A LIMITATION OF THE MICROTOX(R) TEST FOR TOXICITY MEASUREMENTS OF NONIONIC SURFACTANTS

The Microtox(R) test was successfully used to measure EC50 values of n onionic polyethoxylate surfactants. However, toxicity measurements of real samples that contain surfactants above a particular concentration , termed the critical toxicity concentration (CTC) are not valid. Thes e samples require dilution before the test is performed, and because t he relationship between toxicity and concentration is not linear above the CTC, the EC50 cannot be extrapolated back to give the toxicity of the original concentrated sample and a true estimation of toxicity is therefore not possible. This phenomenon may be related to the minimum surface tension requirement of the bacteria or other physical propert ies of the surfactant such as the tendency to assemble at interfaces a nd surfaces and the tendency to form micelles.