Retrograde dissociation

For example, at 278.2 K, hydrates form at a pressure of approximately 5 bar and dissociate upon pressurization at approximately 600 bar. A more detailed explanation of the pseudo-retrograde hydrate phenomena can be found in the binary hydrates section which follows. Note that the hydrate formation pressure of propane hydrates along the Aq-sII-V line at 277.6 K is predicted to be 4.3 bar. 

The pressure at which this dissociation is predicted to occur is called the hydrate pseudo-retrograde pressure at T. Pseudo-retrograde behavior is defined as the disappearance of a dense phase upon pressurization, which is counter-intuitive. This behavior resembles, but is not strictly the same as, vapor-liquid retrograde phenomena (de Loos, 1994). 

Pseudo-retrograde phenomena are predicted to occur between the temperatures of 277.6 and 278.3 K. With a pressure increase of up to 5 bar, sll hydrates will dissociate at any temperature in this range. The lines are model predictions and the circles are experimental observations of hydrate dissociation obtained in the TUD laboratory. As can be seen in Figure 5.19, the TUD hydrate dissociation data do confirm the pseudo-retrograde melting. However, note that the Aq-sII-Lhc predictions deviate 0.2 K from the data. It is usually assumed that hydrates never dissociate with an increase in pressure. However, both measurements and... 

Kok JW, Bahia T, Fihpeanu CM, Nelemans A, Egea G, Hoekstra D. PDMP blocks Brefeldin A-induced retrograde membrane transport from Golgi to ER Evidence for involvement of calcium homeostasis and dissociation from sphingolipid metabolism. J. CeU Biol. 1998 142 25-38. 

A separate problem is presented by starch that contains fatty acids (lipids) for instance, maize, rice, and wheat starch. The naturally occurring lipids form inclusion complexes vnth amylose that exhibit texture and morphology different from those of native starch granules. These differences are reflected by the behavior of starch at a relatively low temperature for instance, gelatinization at — 120 °. The process involved is a high-temperature retrogradation, with participation of the proton dissociated from the com-plexed fatty acid residue. A Cjj acid complexed in the helical structure of... 

The radical catalyzed homopolymerization of the furan-maleic anhydride (F-MAH) Diels-Alder adduct yields a saturated homopoly-mer at temperatures below 60 C, and an unsaturated equimolar alternating copolymer at elevated temperatures, due to retrograde dissociation of the adduct (10, 11). The copolymerization of monomeric furan and maleic anhydride yields the same unsaturated alternating copolymer, independent of temperature (1C)). 

In contrast, the radical catalyzed homopolymerization of the cyclopentadiene-maleic anhydride (CPD-MAH) Diels-Alder adduct yields a saturated homopolymer at temperatures as high as 220 C, while retrograde dissociation occurs at even higher temperatures. Nevertheless, the copolymerization of monomeric cyclopentadiene and maleic anhydride yields a saturated 1 2 copolymer (12-15). 

The retrograde dissociation of the Diels-Alder adduct to generate the diene and dienophile, followed by the polymerization of the comonomer charge transfer complex, has previously been proposed (12-15) in the polymerization of the CPD-MAH adduct, in the presence of peroxides having short half-lives at the elevated polymerization temperatures. Under these conditions, the ground state complex, presumably involved in endo-exo isomerization, is converted to the excited state complex which undergoes the indicated polymerization. 

Subsequent to the presentation of the results reported herein (21), 13C and 270 MHz H-NMR studies of the thermally induced polymerization of the N-phenyl-5-norbornene-2,3-dicarboximides (22) and JC-NMR studies of norbornene end-capped polyimide prepolymers (23) were reported. Endo-exo isomerization of the adducts takes place at 200 C and above. Retrograde dissociation... 

Retrograde dissociation clearly demonstrates the reversible nature of protein-surfactant interaction. It may also be noted that maxima in protein-surfactant adsorption isotherms have also been observed in lysozyme-SDS systems under certain conditions [108] as the surfactant cmc is approached these can arise as a consequence of the surfactant solution activity passing through a maximum in the region of the cmc [109]. 

If a dilute starch solution stands for a prolonged time it gradually becomes cloudy and eventually deposits as an insoluble white precipitate. If more concentrated starch dispersion is allowed to cool it will rapidly set to an elastic gel. Both these are processes of retrogradation, whereby the starch molecules go from a dissolved and dissociated state to an associated condition. The mechanisms of retrogradation are schematically shown in Fig. 9.7 [1]. 

The retrogradation of the dissociation equilibrium is brought about by addition to the reaction medium of common ion salts whose cation is inert toward the polymerization system. As for the nature of the counterions, those that bring about a certain covalency of the active centers are preferred. Addition of a weak nucleophile is also an efficient means to curb the reactivity of carbocations. The following systems (monomer, initiating system, solvent) qualify, more or less perfectly, for the category of living polymerizations ... 

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