The preparation of kerosene-based ferrofluid using oleic acid as a stabilizer has been done in two main steps: first, the synthesis of iron oxide (Fe \(_{3}\) O \(_{4}\) ) nanoparticles from Fe \(^{2}\) + and Fe \(^{3}\) + ions in alkaline medium using the co-precipitation method, and second, the formulation of a kerosene-based ferrofluid with varying nanoparticle-surfactant ratios to evaluate its stability. Characterization results showed that the ferrofluid consisted of Fe \(_{3}\) O \(_{4}\) nanoparticles with a cubic inverse spinel structure, a crystallite size of 10.05 nm, a Fe \(_{3}\) O \(_{4}\) purity of 75.6%, a hydrodynamic size of 17.24 nm, a saturation magnetization of 22.93 emu/g, and a remanent magnetization of 2.83 emu/g. Magnetic properties confirm the superparamagnetic nature of the nanoparticles and their responsiveness to an external magnetic field, which is essential for ferrofluid performance in various applications. Kerosene-based Fe \(_{3}\) O \(_{4}\) ferrofluids coated with oleic acid exhibited stable behavior throughout a 29-day observation period. A second phase appeared on the 8th day and showed constant volume through day 29, indicating stability up to 29 days. The 1.75 nanoparticle-to-surfactant ratio showed the optimal composition, exhibiting only a minimal separated phase. UV–Vis measurements showed an approximately 11% increase in absorbance, indicating relative stability. This study identifies the optimal Fe \(_{3}\) O \(_{4}\) -to-oleic ratio for achieving long-term stability in a kerosene medium. Unlike previous studies that primarily focus on short-term dispersion using different surfactant materials, this research provides quantitative insight into phase stability in the systematic optimization of the nanoparticle–surfactant ratio in a low-polarity kerosene carrier and establishes a composition that preserves superparamagnetic behavior.
