Switchable Oil-in-Water Emulsions Stabilized by Like-Charged Surfactants and Particles at Very Low Concentrations

Abstract

A novel CO2/N2 switchable n-decane-in-water emulsion was prepared, which is stabilized by a CO2/N2 switchable surfactant [N′-dodecyl-N,N-dimethylacetamidine (DDMA)] in cationic form in combination with positively charged alumina nanoparticles at concentrations as low as 0.01 mM and 0.001 wt %, respectively. The particles do not adsorb at the oil–water interface but remain dispersed in the aqueous phase between surfactant-coated droplets. A critical zeta potential of the particles of ca. +18 mV is necessary for the stabilization of the novel emulsions, suggesting that the electrical double-layer repulsions between particles and between particles and oil droplets are responsible for their stability. By bubbling N2 into the emulsions, demulsification occurs following transformation of DDMA molecules from the surface-active cationic form to the surface-inactive neutral form and desorption from the oil–water interface. Bubbling CO2 into the demulsified mixtures, cationic DDMA molecules are re-formed, which adsorb to the droplet interfaces, ensuring stable emulsions after homogenization. Compared with Pickering emulsions and traditional emulsions, the amount of switchable surfactant and number of like-charged particles required for stabilization are significantly reduced, which is economically and environmentally benign for practical applications.

Behavior of Smart Surfactants in Stabilizing pH‐Responsive Emulsions

“…Chemie Communications is higher than the critical zeta potential (AE 18 mV) required for stabilization of these oil-in-dispersion emulsions. [7,41] However, demulsification did occur in the above systems. This suggests that the bending configuration of Bola form surfactant is also inferior for stabilization of the novel oil-indispersion emulsions.…”

Charge‐Reversible Surfactant‐Induced Transformation Between Oil‐in‐Dispersion Emulsions and Pickering Emulsions

“…However, the surface tension of the silica nanoparticle dispersion in DMUa solutions is observed to be lower than that of DMUa solutions (without particles) (Figure S15), [21] as in other oil‐in‐dispersion emulsions [29, 30] . This has been attributed to the fact that the addition of similarly charged nanoparticles promotes diffusion of the similarly charged ionic surfactant from bulk to interface due to repulsive interactions in bulk [21, 22] …”

Behavior of Smart Surfactants in Stabilizing pH‐Responsive Emulsions

“…Zuschriften is higher than the critical zeta potential (AE 18 mV) required for stabilization of these oil-in-dispersion emulsions. [7,41] However, demulsification did occur in the above systems. This suggests that the bending configuration of Bola form surfactant is also inferior for stabilization of the novel oil-indispersion emulsions.…”

“…Reducing the pH may also affect the zeta potential of the particles. In the above system, however, the zeta potential of the alumina particles is always highly positive between pH 3 and 10 [6,7] and…”

“…Reducing the pH may also affect the zeta potential of the particles. In the above system, however, the zeta potential of the alumina particles is always highly positive between pH 3 and 10 [6,7] and 16)-N at different concentrations (given), taken (B) 24 h and (A) 1 month after preparation at room temperature (23 8C). The tertiary amine group makes aqueous solution weakly alkaline (pH 7.0 at 0.1 mM).…”