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The role of cohesive r−4 interactions on the existence of a vapor phase and the formation of vapor–liquid equilibria is investigated by performing molecular simulations for the n-4 potential. The cohesive r−4 interactions delay the emergence of a vapor phase until very high temperatures. The critical temperature is up to 5 times higher than normal fluids, as represented by the Lennard-Jones potential. The greatest overall influence on vapor–liquid equilibria is observed for the 5–4 potential, which is the lowest repulsive limit of the potential. Increasing n initially mitigates the influence of r−4 interactions, but the moderating influence declines for n > 12. A relationship is reported between the critical temperature and the Boyle temperature, which allows the critical temperature to be determined for a given n value. The n-4 potential could provide valuable insight into the behavior of non-conventional materials with both very low vapor pressures at elevated temperatures and highly dipolar interactions.

The vapor‐liquid coexistence properties of mercury are determined from molecular simulation using empirical intermolecular potentials, ab initio two-body potentials, and an effective multibody intermolecular potential. Comparison with experiment shows that pair-interactions alone are inadequate to account for the vapor‐liquid coexistence properties of mercury. It is shown that very good agreement between theory and experiment can be obtained by combining an accurate two-body ab initio potential with the addition of an empirically determined multibody contribution. As a consequence of this multibody contribution, we can reliably predict mercury’s phase coexistence properties and the heats of vaporization. The pair distribution function of mercury can also be predicted with reasonable accuracy. © 2003 American Institute of Physics. @DOI: 10.1063/1.1605381#

The statistical associating fluid theory (SAFT), with monomer and dimer Lennard-Jones (LJ) reference fluids, is used to predict the phase equilibria of pure chains with different lengths. Predictions from the two versions of the theory are compared with Monte Carlo simulation results taken from the literature. We find that the additional structural information from the dimer version of the theory gives predictions in better agreement with simulation values. It is also found that the dimer version provides a much better description of the vapor pressure than the monomer one for long chains for which simulation data are available.

Complete ab initio three-body nonadditivity, including the short-range contributions, has been used for the first time in Monte Carlo simulations of liquid, gas, and liquid–vapor equilibrium of argon. Very good overall agreement with experiment has been achieved for phase equilibrium parameters and pV data of pure phases. The importance of various three-body interaction components has been assessed and their relationship to the liquid structure has been investigated. Although the short-range contributions to the nonadditivity are quite large, cancellations occurring between these and other components at the interatomic distances typical for the liquid make the total three-body nonadditive effect very similar to that given by a simple triple-dipole potential. Performance of the three-term virial equation of state is assessed based on the results of simulations.

Upon incorporation of contributions from long-wavelength density fluctuations by a renormalization-group theory, a crossover equation of state is developed for describing thermodynamic properties of chain fluids. Outside the critical region, the crossover equation of state reduces to the classical equation; inside the critical region, it gives nonclassical universal critical exponents. The crossover equation of state correctly represents phase equilibria and pVT properties of chain fluids in both regions. Good agreement is obtained upon comparisons with computer simulations for square-well chain fluids. As obtained from experimental vapor–pressure and density data, the square-well segment–segment parameters for n-alkanes from ethane to eicosane are linear functions of molecular weight. Calculated thermodynamic properties agree well with experiment for n-alkanes from methane to hexatriacontane.

The conditions, i.e., the interparticle potentials, for which vapor–liquid coexistence is supplanted by microphase separation are examined. The liquid phase is found for potentials with a steeply repulsive core and an attraction of not-too-short a range. To this potential we add a repulsion of longer range than the attraction and find that it breaks up and mixes coexisting bulk vapor and liquid to form a single microphase-separated phase. Our spherical particles are perhaps the simplest continuum model to show a Lifshitz point and microphase separation. In the microphase-separated phase the density is modulated with a period typically ten times the size of the particle’s core. Microphase separation, apart from providing a rich and interesting phase behavior, may have a potential use in nanoelectronic applications.

A Monte Carlo simulation study has been conducted to assess the ability of recently proposed force fields to predict orthobaric densities, second virial coefficients, and P-V-T data for short and long alkanes. A new, modified force field is proposed that provides good agreement with experimental phase equilibrium and second virial coefficient data over wide ranges of temperature and chain length.

The interfacial curvature free energy is shown to cause a significant barrier height correction to the classical nucleation rate. This correction is found to be temperature dependent, but independent of nucleus size. Density functional (DF) calculations are presented for a nonuniform spherical droplet model of the nucleus. Calculations for the surface tension, as a function of nucleus size, and for the interfacial curvature free energy support theoretical predictions and provide an explanation for systematic discrepancies between classical and DF nucleation theories and between the classical theory and experiment.

Void formation and vapor–liquid condensation in deionized dilute aqueous colloidal dispersions were studied for the same latex sample. The former was observed under density‐matched conditions, while the latter was observed when the particles were denser than the dispersion medium. The critical concentration for void formation was found by direct observation using a confocal laser scanning microscope. Vapor–liquid condensation was observed for a dispersion of the same latex particles below the critical concentration. The critical concentrations for both phenomena were in agreement. The results clearly suggest an important role of a long‐range electrostatic attractive interaction on the stability of the colloidal dispersion.

We have obtained the interfacial properties of short rigid-linear chains formed from tangentially bonded Lennard-Jones monomeric units from direct simulation of the vapour-liquid interface. The full long-range tails of the potential are accounted for by means of an improved version of the inhomogeneous long-range corrections of Janecek [J. Phys. Chem. B 110, 6264–6269 (2006)] proposed recently by MacDowell and Blas [J. Chem. Phys. 131, 074705 (2009)] valid for spherical as well as for rigid and flexible molecular systems. Three different model systems comprising of 3, 4, and 5 monomers per molecule are considered. The simulations are performed in the canonical ensemble, and the vapor-liquid interfacial tension is evaluated using the test-area method. In addition to the sur- face tension, we also obtain density profiles, coexistence densities, critical temperature and density, and interfacial thickness as functions of temperature, paying particular attention to the effect of the chain length and rigidity on these properties. According to our results, the main effect of increasing the chain length (at fixed temperature) is to sharpen the vapor-liquid interface and to increase the width of the biphasic coexistence region. As a result, the interfacial thickness decreases and the sur- face tension increases as the molecular chains get longer. The surface tension has been scaled by critical properties and represented as a function of the difference between coexistence densities relative to the critical density., The authors would like to acknowledge helpful discus- sions with F. J. Martínez-Ruiz, E. de Miguel, C. Vega, and A. Galindo. This work was supported by Ministerio de Ciencia e Innovación (MICINN, Spain) through Grant Nos. FIS2010- 14866 (F.J.B.), FIS2009-07923 (J.M.M. and M.M.P.) and FIS2010-22047-C05-05 (L.G.M.D.). J.M.M. also acknowledges Ministerio de Ciencia e Innovación for the FPU Grant with reference AP2007-02172. Further financial support from Proyecto de Excelencia from Junta de Andalucía (Grant No. P07-FQM02884), Consellería de Educacion e Ordenacion Universitaria (Xunta de Galicia), Comunidad Autónoma de Madrid (Grant No. MODELICO-P2009/EPS-1691), and Universidad de Huelva are also acknowledged.

We measured the thicknesses of the vapor–liquid interfaces near the critical points of carbon dioxide (CO2), sulfur hexafluoride (SF6), and trifluoromethane (CHF3) using ellipsometry. The data (when scaled by the refractive index difference Δn and the correlation length ξ) are in agreement with other ellipticity data for binary and pseudobinary mixtures at low pressures. Fully constrained theories of the interface correctly predict the temperature dependence and scaling of the thickness but systematically overestimate the thickness itself by 15%–20%. The theory can be brought into agreement with experiment when an intrinsic interfacial stiffness is added to the theory. A novel feature of the present measurements is that the effects from pressure‐induced window strain were measured and mitigated by using a cylindrically‐symmetric pressure cell with floating seals.

Vapor-liquid interfacial properties of square-well associating fluids are studied via transition-matrix Monte Carlo simulation. Results for one-site and two-site association models are presented. Coexistence properties, surface tension, cluster distribution, density profile, and orientation profile are presented. Molecular association affects the interfacial properties and cluster fractions more than it affects the bulk densities. We observe that the surface tension exhibits a maximum with respect to association strength. This behavior is in agreement with the recent study of Peery and Evans for one site system using a square-gradient approach.

Grand-canonical transition-matrix Monte Carlo and histogram reweighting techniques are used herein to study the vapor-liquid coexistence properties of two-dimensional (2D) flexible oligomers with varying chain lengths (m = 1-8). The phase diagrams of the various 2D oligomers follow the correspondence state (CS) principle, akin to the behavior observed for bulk oligomers. The 2D critical density is not influenced by the oligomer chain length, which contrasts with the observation for the bulk oligomers. Line tension, calculated using Binder's formalism, in the reduced plot is found to be independent of chain length in contrast to the 3D behavior. The dynamical properties of 2D fluids are evaluated using molecular dynamics simulations, and the velocity and pressure autocorrelation functions are investigated using Green-Kubo (GK) relations to yield the diffusion and viscosity. The viscosity determined from 2D non-equilibrium molecular dynamics simulation is compared with the viscosity estimated from the GK relations. The GK relations prove to be reliable and efficient for the calculation of 2D transport properties. Normal diffusive regions are identified in dense oligomeric fluid systems. The influence of molecular size on the diffusivity and viscosity is found to be diminished at specific CS points for the 2D oligomers considered herein. In contrast, the viscosity and diffusion of the 3D bulk fluid, at a reduced temperature and density, are strongly dependent on the molecular size at the same CS points. Furthermore, the viscosity increases and the diffusion decreases multifold in the 2D system relative to those in the 3D system, at the CS points.

In this work, we used constrained lattice density functional theory to investigate how nanoscale seed particles affect heterogeneous vapor-liquid nucleation. The effects of the physical properties of nanoscale seed particles, including the seed size, the strength of seed-fluid attraction, and the shape of the seeds, on the structure of critical nuclei and nucleation barrier were systemically investigated.

The thermodynamic properties of strong short-range attractive Yukawa fluids, k=10, 9, 8, and 7, are determined by combining the slab technique with the standard and the replica exchange Monte Carlo (REMC) methods. A good agreement was found among the coexistence curves of these systems calculated by REMC and those previously reported in the literature. However, REMC allows exploring the coexistence at lower temperatures, where dynamics turns glassy. To obtain the surface tension we employed, for both methods, a procedure that yields the pressure tensor components for discontinuous potentials. The surface tension results obtained by the standard MC and REMC techniques are in good agreement., 6 pages, 4 figures

in the analytical tail corrections used to estimate the in-teractions beyond a spherical potential truncation.The results reported in Ref. 1 for the OPLS and de Pablomodels are not affected by this error, because a sphericaltruncation at 11.5 A without tail corrections was used for theOPLS model and the minimum image convention was em-ployed for the de Pablo model. However, the numerical re-sults reported for the Toxvaerd model and the new modelproposed in Ref. 1 are in error. The values of the Lennard-Jones parameter sare 3.527 A ~Toxvaerd model! and 3.93 A~new model!. Thus, in the former case the tail correctionswere underestimated by a factor of 44 and in the latter by 61.These large factors mean that the results reported in Ref. 1correspond in practice to calculations performed without tailcorrections.We have repeated the calculations for n-pentane,n-octane, and n-dodecane for the new model using propervalues for the tail corrections. The corrected critical pointsand coexistence densities are reported in Tables I and II. Thecritical temperatures are shifted to higher values ~by4to6%!, but the critical densities and pressures are affected to alesser extent. The coexistence curves for the Toxvaerd modelshould shift in a similar way to higher temperatures, i.e.,improving the agreement with the experimental data. Thequalitative results, such as the chain-length dependencies ofthe critical properties, are not affected by the error.

A Fokker–Planck description, based upon hydrodynamic fluctuation theory, of homogeneous nucleation of liquid drops in a vapor phase is reformulated as a one‐dimensional time‐independent scattering problem. This result is then coupled with an elementary analysis of the interactions between distinct fluctuations to determine the concentration of nuclei produced in a nucleation experiment. An extended comparison between the assumptions of this formulation and those of the classical formulation is given.

The ellipticity of the vapor–liquid interface above mixtures of methanol (CH3OH) and deuterated cyclohexane (C6D12) has been measured in the vicinity of the consolute point. The data above and below Tc are consistent with models for the structure of the interface adapted from the theory of Widom and Ramos‐Gomez and Widom. As three‐phase coexistence is approached, the quantity of methanol adsorbed at the interface increases whether ordinary or deuterated cyclohexane is used in the mixture. If ordinary cyclohexane were used for studies within the three‐phase region below Tc, thick, unstable, lenticular films of the methanol‐rich liquid phase would form at the interface and lead to nonreproducible data. In the present measurements lenticular films do not form when deuterated cyclohexane is used in the mixture. (The entire methanol‐rich phase then forms on top.) The gravitationally stable interface above the deuterated mixture can be studied reliably in the three‐phase region below Tc even though the liquid p...

Infra‐red spectra are compared for equivalent absorbing paths of methyl chloroform (1,1,1‐trichloroethane) as vapor at 25°, liquid at −28°, solid I at −40° and solid II at −55°C. The spectra differ in the respects: (1) P‐Q‐R branches observed on some bands in the vapor disappear in the condensed phases, (2) bands generally are sharpened and ones attributable to combinations are intensified by condensation, and (3) three weak bands at 830, 1288, and 1313 cm−1 are present in the condensed states only. Although selection rules for the vapor exclude only the torsional mode and its combinations with other, totally symmetric ones, these weak bands would yield what appears to be the unreasonably high value of 300 cm−1 for the torsional frequency. The fundamental frequencies are discussed briefly and it is pointed out that the spectra are almost entirely explicable upon the basis of only ten of the twelve expected ones for this molecule.