Kinetic stability of surfactant stabilized water-in-diesel emulsion fuels

Abstract

Commercialization of water-in-diesel emulsion fuels demand investigation of the stability of the emulsions over extended periods of time during their storage in the fuel tank as well as during the engine operation. A detailed analysis of the stability of water-in-diesel emulsion fuels stabilized by a mixture of commercially available surfactants is presented. The hydrophile lipophile balance (HLB) is varied by changing the relative composition of the surfactants, Span®80 and Tween®80, and its influence on the microstructure of the formulations is explored using bright field optical microscopy. Emulsions with smaller droplet sizes can be formulated at intermediate compositions of the mixed surfactant system. The effect of surfactant concentration is investigated using microscopy and dynamic light scattering. At 10% surfactant concentration, the emulsification leads to the formation of nanoemulsions containing water droplets of diameter as small as 159 nm. Finally, time stability of emulsions formed at various surfactant concentrations is investigated. Emulsions formed at 2% and 5% surfactant concentration reveal a macroscopic phase separation of initially homogeneous emulsions into phases containing large emulsion droplets in the bottom phase and smaller emulsion droplets in the top phase. In contrast, the emulsions formed at 10% surfactant concentration, show enhanced time stability, with no macroscopic phase separation, although the droplet size increases from 159 nm to 330 nm upon 50 days of storage. An understanding of the change in microstructure and droplet size as a function of time presented in this work is crucial for the systematic investigation of use of such fuels in practical applications.