Mobile ions and electric surface charges are ubiquitous in water and other polar liquids, but should play no role in apolar liquids, where the energetic cost of charge dissociation is typically too large for thermal activation. Defying this conventional wisdom, some surfactant additives can raise the conductivity of apolar liquids by many orders of magnitude and promote electrical charging of immersed solid surfaces or of suspended colloidal particles. The resulting ability to direct particle motion with applied electric fields is crucial, e.g. for liquid toners and electrophoretic displays but neither the mechanism of particle charging nor the effect of the directing field are currently understood. Building on the PI's experience in measuring and modeling electrostatic interactions, and with strong support from the Hewlett-Packard team developing the next generation of color electrophoretic displays, the investigators propose to unravel the fundamental principles of particle charge and field-driven motion in apolar surfactant solutions.
The working hypothesis in this project is that surfactants simultaneously influence the electric behavior of apolar dispersions in at least three ways: by promoting surface charging via acid-base interaction, by solubilizing surface counterions inside of surfactant aggregates, and by forming micellar ions in the solution bulk via charge disproportionation. They aim at disentangling these mechanisms with an original suite of experimental techniques accessing separately the charge contained in the bulk solution and on the particle surface, the particle-particle and particle-field interaction, as well as the amount of adsorbed surfactant and residual water. Using particles of different surface characteristics and surfactants with systematically varied architecture will allow the investigators to find the elusive answer to the question what makes some surfactants more effective than others at imparting charge and electrophoretic mobility on a given particle type. Thermodynamic models for the charging reaction and surfactant adsorption will be tested against their experiments.
Broader Impact
The proposed study should significantly expand the understanding of charging processes in apolar liquids and on immersed solid surfaces. Applications range from safety measures in fuel transportation and the design of electrophoretic displays and electronic skins to the stabilization of emulsions with solid particles relevant for food formulation and skin care creams as well as for oil recovery and oil spill remediation. The research directly touches on our understanding of surfactants surrounding us in everyday life, and can easily be illustrated with examples accessible to non-experts.
Building on this educational potential, the PI works with pre-schoolers through first graders in a predominantly (97%) African-American school via hands-on science workshops designed to reinforce the natural curiosity in these young children. An ongoing close collaboration with the classroom teachers makes these workshops more impactful by enhancing classroom activities with related science materials; and the most successful demonstrations are captured in science demo kits with appropriate materials and instructions for the teachers to use in later years independently of the PI. The PI also supports diversity at the collegiate level by recruiting female students to his lab, which has had a majority of women at any given time, and has a history of involving African American undergraduates in its research.
