Photographic process, mechanism

Despite the fact that not all details of the photographic process are completely understood, the overall mechanism for the production of the latent image is well known. Silver chloride, AgBr, crystallizes with the sodium chloride structure. While Schottky defects are the major structural point defect type present in most crystals with this structure, it is found that the silver halides, including AgBr, favor Frenkel defects (Fig. 2.5). 

Information about details of various chemical reactions can be obtained by application of radiotracers. The photographic process may be mentioned as an example if a thick cylindrical pellet of Agl is labelled on one side by isotope exchange with I, arranged in such a way that light has no access to the labelled side, and irradiated on the opposite side by visible light, l-labelled iodine is immediately liberated. It may be carried away by a stream of an inert gas and measured continuously in a gas counter. The immediate liberation of h is due to fast transport of defect electrons in Agl, in agreement with the proposed mechanism of the photographic process. 

Rate constants of the process and the nuclearity-redox potential correlation will be compared with corresponding data obtained in another environment, particularly when a surfactant or an associated ligand is present. The complete analysis of the autocatalytic transfer mechanism will also be compared with the photographic process of electron transfer from hydroquinone developer to clusters supported on silver bromide. 

Among the second- and third-row transition metal halide complexes, those of platinum have received considerable study. Both the octahedral platinum(IV) complex PtCli and the square planar platinum(II) complex PtCl have been investigated. Both complexes are substitution inert under thermal conditions, and kinetic studies of their substitution chemistry have been important in the development of a general understanding of the mechanisms of substitution reactions in transition metal chemistry. The photochemistry of PtCli" was one of the earliest such studies to be made, and the early discoveries of the photosensitive nature of platinum halides led to these salts being used in photographic processes. The subsequent decision to use a silver-based process was based more on economical rather than on technical reasons. 

The cleaning process proceeds by one of three primary mechanisms solubilization, emulsification, and roll-up [229]. In solubilization the oily phase partitions into surfactant micelles that desorb from the solid surface and diffuse into the bulk. As mentioned above, there is a body of theoretical work on solubilization [146, 147] and numerous experimental studies by a variety of spectroscopic techniques [143-145,230]. Emulsification involves the formation and removal of an emulsion at the oil-water interface the removal step may involve hydrodynamic as well as surface chemical forces. Emulsion formation is covered in Chapter XIV. In roll-up the surfactant reduces the contact angle of the liquid soil or the surface free energy of a solid particle aiding its detachment and subsequent removal by hydrodynamic forces. Adam and Stevenson s beautiful photographs illustrate roll-up of lanoline on wood fibers [231]. In order to achieve roll-up, one requires the surface free energies for soil detachment illustrated in Fig. XIII-14 to obey... 

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