Response surface optimization of ultrasonic encapsulation parameters to enhance stability of gum arabic–based citronella oil emulsions

Botanical oil-based dispersions are promising candidates for sustainable pest management, but their practical use is limited by the high volatility and poor aqueous stability of active essential-oil constituents. This study optimized gum arabic–stabilized citronella oil dispersions containing patchouli oil as a co-oil using pulsed probe ultrasonication and response surface methodology based on a central composite design. The effects of nominal power, pulse-on duration, and processing time on hydrodynamic diameter were evaluated within the ranges of 550–650 W, 5–15 s, and 8–12 min, respectively. The CCD generated a broad Z-average range of 0.72–5.88 µm, indicating strong sensitivity of droplet breakup to ultrasonic conditions. The quadratic model showed good predictive performance (R² = 0.938; adjusted R² = 0.882) and identified pulse-on duration as the dominant factor, followed by nominal power and their interaction. Processing time was not significant within the tested sonication domain. Numerical optimization predicted an optimum condition of 600 W, 10 s pulse-on duration, and 10 min processing time, giving a predicted Z-average of 958 nm and an experimental value of 897 nm, corresponding to approximately 6–7% deviation. The optimized dispersion exhibited negative zeta potentials of −39 to −44 mV, indicating electrostatic–steric stabilization by gum arabic at the oil–water interface. SEM showed a compact freeze-dried matrix, while FTIR spectral coexistence and minor shifts supported physical incorporation/matrix entrapment. These findings demonstrate that pulsed probe ultrasonication provides a statistically guided, organic solvent-free processing window for controlling droplet size and colloidal stabilization in gum arabic–based citronella oil dispersions.