Assessment of the stability of oregano nanoemulsions by dielectric spectroscopy

Abstract

This study proposes a new methodology to monitor the stability of an oil-in-water (O/W) nanoemulsion based on oregano essential oil (OEO) as the oil phase and the surfactants Cremophor EL and Labrafil M1944CS. To achieve this, an OEO nanoemulsion was prepared using different oil concentrations through self-nanoemulsifying drug delivery systems (SNEDDS). Its stability was evaluated over 80 days using Dynamic Light Scattering (DLS), viscosity measurements, FTIR, and dielectric spectroscopy in the GHz frequency range. The interpretation of dielectric spectroscopy data was carried out using a three-phase model consisting of water as the continuous phase, oil droplets, and an interfacial layer formed by the surfactants, resulting in an O/W system. The results showed that nanoemulsion remained stable during the first 30 days, with droplet sizes ranging between 25 and 30 nm. However, after 80 days, an increase in droplet size was observed, reaching approximately 37 nm. Similarly, the thickness of the interfacial layer also increased, possibly due to the diffusion of the surfactant into the aqueous phase. The polydispersity index (PDI) remained around 0.24, indicating homogeneous droplet distribution and good stability throughout the storage period. The viscosity of the nanoemulsion increased with higher concentrations of the dispersed phase in the continuous phase due to stronger droplet interactions and reduced free space for flow, restricting the movement of both phases. Finally, dielectric spectroscopy was highlighted as a promising technique for assessing emulsion stability due to its high sensitivity and reproducibility.