Drying Agrochemical Droplets on Model Surfaces

Abstract

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.