A novel analytical method for precise monitoring of biodiesel water content and stability

Biodiesel is highly reactive with water, posing significant challenges to its commercialization. Therefore, accurate and safe monitoring of biodiesel is essential to prevent unwanted reactions. A U-shaped optical fiber (UOFE) was developed by coating a Ti₃C₂ MXene/TiO₂ hybrids to improve sensitivity to the refractive index (RI) and detect volatilized moisture vapor from plant-derived biodiesels. The sensitivity of three different UOFEs to RI were compared. The results indicate that UOFE with a core diameter of 200 μm reduced in transmission spectra (TS) to 1500 mV; further, coating the surface of this UOFE with Ti₃C₂ MXene/TiO₂ hybrids resulted in a three-fold reduction in TS to 4230 mV. Changes in water content from 10 to 500 ppm linearly decreased TS for soybean and algal biodiesels. A 30 % increase in TS reduction was observed for biodiesels with higher water content. Gas chromatography-mass spectrometry analysis of the fatty methyl ester contents of both biodiesels confirmed the degradation of fatty esters with higher moisture content in the soybean biodiesel and the presence of volatile compounds in the algal biodiesel. Conventional Fourier-transform infrared spectroscopy cannot detect moisture below 500 ppm in biodiesel, and Raman spectroscopy can detect only 500 ppm of water. The sensor successfully detected low concentrations of water (5–10 ppm) in lipids extracted from three types of wet microalgae. Thus, the proposed sensor shows potential for biodiesel monitoring in terms of water content, stability during storage, and volatile compound content in a rapid, accurate, non-contact, and reproducible manner.