Size-dependent thermodynamic properties of silver aggregates. Simulation of the photographic development process

Pulse radiolysis and time-resolved optical spectroscopy were used to observe the earliest steps of silver aggregation in the presence of an electron donor whose electrochemical potential is close to those of usual photographic developers. A critical size has to be reached by the silver aggregate before it is thermodynamically able to accept electrons from the donor, then to grow by an autocatalytic process with alternate adsorption of Ag⁺ ions and electron scavenging. From a computer simulation the critical aggregation numbern _c is found to be 4 and hence the electrochemical potential of the critical aggregate is ≈?0.4 V/NHE. The size-dependence of the electrochemical potential in an aqueous solution is discussed and compared with that of the ionization potential in the gas phase. A tentative extrapolation of the present interpretation to the mechanism of the photographic development is proposed.