Silver halide model

The scheme is similar to the silver halide model except free electrons are not created. It is also in essential agreement with that proposed to explain the rate acceleration observed by Jacobs et al. [207]. 

In recent years, the silver halide model has been used to explain the growth of silver and gold nanoprisms in a manner analogous to the growth of silver halide... 

Figure 3.29 (a) Silver halide model of a single twin plane containing type A and type B faces. Type A faces contain re-entrant grooves which facilitate the rapid growth of these faces,... 

The reactions which will be discussed here are basic in the application of dyes as sensitizer for photographic materials like silver halides, zinc oxide and others. Model experiments can be performed at electrodes of such materials which help to understand the mechanism of spectral sensitization in photography. 

Sensitization of Model Systems. Silver bromide sheet crystals can be sulfur-sensitized easily, and have been studied extensively (56-58). Evaporated silver halide layers and sprayed emulsions likewise can be sulfur-sensitized. 

Shalitt concludes with the statement that "this theory does not attempt to reject the other models but adds another dimension by proposing an additional factor to the forces that form the L.I. in the silver halides."... 

Particle electrophoresis studies have proved to be useful in the investigation of model systems (e.g. silver halide sols and polystyrene latex dispersions) and practical situations (e.g. clay suspensions, water purification, paper-making and detergency) where colloid stability is involved. In estimating the double-layer repulsive forces between particles, it is usually assumed that /rd is the operative potential and that tf/d and (calculated from electrophoretic mobilities) are identical. 

Two mechanisms are possible for dye sensitization. In one, direct electron injection from the excited dye level into the conduction band of the silver halide occurs. An extensive series of experiments varying the relative positions of the dye HOMO and LUMO levels with respect to the silver halide valence and conduction band positions has established the validity of the direct electron injection model [11], In this mechanism (Fig. 4), the dye molecular orbital levels (HOMO and... 

At the opposite extreme from the oriented gas model for molecular crystals, the neighbouring molecules do interact with each other resulting in spectral properties of the bulk that differ considerably from those of the individual molecule. Interacting molecules of this type often tend to form aggregates even in solution, a phenomenon that has been exploited by the photographic industry for the tuning of the spectral response of silver halide emulsions (Herz 1974 Smith 1974 Nassau 1983). Aggregate formation can lead to the development of new, and often quite intense absorption bands... 

The silver halide growth model has been resurrected as a way of explaining particle growth in many synthesis methods [58,104]. In this model, twin planes form reentrant grooves (A-type faces in Figure 11.44), which are favourable sites for the... 

Brown and Tong [108] proposed an electrochemical model for the development of a single silver halide grain in which it is assumed that ... 

Many of the electrode theories have assumed that the anodic reaction is rate-limiting and that the cathodic reduction of silver ions from silver halide is not rate-limiting and might not present any limitations to the process of development. Hamano et al. [112] contend that there are instances where the cathodic process does influence development. They use the Butler-Volmer equation as the basis for their development rate model and derive Eq. (83),... 

Friedrich and Filers [113] have proposed a corrosion model of development and derived electron transfer equations based on the Butler-Volmer expression which can be simplified into three cases. Case 1 is when both the forward and reverse processes of developer oxidation are important. Case 2 is when the net rate is limited by the forward rate of developer oxidation. Case 3 corresponds to a rate which is limited by the kinetics of both developer oxidation and silver halide reduction. 

The structure and electrochemistry of surface aggregates of silver atoms on silver halides are also topics open for basic surface studies. Here the surface scientist will seek simple model systems and avoid the plethora of surface additives so essential in forming the photographic latent image. Examples will be given. 

The single crystal method of (4.) can be repeated with other host crystals which are transparent in the visible or infrared. Since NaCl is isomorphous with AgBr and AgCl and has a lattice parameter intermediate between the two silver halides, it is a model host of choice for studies on the (100) face (22). It must, of course, be dyed out of nonaqueous media. Gypsum (CaSO t O) has also been... 

In this paper we have attempted to explore useful tools and unfinished business in the surface science of silver halide photographic systems. The sensitizing dye/AgBr system has been explored as a compliant model system for persons who might wish to carry out fundamental investigations in modes of adsorption and surface phase transitions. Many of the experimental and conceptual tools developed within the photographic industry are... 

The understanding of the basic mechanism of photolysis of silver halide is incomplete yet vital for the planning and interpretation of experiments. This fact is illustrated by the ramifications inherent in the contemporary discussions of the latent image in silver halide. There are the conventional Gurney-Mott mechanism (1) and the thermodynamic model ((5,9). We have described the Gurney-Mott model. The thermodynamic view envisages nucleation of a supersaturated concentration of silver atoms in silver halide as induced by light. Obviously the effect of external variables is quite different in the two mechanisms. 

For the nucleation in double-jet precipitation of silver halides, a dynamic model was derived which relates the inal number of stable nuclei to various precipitation variables ... 

The formation of colloidal silver halide dispersions (photographic emulsions) was reviewed as a model system of colloids which are formed by precipitation of sparingly soluble salts. For such systems, models for crystal nucleation and growth were derived which were verified for the AgBr system. These models can probably be extended to the study of nucleation and growth of other highly insoluble colloidal systems. 

Figure 8. Models of positively charged and negatively charged kink sites on a silver halide (AgX) surface.

Although the above, and similar, arguments are qualitative, they provide a helpful means of assessing the pattern in solubilities for series of ionic salts we stress ionic because equations 6.55 and 6.56 assume an electrostatic model. Our discussions in Section 5.15 and earlier in this section indicated how partial covalent character in silver halides affects solubility trends. 

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