What are the main factors affecting the application of pesticide adjuvants?
Surfactants are composed of two parts, hydrophilic and hydrophobic. Their ability to reduce surface tension depends on the hydrophilic-hydrophobic relationship and the distribution of molecules among different phases (spray droplets and the surfaces of plants or animals). The wetting and spreading performance of the pesticide on the target and its retention on the target surface directly affect penetration into the target surface and the expression of biological activity. Therefore, adding surfactants can enhance the spreading of pesticide molecules on leaf surfaces or insect bodies, thereby promoting the absorption of the pesticide. This effect is also related to the temperature, concentration and atmospheric pressure of the solution.
Effects of surfactant structure, polarity and concentration on absorption
When Syuan Tan et al. studied the effects of nonionic surfactants on the foliar adsorption and penetration of 24-D, they found that polyoxyethylene glycol (PEG) series surfactants can greatly improve the penetration of some spray-formulated pesticides through the abaxial apple leaf epidermis, but have no obvious effect on adsorption. The experiment also pointed out that the hydrophilic-lipophilic balance (HLB) value of PEG-type surfactants is negatively correlated with the adsorption of surfactants by the leaf epidermis and the penetration of 24-D. Adding surfactants can improve the penetration of 24-D through the cuticular membrane, and its penetration decreases as the HLB value of the surfactant increases. Therefore, the hydrophilic-lipophilic balance (HLB) value of surfactants is an important factor in changing the penetration of 24-D.
Penetration of compound solutes
During the evaporation process, the amount of a substance (component) that penetrates into the epidermis is unrelated to the total amount of solution. Solute absorption is related to the coefficient and concentration between the epidermis and the residue, and the penetration rate depends on the affinity between the solute and the surfactant. The penetration rate of a compound is not only related to its solubility in the epidermal wax, but also to the evaporation rate of the solvent. After evaporation, the penetration rate of the compound will be affected by its concentration in the cuticular wax layer and epidermis. Surfactants may have the effect of clearing transport barriers, hindering the dispersion of compounds, transferring their absorption sites, changing the properties of compounds, reducing their resistance, and creating conditions for plants to absorb active compounds. The improvement in penetration performance occurs through the physical and chemical interactions between the active ingredients of the pesticide and the surfactants when they penetrate into the leaf (combined dissolution or penetration mechanism), or through changes in the plant's internal transport barriers induced by special surfactants, thereby making it easier for them to undergo particle penetration.