FLUORESCENCE QUENCHING IN DOUBLE-CHAINED SURFACTANTS .2. EXPERIMENTAL RESULTS

The aggregation behavior of single- and double-chained ionic surfactants (C16N3C1Br, C16N3C1OAc, C14N3C1Br, 2C10N2C1Br, 2C12N2C1OAc, 2C12N2C1Br, 2C14N2C1Br, 2C14N2C1OAc, and 2C16N2C1OAc) and zwitterionic phospholipids (dicaproyl-and dipalmkoylphosphatidylcholine (2C6PC and 2C16PC, respectively)) is investigated by time-resolved fluorescence quenching and small-angle neutron scattering measurements as a function of surfactant concentration, salt, and temperature. The double-chain surfactants with acetate as counterions form micelles whose aggregation number increases with surfactant concentration but is relatively independent of temperature. The transformation of the corresponding bromide surfactant from liposomes to micelles with increasing temperature is delineated; at intermediate temperatures, the solution contains mixtures of micelles and vesicles. Mixtures of zwitterionic surfactants transform from liposomes to micelles as the mole fraction of 2C6PC increases, with a vesicle-micelle equilibrium at intermediate values. The aggregation numbers for the single-chain acetate and bromide surfactants, which at low temperatures differ by a factor of 2, appear to approach one another at higher temperatures. These observations are analyzed in terms of specific counterion headgroup interactions and their variation with the field variables. © 1990 American Chemical Society.