minimum density of supercooled water at 190 K and the minimum thermal compressibility at 310 K, close to the experimental values. A discussion is presented on the structural changes of liquid water in the supercooled region where the derivative of density with respect to temperature is a maximum.

RESUMO

The liquid-vapor phase equilibria and surface tension of the TIP4P/2005 water model is obtained by using the Ewald summation method to determine the long range Lennard-Jones and electrostatic interactions. The method is implemented in a straightforward manner into standard simulation programs. The computational cost of using Ewald sums in dispersion interactions of water is estimated in direct simulation of interfaces. The results of this work at 300 K show a dramatic change in surface tension with an oscillatory behavior for surface areas smaller than 5x5sigma(2), where sigma is the Lennard-Jones oxygen diameter. The amplitude of such oscillations substantially decreases with temperature. Finite size effects are less important on coexisting densities. Phase equilibria and interfacial properties can be determined using a small number of water molecules; their fluctuations are around the same size of simulation error at all temperatures, even in systems where the interfaces are separated a few molecular diameters only. The difference in surface tension of this work compared to the results of other authors is not significant (on the contrary, there is a good agreement). What should be stressed is the different and more consistent approach to obtain the surface tension using the Ewald sums for dispersion interactions. There are two relevant aspects at the interface: An adsorption of water molecules is observed at small surface areas and its thickness systematically increases with system size.

RESUMO

The crystal formation of NaCl in water is studied by extensive molecular dynamics simulations. Ionic solutions at room temperature and various concentrations are studied using the SPC/E and TIP4P/2005 water models and seven force fields of NaCl. Most force fields of pure NaCl fail to reproduce the experimental density of the crystal, and in solution some favor dissociation at saturated conditions, while others favor crystal formation at low concentration. A new force field of NaCl is proposed, which reproduces the experimental phase diagram in the solid, liquid, and vapor regions. This force field overestimates the solubility of NaCl in water at saturation conditions when used with standard Lorentz-Berthelot combining rules for the ion-water pair potentials. It is shown that precipitation of ions is driven by the short range interaction between Cl-H pairs, a term which is generally missing in the simulation of ionic solutions. The effects of intramolecular flexibility of water on the solubility of NaCl ions are analyzed and is found to be small compared to rigid models. A flexible water model, extending the rigid SPC/E, is proposed, which incorporates Lennard-Jones interactions centered on the hydrogen atoms. This force field gives liquid-vapor coexisting densities and surface tensions in better agreement with experimental data than the rigid SPC/E model. The Cl-H, Na-O, and Cl-O pair distribution functions of the rigid and flexible models agree well with experiment. The predicted concentration dependence of the electric conductivity is in fair agreement with available experimental data.

Assuntos

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The Wolf method for the calculation of electrostatic interactions is applied in a liquid phase and at the liquid-vapor interface of water and its results are compared with those from the Ewald sums method. Molecular dynamics simulations are performed to calculate the radial distribution functions at room temperature. The interface simulations are used to obtain the coexisting densities and surface tension along the coexistence curve. The water model is a flexible version of the extended simple point charge model. The Wolf method gives good structural results, fair coexistence densities, and poor surface tensions as compared with those obtained using the Ewald sums method.

RESUMO

Molecular dynamics simulations of pure water at the liquid-vapor interface are performed using direct simulation of interfaces in a liquid slab geometry. The effect of intramolecular flexibility on coexisting densities and surface tension is analyzed. The dipole moment profile across the liquid-vapor interface shows different values for the liquid and vapor phases. The flexible model is a polarizable model. This effect is minor for liquid densities and is large for surface tension. The liquid densities increase from 2% at 300 K to 9% at 550 K when the force field is changed from a fully rigid simple point charge extended (SPCE) model to that of a fully flexible model with the same intermolecular interaction parameters. The increases in surface tension at both temperatures are around 11% and 36%, respectively. The calculated properties of the flexible models are closer to the experimental data than those of the rigid SPCE. The effect of the maximum number of reciprocal vectors (h(z) (max)) and the surface area on the calculated properties at 300 K is also analyzed. The coexiting densities are not sensitive to those variables. The surface tension fluctuates with h(z) (max) with an amplitude larger than 10 mN m(-1). The effect of using small interfacial areas is slightly larger than the error in the simulations.