Clay particle charging in apolar media

Advancements in electrophoretic displays (e.g., the Amazon Kindle®) and new high-quality printing technologies (e.g., HP Indigo®) have been made possible by the ability to control and understand particle charging in apolar media. While previous work has investigated the acid-base charging mechanisms of oxides and similar particles which have only one charging mechanism in water, the purpose of this study was to determine if and how clay particles, which have two known aqueous charging mechanisms, charge in apolar media. Characterization of kaolinite and montmorillonite surface charging properties and mechanisms were conducted in apolar media utilizing different surfactants. It was found that clay particle charging behavior as a function of surfactant concentration mimicked the expected trend of previously studied oxides, which can charge only via an acid-base mechanism. Furthermore, by comparing the point of zero charge of the edge groups (edge PZC) or the point of zero net proton charge (PZNPC) of the clay particle to the empirical effective pH (pHEff) of the surfactant, the polarity of the particle charge can be predicted. In addition to particle charging, it was shown that the surfactants also improve the stability against aggregation and settling of the particles.