Effects of temperature on viscosity, stability, and microstructure of water-in-biodiesel microemulsions

Using the promising fuel microemulsions based on diesel fuel (29.41–95.33 vol. %) and the mixture of diesel fuel with the vegetable additive in the form of methyl esters of rapeseed oil (4.80–43.27 vol. %) stabilized by the nonionic surfactant, polyethylene glycol ester of isononylphenol, and the co-surfactant, 2-ethylhexanol (total emulsifier concentration of 2.72–41.18 vol.%), we investigated their thermal stability (up to 100–110 °C). Two methods to determine the thermal stability of the microemulsions are proposed, namely, the express thermostatting method and the long-term one. Based on the results obtained, the analysis of the influence of the microemulsion component composition on the range of their thermal stability is carried out. The introduction of the bioadditive is shown to result in a narrowing of the ranges of existence of the single-phase microemulsion by 10–15 °C. The temperature effect on the dynamic viscosity of the microemulsion systems in their single-phase region is investigated. Based on the fish-cut diagrams for the microemulsions in the coordinates “emulsifier content – temperature,” the color map diagrams are plotted, which show the dependence of the dynamic viscosity of the microemulsions on the specified parameters, namely, temperature, water fraction, and vegetable additive. The assumed regions of existence of microemulsions with a dispersed phase in the form of spherical aggregates are determined in the fish-cut diagrams.