A new technique for determining critical micelle concentrations of surfactants and oil dispersants via UV absorbance of pyrene

Abstract The conventional UV-based methods for determining the critical micelle concentration (CMC) of surfactants often fail for low-solubility surfactants or mixtures of surfactants/solvents or oil dispersants due to baseline uncertainty of the UV spectra. To overcome the limitations, we proposed and tested a new UV-based approach and found that the surfactant concentration, at which the incipient red shift of the strongest UV absorbance peak of pyrene occurs, can be used to quickly and roughly locate the range of the CMC for the surfactant. Furthermore, we developed a more robust and accurate method, which can accurately pinpoint the CMC graphically by following the change of the maximum measurable peak difference (i.e., the strongest UV absorbance peak minus a weaker reference peak) as a function of the surfactant concentration. Regardless of the baseline fluctuations, the method was able to accurately determine CMCs of 8 model surfactants and oil dispersants. Based on the UV-absorbance analysis, the ratio of pyrene to surfactant molecules in micelles was estimated, which further reveals the roles and abilities of various surfactants in dissolution/dispersion of pyrene or other PAHs in water. The new method can be used to measure CMCs of a wide range of surfactants and oil dispersants.