Closed Miscibility Gap Research Articles

In situ measurement of liquid-liquid equilibria by medium field nuclear magnetic resonance

A new method for non-invasive measurement of liquid-liquid equilibria (LLE) using a compact medium field nuclear magnetic resonance (NMR) spectrometer is presented. Mixing of all components, phase separation, and analysis of the composition of the coexisting phases is performed in situ in an NMR glass tube. Thus, the experimental effort is reduced and errors caused by sampling are eliminated. Furthermore, calibration of the analysis method is not necessary as quantitative information is obtained directly from the NMR spectra. The proposed method for studying LLE in situ can be swiftly conducted in standard chemical laboratories as medium field NMR spectrometer do not require dedicated laboratory infrastructure but enable convenient handling and fast analysis of the samples. In the present work, four non-reactive ternary solvent systems with closed miscibility gap (toluene + acetone + water, diethyl ether + acetone + water, diethyl ether + methanol + water, and acetonitrile + ethanol + cyclohexane) and one reactive ternary system (water + acetic acid + acetic anhydride) were investigated at a temperature of 22.0 °C using 1H medium field NMR spectroscopic measurements. For comparison, the composition of the coexisting phases is also examined for one non-reactive system (acetonitrile + ethanol + cyclohexane) using 13C medium field NMR spectroscopy as well as spatially resolved spectroscopy in a conventional high field NMR spectrometer. The comparison of the results of the present work to literature data shows that the new proposed method enables swift and reliable investigations of LLE.

Thermodynamic description of the Mn-Si-Zn system

The experimental phase equilibria of the Mn-Si-Zn system available in the literature were critically evaluated. Thermodynamic assessment of the Mn-Si-Zn system was then performed in the framework of CALPHAD (CALculation of PHAse Diagram) method on the basis of the experimental data in the literature. The optimal thermodynamic parameters of the ternary system were then obtained, yielding a good agreement with most of the experimental data. The complete liquidus projection and reaction scheme was also presented for the Mn-Si-Zn system. It is noteworthy that a stable closed liquid miscibility gap appears in the computed ternary phase diagrams, even though it is metastable in three boundary binaries. The occurrence of such a closed miscibility gap can be predicted by a criterion considering the general thermodynamic rules and the features of the three constituent binary systems.

Phase diagrams of binary mixtures of patchy colloids with distinct numbers and types of patches: The empty fluid regime

We investigate the effect of distinct bonding energies on the onset of criticality of low functionality fluid mixtures. We focus on mixtures of particles with two and three patches as this includes the mixture where "empty" fluids were originally reported. In addition to the number of patches, the species differ in the type of patches or bonding sites. For simplicity, we consider that the patches on each species are identical: one species has three patches of type A and the other has two patches of type B. We have found a rich phase behavior with closed miscibility gaps, liquid-liquid demixing, and negative azeotropes. Liquid-liquid demixing was found to pre-empt the "empty" fluid regime, of these mixtures, when the AB bonds are weaker than the AA or BB bonds. By contrast, mixtures in this class exhibit "empty" fluid behavior when the AB bonds are stronger than at least one of the other two. Mixtures with bonding energies ε(BB) = ε(AB) and ε(AA) < ε(BB), were found to exhibit an unusual negative azeotrope.

Phase diagrams of binary mixtures of patchy colloids with distinct numbers of patches: the network fluid regime

We calculate the network fluid regime and phase diagrams of binary mixtures of patchy colloids, using Wertheim's first order perturbation theory and a generalization of Flory-Stockmayer's theory of polymerization. The colloids are modelled as hard spheres with the same diameter and surface patches of the same type, $A$. The only difference between species is the number of their patches -or functionality-, $f_A^{(1)}$ and $f_A^{(2)}$ (with $f_A^{(2)}>f_A^{(1)}$). We have found that the difference in functionality is the key factor controlling the behaviour of the mixture in the network (percolated) fluid regime. In particular, when $f_A^{(2)}\ge2f_A^{(1)}$ the entropy of bonding drives the phase separation of two network fluids which is absent in other mixtures. This changes drastically the critical properties of the system and drives a change in the topology of the phase diagram (from type I to type V) when $f_A^{(1)}>2$. The difference in functionality also determines the miscibility at high (osmotic) pressures. If $f_A^{(2)}-f_A^{(1)}=1$ the mixture is completely miscible at high pressures, while closed miscibility gaps at pressures above the highest critical pressure of the pure fluids are present if $f_A^{(2)}-f_A^{(1)}>1$. We argue that this phase behaviour is driven by a competition between the entropy of mixing and the entropy of bonding, as the latter dominates in the network fluid regime.

Complexes of 2,6-dimethylpyridine with water in condensed phases and the dynamical co-operative interactions involving hydrogen bonds

The system 2,6-dimethylpyridine-water possesses peculiar phase properties. Three solid phases: C(7)H(9)N, H(2)O and C(7)H(9)N.H(2)O are mutually immiscible, and the liquid system shows a closed miscibility gap. However, the liquid system remains single-phase in the temperature range from the melting point up to 307 K. The present thermodynamic and IR spectroscopy studies showed that the hydrogen bond co-operativity determines the properties of the system in the condensed phases. In the crystals of the hydrate, H/D isotopic self-organisation was observed. This non-conventional phenomenon consists of a non-random distribution of the hydrogen isotopes in the neighbouring hydrogen bonds. This has never been reported for the multicomponent systems. The effects of dynamical couplings involving hydrogen bonds between water molecules suggested a way in which the C(7)H(9)N.H(2)O molecules were arranged in the crystal lattice: chain-like structures with an alternating sequence of the rings. The pronounced co-operativity of the hydrogen bonds, as well as the stability of the hydrates, led to the explanation of the limiting miscibility of the liquids in terms of interactions between the molecules of H(2)O and C(7)H(9)N.H(2)O. In a similar way to crystals, the 1 : 1 complexes probably associate thanks the O-HO bonds. Thus, water molecules are necessary to cause aggregation in the liquid mixture. The higher the O-HN bond's energy, the stronger is the cooperativity and the more stable are the aggregates. Consequently, the propensity to phase separation is also stronger.

Modelling liquid–liquid equilibria for island type ternary systems

Closed miscibility gaps in ternary liquid mixtures, at constant temperature and pressure, are obtained if phase separations occur only in the ternary region, whilst all binary mixtures involved in the system are completely miscible. This type of behaviour, although not very frequent, has been observed for a certain number of systems. Nevertheless, we have found no information about the applicability of the common activity coefficient models, as NRTL and UNIQUAC, for these types of ternary systems. Moreover, any of the island type systems published in the most common liquid–liquid equilibrium data collections, are correlated with any model. In this paper, the applicability of the NRTL equation to model the LLE of island type systems is assessed using topological concepts related to the Gibbs stability test. A first attempt to correlate experimental LLE data for two island type ternary systems is also presented.

Phase and viscosity behaviour of refrigerant–lubricant mixtures

The understanding of thermophysical properties and phase behaviour of refrigerant–lubricant oil mixtures is highly important for the optimal design of refrigeration and air-conditioning systems. Refrigerant–lubricant mixtures, which are likely to have strong asymmetry, can show complex phase behaviour such as closed miscibility gaps, open miscibility gaps, liquid–liquid–vapour equilibrium, and even barotropic phenomena (density inversions). In fact, the type of phase behaviour that refrigerant–lubricant mixtures may show is linked to the transition between different types of phase diagrams, mainly as a function of the molecular asymmetry. This also has a profound effect in the mixture transport properties. Thus, in this work the general aspects of phase and viscosity behaviour linked to the type of asymmetry found in refrigerant–lubricant mixtures are discussed in the context of phase behaviour phenomenology.

Thermodynamic and Fluid-Dynamic Properties of Carbon Dioxide with Different Lubricants in Cooling Circuits for Automobile Application

The application of carbon dioxide as the refrigerant in vehicle air conditioning requires the development of a suitable lubricant for the quickly rotating compressor. The phase behavior of different synthetic lubricants with carbon dioxide forms the basis for developing a CO 2 - cooling circuit. Furthermore, the influence of carbon dioxide upon the viscosity of the oil is essential for evaluating a lubricant. These data of the binary mixtures have been measured at temperatures between 5 and 100 °C under pressures of up to 150 bar. The qualitative observation of phase behavior took place in a high-pressure view cell, whose complete content is monitored. The phase equilibria have been determined in an autoclave based on a static-analytical method. The viscosity of the lubricant which is saturated with carbon dioxide has been measured with an integrated quartz viscosimeter. The phase behavior of oils with carbon dioxide can be divided into three different types: binary systems with closed miscibility gaps, systems with open miscibility gaps, and systems that show barotropic phenomena. Oils that show barotropic behavior in contact with compressed carbon dioxide are not recommended as lubricants. Oils with complete or limited miscibility with carbon dioxide may be used. The knowledge of phase behavior and the resulting viscosity of gas-saturated lubricants allow one to evaluate the applicability of lubricants in carclimatization systems.

Calculation of Interfacial Properties of Demixed Fluids Using Density Gradient Theory

This paper deals with theoretical investigation of interfacial properties between two coexisting liquid or fluid phases in thermodynamical equilibrium. The Cahn−Hilliard gradient theory was combined with an activity coefficient model (Koningsveld−Kleintjens model) or with an equation of state (Sanchez−Lacombe lattice fluid model). Using an activity coefficient model, only one variable (concentration) changed passing the interface. The interfacial tension between demixed liquids was calculated for two systems (water + ethylene glycol isobutyl ether (EIB) and water + diethylene glycol diethyl ether (DDE)). Adjusting one parameter, the theory yields satisfactory estimates of interfacial tensions as a function of temperature. Applying an equation of state, two variables (density and concentration) change in the interface. In this case the theory gives the density profiles of both components. The calculated density profiles demonstrate the ability of the concept to predict preferential adsorption phenomena in binary systems. The theoretical approach was applied to three systems (aniline + cyclohexane, acetone + carbon disulfide, and stearic acid + propane). If the systems show a closed miscibility gap, the interfacial tension passes a maximum. A comparison of theoretical and experimental interfacial tensions of the system aniline + cyclohexane indicates that the theoretical concept is able to describe the experiment if two parameters were fitted.

Critical behavior of the ionic system n-Propyl-tri-n-butylammonium iodide + water near its upper and lower consolute points

Aqueous solutions of tetraalkylammonium salts, R4NX, with sufficiently large chain length of the alkyl groups (R≥3) show liquid-liquid immiscibilities at elevated temperatures. In general, upper consolute points are observed, but the shapes of the miscibility gaps indicate the existence of lower consolute points suppressed by crystallization. In aqueous solutions of the unsymmetrically substituted salt n-propyl-tri-n-butylammonium iodide (PrBu3NI) the lower consolute point is shifted above the crystallization curve, so that the closed miscibility gap can be monitored completely. We have performed accurate measurements of the coexistence curve near both the lower and upper consolute points of this system. The upper critical point lies at approximately 345.73 K and 0.438 in weight fraction of the salt. The lower critical point lies at 331.72 K and 0.446 in weight fraction. In the vicinity of the critical points an Ising-like critical behavior of the coexistence curve is found. The correct critical exponent of the Ising model is however only observed, if account is made for the peculiarities of re-entrant phase transitions with upper and lower critical points close to one another. The results are discussed with regard to recent observations of an apparent meanfield-like behavior of some ionic fluids.

Calculation of miscibility behavior of multinary polymer blends

AbstractA method for the calculation of phase diagrams (tie lines and binodal, spinodal, critical points and their stability) based exclusively on the Gibbs energy of mixing, δG, is presented which does not require the calculation of the derivatives with respect to the composition. The method is demonstrated for ternary mixtures of two homopolymers and the corresponding copolymer, and for quaternary and quinternary blends of five polymers exhibiting a closed miscibility gap. The advantages of the presented method become most obvious in the mathematical description of measured phase diagrams, where complex composition dependencies of the interaction parameter are observed.

Calculation of phase diagrams not requiring the derivatives of the Gibbs energy for multinary mixtures

AbstractA method for the calculation of phase diagrams (tie lines and binodal, spinodal, critical points and their stability) based exclusively on the Gibbs energy of mixing with no need of its derivatives with respect to the composition variables is extended to multinary mixtures for any number of components. The mathematical description of the (K‐1)‐dimensional phase diagram of a K‐component mixture is presented. The method is demonstrated for a quinternary blend of five polymers exhibiting a closed miscibility gap; all binary, ternary and quaternary subsystems are completely miscible. The phase separation in the quinternary system is caused by very favorable interactions in the ternary subsytem 1/2/3 and the binary subsystem 4/5.

(Liquid + liquid) phase equilibria at high pressures: Pressure-limited closed-loop behaviour of { xc-(CH 2) 4O + (1 − x)H 2O} and the effect of dissolved ammonium sulfate

The (liquid + liquid) phase equilibria of the binary closed-loop system { xc-(CH 2) 4O + (1 − x)H 2O} and additionally the effect of small amounts of dissolved (NH 4) 2SO 4were investigated in the temperature range from 315.8 K to 405 K and at pressures up to 135 MPa. The measurements were performed in an optical high-pressure cell with magnetic stirring according to the synthetic method. Phase separation was detected by visual observation. The experimental p(T)isopleths are presented and discussed. They show that the closed miscibility gap of the salt-free system disappears at (24.7 ± 0.4) MPa where the loop degenerates to a point at (376.9 ± 0.2) K and x≈ 0.22. Thus the system corresponds to type-VI phase behaviour according to the classification of van Konynenburg and Scott. Added ammonium sulfate is shown to have a strong out-salting effect.

Calculation of shear influences on the phase separation of polymer blends exhibiting upper critical solution temperatures

Calculations were performed on the basis of a generalized Gibbs energy of mixing G γ, which is the sum of the Gibbs energy of mixing of the stagnant system and E s, the energy stored in the system during stationary flow. With increasing shear rate γ, the demixing temperatures shift to lower values (shear-induced mixing; diminution of the heterogeneous area), then to higher values (shear-induced demixing), and finally to lower values again before the effects fade out. The details of the rather complex phase diagrams resulting for a given shear rate are primarily determined by a band in the T/χ plane (χ = mole fraction) within which (∂2 E s/∂χ2) T <0 (i.e., E S acts towards phase separation). There are two ranges of γ within which closed miscibility gaps can exist: The more common outer islands are partly or totally situated outside the equilibrium gap (and within the above mentioned band). As γ is raised they break away from the “mainland” at the upper end of the first region of shear-induced mixing and shift to T>UCST where they submerge. Bound to a suitable choice of parameters, a second kind of closed miscibility gaps, the inner islands, which always remain within the equilibrium solubility gap (and outside the band of negative curvature of E S) is additionally observed. This time the islands break away from the “mainland” at the lower end of the first region of shear-induced mixing where they also submerge. The present findings are compared with the results of previous calculations for LCSTs.

Interfacial tension between polyisobutylene and poly(dimethylsiloxane): influence of chain length, temperature, and solvents

The interfacial tension σ was determined by means of spinning-drop and sessile-drop measurements for binary mixtures of low molecular weight and comparatively uniform fractions of poly(isobutylene) (PIB: 1.5, 3.3, and 5.0 kg/mol) plus poly(dimethylsiloxane) (PDMS: 11.3 and 19.6 kg/mol) as a function of temperature (20-80°C). All values lie in the range between 2.5 and 4.0 mN/m. For PIB-1.5/PDMS-11.3 σ increases with T, for PIB-3.3/PDMS-11.3 it passes a maximum, and for PIB-5.0/PDMS-11.3 it decreases; these observations are interpreted in terms of closed miscibility gaps

Refined calculation of the phase separation behavior of sheared polymer blends: closed miscibility gaps within two ranges of shear rates

The phase diagrams of a blend of two homopolymers exibiting a lower critical solution temperature (LCST) have been calculated on the basis of the generalized Gibbs energy of mixing (sum of the Gibbs energy of mixing of the stagnant blend and the energy the mixture can store during stationary flow) for various shear rates γ. Here, the temperature dependence of the energy stored in blends ws taken into account. The calculations yield a complex dependence: with increasing γ the heterogeneous region of the phase diagram is first reduced (shear dusolution), then enlarged (shear demixing), and finally reduced again

Phase equilibrium investigations of fluid systems at high pressures

Abstract

(P,T,x) Surface of liquid-liquid immiscibility in aqueous solutions of tetraalkylammonium salts

Aqueous solutions of higher homologues of tetraalkylammonium salts exhibit liquid-liquid phase separation. A model study of the pressure-temperature-composition (p,T,x) surface of immiscibility of tetraisopentylammonium bromide+H 2 O up to 370 K and 200 MPa yielded a closed miscibility gap at atmospheric pressure with its lower part suppressed by crystallization, and a very unusual hyperbolic shape of the p,T,x surface

Calculation of High Pressure Phase Equilibria of Mixtures of Ethylene, Vinylacetate and an (Ethylene‐Vinylacetate) Copolymer

AbstractSome new measurements of the high pressure demixing behaviour have been done in the system ethylene, vinyl acetate and an (ethylene‐vinylacetate) copolymer. Calculations of phase equilibria using the chain‐of‐rotators equation of state are compared with these new experimental data The miscibility gap of the ternary mixture can be predicted on the basis of a group contribution approach using interaction parameters from low molecular weight binary mixtures only. The model describe the phase separation (besides of polydispersity effects) sufficiently accurate up to high pressures. The derivation of the spinodal equation for ternary mixtures facilitates the calculation procedures applied. The model calculations detect a closed miscibility gap at higher pressures than in the binary subsystem composed of ethylene and the (ethylenevinylacetate) copolymer. Experimental evidence for this behaviour is open, however.

Solvent-induced phase separation in polycarbonate blends PC/TMPC

Compatibility of the polycarbonates of bisphenol A (PC) and tetramethyl bisphenol A (TMPC) was studied in glassy films cast from CH2Cl2 at room temperature, and above the glass-transition temperature. Blends with different compositions and of different molecular weights were analyzed by DSC and small-angle neutron scattering (SANS). Solution studies were made with light scattering and microscopy. Some of the blend films were two-phased when cast at room temperature, but all films were one-phased in equilibrium above the glass transition. The SANS data demonstrated that phase separation in the cast films was not caused by inherent incompatibility of PC and TMPC, but was induced by the solvent CH2Cl2. The effect is caused by a closed miscibility gap in the ternary solution system PC/TMPC/CH2Cl2.