Size of Cationic Surfactant Micelles at the Silica−Water Interface:  A Fluorescent Probe Study

We present what is, as far as we are aware of, the first time-resolved fluorescence quenching study of the state of adsorbed cationic surfactants on a silica surface. Three different monovalent and one divalent cationic surfactant are investigated. We employ pyrene as fluorescent probe and quenchers that in themselves are surfactants, viz., alkylpyridinium chlorides. Essential to the analysis of the data is knowledge of the distribution of the quencher between the micelles and the aqueous subphase. We use previously obtained relations that give us this quantity. The results show unequivocally that all of the studied surfactants are adsorbed as discrete micelles with aggregation numbers close to those found in bulk solutions for the same surfactants. The aggregation numbers increase with the amount of adsorbed surfactant.