Rate of dextrinization

The effect of salt addition on the hydrolysis rate of dextrin in the presence of the random copolymer catalyst were investigated. The results are summarized in Table I. The catalytic activity of the copolymer is... 

Hreczuk-Hirst D, Ghicco D, German L, Duncan R Dextrins as potential carriers for drug targeting Tailored rates of dextrin degradation by introduction of pendant groups, Int ) Pharm 2001, 230, 57-66. 

Pyrotechnic mixtures may also contain additional components that are added to modify the bum rate, enhance the pyrotechnic effect, or serve as a binder to maintain the homogeneity of the blended mixture and provide mechanical strength when the composition is pressed or consoHdated into a tube or other container. These additional components may also function as oxidizers or fuels in the composition, and it can be anticipated that the heat output, bum rate, and ignition sensitivity may all be affected by the addition of another component to a pyrotechnic composition. An example of an additional component is the use of a catalyst, such as iron oxide, to enhance the decomposition rate of ammonium perchlorate. Diatomaceous earth or coarse sawdust may be used to slow up the bum rate of a composition, or magnesium carbonate (an acid neutralizer) may be added to help stabilize mixtures that contain an acid-sensitive component such as potassium chlorate. Binders include such materials as dextrin (partially hydrolyzed starch), various gums, and assorted polymers such as poly(vinyl alcohol), epoxies, and polyesters. Polybutadiene mbber binders are widely used as fuels and binders in the soHd propellant industry. The production of colored flames is enhanced by the presence of chlorine atoms in the pyrotechnic flame, so chlorine donors such as poly(vinyl chloride) or chlorinated mbber are often added to color-producing compositions, where they also serve as fuels. 

Fig. 10. Plots of the relative rate constants of the hydrolysis of dextrin vs. the mole ratio of vinylalcohol unit to vinylsulfonic acid unit, o PVS VA, Polyvinylalcohol + HPVS (Ref.

Commercially, lead azide is usually manufactured by precipitation in the presence of dextrine, which considerably modifies the crystalline nature of the product. The procedure adopted is to add a solution of dextrine to the reaction vessel, often with a proportion of the lead nitrate or lead acetate required in the reaction. The bulk solutions of lead nitrate and of sodium azide are, for safety reasons, usually in vessels on the opposite sides of a blast barrier. They are run into the reaction vessel at a controlled rate, the whole process being conducted remotely under conditions of safety for the operator. When precipitation is complete, the stirring is stopped and the precipitate allowed to settle the mother liquor is then decanted. The precipitate is washed several times with water until pure. The product contains about 95% lead azide and consists of rounded granules composed of small lead azide crystals it is as safe as most initiating explosives and can readily be handled with due care. 

Upon extensive hydrolysis of starch by either of these enzymes, only small differences were observed in the concentrations of the products or in the average degrees of polymerization of the dextrins. These comparisons were made for equivalent stages of hydrolyses and are not necessarily related to the rates of the hydrolysis of starch by these two amylases. 

Studies of the rate of the hydrolysis of dextrins isolated from a reaction mixture after the extensive hydrolysis of starch by maltase-free malted barley alpha amylase, led Myrback11 to conclude that the flattening of the reaction curves with this amylase is not due to equilibrium between the amylase and the products of the hydrolysis. As indicated above, similar conclusions have been reached for pancreatic amylase and for the amylase of Aspergillus oryzae.41,7a... 

Musculus and Meyer (12) measured the diffusion rates of some starches and dextrins in 1881. The work was designed to determine the relationship of these "isomeric or polymeric" forms to the simple sugars from which they were formed. They concluded that dextrin molecules must be much larger than those of the sugars. This work, however, preceeded Raoult s (13) development of the cryoscopic technique for the determination of the molecular weights of dissolved substances, and van t Hoff s (14) formulation of the solution laws. Further, since the vapor density method was obviously inapplicable, it was not possible for them to actually determine the degree of polymerization. 

An important feature of the cyclodextrins is that they can also accelerate chemical reactions, and therefore serve as models for the catalytic as well as the binding properties of enzymes. The rapid reaction is not catalysis, since the dextrin enters reaction but is not regenerated presumably it arises from approximation, where complex formation forces the substrate and the cyclodextrin into intimate contact. In particular, cyclodextrins can increase the rate of cleavage of phenyl pyrophosphate by factors of as much as 100 (Cramer, 1961). More recent work has improved upon this early example. 

Notes a) ASA(Service) contains undextrinated LA, whereas ASA(Commercial) contains dextrinated LA b) Ignition temps were detd by heating a sample (0.02g for dextrinated LA and O.Olg for other expis) in a suitable container starting from room temp, and using a controlled rate of rise of temp. 

Figure 3. Change in the relative hydrolysis rates (based on sulfuric acid) of dextrin with the mole ratio of vinyl alcohol to vinylsulfonic acid repeating units at 80°C, keeping the concentration of vinulsulfonic acid unit constant at 5.00 X I0"3N. (O) In the presence of the random copoly (vinyl alcohol-vinylsulfonic acid) (%) in the presence of poly (vinyl alcohol)-poly(vinylsulfonic acid) mixture. [Substrate] = 2.00 X 10"2M.

No. Rate of Salt Added the Dextrin at Catalyst 80°C° ko6s/k°ju /t(rto acid pH... 

The catalytic activities of the block copolymer on the hydrolysis of dextrin also were investigated. Figure 8 shows the plots of reaction rate against the substrate concentration. Similar tendency, but larger rate enhancement of the reaction are found compared with that in the presence of the random copolymer catalyst (Figure 4). 

Figure 9 shows Lineweaver-Burk plots of dextrin hydrolysis rates in the presence of the block copolymer. Again, fairly good straight lines are obtained. Some other kinetical investigations also were made for the catalytic activity of the block copolymer, and similar tendencies of catalytic behavior were found compared with that of the random copolymer. 

Figure 8. Dependence of dextrin hydrolysis rates (v) on the substrate concentration in the presence of the block copolymer at 70°C. [Catalyst] = 1.00 X 10 2N. Catalyst (mole ratio of vinyl alcohol to styrenesulfonic acid units in the copolymer) (O) sulfuric acid (%) block copolymer No. 1 (1-4) (A) block copolymer No. 2 (9.8) (A) block copolymer No. 3 (22.1).

Figure 15. Rate of the debranching enzyme on phosphorylase limit dextrin in the presence of the irreversible inhibitor dimethyiarsenothioglucose in the presence and absence of a reversible inhibitor (Bis-Tris). Adapted from Gillard et al.

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