Gas-liquid Coexistence Research Articles

Deposition of C 60, C 70 and C 84 fullerene molecules, in benzene via a change of the fluid state, from a gas–liquid two phase region to the critical point

We dissolve C 60, C 70 or C 84 molecules in benzene and change the fluid state from a gas–liquid two-phase region (25.0 °C) to the critical point (289.0 °C) and from the critical point to the original state (25.0 °C) along the gas–liquid coexistence curve. We find that particle-like and whisker-like nano/micro clusters, which are composed of C 60 molecules, deposit on the surface of a silicon substrate placed vertically in C 60/benzene solution during the temperature change, whereas no appreciable clusters are detected on the silicon substrate in either C 70/benzene or C 84/benzene solutions. The clusters, in which fcc lattice structures are formed by C 60 molecules, remain stable in the solution. The present result suggests that C 60 molecules can be separated and extracted from a mixture of C 60, C 70 and C 84 molecules dissolved in benzene.

Gas–liquid coexistence in asymmetric primitive models of ionic fluids

The gas–liquid coexistence of z:1 asymmetric primitive models is studied using the collective variable based theory. Coexistence-curve data are examined in terms of the corresponding-state variables τ ∗ = | T* − T c*|/ T c* and Δρ ∗ = | ρ ∗ − ρ c*|/ ρ c*. Calculations are made for 1:1, 2:1, 3:1 valences and for different values of ion size ratio λ = σ +/ σ − ( λ ≥ 1 and λ ≤ 1). We analyze a dependence of the critical amplitude of the coexistence curve and the asymptotic slope of the coexistence-curve diameter on the ion size ratio at a fixed valence. It is shown that the both characteristics take maximum values in the equisize case and demonstrate a nonmonotonous behavior with λ at z ≥ 2. Besides, the critical amplitude depends on λ very weakly. As expected, the approximation considered in this work predicts the mean-field critical behavior for size- and charge-asymmetric primitive models in the vicinity of the critical point. A comparison with the available theoretical and simulation results is given.

Gas-liquid phase coexistence in quasi-two-dimensional Stockmayer fluids: A molecular dynamics study

The Maxwell construction together with molecular dynamics simulation is used to study the gas-liquid phase coexistence of quasi-two-dimensional Stockmayer fluids. The phase coexistence curves and corresponding critical points under different dipole strength are obtained, and the critical properties are calculated. We investigate the dependence of the critical point and critical properties on the dipole strength. When the dipole strength is increased, the abrupt disappearance of the gas-liquid phase coexistence in quasi-two-dimensional Stockmayer fluids is not found. However, if the dipole strength is large enough, it does lead to the formation of very long reversible chains which makes the relaxation of the system very slow and the observation of phase coexistence rather difficult or even impossible.

Gas-liquid coexistence in a system of dipolar soft spheres

The existence of gas-liquid coexistence in dipolar fluids with no other contribution to attractive interaction than dipole-dipole interaction is a basic and open question in the theory of fluids. Here we compute the gas-liquid critical point in a system of dipolar soft spheres subject to an external electric field using molecular dynamics computer simulation. Tracking the critical point as the field strength is approaching zero we find the following limiting values: T(c)=0.063 and ρ(c)=0.0033 (dipole moment μ=1). These values are confirmed by independent simulation at zero field strength.

Investigation of the Local Structure in Sub and Supercritical Ammonia Using the Nearest Neighbor Approach: A Molecular Dynamics Analysis

Molecular dynamics simulations of ammonia were performed in the (N,P,T) ensemble along the isobar 135 bar and in the temperature range between 250 and 500 K that encompasses the sub and supercritical states of ammonia. Six simple interaction potential models (Lennard-Jones pair potential between the atomic sites, plus a Coulomb interaction between atomic partial charges) of ammonia reported in the literature were analyzed. Liquid-gas coexistence curve, critical temperature, and structural data (radial distribution functions) have been calculated for all models and compared with the corresponding experimental data. After choosing the appropriate potential model, we have investigated the local structure by analyzing the nearest neighbor radial, mutual orientation, and interaction energy distributions. The change in the local structure was traced back to the change of the nonlinear behavior (which is more pronounced at low temperatures) of the average distance between a reference ammonia molecule and its subsequent nearest neighbor. Our results suggest to use the position of the maximum in the fluctuation of the average distance to define the border of the first solvation shell (particularly at high temperature when the minimum of the radial distribution is not well-defined). Indeed, the effect of the temperature on the position of this maximum shows clearly that the spatial extent of the solvation shell increases with a concomitant decrease of the involved number of ammonia molecules. Furthermore, our results show that the signature of the hydrogen bonding is mainly observed for temperature below 300 K. This signature is quantified by a short distance contribution to the closest radial nearest neighbor distribution, by a strong mutual orientation (defined by the angles between the axis joining the nitrogen atoms and the molecular axes) and by a strong attractive character of the total interaction energy.

Prediction and simulation of compressive shocks with lower perturbed density for increasing shock strength in real gases

In this paper we predict and simulate a peculiar type of compressive shock in a real gas modeled by the van der Waals constitutive equation. This shock produces a phase transition from the gas to the liquid phase and, under some special circumstances, back to the gas phase. This shock has a quite unusual property that when the perturbed pressure (the pressure after a shock) increases, the perturbed density decreases and tends to a limit value from above, in contrast with the ordinary well-known compressive shock in which the density tends to the limit value from below. To achieve such a compressive upper shock, it is necessary to choose the unperturbed state (the state before the shock) in a very thin region in the ρp plane, characterized by low values of density and pressure, near the gas-liquid coexistence curve. This region, which depends on the internal degrees of freedom of a constituent molecule, is completely identified.

Elucidating the Association of Water in Wet 1-Octanol from Normal to High Temperature by Near- and Mid-Infrared Spectroscopy

Near- (NIR) and mid-infrared (MIR) absorption spectra of pure and water-saturated 1-octanol were measured along the liquid-gas coexistence curve from ambient temperature and pressure up to 300 degrees C and 10 MPa. Density of the mixture as a function of temperature was assessed by spectral analysis in the NIR region. Two distinct regimes of temperature were identified in the evolution of solvent-subtracted spectra: at normal conditions and up to 180 degrees C, water is organized in multimeric H-bonded aggregates, while at higher temperature, mainly dimers and monomers exist. Water-water and alcohol-alcohol H-bonding interactions play a major role in wet octanol at low temperature, with resulting microheterogeneity and water segregation at a molecular level; on the other hand, water-alcohol interactions are relevant at higher temperature, as also revealed by the estimated density. At low temperature, water dissolved in octanol shows features which are comparable to those of interfacial water obtained by vibrational sum frequency (VSF) spectroscopy, while at high temperature the spectra generally reproduce those of water in the supercritical phase. Overall spectral assignment was supported by ab initio calculations at the MP2 level on small water clusters.

Nuclear symmetry energy effects on liquid-gas phase transition in hot asymmetric nuclear matter

The liquid-gas phase transition in hot asymmetric nuclear matter is investigated within relativistic mean-field model using the density dependence of nuclear symmetry energy constrained from the measured neutron skin thickness of finite nuclei. We find symmetry energy has a significant influence on several features of liquid-gas phase transition. The boundary and area of the liquid-gas coexistence region, the maximal isospin asymmetry and the critical values of pressure and isospin asymmetry all of which systematically increase with increasing softness in the density dependence of symmetry energy. The critical temperature below which the liquid-gas mixed phase exists is found higher for a softer symmetry energy.

Association of limited valence patchy particles in two dimensions

We investigate theoretically the phase behavior of particles with limited valence in twodimensions, by solving the first-order Wertheim theory form. As previously found for threedimensions, in two dimensions also the valence has a strong impact on the phasediagram, controlling the location of the gas–liquid coexistence. On decreasing thevalence, the critical density and temperature decrease while the region of gas–liquidinstability shrinks and vanishes. At low temperatures, the system reaches its groundstate with particles forming a fully bonded network which spans the system.

Internal pressure measurements of the binary 0.7393H 2O + 0.2607NH 3 mixture near the critical and maxcondetherm points

The pressure, P, and their temperature derivative ( ∂ P / ∂ T ) V , of binary mixture 0.7393H 2O + 0.2607NH 3 has been measured in the near- and supercritical regions. Measurements were made in the immediate vicinity of the liquid–gas coexistence curve (one- and two-phase regions) using a high-temperature, high-pressure, nearly constant-volume adiabatic piezo-calorimeter. Measurements were made along 40 liquid and vapor isochores in the range from 120.03 kg m −3 to 671.23 kg m −3 and at temperatures from 403 K to 633 K and at pressures up to 28 MPa. Temperatures at the liquid–gas phase transition curve, T S ( ρ ) , for each measured density (isochore) were obtained using the quasi-static thermograms technique. The expanded uncertainty of the pressure, P, and temperature derivative, ( ∂ P / ∂ T ) V , measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be 0.05% and 0.12–1.5% (depending on temperature and pressure), respectively. The direct measured pressures, P, and temperature derivatives, ( ∂ P / ∂ T ) V , has been used to calculate the internal pressure (or energy-volume coefficient) as ( ∂ U / ∂ V ) T = T ( ∂ P / ∂ T ) V − P . We also measured the temperature derivatives of the internal energy ( ∂ U / ∂ T ) V = C V (isochoric heat capacity) using the same apparatus. The measurements were made at isochoric heating of the system at quasi-equilibrium conditions. The effect of pressure, temperature, and concentration on the internal pressure was studied. The measured values of pressure ( P S ), temperature derivative, ( ∂ P / ∂ T ) V S , temperature ( T S ) and density ( ρ S ) at the saturation curve together with isochoric heat capacity measurements were used to calculate other thermodynamic properties of the mixture at the bubble- and dew-pressure points curves. Some unusual “appendix type shape” behavior of the ( ∂ P / ∂ T ) V and internal pressure, P int , near the critical and maxcondetherm points (in the retrograde region) was found.

Towards the equation of state for dense stellar matter

The thermal nuclear multifragmentation reaction can be used to investigate the nuclear liquid-gas phase coexistence region and its relation to the astrophysical processes such as the collapse of massive stars, and the supernova type-II explosions. We have performed some calculations to estimate stellar equation of state (EOS) on the basis of statistical multifragmentation model.

Structural Relaxation of a Gel Modeled by Three Body Interactions

We report a molecular dynamics simulation study of a model gel whose interaction potential is obtained by modifying the three body Stillinger-Weber model potential for silicon. The modification reduces the average coordination number and suppresses the liquid-gas phase coexistence curve. The low density, low temperature equilibrium gel that can thus form exhibits interesting dynamical behavior, including compressed exponential relaxation of density correlations. We show that motion responsible for such relaxation has ballistic character, and arises from the motion of chain segments in the gel without the restructuring of the gel network.

Dipolar particles in an external field: Molecular dynamics simulation and mean field theory

Using molecular dynamics computer simulation we compute gas-liquid phase coexistence curves for the Stockmayer fluid in an external electric field. We observe a field-induced shift of the critical temperature DeltaTc. The sign of DeltaTc depends on whether the potential or the surface charge density is held constant assuming that the dielectric material fills the space between capacitor plates. Our own as well as previous literature data for DeltaTc are compared to and interpreted in terms of a simple mean field theory. Despite considerable errors in the simulation results, we find consistency between the simulation results obtained by different groups including our own and the mean field description. The latter ties the sign of DeltaTc to the outside constraints via the electric field dependence of the orientation part of the mean field free energy.

A new scheme for perturbation contribution in density functional theory and application to solvation force and critical fluctuations

To surpass a traditional mean field density functional approximation for a perturbation term of interparticle potential function in liquid state, a correlation term is introduced by using weighted density approximation to deal with the perturbation free energy beyond the mean field one. Consequently, a free energy density functional approximation is advanced by combining the mean field term and correlation term with a hard sphere term treated with a Lagrangian theorem-based density functional approximation in the present work. The present free energy density functional approximation is applied in the framework of classical density functional theory (DFT) to a hard core attractive Yukawa (HCAY) fluid subject to external fields; comparison of the resulted predictions for density profiles with available simulation data is favorable for the present DFT approach as a highly accurate predictive approach. Then, the DFT approach is employed to investigate influencing factors for solvation forces between two infinite planar surfaces immersed in an intervening solvent with the HCAY potential function. It is found that (i) critical fluctuations induce negative adsorptions and long-ranged solvation forces; (ii) for narrow slit, the effect of external potential range is kept down; instead strength of the external field contact potential plays dominating role; (iii) state point in the bulk phase diagram, where the most remarkable critical effects are displayed, is the one with a bulk density a little higher than the critical density; remnants of critical fluctuations remain close to the bulk gas-liquid coexistence curve.

Tracking gas-liquid coexistence in fluids of charged soft dumbbells

The existence of gas-liquid coexistence in dipolar fluids with no other contribution to attractive interaction than dipole-dipole interaction is a basic and open question in the theory of fluids. Recent Monte Carlo work by Camp and co-workers indicates that a fluid of charged hard dumbbells does exhibit gas-liquid (g-l) coexistence. This system has the potential to answer the above fundamental question because the charge-to-charge separation, d , on the dumbbells may be reduced to, at least in principle, yield the dipolar fluid limit. Using the molecular-dynamics technique we present simulation results for the g-l critical point of charged soft dumbbells at fixed dipole moment as function of d . We do find a g-l critical point at finite temperature even at the smallest d value (10;{-4}) . Reversible aggregation appears to play less a role than in related model systems as d becomes small. Consequently attempts to interpret the simulation results using either an extension of Flory's lattice theory for polymer systems, which includes reversible assembly of monomers into chains, or the defect model for reversible networks proposed by Tlusty and Safran are not successful. The overall best qualitative interpretation of the critical parameters is obtained by considering the dumbbells as dipoles immersed in a continuum dielectric.

Tuning colloidal interactions in subcritical solvents by solvophobicity: Explicit versus implicit modeling

The distance-resolved effective interaction between two colloidal particles in a subcritical solvent is explored both by an explicit and implicit modeling. An implicit solvent approach based on a simple thermodynamic interface model is tested against grand-canonical Monte Carlo computer simulations using explicit Lennard-Jones solvent molecules. Close to liquid-gas coexistence, a joint gas bubble surrounding the colloidal particle pair yields an effective attraction between the colloidal particles, the strength of which can be vastly tuned by the solvophobicity of the colloids. The implicit model is in good agreement with our explicit computer simulations, thus enabling an efficient modeling and evaluation of colloidal interactions and self-assembly in subcritical solvent environments.

Local structure in sub- and supercritical CO 2: A Voronoi polyhedra analysis study

The local structure of CO 2 along the liquid–gas coexistence curve and along the critical isochore is analyzed through the metric properties of the Voronoi polyhedra (VP) of the molecules, such as the volume, density, and asphericity of the VP as well as the radius of the vacancies between the molecules. The behaviour of these properties (i.e., their distributions as well as the associated average and fluctuation values) provides a clear evidence of the inhomogeneous character of the distribution of the CO 2 molecules in the system when the temperature becomes close the critical one.

The confirmation of the critical point-Zeno-line similarity set from the numerical modeling data for different interatomic potentials

We use numerical simulation data for several model interatomic potentials to confirm the critical point-Zeno-line relations of similarity (CZS) for the liquid branch of the coexistence curve suggested earlier [E. M. Apfelbaum and V. S. Vorob'ev, J. Phys. Chem. B 112, 13064 (2008)]. These relations have been based on the analysis of experimental values for the critical point parameters and liquid-gas coexistence curves for a large number of real substances and two model systems. We show that the numerical modeling data as a whole confirm the CZS in the domain of the existence of liquid state. The deviations from CZS relations take place for two cases: (a) the numerically calculated coexistence curve gets into domain corresponding to solidification; (b) the liquid-vapor transition becomes metastable with respect to freezing.

Experimental Evidence for Two-Step Nucleation in Colloidal Crystallization

We investigated the freezing of colloidal spheres in two dimensions with single-particle resolution. Using micron-size, charge-stabilized polystyrene spheres with a temperature-dependent depletion attraction induced by surfactant micelles, we supercooled an initially amorphous (gaslike) system. Particle motions were monitored as crystallization proceeded. At low concentrations, freezing occurred in a single step in a manner consistent with classical nucleation theory. In other samples two-step nucleation was found, in which amorphous clusters grew to approximately 30 particles, then rapidly crystallized. Measured free energies show the role of metastable gas-liquid coexistence, which also enhanced the rate of nucleation following deeper quenches.

Density functional approach to finite temperature nuclear properties and the role of a momentum dependent isovector interaction

Using a density functional approach based on a Skyrme interaction, thermodynamic properties of finite nuclei are investigated at nonzero temperture. The role of a momentum-dependent isovector term is now studied in addition to volume, symmetry, surface, and Coulomb effects. Various features associated with both mechanical and chemical instability and the liquid-gas coexistence curve are sensitive to the Skyrme interaction. The separated effects of the isoscalar term and the isovector term of momentum-dependent interaction are studied for a modified $\mathrm{SKM}({m}^{*}=m)$ interaction. The frequently used Skyrme interaction SLy4 is one of the cases considered and is shown to have better features for neutron star studies due to a larger symmetry energy.