Adsorption of pseudoisocyanine on nanoparticles of silver halides

Silver halide particles were synthesized in the water in oil microemulsions consisting of AOT (sodium bis-2 ethylhexylsulfosuccinate)/n-heptane/water. The diameter of the particles is equal to 46 Angstrom for the silver bromide particles and 32 Angstrom for the silver chloride particles. Molecules of pseudoisocyanine (PIC) are adsorbed on the silver halide nanoparticles where they replace the molecules of the surfactant (AOT). The pseudoisocyanine, as other cyanines, forms aggregates (called J-aggregates) on the silver halide particles. This causes an instability in the silver bromide dispersion. Indeed, a large part of the surface will be unprotected by the surfactant and the particles finally coalesce. The stability is however improved if the size of the J-aggregates is smaller. This can be achieved by using an inhibitor, such as p-xylene or by adding another surfactant CTAB (cetyltrimethylammonium bromide) which is more strongly adsorbed than AOT. The concentration of CTAB influences the stability of the particles and the size of the J-aggregates. If an appropriate concentration of CTAB is used, the particles remain stable for 30 days after the adsorption of pseudoisocyanine. The behaviour of pseudoisocyanine on silver chloride particles is significantly different. Indeed PIC was desorbed as a function of time. The adsorption can be improved if the reaction is carried out at low temperature. (C) 1999 Elsevier Science B.V. All rights reserved.