Phase separation types

In most of the work, the physicochemical behaviour of the formulations was studied by gas chromatographic estimation at intervals of the residual pheromone remaining in formulations sprayed onto filter papers. The latter were of the silicone-treated "phase-separating" type as these best simulated a leaf surface. When the formulations were exposed in a laboratory wind-tunnel there was little pheromone loss other than by release, at least for the monounsaturated acetates used in most of the preliminary work, and such analyses provided accurate information on release rates under these conditions. However, use of this technique in the field showed that loss of pheromone was very much more rapid than under comparable conditions of temperature and windspeed in the wind-tunnel (2). These results were taken to indicate that there was significant loss of pheromone by degradation under field conditions. 

The more common situation, Illustrated in Fig. lb, is where the two polymers are immiscible but form a homogenous solution in a common solvent. In this case, film casting along the line C to D generates a variety of structures depending on the selected solvent (and its interaction parameters X o Xij), the chemical nature of the two polymers (X23) as well as on the kinetics of the process. Three phase-separated types of morphologies can result co-continuous, dispersed, and layered. 

Polymer phase Inorganic phase Separation type Separation target References... 

In Winsor s type 3 systems (r = 1), the surfactant s affinity for the oil and the water phases is balanced. The interface will be flat. A type 3 nSOW system can have one, two or three phases depending on its composition. In the multiphase region the system can be (a) two phase—a water phase and an oleic microemulsion (b) two phase—an oil phase and an aqueous microemulsion (c) three phase—a water phase containing surfactant monomers at CMC, an oil phase containing surfactant at CMC and a surfactant phase . The surfactant phase may have a bicontinuous structure, being composed of cosolubilised oil and water separated from each other by an interfacial layer of surfactant. The surfactant phase is sometimes called the middle phase because its intermediate density causes it to appear between the oil and the water phases in a phase-separated type 3 nSOW system. 

Matsushita, T Kobayashi, H. Phase separation-type liquid detergent compositions. Jpn. Kokai Tokkyo Koho JP 62138595,1987. 

Technologies on PVDF based GPEs have been disclosed by Bellcore [3,4], A polymer matrix is a copolymer of vinyMene fluoride (VDF) and hexafluoro-propylene (HFP) and the LiPFg solution in EC/DMC (dimethyl carbonate) (2 1) is added to the matrix to make up a GPE of the phase separation type. HFP plays an important role in the reduction of the crystallinity of the resulting copolymer, increasing its capacity to hold the electrolyte solution. It is claimed that ionic conductivity of their GPE is almost 10 S-cm ... 

Most LB-forming amphiphiles have hydrophobic tails, leaving a very hydrophobic surface. In order to introduce polarity to the final surface, one needs to incorporate bipolar components that would not normally form LB films on their own. Berg and co-workers have partly surmounted this problem with two- and three-component mixtures of fatty acids, amines, and bipolar alcohols [175, 176]. Interestingly, the type of deposition depends on the contact angle of the substrate, and, thus, when relatively polar monolayers are formed, they are deposited as Z-type multilayers. Phase-separated LB films of hydrocarbon-fluorocarbon mixtures provide selective adsorption sites for macromolecules, due to the formation of a step site at the domain boundary [177]. 

Much later, experiments on model colloids revealed tliat tire addition of polymer may eitlier induce a gas-liquid type phase separation or a fluid-solid transition [94, 95, 96 and 97]. Using perturbation tlieories, tliese observations could be accounted for quite well [97, 98]. 

Thus far all the separations we have considered involve a mobile phase and a stationary phase. Separation of a complex mixture of analytes occurs because each analyte has a different ability to partition between the two phases. An analyte whose distribution ratio favors the stationary phase is retained on the column for a longer time, thereby eluting with a longer retention time. Although the methods described in the preceding sections involve different types of stationary and mobile phases, all are forms of chromatography. 

Another example is the purification of a P-lactam antibiotic, where process-scale reversed-phase separations began to be used around 1983 when suitable, high pressure process-scale equipment became available. A reversed-phase microparticulate (55—105 p.m particle size) C g siUca column, with a mobile phase of aqueous methanol having 0.1 Af ammonium phosphate at pH 5.3, was able to fractionate out impurities not readily removed by hquid—hquid extraction (37). Optimization of the separation resulted in recovery of product at 93% purity and 95% yield. This type of separation differs markedly from protein purification in feed concentration ( i 50 200 g/L for cefonicid vs 1 to 10 g/L for protein), molecular weight of impurities (<5000 compared to 10,000—100,000 for proteins), and throughputs ( i l-2 mg/(g stationary phasemin) compared to 0.01—0.1 mg/(gmin) for proteins). 

Blends of poly(vinyl chloride) (PVC) and a-methylstyrene—acrylonitrile copolymers (a-MSAN) exhibit a miscibiUty window that stems from an LCST-type phase diagram. Figure 3 shows how the phase-separation temperature of 50% PVC blends varies with the AN content of the copolymer (96). This behavior can be described by an appropriate equation-of-state theory and interaction energy of the form given by equation 9. 

Solvent Process. In the solvent process, or solvent cook, water formed from the reaction is removed from the reactor as an a2eotropic mixture with an added solvent, typically xylene. Usually between 3 to 10 wt % of the solvent, based on the total charge, is added at the beginning of the esterification step. The mixed vapor passes through a condenser. The condensed water and solvent have low solubiUty in each other and phase separation is allowed to occur in an automatic decanter. The water is removed, usually to a measuring vessel. The amount of water collected can be monitored as one of the indicators of the extent of the reaction. The solvent is continuously returned to the reactor to be recycled. Typical equipment for this process is shown in Figure 2. The reactor temperature is modulated by the amount and type of refluxing solvent. Typical conditions are ... 

Emulsification is essential for the development of all types of skin- and hair-care preparations and a variety of makeup products. Emulsions (qv) are fine dispersions of one Hquid or semisoHd ia a second Hquid (the contiauous phase) with which the first substance is not miscible. Generally, one of the phases is water and the other phase is an oily substance oil-ia-water emulsions are identified as o/w water-ia-oil emulsions as w/o. When oil and water are mixed by shaking or stirring ia the absence of a surface-active agent, the two phases separate rapidly to minimize the iaterfacial energy. Maintenance of the dispersion of small droplets of the internal phase, a requirement for emulsification, is practical only by including at least one surface-active emulsifier ia the oil-and-water blend. 

Truly porous, synthetic ion exchangers are also available. These materials retain their porosity even after removal of the solvent and have measurable surface areas and pore size. The term macroreticular is commonly used for resins prepared from a phase separation technique, where the polymer matrix is prepared with the addition of a hq-uid that is a good solvent for the monomers, but in which the polymer is insoluble. Matrices prepared in this way usually have the appearance of a conglomerate of gel-type microspheres held together to... 

The second type of mass-exchange units is the differential (or continuous) contactor. In this category, the two phases flow through the exchanger in continuous contact throughout without intermediate phase separation and recontacting. Examples of differential contactors include packed columns (Fig. 2.6), spray towers (Fig. 2.7), and mechanically agitated units (Fig. 2.8). 

Another example of phase transitions in two-dimensional systems with purely repulsive interaction is a system of hard discs (of diameter d) with particles of type A and particles of type B in volume V and interaction potential U U ri2) = oo for < 4,51 and zero otherwise, is the distance of two particles, j l, A, B] are their species and = d B = d, AB = d A- A/2). The total number of particles N = N A- Nb and the total volume V is fixed and thus the average density p = p d = Nd /V. Due to the additional repulsion between A and B type particles one can expect a phase separation into an -rich and a 5-rich fluid phase for large values of A > Ac. In a Gibbs ensemble Monte Carlo (GEMC) [192] simulation a system is simulated in two boxes with periodic boundary conditions, particles can be exchanged between the boxes and the volume of both boxes can... 

Models of a second type (Sec. IV) restrict themselves to a few very basic ingredients, e.g., the repulsion between oil and water and the orientation of the amphiphiles. They are less versatile than chain models and have to be specified in view of the particular problem one has in mind. On the other hand, they allow an efficient study of structures on intermediate length and time scales, while still establishing a connection with microscopic properties of the materials. Hence, they bridge between the microscopic approaches and the more phenomenological treatments which will be described below. Various microscopic models of this type have been constructed and used to study phase transitions in the bulk of amphiphihc systems, internal phase transitions in monolayers and bilayers, interfacial properties, and dynamical aspects such as the kinetics of phase separation between water and oil in the presence of amphiphiles. 

The phase separation process at late times t is usually governed by a law of the type R t) oc f, where R t) is the characteristic domain size at time t, and n an exponent which depends on the universality class of the model and on the conservation laws in the dynamics. At the presence of amphiphiles, however, the situation is somewhat complicated by the fact that the amphiphiles aggregate at the interfaces and reduce the interfacial tension during the coarsening process, i.e., the interfacial tension depends on the time. This leads to a pronounced slowing down at late times. In order to quantify this effect, Laradji et al. [217,222] have proposed the scaling ansatz... 

Molecules of two different species will be able to co-exist if the force of attraction between different molecules is not less than the forces of attraction between two like molecules of either species. This is shown more clearly by reference to Fig. 18.11 which shows two types of molecules, A and B. The average forces between the like molecules are F"aa nd BB > and the average forces between dissimilar molecules are F ab- If was the largest of these three forces then the A molecules would tend to congregate or cohere, pushing away the B molecules. A similar phase separation would occur if f BB was the greatest. 

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