Effects of Different Oxygen-Containing Co-solvents’ Molecular Structure and Intermolecular Forces on the Solubility of Methanol-Renewable Diesel Fuel Blends

The limited solubility of methanol in diesel fuels necessitates the use of co-solvents to achieve homogeneous and stable blends. This study investigated the effectiveness of two higher alcohols (1-octanol and 1-dodecanol) and one short-chain ester (methyl butyrate) in improving the miscibility of methanol with a renewable diesel. Methanol-renewable diesel blends with methanol energy shares ranging from 5% to 90% were prepared, and co-solvents were titrated until a clear single-phase mixture was obtained. Ternary phase diagrams were used to characterize the phase behavior at room temperature. Intermolecular interactions governing blend stability were analyzed using Hansen solubility parameters, complemented by ATR-FTIR spectroscopy to assess changes in hydrogen bonding and molecular polarity. The results demonstrate that the co-solvent molecular structure strongly influences blending performance. Higher alcohols exhibited superior effectiveness compared to the ester, with 1-dodecanol providing the highest stabilization efficiency, followed by 1-octanol, while methyl butyrate showed the poorest performance. The enhanced performance of higher alcohols is attributed to their ability to balance dispersion forces and hydrogen-bonding interactions, as supported by Hansen solubility parameter analysis. These findings offer mechanistic insight into methanol-renewable diesel miscibility and provide practical guidance for co-solvent selection in methanol-based compression-ignition fuel formulations.