Rheological behavior of water-in-diesel fuel nanoemulsions stabilized by mixed surfactants

In the present study, water-in-diesel fuel nanoemulsions stabilized by mixed nonionic surfactants were prepared. The mean droplet size of the prepared water-mixed surfactant-diesel fuel nanoemulsions was determined using dynamic light scattering (DLS) and found to be in the range of 49–140 nm. Nanoemulsion systems were then subjected to rheological evaluation. The effect of water loading percentage (5, 6, 7, 8 and 9 wt.%) on rheology was assessed at temperatures 10, 20, 30, 40 and 50 °C for time lapse 0, 1 and 2 weeks, respectively. Results showed that the prepared water-in-diesel fuel nanoemulsions exhibited low viscosity Newtonian character at all water ratios (5–9 wt.%) and at all test temperatures (10–50 °C) for time ageing up to 2 weeks. However, the viscosity of nanoemulsion systems is higher than that for either water or diesel fuel per se at the same conditions. The viscosity–temperature relationship of all nanoemulsions fitted well Arrhenius equation with high correlation coefficients that ascertain their Newtonian behavior. In addition, the higher the volume fraction of the water dispersed phase in the nanoemulsion, the larger is the water droplet size and the higher is the viscosity of the nanoemulsion system. The time influence on the rheology of the nanoemulsions has led to evolution of water droplet size due to Ostwald ripening phenomenon and a slight decrease of viscosity was observed that indicate the change of rheological character of the nanoemulsion system into time-dependent non-Newtonian character as a result of the interfacial relaxation stress by time.