On the Molecular Mechanism of Photographic Stabilization

Abstract

Measurements of the kinetics of sulphur sensitization and its retardation by stabilizers support a mechanism for photographic stabilization in which the essential stabilization efTect involves the interaction of the stabilizer molecule with adsorbed sulphide rather than its precursor, the sulphur sensitizer. The effect of stabilizer on the sulphur sensitizer is incidental. It is proposed that the growth of sulphide specks is retarded by stabilizer. A mechanism is suggested in which the decreased capture rate of adsorbed sulphide by the growing speck is due to the formation of an equilibrium controlled ternary surface chelate complex of stabilizer and sulphide with a bridging metal (silver) ion. Because this complex is immobile the diffusion rate of the adsorbed sulphide is decreased. This modifies the speck size distribution, besides retarding the digestion rate, so that above a certain stabilizer quantity the sensitivity achieved is reduced. The structural requirement for stabilizers is that they arebi(or multi)-dentate nucleophilic ligands which can coordinate metal ion bonded to adsorbed sulphide as well as that on the crystal surface. Adsorption is only part of the requirement for stabilization. This treatment supports the model of sulphur sensitization by nucleation and growth of specks from adsorbed sulphide. The evidence described includes the consideration of compounds which are both stabilizers and sulphur sensitizers and sensitization kinetics measured with stabilizer added after as well as before sulphide formation.