Sunita Kumari1, Sayantan Halder1, Rishika Aggrawal1, Ganapathisubramanian Sundar2, Subit K Saha1. 1. Department of Chemistry, Birla Institute of Technology & Science (BITS), Pilani, Pilani Campus, Pilani, Rajasthan 333031, India. 2. Department of Chemistry, Birla Institute of Technology & Science (BITS), Pilani, Hyderabad Campus, Hyderabad, Telangana 500078, India.
Abstract
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 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.
Micelles, the assemblies of surfactant molecules, up to certain
extent mimic the environment of lipid bilayers. The studies on micellar
organization and dynamics are paid special attention because the general
principle for the micelles formation is related to other molecular
assemblies, such as bilayers, reverse micelles, liposomes, and biological
membrane.[1−4] For biophysical or structure biological studies, surfactants are
used to maintain the purified protein in its native and functional
states.[5,6] The presence of an additive modifies various
aggregating properties of the micellar solution of surfactant.[7−11] Urea, a well-known protein denaturant, with ionic and nonionic surfactants
increases the critical micellar concentration (cmc) values,[12−16] reduces the aggregation number of micelles,[17,18] etc. The solubility of hydrocarbons in aqueous medium is enhanced
in the presence of urea. Mixtures of surfactant and urea are used
for various applications in membrane-protein research.[19−23] Urea facilitates the transfer of purified membrane protein from
detergent micelles to lipid bilayers.[23] Two different mechanisms are proposed regarding urea-induced protein
denaturation.[24−27] In one case, urea acts as a water structure breaker or “chaotrope”
by facilitating the solvation of nonpolar solute. In another case,
urea weakens the intramolecular proteinous bond on directly binding
to proteins.[24−27] Various studies have been carried out to understand the properties
of urea–water system. Few studies reported that urea acts as
a water “structure breaker”,[28,29] whereas other studies showed that urea acts as a water “structure
maker”.[30,31] Some other studies showed the
combination of these two effects in urea.[32,33] Hayashi et al.[34] showed that urea retains
the weakly associated water in the tetrahedral structure and thus
it is not a strong structure breaker of water. Recently, Choudhury
et al. have reported that urea can substitute water in the hydrogen-bonded
network without breaking its tetrahedral, hydrogen-bonded structure.[35]Water is a very important part of all
biological systems. In these
systems, water may exist in free and bound states.[36] The study of water dynamics in organized assemblies possesses
immense significance. It is known that the properties of solvent surrounding
the reactants present in different biological systems and biomimicking
organized assemblies significantly control various chemical reactions,
such as charge transfer, electron transfer, etc.[7,8,37] Thus, the study of the physicochemical properties
of a confined medium surrounding probe molecules needs to be carried
out to understand and control various processes. The study of solvation
dynamics provides us with the information regarding the behavior of
water molecules surrounding probe molecules, giving in-depth knowledge
about the microenvironment. Pure water exhibits very fast solvation
dynamics. In organized assemblies, solvation dynamics retarded many
folds compared to that in the bulk.[36] Many
techniques have been used to study the dynamics of water in water–urea
mixture. Infrared pump-probe spectroscopy has been used to study the
effect of urea on the structure and dynamics of water, and it was
found that even at high concentration of urea, dynamics of water has
not been altered.[38] Only a small fraction
of water molecules displays slower dynamics than bulk water molecules
by forming specific water–urea complex. Idrissi et al.[39] studied the short-time dynamics of water–urea
mixture and the results showed that addition of urea leads to an overall
isotropy and stiffening of the short-time dynamics of both the species.
Many of these studies concentrated on the study of water dynamics
in water–urea mixture.[38−41] Few reports are available on the study of urea dynamics
as a function of urea concentration in water–urea mixture.[42,43] The effect of urea concentration on the dynamics of water in organized
assemblies of surfactant molecules is scarcely studied. Hazra et al.
studied the dynamics of water, urea, and water–urea mixture
inside the reverse micelles of anionic surfactant.[44] Chattopadhyay et al. studied the effect of urea on the
organization and dynamics of Triton X-100 micelles.[16] Ruiz et al. studied the microenvironmental properties of
sodium dodecyl sulfate in aqueous urea solution.[45]Nowadays, a class of surfactants called as “gemini
surfactant”
is attracting much interest in materials science, biological sciences,
nanotechnology, supramolecular chemistry, etc. Gemini surfactants
are a special class of surfactants comprising two hydrophobic tails
and hydrophilic headgroups covalently connected by a spacer group
at their headgroups.[46] Some of the physical
properties of gemini surfactants are far more superior to those of
their conventional counterparts.[46] Different
groups have studied the effect of urea on various physical properties
of conventional surfactants. Dey et al. studied the effects of urea
on the aggregation properties of chiral surfactants.[47] Keller et al.[48] reported the
impact of urea on nonionic sugar-based surfactants. The effect of
urea on the aggregation behavior of gemini surfactants and their mixed
micelles with pluronic surfactant was reported by Mahajan et al.[49] Studies on the effect of urea concentration
on the solvation dynamics in the micellar environment are scarce.
Considering the importance of gemini surfactants over conventional
surfactants and urea in the protein denaturation process, it would
be valuable to study the effect of urea on the solvation and rotational
relaxation dynamics in aqueous micelles of gemini surfactant. If there
is any effect of urea on the micellization behavior of gemini surfactants
with different spacer groups, then how does it affect the solvation
dynamics and rotational relaxation processes of the probe molecule?
Whether the water structure breaker or maker properties of urea or
some other factor(s) guide(s) the rate of solvation process? Is the
effect of urea on the solvation dynamics and rotational relaxation
and micellization behavior of gemini surfactant with different spacer
groups same or different? Answers to these questions are important
as surfactant–urea mixtures are used in various membrane-protein
research works.[19−23]Recently, we have studied the solvation dynamics in the presence
of pure gemini surfactants and mixed micelles of gemini surfactant
and conventional surfactants.[50−53] In the present study, we have investigated the effect
of urea concentration on the aggregation properties of gemini surfactants
with different spacer groups (with and without −OH group(s))
and also its effect on the solvation dynamics and rotational relaxation
of Coumarin 480 (C-480) in aqueous micelles of those surfactants (Scheme ). To probe the solvation
dynamics, the solute chosen should have zero or low dipole moment
in the ground state and very high dipole moment in the excited state,[36] as C-480 is such kind of molecule. Moreover,
because of the presence of several basic centers, it can form hydrogen
bonds.[54−59] There are arguments on the cleavage of hydrogen bond upon excitation.[56,58] However, a more convincing report by Zhao et al.[59] shows that the early time of photoexcitation of C-480 to
the excited state is strengthened due to the intermolecular hydrogen
bonding between C-480 and hydrogen-bond-donating solvents/groups.
The surfactants used in the present study have been explored before
to demonstrate the effect of −OH group(s) on solvation dynamics.[50] To the best of our knowledge, the solvation
dynamics and rotational relaxation in the mixed systems of urea and
gemini surfactants with different spacer groups have not been reported
to date. The microenvironmental properties of C-480 within the micelles
of gemini surfactants with varying urea concentration have been studied.
Two-step and wobbling-in-a-cone models have been explored to show
the bimodal behavior of rotational relaxation processes and to demonstrate
the wobbling motions in the micelles. Results obtained from this study
could be useful to understand the water dynamics in biological systems
in the presence of urea.
Scheme 1
Molecular Structures of Gemini Surfactants
and Coumarin 480
Results and Discussion
Effect
of Urea on cmc
Cmc and degree
of counterion dissociation (α) values of all studied gemini
surfactants in the absence and presence of various concentrations
of urea have been determined by conductivity measurements performed
at 298.15 K following Williams’s method.[60] A plot of specific conductivity, κ, versus concentration
of surfactant with varying concentrations of urea for 12-4-12 is given
in Figure as a representative
one. Similar changes in κ with change in concentration of surfactant
at varying concentrations of urea have been noted in cases of other
two surfactants as well. Although a cmc value has been calculated
from the break point of two intersecting lines, a value of α
has been calculated by calculating the ratio of the slopes of the
straight lines in the postmicellar region to the premicellar region.[61] The values of cmc obtained are given in Table , and the values of
α are given later (Table ), where relevant discussion has been made. Cmc values of
all of the pure gemini surfactants are in good agreement with the
reported values,[50,62] indicating the goodness of cmc
values in the presence of urea as well. It has been noted that the
cmc value of gemini surfactants enhances with increasing concentration
of urea. It is reported in the literature that the solubility of a
surfactant increases many folds in the presence of urea.[63] Increased solubility of surfactant ions in the
solution, i.e., their reduced solvophobicity, delays the aggregation
process of surfactant molecules. In other words, the hydrophobic interactions
between surfactant molecules are decreased in the presence of urea.[39] Consequently, the concentration at which surfactant
molecules start to form aggregates is increased.
Figure 1
Plot of κ vs concentration
of 12-4-12 with varying concentrations
of urea at 298.15 K.
Table 1
Critical Micelle Concentration (cmc),
Mole Fraction Partition Coefficient (Kmic), and Standard Molar Gibbs Free Energy of Micellization (ΔG°, mic) of Gemini Surfactants in the
Presence of Urea
urea (M)
Gemini-A cmc (mM)
Gemini-B cmc (mM)
Gemini-C cmc (mM)
Gemini-A Kmic (104)
Gemini-B Kmic (104)
Gemini-C Kmic (104)
Gemini-A ΔG°, mic (kJ mol–1)
Gemini-B ΔG°, mic (kJ mol–1)
Gemini-C ΔG°, mic (kJ mol–1)
0.0
1.06 ± 0.03
0.96 ± 0.03
0.88 ± 0.02
5.24 ± 0.15
5.78 ± 0.17
6.31 ± 0.14
–26.93 ± 0.07
–27.18 ± 0.07
–27.40 ± 0.05
0.5
1.18 ± 0.04
1.07 ± 0.03
0.96 ± 0.03
4.70 ± 0.15
5.19 ± 0.14
5.78 ± 0.17
–26.67 ± 0.07
–26.91 ± 0.07
–27.18 ± 0.07
1.0
1.23 ± 0.03
1.21 ± 0.03
1.17 ± 0.03
4.51 ± 0.11
4.59 ± 0.11
4.74 ± 0.11
–26.57 ± 0.06
–26.61 ± 0.06
–26.69 ± 0.05
2.0
1.31 ± 0.05
1.47 ± 0.07
1.67 ± 0.09
4.24 ± 0.16
3.78 ± 0.18
3.32 ± 0.17
–26.41 ± 0.09
–26.12 ± 0.12
–25.81 ± 0.13
3.0
1.66 ± 0.08
1.87 ± 0.08
1.78 ± 0.11
3.34 ± 0.15
2.97 ± 0.12
3.12 ± 0.18
–25.82 ± 0.11
–25.53 ± 0.11
–25.65 ± 0.15
4.0
2.19 ± 0.10
2.12 ± 0.11
2.02 ± 0.12
2.53 ± 0.11
2.62 ± 0.13
2.75 ± 0.16
–25.14 ± 0.11
–25.22 ± 0.13
–25.34 ± 0.15
5.0
3.05 ± 0.12
2.63 ± 0.12
2.46 ± 0.10
1.82 ± 0.07
2.11 ± 0.09
2.26 ± 0.09
–24.31 ± 0.09
–24.68 ± 0.11
–24.85 ± 0.11
Table 4
Degree of Counterion Dissociation
(α) for Micelles of Gemini Surfactants
urea (M)
Gemini-A α
Gemini-B α
Gemini-C α
0.0
0.32 ± 0.03
0.29 ± 0.02
0.28 ± 0.02
0.5
0.36 ± 0.02
0.37 ± 0.03
0.29 ± 0.02
1.0
0.42 ± 0.04
0.38 ± 0.03
0.32 ± 0.02
2.0
0.46 ± 0.04
0.39 ± 0.03
0.37 ± 0.03
3.0
0.47 ± 0.04
0.36 ± 0.02
0.34 ± 0.02
4.0
0.34 ± 0.02
0.33 ± 0.02
0.32 ± 0.02
5.0
0.31 ± 0.02
0.30 ± 0.02
0.27 ± 0.02
Plot of κ vs concentration
of 12-4-12 with varying concentrations
of urea at 298.15 K.To support the increment in cmc value with increasing
concentration
of urea in aqueous phase, the mole fraction partition coefficient
of gemini surfactants from aqueous phase to micellar phase (Kmic) has been determined using eq for all micellar systems[48]where 55.5 M is the molar concentration of
water. The Kmic values calculated are
given in Table , which
are found to be decreased with increasing concentration of urea. In
the presence of urea, because of decrease in the water-to-micelles
partition coefficient, the cmc value of a gemini surfactant is increased.
The values of standard molar Gibbs free energy of micellization have
been calculated using eq (48)ΔG°,mic values at 298.15 K and various urea concentrations are
given in Table , which
are found
to be increased with increasing concentration of urea. This also indicates
that the micellization process becomes less favorable with increasing
urea concentration.
UV–Visible Absorption
and Steady-State
Fluorescence Study
UV–visible absorption and steady-state
fluorescence spectra of C-480 have been obtained in the presence of
pure gemini surfactants and also in the presence of gemini surfactant
at various concentrations of urea. Figure depicts the absorption spectra of C-480
in the presence of 10 mM 12-4-12 and 12-4-12 (10 mM) at various concentrations
of urea as a representative. Figure demonstrates the fluorescence spectra of C-480 in
the presence of pure 10 mM 12-4-12 and 12-4-12 (10 mM) at various
concentrations of urea. Similar absorption and fluorescence spectra
are observed in the presence of each of the other two gemini surfactants
and urea systems as well. In both 10 mM Gemini-A and Gemini-B, the
absorption and fluorescence peak maxima of C-480 are 395 and 475 nm,
respectively. In 10 mM Gemini-C, these values are 393 and 474 nm,
respectively. There is no significant change in peak maxima values
upon addition of urea. Absorption and fluorescence peak maxima values
of C-480 in all of the studied systems are given in Table S1. It is noteworthy that an increase in the absorbance
and fluorescence intensity with increasing concentration of urea has
been observed in pure water as well (spectra not shown). It could
be because of either the effect of urea on hydrogen bonding between
C-480 and water and/or the salting-in effect of urea (a thorough study
will be done later). However, this effect can be ruled out in the
presence of high concentration of surfactant (10 mM), where C-480
molecules are completely solubilized in micelles (excitation wavelength-independent
fluorescence peak maximum and very little change in absorption and
fluorescence peak maxima in the presence of urea are noted). Figure shows absorption
and fluorescence spectra of C-480 in pure water, methanol, cyclohaxane,
and 10 mM Gemini-A. Absorption and fluorescence peak maxima of C-480
in pure water are observed at 389 and 489 nm, respectively, and the
same in cyclohaxane appeared at 362, 378, and 409 nm, respectively
(Table S1). In water, fluorescence of C-480
occurs from the stabilized intramolecular charge transfer (ICT) state.[64−67] Low fluorescence intensity in a polar medium is due to the high
rate of nonradiative processes as triplet and ground states are very
close to the ICT state. Compared to water, in a comparatively less
polar medium like micelles, fluorescence intensity increases with
a concomitant blue shift in peak maximum. This is because the emitting
state now gets destabilized and goes away from the triplet and ground
states. The blue shift in the absorption peak maximum in water with
respect to that in micellar medium is due to the intermolecular hydrogen
bonding between C-480 and water molecules, which is generally observed
for molecules with ICT characteristics.[64−67] The ICT fluorescence of C-480
has also been demonstrated by recording fluorescence spectra in different
percentages of dioxane–water mixtures. Figure S1a represents fluorescence spectra and Figure S1b shows changes in fluorescence intensity
and fluorescence peak maxima of C-480 with increasing % of water in
dioxane–water mixtures. In the presence of pure gemini surfactants
and gemini surfactants plus urea, the absorption and fluorescence
peak maxima of C-480 are different from those in the presence of pure
water. These results indicate that the microenvironment around C-480
in the micelles of pure gemini surfactant and that of gemini surfactant
plus urea are different from that in the presence of pure water. The
blue shift in the fluorescence peak maxima in a micellar medium as
well as in a micelle–urea mixed system compared to pure water
suggests that C-480 feels less polar environment in micelle and micelle–urea
mixed media than that in bulk water.
Figure 2
Absorption spectra of C-480 in the presence
of pure 12-4-12 and
12-4-12 at varying concentrations of urea. [C-480] = 5 μM.
Figure 3
Fluorescence spectra of C-480 in the presence
of pure 12-4-12 and
12-4-12 at varying concentrations of urea. [C-480] = 5 μM.
Figure 4
Absorption (solid lines) and fluorescence (dashed
lines) spectra
(λex = 375 nm) of C-480 in pure water, methanol,
cyclohexane, and 10 mM Gemini-A.
Absorption spectra of C-480 in the presence
of pure 12-4-12 and
12-4-12 at varying concentrations of n class="Chemical">urea. [C-480] = 5 μM.
Fluorescence spectra of C-480 in the presence
of pure 12-4-12 and
12-4-12 at varying concentrations of urea. [C-480] = 5 μM.Absorption (solid lines) and fluorescence (dashed
lines) spectra
(λex = 375 nm) of C-480 in pure n class="Chemical">water, methanol,
cyclohexane, and 10 mM Gemini-A.
Study on Microenvironment of Micellar Systems
in the Presence and Absence of Urea
Micropolarity
Determination of
micropolarity around the probe molecule gives valuable information
about the location of the probe molecule inside the micellar environment.
Fluorescence active probe molecules play an important role in the
determination of micropolarity of biological and biologically related
environments.[50,51] In the present study, the micropolarity
values expressed in terms of ET(30), which
is an empirical solvent polarity parameter developed by Reichardt
et al.,[68] have been estimated to obtain
the information about microenvironment surrounding the probe molecule
inside the micelles. To determine the micropolarity around C-480 in
various studied systems, the fluorescence behaviors of C-480 in the
absence and presence of various concentrations of urea have been compared
to those in different compositions of dioxane–water mixture.[50] Fluorescence energies of C-480 at peak maxima
(εmaxfl) after correction of λ2 factor in different percentages of dioxane–water mixture
have been plotted against the ET(30) values
of the dioxane–water mixture (Figure S2). The values of εmaxfl of C-480 in the
absence and presence of various concentrations of urea have been estimated,
and the ET(30) values have been estimated
from Figure S2. The ET(30) values for all of the systems are found to vary between
53.8 and 55.8 kcal mol–1. The calculated ET(30) values in the investigated systems indicate
that micropolarity around C-480 in these systems is less than that
of water (ET(30) = 63.1 kcal mol–1). The ET(30) value in all of the studied
systems is similar to that of methanol (55.5 kcal mol–1). It depicts that C-480 is present neither at the core of the micelles
nor in the bulk water, but somewhere in between these two. According
to literature reports, the micropolarity of the Stern layer is similar
to methanol,[50,51,53,69,70] on the basis
of which we state that C-480 molecules are mostly residing in the
Stern layer. It is pertinent to note that C-480 being present in the
Stern layer of micelles, microenvironment remains more or less same
with the addition of urea in micellar systems. Consequently, both
fluorescence peak maximum and ET(30) values
are found to be the same in gemini micelles with varying concentrations
of urea.
Microviscosity
The measurement
of fluorescence anisotropy of a probe molecule in a microenvironment
gives information about microviscosity around the molecule. As we
did not get a proper trend of microviscosity using C-480 as a probe
following our earlier method,[50] we explored
1,6-diphenyl-1,3,5-hexatriene (DPH), which is a well-known viscosity-sensitive
fluorescence probe molecule.[51] The absolute
values of microviscosities of micelles, ηm, using
DPH have been estimated following the Debye–Stokes–Einstein
relation (eq )[51,71]where τR is the rotational
correlation time of DPH, k is the Boltzmann constant, T is the kelvin scale temperature, and υh is the hydrodynamic volume of DPH. The υh value
has been taken as 313 Å3.[71] The rotational correlation time, τR, has been calculated
using Perrin’s equation (eq )[72]where r and ⟨τf⟩ are steady-state fluorescence
anisotropy and excited-state
lifetime of DPH, respectively, and ro is
the steady-state fluorescence anisotropy of DPH in an extremely viscous
solvent, whose value is taken as 0.362.[73] The values of r, ⟨τf⟩,
τR, and ηm are given in Table . The data in Table show that in case
of each micellar system the fluorescence anisotropy and microviscosity
decrease with increasing concentration of urea. It further supports
our above-mentioned discussion based on Kmic and ΔG° values (Table ) that urea disfavors the formation of micelles.
As a result, probe molecules get more and more exposed to the less
viscous environment. Although this result does not provide the actual
picture of microenvironment around C-480, it serves our purpose to
demonstrate the effect of urea concentration on the microviscosity
of micelles.
Table 2
Fluorescence Anisotropy (r), Average Excited Singlet-State Lifetime ⟨τf⟩, Rotational Correlation Time (τR) of DPH,
Microviscosities (ηm) of Micelles in the Presence
of Various Gemini Surfactants (10 mM) at Various Concentrations of
Ureaa,b
Gemini-A
Gemini-B
Gemini-C
urea (M)
r
⟨τf⟩
(ns)
τR (ns)
ηm (mPa s)
r
⟨τf⟩ (ns)
τR (ns)
ηm (mPa s)
r
⟨τf⟩ (ns)
τR (ns)
ηm (mPa s)
0
0.086 ± 0.012
5.41
1.69 ± 0.31
22.2 ± 0.2
0.087 ± 0.012
5.76
1.82 ± 0.33
24.0 ± 0.2
0.090 ± 0.010
6.31
2.09 ± 0.31
27.4 ± 0.2
0.5
0.080 ± 0.010
4.64
1.32 ± 0.21
17.3 ± 0.2
0.083 ± 0.013
5.61
1.78 ± 0.36
23.4 ± 0.2
0.085 ± 0.012
6.03
1.85 ± 0.34
24.3 ± 0.2
1
0.070 ± 0.010
4.56
1.09 ± 0.19
14.4 ± 0.2
0.082 ± 0.014
5.45
1.60 ± 0.35
21.0 ± 0.2
0.084 ± 0.014
5.89
1.78 ± 0.39
23.4 ± 0.2
2
0.065 ± 0.012
4.46
0.98 ± 0.21
12.8 ± 0.1
0.077 ± 0.011
5.08
1.37 ± 0.25
18.1 ± 0.2
0.079 ± 0.013
5.54
1.55 ± 0.32
20.3 ± 0.2
3
0.061 ± 0.011
4.29
0.87 ± 0.19
11.4 ± 0.1
0.071 ± 0.011
4.91
1.20 ± 0.23
15.8 ± 0.1
0.073 ± 0.011
5.38
1.36 ± 0.26
17.9 ± 0.1
4
0.057 ± 0.012
4.20
0.78 ± 0.19
10.3 ± 0.1
0.064 ± 0.011
4.76
1.02 ± 0.21
13.4 ± 0.2
0.066 ± 0.011
5.04
1.12 ± 0.23
14.9 ± 0.1
5
0.054 ± 0.012
4.08
0.72 ± 0.18
9.4 ± 0.1
0.057 ± 0.010
4.60
0.86 ± 0.17
11.3 ± 0.1
0.064 ± 0.012
4.89
1.05 ± 0.24
13.8 ± 0.1
λex = 375 nm. [DPH]
= 5 μM. A solution of DPH was prepared in tetrahydrofuran.
λex = 375
nm, λem = 429 nm.
λex = 375 nm. [DPH]
= 5 μM. A solution of n class="Chemical">DPH was prepared in tetrahydrofuran.
λex = 375
nm, λem = 429 nm.To see whether there is any effect of the nature of
spacer group
on the microviscosity of micelles, we have plotted ηm against varying concentrations of urea for micelles of all three
surfactants. Figure shows that at a given concentration of urea microviscosity increases
on substitution of the −OH group in the spacer group and it
also increases with increasing number of −OH groups. This result
could be depicting that the hydroxyl group(s) protect(s) the probe
molecules from contact with the water molecules due to which the microviscosity
of micelles increases. Further evidence in support of this phenomenon
has been discussed below.
Figure 5
Plot of variation of microviscosity of micelles
with increasing
concentration of urea in the micellar media of Gemini-A, -B, and -C.
Plot of variation of microviscosity of micelles
with increasing
concentration of urea in the micellar media of n class="Chemical">Gemini-A, -B, and -C.
To further support the effect
of urea concentration on the microviscosity
of micelles, the excited singlet-state lifetime values of C-480 have
been calculated using the time-correlated single-photon counting (TCSPC)
method. Fluorescence decays in all micelle–urea mixed systems
are biexponential in nature. The average lifetime has been calculated
using eq where a1 and a2 are the pre-exponential factors for the corresponding
lifetimes τ1 and τ2 of the two components.
The lifetime values of all components along with the average lifetime
of C-480 and χ2 values in all micelle–urea
mixed systems are given in Tables S2a–c. The fact that all C-480 molecules are solubilized in micelles at
10 mM concentration of gemini, this biexponential decay could be because
of two species: (1) locally excited (LE) and (2) intramolecular charge
transfer (ICT) as it is known that C-480 has ICT characteristics.[55] A detailed study will be carried out later to
confirm this. Data in Tables S2a–c show that ICT species is mostly contributing to the decay. It can
be seen that average excited-state lifetime values (⟨τf⟩) decreased with increasing urea concentration in
all micellar systems. Figure shows the variation of average excited-state lifetime values
of C-480 in the presence of all studied surfactants and urea mixed
systems. The excited-state lifetime values decreased with increasing
microviscosity of gemini surfactants. It has been mentioned (Table S1) that the fluorescence peak maxima of
C-480 remains unchanged in the presence and absence of urea. It infers
that variation in excited singlet-state lifetime is mostly due to
change in microviscosity of the systems. Of course, a comparatively
less effect due to the change in micropolarity cannot be ruled out.
Figure 6
Average
excited-state lifetime (⟨τf ⟩)
value of C-480 in the presence of the studied mixed systems. [Surfactant]
= 10 mM. λex = 375 nm, λem = 475
nm.
Average
excited-state lifetime (⟨τf ⟩)
value of C-480 in the presence of the studied mixed systems. [Surfactant]
= 10 mM. λex = 375 nm, λem = 475
nm.
Solvation
Dynamics
Solvation dynamics
of C-480 has been studied in the presence of 10 mM concentration of
all three studied gemini surfactants and in the presence of gemini
surfactant with various concentrations of urea in the range mentioned
above. Fluorescence decays of C-480 in 10 mM concentration of all
of the three gemini surfactants with varying concentrations of urea
have been recorded. In these systems, emission wavelength-dependent
decays of C-480 have been noted. Wavelengths are selected from the
entire range of a steady-state fluorescence spectrum of C-480 (Figure ). Figure shows the emission wavelength-dependent
decays of C-480 in 12-4-12 (10 mM) at 5 M urea concentration as a
representative one. A fast decay is noted at a short wavelength (say
430 nm). Fluorescence occurring from the unsolvated dipole generated
at the excited state is responsible for fast decay. Of course, there
is a possibility of contribution of high percentage of solvated dipoles
to the fast decay, which could not be detected due to the shortcomings
of our TCSPC setup (instrument response function = 165 ps). At a longer
wavelength (say 565 nm), fluorescence decay shows a clear growth in
the spectrum, followed by the decay. The growth in the decay indicates
the solvation of the probe molecule in the exited state.[74] Decays were fitted triexponentially at a shorter
wavelength, whereas biexponentially at a longer wavelength. Figure S3a,b shows decays of C-480 in 12-4(OH)-12
(10 mM) at 5 M urea concentration and 12-4(OH)2-12 (10
mM) at 5 M urea concentration as representatives. Similar behaviors
have also been observed for all gemini surfactants and urea mixed
systems. It is mentioned above that the most probable location of
C-480 molecules is the Stern layer. Therefore, the probe molecules
located at the Stern layer of the micelles are mostly contributing
to the said solvation process.[50,51,75] The solvation processes occurring in the bulk are too fast to be
measured by our instrumental setup (time resolution, ∼165 ps).
On the other hand, the probe molecules located in the hydrocarbon
core of the micelles are not supposed to contribute to any solvation
process.[50,51]
Figure 7
Fluorescence decays of C-480 in 12-4-12 (10
mM) at 5 M urea concentration.
λex = 375 nm.
Fluorescence decays of C-480 in 12-4-12 (10
mM) at 5 M n class="Chemical">urea concentration.
λex = 375 nm.
Time-resolved emission spectra (TRES) have been constructed
by
following the method of Fleming and Maroncelli.[76] Dynamic Stokes shifts in the emission spectra of C-480
can be seen after constructing TRES. Figure shows the TRES of C-480 in the presence
of 12-4-12 (10 mM) at varying concentrations of urea as a representative
one. TRES for other systems are given in the Supporting Information (Figure S4a,b). The
peak wavenumber, υ(t), for all TRES at different
times was obtained after fitting the TRES to a log-normal function.[66,76] The peak wavenumber (υ(t)) values have been
used to calculate the solvent response function (SRF), C(t), using eq given by Fleming and Maroncelli[76] for the quantitative measurement of solvation dynamicswhere υ(0), υ(t), and υ(∞)
are the peak wavenumbers at times zero, t, and infinity,
respectively. Figure shows the plot of C(t) versus
time for the system of 12-4-12 (10 mM) at varying
concentration of urea. The C(t)
decays for other systems are given in the Supporting Information (Figure S5a,b). The C(t) decays are generally biexponential
in nature in all of the studied systems. The biexponential fitting
of decays of C(t) has been done
using eq where τ1s and τ2s represent the solvent relaxation times,
and a1s and a2s are the corresponding
amplitudes. The decay characteristics of C(t) are given in Tables –3c for all of the studied systems. The average solvation time, ⟨τs⟩, for a biexponential decay has been estimated by eq and are also given in Tables –3cAs mentioned in the literature,
the bimodal
behavior of solvation in the microheterogeneous systems arises due
to the presence of free and bound water molecules.[77] The strength of hydrogen bond between the water molecule
and polar headgroup is stronger than that between two water molecules.[70,78,79] The water molecules that are
hydrogen-bonded with the polar headgroups are called bound water.
Free water molecules lead to fast solvation, whereas the bound water
molecules are responsible for slow solvation.[80] In all micellar systems, the fast components are the major components
to contribute to the solvation processes. In case of gemini surfactant
systems, polar headgroups, counterions, and spacer group can also
contribute to the solvation process. For a gemini surfactant, the
polar headgroups are joined to the long hydrocarbon tails as well
as to the spacer part. The mobilities of polar headgroups as well
as spacer group are restricted greatly as they are connected directly/indirectly
to the long hydrocarbon tails, and it is well reported that polymer
chain dynamics takes place on a very slow time scale (∼100
ns).[50−53,70,75] Therefore, essentially counterions and water molecules are responsible
for the solvation processes.
Figure 8
Time-resolved emission spectra of C-480 in 12-4-12
(10 mM) at X M urea (from right to left: 0, 500,
5000, and 10 000
ps): X = 0 M (a), 0.5 M (b), 1.0 M (c), 2.0 M (d),
3.0 M (e), 4.0 M (f), and 5.0 M (g).
Figure 9
Decays of solvent response function, C(t), of C-480 in the micelles of Gemini-A and urea. (a) 0–1
M urea and (b) 2–5 M urea.
Table 3a
Decay Characteristic of Solvent Response
Function, C(t), of C-480 in the
Presence of Pure 12-4-12 and 12-4-12 at Various Concentrations of
Ureaa
urea (M)
a1s
τ1s (ps)
a2s
τ2s (ps)
⟨τs⟩ (ps)
Δν
(cm–1)
0.0
0.64 ± 0.01
193.15 ± 0.01
0.36 ± 0.01
1709.25 ± 0.04
738.95
1855
0.5
0.66 ± 0.02
258.12 ± 0.01
0.34 ± 0.02
1706.93 ± 0.12
750.72
1917
1.0
0.73 ± 0.02
321.99 ± 0.01
0.27 ± 0.02
1986.20 ± 0.21
771.33
1956
2.0
0.69 ± 0.01
270.65 ± 0.01
0.31 ± 0.01
1919.62 ± 0.11
781.83
1995
3.0
0.69 ± 0.01
373.04 ± 0.01
0.31 ± 0.01
1869.15 ± 0.08
836.83
1989
4.0
0.71 ± 0.01
285.66 ± 0.01
0.29 ± 0.01
1778.84 ± 0.10
718.68
1957
5.0
0.67 ± 0.01
115.14 ± 0.01
0.33 ± 0.01
1431.39 ± 0.07
549.50
2314
Δν = ν(0) –
ν(∞).
Table 3c
Decay Characteristic of Solvent Response
Function, C(t), of C-480 in the
Presence of Pure 12-4(OH)2-12 and 12-4(OH)2-12
at Various Concentrations of Ureaa
urea (M)
a1s
τ1s (ps)
a2s
τ2s (ps)
⟨τs⟩ (ps)
Δν
(cm–1)
0.0
0.80 ± 0.04
493.77 ± 0.02
0.20 ± 0.03
2589.21 ± 0.61
912.86
2075
0.5
0.82 ± 0.03
521.95 ± 0.02
0.18 ± 0.03
2847.76 ± 0.69
940.60
2050
1.0
0.82 ± 0.04
535.06 ± 0.02
0.18 ± 0.03
2930.32 ± 0.71
966.21
2030
2.0
0.86 ± 0.03
515.85 ± 0.03
0.14 ± 0.02
4721.38 ± 0.73
1104.62
1997
3.0
0.70 ± 0.02
391.78 ± 0.01
0.30 ± 0.02
1743.48 ± 0.11
797.29
1930
4.0
0.68 ± 0.02
392.63 ± 0.01
0.32 ± 0.02
1580.51 ± 0.07
772.75
1885
5.0
0.75 ± 0.02
376.72 ± 0.01
0.25 ± 0.02
1867.86 ± 0.13
749.51
1820
Δν = ν(0) –
ν(∞).
Time-resolved emission spectra of C-480 in 12-4-12
(10 mM) at X M n class="Chemical">urea (from right to left: 0, 500,
5000, and 10 000
ps): X = 0 M (a), 0.5 M (b), 1.0 M (c), 2.0 M (d),
3.0 M (e), 4.0 M (f), and 5.0 M (g).
Decays of solvent response function, C(t), of C-480 in the micelles of n class="Chemical">Gemini-A and urea. (a) 0–1
M urea and (b) 2–5 M urea.
Δν = ν(0) –
ν(∞).Δν = ν(0) –
ν(∞).Δν = ν(0) –
ν(∞).The values
of average solvation time (⟨τs⟩) of
C-480 in the presence of gemini surfactants and urea
mixed systems have been calculated (Tables –3c) using eq , and their variations
with urea concentration are shown in Figure . It can be observed from the data in Tables –3c as well as from Figure that in the absence of urea and also up
to 2.0 M concentration of urea, the average solvation time increases
with increasing number of hydroxyl groups in the spacer group of gemini
surfactants.[50] As reported by us earlier,[50] the hydroxyl group(s) present in the spacer
group of gemini surfactant might be forming hydrogen bonds with the
water molecules. It protects the probe molecules from contact with
the water molecules and that is why solvation time is increased with
increasing number of hydroxyl groups in the spacer group of gemini
surfactants.[50] Recently, we have reported
comparatively slower solvation processes in case of micelles of gemini
surfactants containing diethyl ether spacer group causing similar
effect.[74] In case of micelles of Gemini-A,
i.e., 12-4-12 surfactant, because of the presence of hydrophobic spacer
group, the extent of free water molecules is greater than that in
the micelles of gemini surfactants possessing hydroxyl groups in the
spacer group. Evidence to this hypothesis has been discussed below.
Figure 10
Average
solvation time of C-480 in the presence of gemini surfactant
and urea mixed systems.
Average
solvation time of C-480 in the presence of gemini surfactant
and n class="Chemical">urea mixed systems.
We have carried out a separate fluorescence experiment to
further
support the fact that the hydroxyl group(s) present in the spacer
group of gemini surfactants protects the probe molecules from contact
with a certain amount of water molecules.[74] It has been reported[61,81] that the significant change in
micelle size and cmc occurs only when the percentage of organic co-solvent
in water–organic solvent mixture is above 20%. In view of this
fact, we have recorded fluorescence spectra of C-480 in aqueous micelles
and also in micelles in the presence of various percentages of organic
co-solvent not exceeding 20%. By performing these experiments, we
wanted to demonstrate the extent of interactions between the hydroxyl
group of methanol and C-480. For this purpose, we have presented here Figure a for Gemini-A,
which has been reported earlier.[74] This
figure shows that in the presence of 10 mM Gemini-A the fluorescence
peak maximum gets red-shifted from 477 to 480 nm on increasing the
percentage of methanol up to 20%. However, Figure b,c shows that there is no such change in
fluorescence peak maximum in 10 mM Gemini-B and Gemini-C, respectively,
in the presence of the same percentage of methanol. Thus, these results
depict that hydroxyl group(s) present in the spacer group of Gemini-B
and Gemini-C protect molecules containing −OH groups from contact
with probe molecule. Also this experiment is an extra step compared
to our previous study[50] to support our
hypothesis of protection of water molecules by −OH group(s)
in the spacer groups.
Figure 11
Fluorescence spectra of C-480 in 10 mM (a) Gemini-A, (b)
Gemini-B,
and (c) Gemini-C in the presence of various percentages of methanol
in water–methanol mixture. λex = 375 nm.
Fluorescence spectra of C-480 in 10 mM (a) n class="Chemical">Gemini-A, (b)
Gemini-B,
and (c) Gemini-C in the presence of various percentages of methanol
in water–methanol mixture. λex = 375 nm.
One can see in Figure that with increasing concentration
of urea the average solvation
time initially increases, reaches a maximum, and then decreases for
all three gemini micellar systems. For Gemini-B and Gemini-C, the
solvation time is maximum at 2.0 M of urea, whereas for Gemini-A,
the solvation time is maximum at 3.0 M of urea, which can be due to
the fact that the concentration of urea has an effect on the degree
of counterion dissociation (α) (Table ) as well.Figure represents
the variation of α with increasing concentration of urea, that
is, α increases, reaches a maximum, and then decreases with
increasing concentration of urea. The concentration of urea at which
α is maximum is the same as that of average solvation time.
There is a good correlation between the variation of average solvation
time and α with increasing concentration of urea in each case
of gemini surfactant. These results show that counterions have an
impact on the solvation dynamics. In our previous study with gemini
surfactants with polymethylene spacer group and also with mixed surfactant
systems, we have found that the counterions have an impact on solvation
dynamics.[51,53] With increasing α, the extent of free
counterions increases. It is known that water molecules can form hydrogen
bonds with anions.[82] Thus, up to a certain
concentration of urea with increasing number of counterions, the clustering
of water molecules increases.[83] With a
greater degree of clustering of water molecules, the number of free
water molecules is expected to decrease. This could be the reason
for the solvation process becoming slower. However, above a certain
concentration of urea with decreasing α (decreasing number of
free counterions), the clustering of water molecules is reduced. Because
of this, the number of free water molecules probably starts increasing,
which results in an increase in the rate of solvation process, i.e.,
the solvation time starts decreasing. Thus, free counterions indirectly
control the solvation dynamics via the formation of water clusters.
It is noteworthy that the ascending order of α observed in the
micelles of surfactants at a given concentration of urea is as follows:
Gemini-C < Gemini-B < Gemini-A. However, the order is reversed
as far as the average solvation times are concerned at a given concentration
of urea. It infers that ultimately the net availability of free water
molecules toward the solvation process of solute molecules probably
controls the rate of solvation. Although the clustering of water molecules
is higher in case of Gemini-A due to comparatively greater α,
because of the presence of hydrophobic spacer group, the accessibility
of free water molecules for effective solvation might be larger in
this case. Thus, spacer group of gemini surfactant plays an important
role in solvation dynamics. It is to be mentioned here that even high
concentration of urea has very less effect on the strength of hydrogen-bonding
interactions between water molecules.[38] Only a small fraction of water molecules gets immobilized by urea.
Therefore, any significant direct effect of urea on the solvation
dynamics can be ruled out.
Figure 12
Degree of counterion dissociation (α)
vs concentration of
urea for the micelles of Gemini-A, Gemini-B, and Gemini-C.
Degree of counterion dissociation (α)
vs concentration of
urea for the micelles of n class="Chemical">Gemini-A, Gemini-B, and Gemini-C.
Time-Resolved Fluorescence
Anisotropy
To obtain further information about the microenvironment
of micelles,
the time-resolved fluorescence anisotropy, r(t), measurement in molecular assemblies has been carried
out.[51−53] The r(t) values
have been determined using eq where I⊥(t) and I∥(t) are the fluorescence decays polarized perpendicular and
parallel to the polarization of the excitation light, respectively. G in eq is
the correction factor for detector sensitivity to the polarization
detection of emission. For our instrument setup, G factor is ∼0.6. Rotational relaxation behaviors of C-480
in micelles of gemini surfactants and in the presence of various concentrations
of urea have been characterized. Figure a–c shows the fluorescence anisotropy
decays of C-480 in Gemini-A, -B, and -C, respectively, in the presence
of various concentrations of urea. In pure water, the anisotropy decay
of C-480 is monoexponential, whereas that in the presence of pure
gemini surfactants and also in the presence of gemini surfactant with
various concentrations of added urea is biexponential in nature. The
decay function represented by eq has been applied to fit the biexponential anisotropy
decaywhere r0 is the
limiting value of anisotropy, which represents the inherent depolarization
of the probe molecule, τ1r and τ2 represent the time constants for the fast and slow
rotational relaxation components, respectively, and a1r and a2r are the corresponding
relative amplitudes. The rotational relaxation times for the fast
and slow components have been obtained from the fitted decays. The
average rotational relaxation time has been determined using eq where τ1r and τ2r are the rotational relaxation times, and a1r and a2r are the corresponding
amplitudes.
Figure 13
Fluorescence anisotropy decays of C-480 in the micelles
of gemini
surfactants: (a) Gemini-A, (b) Gemini-B, and (c) Gemini-C in the presence
of various concentrations of urea. λex = 375 nm.
λem = 470 nm.
Fluorescence anisotropy decays of C-480 in the micelles
of gemini
surfactants: (a) Gemini-A, (b) Gemini-B, and (c) Gemini-C in the presence
of various concentrations of urea. λex = 375 nm.
λem = 470 nm.Tables –5c present the
rotational relaxation
parameters along with the average rotational relaxation times of C-480
in the micelles of Gemini-A, -B, and -C, respectively, in the presence
of various concentrations of urea. The rotational relaxation time
of C-480 in the presence of pure water is found to be 132 ps, which
is in good agreement with the reported value of 125 ps.[84] The rotational relaxation time of C-480 in micelles
is many fold slower compared to that in pure water. This suggests
that random motions of the probe molecules are restricted in a micellar
medium. It can be seen that for each micellar system the fast rotational
relaxation component has major contribution to the fluorescence depolarization
than the slow component. The fact that the fast motions are responsible
for the loss of anisotropy is evidenced by the values of time-zero
anisotropy, ro < 0.40. It is known
that the maximum possible value of ro is
0.40.[72] The data in Tables –5c show that
fast as well as slow relaxation times along with the average relaxation
time decrease with increasing concentration of urea in a given micelle.
The variations of ⟨τr⟩ of C-480 with
increasing concentration of urea in all three micellar systems are
shown in Figure . The decreases in ⟨τr⟩ with increasing
concentration of urea in a given micellar system is in the harmony
of microviscosity of the systems. At a higher concentration of urea,
the solvent molecules seep in the micelle. The urea molecules reduce
the hydrophobic interaction between the surfactant molecules, which
causes demicellization. As a result, the microviscosity of micelles
decreases with increasing concentration of urea, which results in
decrease of the rotational relaxation time. This phenomenon is also
evidenced by almost continuous decrease in ro value with increasing concentration of urea in each micellar
system. Mukherjee et al.[85] found that the
rotational relaxation process of the probe molecules became faster
when concentration of the urea is increased in F127 micelles. It is
also pertinent to note that at a given concentration of urea the increase
in rotational relaxation time is according to the increasing order
of microviscosity of micelles in the presence of more number of −OH
groups starting from 0 to 2.
Table 5a
Rotational Relaxation Parameters
of C-480 in the Micelles of Gemini-A and Urea Mixed Systemsa
urea (M)
r0
a1r
τ1r (ns)
a2r
τ2r (ns)
⟨τr⟩ (ns)
0.0
0.34 ± 0.04
0.87 ± 0.02
0.61 ± 0.06
0.13 ± 0.02
6.91 ± 0.53
1.43
0.5
0.33 ± 0.05
0.90 ± 0.02
0.67 ± 0.07
0.10 ± 0.02
7.41 ± 0.61
1.34
1.0
0.31 ± 0.03
0.92 ± 0.01
0.66 ± 0.06
0.08 ± 0.01
7.74 ± 0.63
1.23
2.0
0.29 ± 0.03
0.93 ± 0.01
0.67 ± 0.07
0.07 ± 0.01
6.96 ± 0.56
1.11
3.0
0.28 ± 0.04
0.62 ± 0.02
0.36 ± 0.04
0.38 ± 0.01
1.73 ± 0.12
0.88
4.0
0.27 ± 0.03
0.74 ± 0.02
0.45 ± 0.05
0.26 ± 0.01
2.09 ± 0.14
0.88
5.0
0.27 ± 0.03
0.70 ± 0.02
0.39 ± 0.04
0.30 ± 0.01
1.96 ± 0.13
0.86
λex = 375 nm,
λem = 470 nm.
Table 5c
Rotational Relaxation Parameters
of C-480 in the Micelles of Gemini-C and Urea Mixed Systemsa
urea (M)
r0
a1r
τ1r (ns)
a2r
τ2r (ns)
⟨τr⟩ (ns)
0.0
0.30 ± 0.04
0.85 ± 0.01
0.76 ± 0.08
0.15 ± 0.02
12.82 ± 0.97
2.57
0.5
0.28 ± 0.03
0.86 ± 0.02
0.75 ± 0.08
0.14 ± 0.01
12.38 ± 0.89
2.38
1.0
0.28 ± 0.03
0.90 ± 0.02
0.79 ± 0.08
0.10 ± 0.01
12.68 ± 0.98
1.98
2.0
0.27 ± 0.03
0.91 ± 0.01
0.78 ± 0.08
0.09 ± 0.01
13.25 ± 0.99
1.90
3.0
0.27 ± 0.03
0.87 ± 0.02
0.73 ± 0.07
0.13 ± 0.01
7.19 ± 0.59
1.57
4.0
0.26 ± 0.03
0.89 ± 0.02
0.72 ± 0.07
0.11 ± 0.01
7.84 ± 0.63
1.50
5.0
0.26 ± 0.03
0.57 ± 0.02
0.39 ± 0.05
0.43 ± 0.02
1.89 ± 0.12
1.04
λex = 375 nm,
λem = 470 nm.
Figure 14
Average rotational relaxation time of C-480
in the presence of
gemini surfactant and urea mixed systems.
Average rotational relaxation time of C-480
in the presence of
gemini surfactant and n class="Chemical">urea mixed systems.
λex = 375 nm,
λem = 470 nm.λex = 375 nm,
λem = 470 nm.λex = 375 nm,
λem = 470 nm.From literature reports, it can be stated that the observed biexponential
behavior of the anisotropy decay is mainly due to the different types
of rotational motions.[86,87] The two-step and wobbling-in-a-cone
models[87,88] are very suitable to explain this kind of
bimodal behavior of anisotropy decay. According to the two-step model,
the tumbling motion of the entire micelle (τm) and
the lateral diffusion (τD) of the probe along the
micelle surface contribute to the slow rotational relaxation (τ2r). The time corresponding to the slow rotational relaxation
is related to that of the overall motion of micelle as a whole and
lateral diffusion as eq The Debye–Stokes–Einstein equation
is used to calculate τm after estimating the hydrodynamic
radii (r) of micelle
(Note 1, Supporting Information). The estimated rh values for all three micellar media in the
presence of urea are given in Tables –6c. Figure S6 (Supporting Information) represents
the size distribution plot for Gemini-B as a representative one. The
τm values at 298.15 K have been calculated using eq S1 (Supporting Information). After knowing
the τm values, the τD values have
been calculated using eq . Both the τm and τD values
are tabulated in Tables –6c. One can see that the τD values (Tables –6c) are almost same as slow
rotational relaxation time (τ2r) (Tables –5c). Thus, the lateral diffusion of the probe occurring along
the surface of the micelle mainly contributes to the slow rotational
relaxation. We have found that in case of Gemini-A there is an increasing
tendency of the hydrodynamic radius of micelles with increasing concentration
of urea. It infers that with increasing concentration of urea the
micelles become less compact as the stabilization of micelle formation
is reduced. As expected, the time constant for the tumbling motion
of the micelle (τm) increases with increasing size
of micelles. However, we have not found any particular trend for hydrodynamic
radii values of Gemini-B and Gemini-C with increasing concentration
of urea, although there is a relationship between the size of the
micelle and the τm value. There is a diminishing
tendency of τD with increasing concentration of urea
for a given micellar system. This trend is due to the decrease in
the microviscosity of micelles with increasing concentration of urea.
However, as τD is related to τm,
this correlation is also found to be comparatively better for Gemini-A
than for Gemini-B and Gemini-C. The lateral diffusion of the probe
molecule is much faster than the tumbling motion of the micelle as
a whole. The time constants for the wobbling motion, τw, of the probe molecule C-480 in all of the micelles calculated using eq S2 (Note 2, Supporting Information) are also tabulated in Tables –6c. As the
wobbling motion gives an account of the relaxation of local structure
in a micelle, there is a decreasing tendency of the value of τw with decreasing microviscosity of micelles.
Table 6a
Hydrodynamic Diameter, Time for Overall
Rotational Motion of the Micelle (τm), Lateral Diffusion
Time (τD), Wobbling Motion Time (τw), Wobbling Diffusion Coefficient (Dw), Cone Angle (θo), and Order Parameter (|S|) Obtained from the Anisotropy Decays of C-480 in the
Micelles of Gemini-A and Urea
Gemini-A + urea
hydrodynamic
diameter (nm)
τw (ns)
τm (ns)
τD (ns)
Dw × 10–8 (s–1)
θo (deg)
|S|
θo (deg)a
0.0
1.25 ± 0.06
0.67 ± 0.06
221.46 ± 0.53
7.13 ± 0.53
4.93 ± 0.77
61.0 ± 0.9
0.36 ± 0.03
58.1 ± 0.9
0.5
1.10 ± 0.03
0.74 ± 0.07
150.91 ± 0.61
7.79 ± 0.61
4.87 ± 0.78
63.7 ± 0.9
0.32 ± 0.03
53.1 ± 0.9
1.0
1.38 ± 0.07
0.72 ± 0.06
297.99 ± 0.63
7.95 ± 0.63
5.44 ± 0.69
66.4 ± 0.8
0.28 ± 0.02
48.5 ± 0.8
2.0
1.44 ± 0.08
0.74 ± 0.07
338.57 ± 0.56
7.11 ± 0.56
5.52 ± 0.76
67.8 ± 0.8
0.26 ± 0.02
46.1 ± 0.8
3.0
2.07 ± 0.12
0.46 ± 0.04
1005.71 ± 0.12
1.73 ± 0.12
3.72 ± 0.45
43.9 ± 0.7
0.62 ± 0.01
4.0
2.13 ± 0.14
0.57 ± 0.05
1095.72 ± 0.14
2.09 ± 0.14
4.08 ± 0.47
51.2 ± 0.6
0.51 ± 0.01
5.0
2.14 ± 0.15
0.49 ± 0.04
1111.23 ± 0.13
1.96 ± 0.13
4.27 ± 0.47
48.6 ± 0.6
0.55 ± 0.01
θo is calculated
using the spinning-in-equatorial-band model.
Table 6c
Hydrodynamic Diameter, Time for Overall
Rotational Motion of the Micelle (τm), Lateral Diffusion
Time (τD), Wobbling Motion Time (τw), Wobbling Diffusion Coefficient (Dw), Cone Angle (θo), and Order Parameter (|S|) Obtained from the Anisotropy Decays of C-480 in the
Micelles of Gemini-C and Urea
Gemini-C + urea
hydrodynamic
diameter (nm)
τw (ns)
τm (ns)
τD (ns)
Dw × 10–8 (s–1)
θo (deg)
|S|
θo (deg)a
0.0
1.28 ± 0.05
0.81 ± 0.08
237.79 ± 0.97
13.55 ± 0.97
3.82 ± 0.56
59.0 ± 0.8
0.39 ± 0.02
62.0 ± 0.9
0.5
1.28 ± 0.05
0.79 ± 0.08
237.79 ± 0.89
13.06 ± 0.89
4.09 ± 0.59
60.3 ± 0.8
0.37 ± 0.02
59.3 ± 0.9
1.0
1.15 ± 0.02
0.84 ± 0.08
172.45 ± 0.98
13.69 ± 0.98
4.29 ± 0.51
63.7 ± 0.7
0.32 ± 0.01
53.1 ± 0.7
2.0
1.22 ± 0.04
0.83 ± 0.08
205.89 ± 0.99
14.16 ± 0.99
4.53 ± 0.64
65.0 ± 0.6
0.30 ± 0.02
50.8 ± 0.8
3.0
0.88 ± 0.02
0.81 ± 0.07
77.27 ± 0.59
7.93 ± 0.59
4.08 ± 0.45
61.0 ± 0.7
0.36 ± 0.01
58.1 ± 0.7
4.0
1.05 ± 0.03
0.79 ± 0.07
131.26 ± 0.63
8.34 ± 0.63
4.46 ± 0.59
63.0 ± 0.8
0.33 ± 0.02
54.3 ± 0.8
5.0
1.34 ± 0.06
0.49 ± 0.05
272.82 ± 0.12
1.91 ± 0.12
3.07 ± 0.29
41.1 ± 0.6
0.66 ± 0.01
θo is calculated
using the spinning-in-equatorial-band model.
θo is calculated
using the spinning-in-equatorial-band model.θo is calculated
using the spinning-in-equatorial-band model.θo is calculated
using the spinning-in-equatorial-band model.The values of wobbling diffusion coefficient (Dw), order parameter (|S|),
and cone angle
(θo) have been calculated using eqs S3–S5, respectively, by applying the wobbling-in-a-cone
model[50,87] (Note 3, Supporting Information) to obtain more information about the motion restriction
of the probe molecules within the micelles. These values are tabulated
in Tables –6c. Quite large values of spatial restriction parameter
(|S|) show that the probe molecules are located in
a restricted environment. The higher values of θo and lower values of (|S|) at lower concentration
range of urea could be due to the high microviscosity of micelles
in this range. It is possible that the wobbling-in-a-cone model is
not appropriate for micelles with high microviscosity. To solve this
apparent problem, sometimes the spinning-in-equatorial-band model[89,90] is used (Note 4, Supporting Information). We have applied the spinning-in-equatorial-band model for micellar
systems for which |S| < 0.5, which is the requirement
to apply this model. The calculated values of θo after
applying this model are also given in Tables –6c. Thus,
possibly, the probe molecule is oriented in such a way that the emission
moment remains perpendicular to the long axis like that in the spinning-in-equatorial-band
model for micelles with high microviscosity. On the other hand, for
micelles of comparatively lower microviscosity, the alignment of the
C-480 molecules would be such that the emission moment is parallel
to the long axis like that is in the wobbling-in-a-cone model.
Conclusions
The effect of addition of urea in the aqueous
micelles of all three
cationic gemini surfactants with varying spacer groups on the micellization
behavior of surfactants, solvation dynamics, and rotational relaxation
of C-480 has been studied. With increasing concentration of urea,
the hydrophobic interactions between surfactant molecules are reduced
due to lesser solvophobicity. This effect disfavors the formation
of micelles, so cmc is increased with increasing concentration of
urea. The most probable location of the C-480 molecules is the Stern
layer of micelles. Solvation dynamics is found to be bimodal in nature.
The average solvation time increases, reaches a maximum, and then
decreases with increasing concentration of urea. It has been observed
that the degree of counterion dissociation also increases, reaches
a maximum, and then decreases with more and more addition of urea
in the micellar solution. With increasing concentration of free counterions,
the extent of clustering of water molecules is expected to be increased,
resulting in longer solvation times. Thus, the rate of solvation process
can be correlated with the degree of counterion dissociation. The
presence of −OH group in the spacer group has an impact on
the rate of solvation process. The −OH groups protect the water
molecules from contact with the probe molecules through hydrogen bonding.
As a result, the solvation process is slowed down. The microviscosity
of micelles is decreased with increasing concentration of urea, as
a result of which the rotational relaxation process becomes faster.
At a given concentration of urea, the rotational relaxation process
slows down with the introduction of the −OH group in the spacer
group due to enhanced microviscosity of micelles. Rotational relaxation
is bimodal in nature. The fast rotational relaxation component has
major contribution to the fluorescence depolarization compared to
the slow component. The lateral diffusion of the probe molecules along
the surface of the micelle is mainly responsible for slow rotational
relaxation. The tumbling motion of micelle as a whole is much slower
than the lateral diffusion of the probe molecules. Wobbling motion
also becomes faster with increasing concentration of urea as a result
of the decreased microviscosity of micelles. The C-480 molecules might
change their alignment in the micelles with changing microviscosity.
Experimental Section
Materials
Fluorescence
probe C-480
was procured from Exciton (laser grade) and used as received. Gemini
surfactants were synthesized according the reported procedure.[46,50−53] Synthesized gemini surfactants were recrystallized several times
with a methanol and ethyl acetate mixture. Fourier transform infrared
(FT-IR) and NMR spectroscopy methods were used to confirm the structure
of the synthesized gemini surfactants given in the previous report.[50] 1,6-Diphenyl-1,3,5-hexatriene (DPH) was obtained
from Aldrich Chemical Co. Extrapure urea was procured from SD Fine-Chem
Limited, Mumbai, India. Aqueous solution of Ludox was obtained from
Aldrich Chemical Co. This solution was used as a scatterer to record
the lamp profile for time-correlated single-photon counting (TCSPC)
measurements.
Methods
Solutions
of pure gemini
surfactants and gemini surfactants with urea at various concentrations
were prepared in Milli-Q water obtained from Millipore water filtration
system. The concentration of gemini surfactants in all of the measurements
was fixed at 10 mM, which is quite higher than the cmc value of the
respective gemini surfactants to make sure that the probe molecule
completely solubilized in the micellar medium.[50] A JASCO (Model V-650) UV–visible spectrophotometer
was used to measure the absorption spectra of C-480 in pure solvents
(UV-grade solvents) and micellar solutions. A HORIBA Jobin Yvon Fluoromax-4
scanning spectrofluorimeter was used to record to all of the fluorescence
spectra. All of the fluorescence spectra were corrected for instrument
sensitivity. For all of the fluorescence measurements, a slit width
of 3 nm was kept for both the slits. For time-resolved measurements,
the HORIBA Jobin Yvon Fluorocube-01-NL picosecond TCSPC experimental
setup was used. A picosecond diode laser of wavelength 375 nm (NanoLED
375L, IBH, U.K.) was used as the light source. The details of the
instruments is available in previous reports.[50,51,91] The same setup was also used to analyze
the time-resolved fluorescence anisotropy measurements. The time constants
for solvation process have been estimated after constructing the TRES
and calculating the solvent response function, C(t), using the method of Fleming and Maroncelli,[76] the details of which are available elsewhere.[51] The time-resolved fluorescence anisotropy values, r(t), have been calculated using eq . Hydrodynamic radii of
micellar aggregates were measured using Zetasizer, model Nano ZS (ZEN
3600, Malvern Instruments, U.K.). Descriptions of dynamic light scattering
(DLS) measurements are available in our earlier report.[92] All samples were filtered before the measurements
with a 0.22 μm filter (Durapore, poly(vinylidene difluoride)).
The wavelength of the laser light was 632.8 nm, and the scattering
angle was 173°. The corresponding G function
was considered carefully to judge the size distribution. The cmc values
of all gemini surfactants in the absence and presence of different
concentrations of urea were estimated by conductivity method using
the Systronics conductivity meter 304. The conductivity cell with
cell constant of 1 cm–1 was calibrated with the
standard KCl solution of specific conductivity 1412 μS cm–1 prior to the measurement. Standard KCl solution was
purchased from Merck, India. The 1H NMR spectra were recorded
with a Bruker Avance instrument (400 MHz), and the FT-IR spectra were
recorded in ABB Boman MB 300 instrument. All measurements were carried
out at 298.15 ± 1 K.
Table 3b
Decay Characteristic of Solvent Response
Function, C(t), of C-480 in the
Presence of Pure 12-4(OH)-12 and 12-4(OH)-12 at Various Concentrations
of Urea
urea (M)
a1s
τ1s (ps)
a2s
τ2s (ps)
⟨τs⟩ (ps)
Δν
(cm–1)
0.0
0.68 ± 0.01
355.43 ± 0.01
0.28 ± 0.01
1861.01 ± 0.08
762.76
1910
0.5
0.56 ± 0.04
297.45 ± 0.01
0.44 ± 0.04
1409.21 ± 0.25
786.62
1930
1.0
0.66 ± 0.02
347.02 ± 0.01
0.34 ± 0.02
1735.24 ± 0.17
819.01
2057
2.0
0.52 ± 0.03
337.47 ± 0.02
0.48 ± 0.03
1498.59 ± 0.13
894.81
1954
3.0
0.73 ± 0.02
371.56 ± 0.01
0.27 ± 0.02
1768.15 ± 0.16
748.64
1911
4.0
0.51 ± 0.01
262.44 ± 0.01
0.49 ± 0.01
1230.69 ± 0.02
736.88
2098
5.0
0.67 ± 0.01
175.85 ± 0.01
0.33 ± 0.01
1350.64 ± 0.03
563.53
2366
Δν = ν(0) –
ν(∞).
Table 5b
Rotational Relaxation Parameters
of C-480 in the Micelles of Gemini-B and Urea Mixed Systemsa
urea (M)
r0
a1r
τ1r (ns)
a2r
τ2r (ns)
⟨τr⟩ (ns)
0.0
0.33 ± 0.05
0.91 ± 0.02
0.67 ± 0.07
0.09 ± 0.01
13.13 ± 0.98
1.79
0.5
0.32 ± 0.04
0.93 ± 0.02
0.69 ± 0.07
0.07 ± 0.01
10.39 ± 0.87
1.37
1.0
0.33 ± 0.05
0.91 ± 0.01
0.63 ± 0.06
0.09 ± 0.01
7.97 ± 0.65
1.29
2.0
0.32 ± 0.03
0.89 ± 0.01
0.62 ± 0.05
0.11 ± 0.01
5.29 ± 0.44
1.13
3.0
0.31 ± 0.03
0.90 ± 0.01
0.65 ± 0.06
0.10 ± 0.01
5.09 ± 0.43
1.09
4.0
0.30 ± 0.03
0.78 ± 0.01
0.49 ± 0.05
0.22 ± 0.01
2.51 ± 0.15
0.93
5.0
0.27 ± 0.03
0.78 ± 0.01
0.52 ± 0.05
0.22 ± 0.01
2.25 ± 0.14
0.90
λex = 375 nm,
λem = 470 nm.
Table 6b
Hydrodynamic Diameter, Time for Overall
Rotational Motion of the Micelle (τm), Lateral Diffusion
Time (τD), Wobbling Motion Time (τw), Wobbling Diffusion Coefficient (Dw), Cone Angle (θo), and Order Parameter (|S|) Obtained from the Anisotropy Decays of C-480 in the
Micelles of Gemini-B and Urea
Gemini-B + urea
hydrodynamic
diameter (nm)
τw (ns)
τm (ns)
τD (ns)
Dw × 10–8 (s–1)
θo (deg)
|S|
θo (deg)a
0.0
1.30 ± 0.06
0.71 ± 0.07
249.11 ± 0.98
13.86 ± 0.98
5.29 ± 0.75
65.0 ± 0.8
0.30 ± 0.02
50.8 ± 0.8
0.5
1.50 ± 0.07
0.74 ± 0.07
382.68 ± 0.87
10.68 ± 0.87
5.52 ± 0.81
67.8 ± 0.8
0.26 ± 0.02
46.1 ± 0.8
1.0
1.28 ± 0.06
0.68 ± 0.06
237.79 ± 0.65
8.25 ± 0.65
5.53 ± 0.74
65.0 ± 0.8
0.30 ± 0.02
50.8 ± 0.8
2.0
1.38 ± 0.06
0.71 ± 0.05
297.99 ± 0.44
5.39 ± 0.44
4.96 ± 0.58
63.0 ± 0.8
0.33 ± 0.02
54.3 ± 0.8
3.0
1.78 ± 0.08
0.75 ± 0.06
639.47 ± 0.43
5.13 ± 0.43
4.81 ± 0.51
63.7 ± 0.6
0.32 ± 0.01
53.1 ± 0.7
4.0
1.58 ± 0.08
0.61 ± 0.05
447.23 ± 0.15
2.52 ± 0.15
4.21 ± 0.45
53.8 ± 0.6
0.47 ± 0.01
75.8 ± 0.9
5.0
1.34 ± 0.07
0.68 ± 0.05
272.82 ± 0.14
2.27 ± 0.14
3.78 ± 0.37
53.8 ± 0.6
0.47 ± 0.01
75.8 ± 0.9
θo is calculated
using the spinning-in-equatorial-band model.
Scheme 1
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12
Figure 13
Figure 14