Binary mixture critical behavior

An occurrence of several critical points for monocomponent fluid leads to complication of binary mixture phase behavior. Following Varchenko s approach", generic phenomena encountered in binary mixtures when the pressure p and the temperature T change, correspond to singularities of the convex envelope (with respect to the x variable) of the front (a multifunction of the variable x) representing the Gibbs potential G(p,T,x). Pressure p and temperature T play the role of external model parameters like ki2. A total... 

The Class I binary diagram is the simplest case (see Fig. 6a). The P—T diagram consists of a vapor—pressure curve (soHd line) for each pure component, ending at the pure component critical point. The loci of critical points for the binary mixtures (shown by the dashed curve) are continuous from the critical point of component one, C , to the critical point of component two,Cp . Additional binary mixtures that exhibit Class I behavior are CO2—/ -hexane and CO2—benzene. More compHcated behavior exists for other classes, including the appearance of upper critical solution temperature (UCST) lines, two-phase (Hquid—Hquid) immiscihility lines, and even three-phase (Hquid—Hquid—gas) immiscihility lines. More complete discussions are available (1,4,22). Additional simple binary system examples for Class III include CO2—hexadecane and CO2—H2O Class IV, CO2—nitrobenzene Class V, ethane—/ -propanol and Class VI, H2O—/ -butanol. 

There is a large body of experimental work on ternary systems of the type salt + water + organic cosolvent. In many cases the binary water + organic solvent subsystems show reentrant phase transitions, which means that there is more than one critical point. Well-known examples are closed miscibility loops that possess both a LCST and a UCST. Addition of salts may lead to an expansion or shrinking of these loops, or may even generate a loop in a completely miscible binary mixture. By judicious choice of the salt concentration, one can then achieve very special critical states, where two or even more critical points coincide [90, 160,161]. This leads to very peculiar critical behavior—for example, a doubling of the critical exponent y. We shall not discuss these aspects here in detail, but refer to a comprehensive review of reentrant phase transitions [90], We note, however, that for reentrant phase transitions one has to redefine the reduced temperature T, because near a given critical point the system s behavior is also affected by the existence of the second critical point. An improper treatment of these issues will obscure results on criticality. 

Compared to binary mixtures of low molecular fluids, the critical behavior of polymer blends has been much less explored so far. However, a number of interesting static and dynamic critical phenomena in polymer blends attract increasing attention [4, 5], Neutron, X-ray, and static light scattering experiments belong to the major techniques for characterizing the static properties of polymer blends. Photon correlation spectroscopy (PCS) has traditionally been the method of choice for the investigation of the dynamics of critical [6-9] and noncritical [10-12] polymer blends. 

The potential of supercritical extraction, a separation process in which a gas above its critical temperature is used as a solvent, has been widely recognized in the recent years. The first proposed applications have involved mainly compounds of low volatility, and processes that utilize supercritical fluids for the separation of solids from natural matrices (such as caffeine from coffee beans) are already in industrial operation. The use of supercritical fluids for separation of liquid mixtures, although of wider applicability, has been less well studied as the minimum number of components for any such separation is three (the solvent, and a binary mixture of components to be separated). The experimental study of phase equilibrium in ternary mixtures at high pressures is complicated and theoretical methods to correlate the observed phase behavior are lacking. 

Several different measurements were made as a result of the different phase behavior observed for the two binary mixtures. Gas-solid equilibrium was observed for mixtures of TPP and pentane at conditions near the critical point of pentane. Hence, solid solubilities for TPP in supercritical pentane were measured. 

In recent years, studies of the phase behavior of salt-water systems have primarily been carried out by Russian investigators (headed by Prof. Vladimir Valyashko) at the Kurnakov Institute in Moscow, particularly for fundamental understanding of the phase behavior of such systems. Valyashko [37,39,42,43], Ravich [38], Urosova and Valyashko [40], and Ravich et al. [41] have given a classification of the existence of two types of salts, depending on whether the critical behavior is observed in saturated solutions. Type 1 does not exhibit critical behavior in saturated solutions. The classic example of Type 1 is the NaCl-water system and has been studied by many authors [36,37,44-47]. The Type 2 systems exhibit critical behaviors in saturated solutions, and therefore have discontinuous solid-liquid-vapor equilibria. Table 1 shows the classification of binary mixtures of salt-water systems. 

Binary-Liquid Option. As an alternative to this study of critical behavior in a pure fluid, one can use quite a similar technique to investigate the coexistence curve and critical point in a binary-liquid mixture. Many mixtures of organic liquids (call them A and B) exhibit an upper critical point, which is also called a consolute point. In this case, the system exists as a homogeneous one-phase solution for all compositions if Tis greater than... 

Solid-Fluid Equilibria The phase diagrams of binary mixtures in which the heavier component (the solute) is normally a solid at the critical temperature of the light component (the solvent) include solid-liquid-vapor (SLV) curves which may or may not intersect the LV critical curve. The solubility of the solid is very sensitive to pressure and temperature in compressible regions where the solvent s density and solubility parameter are highly variable. In contrast, plots of the log of the solubility versus density at constant temperature exhibit fairly simple linear behavior. 

The full extent and variety of the phase behavior for water-isopropanol-C02 mixtures observed experimentally and calculated with the Peng-Robinson equation of state was not anticipated based on known phase behavior for the constituent binary mixtures or similar ternary mixtures. These results suggest that multiphase behavior for related model surfactant systems could also be complex. Measurements of all the critical endpoint curves, the tricritical points, and secondary critical endpoint for such systems would be tedious and are extremely difficult. However, by coupling limited experimental data with a thermodynamic model based on this cubic equation of state, complex multiphase behavior can be comprehensively described. 

It is interesting to investigate the lineshape behavior of a long-living resonance near the phase separation critical point in a binary mixture. In binary mixtures, the differing molecules push out the coordinate sphere of the... 

Besides these thermodynamic criteria, the most common approach used in the literature is based on the operation at pressures above the binary (liquid - SC-CO2) mixture critical point, completely neglecting the influence of solute on VLEs of the system. But, the solubility behavior of a binary supercritical COj-containing system is frequently changed by the addition of a low volatile third component as the solute to be precipitated. In particular, the so-called cosolvency effect can occur when a mixture of two components solvent+solute is better soluble in a supercritical solvent than each of the pure components alone. In contrast to this behavior, a ternary system can show poorer solubility compared with the binary systems antisolvent+solvent and antisol-vent+solute a system with these characteristics is called a non-cosolvency (antisolvent) system. hi particular, in the case of the SAS process, they hypothesize that the solute does not induce cosolvency effects, because the scope of this process lies in the use of COj as an antisolvent for the solute, inducing its precipitation. 

In the light of these considerations, a different approach based on ternary system thermodynamics could be considered. However, the phase behavior of temaiy systems could be very complex and there is a considerable lack of data on ternary systems containing a component of low volatility therefore, a possible compromise could be to consider that the solute addition can produce the shift of the mixture critical point (MCP) (i.e., the pressure at which the ternary mixture is supercritical) with respect to binary system VLEs and the modification of this kind of system that is formed according to the van-Konynenburg and Scott classification. ... 

In work with a ternary mixture of ethylene, naphthalene, and hexachloroethane, van Gunst et. al.(8) observed an analogous temperature depression of the four phase (S-S-L-G) minimum melting solid line. As shown in Figure 3, such system exhibited two ternary critical end points, designated as the "p (lower temperature) and "q" (higher temperature) points. Ternary systems may display such an interruption of the critical locus if the binaries mixtures of the individual solids with the solvent gas also have interrupted critical loci. The existence of the UCEP and LCEP for each of the binaries does not, however, mandate such behavior for the ternary critical locus. 

Heimburg, T., Mirzaev, S.Z., and Kaatze, U. Heat capacity behavior in the critical region of the ionic binary mixture ethylammonium nitrate - n-octanol. Phys. Rev. E, 2000, 62, p. 4963-76. 

Type II Ternary (Liquid-Liquid-Fluid). Type II ternary phase behavior is characterized by the appearance within the P-x prism of the L-L-V region (13), which does not extend to the sides of the prism, each side depicting the P-x behavior of a constituent binary, as schematically shown in Figure 5c. At pressures below the critical pressure of the SCF component, two liquid phases appear along with an L-L-V region and expand considerably with increasing pressure. At pressures above the mixture critical pressure of the SCF component and one liquid component, the L-L-V region disappears and the phase behavior becomes identical with that for a type I ternary system. 

In ref. ( °) the applications of the analogous dependences (eq. (1) and (2)) for the pressure evolution of the glass temperature and the melting temperature in supercooled liquids were shown. It is noteworthy that both alcohols and water are important technological agents, also used as additives to the CO2 basic critical system. For the discussed case of binary mixtures of limited miscibility the critical behavior is the inherent feature of the system containing water and alcohol or nitrobenzene or nitrotoluene and alkanes, even under atmospheric pressure. When critical binary mixtures are considered as the base for the SCF technologies, no additional component is needed. 

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