This study attempts to enhance the mixture instability of methanol/hydrous methanol mixed with diesel fuel, waste cooking oil (WCO) biodiesel, and Jet A-1 fuel using n-octanol and n-decanol as cosolvent at numerous temperatures of 10 °C, 20 °C, and 30 °C. The experiment is divided into two stages: first, blending pure methanol with diesel oil, Jet A-1, and WCO biodiesel individually utilizing n-octanol and n-decanol as cosolvent at various temperatures. Second, combining hydrous methanol (90% methanol + 10 wt% water) with diesel oil, Jet A-1, and WCO biodiesel independently and applying n-octanol and n-decanol as cosolvent at different temperatures. Pure methanol or hydrous methanol is mixed with the base fuels at different mixing proportions varying from 0 to 100 vol% with 10 vol% increments. The co-solvent, mainly n-octanol and n-decanol (titrant), is progressively and separately inserted into the tube with continuous shaking by utilizing a high-precision pipette until the ternary mixtures' phase borders seem. The findings demonstrate phase separation in pure methanol-diesel and pure methanol-Jet A-1 combinations even when the blend temperature increased to 60 °C. The pure methanol/biodiesel combination proves complete solubility without adding an external agent. The results also illustrate that the ambient temperature considerably affects the stability of mixture and amount of cosolvent in the blend. n-Octanol and n-decanol showed promising performance in enhancing the phase stability issue of methanol and hydrous methanol with the base fuels. It can be deduced that the minimum amount of cosolvent is recorded for biodiesel-hydrous methanol, Jet A-1-hydrous methanol, and diesel-hydrous methanol, respectively.
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