(221b) Micelle's Role in Contributing to Reduce the Dynamic Surface Tension at An Air/Water Interface

In the conventional studies of the surfactant transport from a micellar solution onto an air/water interface, the micelle's contribution to the dynamic surface tension reduction is solely taken as a reservoir which can release monomer when the micelle and monomer equilibrium in the bulk is disturbed by the depletion of monomer from the bulk due to the adsorption of monomer onto the interface. The fact that the highest concentration of the monomer can be replenished by micelle breakup in the bulk is the critical micelle concentration (CMC) imposes a kinetic limit on this route. There is observed contradiction between this kinetic limit and the experimental results obtained by the pendant bubble apparatus showing that highly concentrated micellar solution can relax the surface tension much faster than the kinetic limit can predict.

In this paper, the theory and experimental study of the surfactant transport from the micelle solution includes the novel route that micelle can directly adsorb onto the interface and then break up into monomers which can be incorporated into the surface monolayer later. Fluorescence resonance energy transfer (FRET) study using confocal laser scanning microscope is carried out to show the experimental evidence of the micelle direct adsorption. The FRET donor (NBD labeled phospholipid) is spread on a clean air/water interface of a pure water layer. The FRET acceptor (Nile red) is incorporated into micelle core of Hexaethylene glycol monotetradcyl ether surfactant (C14E6). After the introduction of the acceptor labeled micelle solution underneath the excited-donor labeled air water interface, FRET emission with excited acceptor single is detected by confocal. The excited acceptor has to be in the vicinity of the donor (on the surface) to be excited, and the acceptor can only be transported onto the surface by the direct micelle adsorption route, in which micelle direct adsorb onto the surface, break up and release the acceptor, which than can be incorporated into the surface monolayer.