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Water-in-diesel emulsions have garnered attention in recent years as a potential route to mitigate vehicular pollution. In this work, the formation, flow behavior, and microstructure of water-in-diesel microemulsions using an anionic surfactant, sodium bis­(2-ethylhexyl) sulfosuccinate (AOT), is investigated and compared with the water–decane–AOT microemulsion system. Rheological measurements on the microemulsions indicate Newtonian behavior over a moderate range of shear rate. The viscosity of the microemulsions shows a weak dependence on the molar ratio of water-to-surfactant (ω) but increases significantly with the increase in the dispersed phase volume fraction (Φ). Dynamic light scattering (DLS) investigation of the microemulsions indicates a single exponential decay of the intensity autocorrelation functions over a wide range of water and AOT concentrations, indicating that the dispersed phase consists of a monodispersed population of water droplets. Results suggest that optimizing ω can regulate the mean diameter of droplets in the water-in-diesel emulsion fuel and a control over the number density of droplets can be achieved by independently varying Φ without altering the droplet dimension. Assessment of storage stability of the emulsified fuels showed no change in the diameter of the droplets for more than six months. We show that thermoreversible formation of water-in-diesel emulsions of nearly identical diameter when the diesel based microemulsions are subjected to repeated cooling and heating cycles provides further proof of their thermodynamic stability. Therefore, microemulsion fuels with excellent time-stability, compared to the more studied kinetically stable emulsion fuels, could be more suitable candidates as alternative fuel.

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The ability to modulate the size, the nanostructure, and the macroscopic properties of water-in-oil microemulsions is useful for a variety of technological scenarios. To date, diverse structures of water-in-alkane microemulsions stabilized by sodium bis(2-ethylhexyl) sulfosuccinate (AOT) have been extensively studied. Even though the decisive parameter which dictates the phase behavior of micremulsions is the nature of the continuous phase, relatively very few reports are available on the structure and interactions in the microemulsions of aromatic oil. Here, we present a fundamental investigation on water-in-xylene microemulsions using small-angle neutron scattering (SANS) at a fixed molar ratio (ω) of water to AOT. We elucidate the microstructural changes in the water-AOT-xylene ternary system at dilute volume fractions (Φ = 0.005, 0.01, 0.03), where the droplet−droplet interactions are absent, to moderately concentrated systems (Φ = 0.05, 0.10, 0.15, and 0.20), where colloidal interactions become important. We also characterize the reverse microemulsions (RMs) for thermally induced microstructural changes at six different temperatures from 20 to 50 °C. Depending on the magnitude of Φ, the scattering data is found to be well described by considering the RMs as a dispersion of droplets (with a Schulz polydispersity) which interact as sticky hard spheres. We show that while the droplet diameter remains almost constant with increase in the volume fraction, the attractive interactions become prominent, much like the trends observed for water-in-alkane microemulsions. With increase in temperature, the RMs showed a marginal decrease in the droplet size but no pronounced dependence on the interactions was observed with the overall structure remaining intact. The fundamental study on a model system presented in this work is key to understanding the phase behavior of multiple component microemulsions as well as their design for applications at higher temperatures, where the structure of most RMs breaks down.