Adsorption of Aerosol-OT on graphite from aqueous and non-aqueous media

Efficient dispersion of carbonaceous particles in polar and non-polar liquids is important in the processing and use of paints, pigments, printing inks and tertiary oil. In this study we examined the adsorption of an anionic surfactant, Aerosol-OT (AOT), on graphite particles in cyclohexane and water and the ensuing changes in their dispersion properties. The adsorption of AOT on hydrophobic graphite particles in cyclohexane results from the tendency of the polar head group of the surfactant molecules to partition into hydrophilic domains away from the solvent which is hydrophobic. Fluorescence studies using 1-methyl-8-oxyquinolium betaine as probe reveals the presence of “reverse-micelle”-like surfactant aggregates at the interface at all surfactant concentrations. The observed changes in dispersion stability suggest the formation of interparticle surfactant aggregates at low surface coverages while at higher coverages the aggregates are formed on individual particles. AOT adsorbs on graphite from aqueous solutions as well, despite the unfavorable electrostatic repulsion, evidently through hydrophobic interactions. Such adsorption stabilizes the aqueous dispersions by providing increased electrostatic repulsion among the particles. Desorption of preadsorbed AOT from graphite when contacted with different solvents showed a marked solvent effect with maximum desorption occurring in solvents of intermediate polarity where the hydrophilic and hydrophobic driving forces are balanced.