Effect of Hydrophobicity of Tails and Hydrophilicity of Spacer Group of

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Effect of Hydrophobicity of Tails and Hydrophilicity of Spacer Group of Cationic Gemini Surfactants on Solvation Dynamics and Rotational Relaxation of Coumarin 480 in Aqueous Micelles Sunita Kumari,† Rishika Aggrawal,† Sonu,†,§ Sayantan Halder,† Ganapathisubramanian Sundar,*,‡ and Subit K. Saha*,† †

Department of Chemistry, Birla Institute of Technology & Science (BITS) Pilani, Pilani Campus, Pilani, Rajasthan 333031, India Department of Chemistry, Birla Institute of Technology & Science (BITS) Pilani, Hyderabad Campus, Hyderabad, Telangana 500078, India

‡

S Supporting Information *

ABSTRACT: Solvation dynamics and rotational relaxation of coumarin 480 in aqueous micelles of cationic gemini surfactants with diethyl ether (EE) spacer group (m−EE−m) and tails with varying tail lengths (m = 12, 14, and 16) have been studied. Studies have been carried out by measuring UV−visible absorption, steadystate fluorescence and fluorescence anisotropy, time-resolved fluorescence and fluorescence anisotropy, 1H NMR spectroscopy, and dynamic light scattering. Effects of hydrocarbon tail length and hydrophilicity of spacer group on solvation dynamics and rotational relaxation processes at inner side of the Stern layer of micelles have been studied. With increasing hydrophobicity of tails of surfactants, water molecules in the Stern layer become progressively more rigid, resulting in a decrease in the rate of solvation process with slow solvation as a major component. With increasing hydrophilicity of the spacer group of gemini surfactant, the extent of free water molecules is decreased, thereby making the duration of the solvation process longer. Solvation times in the micelles of gemini surfactants with hydrophilic spacer are almost 4 times longer compared to those in the micelles of their conventional counterpart. Rotational relaxation time increases with increasing tail length of surfactant as a result of increasing microviscosity of micelles with fast relaxation as a major component. With increasing hydrophilicity of the spacer group, the anisotropy decay becomes slower due to the formation of more compact micelles. Rotational relaxation in gemini micelles is also slower compared to that in their conventional counterpart. The anisotropy decay is found to be biexponential with lateral diffusion of the probe along the surface of the micelle as a slow component. Rotational motion of micelle as a whole is a very slow process, and the motion becomes further slower with increasing size of the micelle. The time constants for wobbling motion and lateral diffusion of the probe become longer with increasing microviscosity of micelles.

1. INTRODUCTION The dynamics of water molecules in the bulk is much different from that in biological systems.1−8 The study of dynamics in biological systems is important because it controls various processes in those systems.9 Because of this importance, a substantial amount of work on the dynamics of water has been carried out in various confined media, such as micelles,3,10−12 reverse micelles,13−15 mixed micelles,16−19 and cyclodextrin,4,20 those mimic the biological systems. Water molecules in these confined media behave as “free” and “bound” water molecules those are in dynamic exchange with each other.21,22 In most of the cases, the solvation dynamics has been studied by the timedependent fluorescence Stokes shift method.1−3,9,10,15,23,24 Vajda et al.4 noticed a very fast (315 fs) solvation process in the bulk water phase using coumarin 480 (C-480) as a probe with a single solvation time. However, the solvation dynamics is slowed by © 2017 American Chemical Society

manifolds in restricted environment as compared to that in the bulk phase.3,4 Bhattacharyya et al.3,25 first investigated the solvation dynamics of C-480 and 4-aminopthalimide (4-AP) in the micelles of various types of conventional surfactants, such as cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), and Triton X-100 (TX-100), and concluded that with different probe molecules solvation time remained the same in similar environment. Hazra et al.26 reported the solvation dynamics and rotational relaxation of coumarin 153 (C-153) in SDS-dispersed single-walled carbon nanotubes. They found that the rates of both solvation and rotational relaxation processes become lower in this system as compared to those in bulk water. Received: June 19, 2017 Accepted: August 25, 2017 Published: September 18, 2017 5898

DOI: 10.1021/acsomega.7b00818 ACS Omega 2017, 2, 5898−5910

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Scheme 1. Chemical Structures of Gemini Surfactants with Numbering of Spacer Protons and Coumarin 480 with Names in Short Form in Parenthesis

Shirota et al.27 studied the solvation dynamics of C-480 in the aqueous micelles of cationic and anionic surfactants and observed that the solvation dynamics is faster in the micelles of cationic surfactant as compared to that in the micelles of anionic surfactant due to weaker hydrogen bonding between headgroup of the former surfactant and water molecules. The effect of roomtemperature ionic liquid (RTIL) on the solvation dynamics as well as on the rotational relaxation of C-153 in the aqueous micelles of Triton X-100 was studied by Sarkar and co-workers.28 They found that the presence of ionic liquid increases the microfluidity of micelles that causes faster solvation and rotational relaxation processes. Sarkar et al.18 noticed that both rotational relaxation and solvation times increase with increasing hydrocarbon tail length of conventional surfactants. This happened because of increasing microviscosity of micelles with increasing tail length of surfactants as a result of formation of more closely packed micelles. Earlier, the direct correlation between rotational relaxation time and microviscosity of micelles was observed by

Maroncelli and co-workers.29 Studies have also been carried out by Sarkar et al.30 to see the effect of tail length of ionic liquids on the rotational motions of C-153 and rhodamine 6G in micelles. It has been observed that the rotational relaxation process becomes slower in the micelles of C16mimCl as compared to C12mimCl ionic liquid as the rotational motions are more restricted in the former system due to the longer alkyl chain length. Also, in another study, they found decrease in the rates of solvation and rotational relaxation processes in the neat micelles and microemulsions of each of ionic liquids [C2mim][C4SO4], [C2mim][C6SO4], and [C2mim][C8SO4].31 Samanta and coworkers32 reported the effect of alkyl tail length on the rotational dynamics of polar and nonpolar solutes in a series of N-alkyl-Nmethylmorpholinium ionic liquids. Studies have revealed the location of these probes in distinct environment of the ionic liquids from their contrasting rotational dynamics. In recent times, gemini surfactants have attracted attention because of their countless applications in nanotechnology, biotechnology, material science, supramolecular chemistry, and 5899


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pharmaceutics, such as drug delivery, gene delivery, and so forth.33−37 Gemini surfactants are more surface-active than their conventional counterparts.38 Aggregation properties of a gemini surfactant are dependent on the various parts of the surfactant, such as headgroup,39−41 spacer group,42−47 counterion,48 and alkyl chain length.49,50 Gemini surfactants are reported to have different types of spacer groups like hydrophilic, hydrophobic, rigid, and flexible.38,51,52 Saha et al. studied the effect of hydroxylgroup-substituted10 and polymethylene12 spacer groups on the solvation dynamics and rotational relaxation processes in aqueous micelles. They demonstrated that hydrogen bonding through hydroxyl groups on the spacer group has a significant effect on retardation of solvation process in aqueous micelles. Also, average solvation time increases with increasing polymethylene spacer chain length as a result of increasing degree of counterion dissociation. Looking into the fact of interesting aggregation behavior of gemini surfactants and the effects of chemical nature of its spacer groups as well as the effect of hydrocarbon tail length of conventional surfactants on solvation dynamics and rotational dynamics, the present study aims to see how these dynamics depend on the hydrophobicity of the tails of gemini surfactants using C-480 as a probe (Scheme 1). In the present study, we have selected a series of diethyl ether (EE) spacer group containing gemini surfactants with varying hydrocarbon tail lengths, C12 (Gemini-X), C14 (Gemini-Y), and C16 (Gemini-Z) (Scheme 1). We have also compared our present dynamics data with those in the micelles of gemini surfactant containing C12 tails and tetramethylene spacer group (Gemini-A) (Scheme 1) studied earlier.10 With these compounds, we could simultaneously demonstrate how the rates of solvation and rotational relaxation processes in aqueous micelles vary with increasing tail length of gemini surfactants, that is, increasing hydrophobicity of tails, and also how these rates change with increasing hydrophilicity of spacer group with similar tails. We have also shown how these rates in gemini micelles with hydrophobic spacer group as well as with hydrophilic spacer group differ from those in the micelles of their conventional counterparts, dodecyltrimethylammonium bromide (DTAB), tetradecyltrimethylammonium bromide (TTAB), and cetyltrimethylammonium bromide (CTAB) reported earlier.18

Figure 1. UV−visible absorption and steady-state fluorescence spectra of C-480 in aqueous micellar media of Gemini-X, Gemini-Y, and Gemini-Z surfactants. λex = 375 nm and slit width = 3 nm (both excitation and emission).

surfactants. The peak positions of absorption and fluorescence bands of C-480 in each of the micellar media of Gemini-X, -Y, and -Z are found to be at 394 and 482 nm, respectively. Although there is a change in the absorption and fluorescence intensity, no change in peak position has been noted with the change in hydrophobic tail length of these surfactants. It could be due to constrained structure of C-480.10,18,54 With changing the tail length of surfactants, there may not be significant change in the polarity of the microenvironment, but motional restriction is increased. It has been documented that C-480 molecules show fluorescence peak maxima at 494, 473, 417, and 408 nm in pure water, methanol, hexane, and cyclohexane, respectively.3 By comparing fluorescence peak maxima of C-480 in studied micellar systems to those in all pure solvents, one can say that the polarity of microenvironment around C-480 is similar to that of methanol. Therefore, the C-480 molecules are mostly located in the Stern layer of micelles.3,17 To further support this, the micropolarity expressed in the form of empirical solvent polarity parameter, ET(30) has been estimated by the reported method.10,55−57 The value of ET(30) for the micelles of Gemini-X, -Y, and -Z is found to be 56.5 kcal mol−1, which is close to that of methanol (55.4 kcal mol−1).10 As reported earlier,10 absorption and fluorescence peak maxima of C-480 appear at 390 and 477 nm, respectively, in the micelles of Gemini-A. It implies that there is a blue shift in the fluorescence peak position of C-480 in the micelles of Gemini-A as compared to that in the present micellar systems. Different values of both λmaxabs and λmaxfl of C-480 in the micelles of Gemini-X, -Y, and -Z as compared to those in the micelles of Gemini-A (Table 1) show the effect of hydrophilic spacer on the location of probe molecules. The solvent polarity parameter, ET(30), for the micelles of Gemini-A is found to be 53.2 kcal mol−1, which is lesser than that for the present micellar systems. Li et al.49 described that the hydrophilic spacer group can easily be located in the Stern layer of micelles, favoring the formation of micelles. The data in Table 1 show that the cmc value of each of Gemini-X, -Y, and -Z is lower than that of Gemini-A.10 These results infer that C-480 molecules feel comparatively more polar environment in the micelles of surfactants Gemini-X, -Y, and -Z with hydrophilic diethyl ether spacer group as compared to the

2. RESULTS AND DISCUSSION 2.1. UV−Visible Absorption and Steady-State Fluorescence Study. The UV−visible absorption and steady-state fluorescence spectra of C-480 have been recorded in the aqueous micelles of Gemini-X, Gemini-Y, and Gemini-Z surfactants (Figure 1). To ensure complete solubilization of C-480 molecules in the micelles, the concentration of each surfactant was chosen to be 15 times of respective critical micelle concentration (cmc). To see the effect of spacer group of gemini surfactants, we have also compared our present results to those for Gemini-A studied earlier.10 The peak maxima of absorption and fluorescence bands of C-480 in all of these micellar systems are tabulated in Table 1. The cmc values of all gemini surfactants determined in our previous study using conductometric method are given in Table 1.53 To support those values, the cmc values have also been determined by fluorescence method in the present study (Figure 2). The solubilization of C-480 in the micelles is evidenced by the change in fluorescence intensity with increasing concentration of a surfactant (Figure 2). The cmc values determined by these two different methods corroborate very well, which further supports the purity of the synthesized 5900


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Table 1. Cmc of Gemini Surfactants, Experimental Concentration of Surfactants, Absorption and Steady-State Fluorescence Peak Maxima and Average Excited Singlet-State Lifetime and Fluorescence Anisotropy of C-480 in Micelles of Gemini-X, Gemini-Y, Gemini-Z, and Gemini-Aa (λex = 375 nm and λem = 475 nm), and Microviscosity and Micropolarity [ET(30)] of Micelles system a

Gemini-A Gemini-X Gemini-Y Gemini-Z a

cmcb (mM)

cmcc (mM)

conc. (mM)

λmaxabs (nm)

λmaxfl (nm)

⟨τf⟩ (ns)

χ2

r

microviscosity (cP)

ET(30) (kcal mol−1)

1.170 0.966 0.158 0.024

1.145 0.950 0.154 0.023

18.00 14.49 2.37 0.36

390 394 394 394

477 482 482 482

5.05 5.29 5.31 5.35

1.01 1.03 1.06 1.07

0.022 0.026 0.028 0.041

15.1 ± 0.5 19.2 ± 0.5 23.3 ± 0.6 27.3 ± 0.5

53.2 56.5 56.5 56.5

All data for Gemini-A are taken from the ref 10. bCmc determined by conductometric method. cCmc determined by fluorescence method.

increasing tail length of the surfactants. One can also see that the microviscosities of the micelles of Gemini-X, -Y, and -Z are greater than the microviscosity of the micelles of Gemini-A. It is true that with increasing tail length of the surfactants the formation of micelles becomes favored due to enhanced hydrophobic interactions and as a result the compactness of surfactant molecules in the micelles increases.53 In fact, it is supported by the decrease in cmc values with increasing tail length (Table 1). The fact of increase in microviscosity has been probed by C-480, which, however, is located in the Stern layer of micelles. Thus, this result infers that the C-480 molecules penetrate up to a certain depth in the Stern layer of micelles. They feel comparatively more viscous environment with increasing tail length of the surfactants as more compact micelle structure inhibits the penetration of water molecules deep inside the Stern layer. Thus, compact micelle structure has indirect effect on the microviscosity in the Stern layer of micelles. The microviscosity of the micelles of Gemini-A is lower than that of other three gemini due to the less compact structure of the former than the latter. Formation of a micelle is favored by hydrophobic attractive interactions between the tails of surfactant molecules and disfavored by the repulsive interactions between the hydrophilic headgroups of surfactants. These repulsive interactions are reduced by the solvation of headgroups by the water molecules in aqueous micelles. Therefore, in the present case, when a micelle is stabilized by a greater extent of hydrophobic interactions between the longer tails, the requirement of solvation of headgroups becomes less important. In addition, the water molecules near hydrophilic spacer group are more restricted than those near hydrophobic spacer group. These could be the reasons for why microviscosities of micelles of Gemini-X, -Y, and -Z are greater than the microviscosity of the micelles of Gemini-A. The greater micropolarity of the micelles of Gemini-X, -Y, and -Z as compared to that of the micelles of Gemini-A is because of the presence of hydrophilic spacer groups in the former. 2.4. Solvation Dynamics. In the micelles of all gemini surfactants, C-480 molecule exhibits wavelength-dependent fluorescence decay. Fluorescence decays of C-480 have been recorded at different intervals in the whole wavelength range of fluorescence spectrum. A decay at a shorter wavelength corresponds to the fluorescence from the unsolvated dipole created at the excited state. However, due to the limitation of our TCSPC setup (instrument response function = 165 ps), in the present case, there is a possibility that a high percentage of solvated dipoles also contributes to the fast decays observed at shorter wavelength range. However, at a longer wavelength, the fluorescence decay is from a solvated dipole, which is delayed by solvent relaxation process showing a clear growth in the decay.2,60 Figure 3 represents the wavelength-dependent decays of C-480 in the micelles of Gemini-X as a representative one. Similar decays have been noticed in the micelles of other gemini

Figure 2. Plot of variation of fluorescence intensities with increasing concentration of gemini surfactants. λex = 375 nm and slit width = 3 nm (both excitation and emission).

micelles of surfactant Gemini-A with hydrophobic tetramethylene spacer group. 2.2. Excited Singlet-State Lifetime. Excited singlet-state lifetime values (τf) of C-480 molecule in the aqueous micelles of all gemini surfactants at ∼15cmc have been determined using time-correlated single photon counting (TCSPC) method. All decays were fitted biexponentially. The average lifetime has been calculated using eq 1 ⟨τf ⟩ = a1τ1 + a 2τ2

(1)

where a1 and a2 are the pre-exponential factors for the corresponding lifetimes τ1 and τ2 of the two components. The average lifetimes of C-480 in the all studied micelles along with the χ2 values are given in Table 1. The lifetime values of all components along with corresponding pre-exponential factors are given in Table S1 (Supporting Information). The lifetime values of C-480 molecules in pure solvent have been reported earlier,10 and the values are 5.89, 4.90, and 3.13 ns in water, methanol, and cyclohexane, respectively. By comparing the lifetime value in a particular micellar system to that in pure solvents, one can conclude that the microenvironment around C480 is similar to that of methanol. 2.3. Steady-State Anisotropy and Microviscosity. The steady-state fluorescence anisotropies of C-480 in the micelles of Gemini-X, -Y, and -Z have been determined, and the values obtained along with the same in the micelles of Gemini-A reported earlier10 are given in Table 1. Using these values, the microviscosities of environment around C-480 in all micelles have been estimated by the method described earlier.10,58,59 The data given in Table 1 show that the microviscosity increases with 5901


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Figure 3. Fluorescence decays of C-480 in the micelles of Gemini-X at λem = 430, 460, 490, and 565 nm along with instrument response function; λex = 375 nm.

surfactants as well (Figure S1, Supporting Information). For the quantitative measurement of solvation dynamics, the solvent response function (SRF), C(t) (eq 2), given by Fleming and Maroncelli61 has been used C(t ) =

υ(t ) − υ(∞) υ(0) − υ(∞)

(2)

where υ(0), υ(t), and υ(∞) are the peak wavenumbers at times 0, t, and ∞, respectively. Time-resolved emission spectra (TRES) have been constructed to determine the peak wavenumbers following the method proposed by Fleming and Maroncelli.61 TRES for Gemini-X, -Y, and -Z are shown in Figure 4. The peak wavenumber, υ(t), for each TRES at different times was obtained after fitting the TRES to a log−normal function.61,62 After calculating solvent response function, C(t), using eq 2, the decays of C(t) are shown in Figure 5. The biexponential fitting of the decay of C(t) with time is done using eq 3 C(t ) = a1s e−t / τ1s + a 2s e−t / τ2s

(3)

where τ1s and τ2s are the solvent relaxation times and a1s and a2s are the corresponding amplitudes. The values of solvation times obtained from the fitted data are given in Table 2. The average solvation time ⟨τs⟩ for a biexponential decay is calculated by eq 4 and given in Table 2

⟨τs⟩ = a1sτ1s + a 2sτ2s

(4)

The time-dependent Stokes shift occurs because of the presence of probe molecules in the Stern layer of the micelles,63 neither for the probes existing in the core of the micelles (no solvation)3,10 nor for the probes present in the bulk water (too fast to be detected by the present setup).4 The fast and slow (bimodal behavior of solvation dynamics) solvation components (Table 2) are due to the free and bound water molecules, respectively.21 Apart from water molecules, polar headgroups, spacer groups, and counterions may also be responsible for the solvation of the probe molecule in the Stern layer of aqueous micelles. However, polar headgroups are directly attached to the tails and spacer group is indirectly attached to the tails, which restrict their mobility. Therefore, the solvation process contributed by the headgroups and the spacer group is very slow because it has been reported that dynamics of polymer chains occurs in ∼100 ns time

Figure 4. Time-resolved emission spectra (TRES) of C-480 in the micelles of (a) Gemini-X, (b) Gemini-Y, and (c) Gemini-Z at different times (0−10 000 ps).

scale.3,64 Thus, water molecules and counterions are mainly responsible for the present solvation process. It has been reported that the strength of a hydrogen bond between water and polar headgroup is much stronger than that between two water molecules.3,65,66 Thus, those water molecules interacting with the polar headgroups contribute to the slow component of solvation and the water molecules hydrogenbonded among themselves contribute to the fast component of solvation. The data in Table 2 show that fast and slow solvation times as well as average solvation time increase with increasing 5902


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of free water molecules, the solvation dynamics in the micelles of Gemini-A is faster than that in the micelles of other three surfactants. It is noteworthy that although the contribution of slower component in Gemini-A is similar to that in other three micelles, bound water molecules contributing to the slow solvation is expected to be less rigid in the micelles of the former than in the micelles of the latter. The average solvation times noted in the present micelles are almost four times longer than those in the micelles of conventional surfactants with same tail length.18 This difference could be due to two reasons. First, the formation of micelles by gemini surfactants having two tails is thermodynamically more feasible; cmc values of Gemini-X, -Y, and -Z are much smaller than those of their conventional counterparts,18 DTAB (15 mM), TTAB (3.5 mM), and CTAB (0.8 mM), respectively, and therefore micelles of gemini surfactants are expected to be more compact than the micelles of conventional surfactants. Second, the hydrophilic spacer groups present in the present gemini surfactants easily get hydrated and protect a significant amount of water molecules from their contact with the probe molecules. It can be mentioned here that Shirota and co-workers27 also found that solvation dynamics in the micelles of anionic surfactant, sodium alkyl sulfate (Cn = 8, 10, 12, and 14), and cationic surfactant, alkyltrimethylammonium bromide (Cn = 10, 12, 14, and 16) becomes slower with increasing alkyl chain length. As there is a limitation of time resolution of our TCSPC setup (instrument response function = 165 ps), we are unable to detect a percentage of ultrafast solvation. The quantification of these missing components has been done following the method proposed by Fee and Maroncelli,61,68 and the values are listed in Table 2. To further support the fact that the hydrophilic spacer groups present in the gemini surfactants protect a certain amount of water molecules from their contact with the probe molecule, C480, we have carried out a fluorescence experiment by forming micelles in the water−methanol mixture. In our previous study52 and also a study carried out by other groups,69 it has been observed that significant change in cmc values or change in the size of micelles occurred when the percentage of an organic cosolvent especially hydrogen-bond-donating solvent in water− organic solvent mixture is above 20%. Keeping this in mind, we have recorded fluorescence spectra of C-480 in pure water and also in the presence of various percentages of organic cosolvent not exceeding 20%. In this experiment, we wanted to see the extent of interactions between the −OH group of methanol and C-480. Figure 6a shows that in the presence of 15cmc of GeminiA the fluorescence peak maximum gets red-shifted from 477 to 480 nm on increasing the percentage of methanol up to 20%. On the other hand, Figure 6b shows that there is no change in the fluorescence peak maximum for the same increase in the percentage of methanol in case of 15cmc of Gemini-X. This result is a clear evidence of protection of molecules containing

Figure 5. Decays of solvent response function C(t) of C-480 in the micelles of Gemini-A, -X, -Y, and -Z.

tail length from C12 to C16 in Gemini-X, -Y, and -Z. In all cases, the slow components have major contribution to the solvation than the fast component. The reason behind this could be that water molecules taking part in this solvation process are present deep inside the Stern layer and are quite rigid. Rigidness of water molecules increases with increasing tail length, which is evidenced by decrease in Stokes shift, Δυ (Table 2). Chattopadhyay et al.67 reported that water can penetrate the micelles up to a certain depth depending on the compactness of the micelle. Although the increase in solvation time is not very significant for increase in tail length from C14 to C16, it is comparatively more significant for increase in tail length from C12 to C14, which is unlike conventional surfactants studied by Sarkar et al.18 They noticed average solvation times of 273, 286, and 341 ps for conventional surfactants with tails C12, C14, and C16, respectively. From these results, it can be suggested that there is a little effect of increasing tail length on the solvation time and that this effect is not linearly proportional to the increase in the number of carbon atoms in the tail. On the other hand, when one compares these results to the result of Gemini-A, it can be seen that solvation process is much faster in Gemini-A with a larger value of Stokes shift, Δυ (Table 2), as compared to other three gemini surfactants. It is even faster than that in Gemini-X, which has tails of the same length as Gemini-A. This difference must be due to the difference in the chemical nature of their spacer group. Results depict that solvation process in the Stern layer is faster in case of micelles of gemini surfactants with comparatively hydrophobic spacer group. Gemini-A micelle has a less compact structure (hydrodynamic radius = 1.92 nm) than the other three surfactants (hydrodynamic radius = 0.52−0.63 nm). Moreover, hydrophobic spacer group in Gemini-A is not hydrated, whereas the hydrophilic diethyl ether spacer group in other surfactants is easily hydrated. Therefore, due to the presence of a greater extent

Table 2. Decay Characteristics of C(t) of C-480 in Different Micelles system c

Gemini-A Gemini-X Gemini-Y Gemini-Z a

a1s

τ1s (ps)

a2s

τ2s (ps)

⟨τs⟩a (ps)

Δυb (cm−1)

missing component (%)

0.34 0.40 0.43 0.45

233 254 331 359

0.66 0.60 0.57 0.55

574 1742 1834 1895

458 1147 1188 1204

1354 1005 955 819

16 16 29 39

⟨τs⟩ = a1τ1 + a2τ2. bΔυ = υ(0) − υ(∞). cAll data for Gemini-A are taken from ref 10. 5903


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acidic in nature making those protons highly deshielded. Therefore, it is expected that the extent of H-bonding between a proton in the spacer and water molecule would be more in case of Gemini-X as compared to Gemini-A. Thus, the presence of oxygen atom in the spacer group of Gemini-X can protect the water molecules from their contact with the probe molecules present in the inner side of the micelles to a greater extent as compared to Gemini-A. The 1H NMR spectra of Gemini-Y and -Z in D2O are given in Figure S2, and the data are given in Note 1 in Supporting Information. These data are also in the same line as Gemini-X. Thus, 1H NMR spectroscopy results support our above-mentioned discussion that the hydrophilic spacer has effect on the rate of solvation dynamics in the micelles. 2.5. Time-Resolved Fluorescence Anisotropy. To have better idea about the microenvironment of micelles, timeresolved fluorescence anisotropy measurements of C-480 have been carried out in micellar media. The time-resolved fluorescence anisotropy, r(t), values have been calculated by eq 5 r (t ) =

I (t ) − GI⊥(t ) I (t ) + 2GI⊥(t )

(5)

where I∥(t) and I⊥(t) are the fluorescence decays polarized parallel and perpendicular to the polarization of the excitation light, respectively. G is the correction factor for detector sensitivity to the polarization detection of emission. The G factor for the present instrument setup is ∼0.6. The anisotropy decays in the micelles of Gemini-A, -X, -Y, and -Z are shown in Figure 8. Although the anisotropy decay of C-480 is found to be single exponential in water, it is biexponential in micellar media. Earlier, we have reported that the anisotropy decay of C-480 in pure water is single exponential with rotational relaxation time of 132 ps.10 The biexponential anisotropy decay has been fitted to the decay function represented by eq 6 r(t ) = r0[a1r e−t / τ1r + a 2r e−t / τ2r] Figure 6. Fluorescence spectra of C-480 in (a) 15cmc Gemini-A and (b) 15cmc Gemini-X in the presence of various percentages of methanol in water−methanol mixture; λex = 375 nm.

(6)

where r0 is the limiting value of anisotropy to represent the inherent depolarization of the probe molecule, τ1r and τ2r are the time constants for the fast and slow rotational relaxation components, respectively, and a1r and a2r are relative amplitudes of these two components, respectively. The rotational relaxation times for the fast and slow components have been calculated from the fitted decay. The following equation (eq 7) has been used to estimate the average rotational relaxation time

−OH groups by hydrophilic spacer group from their contact with the probe molecule. We have also carried out 1H NMR spectroscopic study to demonstrate the interactions between spacer part of gemini surfactants and water molecules. The 1H NMR spectra of Gemini-A, -X, -Y, and -Z have been recorded in D2O as solvent. A detailed discussion in this regard is given below for Gemini-A and Gemini-X. The same is true for Gemini-Y and Gemini-Z as well. The chemical shifts for the S1 protons and S2 protons (Scheme 1) of Gemini-A are observed to be at δ 3.34 and 1.81, respectively, in D2O, whereas those for Gemini-X are found to be at δ 3.59 and 3.95, respectively, in D2O. The 1H NMR spectra of Gemini-A and Gemini-X in D2O are given in Figure 7a and b, respectively, and data are given in Note 1 in Supporting Information. Both S1 and S2 protons are more deshielded in Gemini-X as compared to Gemini-A. It has been reported in the literature70,71 that in the presence of an electronegative atom in a molecule close to a proton the latter becomes more acidic, resulting in showing a downfield 1H NMR signal, which is observed in the present case as well. It is also noteworthy that S1 protons are more deshielded as compared to S2 protons in case of Gemini-A, whereas S2 protons are more deshielded as compared to S1 protons for Gemini-X. Because of the presence of an oxygen atom in between two carbon atoms in the spacer of Gemini-X, S2 protons are

⟨τr⟩ = a1rτ1r + a 2rτ2r

(7)

where τ1r and τ2r are the rotational relaxation times and a1r and a2r are the corresponding amplitudes. The values of rotational relaxation times for the fast and slow components along with average rotational relaxation time are given in Table 3. Longer rotational relaxation time in a micellar environment as compared to pure water represents that the free rotational motions of C-480 in water get restricted in micelles. The rotational relaxation data for C-480 in the micelles of Gemini-A obtained in our previous study10 are also given in Figure 8 and Table 3 for the purpose of comparison. Data in Table 3 show that both fast and slow relaxation times along with average relaxation time increase with increasing tail length of surfactants. This result can be corroborated with the increase in microviscosity of micelles with increasing tail length. Recently, Samanta et al.32 have found that the rotational dynamics of 4-aminopthalimide (4-AP) becomes slower when the chain length of the cationic part of N-alkyl-N-methylmor5904


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Figure 7. 1H NMR spectra of (a) Gemini-A (18.0 mM) and (b) Gemini-X (14.5 mM) in D2O.

contribution (29−39%) in each micellar system. Although C-480 is apparently present in a rigid environment, the presence of fast motions responsible for the loss of anisotropy is evidenced by the

pholinium ionic liquid increases. It can be noted that the fast component has major contribution (61−71%) to the fluorescence depolarization, whereas the slow component has minor 5905


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with higher microviscosity as compared to their conventional counterparts. The observed biexponential behavior of the anisotropy decay is not due to the different location of the probe molecule in the micelles but due to the different types of rotational motions.72,73 This type of bimodal anisotropy decay is well explained by a twostep model and a wobbling-in-a-cone model73,74 (Note 2, Supporting Information). The two-step model explains that the slow relaxation time (τ2r) can be related to the times corresponding to the tumbling motion of the micelle as a whole (τm) and the lateral diffusion (τD) of the probe along the micelle surface following the relation (eq 8) 1 1 1 = + τ2r τD τm

where τm is calculated following the Debye−Stokes−Einstein equation using the value of hydrodynamic radius of micelle (Note 3, Supporting Information). The hydrodynamic radii (rh) estimated in the present study and size distribution plots of all three micellar media are given in Table 4 and Figure S3 (Supporting Information), respectively. The τm values have been calculated using eq S1 (Supporting Information) and τD values have been calculated using the values of τm and eq 8 at 298.15 K and are tabulated in Table 4. By comparing the values of τD (Table 4) to the values of slow rotational relaxation time (τ2r) (Table 3), one can see that these values are almost same. Therefore, the slow rotational relaxation is mainly due to the lateral diffusion of the probe along the surface of the micelle. The time constant for the lateral diffusion of the probe increases with increasing tail length of the gemini surfactants, that is, with increasing microviscosity of micelles. The tumbling motion of the micelle as a whole is much slower than the lateral diffusion of the probe in the micelle. As expected, the time constant for the overall motion of the micelle increases with increasing size of the micelle. The time constant for the wobbling motion (τw) of C480 in the micelles has been calculated using eq S2 (Note 4, Supporting Information). The calculated τw values are also tabulated in Table 4. The wobbling motion time (τw) is a measurement of relaxation of local structure in a micelle. The value of τw increases with increasing microviscosity of micelles. Applying the wobbling-in-a-cone model73 (Note 5, Supporting Information), the values of wobbling diffusion coefficient (Dw), order parameter (|S|), and cone angle (θo) have been calculated using eqs S3−S5, respectively, to have further information about the motional restriction of C-480 molecules within the micelles. The values obtained are summarized in Table 4. A large value (greater than 0.5) of spatial restriction parameter (|S|) shows that the probe molecules are located in a restricted environment. The higher values of θo and lower values of (|S|) for Gemini-Y and -Z as compared to Gemini-X might be indicating that the wobbling-in-a-cone model is not suitable for micelles with high microviscosity. To resolve this apparent discrepancy, often a spinning-in-equatorial-band model75,76 is used. However,

Figure 8. Fluorescence anisotropy decays of C-480 in the micelles of gemini surfactants; λex = 375 nm and λem = 470 nm.

Table 3. Rotational Relaxation Data of C-480 in Different Systems

a

system

a1r

τ1r (ps)

a2r

τ2r (ps)

⟨τr⟩ (ps)

Gemini-Aa Gemini-X Gemini-Y Gemini-Z

0.64 0.61 0.65 0.71

372 304 375 407

0.36 0.39 0.35 0.29

1663 1853 2174 2856

837 908 1005 1117

(8)

All data for Gemini-A are taken from ref 10.

values of time-zero anisotropy, ro = 0.30, 0.31, and 0.31 for Gemini-X, -Y, and -Z, respectively, which are less than the maximum possible value of ro = 0.40.60 One can note that the change in average relaxation time is more for increasing tail length from C14 to C16 than that for C12 to C14, which is unlike solvation time. As expected, the relaxation time is lower in case of micelles of Gemini-A as compared to micelles of other surfactants due to lower microviscosity of the former than the latter.46 It has been discussed above that the slow solvation component has major contribution to the solvation dynamics of C-480 in the Stern layer. Thus, these results infer that although solvation dynamics is a slow process, C-480 molecules are quite freely movable deep inside the Stern layer of micelles. In a complex micellar environment, the rate of rotational relaxation process may be directly correlated with microviscosity, but it may not be true for the rate of solvation process. Sarkar et al.18 reported that the average rotational relaxation times for C-480 in the micelles of conventional surfactants with C12, C14, and C16 tails are 481, 687, and 812 ps, respectively. However, in the present surfactant systems with hydrophilic spacer groups, these values are almost 1.5 times longer. It is even longer in the presence of hydrophobic spacer group as well. Thus, the presence of spacer group in a gemini surfactant makes the rotational relaxation process slower in more compact micelles

Table 4. Hydrodynamic Radius (rh), Wobbling Motion Time (τw), Time for Overall Rotational Motion of the Micelle (τm), Lateral Diffusion Time (τD), Wobbling Diffusion Coefficient (Dw), Cone Angle (θo), and Order Parameter (|S|) Obtained from the Anisotropy Decay of C-480 in the Different Micelles system

rh (nm)

τw (ps)

τm (ns)

τD (ps)

Dw × 10−8 (s−1)

θo (deg)

|S|

Gemini-X Gemini-Y Gemini-Z

0.52 0.61 0.63

364 453 475

126 208 226

1880 2200 2890

4.64 4.11 4.55

43.6 45.8 49.3

0.62 0.59 0.54

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rotational component is mainly due to the lateral diffusion of the probe along the surface of micelle. The tumbling motion of micelles as a whole is a very slow process as compared to the lateral diffusion of the probe, and the time constant for the former motion increases with increasing size of the micelle. Both wobbling motion and lateral diffusion of the probe become slower with increasing microviscosity of micelles. There is an indication of different orientation of probe molecules in the micelles of high microviscosity.

in the present case, it is not applicable as for this model the value of |S| should be less than 0.5. The possible reason could be that the probe molecule is aligned in such a way that the emission moment is neither perpendicular to the long axis like that in the spinning-in-equatorial-band model nor parallel to the long axis like that in the wobbling-in-a-cone model but oriented in between these two possibilities. More importantly, the orientation of C-480 molecules in more viscous Gemini-Y and Gemini-Z micelles is possibly different from that in Gemini-X micelles. It is noteworthy that hydrodynamic radii data show that the micelles are spherical in nature and that is why we could use twostep and wobbling-in-a-cone models as they are applicable for spherical micelles. Data also show that size variation is not very large. Therefore, there is no chance of effect of aggregation states, that is, change in micellar shapes from spherical to wormlike or so on, solvation dynamics, or rotational relaxation. Also, concentration of surfactant taken is much higher than cmc, so there would be hardly any chance of existence of premicellar aggregates.

4. EXPERIMENTAL SECTION 4.1. Materials. The detailed procedures for the synthesis of cationic gemini surfactants and their characterization have been reported earlier.53 In addition, a detailed NMR analysis has been carried out afterward to further check the purity of synthesized surfactants (Note 6, Supporting Information). C-480 was procured from Exciton laser grade and used as received. All organic solvents used in spectral measurements were from Spectrochem Chemical Company, India, and were of spectroscopic grades. To record the lamp profile in TCSPC measurements, Ludox in aqueous medium was used as a scatterer, details of which are available elsewhere.12 4.2. Methods. The concentration of gemini surfactants used for the present study has been kept 15 times of their respective cmc values to ensure that all probe molecules are completely solubilized in micelles.10 With these concentrations of surfactants, we did not notice any excitation-wavelengthdependent fluorescence peak maxima further supporting the fact of complete solubilization of probe molecules. The stock solution of C-480 was prepared in UV-grade methanol as solvent. The final concentration of C-480 was set at 5 μM in all experimental solutions. The details of the method of preparation of solutions are given elsewhere.10 Milli-Q water has been used to prepare all aqueous solution for spectral measurements. The UV−visible absorption and fluorescence spectra of C-480 in aqueous micelles were recorded with JASCO V-630 UV−visible spectrophotometer and Horiba Jobin Yvon FluoroMax-4 scanning spectrofluorometer, respectively. Other details of these measurements have been reported earlier.10,12 Steadystate fluorescence anisotropy measurements were carried out with the same instrument using additional polarizer attachment.10 The excited-state lifetimes were recorded from intensity decays using a Horiba Jobin Yvon FluoroCube-01-NL picosecond time-correlated single photon counting (TCSPC) experimental setup. A detailed description is available in our previous publication.10,12,77 An identical setup was used to analyze the time-resolved anisotropy measurements as well. The time constants for solvation process have been determined after constructing the TRES and calculating the solvent response function, C(t), following the method described by Fleming and Maroncelli,61 details of which are available elsewhere.12 As mentioned above, the same TCSPC setup has been used for the determination of the time-resolved fluorescence anisotropy, r(t), which has been calculated using eq 5. To measure the hydrodynamic radii of the micelles, dynamic light scattering (DLS) measurements were carried out using Zetasizer Nano ZS (ZEN 3600; Malvern Instruments, U.K.).10 The corresponding G function was considered with care to judge the size distribution. 1H NMR spectra were recorded on a Bruker Avance instrument (400 MHz). All spectral measurements were performed at 298.15 ± 1 K.

3. CONCLUSIONS The present study demonstrates the effect of hydrophobicity of tails and hydrophilicity of the spacer group of gemini surfactants on the rates of solvation and rotational relaxation processes of C480 in aqueous micelles. Study shows that C-480 molecules are located in the Stern layer. Micelles become progressively more compact with increasing hydrocarbon tail length, which results in lesser extent of penetration of water molecules, thereby increasing microviscosity of the micelles. The rate of solvation process becomes lower with increasing microviscosity of micelles with slow solvation as a major component. It infers that C-480 molecules are located inside the Stern layer of micelles and water molecules become progressively more rigid with increasing compactness of micelles. Micelles with hydrophilic spacer group like diethyl ether in the present case are more compact than those with hydrophobic spacer group. Moreover, hydrophilic spacer group gets easily hydrated. Therefore, the microviscosity of micelles of a gemini surfactant with hydrophobic spacer group is lower than that of micelles with hydrophilic spacer group. Also, the extent of free water molecules is more in the Stern layer of micelles of the former than that of the latter. As a result, solvation dynamics is faster in case of micelles of gemini surfactants with hydrophobic spacer. The rate of rotational relaxation process also decreases with increasing hydrocarbon tail length of surfactants as a result of increased microviscosity of micelles. However, unlike solvation process, the fast rotational motion is a major component for depolarization. Thus, although solvation process is slow, the rotational motion of C-480 is quite feasible inside the Stern layer of a compact micelle. More compact micelles are formed with increased hydrophilicity of the spacer group of gemini surfactants, which results in decrease in the rate of rotational relaxation process. The hydrophilic spacer group of gemini molecules protects the water molecules from their easy contact with the probe molecules present inside the Stern layer and also favors the formation of compact micelle. That is why solvation and rotational relaxation processes are almost 4 and 1.5 times longer in case of present micelles of surfactants with hydrophilic spacer groups as compared to micelles of their conventional counterparts, respectively. Thus, the spacer group of a gemini surfactant has an effect on the rates of solvation and rotational relaxation processes in the micelles and the effect is more pronounced in the case of solvation process. The slow 5907


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(8) Gearheart, L. A.; Somoza, M. M.; Rivers, W. E.; Murphy, C. J.; Coleman, R. S.; Berg, M. A. Sodium-ion binding to DNA: Detection by ultrafast time-resolved Stokes-shift spectroscopy. J. Am. Chem. Soc. 2003, 125, 11812−11813. (9) Zhong, D.; Pal, S. K.; Zewail, A. H. Biological water: A critique. Chem. Phys. Lett. 2011, 503, 1−11. (10) Tiwari, A. K.; Sonu; Saha, S. K. Effect of hydroxyl group substituted spacer group of cationic gemini surfactants on solvation dynamics and rotational relaxation of coumarin-480 in aqueous micelles. J. Phys. Chem. B 2014, 118, 3582−3592. (11) Chakrabarty, D.; Hazra, P.; Chakraborty, A.; Sarkar, N. Dynamics of solvation and rotational relaxation in neutral Brij 35 and Brij 58 micelles. Chem. Phys. Lett. 2004, 392, 340−347. (12) Sonu; Kumari, S.; Saha, S. K. Effect of polymethylene spacer of cationic Gemini surfactants on solvation dynamics and rotational relaxation of coumarin 153 in aqueous micelles. J. Phys. Chem. B 2015, 119, 9751−9763. (13) Hazra, P.; Sarkar, N. Solvation dynamics of Coumarin 490 in methanol and acetonitrile reverse micelles. Phys. Chem. Chem. Phys. 2002, 4, 1040−1045. (14) Correa, N. M.; Silber, J. J.; Riter, R. E.; Levinger, N. E. Nonaqueous polar solvents in reverse micelle systems. Chem. Rev. 2012, 112, 4569−4602. (15) Sonu; Tiwari, A. K.; Kumari, S.; Saha, S. K. Study on intramolecular charge transfer processes, solvation dynamics and rotational relaxation of coumarin 490 in reverse micelles of cationic gemini surfactant. RSC Adv. 2014, 4, 25210−25219. (16) 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. (17) Chakrabarty, D.; Hazra, P.; Sarkar, N. Solvation dynamics of coumarin 480 in TritonX-100 (TX-100) and bile salt mixed micelles. J. Phys. Chem. A 2003, 107, 5887−5893. (18) 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, No. 184516. (19) Sonu; Kumari, S.; Saha, S. K. Solvation dynamics and rotational relaxation of coumarin 153 in mixed micelles of Triton X-100 and cationic gemini surfactants: Effect of composition and spacer chain length of gemini surfactants. Phys. Chem. Chem. Phys. 2016, 18, 1551− 1563. (20) Sen, P.; Roy, D.; Mondal, S. K.; Sahu, K.; Ghosh, S.; Bhattacharyya, K. Fluorescence anisotropy decay and solvation dynamics in a nanocavity: Coumarin 153 in methyl β-cyclodextrins. J. Phys. Chem. A 2005, 109, 9716−9722. (21) Nandi, N.; Bagchi, B. Dielectric relaxation of biological water. J. Phys. Chem. B 1997, 101, 10954−10961. (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) Dey, S.; Adhikari, A.; Mandal, U.; Ghosh, S.; Bhattacharyya, K. A femtosecond study of excitation wavelength dependence of a triblock copolymer-surfactant supramolecular assembly: (PEO)20−(PPO)70− (PEO)20 and CTAC. J. Phys. Chem. B 2008, 112, 5020−5026. (24) 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. (25) Datta, A.; Mandal, D.; Pal, S. K.; Das, S.; Bhattacharyya, K. Solvation dynamics in organized assemblies, 4-aminophthalimide in micelles. J. Mol. Liq. 1998, 77, 121−129. (26) Sengupta, A.; Hazra, P. Solvation dynamics of Coumarin 153 in SDS dispersed single walled carbon nanotubes (SWNTs). Chem. Phys. Lett. 2010, 501, 33−38. (27) 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.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.7b00818. Excited-state lifetime of C-480 in different pure solvents and micelles and fluorescence decays plots of C-480 in the micelles of Gemini-Y and Gemini-Z; short discussion on the wobbling-in-a-cone model to explain the bimodal behavior of anisotropy by calculating the rotational motion of the micelle as a whole (τm), wobbling motion of the probe (τw), wobbling diffusion coefficient (Dw), order parameter |S|, and cone angle (θo); size distribution graphs for the micelles of the gemini surfactants obtained from DLS measurement; 1H NMR data for Gemini-A, -X, -Y, and -Z in D2O; 1H NMR spectra of Gemini-Y and -Z in D2O; and 1H NMR spectra of N,N-dimethyldodecyl amine and all synthesized gemini surfactants in CDCl3 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G.S.). *E-mail: [email protected], [email protected]. Tel: +91-1596-515731. Fax: +91-1596-244183 (S.K.S.). ORCID

Subit K. Saha: 0000-0001-8278-2912 Present Address §

Department of School Education, Haryana, India (S.).

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K.S. acknowledges the Council of Scientific and Industrial Research (CSIR) (01(2839)/16/EMR-II) for financial support, Department of Science and Technology (DST) FIST program, Government of India, and the University Grants Commission (UGC) for special assistance program [F.540/14/DRS/ 2007(SAP-I)]. S.K. acknowledges the UGC-BSR and Birla Institute of Technology & Science (BITS), Pilani, for financial support to carry out this research. R.A. acknowledges CSIR for fellowship and S. acknowledges UGC, Government of India, for senior research fellowship.

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REFERENCES

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Effect of Hydrophobicity of Tails and Hydrophilicity of Spacer Group of

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