Effects of surfactant adjuvants on plant leaf cuticle barrier properties
Abstract: The focus of this project has been the mechanisms of action of surfactants as agricultural adjuvants and the physico-chemical interactions between adjuvant, carrier formulation, and leaf surface. To increase the understanding of this complex system, model systems have been evaluated in parallel to in vitro studies of plant leaf cuticle. Investigations on how thermodynamic, structural and rheological properties of leaf surface constituents are affected by surfactant absorption and hydration have been central. The main techniques employed in the project are: Environmental Scanning Electron Microscopy, Differential Scanning Calorimetry, Optical Phase Contrast Microscopy with Temperature Resolution, Quartz Crystal Microbalance with Dissipation, Small- and Wide-angle X-ray Diffraction, and Franz type diffusion cells. The effects that surfactants exert on the structure of native intact plant leaf cuticle were investigated by Small- and Wide-Angle X-ray diffraction (SWAXD). The wax has a broad melting interval between 40 and 80 C which comprises a crystalline transition from orthorhombic to hexagonal sub-cell. This transition is facilitated by addition of surfactants. Both intact cuticle and extracted wax also possess lamellar long range order. Clivia is an appropriate model plant since it is related to, and has similar leaf characteristics as, some of the most important crop plants, wheat and barley. It is easy to cultivate indoors, and the leaves are wide enough to be evaluated in vitro through diffusion cell experiments. The barrier is very tough; if it works on Clivia it most probably will work in the field as well. The model of plant leaf intracuticular wax can be used to estimate formulations effects on the cuticle structure. A model was based on a leaf wax extract and comprised 1-docosanol (C22H45OH) and dotriacontane (C32H66). The thermotropic phase behaviour of the model was investigated, and the structure of individual phases in the model wax - water system was determined. The thermotropic transitions of the model wax fit in the window of the extracted leaf waxes, but the model wax would benefit from further development, striving for a more amorphous system. The effects of surfactants on the phase behaviour and the rheological characteristics of the model wax were quantified. This was done to address the current lack of understanding of how surfactants affect the barrier properties of plant leaf cuticles on a molecular level. The model wax used is crystalline at ambient conditions, yet it is clearly softened by the surfactants. The softness of the wax film increased in irreversible steps after surfactant exposure and of the two surfactants used, C10EO7 has a stronger fluidizing effect than C8G1.6. Intracuticular waxes (IW) comprise both crystalline and amorphous domains. Surfactants mainly exercise their fluidizing effects in amorphous regions. A mechanism is suggested to explain the fluidizing effects seen on a largely crystalline model wax. It is proposed that surfactants may enter the crevices in between crystalline domains to establish an expanded continuous amorphous network. Surfactants allow higher amounts of active ingredients in solution, available for penetration. Commercial products (normally concentrates) may contain such high amounts of active ingredient that complete solubilisation is never reached, even after dilution. Crystalline active ingredients cannot enter the cuticle and may lead to an unnecessary environmental burden when dislocated from the leaf. The rate of active ingredient leaf uptake may be increased by an appropriate surfactant. Surfactants increase the flux of active ingredients over the cuticle barrier by increasing the diffusion coefficient inside the cuticle. Based on Fick’s first law, an algorithm that accommodates changes in boundary conditions and takes partition into account was developed. It thereby provides a more accurate method, compared to the standard equations normally used for calculating solute diffusion coefficients in membranes. The same quantitative increase in both flux (Ji) and diffusion coefficient (Di) was observed with surfactants present, while the cuticle-water partition coefficient (lg Kcw) remained unchanged. Evaluation tools have been developed by the establishment of QCM-D and membrane diffusion protocols, and the investigations on model wax. These tools can facilitate the determination of desired properties of new and better adjuvants in agriculture. Subsequently, it may contribute to a more cost-efficient and environmentally friendly usage of pesticides in foliar spray applications. The wider aim of this project was to contribute to a more efficient and optimized pesticide application through investigation of the cuticle and its interplay with surfactant solutions.
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