The
present work highlights the effect of urea on solvation dynamics
and the rotational relaxation of Coumarin 480 (C-480) in the Stern
layer of aqueous micelles of cationic gemini surfactants, 12-4(OH)n-12 (n = 0, 1, 2). UV–visible
absorption, steady-state fluorescence and fluorescence anisotropy,
time-resolved fluorescence and fluorescence anisotropy, and dynamic
light scattering measurements have been carried out for this study.
The formation of micelles becomes disfavored in the presence of urea
at high concentration. Solvation dynamics is bimodal in nature with
fast solvation as a major component. The average solvation time increases,
reaches a maximum, and then decreases with increasing concentration
of urea because the degree of counterion dissociation also follows
the same order with the addition of urea in the micellar solution.
With increased degree of counterion dissociation, the extent of clustering
of water molecules is increased, resulting in slower solvation process.
The −OH group present in the spacer group of gemini surfactant
controls the rate of solvation by shielding the water molecules from
the probe molecules forming hydrogen bond. The microviscosity of micelles
is decreased with increasing concentration of urea, as a result of
which the rotational relaxation process becomes faster. In the presence
of the −OH group in the spacer group, the microviscosity of
micelles is enhanced, resulting in longer rotational relaxation time.
Rotational relaxation process is bimodal in nature with the major
contribution from the fast component to the fluorescence depolarization.
Slow rotational relaxation is mainly due to the lateral diffusion
of C-480 molecules along the surface of the micelle. The tumbling
motion of the micelle as a whole is much slower than the lateral diffusion
of C-480. Wobbling motion of C-480 becomes faster with increasing
concentration of urea as a result of decreased microviscosity of micelles.
The alignment of C-480 molecules in micelles might change with changing
microviscosity.