The physicochemical properties of adjuvants determine their function and impact upon biological activity. Various physicochemical parameters are key to modifying both the preretention events and postretention consequences of adjuvant usage, irrespective of whether the adjuvants are tank-mix additives or built into a formulation. This paper discusses several key adjuvant parameters for a range of adjuvant chemistries alone and in mixtures. In addition, the misleading use of terms such as nonionic surfactant and hydrophile–lipophile balance is addressed. From a more coherent understanding of the parameters involved, it can be shown that there are ways of predicting the required properties of an adjuvant to solve specific delivery problems. The recognition that different problems often require quite different approaches illustrates that good adjuvants do not exist per se, only materials that should be rationally selected for specific reasons. The chemistry of the herbicide and the nature of its targets will dictate adjuvant selection criteria.
