Cooling Immiscibility

PUCI3, and MgCl2 to form a 50/50 mole % NaCl-CaCl salt phase and a molten Am-Pu-Mg-Ca alloy which is immiscible in the above salt(lO). After cooling, the metal phase is cleaved away from the salt phase and the salt phase is analyzed. Little, if any, Am or Pu remains in the salt phase and the salt residues can be discarded to waste. Metal recovery begins by evaporating magnesium and calcium from the residual metal button at about 800°C in vacuum. The americium can then be distilled away from the plutonium in a vacuum still operated at 1200°C, using yttria ceramic vessels to contain the molten metal fraction. The bottoms fraction contains the plutonium which is recycled back into the main plutonium stream. 

Since these mixtures are immiscible at room temperature but miscible at the higher reaction temperature, there is excellent contact between the rhodium catalyst and the olefin when the reaction is carried out, increasing the reaction rate by orders of magnitude. After the reaction is complete, the reaction mixture is cooled and the phases completely separate. The product can be simply recovered by decantation and the catalyst can be recycled (Fignre 28.1). 

Orford An obsolete metallurgical process for separating copper and nickel. Ores containing these metals were smelted in a Bessemer converter, forming matte. This was melted with sodium sulfate and coke, which yielded copper and nickel sulfides, which are immiscible and easily separated on cooling. 

The preparation of immiscible polymer blends is another way to disperse a bulk polymer into fine droplets. It has been reported for several polymers that when they are dispersed in immiscible matrices into droplets with average sizes of around 1 pm, they usually exhibit multiple crystallization exotherms in a differential scanning calorimetry (DSC) cooling scan from the melt (at a specific rate, e.g., 10 Cmin ). Frensch et al. [67] coined the term fractionated crystallization to indicate the difference exhibited by the bulk polymer, which crystallizes into a single exotherm, in comparison with one dispersed in a large number of droplets, whose crystallization is fractionated temperature-wise during cooling from the melt. 

In order to illustrate the fractionated crystallization behavior we will present here previous results on immiscible atactic PS and isotactic polypropylene blends (iPP) [68]. The cooling behavior of PS, iPP and an 80/20 PS/iPP blend is presented in Fig. 1, as well as that of an unmixed blend , labeled... 

Sudden Cooling of Emulsion (Thermal Shock) Sudden temperature drop or freezing i.e., giving a thermal shock) of an emulsion mostly enhances the interfacial tension between the two immiscible phases thereby causing coalescence. 

Kwong, P. D. and Liu, Y. (1999). Use of cryoprotectants in combination with immiscible oils for flash cooling macromolecular crystals. J. Appl. Cryst. 32,102-105. 

In some cases, a combination of spray precipitation (see Sect. 22.5.6) and freeze-drying is recommended. For example, one can spray the polymer solution into a mortar, the bottom of which is covered with pieces of solid carbon dioxide the size of a hazel nut. The pieces are then ground more finely, the mortar placed in a desiccator and evacuated with an oil pump. The polymer solution can also be sprayed into a liquid cooled to low temperature, the liquid being immiscible with the solvent of the polymer, e.g., spraying an aqueous solution into cold ether. The polymer then precipitates in the form of a light flaky snow decantation of the ether is followed by evacuation as described above. 

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