Immiscible systems, examples

All the phase diagrams reported above show a complete mutual solubility in the liquid state. The formation of a single phase in the liquid state corresponds to behaviour frequently observed in intermetallic (binary and complex) systems. Examples, however, of a degree of immiscibility in the liquid state are also found in selected intermetallic systems. Fig. 2.16 shows a few binary systems in which such immiscibility can be observed (existence of miscibility gaps in the liquid state). All the three... 

The Class I binary diagram is the simplest case (see Fig. 6a). The P—T diagram consists of a vapor—pressure curve (solid 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, Ca , to the critical point of component two,Cp . Additional binary mixtures that exhibit Class I behavior are C02 -hexane and C02 benzene. More complicated behavior exists for other classes, including the appearance of upper critical solution temperature (UCST) lines, two-phase (liquid—liquid) immiscibility lines, and even three-phase (liquid—liquid—gas) immiscibility lines. More complete discussions are available (1,4,22). Additional simple binary system examples for Class III include C02—hexadecane and C02 H20 Class IV, C02 nitrobenzene Class V, ethane— -propanol and Class VI, H20— -butanol. 

Nonaqueous two-phase systems employ two largely immiscible liquids. Examples include dissolving the Lewis base in acetonitrile, nitrobenzene, or 2,5-hexadione and the organometallic acid chloride in a nonpolar organic liquid such as hexane, decane, or carbon tetrachloride. 

Ill-behaved dispersions nsnally drift with time, are sensitive to incoming drop-diameter distribution and to npstream energy inpnt. Examples inclnde most systems with kerosene-based immiscible systems. First approximation allow 20-min residence time or total overflow velocity of 0.35 L/s m. Feed concentration <10% v/v, size as sedimentation-controlled provided surfactants and contamination negligible and mixtnre is not ill-behaved. Use overflow total flow rate velocity of 0.5 to 3 L/s m based on horizontal cross-sectional area with a usual value of 1.4 L/s m. This is for a horizontal cylinder with length-to-diameter ratios of 3.5. Allow both phases to have >20% of the diameter and no less than 0.2 m to ensnre that the exit phases do not become cross contaminated. For process conttol, the minimnm distance between the high and low levels of the interface should be 0.36 m or at least 2 min residence time. 

EXAMPLE 6.6 Data regression with immiscible systems... 

Ill-behaved dispersions usually drift with time are sensitive to incoming drop-diameter distribution and to upstream energy input. Examples include most systems with kerosene-based immiscible systems. 

Extensive sets of experimental data were obtained on spray columns with all the systems mentioned earlier. The work was done using columns with diameters of 80 and 100 mm and lengths of 2 and 3.4 m. Examples of concentration profiles and their agreement with the best fitting simulated curves are shown in Figure 11 for an immiscible system and in Figure 12 for a system with partially miscible solvents. In the latter case three components were transferred in the column and three pairs of concentration profiles simulated. 

A prime example of an immiscible system a mixture of the polar, aliphatic PA66 and the nonpolar, aromatic PPE [74-76]. There is no significant interfacial attraction between PA66 and PPE. 

An unusual feature is found in immiscible systems when the crystallization rates are examined as a function of concentration. An example is given in Fig. 11.38 for the blend of PEKEKK and poly (ether sulfone). Here the isothermal exotherm is plotted against the composition of the noncrystallizing component at various temperatures. The initial addition of poly(ether sulfone) results in a decrease in the crystallization rate. A maximum is reached at about 30% of the added component. Eurther additions of the diluent polymer result in an increase in the crystallization rate. These results contrast rather sharply with those of the miscible blend of the same crystallizing polymer and poly(ether imide).(82) Eor the miscible blend, as illustrated in Fig. 11.39, the peak times increase continuously with added poly(ether imide). Thus, the structure of the immiscible melt, both initially and as crystallization progresses, is an important factor in governing the rate-composition relation for isothermal crystallization. 

The enhancement of the overall crystallization rate with increasing concentration of the noncrystallizing component in immiscible systems is found in many immiscible blends.(80,87,91-93) A striking example is illustrated in Fig. 11.40 for poly(ethylene terephthalate)-poly (carbonate) blends. (92) For blends containing less than 80 wt percent of the poly(carbonate) component the crystallization rate of the poly(ethylene terephthalate) is greatly enhanced. Kinetic studies for... 

Liquid—Hquid equiHbria having more than three components caimot as a rule be represented on a two-dimensional diagram. Such systems are important in fractional extraction, for example, operations in which two consolute components C and D are separated by means of two solvents A and B. For the special case where A and B are immiscible, the linear distribution law can be appHed to components C and D independendy ... 

Figure 4c illustrates interfacial polymerisation encapsulation processes in which the reactant(s) that polymerise to form the capsule shell is transported exclusively from the continuous phase of the system to the dispersed phase—continuous phase interface where polymerisation occurs and a capsule shell is produced. This type of encapsulation process has been carried out at Hquid—Hquid and soHd—Hquid interfaces. An example of the Hquid—Hquid case is the spontaneous polymerisation reaction of cyanoacrylate monomers at the water—solvent interface formed by dispersing water in a continuous solvent phase (14). The poly(alkyl cyanoacrylate) produced by this spontaneous reaction encapsulates the dispersed water droplets. An example of the soHd—Hquid process is where a core material is dispersed in aqueous media that contains a water-immiscible surfactant along with a controUed amount of surfactant. A water-immiscible monomer that polymerises by free-radical polymerisation is added to the system and free-radical polymerisation localised at the core material—aqueous phase interface is initiated thereby generating a capsule sheU (15). 

Figure 4d represents in situ encapsulation processes (17,18), an example of which is presented in more detail in Figure 6 (18). The first step is to disperse a water-immiscible Hquid or soHd core material in an aqueous phase that contains urea, melamine, water-soluble urea—formaldehyde condensate, or water-soluble urea—melamine condensate. In many cases, the aqueous phase also contains a system modifier that enhances deposition of the aminoplast capsule sheU (18). This is an anionic polymer or copolymer (Fig. 6). SheU formation occurs once formaldehyde is added and the aqueous phase acidified, eg, pH 2—4.5. The system is heated for several hours at 40—60°C.

Sedimentation is also used for other purposes. For example, relative motion of particles and Hquid iacreases the mass-transfer coefficient. This motion is particularly useful ia solvent extraction ia immiscible Hquid—Hquid systems (see Extraction, liquid-liquid). An important commercial use of sedimentation is ia continuous countercurrent washing, where a series of continuous thickeners is used ia a countercurrent mode ia conjunction with reslurrying to remove mother liquor or to wash soluble substances from the soHds. Most appHcations of sedimentation are, however, ia straight sohd—Hquid separation. 

The small size of lithium frequently confers special properties on its compounds and for this reason the element is sometimes termed anomalous . For example, it is miscible with Na only above 380° and is immiscible with molten K, Rb and Cs, whereas all other pairs of alkali metals are miscible with each other in all proportions. (The ternary alloy containing 12% Na, 47% K and 41% Cs has the lowest known mp, —78°C, of any metallic system.) Li shows many similarities to Mg. This so-called diagonal relationship stems from the similarity in ionic size of the two elements / (Li ) 76pm, / (Mg ) 72pm, compared with / (Na ) 102pm. Thus, as first noted by Arfvedson in establishing lithium as a new element, LiOH and LiiCOs are much less soluble than the corresponding... 

---