A femtosecond study of solvation dynamics and anisotropy decay in a catanionic vesicle: excitation-wavelength dependence.

The structure and dynamics of a catanionic vesicle are studied by means of femtosecond up-conversion and dynamic light scattering (DLS). The catanionic vesicle is composed of dodecyl-trimethyl-ammonium bromide (DTAB) and sodium dodecyl sulphate (SDS). The DLS data suggest that 90 % of the vesicles have a diameter of about 400 nm, whereas the diameter of the other 10 % is about 50 nm. The dynamics in the catanionic vesicle are compared with those in pure SDS and DTAB micelles. We also study the dynamics in different regions of the micelle/vesicle by varying the excitation wavelength (lambda(ex)) from 375 to 435 nm. The catanionic vesicle is found to be more heterogeneous than the SDS or DTAB micelles, and hence, the lambda(ex)-dependent variation of the solvation dynamics is more prominent in the first case. The solvation dynamics in the vesicle and the micelles display an ultraslow component (2 and 300 ps, respectively), which arises from the quasibound, confined water inside the micelle, and an ultrafast component (<0.3 ps), which is due to quasifree water at the surface/exposed region. With an increase in lambda(ex), the solvation dynamics become faster. This is manifested in a decrease in the total dynamic solvent shift and an increase in the contribution of the ultrafast component (<0.3 ps). At a long lambda(ex) (435 nm), the surface (exposed region) of a micelle/vesicle is probed, where the solvation dynamics of the water molecules are faster than those in a buried location of the vesicle and the micelles. The time constant of anisotropy decay becomes longer with increasing lambda(ex), in both the catanionic vesicle and the ordinary micelles (SDS and DTAB). The slow rotational dynamics (anisotropy decay) in the polar region (at long lambda(ex)) may be due to the presence of ionic head groups and counter ions.