Spectroscopic Studies on the Interaction of Dye and Surface Active

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Spectroscopic Studies on the Interaction of Dye and Surface Active Ionic Liquid Nitai Patra, Bithika Mandal, and Soumen Ghosh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02052 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 16, 2017

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Industrial & Engineering Chemistry Research

Spectroscopic Studies on the Interaction of Dye and Surface Active Ionic Liquid Nitai Patra, Bithika Mandal* and Soumen Ghosh* Centre for Surface Science, Department of Chemistry, Jadavpur University, Kolkata700032, India, E-mail: [email protected] Abstract The interaction of both cationic dye Safranine T (ST) and anionic dye Congo red (CR) with anionic and cationic surface active ionic liquid 1-butyl-3-methyl imidazolium octyl sulphate ([BMIM] [OS]) and 1-decyl-3-methyl imidazolium chloride ([DMIM] [Cl]) has been investigated by absorbance and emission spectroscopy, time resolved fluorescence study and anisotropy method at premicellar and post micellar region. The interaction of dye with both the ionic liquids occurred electrostatically as well as hydrophobically. In case of ST, initially absorbance decreases up to a certain concentration without any shift of λmax, and then it increases with red shift of λmax. Absorption spectra of CR gave red shifted wavelength with addition of [BMIM] [OS], but at higher concentration of surface active ionic liquid [SAIL], no shifting was observed. Again, blue shifted λmax was found in lower range of [DMIM] [Cl]; but at higher concentration, it was further red shifted. Emission intensity increases in both dye-SAIL systems; for ST in both SAIL media, blue shifted spectra were observed, but there was no shift of emission maxima in case of CR in those media. Dye-IL binding ratio, binding sites and binding constants have also been calculated from fluorescence measurement. Anisotropy measurement showed that movement of dye in pre and post-micellar regions was different in different SAIL systems. Time resolved fluorescence lifetime confirmed microenvironment of dye-SAIL systems.

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1. Introduction Room temperature ionic liquids are composed of cations and anions. They have unique physical properties such as low vapor pressure, low melting point, high thermal stability, high ionic conductivity, favorable solvating properties for a range of polar and non-polar compounds and a wide liquidous temperature range.1-6 SAILs are used in various fields, such as, fuel, catalysis, electrolysis, photochemical and rechargeable cells, optical fluids, nanoparticle synthesis, etc.4-10 These are considered as environmentally benign solvents due to their nonvolatility to prevent pollution.11-13 Photophysical, theoretical and ultrafast spectroscopic studies in SAILs have been reported in literature.14-18 Self-aggregation behavior of SAILs in aqueous solution has been subject of researchers on mixtures19-21 and such research contributed to understand the properties, structure and interactions in IL-containing mixtures, and even in pure SAILs. Long chain imidazolium based SAILs have physicochemical versatility, easier availability and such SAILs exist in hydrophilic, hydrophobic, and amphiphilic forms.12, 22 Among them, 1-alkyl-3-methyl imidazolium based SAILs have attracted much attention for both academic and application purposes.23, 24 These SAILs possess an inherent amphiphilicity. In our work, anionic 1-butyl-3-methyl imidazolium octyl sulphate [BMIM] [OS] and cationic 1-decyl-3-methyl imidazolium chloride [DMIM] [Cl] have been used where surface activity and aggregation behavior are available in literature.25, 26 Generally, amphiphiles behave strongly with oppositely charged dyes due to electrostatic, hydrophobic interactions. Sometimes the electrostatic repulsion can be reduced by increasing the attractive interactions through hydrogen bonding between ionic part of alkyl chains and solvents. Such types of interactions have wide applications in medicinal, environmental, biological and analytical fields.12, 27, 28The interactions of dyes with amphiphilic SAILs which called surface active ionic liquid (SAIL) have been investigated spectroscopically in the present work. Regarding this, congo red (CR) and safranine T (ST)


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have been chosen as dyes. CR is an anionic red colored benzidine dye (scheme 1) that may interact with SAIL by electrostatic forces through the sulfonic groups or by hydrophobic interactions through the conjugated π-electron system. CR is used in histological staining especially as a cytoplasm and erythrocyte stain to demonstrate the presence of amyloidosis in fixed tissues.29-30 Safranine T (Scheme 1) is an aromatic reddish brown photo sensitive phenazine water soluble cationic dye whose ground and excited state spectra are affected by the SAILs.32 Safranine T is also used in histology and cytology as a biological stain. Here we carry out a systematic investigation by UV-visible and fluorescence spectroscopy, time resolved fluorescence study and anisotropy measurement to explore the nature of interaction and spectral changes of these two dyes with cationic and anionic surface active ionic liquids (SAILs). Literature survey shows that there are no interactions between same charged dyemicelle systems; but in the present study same charged IL micelle-dye exhibit some kind of interaction.

Scheme 1: Structures of dyes and surface active ionic liquids



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2. Materials and Methods 2.1 Materials Safranine T (ST) and congo red (CR) are purchased from Fluka. SAILs 1- butyl – 3 – methyl imidazolium octyl sulphate - ([BMIM] [OS]) and 1- decyl - 3 – methyl imidazolium chloride (purities > 98%) were purchased from Sigma Aldrich (USA). SAILs were dried under vacuum at 300 C for 48 hours to remove moisture. These reagents were used as received. Double distilled deionized water is used for preparations of all samples. 2.2. Preparation of dye solution A 1.0 mM of stock solution of safranine T and 2.5 mM of congo red were prepared by addition of known weight of the respective compound in water. Then the mixture was sonicated to produce a clear solution. The required experimental concentration of solution of dye was prepared from stock solution by dilution. 2.3. UV–visible absorption studies Absorption measurements have been performed using a UV 1601 Shimadzu (Japan) spectrophotometer where a 10 mm path length quartz cuvette was taken for solution. The spectra have been taken in 400–600 nm wave-length range. The 10 µl stock solution of dye was added to 2.5 ml of water to reach a final concentration of 0.004 mM for safranine T and 0.01 mM of congo red. Regarding this, technical details are available elsewhere32, 33. The absorbance intensity was measured at 521 nm wavelength for ST and at 499 nm for CR maintaining temperature at 298 K. 2.4. Fluorescence emission studies The fluorescence emission spectra and anisotropy of dye solution have been determined using a Perkin Elmer LS 55 fluorescence spectrophotometer with a quartz cuvette of 10 mm path length. A 10 µl of the stock solution of dye was added to 2.5 ml of water to attain a concentration of 0.004 mM for ST and 0.01 mM for CR. The details are reported in


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the literature.32-35 Emission spectra are recorded after excitation at 521 nm for ST and at 340 nm for CR. For ST, the fluorescence spectra were measured from 540 to 700 nm with excitation and emission slit widths fixed at 9 nm and 2.5 nm, respectively. For CR, fluorescence spectra were determined from 365 - 650 nm with excitation and emission slit widths fixed at 12 nm and 6 nm, respectively. The scan time was fixed at 250 nm per minute. Anisotropy is measured at the wavelengths of excitation at 521 nm and emission at 580 nm for ST whereas that is at 340 nm and 420 nm for CR respectively. The measured anisotropy value was the average of six consecutive values. The sample temperature was maintained at 298 K before each measurement. 2.5. Time resolved fluorescence study Time resolved fluorescence decay was measured by the time-correlated single photon counting (TCSPC) technique in Horiba–Jobin–Yvon FluoroCube fluorescence lifetime system using NanoLED at 370 nm and 490 nm (IBH, UK) as the excitation sources for ST and CR respectively and TBX photon detection module as the detector. The decays data were fitted using IBH DAS-6 decay analysis software. The lamp profile was collected by placing dilute micellar solution of sodium dodecyl sulfate in water as a scatterer in place of the sample. Accuracy of fits was evaluated from χ2 criterion and visual inspection of the residuals of the fitted function to the data. Mean (average) fluorescence lifetimes (τavg) for biexponential iterative fittings were calculated from the decay times (τ1 and

τ2) and the pre-

exponential factors (a1 and a2) by using the following relation36. τavg = a1τ1 + a2 τ2 3. Results and Discussions 3.1 Absorption Spectra: In aqueous solution, cationic ST shows absorption maxima at 521 nm. The effect of ST in anionic [BMIM] [OS] was shown in Fig 1. (A).With the addition of SAIL up to 12.4 mM 5|Page ACS Paragon Plus Environment


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concentrations, the absorbance decreases without any shift of λmax. After that concentration, a gradual red shift from 521 nm to 533 nm was observed up to 47 mM [SAIL]; with further addition, absorbance increases without any change of λmax (533 nm). Absorbance vs [SAIL] plot was given in the inset of Fig 1(A) which shows that up to 40.4 mM [SAIL] (CMC of [BMIM] [OS] = 34 mM25) absorbance decreases, then increases and becomes constant at above 53 mM [SAIL]. The interaction pattern between ST with cationic SAIL was quite different from anionic SAIL. Literature survey showed that there was no change of absorption spectra with the interaction between ST and conventional cationic surfactant, 37, 33 but our investigation (Fig 1B) shows that cationic SAIL [DMIM] [Cl] interacts with ST although the interaction was weak compared to anionic SAIL. Absorbance vs. [SAIL] plot was shown in inset of Fig 1 (B).

Fig. 1: Absorbance of ST with varying [BMIM][OS] (A) and [DMIM][Cl] (B) at 521 nm. (Inset shows the corresponding absorbance vs. [SAIL], standard deviation in the Fig. is in the range of 0.2-0.5%). Initially, the absorbance decreases up to 30.74 mM (although the rate of decrease was very low) without any shift of λmax; after that, λmax started to shift at higher wavelength from 521 to 528 nm at 80.5 mM SAIL. With further addition of SAIL, λmax was not changed. 6|Page ACS Paragon Plus Environment

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Absorbance decreases up to 51.73 mM (CMC of [DMIM] [Cl] is 51.7 mM), 38 then increases up to 80.5 mM and then became constant. The initial decrease of absorbance in both the cases is due to formation of dye – SAIL aggregates. Red shift of the λmax indicates the aggregation process where ion-pair aggregates are formed. Such types of spectral change of ST can be explained on the basis of electrostatic interaction adjacent chains of SAIL and organic portion of dye following the changes of the microenvironment of chromophore32.

Fig. 2: Absorbance of CR with varying (A) [BMIM][OS] and (B) [DMIM][Cl] at λmax 343 nm and 499 nm. (Inset shows the corresponding absorbance vs. [SAIL], standard deviation in the Fig. is in the range of 0.2-0.5%).



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With increasing [SAIL], these molecules tend to form the micelle and leave the dye molecule; as a result, absorbance increases up to a certain concentration, and after that, absorbance intensity was constant at high micellar concentration, indicating solubilisation of dye molecules into the micelles. The absorbance spectra of anionic dye congo red (CR) with addition of SAILs were shown in Fig 2 (A, B). With the addition of the anionic SAIL [BMIM] [OS], absorbance decreases with shifting of λmax at higher wavelength. CR shows λmax at the wavelength of 499 nm; at 33.7 mM of [BMIM] [OS], λmax shifts 11 nm and then no change of λmax is observed, only intensity of CR increases. Here, actually dye–SAIL complex is formed and its intensity decreases with red shift. With increasing concentration of anionic SAIL, micelle is formed and then SAIL molecules leave the dye molecule following the increase of value. In this case, at higher concentration dye molecules are not solubilised into the micelle due to strong electrostatic repulsion.

Fig. 3: Change in absorbance maxima (shifting of frequency) as a function of concentration of ionic liquids-(A) ST and (B) CR. The interaction between cationic SAIL with CR was totally different from other cases. It shows one peak at 343 nm and another at 499 nm. Fig 2(B) shows that the addition of SAIL (2.39 mM to 7.11 mM) resulted in a blue shift of the CR absorption peak by 35 nm (from 499 8|Page ACS Paragon Plus Environment

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nm to 464 nm ) accompanied by decrease of absorption intensity. Further addition of SAIL, a gradual red shift of spectra (466 nm to 482 nm) along with slight increase in the absorbance intensity was observed although it was blue shifted relative to λmax of free CR. Similar types of dye – surfactant interaction are also reported in literature.29 The initial blue shifted spectra most likely cause dye aggregation with formation of an ion pair. This is probably owing to the formation of H – aggregated structure in parallel arrangement.39 The parallel orientation introduced a blue shift due to π – π stacking interactions, as there was no isosbestic point observed in the spectra, signifying the higher order dye-aggregate formation.40 At higher [SAIL], the electrostatic interaction between cationic head group, [DMIM] + of the [DMIM] [Cl] with CR prefers to the cluster association within the polar region of the hydrated surface of the [DMIM] [Cl] head group, resulting to red shift spectra. With increase in concentration of SAIL, micelle formation starts and finally, the aggregates dissociate. The isolated dye molecule is then associated with the surface of micelles, as a result, the intensity increases. Fig. 3 represents the change in absorbance maxima in terms of shifting of frequency of the spectra of dye molecules in different concentrations of SAILs. In Fig. 3 (A), higher shifting of frequency is observed in ST[BMIM][OS] system compared to ST-[DMIM][Cl]. In both cases, a decreasing tendency of shifting of frequency is observed with increasing [SAIL]. In Fig. 3 (B), the opposing tendency of shifting of frequency is observed in cases of CR with two SAILs. 3.2 Steady State Emission Spectra Measurements: ST shows maximum emission maxima at 580 nm. With addition of [BMIM] [OS], initially intensity was not changed up to 5 mM, then increases with a considerable hypsochromic shift (~15 nm) in spectral maxima shown in Fig. 4(A). Due to increasing the hydrophobicity of the media for the presence of aromatic groups of ST, probe molecules were moving from polar aqueous phase to the relatively non-polar surface of the micelles; as a result, blue shift of the



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emission spectra was observed. At micellar region, from 33 to 53 mM [SAIL], the intensity sharply increases for strong interaction between anionic micelles with cationic dye. After micellisation, dye-SAIL complex breaks down and intensity does not increase; it remains almost constant after 53 mM of [BMIM] [OS]. The dramatic change of emission intensity and blue shift are due to strong interaction of ST with [SAIL]. In case of [DMIM] [Cl], emission spectra gradually increased, up to 13 mM, λmax remained fixed at 581 nm and after that, it was ~10 nm blue shifted up to the maximum concentration. Literature survey32 showed that there was no change of emission intensity with the interaction of same charged dye with surfactant system.

Fig. 4: Fluorescence intensity of dye with change in [SAIL], (A) ST-[BMIM][OS], (B) ST[DMIM][Cl] , (C) CR-[BMIM][OS] and (D) CR-[DMIM][Cl]. (In inset fluorescence spectra of corresponding dye-SAIL are given.)

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Here, the interaction pattern of ST with cationic SAIL was quite different; owing to electrostatic repulsion and hydrophobic interaction, emission intensity increases with hypsochromic shift. In this case, blue shift of λmax is less compared to ST-[BMIM][OS], results the situation of the dye in less polar region, shown in Fig. 4 (B). Emission spectra of CR (Fig 4 C, D) show λmax in water at 417 nm, with addition of SAILs in both the cases, emission intensity increases without any shift of λmax. CR does not experience any polarity change when both of SAILs are added from pre- to post-micellar concentration. But, emission intensity increases in both cases of SAILs. The change in emission intensity is comparatively low due to hydrophobic repulsion between dye and SAILs. A pattern of change of emission intensity was observed when a close look will be there in intensity vs [SAIL] graphs. After micellisation of [BMIM][OS] there is increase in emission intensity is saturated to some extent but smoothly with the addition of [DMIM][Cl].

Fig. 5: Modified Stern-Volmer plot for (A) ST-[BMIM][OS], (B) ST-[DMIM][Cl], (C) CR[BMIM][OS] and (D) CR-[DMIM][Cl] systems.



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Modified Stern - Volmer equation (1) was used for calculating the number of binding sites at dye molecule and binding constant of dye-surfactant system. The equation is shown below:41, 42

log[( F − F0 ) / F ] = log K ' + β log[SAIL]

(1)

where F0 and F denote the fluorescence intensities in the absence and presence of quencher, respectively K' is the binding constant and β is the binding affinity, i.e., the number of quencher molecules interacting per site. The values of binding constant (K') and binding sites are listed in Table 1. This shows that in case of CR-SAILs systems, three and two binding sites are present in [BMIM][OS] and [DMIM][Cl] systems respectively, but in case of ST[BMIM] [OS] system three binding sites whereas in ST- [DMIM] [Cl] combination,one binding site was present. Binding constant (K') obtained in the second region of ST[BMIM][OS] system is much higher compared to others dye-SAIL. Results are consistent with the outcomes of the emission intensity vs [SAIL] plot, where similar regions are obtained, as other groups have shown.42-45 Table 1: Binding constants and regions of the dye molecules with SAILs. System

ST-[BMIM][OS]

I

Binding Constant (K') (M-1) 12.60

1.08

II

6112.93

2.91

III

0.83

0.04

3.26

0.85

I

10.13

0.52

II

1.34

0.11

III I

1.04 5.01

0.03 0.49

II

1.17

0.11

Region

ST-[DMIM] [Cl]

CR-[BMIM][OS]

CR-[DMIM] [Cl]


β

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Modified Benesi-Hildebrand equation46,

47

gives an idea about the binding between dye-

surfactant interactions. We have exploited titration data to determine the binding pattern of dye-SAIL systems following the equation (2): 1 1 1 1 = + ∆F ∆Fmax K∆Fmax [ SAIL]n

(2)

Where, ∆F = Fx-F0 and ∆Fmax = F∞-F0; F0, Fx, and F∞ are fluorescence intensities of dye in absence of SAIL, at an intermediate SAIL concentration and final SAIL concentration respectively. K and n denote the binding constant and stoichiometric coefficient respectively.

Fig. 6: Double reciprocal plot for the binding of ST with (A) [BMIM][OS] and (B) [DMIM][Cl]; combinations and reciprocal profile of dependence of CR with (C) [BMIM][OS] and (D) [DMIM][Cl] systems. Plots of

1 1 vs. gives a straight line indicating 1:1 interaction between CR and ∆F [ SAIL]n

[BMIM] [OS], the binding constant extremely low; for CR-[DMIM] [Cl] it is 7.11 M-1. The 13 | P a g e ACS Paragon Plus Environment


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ratio of interaction of ST with SAIL is 1:2. Binding constants for ST- [BMIM] [OS] and ST[DMIM][Cl] systems were 5.29 ×103 M-1 and 158.1 M-1 respectively. Relatively, low value (normal value being in the range of 103 to 104 M-1)33,

48

of binding constant of ST-

[DMIM][Cl] is due to unfavourable situation because of similar electrostatic character of the probe and the SAIL (being both positive). The graphs are given in Figure 6. The binding constants are comparable to the data obtained from Modified Stern Volmer equation analysis.

3.3 Fluorescence polarization anisotropy in surfactant solution: The fluorescence anisotropy (r) can be defined as

and G factor is defined as,

r=

( I v − GI h ) ( I v + 2GI h )

G=

Iv Ih

where, Iν and Ih indicate the respective fluorescence intensities of the vertically and horizontally polarized emission when the sample is excited with vertically polarized light. The G factor denotes the ratio of the sensitivities of the detection system for vertically and horizontally polarized light32, 34 with fluorescence polarization anisotropy.

Fig. 7: Anisotropy of ST and CR with varying (A) [BMIM][OS] and (B) [DMIM][Cl]. In case of ST, with addition of SAIL initially anisotropy was constant up to 20 mM for [BMIM] [OS]; after that anisotropy increases and becomes constant at 64.4 mM; for [DMIM] 14 | P a g e ACS Paragon Plus Environment

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[Cl] up to 47.29 mM anisotropy values remain almost constant and then increase till the concentration of the solution reached at 110.6 mM and the constant plot was observed. At low concentration of SAIL, no significant restriction was maintained and so same value of r was observed (Fig. 7A). After a certain concentration (33 mM for [BMIM] [OS] and 48 mM for [DMIM] [Cl]), SAIL started to form micelles and dye molecules entered into the micellar hydrophobic core following the decrease of rotational movement and then anisotropy increased. After complete micellisation (64 mM for [BMIM] [OS] and 110 mM for [DMIM] [Cl]), saturated restriction of rotational movement was possible and as a result, anisotropy was constant.49 The anisotropy value was greater in oppositely charged dye – SAIL system than the same charged dye–SAIL one. In oppositely charged system, due to electrostatic attraction, the rotation was more restricted than same charging species for electrostatic repulsion. In case of CR dye, anisotropy value was initially constant up to 5 mM with addition of [BMIM] [OS] and 2 mM for [DMIM] [Cl]; after this concentration, ‘r’ increases with a hump at 19.8 mM for [DMIM] [Cl] and 30.36 mM for [BMIM] [OS]; and remains constant after 64 mM for [BMIM] [OS] and 94.8 mM for [DMIM] [Cl]. Table 2: Time resolved decay parameter of ST in water and different SAIL concentrations. [BMIM][OS]/mM

τ (ns)

a

χ2

[DMIM][Cl]/mM

τ (ns)

a

χ2

0

1.22

1.00

1.05

0

1.22

1.00

1.05

19.77

1.44

1.00

1.08

30.74

1.40

1.00

1.15

40.29

1.64

1.00

1.13

68.08

1.68

1.00

0.97

69.97

2.46

1.00

1.08

97.48

1.82

1.00

1.11

3.4. Time resolved fluorescence decay measurements Fluorescence lifetime measurement is a sensitive indicator of a probe in local environment. Fluorescence lifetime often is used as the stability of probe in the excited state. There are 15 | P a g e ACS Paragon Plus Environment


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very rare lifetime studies of ST50 and CR51 in literature. This experiment shows how the rates of radiative and non-radiative deactivation channels are affected upon penetration of dye in micelle of the SAILs. All the investigations are done in pre micellar and post micellar regions of two SAILs for each dye. ST shows a lifetime of 1.22 ns in aqueous solution which is consistent with the literature. The typical decay of ST in both SAILs follows the mono exponential pattern. ST, being a cationic dye, has tendency to penetrate deeper into [BMIM][OS] micelle but the dye remains at outer sphere of micelle of [DMIM][Cl] due to same cationic charges of both dye and SAIL.52, 53 As a result, lifetime value of ST increases gradually in case of both of SAILs, but is observable in [BMIM][OS] media; change of lifetime (τ) value is more (1.22 ns to 2.46 ns) where in presence of [DMIM][Cl] it is not much more (1.22 ns to 1.82 ns). The change in τ (ns) value indicates that binding of cationic dye ST with anionic SAIL [BMIM][OS] is stronger compared to cationic SAIL [DMIM][Cl]. The combined plot of ST in higher SAIL concentration is given in Figure 9 (A) and results are listed in Table 2.

Fig. 8: Lifetime change of ST and CR as a function of concentration of SAIL. The change in lifetime with increase in [SAIL] is in conformity with discussion of emission intensity change and results obtained from Modified Stern Volmer plot. There are three and


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one regions for ST-[BMIM][OS] and [DMIM][Cl] systems respectively in Fig. 8 as similar results are obtained from steady state emission spectra measurement and Modified Stern Volmer model.45 There are two components of lifetime of CR, the shorter (τ1) and higher (τ2) components may be due to monomer and dimer formation of the dye in aqueous solution respectively. The shorter component is of about 69% and higher is of 31%. With increase in [BMIM] [OS], lifetime of CR increases before cmc but after that decreases possibly due to hydrophobic interaction between octyl sulphate group of [BMIM] [OS] micelle and CR. There is smooth increase of lifetime with increase [DMIM][Cl] up to a certain concentration followed by constant lifetime component upon further addition of SAIL. In both cases of SAILs, it is easy to interpret that CR dye could not enter into the interior part of micelle, indicates that CR molecules are in the stern layer of micelles.54, 55 These results indicate weak binding of CR with SAILs. All data of time resolved lifetime measurement is consistent with other micro heterogeneous experiment discussed in results and discussion section. All data of CR in different concentration of SAILs are depicted in Table 3 and the corresponding graphs are presented in Figure 9 (B). Table 3- Time resolved decay parameter of CR in water and different SAIL concentrations. Solution

τ1 (ns)

a1

τ2 (ns)

a2

τavg (ns)

χ2

Water

1.46

0.69

10.84

0.31

4.37

1.09

19.77

2.12

0.44

8.29

0.56

5.57

1.12

52.78

2.37

0.46

7.27

0.54

5.02

1.10

30.74

2.25

0.50

7.22

0.50

4.74

1.06

97.48

2.49

0.52

6.91

0.48

4.61

1.09

[BMIM][OS]/mM

[DMIM][Cl]/mM



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Fig. 9: Time resolved fluorescence decay of (A) ST and (B) CR in absence and presence of different [SAIL].

4. Conclusion The interaction between two different charged ionic dyes and two different surface active ionic liquids has been performed. Generally, similar charged dye does not interact with similar charged surfactant but here, there are some sort of interaction of similar charged dye and SAIL. Decrease of absorbance intensity by addition of SAIL into ST solution is due to dye-SAIL aggregation. At higher [SAIL], intensity increases due to formation of micelle and it leaves the dye molecule of ST. Red shift of λmax indicates ion-pair aggregation process. The interaction of dyes with [BMIM][OS] is stronger than [DMIM][Cl] due to higher electrostatic interaction between [BMIM][OS] having larger octyl sulphate group compared to smaller Clion present in [DMIM][Cl]. Absorbance spectra of CR is more complicated than ST. Interaction with [BMIM][OS] was different from [DMIM][Cl]. Red shifted spectra were observed due to formation of dye-SAIL complex, but interaction with oppositely charged SAIL at lower concentration region was due to H-aggregated structure whereas at higher [SAIL], red shifted spectra appeared for cluster association with the SAIL head group. Steady state fluorescence intensity measurement shows that ST is more polarity sensitive dye 18 | P a g e ACS Paragon Plus Environment

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compared to CR. As a result, in [SAIL] media, there is blue shift of emission maxima in ST but not in the case of CR. Electrostatic and hydrophobic interactions are responsible for solubilization of ST and CR in micelle. This method shows that ST strongly interacts with [BMIM][OS], but weakly with [DMIM][Cl] comparatively; whereas there are weak interactions of CR with both SAILs. Binding interaction ratios of ST and CR are 1:2 and 1:1 respectively with both SAILs.

Anisotropy values are greater in oppositely charged dye-

SAIL system compared to same charged dye-SAIL system. Time resolved fluorescence lifetime change from pre micellar to post-micellar stages for ST-[BMIM][OS] system is higher and of different nature from all other dye-SAIL systems in accordance with emission intensity change and binding constant obtained from the corresponding plots.

5. Acknowledgement N. P. and B. M. thank University Grant Commission for their Senior Research fellowships.

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