Liquid phase, limited miscibility

The preparation of air- and water-stable imidazolium-based ionic liquids as solvents for transition metal catalysis have received attention for last 15 years (119). In addition, these ionic liquids show limited miscibility with most of the common organic solvents offering potential for efficient catalyst recovery by facile phase separation. Therefore, ionic liquids have been recently recognized as potential media for the immobilization of catalysts with considerable success in a wide range of laboratory and even industrial-scale reactions (119,120). 

The situation described above is well known, as are its implications and appHcation. If the second liquid with limited miscibility for the polymer is added to a solution of the polymer in the good solvent, one finds that when a certain volume fraction is reached, phase separation begins. One phase largely consists of a mixture of the two low molecular weight liquids and the other of the polymer (in a solution primarily of the good solvent). The polymer is thus precipitated out by the second liquid. 

Both liquid and vapor phases are totally miscible. Conventional vapor/liqiiid eqiiilihriiim. Neither phase is pure. Separation factors are moderate and decrease as purity increases. Ultrahigh purity is difficult to achieve. No theoretical limit on recovery. Liquid phases are totally miscible solid phases are not. Eutectic system. Sohd phase is pure, except at eutectic point. Partition coefficients are very high (theoretically, they can be infinite). Ultrahigh purity is easy to achieve. Recovery is hmited by eutectic composition. 

It has been known since the beginning of recorded history that not all liquids are completely miscible with one another. But only in recent times have we learned that gases may also, under suitable conditions, exhibit limited miscibility. The possible existence of two gaseous phases at equilibrium was predicted on theoretical grounds by van der Waals as early as 1894, and again by Onnes and Keesom in 1907 (see R8). Experimental verification, however, was not obtained until about forty years later, primarily by Krich-evsky, Tsiklis, and their co-workers in Russia (see Gl, SI), by Lindroos and Dodge at Yale (L5), and, more recently, by de Swaan Arons and Diepen at Delft (D3). 

Figure 5.2. Miscibility diagram (and solubility gaps) of water and organic-phase liquids. Solvents not connected by a binding line in Figure 5.2 are immiscible solvents of unlimited miscibility are connected by a solid line, those of limited miscibility by a dotted line [16]...

Following the publication of the first example of fluorous biphase catalysis by Horvath and Rabai in 1994 [1], the immediate focus was to develop catalysts that would exhibit very biased partition coefficients with respect to fluorous and organic solvents. Such liquids are normally immiscible at room temperature. This was done by attaching ponytails of the formula (CH2)m(CF2) -iCF3 (abbreviated (CH2)mRf )> including arrays emanating from silicon atoms [2]. Catalysis was then effected at elevated temperatures, where fluorous and organic solvents are commonly miscible, with prod-uct/catalysis separation at the low-temperature two-phase limit. 

Ionic liquids in general have higher densities than most organic reactants and products. They are also quite different in other physical and chemical properties, as discussed below. Therefore, they usually have limited miscibility with most reactants and products of practical interest. Ionic liquids have been used to carry catalysts that are charged or bear polar functional groups the catalysts are retained in the ionic liquid phase after separation of the product phase. It has even been reported that an inert ionic liquid could be used as a medium to make and stabilize metallic nanoclusters 12). 

Building on earlier work in these laboratories (8) we have overcome the typical mass transfer limitations of phase transfer catalysis for propylene oxidation by the use of 3-component liquid phases based on CO2 expanded liquids (CXLs). For the application to oxidations by aqueous H2O2, the organic component of the CXL is chosen because it is miscible with both dense CO2 and water. In this way, homogeneous systems are produced which decrease mass-transfer limitations and intensify chemical reactions. Previous reports using CXL systems have shown that they enhance the oxidation of the substrate and improve the selectivity at moderate reaction temperatures and pressures (3, 8, 9). 

A general formulation of the problem of solid-liquid phase equilibrium in quaternary systems was presented and required the evaluation of two thermodynamic quantities, By and Ty. Four methods for calculating Gy from experimental data were suggested. With these methods, reliable values of Gy for most compound semiconductors could be determined. The term Ty involves the deviation of the liquid solution from ideal behavior relative to that in the solid. This term is less important than the individual activity coefficients because of a partial cancellation of the composition and temperature dependence of the individual activity coefficients. The thermodynamic data base available for liquid mixtures is far more extensive than that for solid solutions. Future work aimed at measurement of solid-mixture properties would be helpful in identifying miscibility limits and their relation to LPE as a problem of constrained equilibrium. 

Partial Miscibility in the Solid State So far, we have described (solid + liquid) phase equilibrium systems in which the solid phase that crystallizes is a pure compound, either as one of the original components or as a molecular addition compound. Sometimes solid solutions crystallize from solution instead of pure substances, and, depending on the system, the solubility can vary from small to complete miscibility over the entire range of concentration. Figure 14.26 shows the phase diagram for the (silver + copper) system.22 It is one in which limited solubility occurs in the solid state. Line AE is the (solid -I- liquid) equilibrium line for Ag, but the solid that crystallizes from solution is not pure Ag. Instead it is a solid solution with composition given by line AC. If a liquid with composition and temperature given by point a is... 

Liquids dissolved in liquids similarly may form homogeneous solutions. Some liquids have limited solubility in water. Diethyl ether, CH3CH2OCH2CH3 (an organic liquid), is soluble to the extent of 4 g per 100 g of water at 25°C an excess of the diethyl ether will result in a separation of phases with the less dense organic liquid floating on the water. Some liquids mix in all proportions these liquids are completely miscible. The mixture of commercial antifreeze, ethylene glycol, HOCH2CH2OH, and water, used as a coolant in automobile radiators, is such a solution. 

The compositions of the vapor and liquid phases in equilibrium for partially miscible systems are calculated in the same way as for miscible systems. In the regions where a single liquid is in equilibrium with its vapor, the general nature of Fig. 13.17 is not different in any essential way from that of Fig. I2.9< Since limited miscibility implies highly nonideal behavior, any general assumption of liquid-phase ideality is excluded. Even a combination of Henry s law, valid for a species at infinite dilution, and Raoult s law, valid for a species as it approaches purity, is not very useful, because each approximates real behavior only for a very small composition range. Thus GE is large, and its composition dependence is often not adequately represented by simple equations. However, the UNIFAC method (App. D) is suitable for estimation of activity coefficients. 

At the molecular level, appreciable negative deviations from Raoult s law reflect stronger forces of intermolecular attraction in the liquid phase between unlike than between like pairs of molecules. Conversely, appreciable positive deviations result for solutions in which intermolecular forces between like molecules are stronger than between unlike. In this latter case the forces between like molecules may be so strong as to prevent complete miscibility, and the system then forms two separate liquid phases over a range of compositions. Systems of limited miscibility are treated in Sec. 13.9. 

Related Calculations. This illustration outlines the procedure for obtaining coefficients of a liquid-phase activity-coefficient model from mutual solubility data of partially miscible systems. Use of such models to calculate activity coefficients and to make phase-equilibrium calculations is discussed in Section 3. This leads to estimates of phase compositions in liquid-liquid systems from limited experimental data. At ordinary temperature and pressure, it is simple to obtain experimentally the composition of two coexisting phases, and the technical literature is rich in experimental results for a large variety of binary and ternary systems near 25°C (77°F) and atmospheric pressure. Example 1.21 shows how to apply the same procedure with vapor-liquid equilibrium data. 

Miscibility with the Feed Components. In a solute—solvent system exhibiting strong positive deviations from Raoult s Law, the solute has only limited solubility in the solvent. Above a certain solute concentration, two liquid phases are formed. The presence of two liquid phases on the plates of a distillation column leads to instability and operational problems. It is therefore necessary to ensure that the solute concentration in the liquid phase never exceeds its solubility limit. This solubility limit gives a minimum feasible solvent concentration in the section below the solvent addition plate of the primary column. 

Renon and Prausnitz (8) proposed another equation, based also on the local mole fraction concept, which would avoid this limitation and could be applied to partially miscible mixtures. The relationship between activity coefficient and liquid phase composition is given by the equation... 

On mixing solutions of a protein and a polysaccharide, four kinds of mixed solutions can be obtained. Figure 3.1 shows that two single-phase systems (1 and 3) and two-types of biphase systems (2 and 4) can be produced. The two-phase liquid systems 2 and 4 differ in the distribution of biopolymers between the co-existing phases. The biopolymers are concentrated either in the concentrated phase of system 2 because of interbiopolymer complexing, or within separated phases of system 4 because of incompatibility of the biopolymers. The term biopolymer compatibility implies miscibility of different biopolymers on a molecular level. The terms incompatibility or limited thermodynamic compatibility cover both limited miscibility or limited cosolubility of biopolymers (i.e., system 2) and demixing or phase separation... 

The partitioning of a substance between two liquid phases (multistage partitioning, partition chromatography) and the extraction of solids require similar properties of a solvent [50-55]. When a substance has to be partitioned, a solvent system with limited miscibility of the components is required in order that the substance dissolves to a different extent in the two phases. The greater the chemical differences between any two solvents, the more limited their miscibility. Other requirements that the solvent system must fulfil are, inter alia, a favorable partition coefficient (the average partition coefficient of the component mixture should be between ca. 0.2 and 5), as high a separation... 

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