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Title: Drying agrochemical droplets on model surfaces
Author: Pineiro-Romero, Axa Azahara
ISNI:       0000 0004 7962 590X
Awarding Body: Durham University
Current Institution: Durham University
Date of Award: 2019
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This project addressed the mechanism of the action of surfactants, used as agrochemical adjuvants, and the physico-chemical interaction between adjuvants and a fungicide active ingredient (AI), on a model surface. The first part of the project studied the influence of surfaces with different wettabilities on the mode of evaporation for water droplets, as a reference, and then with different surfactant solutions at different concentrations with and without the addition of AI. In order to do that, a reproducible method to print droplets with the specific size for agrochemical applications was developed. The internal flows for different agrochemical solutions were studied to understand the transport of surfactants and particles within the droplets. Two main agrochemical formulations were used: an alkyl ethoxylate surfactant (Surf1) and an amidoamine-based surfactant (Surf2) both with the addition of a fungicide called Tebuconazole resulting in a suspension and an emulsion respectively. The properties of the bulk solutions were analysed by surface tensiometry and proton nuclear magnetic resonance (1H-NMR) to determine the ability of these surfactants to form micelles and solubilise a hydrophobic active ingredient (AI). Diffusion-ordered spectroscopy (DOSY) showed that only Surf1 formed micelles. There was no difference in the diffusion coefficient for Surf2 at any of the concentrations tested, from which it can be concluded that Surf2 does not form micelles. The evaporation of droplets made of different solutions gave different dried deposits on a substrate. Different strategies were developed to control the deposit structure in order to inhibit the coffee-ring effect (CRE): i) a sol-gel transition in a suspension of a nanoparticle clay (Laponite) and ii) silica particles; both added to the alkyl ethoxylate surfactant. The addition of Laponite and silica particles increased the surface tension of the final formulations at any of the concentrations. The purpose of these two strategies was to obtain more uniform deposits so that the amount of surfactant and AI were more equal along the deposit. However, Laponite formed uniform deposits because the contact line (CL) receded producing deposits of a smaller area and silica particles did not suppress the CRE. "Superspreaders" such as Silwet Gold, an vi organosilicone surfactant, and Capstone® FS30, a fluorosurfactant, were added to the amidoamine-based surfactant in order to lower the contact angle of the oil drops after drying to increase the contact between the agrochemical solution and the surface, and thus increase the efficacy. The contact angle of the small droplets inside the deposit was lower when Silwet Gold was added to Surf2 + AI at 0.03 wt%. The morphology of the dried deposit and the spatial distribution of the AI particles were analysed by scanning electron microscopy (SEM) and the chemical composition was analysed by energy dispersive X-ray spectroscopy (EDS) and Raman spectroscopy. A Raman imaging system was developed to improve the ability to map compounds on a surface. Raman imaging had the required sensitivity to confirm the co-localization of surfactant and AI molecules in the dried deposit. A quantitative method to achieve compositions by Raman spectroscopy was developed. The last part of the thesis consists of a study of the penetration of AI through the cuticle of Clivia Regel Minata in a Franz diffusion cell by two different methods: infinite dose system and simulation of foliar penetration (SOFP). Clivia is selected as a model plant as it does not have stomata that might affect the transport of AI1. The penetration of AI was improved by the addition of surfactant to the formulation. Surfactants below the CMC behaved very similarly to the surfactant-free formulations. Surfactants above CMC or above the solubility limit showed the highest penetration for the infinite dose experiments. In SOFP, the difference in AI penetration between the formulations were not significantly different. Franz cell diffusion is a useful method to study the trends for the penetration of AI through the cuticles of the leaves, however, the leaf-to-leaf variation is still too large to draw firm conclusions about foliar efficiency.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available