Miscibility limits

Among the properties sought in the solvent are low cost, avadabihty, stabiUty, low volatiUty at ambient temperature, limited miscibility in aqueous systems present in the process, no solvent capacity for the salts, good solvent capacity for the acids, and sufficient difference in distribution coefficient of the two acids to permit their separation in the solvent-extraction operation. Practical solvents are C, C, and alcohols. For industrial process, alcohols are the best choice (see Amyl alcohols). Small quantities of potassium nitrate continue to be produced from natural sources, eg, the caUche deposits in Chile. 

HBSA + HBD HBAD + HBD Always positive deviations, HBSA -1- HBD often limited miscibility H-bonds broken and formed dissociation of HBSA or HBAD liquid most important effect... 

N2 and 0 liquids. The limited miscibility of ozone in oxygen is of practical importance because the dense, ozone-rich layer which settles to the bottom, is easily expld. The mutual solubility of the two liqs decreases when the temp is reduced. Thus, liq ozone and oxygen are completely miscible above 93.2°K (at which temp the total pressure is 1.25 atm), but at 90.2°K (the atm-bp of liq oxygen), there is separation into two layers, containing 17.6 and 67.2 mole % ozone, respectively. 

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). 

In certain cases the organic dibasic acid is not sufficiently reactive for the purpose of polymerisation, and so it is replaced either with its anhydride or its acid chloride. For example polyamides (nylons) are often prepared by reaction of the acid chloride with the appropriate diamine. In the spectacular laboratory prepatation of nylon 6,6 this is done by interfacial polymerisation. Hexamethylenediamine is dissolved in water and adipyl chloride in a chlorinated solvent such as tetrachloromethane. The two liquids are added to the same beaker where they form two essentially immiscible layers. At the interface, however, there is limited miscibility and nylon 6,6 of good molar mass forms. It can then be continuously removed by pulling out the interface. 

The ruthenium-copper and osmium-copper systems represent extreme cases in view of the very limited miscibility of either ruthenium or osmium with copper. It may also be noted that the crystal structure of ruthenium or osmium is different from that of copper, the former metals possessing the hep structure and the latter the fee structure. A system which is less extreme in these respects is the rhodium-copper system, since the components both possess the face centered cubic structure and also exhibit at least some miscibility at conditions of interest in catalysis. Recent EXAFS results from our group on rhodium-copper clusters (14) are similar to the earlier results on ruthenium-copper ( ) and osmium-copper (12) clusters, in that the rhodium atoms are coordinated predominantly to other rhodium atoms while the copper atoms are coordinated extensively to both copper and rhodium atoms. Also, we conclude that the copper concentrates in the surface of rhodium-copper clusters, as in the case of the ruthenium-copper and osmium-copper clusters. 

Reciprocals of the critical temperatures, i.e., the maxima in curves such as those in Fig. 121, are plotted in Fig. 122 against the function l/x +l/2x, which is very nearly 1/x when x is large. The upper line represents polystyrene in cyclohexane and the lower one polyisobutylene in diisobutyl ketone. Both are accurately linear within experimental error. This is typical of polymer-solvent systems exhibiting limited miscibility. The intercepts represent 0. Values obtained in this manner agree within experimental error (<1°) with those derived from osmotic measurements, taking 0 to be the temperature at which A2 is zero (see Chap. XII). Precipitation measurements carried out on a series of fractions offer a relatively simple method for accurate determination of this critical temperature, which occupies an important role in the treatment of various polymer solution properties. 

Two kinds of solid solutions crystallize, a solution of metal 1 in metal 2 and vice versa (limited miscibility). 

Two metals that are chemically related and that have atoms of nearly the same size form disordered alloys with each other. Silver and gold, both crystallizing with cubic closest-packing, have atoms of nearly equal size (radii 144.4 and 144.2 pm). They form solid solutions (mixed crystals) of arbitrary composition in which the silver and the gold atoms randomly occupy the positions of the sphere packing. Related metals, especially from the same group of the periodic table, generally form solid solutions which have any composition if their atomic radii do not differ by more than approximately 15% for example Mo +W, K + Rb, K + Cs, but not Na + Cs. If the elements are less similar, there may be a limited miscibility as in the case of, for example, Zn in Cu (amount-of-substance fraction of Zn maximally 38.4%) and Cu in Zn (maximally 2.3% Cu) copper and zinc additionally form intermetallic compounds (cf. Section 15.4). 

It is possible to employ either multiple individual tanks in series or units containing multiple stages within a single shell (see Figure 8.2). Multiple tanks are more expensive, but provide more flexibility in use, since they are more readily altered if process requirements change. In order to minimize pump requirements and maintenance, one often chooses to allow for gravity flow between stages. When the reactants are of limited miscibility, but differ... 

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]...

The calcium ion is of such a size that it may enter 6-fold coordination to produce the rhombohedral carbonate, calcite, or it may enter 9-fold coordination to form the orthorhombic carbonate, aragonite. Cations larger than Ca2+, e.g., Sr2+, Ba2+, Pb2+, and Ra2 only form orthorhombic carbonates (at earth surface conditions) which are not, of course, isomorphous with calcite. Therefore these cations are incapable of isomorphous substitution in calcite, but may participate in isodimorphous or "forced isomorphous" substitution (21). Isodimorphous substitution occurs when an ion "adapts" to a crystal structure different from its own by occupying the lattice site of the appropriate major ion in that structure. For example, Sr2+ may substitute for Ca2 in the rhombohedral lattice of calcite even though SrC03, strontianite, forms an orthorhombic lattice. Note that the coordination of Sr2 to the carbonate groups in each of these structures is quite different. Very limited miscibility normally characterizes such substitution. 

Ideal solutions are miscible over the whole range of composition. We can show, however, that regular solutions exhibit limited miscibility. From Equations (16.72) and (16.6), we obtain the relation for a regular solution... 

The curve at B/RT= 2.50 shows a convex upward curvature, which indicates that Xi is a two-valued function of a. Thus, in that system, two phases with different compositions exist. Thus, B is a parameter that describes phase separation and limited miscibility. 

Miscibility water, VAc, ether, CH2CI2, toluene limited miscibility alkanes... 

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). 

Catalysis in ionic liquids is not limited to biphasic reaction systems. When the reaction mixture is homogeneous, an extraction solvent that is immiscible with the ionic liquid can be used to remove the product. A number of organic solvents display little or only limited miscibility with these liquids. However, this advantage is of limited value in practice, because one major incentive for using ionic liquids is to avoid volatile organic compounds. 

In the hatched two-phase region of limited miscibility, the system separates heterogeneously into water-rich and nicotine-rich layers. However, at temperatures below the lower consolute point (about 61°C) or above the upper consolute point (about 210°C), the components become miscible in all proportions, resulting in a uniform homogeneous phase. The molecular-level origins of this extraordinary behavior, as well as more general aspects of consolute behavior in other (typically, hydrogen-bonded) systems, remain deeply obscure. 

LiZnCl3 and Li2ZnCl4, both of which are less active catalysts toward epimerization.25 Through the use of tert-butyl methyl ether (TBME) as an alternative to water miscible tetrahydrofiiran (THF), the process route avoids the need to carry out a distillation to exchange solvents, a process that is detrimental to the diastereomeric ratio (dr) of the product. THF is instead added as a co-solvent to aid the reaction conversion but limit miscibility of the aqueous washes during workup. A methylcyclohexane solution of 17 is used directly in the next step. 

Polyfvinyl chloride)/poly(MMA-ran-butyl methacrylate) for 50/50 blends complete miscibility up to 190 C, immiscibility above 240 C, limited miscibility in between these temperatures [49]... 

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. 

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