Interaction Between Surfactant, Agrochemical and Target Species

Interaction Between Surfactant, Agrochemical and Target Species 

In selecting adjuvants that can be used to enhance biological efficacy one has to consider the specific interactions that may take place between the surfactant, agrochemical and target species. This is usually described in terms of an activation process for uptake of the chemical into the plant. This mechanism is particularly important for systemic agrochemicals. 

Several key factors may be identified in the uptake activation process (1) In the spray droplet, (2) in the deposit formed on the leaf surface, (3) in the cuticle before or during penetration and (4) in tissues underlying the site of application. Four main sites were considered by Stock and Holloway [125] for increasing the uptake of the agrochemical into a leaf (1) Surface of the cuticle (2) within the cuticle itself, (3) the outer epidermal wall underneath the cuticle and (4) the cell membrane of internal tissues. 

The activator surfactant is initially deposited together with the agrochemical and it can penetrate the cuticle, reaching other sites of action and, hence, the role of surfactant in the activation process can be very complex. The net effect of surfactant interactions at any of the sites of action is to enhance the mass transfer of an agrochemical from a solid or liquid phase on the outside of the cuticle to the aqueous phase of the internal tissues of the treated leaf. As discussed above, solubilisation can play a major role in activating the transport of the agrochemical molecules. With many non-polar systemic fungicides, which are mostly applied as suspension concentrates, the presence of micelles can enhance the rate of dissolution of the chemical and this results in increased availability of the molecules. It also leads to an increase in the flux as discussed above. 

It has been suggested that cuticular wax can be solubilized by surfactant micelles (by the same mechanism of solubilization of the agrochemical). However, no evidence could be presented (for example using SEM) to show the wax disruption by the micelles. Schonherr [126] suggested that the surfactants interact with the waxes of the cuticle and thus increase the fluidity of this barrier. This hypothesis is sometimes referred to as wax Plasticization (similar to the phenomenon of the glass transition temperature reduction of polymers by addition of plasticizers). Some measurements of uptake using surfactants with various molecular weights and HLB numbers offered some support for this hypothesis. 

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