The response of carbon black stabilized oil-in-water emulsions to the addition of surfactant solutions.

We use carboxyl-terminated, negatively charged, carbon black (CB) particles suspended in water to create CB-stabilized octane-in-water emulsions, and examine the consequences of adding aqueous anionic (SOS, SDS), cationic (OTAB, DTAB), and nonionic (Triton X-100) surfactant solutions to these emulsions. Depending upon the amphiphile's interaction with particles, interfacial activity, and bulk concentration, some CB particles get displaced from the octane-water interfaces and are replaced by surfactants. The emulsions remain stable through this exchange. Particles leave the octane-water interfaces by two distinct modes that depend on the nature of particle-surfactant interactions. Both happen over time scales of the order of seconds. For anionic and nonionic surfactants that bind to the CB through hydrophobic interactions, individual particles or small agglomerates stream away steadily from the interface. Cationic surfactants bind strongly to the carboxylate groups, reduce the magnitude of the surface potential, and cause the CB particles to agglomerate into easily visible chunks at the droplet interfaces. These chunks then leave the interfaces at discrete intervals, rather than in a steady stream. For the longer chain cationic surfactant, DTAB, the particle ejection mode reverts back to a steady stream as the concentration is increased beyond a threshold. This change from chunks of particles leaving intermittently to steady streaming is because of the formation of a surfactant bilayer on the particles that reverses the particle surface charge and makes them highly hydrophilic. The charge reversal also suppresses agglomeration. Zeta potentials of CB particles measured after exposure to surfactant solutions support this hypothesis. These results are the first systematic observations of different particle release modes from oil-water interfaces produced by variations in interactions between surfactants and particles. They can be generalized to other particle-surfactant systems and exploited for materials synthesis.