Water-in-diesel system is one of the most conceptual promising proposed solutions to overcome nitrogen oxide and smoke emission which environmental harmfully produced from using conventional fuel systems. Stability of water-in-diesel emulsion (WIDE) is a major issue in applying this system. In this research, stability of WIDE was investigated by using two surfactants, namely, Span 20 and Tween 85. Two stages of experimental investigation were conducted. The first stage was a screening to select the proper surfactant and mixing time to obtain the best emulsification. Span 20 was found to give higher stability as compared to that of tween 85. In addition, WIDE showed higher stability when the mixing time was thirty minutes. The second stage, response surface method (RSM) with central composite design (CCD) were applied for modelling and optimization of WIDE mixing conditions. Three parameters (water/diesel percentage, surfactant percentage and pH value) were studied in this technique to obtain the optimum stability. ANOVA analysis showed that the model was significant and the water percentage of 5% and surfactant percentage of 1% showed the highest emulsion stability. pH affected on WIDE system under mild acidic and alkali conditions. The results showed that the pH plays an important role for the stability of WIDE.
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