Immiscible liquids, phase diagrams

Figure 7.13 Continuous changes of solid-liquid phase-diagram type from (a) ideal to (f) eutectic, showing (b) positive nonideality that progressively results in (c) a low-melting minimum, (d) a region of solid-solid immiscibility, or (e) complete closing of the solubility gap, culminating in (f) complete eutectic immiscibility.

Figure 4. Charge-transfer processes at the liquid-liquid interface, (a) Probing ET at the liquid-liquid interface with the SECM. The kinetics of ET between two redox couples confined to different immiscible liquid phases can be measured with the SECM operating in the conventional feedback mode. Electroneutrality is maintained by transfer of the common ion (shown as an anion) across the interface (IT). Adapted with permission from Ref. [38]. Copyright 1995, American Chemical Society, (b) Schematic diagram of facilitated ion transfer reaction studied by SECM.

In some systems there is a total miscibility between metal and salt, in others, the metal solubility. All the phase diagrams are characterized by the lowering of the melting point of the salt when the metal is added. This phenomenon is indicative of true solutions. Several systems exhibit a region with two immiscible liquid phases, i.e. the miscibility gap . Systems with miscibility gap show positive deviation from Raoult s law, i.e. the activity coefficient of the salt is larger than unity. Above a certain temperature, which is called the critical temperature of miscibility or above the consolute temperature , salt and metal are completely miscible at all compositions. 

The phase diagram for a mixture of acetone, methyl isobutyl ketone (MIK) and water at 25°C is shown at right. The shaded region (below the curve) represents compositions for which two immiscible liquid phases form, while the unshaded region (above the curve) are compositions for which only one phase forms. The nearly horizontal lines with the shaded region represent tie lies connecting the compositions of two liquid phases that co-exist at equilibrium. 

The equilibrium pressures (0.5—760Torr) of hydrogen existing above mixtures of lithium with lithium hydride (0.5—99 mol% LiH) sealed in iron capsules have been measured from 983 to 1176 K. The isotherms confirm the phase diagram to consist of two immiscible liquid phases with boundaries 25.2 and 98.4 mol% LiH at 983 K and 45.4 and 85.8 mol% LiH at 1176 K. For dilute solutions of lithium hydride in liquid lithium, the relationship between the mole fraction in solution, Xi.iH, and the equilibrium pressure, (phj)S at T(K) is given by... 

In Fig. 9.26, the thermodynamic equilibrium, solid-liquid phase diagram of a binary (species A and B) system is shown for a nonideal solid solution (i.e., miscible liquid but immiscible solid phase). The melting temperatures of pure substances are shown with Tm A and Tm B. At the eutectic-point mole fraction, designated by the subscript e, both solid and liquid can coexist at equilibrium. In this diagram the liquidus and solidus lines are approximated as straight lines. A dendritic temperature T and the dendritic mass fractions of species (p)7(p)s and (p)equilibrium partition ratio kp is used to relate the solid- and liquid-phase mass fractions of species (p)7(p)J and (p)f/(p)f on the liquidus and solidus lines at a given temperature and pressure, that is,... 

Some important aspects of the mixing behavior of selected systems should be mentioned. Non-ideal mixing is not only found for lipids in the ordered gel phases but also for liquid-crystalline lipids. Gel phase immiscibility and phase diagrams which are of the peritectic or eutectic type are regularly observed when the two lipids in the mixture have differences in chain length of 4 or more CH2-groups or when the gel phase stnictures are different. For instance, lipids that form inter-... 

Case I. At sufficiently low pressures, the solubility curve does not intersect the coexistence curve. In this case, the gas solubility is too low for liquid-liquid immiscibility, since the coexistence curve describes only liquid-phase behavior. Stated in another way, the points on the coexistence curve are not allowed because the fugacity f2L on this curve exceeds the prescribed vapor-phase value f2v. The ternary phase diagram therefore consists of only the solubility curve, as shown in Fig. 28a where V stands for vapor phase. 

On an atomic basis, Li is much more soluble in K than vice versa (0.07, 0.43, 1.29 and 1.99 mol% Li compared with 0.007, 0.02, 0.05 and 0.07 mol% K). Although these solubilities are larger than those given by eqs. (a) and (b), both investigations point to a two-immiscible-liquid-type of phase diagram with a consolute T > 1000°C, and with the consolute composition being on the Li-rich side as in the Li-Na system. 

Figure 5.9 Phase diagram showing liquid immiscibility in the Na2BgOi3-Si02 system below the liquidus [16].

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... 

For the niobium-copper system different phase diagrams of the simple eutectic type (with the eutectic point very close to Cu) have been proposed, either with an S-shaped near horizontal liquidus line or with a monotectic equilibrium. It was stated that the presence of about 0.3 at.% O can induce the monotectic reaction to occur, whereas if a lesser amount of oxygen is present no immiscibility gap is observed in the liquid. 

The SEE diagram for a longer alkyl chain IL (2-hydroxy-ethyl)dimethyl undecyloxymethylammonium dicyanamide, [CnOCiEtOH(Ci)2N][dca] (1) in 1-octanol presents a typical SLE/LLE phase diagram—a simple eutectic system with immiscibility in the liquid phase with the UCST. The influence of the [dca] anion in spite of the long alkyl chain makes this salt liquid at room temperature = 283.5 K). Therefore, the choice of the anion can have a huge effect on the phase behavior of ammonium and imidazolium ILs. 

The interesting influence of the cation on the SLE diagram IL + water can be observed [99,100] from the diagrams of ammonium salts [(Cio)2(Q)2N][N03] and [Be(Ci)2C N][N03]. Simple liquidus curve and no immiscibility in the liquid phase for the didecyldimethylammonium cation with the eutectic point shifted strongly to the solvent-rich side was noted (see Figure 1.11). 

The phase diagrams of an ammonium IL, [Be(Ci)2C N] [NO3], with benzene and toluene have shown low immiscibility in the liquid phase with UCST at high IL mole fraction [99]. 

Three-Phase Transformations in Binary Systems. Although this chapter focuses on the equilibrium between phases in binary component systems, we have already seen that in the case of a entectic point, phase transformations that occur over minute temperature fluctuations can be represented on phase diagrams as well. These transformations are known as three-phase transformations, becanse they involve three distinct phases that coexist at the transformation temperature. Then-characteristic shapes as they occnr in binary component phase diagrams are summarized in Table 2.3. Here, the Greek letters a, f), y, and so on, designate solid phases, and L designates the liquid phase. Subscripts differentiate between immiscible phases of different compositions. For example, Lj and Ljj are immiscible liquids, and a and a are allotropic solid phases (different crystal structures). 

Figure 7.11 Schematic T-x phase diagram for a binary A/B solution exhibiting partial immiscibility and liquid-liquid phase separation below the consolute temperature Tc. The horizontal tie-line (heavy solid line) connects the compositions of coexisting A-rich and B-rich liquid phases (small circles) in the lower liquid-liquid coexistence dome. (See text for description of behavior along vertical dashed and dotted lines.)...

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