Electric Charging of Dispersion Particles In Nonpolar Solutions

The electrostatic stabilization of colloidal dispersions is usually considered the domain of polar media only, because the energetic cost of introducing charges in nonpolar liquids is prohibitively high. The presence of reversed surfactant micelles, however, can reduce this cost, and some surfactant additives are known to raise solution conductivity and to promote particle charging and electrostatic stabilization even in solvents of very low dielectric constant. In the present study we have investigated the effect of ionic and nonionic surfactants on alkane based dispersions of polymer particles with different surface headgroups. Our results confirm that both ionic and nonionic surfactants can mediate particle charging, but suggest different charging mechanisms. They also show that, contrary to conclusions drawn from earlier studies, electrostatic stabilization of nonpolar dispersions can be achieved well below the critical surfactant concentration for micellization. We shall discuss the implications for possible charging pathways and point out some difficulties and pitfalls in characterizing the charging state of particles in nonpolar liquids.