1. Introduction {#sec1}
===============

Surfactant molecules can self-assemble to form various organized assemblies such as vesicles, reverse micelles, micelles, and so forth, which can mimic biological systems.^[@ref1]^ Reverse micelles are one of the simplest systems which can mimic the membrane architecture.^[@ref2]^ The interior of the reverse micelle which comprises the polar or charged headgroups and counterions can readily hold water or other polar solvents such as methanol, acetonitrile, dimethylformamide, and so forth.^[@ref3]−[@ref5]^*W*~o~ is the molar ratio of the polar solvent to the surfactant (\[polar solvent\]/\[surfactant\]) and controls the size of the reverse micelles. If the pool contains water, it is known as the "water pool."^[@ref2]^ The water molecules present in the reverse micelles are highly structured but heterogeneous.^[@ref6]^ The water molecules entrapped in the pool of the reverse micelles behave similarly to the water molecules evident at the interface of the proteins or in the biological membranes.^[@ref7]^ In biological systems, some water molecules, called bound water, are directly bound to the interface of biomolecules such as proteins and spend a longer time in their vicinity. However, some water molecules experience a very fast rotational and translational diffusion as compared to the former and are usually referred to as free water.^[@ref8]^ There is a dynamic exchange between these two types of water.^[@ref8],[@ref9]^ The hydrogen bonding interactions among water molecules and with several proton donating or proton accepting groups of biomolecules form a continuous hydrogen bonded path with a well-defined structure in the hydration layer, which boosts the proton transfer along the macromolecular chain.^[@ref10]^ Thus, reverse micelles as biomimicking molecular assemblies with different types of water molecules present within them provide a great opportunity to investigate the dynamics of molecules with various degrees of hydration.^[@ref11]^ Techniques such as small-angle neutron scattering,^[@ref12],[@ref13]^ Fourier transform infrared (FT-IR),^[@ref14],[@ref15]^ nuclear magnetic resonance (NMR),^[@ref10],[@ref16]−[@ref18]^ dielectric relaxation,^[@ref19],[@ref20]^ and solvation dynamics^[@ref9],[@ref21]−[@ref29]^ have been utilized to understand the properties of water in many molecular assemblies. However, solvation dynamics has the edge over the other techniques in terms of both spatial and temporal resolution.^[@ref9]^ Most interestingly, the ultraslow component of water, which is slower by many orders of the magnitude of the bulk water, has also been observed with the help of solvation dynamics in a confined environment.^[@ref20]^ The bulk water exhibits very fast solvation dynamics.^[@ref30]^ Barbara et al.^[@ref31]^ found the biexponential nature of solvation dynamics in water with two components, 0.16 and 1.2 ps. Fleming et al.^[@ref32]^ also reported the biexponential decay with components, 126 and 880 fs.

The solvation dynamics studied in several biomimicking systems^[@ref33]−[@ref35]^ is found to be dependent on the structures of the molecular assemblies.^[@ref36]−[@ref49]^ Bhattacharya and group^[@ref50]^ investigated the solvation dynamics in the reverse micelles of Aerosol OT (AOT) using Coumarin 480 (C-480) as the fluorescent probe molecule and observed faster solvation with increasing water content, suggesting that the mobility of the water molecules in the water pool increased with increasing *W*~o~. In contrast, Hazra et al.^[@ref51]^ observed that the solvation dynamics was almost insensitive to the water loading in the microemulsions of sodium dodecyl sulfate (SDS), TX-100, and cetyltrimethylammonium Bromide (CTAB) as the probe, C-480, was located in the interfacial region.

Gemini surfactant, a special kind of surfactant, has two monomeric surfactant units chemically bonded to each other at the headgroups by a spacer group showing much superior properties than their conventional counterparts.^[@ref52],[@ref53]^ The chemical nature of the spacer group of the gemini surfactant has a vital role in controlling its physical properties such as critical micelle concentration (cmc), counterion binding, aggregation number, and so forth.^[@ref54]−[@ref61]^ Saha and co-workers^[@ref62],[@ref63]^ investigated the effects of the chemical nature of the spacer group on the solvation dynamics and rotational relaxation processes in aqueous micelles by varying the number of hydroxyl group substitution and the number of −CH~2~-- units in the spacer group. It was seen that the solvation time increased with increasing the hydroxyl group substitution in the spacer group.^[@ref62]^ The average solvation time was found to be increased with increasing the number of methylene units in the spacer group.^[@ref63]^

Recently, Saha and group^[@ref64]^ reported the solvation dynamics and rotational relaxations of Coumarin 490 (C-490) in the aqueous reverse micelles of the gemini surfactant, 12-4-12.2Br^--^. The rates of solvation and rotational relaxation processes were found to be increased with increasing *W*~o~ as more C-490 molecules moved toward the interior of water pool and experienced a more flexible environment around them. Moreover, the solvation time in the reverse micelles of cationic gemini surfactants was found to be faster than that observed in the reverse micelles of AOT using the same probe.^[@ref65]^ The AOT surfactant having oxygen atoms in the headgroup possesses a strong hydrogen bonding with water molecules, which is responsible for slow solvation dynamics observed in its reverse micelles.

Zhao and group^[@ref66]^ studied the effects of the spacer chain length on the aggregation properties and cmc of gemini surfactants, C~12~-*s*-C~12~ in *n*-heptanol/*n*-hexane solution. They found the results similar to what Zana and group^[@ref67]^ had observed. The cmc of reverse micelles is directly proportional to the spacer chain length up to *s* = 4 and decreases with further increment in *s*.^[@ref68]^ The spacer stretches itself for a shorter length, but for longer lengths (*s* \> 6), it prefers to bend toward the hydrophobic phase in order to pack the quaternary ammonium headgroups as close as possible on the surface, while maintaining an equilibrium distance between the headgroups. Zhao's group also investigated the different states of the water using FT-IR spectroscopy within the same reverse micelles.^[@ref69]^ The OH-stretching band was assigned to four species of water: quaternary ammonium headgroup-bound, bulklike, counterion-bound, and free water species. There is a sharp increase in the number of bulklike water molecules, but a small increase in the number of headgroup-bound water molecules per surfactant molecule with increasing *W*~o~. It is also reported that the water pool size reduces with an increase in the spacer chain length at a fixed amount of water.^[@ref70]^ This is due to the effect in the molecular geometry because of the increased size of the headgroup with *s*. The number of bulklike water molecules per surfactant molecules decreases with increasing *s*. However, with a decrease in the size of the water pool with increasing *s*, the interfacial curvature increases, which restricts the hydration of the headgroup. As a result, the headgroup-bound water molecules remain unchanged. The number of counterion-bound water molecules per surfactant molecule increases with an increase in the spacer chain length, corresponding to an increased degree of counterion dissociation, as reported by Zana et al.^[@ref71],[@ref72]^

Looking into the relationships between the number of water molecules of different states and water loading (*W*~o~) in the reverse micelles and the spacer chain length at a given water loading, the present work is focused on to see how these relationships control the rates of solvation and rotational relaxation of C-490 ([Scheme [1](#sch1){ref-type="scheme"}](#sch1){ref-type="scheme"}). A study has been carried out in the reverse micelles of gemini surfactants, water/12-*s*-12.2Br^--^/*n*-propanol/cyclohexane with varying spacer groups (*s* = 5, 6, 8) ([Scheme [1](#sch1){ref-type="scheme"}](#sch1){ref-type="scheme"}) with different water loadings (*W*~o~) in each case. It is known that the aggregates with *W*~o~ \< 15 are called reverse micelles and those with *W*~o~ \> 15 are designated as microemulsions.^[@ref73]^ In this work, although *W*~o~ varies from 2 to 25, for our convenience, all aggregates would be called as reverse micelles only. The sharp rise in the number of bulklike water molecules as compared to a small increase in headgroup- and counterion-bound water molecules per surfactant molecule with increasing *W*~o~ controls the microenvironment around C-490 and also the rates of solvation and rotational relaxation. At a given water loading, the relative abundance of bulklike water molecules is mostly more than that of the headgroup- and counterion-bound water molecules per surfactant molecule because of which the fast component is the major component for solvation. The rate of rotational relaxation process is increased with increasing *W*~o~ because of the gradual swelling of reverse micelles. At a given *W*~o~, with increasing spacer chain length, the average solvation time is increased because of the gradual increase in the number of counterion-bound water molecules and the concomitant decrease in the number of bulklike water molecules per surfactant molecule. Because of the fact that the size of the water pool is reduced with an increase in the spacer chain length at a given *W*~o~, the average rotational relaxation time of C-490 is gradually increased. Because the reverse micelles behave as biomimicking systems and water molecules control various biological processes, the study of comprehensive effects of different states of solubilized water in the reverse micelles on the solvation dynamics and rotational relaxation processes and the role of the spacer chain length of gemini surfactants on relative abundances of various states of water thereby affecting the dynamics have practical and fundamental importance.

![Chemical Structures of Gemini Surfactants with Increasing Methylene Groups in the Spacer (These Three Surfactants Will be Written as 12-5-12, 12-6-12, and 12-8-12 Hereafter) and Coumarin 490](ao0c00035_0017){#sch1}

2. Results {#sec2}
==========

2.1. Scanning Electron Microscopy Images {#sec2.1}
----------------------------------------

[Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"} represents the field emission scanning electron microscopy (FE-SEM) image of reverse micelles of 12-8-12 at *W*~o~ = 5 as a representative. Similar reverse micelles are obtained in the case of other gemini surfactants with varying *W*~o~ as well.

![FE-SEM image of reverse micelles of the gemini surfactant, 12-8-12 at *W*~o~ = 5.](ao0c00035_0001){#fig1}

2.2. UV--Visible Absorption and Steady-State Fluorescence Spectra {#sec2.2}
-----------------------------------------------------------------

UV--Visible absorption spectra of C-490 have been recorded in the reverse micellar media of 12-5-12, 12-6-12, and 12-8-12 with varying *W*~o~ and also in pure solvents such as water, *n*-propanol, and the bulk solvent \[cyclohexane plus 10% *n*-propanol (CYH + prop)\]. [Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"} displays the absorption spectra of C-490 in various solvents and reverse micelles of 12-5-12 with varying *W*~o~ as a representative. Similar absorption spectra are obtained for reverse micellar systems of the other two gemini surfactants as well. The peak maxima of absorption bands of C-490 have been tabulated in [Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"}. C-490 has a broad absorption peak at 366 nm in pure water. The peak maximum of C-490 appears at 373 nm in the bulk solvent and 383 nm in pure *n*-propanol. The steady-state fluorescence spectra of C-490 have been recorded for all the systems mentioned above. [Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}a represents steady-state fluorescence spectra in various solvents and reverse micelles of 12-5-12 with varying *W*~o~ excited at 340 nm as a representative. In bulk solvent (CYH + prop), the fluorescence peak maximum appears at 467 nm, while in *n*-propanol, the peak maximum is observed at 476 nm. The fluorescence band, when recorded in water, undergoes a bathochromic shift to 491 nm as compared to bulk solvent and *n*-propanol. The steady-state fluorescence spectra of C-490 excited at 375 and 412 nm have also been recorded for all the systems to see any effect of excitation wavelength on peak positions and intensities and are given in [Figures [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}b and [S1](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf), respectively. The normalized fluorescence spectra along with the normalized absorption spectra are provided in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"} to have a better picture of the effect of static polarity of solvents and water content (*W*~o~) in reverse micelles on peak positions of the bands. All fluorescence peak maxima data have been tabulated in [Table [1](#tbl1){ref-type="other"}](#tbl1){ref-type="other"}, and the values of full width at half-maximum (FWHM) for fluorescence bands of C-490 in different reverse micellar systems at various excitation wavelengths are given in [Table S2](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf).

![UV--visible absorption spectra of C-490 in pure water, bulk solvent, *n*-propanol, and in reverse micellar systems of 12-5-12 with varying *W*~o~.](ao0c00035_0009){#fig2}

![Steady-state fluorescence spectra of C-490 in pure water, bulk solvent, *n*-propanol, and in reverse micellar systems of 12-5-12 with varying *W*~o~ at λ~exc~ = 340 nm (a) and at λ~exc~ = 375 nm (b).](ao0c00035_0010){#fig3}

![](ao0c00035_0011){#fig4}

###### UV--Visible Absorption and Steady-State Fluorescence Peak Maxima of C-490 in Pure Water, *n*-Propanol, Bulk Solvent (CYH + Prop), and in Reverse Micellar Systems with Varying *W*~o~

  systems        λ~max~^abs^ (nm)   λ~max~^flu^\*^^ (nm)   λ~max~^flu^\#^^ (nm)   λ~max~^flu^\$^^ (nm)   λ~max~^abs^ (nm)   λ~max~^flu^\*^^ (nm)   λ~max~^flu^\#^^ (nm)   λ~max~^flu^\$^^ (nm)   λ~max~^abs^ (nm)   λ~max~^flu^\*^^ (nm)   λ~max~^flu^\#^^ (nm)   λ~max~^flu^\$^^ (nm)
  -------------- ------------------ ---------------------- ---------------------- ---------------------- ------------------ ---------------------- ---------------------- ---------------------- ------------------ ---------------------- ---------------------- ----------------------
  water          366                491                    491                    492                                                                                                                                                                              
  bulk solvent   373                467                    467                    467                                                                                                                                                                              
  *n*-propanol   383                476                    476                    479                                                                                                                                                                              

                12-5-12   12-6-12   12-8-12                                                   
  ------------- --------- --------- --------- ----- ----- ----- ----- ----- ----- ----- ----- -----
  *W*~o~ = 2    383       474       473       474   384   474   473   474   384   476   473   475
  *W*~o~ = 5    383       474       474       475   383   474   474   475   384   475   474   477
  *W*~o~ = 10   382       475       475       475   383   475   475   475   383   476   474   477
  *W*~o~ = 15   382       476       476       478   382   477   476   478   383   477   474   477
  *W*~o~ = 20   381       477       476       478   382   477   476   478   382   477   475   478
  *W*~o~ = 25   381       478       477       478   380   478   477   478   382   477   476   478

λ~exc~ = 340 nm. ^\#^λ~exc~ = 375 nm. ^\$^λ~exc~ = 412 nm.

The fluorescence excitation spectra of C-490 have also been recorded in the reverse micelles. [Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"} displays the excitation spectra in the reverse micelles of 12-8-12 at *W*~o~ = 25 monitored at two different emission wavelengths, 435 and 550 nm. The excitation spectrum monitored at 435 nm was compared with the UV--visible absorption spectrum of C-490 in the bulk solvent, while the one monitored at 550 nm was compared with the absorption spectrum of C-490 in the reverse micelles at *W*~o~ = 25.

![](ao0c00035_0012){#fig5}

2.3. Excited State Lifetime {#sec2.3}
---------------------------

Excited singlet-state lifetime values of C-490 in various solvents and reverse micelles of all the gemini surfactants have been determined using the TSCPC method. Fluorescence decays in all selected solvents are found to be monoexponential. All the decays for the reverse micellar systems were fitted biexponentially. The average lifetime (⟨τ~f~⟩) has been calculated using [eq [1](#eq1){ref-type="disp-formula"}](#eq1){ref-type="disp-formula"}, as given later. The average lifetimes of C-490 in all the reverse micellar systems and pure solvents have been tabulated in [Table S3](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf). [Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"} represents a comparison of the average lifetime of C-490 in reverse micelles of different surfactants at varying *W*~o~. ⟨τ~*f*~⟩ decreases with increasing *W*~o~ for reverse micelles of a given surfactant, whereas it enhances with an increase in the spacer chain length of gemini surfactants at a given *W*~o~.

![Comparison of average lifetime of C-490 in reverse micelles of different surfactants at varying *W*~o~. λ~exc~ = 375 nm.](ao0c00035_0013){#fig6}

2.4. Solvation Dynamics {#sec2.4}
-----------------------

To study the solvation dynamics in a particular reverse micelle, the fluorescence decays of C-490, excited at 375 nm, were monitored at different emission wavelengths in the entire range of the fluorescence emission spectrum. The representative fluorescence decays of C-490 in reverse micelles of 12-5-12 at *W*~o~ = 2, along with the corresponding residuals for every decay, are shown in [Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}.

![Fluorescence decays of C-490 in the reverse micelles of 12-5-12 at *W*~o~ = 2 at IRF, 415, 445, 475, and 580 nm and corresponding residual (lower panels) for each decay. λ~exc~ = 375 nm.](ao0c00035_0014){#fig7}

Time-Resolved Emission Spectra (TRES) have been constructed using the method given by Fleming and Maroncelli^[@ref74]^ for reverse micellar systems of all three gemini surfactants at different *W*~o~. TRES plots of C-490 in reverse micelles of all three gemini surfactants at *W*~o~ = 10 have been shown in [Figure [8](#fig8){ref-type="fig"}](#fig8){ref-type="fig"} as a representative. The solvent correlation function (SCF), *C*(*t*) ([eq [2](#eq2){ref-type="disp-formula"}](#eq2){ref-type="disp-formula"}), explained by Fleming and Maroncelli has been calculated utilizing TRES and has been used to study the dynamics of the solvation.^[@ref74]^ The decays of *C*(*t*) for reverse micellar systems of a particular surfactant for varying *W*~o~ are shown in [Figure [9](#fig9){ref-type="fig"}](#fig9){ref-type="fig"}. The decay characteristics of *C*(*t*) obtained after biexponential fitting ([eq [3](#eq3){ref-type="disp-formula"}](#eq3){ref-type="disp-formula"}) of decay curves along with the average solvation time, ⟨τ~s~⟩, calculated using [eq [4](#eq4){ref-type="disp-formula"}](#eq4){ref-type="disp-formula"}, are tabulated in [Table [2](#tbl2){ref-type="other"}](#tbl2){ref-type="other"}. The error bars associated with the biexponential fitting of *C*(*t*) decays are given in [Table [2](#tbl2){ref-type="other"}](#tbl2){ref-type="other"}. Decay characteristics in *n*-propanol and bulk solvent are also given in [Table [2](#tbl2){ref-type="other"}](#tbl2){ref-type="other"}. [Figure [10](#fig10){ref-type="fig"}](#fig10){ref-type="fig"} displays the comparison of ⟨τ~s~⟩ with respect to the water pool size of the reverse micelles and the spacer chain length of the gemini surfactants.

![Representative TRES plots of C-490 in reverse micelles of (a) 12-5-12, (b) 12-6-12, and (c) 12-8-12 for *W*~o~ = 10 at different times (0--10 ns).](ao0c00035_0015){#fig8}

![Decays of SCF, *C*(*t*) of C-490 in reverse micelles of (a) 12-5-12, (b) 12-6-12, and (c) 12-8-12 at varying *W*~o~.](ao0c00035_0016){#fig9}

![Comparison of average solvation time with respect to water pool size of the reverse micelles and spacer chain length of the gemini surfactants 12-*s*-12.2Br^--^ (*s* = 5, 6, 8).](ao0c00035_0002){#fig10}

###### Decay Characteristics of *C*(*t*) of C-490 in *n*-Propanol, Bulk Solvent, and Reverse Micelles of 12-5-12, 12-6-12, and 12-8-12 at Varying *W*~o~

  pure and mixed solvents                                                               
  ------------------------- ------------- --------------- ------------- --------------- --------
  bulk solvent              0.33 ± 0.07   959.26 ± 0.17   0.67 ± 0.07   256.92 ± 0.02   493.83
  *n*-propanol              1.00          139.92 ± 0.01                                 139.92

  12-5-12                                                                               
  --------- ------------- --------------- ------------- --------------- -------- ------ ------
  2         0.15 ± 0.04   943.27 ± 0.32   0.85 ± 0.04   281.12 ± 0.01   380.44   0.13   1140
  5         0.18 ± 0.02   912.64 ± 0.42   0.82 ± 0.03   243.12 ± 0.01   363.63   0.21   1046
  10        0.37 ± 0.01   644.31 ± 0.01   0.63 ± 0.01   145.73 ± 0.01   330.20   0.31   956
  15        0.29 ± 0.01   754.63 ± 0.01   0.71 ± 0.01   140.47 ± 0.01   318.58   0.38   882
  20        0.24 ± 0.01   769.14 ± 0.02   0.76 ± 0.01   133.29 ± 0.01   285.89   0.39   886
  25        0.13 ± 0.01   964.79 ± 0.12   0.87 ± 0.01   155.78 ± 0.01   260.95   0.44   828

  12-6-12                                                                                
  --------- ------------- ---------------- ------------- --------------- -------- ------ ------
  2         0.29 ± 0.01   967.06 ± 0.04    0.71 ± 0.01   243.41 ± 0.01   453.27   0.06   1162
  5         0.27 ± 0.02   906.78 ± 0.04    0.73 ± 0.01   262.38 ± 0.01   436.37   0.11   1126
  10        0.18 ± 0.02   1000.48 ± 0.05   0.82 ± 0.02   233.18 ± 0.01   371.29   0.23   1045
  15        0.21 ± 0.02   867.61 ± 0.04    0.79 ± 0.02   221.31 ± 0.01   357.03   0.30   1001
  20        0.20 ± 0.01   925.28 ± 0.05    0.80 ± 0.01   172.00 ± 0.01   322.26   0.42   872
  25        0.19 ± 0.01   840.78 ± 0.03    0.81 ± 0.02   158.30 ± 0.01   287.97   0.52   729

  12-8-12                                                                                
  --------- ------------- ---------------- ------------- --------------- -------- ------ ------
  2         0.19 ± 0.03   1149.60 ± 0.09   0.81 ± 0.02   335.54 ± 0.01   490.21   0.15   1011
  5         0.33 ± 0.02   948.58 ± 0.03    0.77 ± 0.01   198.25 ± 0.01   465.68   0.22   944
  10        0.44 ± 0.01   812.52 ± 0.02    0.56 ± 0.01   137.06 ± 0.01   434.26   0.34   818
  15        0.27 ± 0.01   1013.60 ± 0.03   0.73 ± 0.01   206.77 ± 0.01   424.61   0.37   837
  20        0.39 ± 0.01   814.53 ± 0.02    0.61 ± 0.01   128.96 ± 0.01   396.33   0.57   602
  25        0.32 ± 0.01   741.05 ± 0.01    0.68 ± 0.01   101.86 ± 0.01   306.40   0.60   571

Δυ̅ = υ̅(0) -- υ̅(∞).

2.5. Rotational Relaxation {#sec2.5}
--------------------------

The time-resolved fluorescence anisotropy (*r*(*t*)) measurements were performed to know more about the microenvironment of C-490 in the reverse micelles. The rotational relaxations of the probe residing in an organized assembly can be known by the time-resolved fluorescence anisotropy measurements. The *r*(*t*) of C-490 has been calculated using [eq [5](#eq5){ref-type="disp-formula"}](#eq5){ref-type="disp-formula"}, monitoring fluorescence emission at 470 nm. The anisotropy decays in reverse micelles of each of the three gemini surfactants with varying *W*~o~ have been shown in [Figure [11](#fig11){ref-type="fig"}](#fig11){ref-type="fig"}. The anisotropy decays in all the reverse micellar systems have been fitted biexponentially using [eq [6](#eq6){ref-type="disp-formula"}](#eq6){ref-type="disp-formula"}. The average rotational relaxation times, ⟨τ~r~⟩ , for all the systems have been determined by using [eq [7](#eq7){ref-type="disp-formula"}](#eq7){ref-type="disp-formula"}.

![Time-resolved fluorescence anisotropy decay of C-490 in reverse micelles of (a) 12-5-12, (b) 12-6-12, and (c) 12-8-12 at *W*~o~ = 2, 5, 10, 15, 20, and 25; λ~exc~ = 375 nm and λ~em~ = 470 nm.](ao0c00035_0003){#fig11}

The fast and slow rotational relaxation times, along with the corresponding relative amplitudes and average rotational relaxation times, have been tabulated in [Table [3](#tbl3){ref-type="other"}](#tbl3){ref-type="other"}. The rotational relaxation times have also been measured in *n*-propanol and bulk solvent, and the data are included in [Table [3](#tbl3){ref-type="other"}](#tbl3){ref-type="other"}. As reported in the literature,^[@ref75]^ the error bars associated with the rotational relaxation times are found to be high in the present study as well. The dependence of ⟨τ~r~⟩ of C-490 on *W*~o~ and spacer chain length has been shown in [Figure [12](#fig12){ref-type="fig"}](#fig12){ref-type="fig"}.

![Dependence of average rotational relaxation time (⟨τ~r~⟩) of C-490 on *W*~o~ and spacer chain length of gemini surfactants, 12-*s*-12.2Br^--^ (*s* = 5, 6, 8).](ao0c00035_0004){#fig12}

###### Rotational Relaxation Time of C-490 in *n*-Propanol, Bulk Solvent, and Reverse Micelles of 12-*s*-12, 2Br^--^

  pure and mixed solvents                                   
  ------------------------- ------ ---------- --- --- ----- ------
  bulk solvent              1.00   278 ± 27           278   1.10
  *n*-propanol              1.00   356 ± 43           356   1.07

  12-5-12                                                             
  --------- ------------- ------------ ------------- ---------- ----- ------
  2         0.06 ± 0.01   6211 ± 498   0.94 ± 0.03   448 ± 65   794   1.04
  5         0.06 ± 0.01   6469 ± 511   0.94 ± 0.03   411 ± 48   774   1.02
  10        0.05 ± 0.01   6334 ± 502   0.95 ± 0.04   387 ± 58   684   1.10
  15        0.04 ± 0.01   6725 ± 525   0.96 ± 0.04   367 ± 62   621   1.06
  20        0.03 ± 0.01   5630 ± 478   0.97 ± 0.04   377 ± 59   535   1.04
  25        0.03 ± 0.01   4655 ± 470   0.97 ± 0.04   340 ± 58   469   1.04

  12-6-12                                                             
  --------- ------------- ------------ ------------- ---------- ----- ------
  2         0.10 ± 0.01   4456 ± 438   0.90 ± 0.03   441 ± 63   843   1.07
  5         0.07 ± 0.01   5573 ± 435   0.93 ± 0.04   457 ± 63   815   0.97
  10        0.06 ± 0.01   5954 ± 477   0.94 ± 0.03   412 ± 58   745   1.07
  15        0.04 ± 0.01   7203 ± 553   0.96 ± 0.04   418 ± 64   689   0.99
  20        0.04 ± 0.01   6002 ± 485   0.96 ± 0.04   400 ± 61   624   1.09
  25        0.03 ± 0.01   7286 ± 580   0.97 ± 0.04   376 ± 58   583   0.99

  12-8-12                                                             
  --------- ------------- ------------ ------------- ---------- ----- ------
  2         0.07 ± 0.01   6848 ± 529   0.93 ± 0.04   538 ± 84   980   1.07
  5         0.07 ± 0.01   6638 ± 519   0.93 ± 0.04   492 ± 73   922   1.06
  10        0.06 ± 0.01   6918 ± 549   0.94 ± 0.03   489 ± 67   875   1.07
  15        0.05 ± 0.01   7306 ± 560   0.95 ± 0.03   437 ± 68   780   1.02
  20        0.05 ± 0.01   6910 ± 548   0.95 ± 0.04   423 ± 65   747   1.08
  25        0.05 ± 0.01   5778 ± 520   0.95 ± 0.04   399 ± 68   668   1.07

2.6. Different States of Water Solubilized in Reverse Micelles: FT-IR Studies {#sec2.6}
-----------------------------------------------------------------------------

In order to quantify different states of solubilized water in reverse micelles, the FT-IR band was recorded for O--H stretch in each of the reverse micellar systems \[[Figure S2a--c](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf)\] and was deconvoluted by a Gaussian program, PRESENTS, on the basis of second derivative. Four sub-bands are obtained for each FT-IR spectrum after curve fitting. As a representative, [Figure [13](#fig13){ref-type="fig"}](#fig13){ref-type="fig"} shows the deconvolution result for the O--H stretching band of water in the reverse micelles of 12-5-12 at *W*~o~ = 20. For each FT-IR band, the fitting with the experimental line was very good with the correlation coefficient value close to ∼0.999.

![Deconvoluted O--H stretching band of water in the reverse micelles at *W*~o~ = 20 for 12-5-12 where (i), (ii), (iii), and (iv) represent the O--H stretching modes of quaternary ammonium headgroup-bound, bulklike, counterion Br^--^ bound, and free water species, respectively.](ao0c00035_0005){#fig13}

After getting sub-bands corresponding to four different states of water, the fractions of area (*F*~i~) for each state of water with varying *W*~o~ in the reverse micelles of a particular gemini surfactant have been calculated and plotted in [Figure S3a--c](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf). Using *F*~i~ value, the average number of water molecules of each state per surfactant molecule is then calculated (see Note 1 in [Supporting Information](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf))^[@ref69],[@ref76],[@ref77]^ and plotted in [Figure [14](#fig14){ref-type="fig"}](#fig14){ref-type="fig"} for different *W*~o~. [Figure [15](#fig15){ref-type="fig"}](#fig15){ref-type="fig"}a represents the changes in the area fraction of each state of water with an increase in the spacer chain length, and [Figure [15](#fig15){ref-type="fig"}](#fig15){ref-type="fig"}b represents the change in the average number of water molecules of each state of water with an increase in the spacer chain length at *W*~o~ = 20 as representatives.

![Average number of water molecules of each state of water per surfactant molecule as a function of water content in the reverse micelles of all three surfactants, (a) 12-5-12, (b) 12-6-12, and (c) 12-8-12.](ao0c00035_0006){#fig14}

![(a) Variation in area fraction of different water species (*F*~i~) with increasing spacer chain length and (b). Variation in the average number of water molecules of each state of water with increasing spacer chain length of gemini surfactants in the reverse micelles at *W*~o~ = 20.](ao0c00035_0007){#fig15}

3. Discussion {#sec3}
=============

The absorption peak maximum of C-490 in pure water is blue-shifted by 17 nm with respect to that in *n*-propanol. The water being highly polar, the blue shift in this solvent with respect to *n*-propanol is owed to the intermolecular hydrogen bonding interactions between water molecules and C-490 which is a common phenomenon for intramolecular charge transfer (ICT) type of molecules.^[@ref78]−[@ref81]^ The fluorescence peak maxima of C-490 in *n*-propanol and water are red-shifted by 9 and 24 nm, respectively, with respect to that in the bulk solvent. This is due to greater stabilization of the excited state by a more polar solvent. Absorption and fluorescence bands of C-490 in reverse micelle at *W*~o~ = 2 get red-shifted by 10 and 6 nm (λ~exc~ = 375 nm), respectively, with respect to that in the bulk solvent ([Figure [2](#fig2){ref-type="fig"}](#fig2){ref-type="fig"}--[4](#fig4){ref-type="fig"}). It implies that C-490 molecules reside inside the reverse micelles. Absorption and fluorescence peak maxima of C-490 in pure water are 366 and 491 nm, respectively. However, these peak maxima are located at 381 and 477 nm in reverse micelles of 12-5-12 at *W*~o~ = 25. Peak maxima values close to these are noted in the case of reverse micelles of other gemini surfactants as well. These results infer that the static polarity of the microenvironment around C-490 inside the reverse micelles is significantly different from that in bulk solvent and bulk water. Normalized spectra in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"} show that with increasing loading of water in the reverse micelles, there is a progressive blue shift in the absorption band and a progressive red shift in the fluorescence band. These results depict that the microenvironment inside the reverse micelles becomes more polar with the increasing content of solubilized water in the pool. The red shift in the fluorescence band noticed for reverse micelles is similar to that observed in *n*-propanol. Thus, polarity-wise, the microenvironment is similar to that of alcohol. Similar polarity of reverse micellar systems has been described in the literature.^[@ref51],[@ref82]^ The dependence of absorption and fluorescence peak positions of C-490 on the spacer chain length of gemini surfactants is found to be insignificant.

In this study, several experiments have been carried out to check the microheterogeneity of the environment around C-490 molecules in the reverse micelles. The excitation spectrum recorded for C-490 in reverse micelles ([Figure [5](#fig5){ref-type="fig"}](#fig5){ref-type="fig"} is for 12-8-12 at *W*~o~ = 25) at λ~em~ = 435 nm almost bears a resemblance to the absorption spectrum of C-490 in bulk solvent; however, the excitation spectrum measured at 550 nm resembles the absorption spectrum of C-490 in the reverse micelles at *W*~o~ = 25. These observations indicate that C-490 molecules are partitioned into different environments of the reverse micelles.^[@ref1],[@ref51],[@ref59],[@ref60],[@ref64]^ The fluorescence spectra of C-490 in the reverse micelles recorded at λ~ex~ = 375 and 412 nm are found to be red-shifted by 2--3 nm as compared to excitation at 340 nm. FWHM of the fluorescence band increases upon increase in the excitation wavelength. Differences are also noted in terms of changes in intensities. While fluorescence intensity increases with increasing *W*~o~ and then moves towards a spectrum with similar intensity in *n*-propanol as solvent upon excitation at 340 nm ([Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}a), the fluorescence intensity decreases when excited at 375 nm ([Figure [3](#fig3){ref-type="fig"}](#fig3){ref-type="fig"}b) or at 412 nm ([Figure S1](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf)). It could be because of the increase in the population of a species at 340 nm and the decrease in the population of the same species at a longer wavelength with increasing *W*~o~. As mentioned above, the absorption bands in reverse micelles at *W*~o~ = 25 and in pure water are blue-shifted as compared to the reverse micelles at *W*~o~ = 2. These results are also in support of the microheterogeneous environment of C-490 in the reverse micelles.

Hydrodynamic diameter values of reverse micelles of all the three gemini surfactants at two representatives *W*~o~ (2 and 10) obtained from dynamic light scattering (DLS) measurements are displayed in [Table [4](#tbl4){ref-type="other"}](#tbl4){ref-type="other"}. These data imply that the size of a given reverse micelle increases with increasing loading of water and the size decreases with increment in the spacer chain length at a given *W*~o~. From FE-SEM images, shapes of all reverse micelles are found to be spherical in nature (given [Figure [1](#fig1){ref-type="fig"}](#fig1){ref-type="fig"} is for 12-8-12 as a representative). The size of a micelle found in FE-SEM images is different from that measured by the DLS method. This difference is due to the two different methods used. To check whether there is any effect of swelling of the reverse micelles on the microenvironment of C-490, changes in absorption and fluorescence peak positions, FWHM of fluorescence bands, and fluorescence decay parameters at varying *W*~o~ of all micelles have been analyzed. Normalized spectra given in [Figure [4](#fig4){ref-type="fig"}](#fig4){ref-type="fig"} show blue shift in the absorption band and red shift in the fluorescence band with increasing *W*~o~. FWHM of fluorescence bands has a decreasing trend with increasing *W*~o~ ([Table S2](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf)). The fluorescence decays are monoexponential in water, *n*-propanol, and bulk solvents;^[@ref83]^ however, all the decays are found to be biexponential in the reverse micellar systems ([Table S3](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf)). It can be seen from the [Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"} that the average lifetime of C-490 decreases with an increase in water pool size. Data in [Table S3](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf) infer that although there is no significant change in the lifetime of the slow component, there is a trend of decrease in the lifetime of the fast component with increasing *W*~o~. FT-IR data ([Figure [14](#fig14){ref-type="fig"}](#fig14){ref-type="fig"}) show that while the average number of bulklike water per surfactant molecule rapidly increases with increasing *W*~o~, the rates of the increase in the number of each of headgroup- and counterion-bound water are comparatively less. Thus, the number of hydrogen-bonded water increases, and it is known that the hydrogen-bonded water is more polar than the water bound to headgroup or counterion.^[@ref9],[@ref20]^ Therefore, the conversion of an ICT to a TICT state, depending on the static polarity of the environment, is facilitated by enhancement in the polarity of the system with increasing water pool size.^[@ref82],[@ref84]−[@ref87]^ The TICT state gets more populated with an increase in the polarity of the environment around the coumarin molecule, which results in a red shift in the fluorescence band with decreased fluorescence intensity upon excitation at a longer wavelength. For C-490, the generation of the TICT state is the main nonradiative pathway.^[@ref64],[@ref65]^ The C-490 molecule with a primary amine group on the 7th position participates in hydrogen bonding, which also results in the stabilization of the excited state. All these results indicate that inside the reverse micelles, C-490 molecules are located in different microenvironments and they are most likely interface and water pool. Also, C-490 molecules get progressively transferred to a more polar environment, that is, in the water pool with increasing *W*~o~. Similar results are reported by Hazra and Sarkar for their study with C-490 in AOT--heptane reverse micelles.^[@ref65]^ It has been discussed below that the degree of counterion dissociation increases with increasing *W*~o~. Hence, there is a possibility that the microenvironment becomes more polar because of a progressive increase in the number of counterions with increasing *W*~o~. However, counterions remain hydrogen-bonded with water. The abundance of bulklike water is more than that of counterion-bound water. The number of bulklike water increases more rapidly than that of counterion-bound water with increasing *W*~o~. Moreover, the bulklike water is more polar than the counterion-bound water, and C-490 migrates to the water pool from the interface with increasing pool size. Thus, the increase in the polarity of the microenvironment with increasing water loading is most likely due to the presence of more abundant bulklike water.

###### Hydrodynamic Diameter of Reverse Micelles of Three Gemini Surfactants at *W*~o~ = 2 and 10

                hydrodynamic diameter (nm)                  
  ------------- ---------------------------- -------------- -------------
  *W*~o~ = 2    11.48 ± 0.11                 8.05 ± 0.39    6.52 ± 0.21
  *W*~o~ = 10   14.09 ± 0.13                 10.77 ± 0.13   8.43 ± 1.70

As mentioned above, the size of water pool of the reverse micelle decreases with an increase in the spacer chain length of the surfactants at a given *W*~o~. It can be seen from FT-IR data ([Figure [15](#fig15){ref-type="fig"}](#fig15){ref-type="fig"}) that while there is a fall in the number of bulklike water per surfactant molecule, the number of counterion-bound water rises with an increase in the spacer chain length. It is evidenced by the increase in the degree of counterion dissociation with an increase in the spacer chain length ([Figure [16](#fig16){ref-type="fig"}](#fig16){ref-type="fig"}). On the other hand, the number of each of headgroup and free water almost remains constant. Counterion-bond water is comparatively less polar than the hydrogen-bonded water.^[@ref9],[@ref20]^ It is thus expected that the polarity of the microenvironment of C-490 would decrease with increasing the chain length of the spacer. That is why the average excited-state lifetime of C-490 becomes longer with an increase in the spacer chain length of the gemini surfactants implied by [Figure [6](#fig6){ref-type="fig"}](#fig6){ref-type="fig"}. This result indicates that the opposing effect of an increase in the polarity due to the increasing number of dissociated counterions is subsided by the effect of clustering of water.

![(a) Change in conductivity of reverse micellar systems of the three gemini surfactants with increasing *W*~o~ and (b) variation of the average number of counterion-bound water molecules with *W*~o~ for each gemini surfactant.](ao0c00035_0008){#fig16}

[Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"} shows that a fluorescence decay at a short emission wavelength (e.g., 415 nm) is very fast. Initially, the solvent dipoles are oriented randomly around the solute dipoles created at the excited state and the energy of the system is high. Ideally, the fast decay corresponds to the fluorescence from these unsolvated dipoles. These unsolvated dipoles do not undergo any relaxation process. Of course, because of the limitation of our present time-correlated single-photon counting (TCSPC) set-up (IRF 165 ps), there would be significant contributions from the solvated dipoles as well to the initial fast decay, which could not be detected. The missing components have been analyzed by the method proposed by Fee and Maroncelli,^[@ref88]^ and the values obtained have been tabulated in [Table [2](#tbl2){ref-type="other"}](#tbl2){ref-type="other"}. The values of Δυ̅(=υ̅(0) -- υ̅(∞))are also given in [Table [2](#tbl2){ref-type="other"}](#tbl2){ref-type="other"}. The missing component is found to be increasing, and the Δυ̅ value is decreasing with increasing *W*~o~ in reverse micelles of a given surfactant because of progressive increase in the number of bulklike water.^[@ref49],[@ref50]^ However, a clear growth followed by a slow decay is observed when the decay is recorded at a longer emission wavelength (e.g., 580 nm in [Figure [7](#fig7){ref-type="fig"}](#fig7){ref-type="fig"}). This decay is explicitly from the solvated dipoles. The dipoles created in the excited state first undergo a solvation process followed by a fluorescence decay, which is delayed by the relaxation time^[@ref9],[@ref89]^ and found to have a negative pre-exponential factor. Before measuring solvation dynamics in all reverse micelles, the solvation time of C-490 in the respective bulk solvents has been measured ([Table [2](#tbl2){ref-type="other"}](#tbl2){ref-type="other"}). The solvation in pure *n*-propanol is found to be single exponential with a solvation time of ∼140 ps. However, the average solvation time in bulk solvent (CHX + *n*-propanol) is ∼494 ps with a time constant of ∼257 ps (67%) and ∼959 ps (33%). For a mixture of solvents of different polarities, preferential solvation of the dipolar solute takes place in the solvation shell. As a result, slower solvation occurs in mixed solvent as compared to *n*-propanol.^[@ref51],[@ref90]^ The data in [Table [2](#tbl2){ref-type="other"}](#tbl2){ref-type="other"} show that the solvation dynamics of C-490 in reverse micelles has two solvation components, fast and slow. For 12-5-12 at *W*~o~ = 2, the average solvation time (⟨τ~s~⟩) is ∼380 ps, with time constants for solvation being ∼943 ps (15%) and ∼281 ps (85%). For 12-6-12, the ⟨τ~s~⟩ is ∼453 ps, with time constants for solvation being ∼967 ps (29%) and ∼243 ps (71%). In case of 12-8-12, the ⟨τ~s~⟩ is ∼490 ps, with time constants for solvation being ∼1150 ps (19%) and ∼336 ps (81%). However, at *W*~o~ = 25, the average solvation times are reduced to ∼261, ∼288, and ∼306 ps for 12-5-12, 12-6-12, and 12-8-12, respectively. The corresponding time constants for solvation are as follows: ∼ 965 ps (13%) and ∼156 ps (87%); ∼841 ps (19%) and ∼158 ps (81%); and ∼741 ps (32%) and ∼102 ps (68%). It is noteworthy that the solvation dynamics in bulk water occurs in less than 1 ps time scale.^[@ref31],[@ref32]^ Thus, the results for present reverse micelles imply that the water molecules present in the water pool inside the reverse micelles are responsible for the solvation of C-490. As mentioned above, the solvation dynamics of water is found to be bimodal in nature. Bagchi and co-workers^[@ref20],[@ref91]^ have proposed a multishell continuum model to describe the solvation dynamics in protein and reverse micelles. Their model^[@ref20]^ suggests that the fast and slow solvation components are due to "free" and "bound" water molecules, respectively, those are in dynamic exchange with each other. Apart from water molecules, counterions, spacer group, and polar headgroups of gemini surfactants may also contribute to solvation. However, the polar headgroups of the surfactant molecule are attached to the hydrocarbon tails, and the spacer group is connected to the tails through headgroups. Thus, their mobility is constrained to a considerable extent as they are connected to the long hydrocarbon tails, and it has been reported that the polymer chain dynamics occurs on a very long time scale (∼100 ns).^[@ref62],[@ref92],[@ref93]^ Therefore, water molecules and counterions mostly contribute to solvation. It can be seen from [Figure [10](#fig10){ref-type="fig"}](#fig10){ref-type="fig"} that for the reverse micellar system of each of the gemini surfactants, the average solvation time decreases with increasing *W*~o~. It is due to an increase in the number of "free" water, as described in Bagchi and co-workers' model.^[@ref20]^

It is to be noted that the solvation dynamics in the present reverse micelles of gemini surfactants is found to be much faster than that in the reverse micelles of CTAB, as reported by Hazra et al.^[@ref51]^ There are two possible reasons for the same. First, the solute molecule used by them is Coumarin 480 (C-480), which is comparatively more hydrophobic than C-490 used in the present study. C-480 is mostly located at the interface and reflects the dynamics, which is a collective motion at the interface. That is why they did not observe any significant effect of water loading on solvent relaxation times. However, C-490 migrates from the interface to the water pool with water loading as micelles swell. Second, it is the difference between the size of reverse micelles of CTAB and gemini surfactants. The diameter of the water pool of isooctane/CTAB/1-hexanol/water at *W*~o~ = 22.8 is 5.82 nm and that of dodecane/CTAB/1-hexanol/water at *W*~o~ = 40.6 is 10.8 nm. However, DLS data given in [Table [4](#tbl4){ref-type="other"}](#tbl4){ref-type="other"} show that the hydrodynamic diameter of gemini reverse micelles is larger even at a comparatively lower *W*~o~. Thus, the mobility of water molecules is expected to be relatively faster in the reverse micelles of cationic gemini surfactants as compared to reverse micelles of cationic conventional surfactants. Also, in the present case, the observed decrease in average solvation time with increasing *W*~o~ reveals that the solvent reorganization is dependent on the water loading and water pool size. Our results are similar to the solvation dynamics in AOT reverse micelles reported before.^[@ref3],[@ref50],[@ref94],[@ref95]^ The rate of solvation becomes faster as a result of the migration of dye molecules to the water pool with increasing loading of water in AOT reverse micelles. Levinger and Corbeil^[@ref35]^ have observed the dependence of the solvation rate on water loading in their study with the dye molecule, Coumarin-343, located at the interface of the reverse micelles of SDS and CTAB. However, the faster rate of solvation in the present reverse micellar systems as compared to that in AOT reverse micelles is again because of the bigger water pool size of the former than the latter.

In order to describe the different states of water molecules responsible for bimodal solvation dynamics and also the effects of reverse micelle pool size and the spacer chain length on the dynamics of solvation, we have analyzed the O--H stretching band of water solubilized in the reverse micelles following the method given by Zhao et al.,^[@ref69]^ using FT-IR spectroscopy. The fact that different solvents have been used for the stabilization of present reverse micelles, as mentioned before, we have carried out experiments using FT-IR to find different states of water molecules instead of directly using the results reported by Zhao et al.^[@ref69]^ In the present case, the peaks of the sub-bands are centered at 3235, 3409, 3525, and 3614 cm^--1^ (at *W*~o~ = 20 for 12-5-12 as an example), which correspond to the O--H stretching modes of water as (i) surfactant headgroup-bound, (ii) bulklike, (iii) counterion-bound, and (iv) free water, respectively. After getting the four sub-bands, the average number of water molecules has been calculated as mentioned above and plotted with varying *W*~o~ in [Figure [14](#fig14){ref-type="fig"}](#fig14){ref-type="fig"}. The figure shows that the number of bulklike water molecules per surfactant molecule (*N*~b~) increases sharply with *W*~o~, the number of each of headgroup-bound water per surfactant molecule (*N*~hg~) and counterion-bound water per surfactant molecule (*N*~ci~) increases slowly with *W*~o~, and the number of free water molecules per surfactant molecule (*N*~f~) almost remains constant. Results are in well agreement with that reported by Zhao et al.^[@ref69]^ for their reverse micellar systems except for the slow increase in the number of counterion-bound water as well observed in our study, especially in the case of 12-8-12. The conductivity experiments have been performed to see any change in conductance of reverse micellar systems with increasing water content (*W*~o~) and spacer chain length. [Figure [16](#fig16){ref-type="fig"}](#fig16){ref-type="fig"}a shows that the conductivity increases with an increase in *W*~o~, which means that the degree of dissociation of counterions rises with an increase in *W*~o~. It can also be seen from the figure that the increase in the degree of counterion dissociation is in correspondence to the change in the number of counterion-bound water per surfactant molecule ([Figure [16](#fig16){ref-type="fig"}](#fig16){ref-type="fig"}b). Similar to that reported by Zhao et al.,^[@ref69]^ we have also observed that the number of free water molecules per surfactant molecule is very low. These free water molecules are different from the bulk water molecules^[@ref77]^ and are dispersed among the hydrophobic tails of surfactant molecules.^[@ref69]^ The number of bulklike water molecules per surfactant molecule (*N*~b~) increases with increasing *W*~o~, which causes the gradual swelling of reverse micelles supported by an increase in the hydrodynamic diameter, as discussed above ([Table [4](#tbl4){ref-type="other"}](#tbl4){ref-type="other"}). With a growing water pool size, the interfacial curvature is reduced. As a result, more number of water molecules can hydrate the headgroups and also some *n*-propanol molecules get expelled from the interface. That is why there is a gradual increase in headgroup-bound water per surfactant molecule with increasing *W*~o~.

As mentioned above, according to the model proposed by Bagchi and co-workers,^[@ref20]^ free water and bound water, those are in dynamic exchange with each other, are responsible for fast and slow solvation components, respectively. In the present case, FT-IR results infer that there are two types of bound water molecules: headgroup-bound and counterion-bound. Water molecules designated as free water in the FT-IR study are those that exist in very small quantity in the reverse micelles and are dispersed along the surfactant tails. It is very unlikely that these molecules would take part in solvation as C-490 molecules are mostly present at the micellar interface and in water pool. It is evidenced by the fact that solvation becomes faster with an increasing number of bulklike water molecules per surfactant molecule. The data in [Table [2](#tbl2){ref-type="other"}](#tbl2){ref-type="other"} imply that the time constants for the fast component become shorter with increasing the *W*~o~. Also, the weightage of the fast component is more than that of the slow component. These results thus support the fact that water molecules are mostly responsible for solvation dynamics, and the bulklike water mainly contributes to the faster solvation observed. Also, C-490 molecules are primarily located inside the reverse micelles, and more and more C-490 molecules get transferred from the interface to the water pool with increasing loading of water. Although the number of bulklike water molecules per surfactant molecule increases, the number of free water molecules per surfactant molecule remains unchanged with increasing pool size. As stated, the number of bulklike water molecules per surfactant molecule increases rapidly, whereas there is a small increase in the number of headgroup-bound and counterion-bound water molecules per surfactant molecule with increasing *W*~o~. The rate of solvation increases with increasing *W*~o~ because the contribution of bulklike water outweighs that of bound water. Thus, as mentioned above, the bulklike water molecules are mostly responsible for the fast solvation and headgroup- and counterion-bound water molecules are responsible for the slow solvation. The data of decay parameters given in [Table [2](#tbl2){ref-type="other"}](#tbl2){ref-type="other"} support the results which show that the relative contribution toward the solvation by the fast component is significantly greater than that by the slow component. It is important to note that in the present case, the bulklike water plays the same role as free water plays in the model proposed by Bagchi and co-workers.^[@ref20]^ Also, the bulklike water solubilized in the reverse micelles is not the same as neat bulk water in the true sense. The dynamics of bulklike water in confined water pool of reverse micelles is much slower than that of the neat bulk water. According to Senapati and Chandra,^[@ref96]^ the retardation of the solvation rate in a cavity is because of the slower orientational relaxation of the solvents.

In order to find out the reasons behind the decrease in the solvation rate with an increase in the spacer chain length at a given *W*~o~ \[[Figure [10](#fig10){ref-type="fig"}](#fig10){ref-type="fig"}\], the dependence of the number of different states of water molecules per surfactant molecule on the spacer chain length has been studied. [Figure [15](#fig15){ref-type="fig"}](#fig15){ref-type="fig"}b represents the relationship between the number of different states of water molecules per surfactant molecule (*N*~i~) and the spacer chain length of gemini surfactants at *W*~o~ = 20 as a representative. One can see from this figure that, with an increase in the spacer chain length, while the number of bulklike water molecules per surfactant molecule decreases, the number of counterion-bound water molecules increases. The degree of counterion dissociation enhances with an increase in the spacer chain length at a given *W*~o~, as evident from the conductivity data shown in [Figure [16](#fig16){ref-type="fig"}](#fig16){ref-type="fig"}a. This result is in support of an increase in the number of counterion-bound water with an increase in the spacer length.^[@ref63],[@ref71]^ Because of this reason, more and more clustering of water molecules occurs with an increasing the chain length of the spacer. Therefore, the proportion of "bound" water molecules increases. On the other hand, the proportion of bulklike water molecules that act as so-called "free" water molecules decreases with increasing spacer chain length. The numbers of headgroup-bound water molecules and that of free water molecules per surfactant molecule remain almost unchanged. DLS data ([Table [4](#tbl4){ref-type="other"}](#tbl4){ref-type="other"}) show that the size of the reverse micelles decreases with an increase in the spacer chain length. As a result of it, the mobility of water molecules becomes restricted and the solvation process becomes slower with an increase in the spacer chain length at a given *W*~o~. Thus counterions indirectly control the rate of the solvation process. These results further indicate that the collective motion at the interface, including the restricted motion of clustered water molecules hydrogen-bonded with Br^--^ ions, constrained *n*-propanol, and a few water molecules bound to the surfactant could contribute to slower solvation. It is noteworthy that some contribution from *n*-propanol to the solvation dynamics in the water pool cannot be totally ruled out.

The time-resolved fluorescence anisotropy decays were recorded to know more about the microenvironment of C-490 in the reverse micelles of gemini surfactants. The rotational relaxation time of C-490 in *n*-propanol is slower than that in the bulk solvent ([Table [3](#tbl3){ref-type="other"}](#tbl3){ref-type="other"}). We could not measure the rotational relaxation time in cyclohexane because of the limitations of our setup. It must be faster than that in the bulk solvent. Hazra et al.^[@ref51]^ reported faster rotational relaxation of C-480 in cyclohexane as compared to cyclohexane/1-hexanol bulk solvent. Comparatively, slower rotational relaxation in *n*-propanol is due to the formation of the hydrogen bonds between C-490 molecules and *n*-propanol. The rotational relaxation of C-490 in reverse micelles is slower than that in the bulk solvent ([Table [3](#tbl3){ref-type="other"}](#tbl3){ref-type="other"}), which implies that C-490 molecules are mostly present inside the micelles. The data in [Table [3](#tbl3){ref-type="other"}](#tbl3){ref-type="other"} also depict that the anisotropy decays in all reverse micelles are bimodal in nature. The fast rotational relaxation is a more abundant (∼95%) component than the slow rotational relaxation in all reverse micellar systems. Therefore, the fast rotational motion of C-490 is mainly responsible for depolarization to take place and the anisotropy to become zero. Data also show that while the rotational relaxation time for the fast component decreases with *W*~o~, there is no clear trend in the change in relaxation time for the slower component. However, the relative amplitude or the relative contribution toward the anisotropy decay from the fast component increases and the same from the slow component decreases with increasing water pool size. The average rotational relaxation time decreases on increasing *W*~o~ ([Figure [12](#fig12){ref-type="fig"}](#fig12){ref-type="fig"}). Rotational relaxation data thus reflect that C-490 molecules progressively experience a more flexible microenvironment with increasing water loading. It is because the number of bulklike water molecules per surfactant molecule increases rapidly with increasing *W*~o~ ([Figure [12](#fig12){ref-type="fig"}](#fig12){ref-type="fig"}), the reverse micelles progressively swell, and the dye molecules migrate from the interface to the water pool. Also, the fact that the fast component is mostly contributing toward the anisotropy decay and there is an increase in the contribution from the fast component and a concomitant decrease in the contribution from the second component with increasing *W*~o~, the bi-exponential anisotropy decay is primarily due to two different locations of C-490 molecules. Even if the slow rotational motion is due to the lateral diffusion along the interface of reverse micelles and/or rotational motions of micelles as a whole, that contribution is negligibly small (only ∼5%). [Figure [12](#fig12){ref-type="fig"}](#fig12){ref-type="fig"} also shows that the average rotational relaxation time increases with an increase in the spacer chain length of gemini surfactants at a given *W*~o~. It has been discussed above that with the increasing the chain length of the spacer, the water pool size is reduced and also the degree of counter ion dissociation is increased at a given *W*~o~. As a result of these, the number of bulklike water per surfactant molecule decreases and the number of counterion-bound water per surfactant molecule increases ([Figure [15](#fig15){ref-type="fig"}](#fig15){ref-type="fig"}). Both these factors are in favor of increasing rigidity of the microenvironment around C-490. That is why, rotational relaxation process becomes slower with increasing spacer chain length of gemini surfactants for a fixed water content.

4. Conclusions {#sec4}
==============

The microenvironment around C-490 in the reverse micelles becomes more polar with an increase in the water pool size because of progressive increment in bulklike water molecules and also the migration of C-490 to the water pool from the interface. At a given *W*~o~, the polarity of the microenvironment decreases with an increase in the spacer chain length of gemini surfactants. The solubilized water in the reverse micelles has four different states of water: headgroup- and counterion-bound water, bulklike water, and free water. While headgroup- and counterion-bound water molecules contribute to slow solvation, the bulklike water molecules contribute to fast solvation. Free water molecules do not contribute to any solvation process. The fast component contributes more to the solvation as compared to the slow component as the bulklike water is more abundant. The solvation rate increases with increasing *W*~o~ because of the sharp increase in the number of bulklike water molecules per surfactant molecule. An increment in the rate of the rotational relaxation process with increasing *W*~o~ is due to gradual swelling of reverse micelles. With an increase in the spacer chain length at a given *W*~o~, the degree of counterion dissociation increases and the size of the water pool decreases. As a result, the number of counterion-bound water molecules increases and that of bulklike water molecules decreases. Because of greater extent of clustering of water molecules with an increment in the spacer chain length, there is progressively an increase in contribution of bound-water molecules toward slow solvation and a concomitant decrease in contribution of bulklike water molecules toward fast solvation. Thus, the rate of solvation decreases. Also, because of the progressive increase in the rigidity of the water molecules, the rate of rotational relaxation process of C-490 decreases with increasing spacer chain length. An indirect effect of counterions on the rates of solvation and rotational relaxation has been noted. The faster rates of solvation and rotational relaxation in the present reverse micelles as compared to the reverse micelles of conventional cationic surfactants is due to a comparatively bigger size of the water pool of the present reverse micelles. The mobility of water molecules is relatively faster in the water pool of reverse micelles of cationic gemini surfactants as compared to conventional cationic surfactants. The fact that reverse micelles behave as biomimicking systems and water has a great role in various biological processes, the study of comprehensive effects of different states of solubilized water in the reverse micelles on the solvation dynamics and rotational relaxation processes and how these are controlled by the spacer chain length of gemini surfactants have practical and fundamental importance.

5. Experimental Section {#sec5}
=======================

Three gemini surfactants with different spacer chain lengths, 12-*s*-12, 2Br^--^ (*s* = 5, 6, and 8) were synthesized following the reported method^[@ref56],[@ref57]^ and recrystallized with methanol and ethyl acetate mixture several times. The structures of the synthesized compounds ([Scheme [1](#sch1){ref-type="scheme"}](#sch1){ref-type="scheme"}) were confirmed by ^1^H NMR and FT-IR data. The details of ^1^H NMR and FT-IR data for 12-5-12 have been provided in Table S1, [Supporting Information](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf), whereas for 12-6-12 and 12-8-12, the details are provided in our previous publication.^[@ref63]^ C-490, procured from Sigma-Aldrich, was used as the fluorescent probe for the studies and was used without any further purification. Zhao et al.^[@ref69]^ prepared reverse micelles in *n*-heptane with the assistance of *n*-hexanol. However, in the present study, with C-490 as a fluorescent solute, we could prepare reverse micelles in cyclohexane (CYH) as a nonpolar bulk phase and *n*-propanol as the cosurfactant. Stable reverse micelles were formed only after the addition of *n*-propanol. All the solvents used were of spectroscopic grade purchased from Spectrochem Chemical Company, India. The water pool of the reverse micelles was prepared using Milli-Q water, obtained from a Millipore water filtration system. The final concentration of C-490 and that of gemini surfactants were maintained as 0.01 and 20 mM, respectively, in all the measurements. A stock solution of the surfactant was prepared in CYH and *n*-propanol. The reverse micelle solutions were prepared by the addition of the calculated amount of each of the surfactant from the stock solution followed by the addition of the estimated amount of water. A final concentration of *n*-propanol was maintained as 10% by adding extra *n*-propanol. Then, CYH was added to make up the solutions to the desired volume. Finally, the solutions were shaken gradually until they were optically transparent. The solutions were transparent once *n*-propanol was completely dispersed in the system at the final. All UV--visible absorption and steady-state fluorescence measurements were performed on a JASCO V-650 UV--visible spectrophotometer and a HORIBA Jobin Yvon FluoroMax-4 scanning spectrofluorometer, respectively. The steady-state fluorescence measurements, both excitation and emission spectra, were recorded with a slit width of 1 nm. The emission spectra were corrected for instrument sensitivity. A HORIBA Jobin Yvon Fluorocube-01-NL picosecond TCSPC experimental setup was used to record the time-resolved measurements. The excited singlet-state lifetimes were determined from the intensity decays using the same setup. The light source used was a picosecond diode laser of a wavelength of 375 nm (NanoLED 375L, IBH, UK) with an instrument response function of about 165 ps. A TBX-photon detection module (TBX-07C) was used to detect the fluorescence signals at a magic angle of 54.7° polarization. The time-resolved fluorescence anisotropy measurements were performed using the same TCSPC set up. All the spectroscopic measurements were carried out at room temperature, 298.15 ± 1 K. The time-resolved fluorescence decays and anisotropy data were analyzed using IBH DAS-6, decay analysis software. Each decay profile was fitted with bi or triexponential fitting, keeping in mind the goodness of the fit, judged by the χ^2^ criterion and visual inspection of the residuals of the fitted function to the data. The χ^2^ value must lie between 1 and 1.2. The average excited singlet state lifetime, ⟨τ~f~⟩, for a biexponential intensity decay has been calculated by using [eq [1](#eq1){ref-type="disp-formula"}](#eq1){ref-type="disp-formula"}where α~1~ and α~2~ are relative amplitudes, and α~1~ + α~2~ = 1. τ~1~ and τ~2~ are the lifetimes of the two components.

The solvation dynamics studies were carried out using the methodology proposed by Fleming and Maroncelli.^[@ref74]^ The decay profiles at different emission wavelengths across the entire range of a steady-state emission spectrum were obtained. The TRES at different times were constructed from the appropriately normalized intensity decay functions for the various wavelengths at various times. The TRES at each time then were fitted to a log normal fitting, and the peak wavenumber at each time, υ̅(*t*), was acquired.^[@ref74],[@ref78],[@ref89]^ The obtained peak wavenumbers were used to get the SCF, *C*(*t*), for the quantitative measurement of solvation dynamics of the probe using [eq [2](#eq2){ref-type="disp-formula"}](#eq2){ref-type="disp-formula"}where, υ̅(0), υ̅(*t*), and υ̅(∞) are the peak wavenumbers at time zero, *t*, and infinity, respectively.

The time constants of the observed solvation were obtained after fitting the plot of SCF, *C*(*t*) versus time. The biexponential function was used to obtain time constants for solvation using [eq [3](#eq3){ref-type="disp-formula"}](#eq3){ref-type="disp-formula"}where τ~1s~ and τ~2s~ are the solvent relaxation times of corresponding relative amplitudes, α~1s~ and α~2s~, respectively; α~1s~ + α~2s~ = 1. The average solvation time, ⟨τ~s~⟩, for a biexponential decay is calculated by using [eq [4](#eq4){ref-type="disp-formula"}](#eq4){ref-type="disp-formula"}

The time-resolved fluorescence anisotropy, *r*(*t*), was calculated using [eq [5](#eq5){ref-type="disp-formula"}](#eq5){ref-type="disp-formula"}where *G* signifies the correction factor for the detector sensitivity to the polarization detection of emission, and *I*~\|\|~(*t*) and *I*~⊥~(*t*) denote fluorescence decays polarized parallel and perpendicular to the polarization of the excitation light, respectively. The value of *G*-factor for our instrument is ∼0.6. The biexponential anisotropy decay function can be described using [eq [6](#eq6){ref-type="disp-formula"}](#eq6){ref-type="disp-formula"}where *r*~o~ stands for the limiting anisotropy which represents the inherent depolarization of the probe molecule, τ~1r~ and τ~2r~ represent first and slow rotational relaxation times, respectively, of the probe molecule, and α~1r~ and α~2r~ are the corresponding relative amplitudes, where α~1r~ + α~2r~ = 1. The average rotational relaxation time for the biexponential anisotropy decay is given by [eq [7](#eq7){ref-type="disp-formula"}](#eq7){ref-type="disp-formula"}

The FTIR spectrum of each reverse micellar system was recorded from 1000 to 4000 cm^--1^ with a ABB Bomen MB 3000 FTIR spectrometer. NaCl (25 × 4 mm) FTIR cell windows with path length 0.05 mm were used to record the spectra. Each spectrum was recorded at 32 scans at a resolution of 1 cm^--1^ at room temperature. The absorption of the bulk solvent (cyclohexane and *n*-propanol) was taken as the reference for all cases. The stretching frequency region of the O--H of water molecules (3000--3800 cm^--1^) was fitted with the help of software, presented according to the Gaussian fittings. The deconvoluted peaks help to study about the different states of water present in the reverse micelles. An instrument Zeta Sizer, model Nano ZS (ZEN 3600, Malvern Instruments, UK) was used to carry out the DLS measurements to know about the hydrodynamic size of the reverse micelles. Each sample was filtered before the measurements with a 0.22 μm filter (Durapore, PVDF). Laser light of wavelength 632.8 nm was used, and 173° was kept as the scattering angle. The size distribution was carefully judged by considering the corresponding *G* function. The shape and size of the reverse micelles were also known with the help of FE-SEM (FEI-Apreo S). The samples were drop-casted over silicon wafer and dried to prepare a thin film. The samples were then spin-coated with 2 nm silver before recording the images.

The Supporting Information is available free of charge at [https://pubs.acs.org/doi/10.1021/acsomega.0c00035](https://pubs.acs.org/doi/10.1021/acsomega.0c00035?goto=supporting-info).FTIR and ^1^H NMR data for gemini surfactant 12-5-12.2Br^--^, steady-state fluorescence spectra of C-490 excited at 412 nm in the pure solvents and reverse micellar systems of 12-5-12.2Br^--^, FWHM of fluorescence bands of C-490 in all reverse micellar systems at various excitation wavelengths, excited singlet state lifetimes of C-490 in pure solvents and all reverse micellar systems, OH-stretching bands of water solubilized in all the reverse micellar systems, plot for variation in the area fraction of different water species with *W*~o~ in reverse micelles of all the three gemini surfactants, and a note on the calculation of the average number of each of the water species per surfactant molecule ([PDF](http://pubs.acs.org/doi/suppl/10.1021/acsomega.0c00035/suppl_file/ao0c00035_si_001.pdf))

Supplementary Material
======================

###### 

ao0c00035_si_001.pdf

^§^ Department of Chemistry, Birla Institute of Technology and Science (BITS) Pilani, Hyderabad Campus, Jawahar Nagar, Kapra Mandal, Medchal District, Telangana-500078, India (S.K.S. and R.A.).

The authors declare no competing financial interest.

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