Lower critical solution temperature Metal

Some polymers (PEG or PEG-like) have low cloud points (the critical solution temperature) in water. If these polymers support the formation of an ABS, metal ion stripping by temperature programming may be possible. Rogers et al. [35] have found that UCON (a random copolymer of ethylene oxide and propylene oxide) can replace PEG-2000 and give better extraction of TcO in certain ABSs. UCON has a cloud point around 50 C and can form an ABS with salt solutions at lower salt concentrations [68]. TcO " can be extracted into the UCON-rich phase, and the UCON-rich phase can be separated, heated, and the two new phases separated again [35]. Thus, metal ions can be stripped from a polymer-rich phase directly into water. This technique shows promise, but more research is needed to define clearly the conditions necessary for a successful separation. 

An application has been found in which a system that exhibits an upper, or lower, critical consolute point, UCST or LCST, respectively, is utilized. At a temperature above or below this point, the system is one homogeneous liquid phase and below or above it, at suitable compositions, it splits into two immiscible liquids, between which a solute may distribute. Such a system is, for instance, the propylene carbonate - water one at 25°C the aqueous phase contains a mole fraction of 0.036 propylene carbonate and the organic phase a mole fraction of 0.34 of water. The UCST of the system is 73 °C (Murata, Yokoyama and Ikeda 1972), and above this temperature the system coalesces into a single liquid. Temperature cycling can be used in order to affect the distribution of the solutes e.g. alkaline earth metal salts or transition metal chelates with 2-thenoyl trifluoroacetone (Murata, Yokayama and Ikeda 1972). 

A plot of the temperatures required for clouding versus surfactant concentration typically exhibits a minimum in the case of nonionic surfactants (or a maximum in the case of zwitterionics) in its coexistence curve, with the temperature and surfactant concentration at which the minimum (or maximum) occurs being referred to as the critical temperature and concentration, respectively. This type of behavior is also exhibited by other nonionic surfactants, that is, nonionic polymers, // - a I k y I s u I Any lalcoh o I s, hydroxymethyl or ethyl celluloses, dimethylalkylphosphine oxides, or, most commonly, alkyl (or aryl) polyoxyethylene ethers. Likewise, certain zwitterionic surfactant solutions can also exhibit critical behavior in which an upper rather than a lower consolute boundary is present. Previously, metal ions (in the form of metal chelate complexes) were extracted and enriched from aqueous media using such a cloud point extraction approach with nonionic surfactants. Extraction efficiencies in excess of 98% for such metal ion extraction techniques were achieved with enrichment factors in the range of 45-200. In addition to metal ion enrichments, this type of micellar cloud point extraction approach has been reported to be useful for the separation of hydrophobic from hydrophilic proteins, both originally present in an aqueous solution, and also for the preconcentration of the former type of proteins. 

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. 

Lithium which had been purified by filtration followed by gettering with titanium and yttrium at 753 K had a lower resistivity than metal purified by other methods. For the liquid, dp/dOc was positive but decreased with increasing temperature, whereas for the solid, the value increased with increasing temperature. The resistance of dissolved oxide and hydride impurities in eutectic alloys of sodium and potassium appears to be a complex function of the concentration of each impurity, which can be attributed to chemical interaction in the metal to form hydroxide. Dissolved hydride causes a considerable increase in the resistance of the alloy but hydroxide has a much smaller effect. Dissolved lithium hydride affects the resistance of the alloy more than does sodium or potassium hydride but, again, hydroxide, as lithium hydroxide, has a smaller effect. Information on the solubility of lithium salts in liquid lithium has been critically reviewed. Recommended solubilities are provided for solutions of oxide and nitride as... 

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