Nanoemulsions are highly effective delivery systems for bioactive compounds with low bioavailability. However, fully understanding their interfacial properties remains challenging, despite their critical role in determining the stability and performance of these nanosystems. In this study, spontaneous nanoemulsions, also known as self-nanoemulsifying drug delivery systems (SNEDDS), were investigated. To characterize their interfacial properties, nanoemulsions with droplet sizes of 20, 30, 50, 70, and 100 nm and varying oil phase volumes were prepared and analyzed using dielectric spectroscopy. The results showed that decreasing droplet size significantly affected the interfacial properties, leading to reductions in the interfacial layer thickness, the interfacial constant K, and the interfacial dielectric constant. These changes are attributed to the increased structural complexity of the interfacial layer as the interfacial area of the droplets increases. FTIR analysis supported these findings by revealing enhanced interactions between water molecules and the interfacial layer with increasing interfacial area. Specifically, nanoemulsions with smaller droplets exhibited a shift and broadening of the C-O and C-O-C bands, indicating stronger hydrogen bonding. Overall, the dielectric spectroscopy and FTIR results demonstrate that the structure of the nanoemulsion interfacial layer varies with droplet size, providing valuable insights for the rational design of nanoemulsions with controlled release properties.
