Characteristics and Stability of Valerian Essential Oil Nanoemulsion: A Comparison Between Ultrasonication and Microfluidization

Nanoemulsions are increasingly recognized as promising delivery systems for essential oils in functional products due to their ability to protect sensitive bioactives. Valerian essential oil (VEO) contains valuable bioactives with notable functional properties; however, its volatility and instability limit its direct incorporation into aqueous environments. This study investigated the preparation of oil-in-water nanoemulsions containing 1.5% (w/w) VEO using two high-energy emulsification techniques, ultrasonication (US) and microfluidization (MF), in phosphate buffer (PB) or distilled water (DW) as dispersion media. Samples were characterized in terms of particle size, polydispersity index (PDI), zeta potential, entrapment efficiency, and physical stability. The most stable sample was further evaluated under environmental stress, including varying ionic strength (up to 150 mM NaCl) and pH (4-8). In the formulation prepared in PB using US (US-PB), smaller particle sizes were observed compared to those prepared using MF (MF-PB) (136.4 ± 0.12 vs. 146.5 ± 0.26 nm), showing a similar trend in both formulations prepared in DW (US-DW and MF-DW) (139 ± 4.57 vs. 142.4 ± 0.47 nm). All samples remained monodispersed (PDI < 0.3). Zeta potential values indicated stronger stabilization in formulations prepared in PB (-56.89 ± 1.61 mV for US-PB and -50.16 ± 5.34 mV for MF-PB) compared with DW (-22.93 ± 5.68 mV for US-DW and -26.17 ± 7.90 mV for MF-DW). Lumisizer analysis showed slightly higher stability for US-PB (instability index 0.37) compared with MF-PB (0.40), whereas DW systems were less stable (0.67 and 0.54 for US-DW and MF-DW, respectively). Formulations prepared in PB, particularly US-PB exhibited the highest entrapment efficiency (> 88%). At 150 mM NaCl concentration and pH 4.0, US-PB stability decreased due to reduced zeta potential and electrostatic repulsion, whereas it remained stable at pH 8.0.