Inverse solubility-temperature

Components in which water temperature changes abruptly with distance, such as heat exchangers, tend to accumulate precipitates. Heater surfaces also accumulate precipitates if the dissolved species have inverse temperature solubilities. Systems in which pH excursions are frequent may accumulate deposits due to precipitation processes. Plenum regions, such as heat exchanger headboxes, tend to collect deposits. 

Calcium carbonate has normal pH and inverse temperature solubilities. Hence, such deposits readily form as pH and water temperature rise. Copper carbonate can form beneath deposit accumulations, producing a friable bluish-white corrosion product (Fig. 4.17). Beneath the carbonate, sparkling, ruby-red cuprous oxide crystals will often be found on copper alloys (Fig. 4.18). The cuprous oxide is friable, as these crystals are small and do not readily cling to one another or other surfaces (Fig. 4.19). If chloride concentrations are high, a white copper chloride corrosion product may be present beneath the cuprous oxide layer. However, experience shows that copper chloride accumulation is usually slight relative to other corrosion product masses in most natural waters. 

The precipitation of anhydrite (anhydrous calcium sulfate, CaS04) may also occur. Under ambient temperatures, anhydrite is much more soluble than calcium carbonate, but because calcium sulfate, in common with other calcium salts such as calcium phosphate (also known as tricalcium phosphate [Ca3(P04)2]), has an inverse-temperature solubility, it deposits more rapidly on the hottest heat transfer surfaces. 

Most salts absorb heat when they go into solution, and their solubility increases with a rise in temperature however, calcium carbonate (CaC03), in common with several other anhydrous salts such as calcium sulfate (CaS04) and calcium phosphate [Ca3(P04)2], has an inverse temperature solubility and thus readily precipitates to form deposits in hot water areas (FW tanks, FW lines, and boiler heat exchange surfaces). 

Several common salts have an inverse temperature solubility and readily precipitate to form deposits on hot boiler surfaces and other heat exchange areas. These include ... 

The compound was used as a catalyst for the hydrogenation of olefins. No rhodium was lost. This type of polymer shows inverse temperature solubility. When the temperature was raised, the polymeric catalyst separated from solution for easy recovery and reuse. This type of smart catalyst will separate from solution if the reaction is too exothermic. The catalytic activity ceases until the reaction cools down and the catalyst redissolves. Poly (A i sop ropy lacrylamide) also shows inverse temperature solubility in water. By varying the polymers and copolymers used, the temperature of phase separation could be varied (e.g., from 25 to 80°C).214 A terpolymer of 2-isopropenylan-thraquinone, A-isopropylacrylamide, and acrylamide has been used in the preparation of hydrogen peroxide instead of 2-ethylanthraquinone.215 The polymer separates from solution when the temperature exceeds 33 C to allow re-... 

The use of catalysts based on polymers with inverse temperature solubility, often copolymers of TV-isopropy-lacrylamide, to allow recovery by raising the temperature to precipitate the polymer for filtration,9 was mentioned in Chap. 5. The opposite, if the catalyst is soluble hot, but not cold, has also been used in ruthenium-catalyzed additions to the triple bonds of acetylenes (7.1).10 The long aliphatic tail of the phosphine ligand caused the catalyst to be insoluble at room temperature so that it could be recovered by filtration. There was no loss in yield or selectivity after seven cycles of use. A phosphine-modified poly(A-iso-propylacrylamide) in 90% aqueous ethanol/heptane has been used in the hydrogenation of 1-olefins.11 The mixture is biphasic at 22°C, but one phase at 70°C, at which the reaction takes place. This is still not ideal, because it takes energy to heat and cool, and it still uses flammable solvents. 

Hydroalkoxylation of alkynes, or the addition of alcohol to alkynes, is a fundamental reaction in organic chemistry that allows the preparation of enol ethers and a variety of oxygen-containing heterocycles such as furan, pyran, and benzofuran derivatives. Bergbreiter et al. found that a Mnear poly-(A-isopropylacrylamide) (PNIPAM) polymer exhibited inverse temperature solubility in water (i.e., soluble in cold water but insoluble in hot water). A recoverable homogeneous palladium catalyst was prepared based on the polymer. The PNIPAM-bound Pd(0) catalyst was effective for the reaction of 2-iodophenol with phenylacetylene in aqueous THE media to give the target product... 

An increase in temperature usually results in a decrease in the adsorption of ionic surfactants, although the change may be small when compared to those due to pH and electrolyte changes. Nonionic surfactants solubilized by hydrogen bonding, which usually have an inverse temperature-solubility relationship in aqueous solution, generally exhibit the opposite effect. In other words, adsorption will increase as the temperature increases, often having a maximum near the Krafft point of the particular surfactant. 

The temperature dependence of the cmc of polyoxyethylene nonionic surfactants is especially important since the head group interaction is essentially totally hydrogen bonding in nature. Materials relying solely on hydrogen bonding for solubilization in aqueous solution are commonly found to exhibit an inverse temperature-solubility relationship. As already mentioned, major manifestation of such a relationship is the presence of the cloud point for many nonionic surfactants. 

The prototypical smart polymer is poly(N-isopropyl acrylamide) (P(NIPAM)), which exhibits an inverse temperature solubility profile in water, that is it is water-soluble below 32 °C but precipitates above 32 °C. The temperature at which this coil-to-globule phase transition occurs is known as the Lower Critical Solution Temperature (LCST), and conveniently this can be modified in P(NIPAM) by incorporation into the polymer chain of more hydrophobic or hydrophilic monomers. Owing to the fact that the LCST is close to body temperature and can readily be modified to just below or just above 37 °C through this co-monomer addition, P(NIPAM) polymers have been widely exploited in biomedical applications. The chemistries and applications of P(NIPAM) have been extensively reviewed elsewhere, [75-81] but even 15 years after one particularly well-cited review, many research groups are working with this remarkably versatile polymer [82-87]. 

Bergbreiter and coworkers (2/) recently described the development of a new polymeric support with interesting physical properties poly(/V-isopropyl-acrylamide) (PNIPAM). Several co- and ter-polymers containing PNIPAM have been successfully prepared. These materials exhibit an inverse temperature solubility profile in water. If heated above their lower critical solution temperature (LOST), they precipitate quantitatively from solution. Unlike a protein, PNIPAM derivatives do not denature. The LOST of the PNIPAMs can... 

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