Investigating the wettability of neem oil nanoemulsion as a green pesticide on leaf surfaces – optimizing formulation, assessing stability, and enhancing wettability

The present study focuses on formulating a stable green oil-in-water nanoemulsion (NE) formulation for pesticide application, thereby addressing the primary challenges of botanical pesticides by improving the stability, efficacy, and wettability of leaf surfaces. Neem oil and rhamnolipid biosurfactant were utilized as the base oil and surfactant, respectively for the NE formulation. Initial screening studies with varying oil-to-surfactant ratios identified an optimal ratio of 7.13 (w/w), corresponding to 15% oil (v/v) of total emulsion volume, for a stable NE. The stability of the formulation was validated in a 25 mL volume over 20 days, maintaining a mean droplet diameter of 139.1 ± 7.34 nm and a polydispersity index of 0.207. Further stability assessments under varying time, temperature, ionic strengths, and centrifugal forces revealed a relatively stable droplet diameter ranging from 141 to 245 nm. The surface tension for the optimal formulation was measured to be 35.7 ± 0.4 mN m^-1. Additionally, experiments with various NE formulations on selected leaves showed that NEs with higher oil content displayed faster spreading, as indicated by a rapid reduction in contact angle. The novelty of the study lies in demonstrating the synergistic interaction between oil and rhamnolipid, which facilitated faster spreading compared to the individual spreading of either component, as revealed by the dynamic spreading studies of the NE formulation on leaf surfaces. Therefore, this research paves the way for future research and development of green pesticides that are sustainable and effective compared to their chemical counterparts.