Anionic and cationic surfactants differentially enhanced the phytotoxicities of four herbicides to corn and soybeans in relation to variations in their structure. Thirteen ionic and two related nonionic surfactants were studied in sprays containing water-soluble forms of 2,2-dichloropropionic acid [dalapon], 2,4-dichlorophenoxyacetic acid [2,4-D], 4,6-dinitro-o-sec-butylphenol [DNBP], and 3-amino-1,2,4-triazole [amitrole]. Effects of surfactant structure, type, and concentration on two other spray properties—surface tension and specific conductivity—were measured at three pH conditions.
Appreciable enhancement of herbicide toxicities occurred only at the highest concentration, 1.0 g per 100 ml (1.0 percent). With straight-chain isomers of an alkylbenzenesulfonate (ABS) anionic surfactant, enhancement of all herbicides increased as the position of the benzene ring was moved to the mid-chain (C5 and C6) carbons of an n-dodecyl alkyl group. DNBP toxicity to both species and surfactant toxicities to soybeans were increased progressively in separate odd- and even-carbon series, i.e. a C1,3,5-series and a C2,4,6-series. Other ABS variations which provided marked enhancement were use of a highly-branched dodecyl alkyl and substitution of an ester oxygen atom for the benzene ring. Among ethoxylated alkylamine cationic surfactants, variations in alkyl structure affected differentially the toxicities of the various herbicides at alkaline, neutral, and acid pH. Surfactants caused marked modification of physical-chemical properties of sprays in relation to their structure and concentration. However, no one of the properties studied could be correlated consistently with the herbicidal effectiveness of a spray. Results suggested that spray performance of herbicide formulations can be improved by using more effective surfactants to achieve other desired formulation characteristics. The increased toxicity of 2,4-D in certain alkaline sprays also suggested that the conditions necessary for absorption of 2,4-D should be examined more critically.