(85a) Surfactant Mediated Charging of Polymer Particles in Apolar Solvents

Apolar solvents do not easily accommodate electric charge, but it has long been known that surfactant additives can cause substantial particle charging in these media. While the phenomenon is commonly exploited, e.g. in liquid toners and electrophoretic displays, the charging mechanisms remain rather unclear. Competing popular hypotheses involve (a) the asymmetric adsorption of (reverse) charged micelles that originate from equilibrium fluctuations around a zero micelle charge, (b) the presence of dissociated ionic surfactant molecules at the particle surface, (c) the dissociation or protonation of ionizable particle surface groups, possibly facilitated by residual water introduced with the surfactant, or (d) surface charging due to acid-base interactions between the surfactant and the particle surface. Here we present measurements of the interaction energy and zeta potential of polymer particles with different acid-base characteristic and different ionizable surface groups in alkane solutions of ionic and of nonionic surfactants. Our results cannot be explained by any one of the proposed mechanisms alone, but they suggest a rather complex interplay of at least two and sometimes three different charging pathways. We will sketch a strategy to systematically disentangle the contributing effects.