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A Femtosecond Study of Solvation Dynamics and Anisotropy Decay in a Catanionic Vesicle: Excitation‐Wavelength Dependence
Author(s) -
Dey Shantanu,
Sasmal Dibyendu Kumar,
Das Dibyendu Kumar,
Bhattacharyya Kankan
Publication year - 2008
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200800658
Subject(s) - micelle , solvation , vesicle , chemistry , dynamic light scattering , solvation shell , chemical physics , sodium dodecyl sulfate , analytical chemistry (journal) , chemical engineering , solvent , materials science , nanoparticle , organic chemistry , aqueous solution , nanotechnology , membrane , biochemistry , engineering
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 ( λ ex ) from 375 to 435 nm. The catanionic vesicle is found to be more heterogeneous than the SDS or DTAB micelles, and hence, the λ 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 λ 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 λ 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 λ 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 λ ex ) may be due to the presence of ionic head groups and counter ions.