Exploring new avenues of surfactant mediated particle charging in apolar media

Abstract

The ability to control and understand particle charging in apolar media has led to advanced digital printers (e.g., HP Indigo®) and electrophoretic displays (e.g., the Amazon Kindle®). Previous work has investigated the surfactant mediated acid-base charging mechanisms of oxides and similar particles where it has been shown that the sign of particle charge can be determined by comparing the acid-base properties of a particle, i.e., point of zero charge (PZC), to an experimentally determined “effective pH” of a surfactant molecule. This work explores the surfactant mediated particle charging mechanisms in four new venues. First, the present work shows that clay particles, which have multiple charging mechanisms in water, can charge only via an acid-base reverse micelle charging mechanism in apolar media. While the majority of clay particle charge is derived from isomorphous substitution in water, this charging mechanism is not operable in apolar media. It is also demonstrated that the charge of clay particles can be predicted by comparing their surface acid-base properties to the acid-base properties of the surfactants used. Second, the effect that reverse micelle size and structure has on particle charging in apolar media is investigated here. Results show that the size of the reverse micelle core dictates its particle charging ability. Third, binary surfactant systems in apolar media are investigated with the objective of tuning the surfactant acid-base properties for the target particles by mixing surfactants with different effective pH values. This would allow the practitioner another method to accurately control the sign and magnitude of nanoparticle charge in apolar media. Finally, while particle charging has been studied in aqueous (ε=80) and apolar (ε=2) media, limited study has been conducted in solvents with intermediate dielectric constants, commonly referred to as leaky dielectrics, where many industrial applications of particle charging exist. The present work explores particle charging in leaky dielectrics where three charging mechanisms are identified arising from particle-solvent, particle-surfactant, and solvent-surfactant interactions. In leaky dielectrics, particles can acquire charge from the solvent as described by their donor numbers, from the adsorption of surfactant ions, and from the formation of reverse micelles which facilitate an acid-base charging mechanism.