Résumé
Model systems of charged colloidal particles with non-uniform and fluctuating charge distribution have been designed. They consist of C12EO5 stabilized decane micoremulsions that have been doped with various cationic gemini-type surfactants. The valence of the doping surfactants varies from 1 to 4 and their molecular structure is either linear or branched. The phase diagrams of the doped microemulsions have been established. Close to the emulsification failure temperature, the systems consist of positively charged spherical droplets of about 8nm with Z structural charges, Z varying from 0 to 100. Static light scattering and small angle neutron scattering show that the droplets doped with multivalent and branched surfactants repel each other much less than the ones doped with monovalent or linear multivalent surfactants. To account for these results several parameters were considered: The particles are probably polydispersed in charge as revealed by the observation of both diffusive modes in dynamic light scattering. However, for a given value of Z, the charge polydispersity is essentially determined by the surfactant valence and does not account for the influence of the molecular structure of the surfactant. Counter-ion condensation has been measured by bromide specific electrode and is more pronounced when the doping surfactants are multivalent and branched. This can be attributed to higher local density of charge. Knowing the effective charge of the particles allows calculation of the osmotic compressibility of the solution S(0). When the Debye length is assumed to be determined by the free counter-ions, it means without any open parameter, calculated S(0) is higher than the one obtained experimentally. To recover the experimental S(0) by calculation, an extra screening that reduce the range of the interaction has to be introduced. It varies linearly with the average distance between the patches of charge at the droplet interface and with the molecular structure of the surfactant.