The Response of Carbon Black Stabilized Oil-in-Water Emulsions to the Addition of Surfactant Solutions

Abstract

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.

Controlling Pickering Emulsion Destabilisation: A Route to Fabricating New Materials by Phase Inversion

“…Another strategy for displacing particles from drop surfaces is to add surfactant which competes for the oil-water interface [32,33,34]. Vella et al [35] observed that adding surfactant could disrupt particle layers attached to planar water surfaces.…”

“…Katepalli et al [33] argued that surfactant addition will cause particle displacement from a drop (of radius, r d ) if a surfactant-stabilised drop (of the same radius) has a lower free energy than the particle-stabilised drop. Thus adding surfactant causes particle displacement due to the emulsion system seeking a lower energy state.…”

“…Thus adding surfactant causes particle displacement due to the emulsion system seeking a lower energy state. Katepalli et al [33] found that for the surfactant-stabilised emulsion to be more stable, the following inequality must be satisfied: $\frac{{\mathsf{\gamma }}_{ow}-{\mathsf{\gamma }}_{os}}{{\mathsf{\gamma }}_{ow}}>f{\left[\frac{1-cos{\mathrm{sans-serif\theta }}_{ow}}{\sin {\mathrm{sans-serif\theta }}_{ow}}\right]}^2$ where γ os is the interfacial tension of a surfactant-stabilised drop and f is the fraction of the interfacial area occupied by particles. For particles of intermediate (or neutral wettability (θ ow = 90°), surfactant addition can cause particle displacement if the fractional change in the oil-water interfacial tension with surfactant adsorption is greater than the fraction of the drop surfaces coated with particles.…”

“…For particles of intermediate (or neutral wettability (θ ow = 90°), surfactant addition can cause particle displacement if the fractional change in the oil-water interfacial tension with surfactant adsorption is greater than the fraction of the drop surfaces coated with particles. Katepalli et al [33] examined the response of octane-in-water emulsions stabilised by carbon black nanoparticles to exposure to solutions of surfactant at their critical micelle concentrations. They found that adding Triton X-100, which reduces the interfacial tension by 94% from 51 to 3 mN·m −1 , was sufficient to displace carbon black particles from the octane drops [33].…”

Stabilization of Oil-in-Water Emulsions with Noninterfacially Adsorbed Particles

“…10 In cases where the particles are attracted to the interfaces, emulsions may be either stabilized or destabilized by the interactions between the particles and surfactants. [11][12][13][14][15] For example, Whitby et al reported on the ability of anionic surfactant to destabilize Pickering emulsions prepared with hexadecylsilane modified silica when added at concentrations greater than the critical micelle concentration (CMC). 13 In cases where synergistic stabilization of the emulsions exist, the effect may be attributed to surfactant-induced flocculation of the particles leading to greater steric hindrance to coalescence, or enhanced 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 4 stabilization due to lowering of the interfacial tension between the oil and water phases without a significant loss of particles from the interface.…”