Polarity of the micelle-water interface as seen by 4-aminophthalimide, a solvent sensitive fluorescence probe

4-aminophthalimide (AP) has been employed as a fluorescence probe to follow the micellar aggregation of three commonly used cationic, anionic and neutral surfactants. From a study of spectral shift and enhancement of fluorescence intensity and lifetime as a function of surfactant concentration, critical micellar concentrations have been evaluated, the location of the probe in the micelles has been identified and the polarity of the micelle-water interface has been measured. It is shown that AP always binds to the micelle-water interface irrespective of the micellar charge. The advantage of AP over other probes is discussed.