Effect of synergists on organic pigment particle charging in apolar media

The current work investigates the apolar charging behavior of organic pigment particles and the role that synergists play in regard to particle charging. Organic pigments are often used in apolar paints, inks, and most recently electrostatic lithography. For electrolithography to work, the particles must be both stable and possess the correct polarity and magnitude of charge. It is therefore important to better understand the charging behavior and potential charging mechanisms of these particles that have received little or no attention in the literature. Unfortunately, these already complex systems are further complicated by the fact that the stability of organic pigments is often improved through the use of synergists. Synergists are designed to enhance the adsorption of steric stabilizers to the particles. However, their effect on particle charging has not been previously published. In this study, the particle zeta potential is determined for apolar dispersions of magenta and cyan particles in heptane (with and without synergist present). The particles are dispersed with three different surfactants commonly used in apolar charging studies: Span 80, Aerosol-OT, and OLOA 11000. Acid-base interactions appear to play an important role, particularly for cyan. However, due to the complexity of these systems, any general rule must be applied with caution as the particle, surfactant, and synergist chemistry all determine the nature of the particle charge.