Spectrophotometric assay of POE nonionic surfactants and its application to surfactant sorption isotherms

The operational range and suitability toward environmental samples of an io dine-iodide (I-I) assay for nonionic surfactants were assessed. The I-I ass ay provides a rapid and repeatable method for determining aqueous nonionic surfactant concentrations. Through a systematic examination of surfactant s tructure, the operational range of the assay was shown to he on the order o f 10(-6) to 10(-3) M-E0, where the concentration unit ME0 is defined as the mol ar surfactant concentration multiplied by the number of ethylene oxide units in the surfactant molecule. For environmental samples, it was shown that the I-I assay can be applied to measurement of surfactant sorption iso therms to aquifer sands and bacteria cultures. A potential limitation of th e I-I assay is interference with humic acids, with the magnitude of the int erference dependent on the concentration of humic acids present The main be nefit of the I-I assay is that its high accuracy and ease of application al lows measurement of low levels of surfactant sorption. Surfactant sorption to aquifer sand could be measured down to the range of 10(-9) mol/g.