Solvation dynamics and rotational
relaxation of coumarin
480 in aqueous micelles of cationic gemini surfactants with diethyl
ether (EE) spacer group (
m
–EE–
m
) and tails with varying tail lengths (
m
= 12, 14, and 16) have been studied. Studies have been carried out
by measuring UV–visible absorption, steady-state fluorescence
and fluorescence anisotropy, time-resolved fluorescence and fluorescence
anisotropy,
1
H NMR spectroscopy, and dynamic light scattering.
Effects of hydrocarbon tail length and hydrophilicity of spacer group
on solvation dynamics and rotational relaxation processes at inner
side of the Stern layer of micelles have been studied. With increasing
hydrophobicity of tails of surfactants, water molecules in the Stern
layer become progressively more rigid, resulting in a decrease in
the rate of solvation process with slow solvation as a major component.
With increasing hydrophilicity of the spacer group of gemini surfactant,
the extent of free water molecules is decreased, thereby making the
duration of the solvation process longer. Solvation times in the micelles
of gemini surfactants with hydrophilic spacer are almost 4 times longer
compared to those in the micelles of their conventional counterpart.
Rotational relaxation time increases with increasing tail length of
surfactant as a result of increasing microviscosity of micelles with
fast relaxation as a major component. With increasing hydrophilicity
of the spacer group, the anisotropy decay becomes slower due to the
formation of more compact micelles. Rotational relaxation in gemini
micelles is also slower compared to that in their conventional counterpart.
The anisotropy decay is found to be biexponential with lateral diffusion
of the probe along the surface of the micelle as a slow component.
Rotational motion of micelle as a whole is a very slow process, and
the motion becomes further slower with increasing size of the micelle.
The time constants for wobbling motion and lateral diffusion of the
probe become longer with increasing microviscosity of micelles.
The
present study demonstrates how the different states of solubilized
water viz. quaternary ammonium headgroup-bound, bulklike, counterion-bound,
and free water in reverse micelles of a series of cationic gemini
surfactants, water/12-s-12 (s =
5, 6, 8).2Br–/n-propanol/cyclohexane,
control the solvation dynamics and rotational relaxation of Coumarin
490 (C-490) and microenvironment of the reverse micelles. The relative
number of solubilized water molecules of a given state per surfactant
molecule decides major and minor components. A rapid increase in the
number of bulklike water molecules per surfactant molecule as compared
to the slow increase in the number of each of headgroup- and counterion-bound
water molecules per surfactant molecule with increasing water content
(W
o) in a given reverse micellar system
is responsible for the increase in the rate of solvation and rotational
relaxation of C-490. The increase in the number of counterion-bound
water molecules per surfactant molecule and the concomitant decrease
in the number of bulklike water molecules per surfactant molecule
with increasing spacer chain length of gemini surfactants at a given W
o are ascribed to the slower rates of both solvation
and rotational relaxation. Relative abundances of different states
of water have a role on the microenvironment of the reverse micelles
as well. Thus, a comprehensive effect of different states of water
on dynamics in complex biomimicking systems has been presented here.
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.
The present work reports the transformation of aqueous micelles of gemini surfactants (12-4-12,2Br − or 12-8-12,2Br − ) to a bilayer of surfactants on the surface of gold nanoparticles (AuNPs) capped with citrate during their in situ synthesis. The synthesized AuNPs are characterized using surface plasmon resonance (SPR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), dynamic light scattering (DLS), scanning transmission electron microscopy (STEM), and high-resolution transmission electron microscopy (HR-TEM) techniques. The transformation of micelles to a surfactant bilayer with time is understood with the help of fluorescence measurements such as steady-state and time-resolved fluorescence and fluorescence anisotropy, probed by tuning the precise locations of two fluorophores, Coumarin-480 (C-480) and rhodamine 6G (Rh6G), present in the micelles. As the formation of AuNPs initiates, the dyes get relocated from micelles to a site near the NP surface, which results in fluorescence quenching and a decrease in lifetime due to the nanomaterial surface energy transfer (NSET) from the donor dye to the acceptor AuNPs. The results indicated that Rh6G lies close to the head groups of the surfactant in micelles. Fast segmental/tumbling motions of Rh6G in micelles/bilayers are primarily responsible for the decay of anisotropy to zero. Lateral diffusion is responsible for slow rotational relaxation. With the growth of rod/needle-shaped NPs, the average lifetime and rotational relaxation time increase with an increase in fluorescence intensity due to the transfer of dye molecules from the NP surface to the interior of the bilayer. A significant change in the weightage of the slow component of C-480 compared to that of Rh6G with the formation of the bilayer supports that the former dye is located deep inside the bilayer with an equal contribution to depolarization from both the rotational motions at equilibrium. The excitation wavelength-dependent rotational relaxation time of Rh6G in the bilayer supports the prolate ellipsoidal shape of AuNPs surrounded by the bilayer. This study can help understand the mechanism of drug loading in forming metal NP-based hybrid drug delivery systems in an aqueous micellar solution of gemini surfactants.
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