(301c) Surfactant-Mediated Electrophoretic Properties of Non-Polar Dispersions

Charging in non-polar dispersions is important in a wide range of applications such as electrophoretic image displays and liquid toners. Compared to the well-known cases of aqueous media where electric charges are ubiquitous, charging in non-polar dispersions is much less common and intuitive because of the large energetic cost of generating or sustaining charges in such low dielectric media. Electric charging in non-polar liquids, however, is nonetheless observed, especially in the presence of surfactant additives. Surfactants are known to stabilize charges in the bulk liquid by incorporating ions in the core of reverse micelles and to promote the surface charging of particles dispersed in non-polar liquids. Despite its academic and industrial interest, the actual mechanism of particle charging in non-polar liquids is still poorly understood. Even though a number of  hypothetical mechanisms have been proposed, these are largely system dependent, and most studies have either focused on a single surfactant type or compared a small number of vastly different systems, making the relative contribution of different charging mechanisms hard to identify. The objective of this study is to achieve a better understanding of the surfactant-mediated charging in non-polar dispersions via thorough characterization of the well-defined systems under systematic variation of the most relevant system parameters. In particular, we investigate the particle charging mediated by a series of custom-synthesized and well-purified polyisobutylene succinimide (PIBS) surfactants with small structural variations. The surface properties of charge-acquiring colloidal particles are also varied. Conductivity and electrophoretic mobility measurements assessing the charging effect of the surfactants are complemented by light scattering studies of the surfactant micelles, and the residual water content is assessed using Karl-Fischer titration. Acid-base interactions between the particle surface and the surfactant are studied quantitatively by interfacial tensiometry and contact angle measurements.  Our experimental results indicate that smallest variations in surfactant chemistry result in an observable differences in the charging behavior, and that particle charging may indeed result from the interplay of several charging mechanisms.