(85b) Electroacoustics, Conductivity & Dispersion Forces in Nonpolar Carbon Black Dispersions

High-volume-loading dispersions of carbon black in hydrocarbon oils are of interest both commercially, where they serve as coatings, pigment-based inks, and model systems for automotive soot; and scientifically, as they form heterogeneous, space-filling, weakly attractive gels with unique rheological properties.  Common dispersants used to stabilize carbon in oils typically utilize both electrostatic and steric repulsion mechanisms.  In this work, we study systems of Cabot Vulcan XC72 carbon black (a highly fractal and conductive black), stabilized using both ionic and nonionic polymer dispersants, to better understand and characterize the interparticle forces at play in these dispersions.

Characterization of the electrostatic stabilization in the dispersions involves measuring the electrophoretic mobility of the carbon particles. Because the properties of these systems are volume fraction dependent, we must make mobility measurements at the particle concentrations we are interested in studying. Instead of light-scattering-based techniques, we instead use electroacoustics to measure particle mobility.  By measuring the electroacoustic response of these dispersions, along with the dispersion rheology, we can correlate the breakup of the carbon black gels with the evolution of surface charging.  We additionally study the conductivity response of the dispersions to low-frequency electric fields. Because the carbon black particles are conductive, they undergo dielectrophoretic chaining in LF fields. Particle chains that are sufficiently stabilized, especially with a thick steric surface layer, tend to have lower conductivities in these fields.  Finally, the sensitivity of the carbon dispersions to solution ionic concentration and valence is observed, recalling some of the earliest analytical techniques in colloid science, including examination of the Schultz-Hardy Rule for nonpolar solvents.