Physico‐chemical studies of pesticides. IV.solubilised pesticides

Abstract

The beliaviour of several relatively insoluble pesticides (y-BHC, carbophenothion, coumaphos, diazinon, fenchlorphos and malathion) solubilised in aqueous soap (sodium dodecyl sulphate, cetyltrimethylammonium bromide and dodecyltrimethylammonium bromide) solutions have been examined by in vitro dipping experiments with flour beetles (Tribolium colzfisum Duval).Median lethal concentrations (C) ,and saturation solubilities (C,) of the pesticides increased with soap concentration whilst their ratios (C/C,) which could be regarded as approximating their thermodynamic activities, decreased. The soaps, alone, whilst being sorbed from solution by the insects, removed cuticular material as did some common solvents (ether, hexane, methyl alcohol and methyl cyanide) but the effects were not sufficient to explain the changes in biological activity of the solubilised pesticides.Changes in surface tension and viscosity of the solutions and the amounts of liquid retained by the beetles after dipping were small and did not appear t o affect the toxicities of the pesticides tested. Biological response appeared t o be determined by the rate of uptake of pesticide by the insect cuticle and hence was affected by the concentration of the soap micelles.