Rheological reversibility and long-term stability of repulsive and attractive nanoemulsion gels

In this research, the stability of sodium dodecyl sulfate (SDS)-stabilized canola oil nanoemulsion gels was investigated as a function of repeated rotational shear, oscillatory strain and storage time. Nanoemulsion gels, termed nanogels, were formed from oil-in-water nanoemulsions, prepared with various concentrations of SDS to get a range of droplet sizes, interdroplet interactions, and gel strengths. Repulsive nanogels were formed with 0.5, 1 and 2 times the critical micelle concentration (CMC), while attractive nanogels were prepared with 5, 10 and 15 times the CMC of SDS. No change in droplet size of the nanoemulsions was observed over a period of 90 days and an accelerated gravitation study indicated extremely high stability against creaming. All nanogels showed a remarkable recovery in viscosity and gel strength during repeated shear and strain sweep experiments, respectively. Interestingly, the elastic storage moduli (G') for the repulsive nanogels significantly decreased, converting the nanogels into flowable weak gels. For attractive nanogels, the decrease in G' was less. It was proposed that generation of surface active components due to lipid oxidation may alter the interfacial composition and ultimately reduce the thickness of the charge cloud leading to a reduction in G' in the repulsive nanogels. For attractive nanogels, the uptake of lipid oxidation products in the excess micelles and their inter-droplet transfer led to a decrease in the attractive depletion interactions and charge cloud, with subsequent loss of the gel structure. The nanogels possess great potential for use in food and related soft materials provided the loss of gel strength with time could be prevented.