Immiscibility boundary

Miscibility doors can be observed when the homopolymer A is miscible with the homopolymer consisting of segments of type 2. Usually, only very near to the miscibility-immiscibility boundary can a temperature dependence of the phase behavior be seen, i.e. an LCST occurs. Figure 5 shows examples for miscibility doors. Further systems are listed in Table 1. Miscibility doors were also observed for blends of styrene copolymers and polyfvinyl methyl ether) (PVME) (Fig. 6, Table 2). In contrast to PPO/PS systems blends of PVME and PS... 

The sodium-lithium phase diagram has been redetermined over the entire composition range by a combination of resistance and thermal methods. Each method is particularly effective for specific parts of the diagram. Two liquid phases separate below the new consolute temperature 578 K and composition 63 mol% Li. The two-liquid immiscibility boundary extends from 10.1 to 97.0mol%Li at the monotectic temperature 443.90 K. The eutectic occurs at 3.0mol%Li and 365.25 K. Positive deviation from ideality is demonstrated for both sodium- and lithium-rich solutions. The tendency to separate into two immiscible liquids makes itself apparent above 578 K in the temperature coefficient of resistivity of these solutions. Above 623 K, dp/dT changes smoothly from sodium to lithium, but at 580—588 K the coefficient shows a maximum at ca. 68 mol% Li which is attributed to incipient immiscibility. A capillary method was... 

The thermodynamic model presented above only predicts when phase separation will occur. There are, however, two mechanisms by which phase separation can actually occur. The first mechanism is similar to that discussed in an earlier chapter for precipitation of crystals from a melt, where a nucleus is formed and then grows with time. By analogy, this mechanism is termed nucleation and growth. Many of the same factors which control crystal formation also affect phase separation by this mechanism. The second mechanism is termed spinodal decomposition. This mechanism involves a gradual change in composition of the two phases until they reach the immiscibility boundary. 

If we initially heat a batch to a temperature above Tc, we will obtain a homogeneous liquid. If this liquid could be instantaneously quenched to a temperature below Tg, theory then indicates that a homogeneous glass would be formed. Since an instantaneous quench would prevent any mass transport, no phase separation would occur. Of course, we cannot actually cool a melt instantaneously. As the temperature decreases, we will encounter the upper immiscibility boundary and the melt will begin to phase separate. If the melt is very fluid at this... 

Determination of immiscibility diagrams usually involves a large number of experimental measurements. The locus of the immiscibility boundary is often determined by heat treating a series of samples of constant bulk composition at different temperatures, quenching these samples, and determining if the sample is phase separated by visual observation of opalescence. The temperature of immiscibility is defined as the temperature bracketed by samples which either are or are not opalescent. The accuracy with which this temperature can be defined is determined by the temperature interval between treatments, and thus depends on the number of samples used. This process must be repeated for other compositions until the complete immiscibility boundary is well defined. 

A number of techniques have been used in the experimental determination of immiscibility boundaries. The method discussed above is often aided by use of clearing studies. If we consider the microstructure of a sample heat treated at a temperature just below the immiscibility boundary, we soon realize that the detection of phase separation may not be very easy. The lever rule predicts that only a very small quantity of the minor phase will be present, which may make detection difficult. A clearing study relies upon pretreatment of our samples at a lower temperature, where the extent of phase separation is much greater and hence more apparent. The pretreated samples are heated to the region of the immiscibility boundary, held for a specified time, and examined for the disappearance of the opalescence. The immiscibility temperature then brackets the temperatures where opalescence remains or disappears. 

Determination of spinodal boundaries is much more difficult than the determination of immiscibility boundaries. Since light scattering is unaffected by the connectivity of the phases, the observation of opalescence tells us nothing about the morphology of the sample. Direct examination of the microstmcture would certainly reveal the... 

Because almost any diacid can be leaddy converted to the acid chloride, this reaction is quite versatile and several variations have been developed. In the interfacial polymerization method the reaction occurs at the boundary of two phases one contains a solution of the acid chloride in a water-immiscible solvent and the other is a solution of the diamine in water with an inorganic base and a surfactant (48). In the solution method, only one phase is present, which contains a solution of the diamine and diacid chloride. An organic base is added as an acceptor for the hydrogen chloride produced in the reaction (49). Following any of these methods of preparation, the polymer is exposed to water and the acid chloride end is converted to a carboxyhc acid end. However, it is very difficult to remove all traces of chloride from the polymer, even with repeated washings with a strong base. 

In processing, it is frequently necessary to separate a mixture into its components and, in a physical process, differences in a particular property are exploited as the basis for the separation process. Thus, fractional distillation depends on differences in volatility. gas absorption on differences in solubility of the gases in a selective absorbent and, similarly, liquid-liquid extraction is based on on the selectivity of an immiscible liquid solvent for one of the constituents. The rate at which the process takes place is dependent both on the driving force (concentration difference) and on the mass transfer resistance. In most of these applications, mass transfer takes place across a phase boundary where the concentrations on either side of the interface are related by the phase equilibrium relationship. Where a chemical reaction takes place during the course of the mass transfer process, the overall transfer rate depends on both the chemical kinetics of the reaction and on the mass transfer resistance, and it is important to understand the relative significance of these two factors in any practical application. 

The system of distinctions and terminology of the thermodynamic and electric potentials introduced by Lange is still very useful and recommended for describing all electrified phases and interphases. Therefore these potentials can be assigned to metal/solution (M/s), as well as the liquid/liquid boundaries created at the interfaces of two immiscible electrolyte solutions water (w) and an organic solvent (s). 

Z. Koczorowski, in The Interface Structure and Electrochemical Processes at the Boundary Between Two Immiscible Liquids, Ed. by V. E. Kazarinov, Springer Verlag, Berlin, 1987,pp, 77-106. 

Boundary membranes play a key role in the cells of all contemporary organisms, and simple models of membrane function are therefore of considerable interest. The interface of two immiscible liquids has been widely used for this purpose. For example, the fundamental processes of photosynthesis, biocatalysis, membrane fusion and interactions of cells, ion pumping, and electron transport have all been investigated in such interfacial systems. 

The liquid-liquid interface is not only a boundary plane dividing two immiscible liquid phases, but also a nanoscaled, very thin liquid layer where properties such as cohesive energy, density, electrical potential, dielectric constant, and viscosity are drastically changed along with the axis from one phase to another. The interfacial region was anticipated to cause various specific chemical phenomena not found in bulk liquid phases. The chemical reactions at liquid-liquid interfaces have traditionally been less understood than those at liquid-solid or gas-liquid interfaces, much less than the bulk phases. These circumstances were mainly due to the lack of experimental methods which could measure the amount of adsorbed chemical species and the rate of chemical reaction at the interface [1,2]. Several experimental methods have recently been invented in the field of solvent extraction [3], which have made a significant breakthrough in the study of interfacial reactions. 

The determination of the standard Gibbs energies of transfer and their importance for potential differences at the boundary between two immiscible electrolyte solutions are described in Sections 3.2.7 and 3.2.8. 

Potential differences at the interface between two immiscible electrolyte solutions (ITIES) are typical Galvani potential differences and cannot be measured directly. However, their existence follows from the properties of the electrical double layer at the ITIES (Section 4.5.3) and from the kinetics of charge transfer across the ITIES (Section 5.3.2). By means of potential differences at the ITIES or at the aqueous electrolyte-solid electrolyte phase boundary (Eq. 3.1.23), the phenomena occurring at the membranes of ion-selective electrodes (Section 6.3) can be explained. 

For semiconductor electrodes and also for the interface between two immiscible electrolyte solutions (ITIES), the greatest part of the potential difference between the two phases is represented by the potentials of the diffuse electric layers in the two phases (see Eq. 4.5.18). The rate of the charge transfer across the compact part of the double layer then depends very little on the overall potential difference. The potential dependence of the charge transfer rate is connected with the change in concentration of the transferred species at the boundary resulting from the potentials in the diffuse layers (Eq. 4.3.5), which, of course, depend on the overall potential difference between the two phases. In the case of simple ion transfer across ITIES, the process is very rapid being, in fact, a sort of diffusion accompanied with a resolvation in the recipient phase. 

Water-immiscible semisolids, such as oleaginous bases, in direct contact with an aqueous receptor or donor fluid represent a free boundary transport system, with transport occurring across the semisolid/liquid interface. 

Liquid/liquid stirred cell (free boundary method) Mass transport between immiscible phases 6-8... 

In emulsion polymerization, a solution of monomer in one solvent forms droplets, suspended in a second, immiscible solvent. We often employ surfactants to stabilize the droplets through the formation of micelles containing pure monomer or a monomer in solution. Micelles assemble when amphiphilic surfactant molecules (containing both a hydrophobic and hydrophilic end) organize at a phase boundary so that their hydrophilic portion interacts with the hydrophilic component of the emulsion, while their hydrophobic part interacts with the hydrophobic portion of the emulsion. Figure 2.14 illustrates a micellized emulsion structure. To start the polymerization reaction, a phase-specific initiator or catalyst diffuses into the core of the droplets, starting the polymerization. 

When the reactants involved in a step growth polymerization process are mutually immiscible, we can employ an interfacial polymerization method. Two solutions, each containing one of the monomers, are layered one on top of the other. This creates a phase boundary that forms wth the least dense liquid on top. The different monomers can then meet and polymerize at the interface. A commonly demonstrated example of this is the manufacture of nylon 610 by the interfacial reaction between an aqueous solution of hexamethylenediamine with sebacoyl chloride dissolved in carbon tetrachloride. Because the reaction only occurs at the interface, it is possible to pull the products from this interface to isolate the final product. 

---