Studies on octylphenoxy surfactants. Part 1: Effects of oxyethylene content on properties of potential relevance to foliar absorption

Selected properties of octylphenol and seven of its ethoxylated derivatives were studied as a preliminary to investigating their effects on the foliar absorption of three model compounds and the surfactants themselves. Surface tension of the surfactants (2 g litre −1 in water) increased with their hydrophile:lipophile balance (HLB) which is a function of oxyethylene (EO) content. The surface tension of the aqueous surfactants was inversely related to the cosine of their contact angle (θ) on maize leaf in the EO range 7.5–30. Cos θ was in turn related to the wetting of maize by aqueous acetone solutions of surfactants with EO >9.5. The critical surface tension required for complete wetting of the adaxial surface of maize leaf was estimated to be 29 mN m −1 . Evaporation of drops of aqueous acetone solutions was biphasic, with preferential loss of acetone during the initial, more rapid phase. The rate of evaporation from the remaining, effectively aqueous systems was inversely related to the HLB of the surfactants. Phytotoxicity of the surfactants, as measured by ethylene production, increased with decreasing EO, while their hygroscopic water retention increased with both log EO and relative humidity (r.h.). Aqueous solubilities of pentacosane, DDT and atrazine were increased up to 8‐fold by surfactant (20 g litre −1 ). Maximum solubilities were observed with surfactants having intermediate HLB values. The solubility of pentacosane (maximum 221 μg litre −1 ) indicated that the surfactants would probably not solubilise more than 0.1% of the epicuticular wax when applied to maize leaf. Scanning electron microscopic observation of leaf surfaces exposed to surfactants for 24 h did not reveal any visible modifications of the wax structure. Volatilisation of DDT was reduced in the presence of surfactants but that ofatrazine was increased, although in inverse proportion to its solubility in aqueous surfactant.