Reaction kinetics, degradation

The speed of autoxidation was compared for different carotenoids in an aqueous model system in which the carotenoids were adsorbed onto a C-18 solid phase and exposed to a continnons flow of water saturated with oxygen at 30°C. Major products of P-carotene were identified as (Z)-isomers, 13-(Z), 9-(Z), and a di-(Z) isomer cleavage prodncts were P-apo-13-carotenone and p-apo-14 -carotenal, and also P-carotene 5,8-epoxide and P-carotene 5,8-endoperoxide. The degradation of all the carotenoids followed zero-order reaction kinetics with the following relative rates lycopene > P-cryptoxanthin > (E)-P-carotene > 9-(Z)-p-carotene. 

The thermal degradation of anthocyanins, both in extracts and model systems, was reported to follow first-order reaction kinetics in all studies. The stability of anthocyanins and all pigments found in foods decreased with increases in temperature. 

It is most fortunate for the development of polymer science that these imagined complications have turned out to be almost wholly illusory. As will be brought out in the course of this chapter, the influence of molecular size and complexity on chemical reactivity may be disregarded in very nearly all polymer reactions. If this were not the case, application of the principles of reaction kinetics to polymerization and polymer degradation reactions would be difficult, and might be so complicated as to be fruitless. Not only would polymer reaction kinetics... 

The rates of degradation can vary widely, from hours to years. The rates observed are a complex function of the permeation rate of the gel by solvent and reactive species as well as the reaction kinetics of the biodegradation reaction [222-224],... 

Study of thermal characteristics, stability, degradation and reaction kinetics on small samples, over a temperature range of- 175°C to 1000°C and above. In some cases precision may be good (1%) but it is variable and may be much poorer. 

Bell et al. (2002) investigated the relationship between water mobility as measured by oxygen-17 NMR (transverse relaxation rate obtained from linewidth at half-height) and chemical stability in glassy and rubbery polyvinylpyrrolidone (PVP) systems. Reported results suggest that water mobility in PVP model systems was not related to Tg. The study did not find a link between water mobility and reaction kinetics data (half-lives) for degradation of aspartame, loss of thiamin and glycine, and stability of invertase. 

Rusche, H. (1973b). Thermal degradation of wood at temperatures up to 200 deg C. Part II. Reaction kinetics of loss of mass during heat treatment of wood. Holz als Roh- und Werkstoff, 31(8), 307-312. 

The chemical stability of an amorphous formulation is usually also a function of its storage temperatme relative to Tg. The enhanced molecular mobility achieved near the glass transition translates into an increase in translational diffusion-dependent degradation pathways, such as aggregation in proteins. It should be noted that the reaction kinetics near the Tg do not obey Arrhenius kinetics, and that extrapolation of the accelerated stability data generated near the Tg to stability at the storage temperature should be viewed with extreme caution. Amorphous materials must be stored well below the glass transition (at least 10°C, and typically 40 to 50°C below Tg) to maintain their physical and chemical stability. 

Can be used to monitor the reaction kinetics of drug degradation. 

The reaction kinetics were studied on the total and separated components (graft and homopolymers) after 24-96 h of grafting at 10° C, see Fig. 7. The total yield increases almost linearly with time, and grafting is the dominant reaction. Extensive milling times has a negative effect, as the graft polymer is subjected to degradation. This was demonstrated by viscosity decreases with time. 

To understand how degradation data are treated, it is convenient to mention the basics of chemical reaction kinetics. The principles of chemical reaction engineering can be found in any reaction engineering or reactor design textbook [26]. A chemical reaction is the process whereby one or more components are transformed into one or more different components. The rate of reaction is the velocity at which the component(s) are being transformed in a chemical reaction. For the chemical reaction... 

Chemical reaction kinetics can be used to evaluate degradation data at accelerated conditions and predict the drug product assay at normal conditions for periods longer than the proposed shelf life. This is applicable to limited cases because the reaction kinetics is often complex for drug products. The following example illustrates the procedure to follow to calculate the API concentration with time for a drug product stored at normal conditions when such data are not yet available. 

Clearly, molecular structure influences the reaction kinetics of organic compounds during their photocatalytic oxidation. This relationship between degradability and molecular structure may be described using quantitative structure-activity relationship (QSAR) models. QSAR models can be developed to predict kinetic rate constants for organic compounds with similar chemical structures. The following section discusses QSAR models developed by Tang and Hendrix (1998) as well as those developed by other researchers. 

Dechlorination is a surface reaction with the zero-valent iron serving as the electron donor. When there is a proton donor, such as water, chlorinated compounds will be dehalogenated. The reaction kinetics depends upon the mass transfer to the surface of the iron, the available surface area, and the condition of the surface. The reaction is pseudo first order, and direct contact with the surface of the iron is required for degradation to take place (Gillham and O Hannesin, 1994). The basic equation for dechlorination by iron metal is as follows ... 

Because nitrobenzene degradation is faster in batch experimental systems than in column studies, a mass-transfer limitation exists therefore, when determining the effectiveness of in situ groundwater treatment systems, hydraulics, mass transfer, and reaction kinetics should be taken into consideration (Burris et al., 1996). 

Our understanding of the iron reactive barrier is, however, still limited. Degradation rates of many chlorinated compounds have been reported, but the exact reaction kinetics and mechanism are much less understood. The long-term stability of the reactive barrier and the effects of other system... 

Cabaniss, S., Madey, G., Leff, L., Maurice, R, and Wetzel, R. (2005). A stochastic model for the synthesis and degradation of natural organic matter. Part I. Data structures and reaction kinetics. Bio geochemistry 76, 319-347. 

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