Factors Influencing Particle Charge in Apolar Media

            A growing number of applications seek to use electrostatic effects in apolar media for aggregation stability and manipulating particle movement in such systems, as exemplified by the Amazon Kindle® reader and HP ElectroInk® printer. Progress is limited, however, by incomplete knowledge of the mechanism(s) of particle charging in these systems. To help shed light on this, the current work focuses on the study of several common factors that affect the polarity and magnitude of particle charge in apolar systems: particle surface functional groups, surfactant functionality, trace water content, and the implementation of synergists.

            One theory for the origin of particle charge in apolar media involves an acid-base charge transfer between the particle surface functional groups and adsorbed surfactant molecules. This theory is explored by measuring the electrophoretic mobility of a series of mineral oxide particles that are dispersed in Isopar-L using common charge-stabilizing surfactants: OLOA 11000 (basic), SPAN 80 (acidic), and AOT (ionic). The mineral oxide series is selected to yield a wide range of point of zero charge (pzc) values when in water (in terms of pH), which serves as an indication of the relative acidity or basicity of the particle surface functional groups. It is found that the polarity and magnitude of the particle charge is strongly dependent on the acid-base functionality of both the particle functional groups and the surfactants.

            Another important factor that affects particle charging is the amount of trace water that is introduced to the system by hygroscopic particles and surfactants. The amount and location of this water can have significant effects on the electrical properties of these systems. The current work not only explores the effect of water content on particle charge in apolar dispersions, but differentiates between water bound to the particles and water located in inverse micelles. This is accomplished by measuring the electrophoretic mobility of silicon dioxide particles dispersed in solutions of Isopar-L and OLOA 11000. The water content is determined for both the dispersion and the supernate after centrifuging the particles out.

            Finally, synergists are often employed to enhance the adsorption of surfactants onto particle surfaces, providing steric stabilization against aggregation. However, these synergists can also have a dramatic effect on particle charging in apolar systems by either adsorbing to the particle surface or becoming incorporated into the surfactant inverse micelles. In this study we examine the electrophoretic mobility of commercially available cyan and magenta pigments with their synergist counterparts dispersed in heptane with charge-stabilizing surfactants OLOA 11000, AOT, and Span 80.