Understanding the phase diagrams of quadrupolar molecules

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236

The vapor-liquid equilibria of three recently proposed water models have been computed using Gibbs-Duhem simulations. These models are TIP4P/Ew, TIP4P/2005, and TIP4P/ice and can be considered as modified versions of the TIP4P model. By design TIP4P reproduces the vaporization enthalpy of water at room temperature, whereas TIP4P/Ew and TIP4P/2005 match the temperature of maximum density and TIP4P/ice the melting temperature of water. Recently, the melting point for each of these models has been computed, making it possible for the first time to compute the complete vapor-liquid equilibria curve from the triple point to the critical point. From the coexistence results at high temperature, it is possible to estimate the critical properties of these models. None of them is capable of reproducing accurately the critical pressure or the vapor pressures and densities. Additionally, in the cases of TIP4P and TIP4P/ice the critical temperatures are too low and too high, respectively, compared to the experimental value. However, models accounting for the density maximum of water, such as TIP4P/Ew and TIP4P/2005 provide a better estimate of the critical temperature. In particular, TIP4P/2005 provides a critical temperature just 7 K below the experimental result as well as an extraordinarily good description of the liquid densities from the triple point to the critical point. All TIP4P-like models present a ratio of the triple point temperature to the critical point temperature of about 0.39, compared with the experimental value of 0.42. As is the case for any effective potential neglecting many body forces, TIP4P/2005 fails in describing simultaneously the vapor and the liquid phases of water. However, it can be considered as one of the best effective potentials of water for describing condensed phases, both liquid and solid. In fact, it provides a completely coherent view of the phase diagram of water including fluid-solid, solid-solid, and vapor-liquid equilibria.

Section: Resultsmentioning

confidence: 99%

Vapor-liquid equilibria from the triple point up to the critical point for the new generation of TIP4P-like models: TIP4P/Ew, TIP4P/2005, and TIP4P/ice

Vega

Abascal²,

Nezbeda³

2006

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“…Considerable effort has been devoted in the last decade to explain this ratio in a number of molecular fluids. 48,49,18 Can we provide a qualitative explanation of the origin of the value 0.14 for T t /T c in fully-flexible LJ chains? The critical temperature of LJ chains increases from T* ϭ1.31 for the monomer up to about 3.5 times this value for very long chains.…”

Section: Global Phase Diagram For Lj Chainsmentioning

confidence: 99%

Extending Wertheim’s perturbation theory to the solid phase of Lennard-Jones chains: Determination of the global phase diagram

Vega

Blas

Galindo

2002

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Wertheim's first order thermodynamic perturbation theory ͑TPT1͒ ͓M. S. Wertheim, J. Chem. Phys. 87, 7323 ͑1987͔͒ is extended to model the solid phase of chains whose monomers interact via a Lennard-Jones potential. Such an extension requires the free energy and contact values of the radial distribution function for the Lennard-Jones reference system in the solid phase. Computer simulations have been performed to determine the structural properties of the monomer Lennard-Jones system in the solid phase for a broad range of temperatures and densities. Computer simulations of dimer Lennard-Jones molecules in the solid phase have also been carried out. The theoretical results for the equation of state, the internal energy, and the sublimation curve of the dimer model in the solid phase are in excellent agreement with the simulation data. The extended theory is used to determine the global ͑solid-liquid-vapor͒ phase diagram of the LJ dimer model; the theoretical estimate of the triple point temperature for the LJ dimer is T*ϭ0.653. Similarly, Wertheim's TPT1 is used to determine the global phase diagram of chains formed by up to 8 monomer units. It is found that the calculated triple point temperature is hardly affected by the chain length, and that for large chain lengths the fluid-solid equilibrium coexistence densities are virtually independent of the number of monomers in the chain when the densities are expressed in monomer units. This is in agreement with experimental indications observed in polyethylene, where both the critical and the triple point temperatures tend to finite values for large molecular weights.

“…Note that this behavior is in relation with the general phase diagram of these molecules 11 and it confirms that the key value in considering quadrupole liquids is not the reduced quadrupole but rather the reduced quadrupole density 15 defined as 5 and 6 and Table III͒ but the very structured SSCF appears only at the highest quadrupole Q *2 = 4.5 and not at Q *2 = 3 even at the lowest temperature, different from the results for the shortest model.…”

Section: Simulation Resultsmentioning

confidence: 66%

“…The most of relevant information giving account of the positions of the atoms in the liquid is, in principle, obtainable from the PCF supposed the intermolecular forces fulfill the above conditions. 10,11 We also find remarkable changes in the SSCF before the solid-liquid phase transition occurs providing a a͒ Author to whom correspondence should be addressed. Moreover, we would make out whether we are in front of a solid or of a determined phase of liquid crystal.…”

Section: Introductionmentioning

confidence: 69%

A new insight on the structural changes of linear quadrupole liquids

Cortada

Calero

Lago

2005

Molecular-dynamics simulations for linear quadrupole liquids are presented. The study is carried out for two different molecular lengths at constant density and a number of temperatures and quadrupole moments. All the simulated thermodynamic states correspond to the condensed phases and some of them show typical features of a solid structure. Furthermore, a change on the preferred intermolecular orientation in the liquid phase is observed from a shifted parallel molecular arrangement to a perpendicular orientation as the quadrupole raises. This change depends on the quadrupole moment as well as on the molecular length and is put in relation with the solid structure of different "diatomic" molecules such as nitrogen, ethane, and acetylene. The appearance of a plastic solid phase at low quadrupole moment and density is also justified. A thoroughly discussion about the availability of classical perturbation theories for this kind of systems is presented.