Influence of droplet size on repulsive and attractive nanoemulsion gelation

The aim of this paper is to understand how the change in droplet size and ionic emulsifier concentration influence cage-driven repulsive jamming and bond driven attractive gelation in nanoemulsions. It was hypothesized that in repulsive regime decrease in droplet size would increase the influence of counterion shell layer around the nanodroplets thereby increasing the effective oil volume fraction (phi(eff)) beyond random jamming. In attractive gelation, decrease in size and a corresponding increase in number of droplets would increase the strength of fractal network leading to gelation at a lower phi(eff). 40 wt% canola oil-in-water nanoemulsions stabilized with sodium dodecyl sulfate (SDS) were prepared by multiple passes through a high-pressure homogenizer at two different emulsifier concentrations, one in repulsive regime (2 times CMC) and the other one in attractive regime (15 times CMC). Rheology of the resulting nanogels was determined using a controlled stress rheometer. Repulsive nanoemulsions transformed from fluids (G' < G") at droplet size >250 nm to weakly gelled glassy state (G' > G") with no yield stress at around 224 nm to jammed state below 200 nm where strong nanogels were formed (G' G"). When the effect of droplet size was transformed to phi(eff), a gradual increase in gel strength was observed where the transition to glassy and jammed state began at 0.58 and 0.65, respectively. For attractive nanoemulsions, a rapid increase in gel strength was observed below 140 nm with only a very small change in phi(eff) from 0.42 to 0.47, which was attributed to the charge screening effect of the higher number of ions from SDS. It was proposed that the interdroplet interactions transformed from oscillatory structural forces generated by higher SDS micelles at larger droplet sizes to depletion attraction where stronger fractal network formed by increased number of smaller nanodroplets led to rapid increase in gel strength. (C) 2015 Elsevier B.V. All rights reserved.