Preparation of water-in-diesel oil nano-emulsion using nonionic surfactants with enhanced stability and flow properties

ABSTRACT

Formation of water-in-diesel oil (w/o) nano-emulsion has been achieved by a low-energy emulsification method by stabilizing a new combination of nonionic sorbitan esters surfactants, that is PEG20-sorbitan monostearate and sorbitan monooleate in mixed proportions. Different combinations of the surfactants (T6 + S8) have been tested and the best possible combination of mixed surfactants is found at a surfactants ratio of 35:65 (wt/wt) for T6:S8 at hydrophile–lipophile balance (HLB) = 8.01, which resulted in smaller droplet size of 44.87 nm. A phase diagram study is performed to identify the zones of formation of transparent, translucent, and opaque emulsions (44 nm < droplet size < 700 nm). The mechanism responsible for the instability of emulsion is explained by Ostwald ripening, with inference describing a decrease in particle size with the Ostwald ripening rate. In case of nano-emulsion of droplet size 64.28 nm, the Oswald ripening rate is found as 0.0874 × 10⁻²⁷ m³ · s⁻¹. Comparison of Ostwald ripening rate with other sets of surfactants obtained by different authors showed the lowest rate among them, indicative of enhanced stability. A rheological study of the tested set of nano-emulsions depicts the Newtonian behavior (1.0371 ≤ n ≤ 1.0826) over a wider range of shear rates (10–1000 s⁻¹) at different temperatures (25–40°C).

GRAPHICAL ABSTRACT

KEYWORDS:

• Nano-emulsion
• nonionic surfactant
• ostwald ripening
• rheology
• stability