Photo-oxidation Profile

It has long been recognized that photochemical damage is more severe at the surface than in the sample interior. The reasons for this inhomogeneity are twofold  

Penetration distance of light For transparent systems, the light intensity l x) decreases with penetration distance x according to the Lambert-Beer law according to Eq. (63), where lo is the incident light intensity, and a the absorption coefficient, which is proportional to the molar absorption coefficient e. 

For materials which scatter light, such as semicrystalline polymers, and for polymers which contain charges or pigments, the Lambert-Beer law should be replaced by the Kubelka-Munk equation [Eq. (64), with K = absorption coefficient, S = scattering coefficient and R = reflectance of the material]. 

The oxygen diffusion constant, D, depends on the state of aging process, the reactant concentrations, and the sample morphology (orientation, crystallinity). It follows approximately an Arrhenius-type temperature dependence as in Eq. (66), where Do is a proportionahty constant, and R the molar gas constant. 

Although simple to measure, the activation spectrum does not consider the amount of incident radiant energy present at each wavelength, and is hence depen- 

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The details of the construction of the photo-oxidation cell are shown in Figure 10.40. Typical photo-oxidation profiles obtained using this apparatus are shown in Figure 10.41. The first portion of each curve shows the differential pressure versus time during the thermal equilibration of the apparatus. If the pressure remains constant for an extended period of time then the apparatus is adequately sealed and thermal equilibrium has been achieved. The pressure in the sample cell decreases as photo-oxidation proceeds. 

For Aspergillus niger extracellular endo-D-galacturonanase, the role of histidine in the enzyme reaction was investigated by the method of photo-oxidative inactivation, catalyzed by Methylene Blue.140 The inactivation of the enzyme was paralleled by the decomposition of histidine. The similarity of pH profiles, as well as the values of the rate constants of enzyme inactivation (4.0 X 10-2 min-1) and of decomposition of histidine (3.9 X 10-2 min-1), indicate that one of the five histidine residues present in the molecule of the enzyme141 is essential for its activity. 

Fig. 9. Comparison of depth-averaged profiles of dissolved organic carbon (DOC) concentrations of samples from the Nova Scotian shelf measured with various oxidation methods. Data from Gershey et al. (1979). = persulfate oxidation (Menzel and Vaccaro, 1964) o = dry oxidation (MacKinnon, 1978) = photo-oxidation (Collins and Williams, 1977).

Figure 11.2. Schematic representation of the heterogeneous photo-oxidation of ferrocene by specifically adsorbed water-soluble metalloporphyrin at the water/EKlE interface. It is postulated that the distance of closest approach is determined by the onset of the solvent density profiles generated by capillary wave fluctuations.

A computer model of polymer photo-oxidation has been developed [878]. This model can generate realistic concentration versus time profiles of the chemical species formed during photo-oxidation of hydrocarbon polymers using, as input data, a set of elementary reactions with corresponding rate constants and initial conditions. A numerical integration procedure on the... 

Assuming that the photo-induced production of OH radicals and the oxidation reactions are very fast and take place only within the volume where UV/VIS radiation is absorbed, two different concentration profiles can be distinguished theoretically (Fig. 8-2). 

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