Interactions between pesticides, surfactants and plant waxes

  • Xuzhi Hu

Student thesis: Phd


Nonionic surfactants are normally added into commercial pesticide formulation to help enhance pesticide solubilisation, increase droplet coverage on plant surface and transport active ingredients across plant outer surfaces, the wax film. Pesticide efficiency is dominated by the interactions between pesticides, surfactants and waxes. However, our knowledge of these interactions at the molecular level still remains very limited. In the work presented in this thesis, key aspects from a typical agri-spray process were followed. The investigations focused on the configurational alterations of surfactant micelles as the pesticide nanocarriers upon pesticide and wax solubilisation, and the structural changes of the reconstituted wax films before and after exposure to pesticides and surfactants. The impact on the structural changes of surfactant micelles upon pesticide and wax solubilisation was investigated by small angle neutron scattering (SANS), cryogenic transmission electron microscopy (cryo-TEM) and nuclear magnetic resonance (NMR). By taking advantages of isotopic contrast variations, the power of NMR to locate the exact amount and positions of pesticide and wax molecules inside surfactant micelles, and the visual support form cryo-TEM images, our studies have revealed that pesticide solubilisation clearly altered micellar structures, by increasing the aggregation number and micellar length, whilst shrinking and dehydrating their shells, leading to consequent decrease in the cloud points. When waxes were further solubilised into the pesticide-loaded micelles, pesticides were partially released from the micelles, resulting in the adjustment of micellar structures by shortening their length, whilst expanding and rehydrating their shell. The thermodynamic equilibrium of pesticide and wax solubilisation in surfactant micelles can also be altered by temperature change. Increasing temperature can drive pesticides further into the micelles and solubilise more waxes. It can also affect the micellar structures by elongation in the total length, shrinkage and dehydration in the shells. The adsorption dynamics of pesticides and surfactants onto model wax films was studied using neutron reflection (NR) in combination with deuterium labelling to wax, surfactant and solvent. The wax films were reconstituted onto hydrophilic and hydrophobic surfaces, respectively. Though the wax films on both surfaces bear similar morphologies composed of top wax crystals and underlying films, which is comparable to the natural wax morphology, the structural configuration differs significantly. Compared to wax films on the hydrophilic surface, the films formed onto the hydrophobic surface are packed more tightly within the substrate. They are thus more stable and robust when exposed to pesticides and surfactants. Clearly, the hydrophobicity of the substrates influences detailed film morphology and integrity. The more stable wax films on the hydrophobic substrate enabled us to observe how surfactants adsorbed onto and penetrated into the wax films and then altered local structures of the wax films. Similar studies also enabled us to follow how pesticides diffused into the model plant waxy barrier. This work altogether has provided a useful rationalisation of interplay between surfactant structures, pesticide structures and environmental factors that affect pesticide loading and release before and after exposure to wax films.
Date of Award31 Dec 2020
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorJian Lu (Supervisor) & Henggui Zhang (Supervisor)


  • Drug delivery
  • Neutron reflection
  • Plant waxes
  • Nonionic surfactant
  • Pesticide formulation
  • Small angle neutron scattering

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