The study of wax film biointerfaces and their interaction with surfactants

The cuticular wax film biointerface is a key component of plants and essential for the well-being of all plant species, from controlling the transport of water and active ingredients (a.i) across the plant surface to protecting against external environmental attacks, fungi and pathogens. A key aim within the field of adjuvant science and the agrochemical industry is to study the epicuticular wax surface structure and transport properties of common crops and develop a better understanding of the role of the leaf wax barrier during pesticide uptake. Surfactants are a key component of pesticide formulations due to their wide ranging and characteristic amphiphilic properties, from reducing surface tension at a liquid or solid interface to the solubilisation and delivery of active ingredients in micelle aggregates dispersed in an aqueous environment. With the development of powerful experimental and thin-film analytical techniques we can now characterise these surfactants and their aggregates in terms of their shape, structure, adsorption and solubilisation properties and how these properties govern the interfacial interactions with cuticular wax surfaces at the molecular level. This knowledge allows us to fully exploit the huge potential for surfactants in formulation and adjuvant science and better understand the surface interaction processes and modifying actions of surfactants on the physical integrity, composition and transport properties of the wax films to improve the efficiency and effectiveness of pesticide formulations. In this thesis we study the structure, composition and barrier properties of cuticular waxes and the interaction of the wax biointerface with non-ionic alcohol ethoxylate (CnEm) surfactants. Representative reconstituted wheat and barley wax film mimics of controlled thickness, from waxes isolated by supercritical CO2 extraction were formed on a silicon surface for accurate and reproducible studies of the wax film properties and interactions with surfactants. Neutron reflectometry (NR), dual polarisation interferometry (DPI) and spectroscopic ellipsometry (SE) were used to carry out an ex-vivo study of the wax film mimics, while atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to compare the reformed films with intact waxes upon the adaxial surface of an excised wheat leaf. The chemical composition of the extracted wax samples was measured by gas-chromatography-mass-spectroscopy (GCMS). Our studies revealed that the reconstituted wax mimics formed a smooth and porous underlying wax film with characteristic nanoscale crystalline extrusions on the outer surface, mimicking the structures of the underlying cuticular wax films and epicuticular crystals found upon adaxial wheat leaf surfaces. Films were modelled as two representative layers strongly correlating with the structures of cuticular waxes seen on the surface of crops using SEM, and in literature. NR modelled the diffuse underlying layer with thicknesses ranging from ~65-70 Å, while the characteristic surface crystalline extrusions formed a second layer of heights exceeding 300 Å. Moisture controlled NR measurements indicated that water penetrated extensively into the wax films measured under saturated humidity and in an aqueous environment, causing them to hydrate and swell significantly. Studies of the wax film interface as surfactants were introduced showed that in an aqueous environment, an increasing mass of surfactant could both adsorb onto and penetrate into the porous wax film. This process was found to be reversible with water-rinsing at low concentrations. Above the critical micelle concentration (CMC), surfactants aggregate to form micelles which act to solubilise and remove the wax film, causing structural damage and influencing the integrity of the transport barrier. These studies provided a useful structural basis for the waxes studied in suggesting that while waxes generally provide a limiting barrier to transport, a hydration pathway does exist. We propose a model for the wax film biointerface and its interaction with surfactants which act to provide more favourable conditions for the transport of a.i. Finally a detailed study of the micelle detergency and solubilising actions of the alcohol ethoxylate surfactant family was carried out. Small-angle neutron scattering (SANS) measurements investigating the effect of temperature, surfactant head- and chain length and wax solubilisation on the size, shape and properties of the micelles formed showed micellar growth to correlate with increasing hydrophobicity of the surfactant (increasing chain length, decreasing head-length) and increasing temperature. As the hydrophobicity and temperature of surfactant solutions was increased, micelles tended to grow increasingly longer, increasing in size (volume) and shape to form increasingly cylindrical and rod-like micelles. The same effect was seen with increasing solubilisation of cuticular wax compounds which acted to further increase the effective micelle hydrophobicity and further promote micellar growth. Surfactants of increasing hydrophobicity formed exceedingly massive aggregates which act to increase to solubilisation capacity of the micelles formed, apparently making them more effective detergents. This work highlights how various surfactant properties can govern solubilisation of the wax film barrier and the uptake and transport of active ingredient. An optimal balance between both must be achieved for a suitable surfactant.