Rates of flocculation and deflocculation in dispersions of carbon black in hydrocarbons

The optical density of carbon black suspensions in heptane, toluene, and solutions of polystyrene in toluene and in cyclohexane, decreases with time and ultimately reaches a steady-state value. This is interpreted in terms of the simultaneous occurrence of flocculation and deflocculation following, respectively, second- and first-order kinetic rate laws. A method is developed for obtaining an absolute value of β, the rate constant for deflocculation, from the time dependence of the optical density, assuming that optical density is proportional to floc concentration.

The rate constants for both flocculation and deflocculation increase with increasing initial average size of the flocs. The specific rate of redispersion, β, increases with increasing polystyrene concentration in cyclohexane, the effect being explicable in terms of decrease of attraction between carbon particles due to the presence of tightly adsorbed polymer on the surface. In toluene, β passes through a minimum with increasing polystyrene concentration, attributable to decreased redispersion resulting from bridging of particles by extended loops of adsorbed polymer.