Herbicide-surfactant-plant interactions

Numerous physical and chemical interactions among herbicides, surfactants, solvents (carrier), and plant surfaces are conceivable and probable, though inadequately studied as yet. 

The word optimize is purposely used here. Herbicides have a certain level of innate activity. Adjuvants do not increase this activity but merely aid in the movement of an herbicide to an area or region of a plant where the herbicide will do the most good. It has clearly been established that surfactants do increase the apparent activity of herbicides (2, 4, 5, 10). The cause of this increase, however, is incompletely understood. Indeed, there may not be a simple explanation of the observed phenomenon. The answer may actually be a complex interaction of the various components. 

The components in a simple penetration experiment consist of a surfactant, water-soluble herbicide, and water. Since the surfactant is at a concentration of 0.5 to 1%, it interacts with water and forms micelles. Since micelles are formed, these could solubilize some of the herbicide inside the micelle. Now we have five components, (1) water, (2) surfactant monomer, (3) surfactant micelle, (4) micelle with solubilized herbicide, and (5) an herbicide in anhydrous or hydrated form which all come in contact with the plant. Which one or more of these components has the greatest effect on the plant Before a thorough understanding of this phenomenon can be achieved, the interaction of each of these components with a plant must be investigated separately, and perhaps the plant is too complex for initial study. Perhaps a homogeneous semipermeable membrane could be used instead. 

Surfactant effects on adsorption of herbicides on to soil have been investigated and suggested to be a factor to be considered in the overall effect of surfactant on toxicity towards the plant. The degradation, mobility and uptake of one such compound, picloram [4-amino-3,5,6-trichloropicolinic acid] (pK = 3.4) is affected by adsorption-desorption processes in solids. Picloram adsorption on to soils at pH 5 was reduced by 1 % anionic surfactant [284]. The mechanism involved in picloram adsorption included protonation of the molecule, metal-ion bridging and interaction with metal ions. Picloram adsorption was enhanced by cationic surfactants, suggesting that hydrophobic adsorption of the cationic monomers on to the soil provides a cationic surface for interaction of the anionic picloram. Different soils with different pH values resulted in some variations in these effects which are presented in Table 10.29. 

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