Pressure-composition Phase Diagram Research Articles

Molecular Simulation of α-Olefins Using a New United-Atom Potential Model: Vapor−Liquid Equilibria of Pure Compounds and Mixtures

A new united atom potential model was developed for the vapor−liquid phase equilibrium of α-olefins ranging from 1-butene to 1-octene. The vapor pressure, normal boiling point, second virial coefficient, heat of vaporization, and structure were calculated with the new model, over a wide temperature range, and compared to experimental data. Critical constants of pure components were reproduced satisfactorily. Furthermore, this model was used to calculate the 1-butene−n-hexatriacontane and 1-hexene−n-hexatriacontane mixture phase equilibria at high pressure. Pressure-composition phase diagrams and lower critical solution temperature curves were constructed and compared to limited literature experimental data and predictions from an equation of state.

Liquid-liquid immiscibility in lipid monolayers

Some binary lipid mixtures form coexisting liquid phases when spread at the air/water interface. This work describes the pressure–composition phase diagrams of binary mixtures of four unsaturated phosphatidylcholines with dihydrocholesterol. These four binary mixtures have critical compositions of approximately fifty mole percent, and average critical exponents of 0.25±0.07. The data can also be approximated by a regular solution thermodynamic model, yielding parameters for the non-ideality of these mixtures.

Theory of lipid monolayers comprised of mixtures of flexible and stiff amphiphiles in athermal solvents: Fluid phase coexistence

Statistical thermodynamic theory developed recently to study self-assembled fluid monolayer films of short diblock copolymeric surfactants at the oil/water interface is applied to spread monolayers comprised of mixtures of amphiphiles that differ only in the flexibility of their chains. Although the oil and water solvents are assumed to be athermal for the two blocks of the amphiphiles, we predict large positive deviations from the ideal free energy of mixing that result in phase separation for sufficiently long chains. Pressure–composition phase diagrams and representative pressure–area isotherms are predicted for monolayer films over a range of chain lengths. By comparing predicted structural properties to the contributions to the mixing free energy, we explore the physical origins of the demixing, and consider its implications for experimental systems such as cholesterol/lipid mixtures in monolayers and bilayers.

Atomistic calculations of hydrogen loading in palladium

Hydrogen loading on nanometer-size palladium clusters and defect crystals are simulated by the Monte Carlo method. The pressure-composition isotherms are calculated. In contrast with simulations of bulk samples, the clusters show no evidence of a phase transformation. Simulation shows that hydrogen is loaded on the surface first and then the interior of the cluster. This behavior is related to the observed narrowing of the plateau in the phase boundary of the pressure-composition phase diagram of nanocrystalline palladium.

A neutron diffraction investigation of the LaNi 5D phase diagram

Phase boundaries in the LaNi 5D system determined by in situ neutron diffraction experiments have shown good agreement with those predicted from the LaNi 5D pressure—composition phase diagram by intersecting extrapolated pressure plateaux and pure phase isotherms. Lattice parameter measurements show strong dilational strain in the α phase and severe compressive strain in the β phase in the two-phase region. In addition, there are systematic changes in linewidths in the two-phase region. Taken together, these provide strong evidence that the partially hydrided powder is microscopically inhomogeneous, i.e. individual micron-sized particles contain coexisting nanoscale α and β phase domains. The alternative model, consisting of a mixture of particles which are wholly α and particles which are wholly β phase, is not supported by these data. We observed hysteresis in the lattice parameters of the β phase and the linewidths of the α phase in the two-phase region, implying a fundamental asymmetry in the D absorption and desorption mechanisms.

Critical lines for type-III aqueous mixtures by generalized corresponding-states models

An algorithm has been developed for calculating the gas-gas critical line of type-III binary fluid mixtures for extended corresponding-states (ECS) models. The algorithm searches for an extremum in pressure on the spinodal curve of an isothermal pressure-composition phase diagram of a binary mixture. The method has been applied to solutions of carbon dioxide and of nitrogen in water. starting at the water critical point. Two variants of ECS have been tested for their ability to represent reliablePVTx data in the nitrogen-water mixture. It is demonstrated that in the latter system both ECS variants produce an artifact in the gas-gas critical line in the range of 0–0.2 mole fraction of nitrogen.

Re-entrant behaviour in pressure—composition phase diagrams of liquid crystals

For liquid crystalline pure compounds exhibiting re-entrant phenomena the chemical potential difference between two successive phases along the isotherm is supposed to have simple pressure dependence. For binary mixtures of such compounds the thermodynamic approach called equal Gibbs energy analysis is applied to the study of their pressure—composition phase diagrams. Many types of isothermal phase diagrams which exhibit re-entrant phase behaviour can be predicted by assuming perfect or non-perfect nature (regular and non-zero excess molar volume) of the solutions. For the mesogenic system hexyloxyoctyloxy cyanobiphenyl, it is shown that the pressure—composition phase diagram is more successfully described with a regular solution hypothesis than with a perfect hypothesis.

Pressure-composition isotherms for nanocrystalline palladium hydride.

We present results from Monte Carlo simulations of the pressure-composition isotherms of nanometer-size palladium clusters under hydrogen loading. In contrast to simulations of bulk samples, the clusters do not show a phase transformation upon hydriding. A study of the hydrogen loading of the clusters shows that they load the surface first and then the interior, with no evidence of a phase transformation. We relate this behavior of our models to the observed narrowing of the plateau (miscibility gap) in the phase boundary of the pressure-composition phase diagram of nanocrystalline palladium.

Phase equilibria of binary Lennard-Jones mixtures with cubic equations of state

Harismiadis, V.I., Panagiotopoulos, A.Z. and Tassios, D.P., 1994. Phase equilibria of binary Lennard-Jones mixtures with cubic equations of state. Fluid Phase Equilibria, 94: 1-18. The ability of cubic equations of state to predict the phase behavior of the Lennard-Jones pure fluid and asymmetric binary mixtures obeying the Lorentz-Berthelot combining rules is examined. It is shown that the commonly used Soave-Redlich-Kwong and Peng-Robinson equations of state give good vapor pressures but incorrect saturated liquid densities for the pure fluid. A modification of the Peng-Robinson equation of state that uses a volume translation term to describe accurately the saturated liquid densities is proposed. For the mixture calculations, we use the van der Waals one-fluid conformal solution mixing rules. All the cubic equations of state tested give excellent pressure-composition phase diagrams for the binary mixtures. Predictions of pressure-density phase envelopes are less satisfactory, but equivalent to those of other more complex equations of state previously developed for the Lennard-Jones fluid. Our results suggest that cubic equations of state can be used to model mixtures of significantly different compounds provided that appropriate combining and mixing rules are used.

Critical mixing in monomolecular films : pressure-composition phase diagram of a two-dimensional binary mixture

The pressure-composition phase diagram of mixed monolayers of dimyristoyl phosphatidylcholine (DMPC) and cholesterol at a temperature of 23.5 °C is derived by numerical analysis of pressure-area isotherms and corroborated by direct fluorescence microscopic observations. We identify a fluid-fluid miscibility gap, terminated by an upper critical point which is accessible near room temperature. We propose that the coexisting mixed phases of cholesterol and DMPC contain the phospholipid in two distinct states. J. Phys. France 51 (1990) 1537-1552 15 JUILLET 1990,

Phase equilibrium diagram of the system MnCrO

A hypothetical oxygen pressure-composition phase diagram and a projection of the oxygen pressure-temperature-composition diagram on the composition triangle were constructed from phase equilibria in the system MnCrO on the basis of the data available in literature. The temperature-composition phase equilibrium diagram of this same system in air was specified. Isomorphism of solid solutions with spinel and hausmannite structure and their intertransformation was studied. Two chemical compounds, MnCr 2O 4 and Cr 4Mn 28O 48, are supposed to exist in the system.

Interpretation of Pressure-Composition Phase Diagrams for CO2/Crude-Oil Systems

Abstract Results of single-contact phase behavior studies for CO2/crude-oil mixtures often are presented as pressurecomposition (P-X) phase diagrams. In such diagrams, regions of pressure and CO2 mole fraction for which more than one phase forms can be identified easily. Phase diagrams for CO2/crude-oil systems can be quite complex, however, since under some conditions such mixtures can form a liquid and a vapor, two liquid phases, or two liquids and a vapor in equilibrium. This paper examines P-X diagrams for two ternary systems, CO2/propane/hexadecane and CO2 /methane/hex- adecane, and describes transitions from one diagram to another that occur with changes in system temperature or changes in oil composition. Nine experimentally determined P-X diagrams are presented for mixtures of Wasson crude oil with CO2. Three different oils, stock-tank oil, stock-tank oil plus 312 scf/bbl [560 std m3/m3] solution gas, and stocktank oil plus 602 scf/bbl [1084 std m3/m3] solution gas, were studied at three temperatures, 90, 105, and 120°F [32, 41, and 49°C]. Comparison of the resulting phase diagrams with those discussed for the simpler ternary systems indicates that the principal features of the crude oil phase diagrams are qualitatively consistent with those of the ternary systems. The results of the CO2/crude-oil experiments indicate that for low-temperature systems (below about 120°F [49°C]), the extrapolated vapor pressure (EVP) of CO2 is a good estimate of the pressure required to produce a dense, relatively incompressible CO2-rich phase that can extract hydrocarbons efficiently from a crude oil. Hence, in the absence of other experimental evidence, the EVP curve can be used as a rough estimate of the minimum miscibility pressure (MMP) for low-temperature reservoirs.

Pressure-composition phase diagrams of cholesterol/lecithin, cholesterol/phosphatidic acid, and lecithin/phosphatidic acid mixed monolayers: A Langmuir film balance study

A highly sensitive Langmuir film balance was built which is free of leakages and which combines very good temperature control, high purity, and quick and easy operation. The isotherms of a pure lipid (dihexadecyl phosphatidylcholine) in the region below and above the critical temperature were studied. Although critical exponents could not be determined more evidence is provided that this is a tricritical point. The monolayer phase diagrams (i) of mixtures of cholesterol with dipalmitoyl phosphatidylcholine (DPPC) and dimyristoyl phosphatidic acid (DMPA), respectively, and (ii) of the DPPC/DMPA alloy were established in the whole accessible region of the lateral pressure π, i.e., between π = 0 and the film breakdown pressures. The pressure-composition phase diagrams were established by analyzing (i) the variation of the bulk lateral compressibility, κ, as a function of both the lateral density ϱ and the composition, (ii) the molecular area, A, as a function of composition, and (iii) by estimating solubilities on the basis of Raoult's law. Breaks in the A-versus-composition plots and discontinuities and (or) breaks in the κ-versus-ϱ plots indicated the location of phase boundaries. The phase diagram of the DPPC/cholesterol mixture decomposes into three regions: (1) a coexistence region of nearly pure crystalline DPPC and a DPPC/cholesterol mixture, (2) a coexistence region of the latter mixture and nearly pure cholesterol, and (3) the mixture of fluid DPPC and cholesterol which seems to be homogeneous. No indication of a special stoichiometric cholesterol/DPPC complex was obtained. At 25°C a phase line runs from 50 mole% cholesterol at π → 0 to 20% at the highest obtainable pressure. At high pressures the 20% phase boundary is nearly vertical, indicating the special role played by this composition for bilayers. Our results compare well with recent calorimetric mixed-bilayer studies. This agreement leads to the conclusion that the mixed-lipid monolayer exhibits long-time stability above zero pressure. The phase diagram of the DMPA/cholesterol system has qualitatively nearly the same form as that of the mixture with DPPC although the position of the phase boundaries is substantially different. The mixture of the single negatively charged DMPA (at pH 5.5 on pure water) with DPPC showed a distinct minimum of the liquidus line in the π-composition diagram at 50 mole% DMPA, demonstrating congruently melting behavior. Strong evidence for a stoichiometric 1/1 DMPA/DPPC complex in the crystalline state is provided. The 1/1 mixture shows an extremely sharp main phase transition with a considerably higher compressibility in the coexistence region as that observed even for pure DMPA. A distinct break in the solid-state region of the isotherm strongly suggests a second-order phase transition which could be an order-disorder transition. A different phase diagram was obtained for bilayer vesicles of the DPPC/DMPA mixture. Only a “cigar-like” diagram with complete miscibility in the solid and the fluid states was observed with the TEMPO spin-label technique.

Pressure-composition phase diagram for hydrides of rare earth-Fe 2 laves compounds

The pressure-composition-temperature phase diagram for the absorption of hydrogen in the cubic Laves phase compound ErFe 2 has been investigated. At least three stable hydrides ErFe 2 H n are found with n ≈ 2, 3.4 and 4. At 20 °C, the equilibrium pressures for these phases are 0.04, 10 and 4000 Torr, respectively. The implications of these results on the preparation of samples for the measurement of physical properties is discussed.

Pressure-induced pyrochlore to perovskite transformations in the Sr 1−xPb xRuO 3 system

The oxygen-deficient pyrochlore Pb 2Ru 2O 7−x transforms to an orthorhombic perovskite near 90 kbar and 1400° C. As x decreases in the system Sr 1−xPb xRuO 3, the pressure for perovskite formation decreases to one atmosphere for x ≈ 0.3. A pressure-composition phase diagram at 1400° C is presented. The quenched high-pressure perovskite phases retransform very slowly at 450° C and very rapidly at 600° C.