Immiscibility Inversion temperature

Shinoda and Kuineda [8] highlighted the effect of temperature on the phase behavior of systems formulated with two surfactants and introduced the concept of the phase inversion temperature (PIT) or the so-called HLB temperature. They described the recommended formulation conditions to produce MEs with surfactant concentration of about 5-10% w/w being (a) the optimum HLB or PIT of a surfactant (b) the optimum mixing ratio of surfactants, that is, the HLB or PIT of the mixture and (c) the optimum temperature for a given nonionic surfactant. They concluded that (a) the closer the HLBs of the two surfactants, the larger the cosolubilization of the two immiscible phases (b) the larger the size of the solubilizer, the more efficient the solubilisation process and (c) mixtures of ionic and nonionic surfactants are more resistant to temperature changes than nonionic surfactants alone. 

Beaded acrylamide resins (28) are generally produced by w/o inverse-suspension polymerization. This involves the dispersion of an aqueous solution of the monomer and an initiator (e.g., ammonium peroxodisulfates) with a droplet stabilizer such as carboxymethylcellulose or cellulose acetate butyrate in an immiscible liquid (the oil phase), such as 1,2-dichloroethane, toluene, or a liquid paraffin. A polymerization catalyst, usually tetramethylethylenediamine, may also be added to the monomer mixture. The polymerization of beaded acrylamide resin is carried out at relatively low temperatures (20-50°C), and the polymerization is complete within a relatively short period (1-5 hr). The polymerization of most acrylamides proceeds at a substantially faster rate than that of styrene in o/w suspension polymerization. The problem with droplet coagulation during the synthesis of beaded polyacrylamide by w/o suspension polymerization is usually less critical than that with a styrene-based resin. 

The square route of the cohesive pressure is termed Hildebrand s solubility parameter (5). Hildebrand observed that two liquids are miscible if the difference in 5 is less than 3.4 units, and this is a useful rule of thumb. However, it is worth mentioning that the inverse of this statement is not always correct, and that some solvents with differences larger than 3.4 are miscible. For example, water and ethanol have values for 5 of 47.9 and 26.0 MPa°-, respectively, but are miscible in all proportions. The values in the table are measured at 25 °C. In general, liquids become more miscible with one another as temperature increases, because the intermolecular forces are disrupted by vibrational motion, reducing the strength of the solvent-solvent interactions. Some solvents that are immiscible at room temperature may become miscible at higher temperature, a phenomenon used advantageously in multiphasic reactions. 

In this process phase inversion is introduced by lowering the temperature of the polymer solution. A polymer is mixed with a substance that acts as a solvent at a high temperature and the polymer solution is cast into a film. When the solution is cooled, it enters into an immiscible region due to the loss of solvent power. Liquid-liquid demixing occurs and the solution is separated into two phases, i.e., the polymer-lean phase is dispersed as droplets in the polymer-rich phase. Further, cooling causes gelation of polymer. Because the solvent is usually nonvolatile, it must be removed with a liquid that is miscible with the solvent but not miscible with the polymer. The membranes made by the TIPS method have pore sizes in the range of 0.1 and 1 pm and the pore structure is uniform in the depth direction. ... 

In contrast to emulsions, which are unstable macrodisperse systems (1-10 pm in droplet diameter), microemulsions are homogeneous, optically transparent, thermodynamically stable systems that can be formed only in specific ranges of temperature, pressure, and composition. They consist of droplets of water tens of nanometers in size dispersed within an immiscible organic (oil) phase [inverse micelles, or water-in-oil (W/0) microemulsions] or vice versa, oil pools dispersed within an aqueous phase [direct micelles, or oil-in water (OAV) microemulsions]. The droplets are encased in a surfactant shell as in emulsions or, more frequently, in a shell consisting of a suitable surfactant and a cosurfactant (usually an alcohol) and are thus stabilized. 

A test of Kerner s equation involving an emulsion polymer blend of low Tg/high Tg components (low and high meaning below and above the testing temperature) was reported, where poly(vinyl acetate) (PVAc)/vinyl acetate-ethylene copolymer (EVAC) immiscible mixtures were prepared [5]. The particle size of the individual components was similar and the cast emulsion blend exhibited a modulus-temperature behavior as noted in Fig. 6.4. As expected, the position where both phases are equally continuous (and discontinuous) is equal to 0.50 volume fraction. In cases where the particle sizes of the emulsion blend are significantly different, the phase inversion position would be expected to be shifted from the 0.5 volume fraction point. 

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