A Structural Study on the Solubilisation of Pesticides into Surfactant Micelles

UoM administered thesis: Phd

  • Authors:
  • Faheem Padia

Abstract

The ability of surfactants to form micelles and solubilise hydrophobic substances inaqueous environments has been widely exploited in formulation science. In spite ofextensive studies over the past few decades by both experimental and theoreticalmethods, however, it remains difficult to predict key micellar parameters such as theirsize, shape and nanostructure which is essential for their successful implementation inthe solubilisation of active ingredients. This is partly due to the vast number ofsurfactants commercially available but, in addition, the fragmentation of the field ofsurfactant science, over recent years, has made it more difficult to identify generaltrends and properties of surfactant systems. A further challenge is in characterisingsystems of heavily mixed surfactants since our knowledge on pure surfactant systemsmay not allow us to predict the behaviour of these systems. The broad aim of thisthesis was to contribute to these aspects of surfactant science.The first part of the thesis reports a systematic study of the surfactant structure-micellar structure relationship of pure alkyl ethoxylate (CmEn) surfactants. This wasdone by independently varying the lengths of the alkyl chain and ethoxylate group andmeasuring the micellar structural properties. The next part of the thesis reports theeffects of solubilisation of two model pesticides, Cyprodinil and Diuron, on the size,shape and internal structure of these surfactant micelles. These pesticides were chosenbecause they were structurally representative of different features of those widely usedin agrochemicals. The final part of the thesis reports the work on binary surfactantmixtures that rationalise the general structural features of mixed micelles and theirimpact on pesticide solubilisation. Various experimental techniques were usedincluding small angle neutron scattering (SANS), nuclear magnetic resonance (NMR),nuclear Overhauser effect spectroscopy (NOESY NMR), dynamic light scattering(DLS) and UV spectroscopy.The key findings of the thesis were that the micellar core volumes could bepredicted with reasonable accuracy using the hydrophilic-lipophilic balance (HLB) ofthe surfactants in pure micelles. NOESY results revealed protrusions of the terminalmethylene groups into the ethoxylate shell, thus providing evidence for thetheoretically predicted phenomenon referred to as the-shell interface. SANS revealed that solubilisation of both pesticides caused micellargrowth, with the long axial lengths of the micelles growing much longer. Thesestructural changes were associated with the dehydration of the ethoxylate shells.Although a partitioning experiment predicted that the pesticides would be solubilisedin the hydrated ethoxylate micellar shell, NOESY measurements revealed that thesolubilisation occurred predominantly in the micellar cores. The discrepancy wascaused by alkyl chain-ethoxylate mixing leading to the formation of dehydratedpalisade regions that entrapped the pesticides towards the cores. The results from thebinary mixed micelles showed some signs of synergistic behaviour but noenhancement of pesticide solubilisation.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
  • Jian Lu (Supervisor)
Award date31 Dec 2012