Aggregation Phenomena in AgX Precipitation in the Presence of Gelatin

Abstract

Despite the continuous development of new synthetic polymers, gelatin remains as the principal component of the binder used in most silver halide photographic films and papers. The main reason for the longevity of gelatin in silver halide photographic systems is its remarkable property to play a unique role in almost every step of the manufacture as well as in the photographic processing of these products. The most important function of gelatin in the preparation of photographic emulsions is to provide colloid protection and stabilization for the AgX crystals. Colloid stability of fine AgX particles is provided through steric stabilization, facilitated by the adsorption of gelatin on the AgX surface. A critical level of gelatin is required for such stabilization. The experimentally estimated critical gelatin surface coverage required to prevent coalescence during precipitation is in good agreement with previous theoretical calculations and with equilibrium adsorption measurements. Inadequate peptization of AgX colloid particles during precipitation can cause aggregation that can lead to undesirable agglomeration and coalescence. But in some cases gelatin can attenuate flocculation and control coalescence resulting in desirable dislocations such as the formation of twinning dislocations, which is the key step in the nucleation and growth of tabular AgX crystals. This is demonstrated by a strong correlation between the degree of flocculation during nucleation and the resulting tabular grain population fraction.