Influence of positively charged nanoparticles on the stability of oil-in-water emulsions stabilized by a cationic surfactant at extremely low concentration

Abstract

Hypothesis

Charged particles and like-charged ionic surfactant can co-stabilize oil-in-dispersion emulsions at very low concentrations. The surfactant molecules adsorb at the oil–water interface endowing droplets with charge whereas particles remain dispersed in the aqueous phase forming a thick lamella between droplets. The reduced van der Waals attraction between droplets together with the electric double layer repulsion between droplets and between droplets and particles prevents droplets from flocculation and coalescence.

Experiments

n-Decane-in-water emulsions co-stabilized by positively charged alumina nanoparticles (0.0001–0.06 wt%) and cationic surfactant cetyltrimethyl-ammonium bromide (CTAB, 0.002–0.1 mM) were prepared and characterized. Specifically, the minimum particle concentration required for emulsion stabilization as a function of CTAB concentration was determined, which describes a V shaped boundary separating stable emulsions from unstable ones.

Findings

A model based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory was proposed to calculate the droplet–droplet and droplet-particle interactions. Along the V shaped boundary particles are attracted by droplets at large distance but may form a monolayer between droplets when they approach at small distance. Both the attraction and repulsion between droplets and particles are proportional to the particle number surrounding unit area of droplet interfaces and the repulsion is always larger than the attraction. This together with the droplet–droplet repulsion ensures emulsion stabilization. Increasing CTAB concentration results in high interfacial potential but also high counterion concentration; to compensate, the particle concentration required for emulsion stabilization is initially reduced and subsequently increased yielding the V shaped boundary.