Article pubs.acs.org/JPCB

Effect of Polymethylene Spacer of Cationic Gemini Surfactants on Solvation Dynamics and Rotational Relaxation of Coumarin 153 in Aqueous Micelles Sonu, Sunita Kumari, and Subit K. Saha* Department of Chemistry, Birla Institute of Technology & Science (BITS), Pilani, Rajasthan 333 031, India S Supporting Information *

ABSTRACT: The present work demonstrates the solvation dynamics and rotational relaxation of Coumarin 153 (C-153) in the micelles of a series of cationic gemini surfactants, 12-s12, 2Br− containing a hydrophobic polymethylene spacer with s = 3, 4, 6, 8, 12. Steady-state and time-correlated singlephoton counting (TCSPC) fluorescence spectroscopic techniques have been used to carry out this study. Steady-state and TCSPC fluorescence data suggest that C-153 molecules are located at the Stern layer of micelles. While probe molecules feel more or less the same micropolarity in the micellar phase, the microviscosity of micelles decreases with spacer chain length. Solvation dynamics at the Stern layer is bimodal in nature with fast solvation as a major component. Counter ions and water molecules bonded with the polar headgroups of surfactant molecules are responsible for the slow component. Average solvation time increases with spacer chain length because of the increased degree of counter ion dissociation. Some water molecules are involved in the solvation of counter ions themselves, resulting in the decrease in “free” water molecules to be available for the solvation of C-153. The hydrophobic spacer chain also has an effect on increasing the solvation time with increasing chain length. The average rotational relaxation time for C-153 decreases with spacer chain length with a rapid decrease at s > 4. The anisotropy decay of C-153 in micelles is biexponential in nature. The slow rotational relaxation is due to the lateral diffusion of C-153 in micelles. Lateral diffusion is much faster than the rotational motion of a micelle as a whole. The rotational motion of the micelle as a whole becomes faster with the decreasing size of micelles. Bhattacharyya and co-workers1 in their time-dependent Stokes’ shift studies with Coumarin 480 (C-480) first showed that the relaxation dynamics of water molecules at the Stern layer of micelles was much slower than that of ordinary bulk water. The solvation dynamics of water molecules in the micelles of conventional surfactants are found to be bimodal in nature.1,13 A dynamic exchange between “free” and “bound” water molecules associated with the self-organized system has been proposed to explain this kind of bimodal behavior.23 Bagchi and co-workers25−28 have proposed that the slower component of solvation dynamics of water in the micellar phase is due to the hydrogen bonding interaction between the water molecules and surfactant headgroups. Sarkar and co-workers6,15,29,30 have also reported solvation dynamics in the micellar systems of various surfactant. They have recently reported the effect of ionic liquid on the solvation dynamics and the rotational relaxation of Coumarin 153 (C-153) in the aqueous micelles of Triton X-100.31 Their study shows that solvation dynamics becomes faster on addition of ionic liquid.

1. INTRODUCTION The study of the solvation dynamics of solvent molecules and the rotational relaxation of an electronically excited molecule in different biologically relevant organized systems has been an area of interest in the last few decades.1−11 Cyclodextrins, vesicles, micelles, reverse micelles, and protein−surfactants complexes are a few biologically relevant systems.1−11 These systems have been chosen for the study of solvation dynamics and rotational relaxation.12−20 Molecular assemblies of surfactant molecules are considered to be a model of biological lipid membranes.21 Water is the essential component of living organisms which when present in the self-organized molecular assemblies of surfactant molecules shows different characteristics than water in the bulk.22 The characteristics of water molecules are changed in the vicinity of molecular aggregates.22,23 In the presence of molecular assemblies, the solvation dynamics of water molecules is retarded many times as compared to that in bulk water. Fleming et al.24 reported the solvation dynamics of Coumarin dyes in bulk water and in γcyclodextrin (γ-CD). While solvation dynamics in bulk water occurs in 315 fs, the dynamics slows down many times in γ-CD and happens on the nanosecond time scale. © XXXX American Chemical Society

Received: March 31, 2015 Revised: June 7, 2015

A

DOI: 10.1021/acs.jpcb.5b03081 J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Scheme 1. Molecular Structures of Cationic Gemini Surfactants (12-s-12, 2Br−) and Fluorescent Probe Molecule C-153

a gemini surfactant can be long or short, hydrophilic or hydrophobic, flexible or rigid.35 Zana and co-workers43 have studied the aggregation behavior and microstructure of gemini surfactants in aqueous solution. It has been reported that the cmc of gemini surfactants, m-s-m with a hydrophobic polymethylene spacer, increases with increasing s reaches a maximum at s = 5−6, irrespective of the value of m, and then decreases.39 It is believed that the increase in cmc with s observed for s ≤ 6 for m-s-m surfactants is due to a conformational change in the surfactant molecule.39 Gemini surfactant with a spacer group of short chain length (12-2-12, 2Br−) shows concentration-dependent micellar growth forming wormlike micelles. 12-3-12, 2Br− gemini surfactant molecules form spheroidal micelles at a concentration of 30 mM. The drastic decrease in cmc with increasing s at s ≥ 10 is attributed to the progressive penetration of the spacer in the micelle hydrophobic core.44 The change in conformation of the gemini surfactant molecule followed by progressive penetration of the spacer with increasing s is expected to affect the number of water molecules per spacer group at the Stern layer or at the micelle−water interface. Moreover, the hydration of micelles decreases with increasing spacer chain length of a gemini surfactant.45 Thomas and coworkers38 have reported that a hydrophilic flexible spacer can easily be located at the micelle−water interface and forms a more closely packed micellar structure than one with a hydrophobic rigid spacer. Zana and co-workers46 have reported that the micropolarity of 12-s-12, 2Br− micelles increases with increasing s, reaches a maximum at s = 5, and then decreases. They have also found that the microviscosity of 12-s-12, 2Br− micelles decreases with increasing s.46,47 We have recently reported the effect of a hydroxyl-group-substituted spacer group of cationic gemini surfactants on the solvation dynamics and rotational relaxation of C-480 in aqueous micelles.48 With these backgrounds our focus in the present work is to study the effect of the hydrophobic polymethylene spacer chain length on solvation dynamics in the micelles of gemini surfactants 12-s-12, 2Br− with s = 3, 4, 6, 8, and 12 (Scheme 1), which has not been reported so far. Generally for the purpose of studying the solvation dynamics and time-dependent Stokes shift, a fluorescent probe is used which has a very small dipole

In the presence of ionic liquid, the microfluidity of micelles is enhanced as a result of the penetration of more water molecules in the micellar phase. Sarkar and co-workers30 in another study have found that the solvation time and rotational relaxation time both increase with increasing alkyl chain length of the surfactant. With increasing alkyl chain length of the surfactant, the micelles become more close-packed, which leads to an increase in the microviscosity of the micelles. Maroncelli and co-workers32 have found that the rotational relaxation times correlate well with the microviscosity but not with solvent’s hydrogen bonding ability. It has been reported that for the micelles of cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS) the solvation rate is much faster than that in neutral micelles such as TX-100 because of the much thicker palisade layer for the latter than for the former two micelles.1 Kumbhakar et al.33 have reported the effect of temperature on solvation dynamics in neutral micelles using the dynamic fluorescence Stokes’ shift of C-153. Hazra and coworkers34 have recently reported the solvation dynamics in SDS-dispersed single-walled carbon nanotubes using C-153 as a probe. They have also studied the dynamics of urea inside the reverse micelles of dioctyl-sulfosuccinate sodium salt (AOT) using Coumarin 343 (C-343).20 Shirota and co-worker’s21 study on C-480 shows that the solvation dynamics in the aqueous micelles of anionic surfactant is slower than that in the aqueous micelles of cationic surfactant due to the stronger hydrogen bonding interaction between the water molecule and the headgroup of the former than that of the latter. Recently, another class of surfactants known as “gemini surfactants” has attracted special attention in material sciences, biological sciences, nanotechnology, supramolecular chemistry, and so forth.35 Gemini surfactants are made up of two hydrophobic tails and two hydrophilic headgroups, covalently joined by a spacer group at their headgroups. Some properties of gemini surfactants are superior to their conventional counterparts.36 The spacer part plays a significant role in the aggregation properties of a gemini surfactant. The critical micelle concentration (cmc), counterion binding, thermodynamic properties, microviscosity and micropolarity, rheological behavior, and aggregation number of gemini surfactants vary with any change in the spacer part.35,37−42 The spacer group of B

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picosecond time-correlated single-photon counting (TCSPC) experimental setup. A picosecond diode laser of 375 nm (NanoLED 375L, IBH, UK) was used as an excitation light source. A TBX photon detection module (TBX-07C) has been used for the detection of fluorescence signals at magic angle (54.7°) polarization. The instrument response function of this laser system is ∼165 ps. The decays have been analyzed using IBH DAS-6 decay analysis software. The goodness of fit has been analyzed by χ2 criterion and a visual inspection of the residuals of the fitted function to the data. The same setup was used for time-resolved fluorescence anisotropy measurements. For multiexponential decays, the average excited singlet state lifetimes have been calculated by using eq 152,53

moment in the ground state and a very large dipole moment in the excited state.49 C-153 is one such molecule that has been used as a probe for a number of solvation dynamics studies31,33,34 and also has been used in the present study (Scheme 1). Because the spacer chain length has an effect on counterion binding, we want to see the effect of this phenomenon on solvation dynamics because counterions also contribute to the solvation dynamics.3 Because the microviscosity of 12-s-12, 2Br− micelles decreases with increasing s,46,47 the present work also aims to study the effect of the spacer group of gemini surfactants on the rotational relaxation of C-153. This study will provide a better picture of the microenvironment around the probe. Because the aggregation behavior of gemini surfactants can be tuned by changing the chemical nature of the spacer group, the study of the dynamics of water molecules and rotational relaxation has practical and fundamental importance.

⟨t ⟩ =

∑ aiτ i

(1)

where ai is the pre-exponential factor of the ith component and τi is the lifetime of the ith component. The quantitative measurement of solvation dynamics of C-153 has been made by using the solvent correlation function (SCF), C(t), given by Fleming and Maroncelli54

2. MATERIALS AND METHODS 2.1. Materials. C-153 was procured from Exciton (laser grade) and was used without any further purification. Gemini surfactants with different spacer chain lengths were synthesized according to the reported procedure38,39 and recrystallized several times with a methanol and ethyl acetate mixture. The structures of synthesized compounds were confirmed by FT-IR and 1H NMR data (Table S1, Supporting Information). Milli-Q water obtained from a Millipore water filtration system was used for the preparation of all aqueous solutions. All solvents used were of spectroscopic grade and were procured from Spectrochem Chemical Company, India. An aqueous solution of Ludox was used as scatterer to record the lamp profile in the case of time-correlated single-photon counting (TCSPC) measurements. Ludox was procured from Aldrich Chemical Co. 2.2. Methods. The solutions of all gemini surfactants in aqueous media were prepared with the same concentration of C-153. A stock solution (0.025 mL) of 1 mM C-153 in methanol was added to the volumetric flask and then held for a few hours to allow the complete evaporation of methanol. The required volume of the aqueous solution of a gemini surfactant was then added to the volumetric flask, and the final volume was adjusted to 5 mL using Milli-Q water. The concentration of C-153 in the final solution was 5 μM. The concentration of solutions of all of the gemini surfactants was kept at 10 mM, well above their cmc values, to ensure that the probe molecules were completely micellized. Absorption spectra were recorded using a Shimadzu (UV1800) UV−visible spectrophotometer. A Horiba Jobin Yvon Fluoromax-4 scanning spectrofluorimeter has been used for all steady-state fluorescence measurements. Quartz cuvettes of 1 cm path length were used for all spectral measurements. The excitation and emission slit widths used for the fluorescence measurements were 3 nm each. Fluorescence spectra were corrected for the spectral sensitivity of the instrument. The steady-state fluorescence anisotropy measurements were performed with the same steady-state spectrofluorimeter fitted with a polarizer attachment, and details are given elsewhere.50,51 Quinine sulfate in 0.1 N sulfuric acid as a standard (ϕf = 0.55) has been used for the determination of the relative fluorescence quantum yield by calculating the area under the corrected fluorescence band of C-153 in the presence of different gemini surfactants and that of quinine sulfate.51 The excited singlet state lifetimes were measured from intensity decays using a Horiba Jobin Yvon Fluorocube-01-NL

C(t ) =

ν(t ) − ν(∞) ν(0) − ν(∞)

(2)

where ν(0), ν(t), and ν(∞) are the peak frequencies at time zero, t, and infinity, respectively. To determine the peak frequencies, the time-resolved emission spectra (TRES) have been constructed using the reported method of Fleming and Maroncelli54 by collecting the decay profiles at various wavelengths across the entire range of an emission spectrum. The instrument response function has been deconvoluted by fitting each decay profile to a bi- or triexponential function to have a χ2 value in between 1 and 1.2 using decay analysis software (DAS 6). The impulse response function, I(λ, t), has been calculated using those best-fit decay profiles. To construct a TRES, a set of H(λ) values was calculated using eq 3 H (λ ) =

F (λ ) ∑ ai(λ) τi(λ)

(3)

where F(λ) is the steady-state intensity, ai(λ) is the preexponential factor, and τi(λ) is the decay time at that wavelength with Σai(λ) = 1. The TRES at different times were constructed from the appropriately normalized intensity decay functions, I′(λ, t), for various wavelengths and at various times using eq 4. ⎡ t ⎤ I ′(λ , t ) = H(λ) × I(λ , t ) = H(λ) ∑ ai(λ) exp⎢ − ⎥ ⎣ τi(λ) ⎦ (4)

The peak frequency ν(t) of each TRES at different times was obtained after fitting the TRES to a log-normal function.54,55 To obtain the solvation time, the decay of C(t) with time was fitted by the following biexponential function C(t ) = a1se−t / τ1s + a 2se−t / τ2s

(5)

where τ1s and τ2s are the solvent relaxation times and a1s and a2s are the amplitudes. The average solvation time ⟨τs⟩ for a biexponential decay has been calculated by using eq 6:

⟨τs⟩ = a1sτ1s + a 2sτ1s C

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C-153 in the micellar environment of five different gemini surfactants with varying spacer chain length at 10 mM have been represented in Figure 1. The chosen concentration (10

The same TCSPC setup was used for the measurement of timeresolved fluorescence anisotropy (r(t)), and the following equation was used for the r(t) calculation: r (t ) =

I (t ) − GI⊥(t ) I (t ) + 2GI⊥(t )

(7)

I∥(t) and I⊥(t) are fluorescence decays polarized parallel and perpendicular to the polarization of the excitation light, respectively, and G represents the correction factor for the detector sensitivity to the polarization detection of emission. The value of G for our instrument is ∼0.6. The anisotropy decay of C-153 is single-exponential in water; however, the decays are biexponential in nature in all micellar media. The biexponential anisotropy decay function can be represented as eq 8 r(t ) = r0[a1r e−t / τ1r + a 2r e−t / τ2r]

(8) Figure 1. Absorption and fluorescence spectra of C-153 in pure water and in aqueous solutions of 12-s-12, 2Br−. λex = 375 nm.

where ro is the limiting anisotropy representing the inherent depolarization of the probe molecule, τ1r and τ2r are the fast and slow rotational relaxation components of the probe molecule in micellar media, respectively, and a1r and a2r are the relative amplitudes of two components, respectively. Equation 9 has been used for the calculation of average rotational relaxation times in micellar media of gemini surfactants ⟨τr⟩ = a1rτ1r + a 2rτ2r

mM) of surfactants is well above the respective cmc values which have been estimated by the conductometric method57 at 298.15 K (Supporting Information, Figure S1 and Note 1). The cmc values obtained are in good agreement with the values available in the literature (Table 1).

(9)

where τ1r and τ2r are the two rotational relaxation times of C153 in micelles and a1r and a2r are the relative amplitudes, respectively. The cmc values of all of the gemini surfactants were determined by conductivity measurements using a directreading Eutech Instruments combined pH and conductometer, model PC 510. The details of the conductivity measurements are available elsewhere.42 The cmc values have been determined from the break points in the plots of specific conductivity (κ) versus the concentration of solutions of gemini surfactants following Williams’s method.56 The degree of counterion dissociation (α) has been determined by taking the ratio of the postmicellar slope to the premicellar slope of the plot of specific conductivity (κ) versus the concentration of solutions of surfactants.57 The micellar aggregation numbers (Nagg) of geminis in an aqueous medium have been estimated by the steady-state fluorescence quenching of pyrene by cetylpyridinium chloride as the quencher reported in the literature.58−63 The concentrations of pyrene and surfactant were kept constant at 3 μM and 10 mM, respectively. The concentration of cetylpyridinium chloride as the quencher was varied from 0 to 0.09 mM, ensuring the full solubilization of the pyrene in the micelles and confirming a Poisson distribution for quencher.58 The dynamic light scattering (DLS) measurements for the estimation of the hydrodynamic radii of the aggregates of gemini surfactants were carried out in Zeta Sizer, model Nano ZS (ZEN 3600, Malvern Instruments, UK). Details of method of DLS measurements are available in the literature.57 The 1H NMR spectra were recorded with a Bruker Avance instrument, and the FT-IR spectra were recorded with an ABB Boman MB 300 instrument. All spectroscopic, DLS, and conductivity measurements were carried out at 298.15 ± 1 K.

Table 1. Absorption (λmaxabs) and Steady-State Fluorescence Peak Maxima (λmaxfl) of C-153 in Pure Solvents and the Micelles of Gemini Surfactants, cmc of Surfactants, and Aggregation Number of Micelles (N) systems

λmaxabs (nm)

λmaxfl (nm)a

water methanol cyclohexane 12-3-12 12-4-12 12-6-12 12-8-12 12-12-12

433 424 394 438 438 438 438 438

552 538 460 542 542 542 542 542

cmc (mM)b

N

(0.96c) (1.17d) (1.09c) (0.83e) (0.46c)

74f 65 52f 48 40f

0.93 1.17 1.03 0.72 0.41

λex = 375 nm. bcmc values are calculated by the conductivity method, and values in parentheses are reported in the literature. cRef 40. dRef 48. eRef 44. fAggregation numbers are taken from ref 40. a

The absorption and fluorescence spectra of C-153 have also been recorded in three pure solvents (cyclohexane, methanol, and water; only spectra in water are shown in Figure 1 for clarity). The peak maxima of these spectra along with the same in micellar solutions of each of five gemini surfactants at 10 mM concentration are also tabulated in Table 1. The absorption peak maximum of C-153 in pure water is found at 433 nm, which is well corroborated with the literature value,64 but in the presence of 10 mM of each of five gemini surfactants it is redshifted by 5 nm (438 nm). The red shift in the absorption maximum of C-153 in the presence of gemini surfactants indicates the destabilization of the ground state of C-153.53 Results also indicate that C-153 feels different environments in water and in the presence of gemini surfactants. The same absorption peak maximum value of C-153 in all micellar media was also observed in the case of a structurally similar compound, coumarin-480 (C-480).30,48 The possible reason could be the structurally constrained nature of C-153.65 Like

3. RESULTS AND DISCUSSION 3.1. UV−Visible Absorption and Steady-State Fluorescence Spectra. The absorption and fluorescence spectra of D

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Table 2. Excited Singlet State Lifetime (τ)a, Pre-exponential Factors (a), Fluorescence Quantum Yield (ϕf), Radiative (kr) and Nonradiative (knr) Rate Constants of C-153 in Various Homogeneous and Micellar Media

a

system

ϕf

a1b

τ1 (ns)

water methanol cyclohexane 12-3-12 12-4-12 12-6-12 12-8-12 12-12-12

0.02 0.57 0.81 0.72 0.68 0.55 0.46 0.37

1.00 1.00 1.00 0.30 0.29 0.34 0.34 0.32

1.73 4.03 4.68 0.32 0.39 0.38 0.43 0.39

a2b

0.70 0.71 0.66 0.66 0.68

τ2 (ns)

⟨τ⟩ (ns)

χ2

kr × 10−8 (s−1)

knr × 10−8 (s−1)

2.64 2.63 2.44 2.42 2.46

1.11 1.10 1.05 1.01 1.01 1.11 1.10 1.01

0.12 1.41 1.73 2.73 2.59 2.25 1.90 1.50

5.66 1.07 0.41 1.06 1.21 1.85 2.23 2.57

3.63 3.55 3.50 3.44 3.43

λex = 375 nm; λem = 475 nm. bAll pre-exponential factors (a) are normalized.

nm in the micelles of 12-3-12, 2Br− and the best fit and residuals are shown by representative Figure S3. All lifetime data along with pre-exponential factors and χ2 values are tabulated in Table 2. The slow component is the major component in all micellar media. On comparison of lifetime values in micelles to that in pure solvents, it can be seen that the lifetimes of the major components in all micellar media are close to that in methanol. These results are also in accordance with the fact that C-153 molecules are residing at the Stern layer of micelles. 3.3. Microviscosity. The above-mentioned fluorescence lifetime data have been used to calculate the radiative (kr) and nonradiative (knr) rate constants using the following two equations:52,70

absorption spectra, the fluorescence peak maximum of C-153 in a 10 mM solution of each of five different gemini surfactants also appears at the same wavelength (542 nm). C-153 has a fluorescence peak maximum at 552 nm in pure water, which is in good agreement with the literature value.64 3.2. Micropolarity. The peak maximum of C-153 in pure water is red-shifted by 10 nm as compared to micelles of gemini surfactants. This result depicts that C-153 is facing a less polar environment in micelles as compared to pure water because the fluorescence peak maxima of C-153 appear at 460 and 538 nm in cyclohexane and methanol, respectively. By comparing the fluorescence peak maxima of C-153 in micellar media to that in pure solvents, it can be concluded that the polarity of the environment around the probe in micellar media is close to that of methanol. Therefore, C-153 molecules reside neither in the core of the micelles (λmaxfl in cyclohexane is 460 nm) nor in the bulk aqueous phase (λmaxfl in water is 552 nm) but at the Stern layer of micelles.1,48 The same values of absorption and fluorescence peak maxima in all five gemini micellar media indicate that C-153 molecules are mainly solubilized at the Stern layer and feel more or less the same polarity in the micellar phase.66 This result is in contrast to Zana et al.’s46 findings on the micropolarity of 12-s-12, 2Br− micelles as measured by the pyrene polarity ratio I1/I3, which shows that the micropolarity goes through a maximum at s ≈ 5. It implies that the location of C-153 is different from that of pyrene. The results of the microenvironment around C-153 in the micelles have been supported by calculating the micopolarity expressed on an equivalent scale of ET(30), an empirical polarity parameter.67,68 The process of estimating the micropolarity is to compare the fluorescence behavior of the probe molecule in micellar media to that in various compositions of a dioxane− water mixture.69 The fluorescence energies of C-153 at the peak maximum, εmaxfl, in different compositions of the dioxane− water mixture have been calculated by correcting εmaxfl for the λ2 factor. These εmaxfl values are then plotted against ET(30) (Figure S2). The εmaxfl value of C-153 in a 10 mM solution of each of the studied gemini surfactants is found to be same (18 248 cm−1), and the value of ET(30) obtained from Figure S2 is 55.1 kcal mol−1. This ET(30) value is close to that of methanol (55.4 kcal mol−1). This result further supports the fact that C-153 molecules reside at the Stern layer of micelles of all five gemini surfactants.1,30,48 To further establish the fact that the probe molecules are located at the Stern layer, the excited singlet state lifetimes of C-153 have been estimated in micelles of 10 mM concentration of each of five gemini surfactants and in three pure solvents. Decays are monoexponential in pure solvents and biexponential in all micellar media. The fluorescence decay of C-153 at 445

kr = k nr =

ϕf τ

(10)

1 − kr τ

(11)

To calculate kr and knr in micellar media, the average lifetime expressed by eq 12 has been used.70 τ = a1τ1 + a 2τ2

(12)

Rate constant values in pure solvents and in micellar media are tabulated in Table 2. It can be seen from the data that kr decreases whereas knr increases with increasing spacer chain length of gemini surfactants. This result shows that the free rotational motions of C-153 in the micelles become progressively less restricted with increasing spacer chain length. It indicates that the microviscosity of the environment around C-153 decreases with increasing spacer chain length. This result is in agreement with that observed by Zana et al.46,47 in their study on the microviscosity of 12-s-12, 2Br− micelles, as determined using fluorescent probe dipyrenylpropane. They have observed that microviscosity decreases rather rapidly at s > 4. To further support the change in the microviscosity of micelles with increasing spacer chain length, 1,6-diphenyl-1,3,5hexatriene (DPH) has been used as a viscosity probe molecule. It is to be noted here that we do not get any proper trend while using C-153 as a probe molecule to determine microviscosity using our earlier method.48 To calculate the absolute values of microviscosities of micelles, ηm using DPH, the following Debye−Stokes−Einstein relation has been used (eq 1371) ηm = E

kTτR υh

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Table 3. Fluorescence Anisotropy (r)a, Excited Singlet State Lifetime (τf),b Rotational Correlation Time (τR) of DPH,c Microviscosities (ηm) of Micelles, and Binding Constant (K) and Standard Gibbs Free Energy of Binding (ΔG°) of C-153 at 298.15 K

a

system

r

τf (ns)

χ2

τR (ns)

ηm (mPa s)

K × 10−5 (M−1)

ΔG° (kJ mol−1)

12-3-12 12-4-12 12-6-12 12-8-12 12-12-12

0.094 0.088 0.075 0.068 0.056

5.38 5.41 5.21 5.50 5.82

1.01 1.10 1.10 1.01 1.02

1.88 1.74 1.36 1.27 1.07

24.7 22.9 17.9 16.7 14.1

3.44 3.22 2.10 1.32 1.06

−31.59 −31.43 −30.37 −29.22 −28.63

Anisotropy measured at 430 nm. λex = 375 nm. bλex = 375 nm; λem = 430 nm. c[DPH] = 5 μM. Solutions of DPH were prepared in tetrahydrofuran.

spacer chain (12-3-12, 2Br−) makes a more compact environment around the probe molecule than does a gemini surfactant with a comparatively longer spacer (12-12-12, 2Br−) because of the higher aggregation number of the former compared to that of the latter. There are reports on the decrease in binding constant for the conventional surfactants with decreasing aggregation numbers.53,75 The standard Gibbs free energy of binding (ΔG°) of C-153 with gemini surfactants has been calculated using the relation ΔG° = −RT ln K, and the values obtained are also complied in Table 3. The values of ΔG° are negative for all of the studied systems. The negative values indicate that the binding of C-153 with the micelles of a gemini surfactant is thermodynamically feasible. The increase in ΔG° values with increasing spacer chain length further implies that the binding interaction becomes progressively less feasible with increasing spacer chain length. 3.5. Solvation Dynamics. To study the solvation dynamics, the fluorescence decays of C-153 in a 10 mM solution of each of the five gemini surfactants have been monitored at different emission wavelengths. The fluorescence decays of C-153 in the presence of gemini surfactants are found to be emission-wavelength-dependent. Figure 2 shows the

where k is the Boltzmann constant, T is temperature in Kelvin, τR is the rotational correlation time of DPH, and υh is the hydrodynamic volume of DPH. The υh value is taken to be 313 Å3.72 The rotational correlation time, τR, is obtained from Perrin’s equation (eq 1470) τ τR = ro f −1 (14) r where τf and r are the fluorescence lifetime and steady-state fluorescence anisotropy of DPH, respectively, and ro is the steady-state fluorescence anisotropy of DPH in a highly viscous solvent and is taken to be 0.362.73 The values of τf, r, τR, and ηm are presented in Table 3. It is evident from the data in Table 3 that the microviscosity decreases with increasing spacer chain length. Even if this result does not give the exact value of the microviscosity of the environment around C-153, it still fulfils our purpose as we could show the change in the microviscosity of 12-s-12, 2Br− micelles with changes in the length of the spacer chain. 3.4. Binding of C-153 with Micelles. To get the information about the effect of the spacer chain length on the binding interaction of probe molecule C-153 with the micelles, the binding constant (K) and the standard Gibbs free energy of binding (ΔGo) have been estimated at 298.15 K. The method given by Almgren et al. has been used in eq 15 for the calculation of K.74 Ii − Io = 1 + (K[M])−1 IC − Io

(15)

where Io, IC, and Ii are the fluorescence intensity of C-153 in the absence of gemini surfactants, at an intermediate concentration, and at saturation, respectively. The micellar concentration [M] is given by [M] =

S − cmc N

Figure 2. Time-resolved fluorescence decays of C-153 in 12-3-12, 2Br− micelles at 655 nm (pink), 520 nm (blue), and 460 nm (red) and the instrument response function (black).

(16)

where S is the gemini surfactant concentration and N is the aggregation number of micelles. The values of cmc and N are given in Table 1. N values of 12-3-12, 12-6-12, and 12-12-12 are taken from the literature,40 and N values of 12-4-12 and 12-812 have been calculated using the method described in section 2.2.58−63 Figure S4 shows the plot of (Ii − Io)/(IC − Io) against [M]−1 for C-153 in the presence of 12-3-12, 2Br− as a representative. Similar plots have also been obtained for other gemini surfactants. The K values have been calculated from the slope and are given in Table 3. The binding constant value is decreasing with the increasing spacer chain length of gemini surfactants. This result reveals that the binding interaction of C153 is decreasing with the increasing spacer chain length of the investigated gemini surfactants. A gemini surfactant with a short

fluorescence decays of C-153 in the presence of 12-3-12, 2Br− surfactant at different wavelengths as a representative. The decay is very fast toward the blue edge of the fluorescence spectrum of C-153 (Figure 2, 460 nm). At a short wavelength, the decay corresponds to the fluorescence from the unsolvated dipoles of C-153 created in the excited state without undergoing any relaxation process. Of course, in the present case, the initial fast decay has not completely arose from the unsolvated dipoles, and there are some contributions from the solvated dipoles as well because there are some limitations of our TCSPC setup. On the other hand, the decay becomes slower toward the red edge of the fluorescence spectrum of CF

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Table 4. Decay Characteristics of C(t) of C-153 in Different Micellar Systems

153 (Figure 2, 655 nm). At a long wavelength, there is a clear growth in the decay followed by a slow decay with a negative pre-exponential factor. It indicates that the dipoles created in the excited state undergo a solvent relaxation process followed by fluorescence emission and are therefore delayed by the relaxation time.49,70 In the case of 12-3-12, 2Br− the decay is biexponential at 460 nm with two components at 887 ps and 1.765 ns, and at 655 nm, C-153 shows a distinct rise at 960 ps followed by a time constant of 3.779 ns. Similar behaviors of fluorescence decays of C-153 have been observed in the presence of other gemini surfactants as well. If probe molecules are mostly present in the hydrocarbon core of the micelles, then they should not show any wavelength-dependent decay. Dynamics exclusively in the bulk water is too fast to be detected by our instrumental setup (time resolution ∼165 ps). Therefore, the observed time-dependent Stokes shift is mostly due to the probe molecules that are present at the Stern layer of the micelles.76 The dynamics of the solvent is studied by calculating the solvent correlation function (SCF), C(t), defined in eq 2 and explained by Fleming and Maroncelli.54 Peak frequencies ν(0), ν(t), and ν(∞) at time zero, t, and infinity, respectively, have been calculated by constructing time-resolved emission spectra (TRES). Figure 3 represents the TRES of C-153 in the micelles

a

system

Δυ (cm−1)a

a1s

τ1s (ps)

a2s

τ2s (ns)

⟨τs⟩ (ps)

12-3-12 12-4-12 12-6-12 12-8-12 12-12-12

976 842 846 720 1186

0.95 0.67 0.83 0.69 0.74

217 146 319 305 175

0.05 0.33 0.17 0.31 0.26

4.494 1.445 2.825 1.980 1.507

431 574 745 824 522

Δυ = ν(0) − ν(∞).

dynamics is bimodal in nature. It has been reported in the literature that these fast and slow solvation components are due to “free” and “bound” water molecules, respectively, those are in dynamic equilibrium with each other.23 As far as the present micellar systems are concerned, many factors such as water molecules, headgroups, spacer groups, and counterions could be responsible for the solvation of dipoles created in the excited state. However, a polar headgroup attached to a long hydrocarbon chain as well as a spacer group of surfactant contributes to a very slow solvation process on the nanosecond time scale.1,77 The hydrophobic spacer group of surfactant is expected not to take part in solvation. Therefore, water molecules and counterions are mostly responsible for the solvation dynamics. Because the hydrogen bond strength between two water molecules is much weaker than the bond strength between water molecules and polar headgroups of surfactant molecules,1,25,26 two kinds of water molecules take part in the solvation process. While water molecules bonded with polar headgroups of surfactant molecules are responsible for the slow component, the other type of water molecule contributes to fast solvation dynamics. In the present case, the fast component is the major component, and the time scale is 146−319 ps, which is slightly on the lower side as compared to that of micelles of conventional surfactants.1 The present time scale is almost 3 orders of magnitude slower than that in bulk water (310 fs) as reported by Vajda et al.24 with C-480 as a probe molecule. While there is no clear trend in solvation time for fast and slow components, an increase in the average solvation time ⟨τs⟩ with increasing spacer chain length of surfactant molecules from s = 3 to 8 in 12-s-12, 2Br− surfactants has been observed. Like Zana et al.,44 we have also noticed that the degree of counterion dissociation, α, increases with increasing spacer chain length (Figure 5). It is known that counterions contribute to slow solvation dynamics. That is why the average solvation time ⟨τs⟩ increases with the increasing spacer chain length of

Figure 3. Time-resolved emission spectra (TRES) of C-153 in 12-312, 2Br− micelles.

of the 12-3-12, 2Br− surfactant as representative. The decays of C(t) versus time for micelles of each of five gemini surfactants are shown in Figure 4. The decay parameters of C-153 in various micellar systems are tabulated in Table 4. The data in Table 4 shows that the solvation dynamics of C153 has two solvation components, fast and slow; i.e., the

Figure 4. Decays of the solvent correlation function, C(t), of C-153 in 12-s-12, 2Br− micelles.

Figure 5. Degree of counterion dissociation (α) versus spacer chain length, s, of 12-s-12, 2Br−. G

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not having any proper trend in solvation times for fast and slow components. Because of the conformational change at s ≤ 5−6, there could be some difference in the morphology of micelles of 12-4-12, 2Br− surfactant as compared to that of other surfactants. This could be the reason that there are some deviations in solvation parameters from the trend observed in the case of 12-3-12, 12-6-12, and 12-8-12. 3.6. Rotational Relaxation or Time-Resolved Fluorescence Anisotropy. Time-resolved fluorescence anisotropy served as a useful method for the study of microenvironment around the probe molecule in molecular assemblies.30,48,53,75 The time-dependent fluorescence anisotropy decay gives additional information about the rotational relaxation of the probe in organized molecular assemblies. The time-resolved fluorescence anisotropy, r(t) has been calculated using eq 7. The time-resolved fluorescence anisotropy decays of C-153 in different gemini surfactants are shown in Figure 6. All of the

surfactant molecules. It could be that in addition to the participation of counterions for solvation, counterions also indirectly affect the solvation process by interacting with water molecules through hydrogen bonding. Possibly with the increasing degree of dissociation of counterions, more and more water molecules get involved in the solvation of the counterions themselves, which results in a decrease in free water molecules available for the solvation of C-153 at the Stern layer of micelles. That is why solvation dynamics becomes slower with an increasing degree of counterion dissociation. Of course the effect of the hydrophobic spacer chain cannot be ruled out. An increase in the hydrophobicity of the spacer with increases in its chain length reduces the availability of water molecules for solvation. As a result, the solvation time increases. The decrease in the availability of water molecules for the solvation process is also supported by the results of the hydration of micelles of 12-s-12, 2Br− surfactants reported by Borse et al.45 They have found that the volume of water molecules per gram of surfactant molecules in the micellar phase decreases with increasing spacer chain length.45 This is the reason that in the case of the 12-3-12, 2Br− surfactant the fast component is the only component that contributes to solvation dynamics. For other geminis from s = 4 to s = 8, there is a tendency for the solvation time of the fast component to increase with a tendency for the relative amplitude to decrease. Consequently, there is a tendency for the relative amplitude of the slow component to increase. It is noteworthy that the microviscosity of the environment around the probe is not responsible for slowing the solvation dynamics for gemini surfactants 12-s-12, 2Br− with s = 3 to 8. This conclusion is based on our observation that the microviscosity decreases with increasing spacer chain length (Table 3). Had the change in microviscosity been a reason for changing the solvation time, there would have been a decrease in solvation time with increasing spacer chain length. It could be that only in the case of 12-12-12 does the viscosity effect dominate the effect of counterions. It is to be noted that the microviscosity is lowest in the case of the 12-12-12, 2Br− surfactant among the five studied gemini surfactants (Table 3). Low values of solvation times for both fast and slow components have been obtained. As a result of that a low average solvation time ⟨τs⟩ has been obtained for 12-12-12, 2Br− as compared to that for 12-8-12, 2Br−. It has been reported for the present series of gemini surfactants 12-s-12, 2Br− that in aqueous solution two covalently linked, positively charged headgroups try to maintain a critical distance between the headgroups to reduce the Coulombic force of repulsion.45 The equilibrium distance between the charged headgroups of gemini surfactants increases with spacer chain length.45 It is expected that with increasing average equilibrium distance between the charged headgroups within micelles the hydration of headgroups would be lowered. Consequently, the number of water molecules responsible for the slow solvation component should be reduced. This could be the reason that there is a tendency toward lowering the slow solvation time with increasing spacer chain length (Table 4). It is noteworthy that at spacer s ≤ 5−6 methylene units there is a conformational change in the surfactant molecule of type m-s-m and that the spacer remains mainly in an extended conformation.39 As a result of that, the cmc increases with s for s ≤ 5−6 (Table 1).40 However, for s > 5−6, the spacer chain tries to form a loop extended toward the hydrophobic core of the micelles. Therefore, a decrease in the cmc is observed for s > 5−6 (Table 1).39 All of these factors could be the reasons for

Figure 6. Fluorescence anisotropy decays of C-153 in pure water and in micelles of 12-s-12, 2Br− surfactants.

decays are found to be biexponential in nature in micellar media and single-exponential in water. The biexponential anisotropy decay function is represented as eq 8. The average rotational relaxation times ⟨τr⟩ of C-153 in all the gemini surfactants are calculated by using eq 9 and tabulated in Table 5. The rotational relaxation time for C-153 in pure water is found to be 148 ps, which is higher than the reported value (100 ps) due to the high resolution time of our instrument.76 The τr values of C-153 in studied micelles of gemini surfactants are many-fold longer than that in pure water, which indicates that the random motion of the probe molecule is restricted in the micellar environment. Data in Table 5 show that the average rotational relaxation time decreases with the increasing spacer chain length of the gemini surfactants. The aggregation number of micelles at 10 mM for each of gemini surfactant is found to be decreased (Table 1) with the spacer chain length. The hydrodynamic diameters of 12-s-12, 2Br− gemini surfactants with s = 3, 4, 6, 8, 12 are found to be 1.24 ± 0.03, 1.23 ± 0.03, 1.01 ± 0.03, 1.00 ± 0.03, and 0.86 ± 0.03 nm, respectively (Figure S5 represents hydrodynamic distributions). To the best of our knowledge there is no report on the hydrodynamic diameter of micelles in this series of gemini surfactants. However, there is a report on the hydrodynamic distributions of 10 mM gemini surfactant 14-4-14, 2Br− as studied by Wang et al.78 They have noticed two hydrodynamic distributions for this gemini surfactant: one centered at 0.82 nm and the other centered at 70 nm. According to them, the former is for micelles and the latter is for vesicles. It is to be noted that we did not find any distribution which corresponds H

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The Journal of Physical Chemistry B Table 5. Rotational Relaxation Parameters for C-153 in Micelles of 12-s-12, 2Br− Gemini Surfactants system

r0

water 12-3-12 12-4-12 12-6-12 12-8-12 12-12-12

0.26 0.35 0.34 0.32 0.29 0.28

α1r 1.00 0.81 0.78 0.54 0.50 0.49

± ± ± ± ± ±

τ1r (ps) 0.02 0.02 0.02 0.02 0.02 0.02

148 659 582 409 349 283

± ± ± ± ± ±

a2r

47 72 63 60 68 44

0.19 0.22 0.46 0.50 0.51

± ± ± ± ±

0.01 0.02 0.01 0.01 0.02

τ2r (ps)

⟨τr⟩ (ps)

χ2

± ± ± ± ±

148 1623 1601 1282 1269 1228

1.05 1.05 1.07 1.08 1.07 0.99

5735 5218 2307 2189 2137

455 426 165 146 140

Table 6. Parameters Obtained from the Anisotropy Decays of C-153 in Different 12-s-12, 2Br− Micelles system

hydrodynamic radius (nm)a

12-3-12 12-4-12 12-6-12 12-8-12 12-12-12

0.620 0.615 0.505 0.500 0.430

τe (ps)b 745 655 497 415 326

± ± ± ± ±

72 63 60 68 44

τm (ns)c 216 218 116 114 073

± ± ± ± ±

0.455 0.426 0.165 0.146 0.140

τD (ps)d

DL × 107 (cm2s−1)e

Dw × 10−8 (s−1)f

θo (deg)g

|S|h

± ± ± ± ±

1.63 1.77 2.71 2.80 2.10

3.70 3.93 2.82 3.03 3.85

55.7 53.8 39.7 37.6 37.6

0.44 0.47 0.68 0.71 0.71

5890 5350 2350 2230 2200

455 426 165 146 140

a

Hydrodynamic radius. bEffective relaxation time (τe). cTime for overall rotational motion of the micelle (τm). dLateral diffusion time (τD). eLateral diffusion coefficient (DL). fWobbling diffusion coefficient (Dw). gCone angle (θo). hOrder parameter (|S|).

discussed in Supporting Information, Note 3 are presented in Table 6. The τD values calculated using eq 17 are also given in Table 6. It can be seen from the data given in Tables 5 and 6 that the τ2r value in a given micellar system is very close to the τD value and very different from τm value in the same micelles. It implies that the slow rotational relaxation in a micellar medium is mainly due to the lateral diffusion of the fluorophore. The main motion which causes the anisotropy to decay to zero in these micellar systems is lateral diffusion. Lateral diffusion of the fluorophore in micelles is a much faster process than rotational motion of the micelle as a whole. τm values are found to be 30 to 50 times longer than τD. Data also show that the lateral diffusion time decreases rather rapidly at s > 4. The lateral diffusion process becomes progressively faster with spacer chain length, i.e., with decreasing microviscosity of the environment around the fluorophore. It is supported by the increasing tendency of the values of the lateral diffusion coefficient (DL) with the spacer chain length (Table 6) calculated using eq 1880

to vesicles for the present gemini surfactants. The fact that the values of the hydrodynamic diameters of the present gemini micelles corroborate well with the reported hydrodynamic diameter of similar cationic gemini surfactant 14-4-14, 2Br− except their tail length supports the authenticity of hydrodynamic diameter values estimated by us. The DLS results depict that with increasing spacer chain length micelles become progressive less compact. As a result of that, the microviscosity of the environment around the probe molecules decreases, which is supported by the above-mentioned microviscosity values given in Table 3. The data in Table 3 also show that the binding constant of C-153 decreases with increasing spacer chain length. All of these results support the fact that C-153 molecules feel a less compact, less viscous environment with the spacer chain length, and as a result of that, the average rotational relaxation time ⟨τr⟩ decreases. The trends in microviscosity (Table 3) and ⟨τr⟩ are very closely correlated. There are significant decreases in microviscosity and ⟨τr⟩ for s > 4. Zana et al.39 have also reported the rapid decrease in the microviscosity of 12-s-12, 2Br− at s > 4. Maroncelli and coworker33 have noticed that the rotational relaxation times correlate well with viscosity. The decrease in the rotational relaxation time of the probe molecule with the decreasing aggregation number of micelles has been reported in the literature.53,75 The fluorescence anisotropy decays of C-153 in all micellar media are biexponential in nature. A biexponential anisotropy decay is due to various kinds of rotational motions but is not due to different locations of the probe.79 The models often used to describe the biexponential behavior of an anisotropic decay as a result of various kinds of rotational motions are the two-step and wobbling-in-a-cone models (Supporting Information, Note 2). According to the two-step model, the slow rotational relaxation time (τ2r) is related to the relaxation time corresponding to the lateral diffusion of the fluorophore (τD) and the relaxation time corresponding to the rotational motion of the micelle as a whole (τm) as follows in eq 17: 1 1 1 = + τ2r τD τm

DL =

rm 2 4τD

(18)

where rm is the hydrodynamic radius of the micelle. It is pertinent to note that like ⟨τr⟩ values, τD values also have a very good correlation with microviscosities. As expected, the rotational motion of the micelles as a whole is found to be faster with a progressive decrease in the size of the micelles. The fast component of rotational relaxation is modeled as restricted rotational diffusion. The effective relaxation time (τe) corresponding to the restricted rotational diffusion is related to the fast relaxation time (τ1r) and slow relaxation time (τ2r) as given in eq 19: 1 1 1 = + τ1r τe τ2r

(19)

The relaxation of the local structure of the micelles can be measured by the effective relaxation time, τe.79 τe values calculated using eq 19 decrease with the spacer chain length. We have discussed that the microviscosity of micelles decreases with the spacer chain length. Thus, the relaxation of local structure becomes faster with decreasing microviscosity of the micelles. Using the wobbling-in-a-cone model (Supporting

(17)

The values of τm at 298.15 K calculated using estimated hydrodynamic radii mentioned above following the method I

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phobic spacer chain also has an effect on increase in solvation time with increasing chain length. The availability of water molecules for solvation is reduced with increasing hydrophobicity of the spacer as a result of increasing its chain length. In the case of micelles of the 12-12-12, 2Br− surfactant, however, low solvation time is due to the fact that the viscosity effect dominates the effect of counterions. The change in the conformation of surfactant at s ≤ 5−6 and the formation of the spacer loop extended toward the hydrophobic core of micelles have some effects on solvation dynamics. The random motion of C-153 is restricted in the micellar environment. The average rotational relaxation time for C-153 decreases with increasing spacer chain length as the micelles become progressively less compact and less viscous. There is a very good correlation between the average rotational relaxation time and the microviscosity of micelles with a rapid decrease in these two parameters at s > 4. Anisotropy decays of C-153 are biexponential in nature because of various kinds of rotational motions. The slow relaxation in micellar media is due to the lateral diffusion of the fluorophore. Lateral diffusion of the fluorophore in the micelles is much faster than the rotational motion of the micelle as a whole. The lateral diffusion process also correlates well with the microviscosity of micelles. The rotational motion of the micelle as a whole becomes faster with decreasing size of the micelles. The effective relaxation time decreases with the spacer chain length. The relaxation of local structure becomes faster with decreasing microviscosity of the micelles. The probe molecules seem to be oriented differently in the micelles of 12-3-12 and 12-4-12 as compared to micelles of the other three surfactants. This could be because of the very compact nature of micelles of the former two surfactants.

Information, Note 2), the values of the wobbling diffusion coefficient (Dw), cone angle (θo), and order parameter (|S|) have been calculated and listed in Table 6. The order parameter |S| is a measure of spatial restriction. In the present study the value of |S| ranges from 0.44 to 0.71. The high values of |S| support the fact that the probe molecules are located at the Stern layer and experience restricted motions. However, the values of |S| and θo do not correlate well with the microviscosity of micelles. It has been observed that for 12-3-12 and 12-4-12 micelles, θo values are higher and |S| values are lower as compared to those of micelles of the other three surfactants. To overcome this discrepancy, another model, the spinning-inequatorial band model,80,81 has been used, for which |S| should not exceed 0.5, which is the case for 12-3-12 and 12-4-12 micelles (Table 6). According to this model the alignment of the rodlike probe molecule is different from that in the wobbling-in-a-cone model. The probe molecule should be aligned in such a way that the emission moment is perpendicular to the long axis and |S| is related to θo as given below: ⎤2 ⎡1 S2 = ⎢ (1 − cos2 θo)⎥ ⎣2 ⎦

(20)

The θo values calculated using eq 20 are found to be 69.7 and 75.8° for 12-3-12 and 12-4-12 micelles, respectively, and are different from that calculated using the wobbling-in-a-cone model. While the values of the wobbling diffusion coefficient, Dw, are in reasonably good agreement with the microviscosities of micelles of 12-6-12, 12-8-12 and 12-12-12, this is not the case for 12-3-12 and 12-4-12 micelles. This could also be because of the fact that probe molecules are oriented differently in the micelles of 12-3-12 and 12-4-12 as compared to the other three micelles. More compact micelles of 12-3-12 and 12-4-12 could be the reason that probe molecules are oriented differently in these micelles than in micelles of the other three surfactants.

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ASSOCIATED CONTENT

S Supporting Information *

FT-IR and 1H NMR data of the synthesized gemini surfactants. A brief note on the determination of the cmc of gemini surfactants by the conductivity method, with a figure showing the plots. Plots showing the variation in the fluorescence energy at the peak maximum of C-153 in a dioxane−water mixture with ET(30) of the dioxane−water mixture and the timeresolved fluorescence decay of C-153 at 445 nm in 12-3-12 micelles along with the best fit and residuals. A size distribution graph for the micelles of gemini surfactants and brief notes on the wobbling-in-a-cone model and methods used to compute the τm. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.jpcb.5b03081.

4. CONCLUSIONS C-153 molecules reside at the Stern layer of 12-s-12, 2Br− micelles and feel more or less the same polarity. The micropolarity of the environment around C-153 in micelles expressed on an equivalent scale of ET(30) is 55.1 kcal mol−1, which is close to that of methanol. The microviscosity of micelles decreases with the increasing spacer chain length of gemini surfactants, 12-s-12, 2Br−. Binding interactions of C-153 decrease with the increasing spacer chain length of surfactants. The negative values of ΔG° indicate that the binding of C-153 with the micelles of gemini surfactants is thermodynamically feasible. Solvation dynamics at the Stern layer of gemini micelles is bimodal in nature. Water molecules and counterions are mostly responsible for solvation dynamics. Water molecules bonded with the polar headgroups of surfactant molecules are responsible for the slow component. Other water molecules contribute to the fast component. The fast component is the major component, and the time scale of solvaltion is ∼3 orders of magnitude slower than that in bulk water. Counter ions are also responsible for the slow component. The average solvation time increases because of the increased degree of counterion dissociation for gemini 12-s-12, 2Br− with s = 3 to 8. A portion of water molecules get involved in the solvation of counterions themselves. As a result, there is a decrease in free water molecules available for the solvation of C-153. The hydro-

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AUTHOR INFORMATION

Corresponding Author

*Tel: +91 1596 515731. Fax: +91 1596 244183. E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K.S. acknowledges the University Grants Commission (UGC) special assistance program (F.540/14/DRS/2007 (SAP-I)), the Department of Science and Technology (DST) FIST program, the Government of India, and also the Aditya Birla Groups for financial support. Sonu acknowledges UGC for financial support under a senior research fellowship, and J

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(20) Sengupta, A.; Khade, R. V.; Hazra, P. How Does the Urea Dynamics Differ from Water Dynamics inside the Reverse Micelle? J. Phys. Chem. A 2011, 115, 10398−10407. (21) Tamoto, Y.; Segawa, H.; Shirota, H. Solvation Dynamics in Aqueous Anionic and Cationic Micelle Solutions: Sodium Alkyl Sulfate and Alkyltrimethylammonium Bromide. Langmuir 2005, 21, 3757− 3764. (22) Nandi, N.; Bhattacharyya, K.; Bagchi, B. Dielectric Relaxation and Solvation Dynamics of Water in Complex Chemical and Biological Systems. Chem. Rev. 2000, 100, 2013−2046. (23) Nandi, N.; Bagchi, B. Dielectric Relaxation of Biological Water. J. Phys. Chem. B 1997, 101, 10954−10961. (24) Vajda, S.; Jimenez, R.; Rosenthal, S.; Fidler, V.; Fleming, G. R.; Castner, E. W. J. Femtosecond to Nanosecond Solvation Dynamics in Pure Water and Inside the γ-Cyclodextrin Cavity. J. Chem. Soc., Faraday Trans. 1995, 91, 867−873. (25) Balasubramanian, S.; Bagchi, B. Slow Solvation Dynamics near an Aqueous Micellar Surface. J. Phys. Chem. B 2001, 105, 12529− 12533. (26) Balasubramanian, S.; Pal, S.; Bagchi, B. Hydrogen-bond Dynamics near a Micellar Surface: Origin of the Universal Slow Relaxation at Complex Aqueous Interfaces. Phys. Rev. Lett. 2002, 89, 115505−1−4. (27) Pal, S.; Balasubramanian, S.; Bagchi, B. Temperature Dependence of Water Dynamics at an Aqueous Micellar Surface: Atomistic Molecular Dynamics Simulation Studies of a Complex System. J. Chem. Phys. 2002, 117, 2852−2859. (28) Pal, S.; Balasubramanian, S.; Bagchi, B. Identity, Energy, and Environment of Interfacial Water Molecules in a Micellar Solution. J. Phys. Chem. B 2003, 107, 5194−5202. (29) Chakrabarty, D.; Hazra, P.; Chakraborty, A.; Sarkar, N. Solvation Dynamics of Coumarin 480 in Bile Salt-Cetyltrimethylammonium bromide (CTAB) and Bile Salt-Tween 80 Mixed Micelles. J. Phys. Chem. B 2003, 107, 13643−13648. (30) Chakrabarty, D.; Chakraborty, A.; Seth, D.; Hazra, P.; Sarkar, N. Effect of Alkyl Chain Length and Size of The Headgroups of the Surfactant on Solvent and Rotational Relaxation of Coumarin 480 in Micelles and Mixed Micelles. J. Chem. Phys. 2005, 122, 184516−1−7. (31) Pramanik, R.; Sarkar, S.; Ghatak, C.; Rao, V. G.; Mandal, M.; Sarkar, N. Effects of 1-Butyl-3-methyl Imidazolium Tetrafluoroborate Ionic Liquid on Triton X-100 Aqueous Micelles: Solvent and Rotational Relaxation Studies. J. Phys. Chem. B 2011, 115, 6957−6963. (32) Horng, M. L.; Gardecki, J. A.; Maroncelli, M. Rotational Dynamics of Coumarin 153: Time-Dependent Friction, Dielectric Friction, and Other Nonhydrodynamic Effects. J. Phys. Chem. A 1997, 101, 1030−1047. (33) Kumbhakar, M.; Goel, T.; Mukherjee, T.; Pal, H. Role of Micellar Size and Hydration on Solvation Dynamics: A Temperature Dependent Study in Triton-X-100 and Brij-35 Micelles. J. Phys. Chem. B 2004, 108, 19246−19254. (34) Sengupta, A.; Hazra, P. Solvation Dynamics of Coumarin 153 in SDS Dispersed Single Walled Carbon Nanotubes (SWNTs). Chem. Phys. Lett. 2010, 501, 33−38. (35) Han, Y.; Wang, Y. Aggregation Behavior of Gemini Surfactants and Their Interaction With Macromolecules in Aqueous Solution. Phys. Chem. Chem. Phys. 2011, 13, 1939−1956. (36) Menger, F. M.; Keiper, J. S. Gemini Surfactants. Angew. Chem., Int. Ed. 2000, 39, 1906−1920. (37) Zana, R. Critical Micellization Concentration of Surfactants in Aqueous Solution and Free Energy of Micellization. Langmuir 1996, 12, 1208−1211. (38) Wang, X.; Wang, J.; Wang, Y.; Yan, H.; Li, P.; Thomas, R. K. Effect of the Nature of the Spacer on the Aggregation Properties of Gemini Surfactants in an Aqueous Solution. Langmuir 2004, 20, 53− 56. (39) Zana, R. Dimeric (Gemini) Surfactants: Effect of the Spacer Group on the Association Behavior in Aqueous Solution. J. Colloid Interface Sci. 2002, 248, 203−220.

S.K. acknowledges UGC for a B.S.R fellowship. Thanks are also due to one of the reviewers for making valuable comments to give the manuscript a better shape.

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