MOLECULAR-CONFORMATION OF IONIC SURFACTANTS IN AQUEOUS-SOLUTION

The molecular conformation of ionic surfactant in aqueous solution is investigated with fluorescent probes which are intrinsic in surfactant molecules or externally introduced. Quenching of pyrene monomer fluorescence by alkyltriphenylphosphonium or N-alkylpyridinium obeys Stern-Volmer equation, being diffusion-controlled dynamic quenching, but the behavior of quenching with different lengths of alkyl chain is "abnormal", i. e. the longer the chain, the greater the quenching rate constant. The pyrene excimer fluorescence is observed in the aqueous solution of cetyltrimethylammonium bromide (CTMAB), and the inhibition (for cationic quenchers) and promotion (for anionic quenchers) effects of CTMAB on the quenching of pyrene fluorescence are also observed. The self-coiling conformation of ionic surfactant molecules in aqueous solution is proposed to be responsible for these observations and the conformation to be dynamic.