Gas-liquid Critical Temperature Research Articles

Infrared spectroscopic study of super-critical water across the Widom line

When density is varied at a constant temperature just above the gas-liquid critical temperature, the system is found to exhibit large scale density fluctuations which are often rationalized in terms of crossing of a Widom line. We use the discrete variable representation (DVR) scheme to construct the spectroscopic maps for transition frequencies and transition dipoles, and obtain the infrared spectrum of the OH stretch in the said temperature-density region of the phase diagram. The infrared lineshape shows a crossover from Lorentzian to Gaussian as we approach the Widom line. The width of the lineshape displays a pronounced maximum.

Monolayer adsorption of noble gases on graphene

We report our results of simulations of the adsorption of noble gases (Kr, Ar, Xe) on graphene. For Kr, we consider two configurations: supported and free-standing graphene, where atoms are adsorbed only on one or two sides of the graphene. For Ar and Xe, we studied only the case of supported graphene. For the single-side adsorption, we calculated the two-dimensional gas-liquid critical temperature for each adsorbate. We determined the different phases of the monolayers and constructed the phase diagrams. We found two-dimensional incommensurate solid phases for krypton, argon and xenon, and a two-dimensional commensurate solid phase for krypton. For double side adsorption of Kr, we do not see evidence of an ordering transition driven by the interlayer forces.

Predicting the gas-liquid critical temperature of binary mixtures based on the quantitative structure property relationship

Mixtures are used widely in the chemical industry and most chemical processes are designed based on mixtures' critical properties. Therefore, it is extremely important to study critical properties of mixtures. In this work, a quantitative structure property relationship (QSPR) study was employed to predict critical temperatures of binary mixtures. Dragon software was used to calculate molecular descriptors of pure chemicals. Descriptors of mixtures were calculated as mole weighted average. The genetic algorithm (GA) was used to select the optimal subset of descriptors which significantly contribute to the critical temperature of binary mixtures. The multiple linear regression (MLR) method was used to build QSPR models. The validations including internal and external validation were used to check the stability and predictive capability of the obtained models. Three different strategies of external validation, including the “points out”, “mixtures out” and “compounds out”, were used to divide the training set and test set. The applicability domain (AD) for the models was also discussed. All the results have shown that the obtained models had great fitness with the experimental data (R2 were 0.922, 0.925 and 0.900, root mean square error were 0.025, 0.029 and 0.030, average absolute error were 0.014, 0.021 and 0.018, respectively), excellent internal robustness (Q2LMO were 0.733, 0.904 and 0.888), and good predictive ability (Q2ext were 0.888, 0.822 and 0.780). The established models offer a reasonable estimation of the critical temperature of binary mixtures, and hence could provide guidance for chemical process design involving binary mixtures.

Quantitative structure-property relationship (QSPR) study for predicting gas-liquid critical temperatures of organic compounds

Gas-liquid critical temperature is an important parameter of critical state. Organic compounds are under rapid phase changes leading to explosions when conditions are changed at their critical states. Therefore, for safety purposes it is important to study the gas-liquid critical properties for different organic compounds, especially their critical temperatures. In this work, critical temperatures of 692 organic compounds were collected and applied to build quantitative structure-property relationship (QSPR) models. Dragon software was used to obtain their molecular structure information. Methods of multiple linear regression (MLR) and support vector machine (SVM) were applied to build the models, combined with genetic algorithm method. Between these two models, the MLR model has better internal robustness and the SVM model has better goodness-of-fit predictive ability. The results show the developed models have great performance in predicting the gas-liquid critical temperatures. With these models, critical temperatures of organic compounds can be predicted solely based on their molecular structures.

Demixing of a binary mixture in slit-like pores at high temperatures

We investigate the fluid—fluid demixing transitions in the case of adsorption of so-called symmetric binary mixtures in slit-like pores at temperatures higher than the bulk gas—liquid critical temperature. The aim of the study is to determine how the demixing of such mixtures in the pores depends on the bulk phase composition and on the parameters characterizing the pore. The calculations have been carried out by means of a density functional theory. In the case of an equimolar bulk mixture, the demixing transition inside the pore occurs only when the adsorption potentials of both species are identical. The occurrence of this transition is manifested by a cusp in the adsorption isotherm. For nonequimolar bulk phase compositions, the transition can also take place if the adsorption energies of both components are different. However, the difference in the adsorption energies should be small enough, otherwise a continuous demixing takes place. For non-equimolar compositions two branches of the grand canonical potential intersect, whereas for equimolar bulk composition they meet tangentially. We have determined phase behaviour for several model systems.

Study of the solid-liquid-vapour phase equilibria of flexible chain molecules using Wertheim's thermodynamic perturbation theory

The phase diagram of flexible molecules formed by freely-jointed tangent spheres is studied using the first-order thermodynamic perturbation theory of Wertheim for both fluid and solid phases. A mean-field term is added to the free energy of the fluid and solid phase in order to account for attractive dispersion forces. The approach is used to determine the global (solid-liquid-vapour) phase diagrams and triple points of chain molecules of increasing chain length. It is found that the triple point temperature is not affected strongly by the length of the chain, whereas the gas-liquid critical temperature increases dramatically. The asymptotic limits of the phase diagram for infinitely long chains are discussed. The reduced critical temperature of infinitely long chains as given by the mean-field theory is 2/3, and the reduced triple point temperature is 0.048 56, so that an asymptotic value of T t/T c = 0.07284 for the ratio of the triple to critical point temperatures is obtained. This indicates that fully-flexible tangent chains present an enormous liquid range. The proposed theory, while being extremely simple, provides a useful insight into the phase behaviour of chain molecules, showing the existence of finite asymptotic limits for the triple and critical point temperatures. However, since n-alkanes present an asymptotic limit of about T t/T c, = 0.40, the agreement With experiment is not quantitative. This suggests that fully flexible models may not be appropriate to model the solid phases of real chain molecules.

Plane-rotator lattice gas in an external orienting field

We consider a lattice gas of particles that carry a two-component classical spin (i.e., a two-component unit vector). The pair interaction, restricted to nearest neighbors, implicitly includes hard-core repulsion, since each site can host one particle at most, and also contains ferromagnetic interaction between spins as well as nonmagnetic attraction between particles. We study thermodynamic properties and phase diagrams of the model by means of mean-field and two-site cluster approximations, and also by grand-canonical Monte Carlo simulations carried out for one value of the chemical potential; these aim at a quantitative check of the named approximations, as well as a more detailed description. Both mean-field and two-site cluster approximations were found to provide a qualitatively correct description for the thermodynamics of the model with nearest-neighbor interactions, and the predictions of the two-site cluster approximation were in satisfactory quantitative agreement with simulation results. According to both mean-field and two-site cluster treatments, the temperature-density phase diagrams of the model at zero external field are similar to those of the Ising lattice gas, but the effect of external field is quite different. In particular, at zero nonmagnetic attraction the field effect on the gas-liquid critical temperature becomes nonmonotonic. Having compared the field effects in the Ising, plane-rotator, and Heisenberg spin fluids, we conclude that the ability of the external field to decrease the gas-liquid critical temperature weakens with increasing number of spin components.

Effect of an external magnetic field on the gas-liquid transition in the Ising spin fluid

The theoretical phase diagrams of the magnetic (Ising) lattice fluid in an external magnetic field is presented. It is shown that, depending on the strength of the nonmagnetic interaction between particles, various effects of external field on the Ising fluid take place. In particular, at moderate values of the nonmagnetic attraction the field effect on the gas-liquid critical temperature is nonmonotoneous. A justification of such behavior is given. If short-range correlations are taken into account (within a cluster approach), the Curie temperature also depend on the nonmagnetic interaction.

Phase behavior of a two-dimensional quenched–annealed lattice gas with nearest-neighbor attraction. Grand canonical Monte Carlo simulation

We performed Monte Carlo simulation of the phase diagram of a two-dimensional lattice with the nearest-neighbor attraction on a hexagonal lattice. A fraction of sites is rigidly fixed in a disordered configuration in order to mimic quenched absorbent (matrix). It is shown that the phase diagram of this partly quenched system is essentially narrower and the critical temperature is lower, compared with the bulk system, i.e. without matrix. Moreover, a liquid-like branch of the coexistence envelope has a specific shape. It is shown that different, thermodynamically equivalent, matrix configurations result in a slightly different gas-liquid critical temperature. We have analyzed profoundly thermodynamic behavior of a fluid in each copy of the matrix realization. In particular, the behavior of the heat capacity, of the fourth Binder cumulant, compressibility of a fluid along coexistence are analyzed. The pair distribution function of fluid species and its blocking term, due to correlations of fluid particles via matrix subsystem, have been obtained. The blocking distribution function appears to be more long ranged than the total fluid–fluid distribution function. The fluctuations of the blocking effects may serve as a measure of the number of matrix realizations necessary to involve in obtaining adequate statistical averages in the partly quenched systems.

Configurational Properties and Corresponding States in Simple Fluids and Water

Using accurate equations of state, contributions to the configurational entropy and energy of water are calculated and compared with six simple fluids and a model Lennard-Jones (LJ) fluid. The reorganizational entropy is calculated for the first time for water and other fluids. Comparisons are made at equal reduced densities from the triple point to the gas-liquid critical temperature. A corresponding states principle (CSP), suggested by the van der Waals model, is satisfied for certain configurational properties among the simple and LJ fluids. Saturated simple liquids in states of equal free volume fractions have comparable configurational properties. Water deviates significantly from the CSP in some but not all cases. The normal boiling point of eight simple fluids is seen to be an isofree volume state that occurs at a reduced density (ρ/ρc) of 2.63 ± 0.02, or equivalently, an occupied volume fraction of 0.42. A nearly invariant entropy of vaporization at the normal boiling point among eight simple fluids ((9.1 ± 0.3)k) is one manifestation of the CSP (Trouton's rule). The LJ model fluid accurately describes the configurational properties of argon and methane. This result lends credibility to the wide spread use of the LJ potential to describe atom−atom (or site−site) interactions in large and complex molecules.

Anomalous solubility of organic solutes in supercritical water: A molecular explanation

In its supercritical state water exhibits anomalous solvent properties, the most important being its ability to solubilize organic solutes of various sizes which are sparingly soluble under ambient conditions. This phenomenon occurs at high pressure where the density is rather large (0.6-0.9 gm/cm(3)). In this work, a microscopic explanation for the anomalous solubility of organic substances in supercritical water is presented by using the quasi-chemical approximation of Bethe and Guggenheim. The theory suggests the enhanced anomalous solubility arises because the critical temperature of the binary mixture (water plus organic solute) could be slightly lower than the gas-liquid critical temperature of pure water. Several exotic solvent properties may arise due to the subtle interplay between these two critical temperatures.

Phase conjugation by stimulated Brillouin scattering in CClF 3 near the gas–liquid critical temperature

Stimulated Brillouin scattering in CClF3 near the gas–liquid critical temperature has been investigated by using a Q-switched Nd:YAG laser operating at 1.06 μm. The resulting phase-conjugate backscattered wave has a frequency shift that is tunable from ~300 MHz at (Tc − T) = 4.5° to ~150 MHz at (Tc − T) = ~0.1°C. Reflectivities in excess of 60% were observed throughout the temperature region studied.

Critical loci for binary chloroalkane-n-alkane mixtures. II. Dichloromethane with C 3-C 9 n-alkanes

Gas-liquid critical temperature and pressure were determined experimentally at several compositions for the seven binary mixtures formed by dichloromethane with propane through n-nonane. The results are presented on the P– T, T– x and P– x planes which reveal a regular pattern of behaviour as the size of the n-alkane varies.

Critical Loci for binary chloroalkane- n-alkane mixtures.I.1,2-dichloroethane with C 3-C 9n-alkanes

Gas—liquid critical temperature and pressure were determined experimentally at several compositions for the seven mixtures formed by 1,2-dichloroethane with propane through to n-nonane. The results are presented on the p - T, T - x and p - x planes which reveal a regular pattern of behaviour as the size of the n-alkane varies.

The thermodynamic properties of the ternary mixture: carbon disulphide +benzene+hexafluorobenzene

The molar excess volumes and excess enthalpies have been measured at 298.15 K over a range of composition for the ternary mixture: carbon disulphide+benzene+hexafluorobenzene. The results indicate a slight weakening of the average intermolecular interactions over most of the composition range although a slight enhancement is observed at high mole fractions of C 6F 6. Complementary measurements of the gas-liquid critical temperature are also reported.

Surface tension theory of pure liquids and polymer melts

Using the lattice fluid model in conjunction with the Cahn-Hilliard theory of inhomogeneous systems, an accurate method has been developed for calculating the surface tension of nonpolar and slightly polar liquids of arbitrary molecular weight. For low molecular weight liquids, the surface tension can be computed from the triple point to 0.7 of the gas-liquid critical temperature with an error of less than 5%. The theory is somewhat less accurate for polymer melts because it tends to overestimate the surface entropy. The molecular weight dependence of the surface tension and entropy is closely related to the dependence of liquid density on molecular weight.