The partition of ethoxylated non-ionic surfactants between two non-miscible phases

The partition of a polydispersed ethoxylated non-ionic surfactant in equilibrated oil-water systems has been studied at 25 degrees C. The model surfactant used was a commercial sample of nonylphenol ethoxylated with 10 moles of ethylene oxide (NPEO10). The partition isotherms over the range of surfactant concentration including the critical micelle concentration (CMC) were made with n-hexane, i-octane and n-decane as oil phases. Each partition isotherm exhibits a change of slope that matched the CMC value of surfactant determined by surface tension measurements on aqueous solutions. During the partition of NPEO10 in the oil-water systems, the oligomer distribution in the oil and water phases changed because of fractionation. Below CMC, the mean ethoxylation degree in the oil phase was smaller, whereas in water it was higher than the mean initial value of the surfactant. Moreover, the mean ethoxylation degree in both oil and water phase was practically independent of surfactant concentration. Above CMC, the mean distribution of ethoxymers decreased in both phases. This was ascribed to the competition between micelles from water and the oil phase for the more hydrophobic species of the surfactant. The mean distribution of ethoxymers in the aqueous phase asymptoted to a value that was the mean of the surfactant itself, whereas it steeply decreased in the organic phase.