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Complexation, dimerisation and solubilisation of methylene blue in the presence of biamphiphilic ionic liquids: a detailed spectroscopic and electrochemical study† Reshu Sanan, Tejwant Singh Kang* and Rakesh Kumar Mahajan* The interactions of methylene blue (MB), a redox active dye with surface active biamphiphilic ionic liquids (BAILs): 1-butyl-3-methylimidazolium dodecylsulfate, [C4mim][C12OSO3] and 1-hexyl-3-methylimidazolium dodecylsulfate [C6mim][C12OSO3] have been investigated in aqueous medium to explore the candidature of surface active ionic liquids (ILs) in the field of dye–surfactant chemistry. Various thermodynamic, spectroscopic and electrochemical techniques such as conductivity, steady-state fluorescence, UV-visible absorption, and cyclic voltammetry (CV) have been used to obtain comprehensive information about MB–BAIL interactions. The presence of MB is seen to enhance the critical micellar concentration (cmc) of BAILs. The extent of interaction between the MB and BAILs varies with the concentration as well as the nature of BAILs. Different interactional phenomena such as the formation of ion-pair complexes,

Received 23rd September 2013, Accepted 2nd January 2014

dimers, and solubilisation of monomers of MB have been observed in different concentration regimes of

DOI: 10.1039/c3cp54030h

terms of various micellar and binding parameters exploiting UV-visible absorption and CV measurements.

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[C4mim][C12OSO3] via hydrophobic and electrostatic interactions.

BAILs. A quantitative evaluation of the process of interaction between MB and BAILs has been made in Comparatively more hydrophobic [C6mim][C12OSO3] interacts strongly with MB as compared to

1. Introduction Methylene blue (MB) is a coplanar, polycyclic, aromatic, redox active, basic dye belonging to the phenothiazine family. Its immense potential manifested as a sensitizer in solar energy conversion,1 redox mediator in catalytic oxidation reactions,2 active electrochromic species,3 ingredients in pharmaceutical preparations,4 and above all as a drug in photodynamic therapy for anticancer treatment,5 has made MB a valuable asset for the scientific community. MB is generally known to undergo self-association which determines its utility for a particular application, as the tendency as well as type of aggregation (whether H-type or J-type aggregates) is strongly dependent on the concentration and structure of the dye, pH, temperature, nature of the solvent and the presence of additives.6–8 In this regard diverse systems such as DNA, peptides, polymers, macrocyclic Department of Chemistry, UGC-Centre for Advanced Studies, Guru Nanak Dev University, Amritsar-143005, India. E-mail: [email protected], [email protected]; Fax: +91 183 2258820 † Electronic supplementary information (ESI) available: Fig. S1–S13 contain supporting plots of CV, UV-visible and fluorescence spectra. Comparison of results for MB + [C4mim][C12OSO3] and MB + [C6mim][C12OSO3] systems from UV-visible absorbance and fluorescence emission spectroscopy is discussed in Table S1. See DOI: 10.1039/c3cp54030h

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hosts, lipid assemblies, and surfactants have been investigated with the objective of controlling and designing the aggregation patterns of MB.9–14 Surfactants have been known to alter the photophysical and aggregation properties of various dyes owing to the hydrophobic and electrostatic forces.15–17 Moreover the vesicles, micelles, reverse micelles commonly referred to as membrane mimetic systems are regarded as useful tools to study several photoinduced and bioenergetic phenomena because here it is possible to change the equilibrium and dynamic properties of the interfaces in a controlled way.18,19 Recently, the field of surfactant chemistry has envisaged the replacement of conventional ionic surfactants by surface active ionic liquids (SAILs) because of their inherent amphiphilicity, greener aspects, low toxicity, superior surface activity, enhanced solubility in various media and versatility in functionalisation.20–22 These SAILs possesses easily tunable physicochemical properties through a myriad combinations of organic and inorganic cation/ anion and can self-assemble into unique and controllable structures which can act as micellar catalysts, and can show diverse templating behavior for the preparation of nanostructured materials.23–25 Numerous surface active ionic liquids (SAILs) based on imidazolium, pyridinium and pyrrolidinium cations with different hydrocarbon chain lengths have been investigated for their aggregation behavior, however imidazolium based ILs

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remain of particular interest. The aggregation behavior of a variety of 1-alkyl-3-methylimidazolium cation based ILs has been well characterized using a variety of thermodynamic and spectroscopic state of art techniques viz. conductivity, surface tension, fluorescence, SAXS, potentiometry, dynamic light scattering, 1 H NMR etc., both in the absence and presence of additives.26–34 Replacing the halide ion in imidazolium based IL surfactants with those of alkyl sulfates gave rise to a newer generation of biamphiphilic ionic liquids (BAILs) which tend to be biodegradable and hence greener,35,36 and are the focus of current research with frequent uses like templates for preparation of anisotropic gold nanostructures,37 synthesis of gold nanocrystals/microplates,38 as supporting electrolytes and additives to construct MEKCECL systems.39 However, SAILs have not yet been studied in conjunction with dyes. Therefore with an aim to explore the candidature of SAILs in the field of dye–surfactant chemistry, we herein investigated the interactional behavior of surface active biamphiphilic ionic liquids (BAILs): 1-butyl-3-methylimidazolium dodecylsulfate, [C4mim][C12OSO3] and 1-hexyl-3-methylimidazolium dodecylsulfate [C6mim][C12OSO3] with MB in its aqueous solutions. Applications of surface active agents as wetting, dispersing and leveling agents are of great concern in the theory and technology of dyeing, so it becomes important to understand their molecular interactions related to this process. As MB acts not only as a dye but also as a drug, it is hereby expected that insights in these interactions will not only be beneficial for the textile industry but will also open new avenues for research in several areas including biology, chemistry and medicine, as SAILs are already known to act as drug carriers and also to enhance the permeability of drugs across biological membranes.40 Moreover a lack of such studies in the literature prompted us to carry out a detailed analysis of interaction phenomena in the MB–BAIL systems, ranging from very low concentrations of BAILs to post-micellar regions through an appraisal of both micellar and binding parameters. Thermodynamic, electrochemical and spectroscopic measurements have been exploited to get better insights into the interaction process and the mechanism behind it. This involves the effect of MB on various micellar parameters of BAILs such as critical micellar concentration (cmc), degree of counterion binding (b), standard free energy of micellization (DGomic) etc. The extent of interactions prevailing in MB–BAIL ion-pair complexes have been quantitatively discussed in the light of the binding constant and Stern– Volmer quenching constant.

2. Experimental 2.1

Materials

1-Butyl-3-methylimidazolium chloride and 1-hexyl-3-methylimidazolium chloride with stated purities 98% were obtained from Sigma Aldrich, sodium dodecylsulfate (SDS) (purity >99%) was procured from Spectrochem, India while methylene blue (purity Z98%) was a product of Fluka. All chemicals were of analytical grade and used as received. An analytical balance (Sartorius analytic) with a precision of 0.0001 g was used for

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weighing the amounts of different substances. The solutions were prepared by dissolving accurately weighed quantities in the requisite volumes of deionised doubly distilled water. 2.2

Methods

2.2.1 Synthesis of biamphiphilic ionic liquids. 1-Butyl-3methylimidazolium dodecylsulfate [C4mim][C12OSO3] and 1-hexyl-3-methylimidazolium dodecylsulfate [C6mim][C12OSO3] were synthesized and purified as per the procedures mentioned elsewhere.38,41 This involved the ion metathesis reaction of 1-alkyl-3-methylimidazolium chlorides with sodium dodecylsulfate where 1 mol equivalent of both were mixed together in dichloromethane in a round bottomed flask and stirred for about 6 hours. This was followed by the filtering of NaCl precipitates and washing of the organic phase with water till the water became chloride free as monitored by argentometric titration. Subsequent evaporation of the solvent under reduced pressure gave the required products which were dried under vacuum at 80 1C for 24 hours prior to use. The purity of the products was ascertained by 1H NMR measurements. 2.2.2 Conductivity measurements. The specific conductivity (k) measurements were carried out at 298.15 K with the help of a Model EQ 661 digital conductivity meter (Equiptronics, Bombay) by the dilution method using concentrated aqueous solutions of BAILs. Concentrated stock solutions of BAILs + MB were prepared in deionised doubly distilled water and successive additions of these stock solutions were made to MB + water systems to obtain the specific conductivity. A dip-type conductivity cell having a cell constant of 1.01 cm1 was used. The temperature of the measurement was controlled using a water circulating thermostat within the accuracy of 0.1 K. The specific conductivity of deionised doubly distilled water used in this study was measured to be 1–2 mS cm1. The reproducibility of conductivity measurements on a single sample was estimated to be within 0.2%. 2.2.3 UV-visible measurements. The absorption spectra were recorded on a UV-1800 Shimadzu UV-visible spectrophotometer with a quartz cuvette having a path length of 1 cm. The titrations were performed at 298.15 K by successive additions of stock solutions of SAILs and MB directly into the quartz cuvette containing 3 mL of 0.05 mM MB solution. After every addition, the solution was equilibrated for 5 minutes to reach thermal equilibrium. The spectra were recorded in the range 400–800 nm. 2.2.4 Steady state fluorescence measurements. The steady state fluorescence measurements were performed on a Varian-Cary Eclipse Fluorescence spectrophotometer using a 10 mm path length quartz cuvette at 298.15 K. The emission spectra of MB were recorded in the range 640–760 nm at an excitation wavelength of 612 nm using an excitation and emission slit width of 5 nm. The fluorescence titrations were carried out in a manner similar to that of UV-visible measurements. 2.2.5 Voltammetric measurements. Cyclic voltammetry (CV) measurements were carried out on a PC controlled CHI660D (Austin, USA) electrochemical workstation equipped with a conventional three electrode system comprising of a working Pt electrode (2 mm in diameter), a reference Ag/AgCl electrode and a counter Pt wire. The CV measurements were carried out

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in the presence of 0.1 M HCl as a supporting electrolyte to ensure the reduction of MB which is completely reversible at low pH.42 Prior to the measurements, the solutions were deoxygenated with N2 and working electrodes were polished with slurry of alumina powder. The titrations were performed analogous to spectroscopic measurements at 298.15 K.

3. Results and discussion 3.1

Conductivity measurements

First of all, the interactional behavior of BAILs with MB was investigated to determine the effect of varying the amounts of MB (0.05 mM, 0.1 mM, 0.5 mM and 1.0 mM) on the cmc of both the BAILs viz. [C4mim][C12OSO3] and [C6mim][C12OSO3] using conductivity measurements. As shown in Fig. 1(a) and (b), the sharp breaks in the specific conductivity (k) vs. molar concentration of the BAILs provided the corresponding cmc values (Table 1). In aqueous medium, the values of cmc for both the BAILs have been found to be in good agreement with the literature values.38,43 As both of the ILs possess similar anionic moieties, the larger chain length of the imidazolium cation, leading to greater hydrophobicity in [C6mim][C12OSO3], is responsible for its lower cmc value as compared to [C4mim][C12OSO3]. The addition of MB brought

remarkable changes in the conductivity behavior of BAILs only when the concentration of MB was equal to or more than 0.1 mM (however the effect of MB below 0.1 mM cannot be ruled out looking at the sensitivity of the technique and very low concentration of MB). At these concentrations of MB, two break points in the k vs. concentration plots were observed which have been assigned as C1 and C2 (cmc), with three regions demarcated as region I (below C1), II (between C1 and C2) and III (behind C2) respectively. In region I, where the constituent ions of BAILs remain obviously in monomeric form, the positively charged MB and negatively charged ions of the BAILs are supposed to form ionpairs. However, at this point using conductivity measurements, it is difficult to speculate to any extent about the physical state of the MB (whether monomeric/dimeric/higher aggregates). This ion-pair formation among oppositely charged dye–surfactant systems has been well exemplified earlier.44–46 Further it can be seen that the concentration C1 is very much proportional to the concentration of MB hinting at the formation of a MB–BAIL ion-pair complex with 1 : 1 stoichiometry. At same concentration of MB, C1 appears at slightly higher concentrations of BAIL in MB–[C6mim][C12OSO3] system as compared to MB–[C4mim][C12OSO3] mixtures raising the possibility of greater interactions among the components in the former case. Also for both the MB–BAIL systems, the

Fig. 1 Variation of specific conductivity (k) with molar concentration of (a) [C4mim][C12OSO3] and (b) [C6mim][C12OSO3] in the presence of varying amounts of MB.

Table 1 Micellar parameters of [C4mim][C12OSO3] and [C6mim][C12OSO3] in the presence of varying amounts of MB (CMB); concentration corresponding to ion-pair formation (C1), critical micellar concentration (cmc), degree of counterion binding (b) and standard free energy of micellization (DGomic) as determined from conductivity measurementsa

[C4mim][C12OSO3]

[C6mim][C12OSO3]

CMB (mM)

C1 (mM)

C2 = cmc (mM)

b

DGomic

0.00 0.05 0.10 0.50 1.00

— — 0.088 0.42 0.81

2.28 2.30 2.38 2.61 2.88

0.63 0.54 0.57 0.60 0.64

40.8 38.5 39.1 39.5 40.1

a

1

(kJ mol )

C1 (mM)

C2 = cmc (mM)

b

DGomic (kJ mol1)

— — 0.098 0.45 0.89

1.12 1.14 1.16 1.49 1.89

0.73 0.71 0.71 0.73 0.76

46.4 45.8 45.7 45.1 44.9

Maximum uncertainty limit in cmc and b is 0.03 mM and 0.02 respectively.

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slope of change in specific conductivity in region I is found to vary from positive to negative as the concentration of MB is increased suggesting an increased extent of interaction among MB and constituent ions of the BAIL at higher MB concentrations. The origin of the second break at C2 is attributed to the cmc of the BAIL and is found to increase marginally at lower concentrations of MB and notably at higher MB concentrations (Table 1). It has been reported earlier that the presence of dye counter ions can induce changes in the electrostatic forces within the micelles leading to a change in the critical properties of a surfactant.47 Also the presence of additives is known to alter the solute–solvent interactions reflecting them in modulating the characteristic properties of surfactants. This increase in cmc of BAIL molecules on addition of MB is in sharp contrast with a conspicuous decrease in the cmc (from B7 mM to B70 mM) of ionic surfactant SDS in the presence of 45 mM MB.48 This suggests that these BAIL molecules behave entirely differently from their conventional counterparts. Further, the increase in cmc on addition of MB (Fig. S1, ESI†) is found to be greater for [C6mim][C12OSO3] as compared to [C4mim][C12OSO3]. Region III suggests the presence of micelles of BAILs formed at C2. In the presence of micelles, we speculate here on the monomeric adsorption of MB molecules onto the micelle surface,49,50 which is further corroborated from other studies as discussed in the forthcoming sections. Further, k is found to vary almost linearly with the concentration of BAILs both in the premicellar (II) and post-micellar region (III), and the ratio of the slope of post- to premicellar regions provides the degree of ionization (a) for the BAILs, from where the degree of counterion binding (b), can be computed as: b=1a

(1)

Employing the charged pseudophase model of micelle formation,51 and using the value of b, the standard free energy of micellization per mole of the surfactant (DGomic), is calculated as: DGomic = (1 + b)RT ln xcmc

(2)

where xcmc is the cmc in mole fraction of the BAIL and the other symbols have their usual meanings. As can be seen from Table 1, b is relatively higher for [C6mim][C12OSO3] than [C4mim][C12OSO3] as the increase in the chain length of the imidazolium counterion is known to reduce its hydration and enhance counterion binding.38 Further addition of different amounts of MB brings about varied changes in b values for both the BAILs, firstly leading to a decrease in b at lower MB concentrations followed by an increase at higher concentrations of MB. This suggests that as the concentration of MB is increased, the relative concentration of free counterions, i.e. alkylimidazolium cation, as a consequence of interactions with MB decreases which could probably account for the change in slope of specific conductivity in region I, as mentioned earlier. Moreover quantitatively the effect of MB addition on b is less for [C6mim][C12OSO3], which is again due to the longer alkyl chain length of the imidazolium cation being capable of exercising greater hydrophobic interactions. The DGomic values are found

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to be negative indicating the spontaneity of the micellization process. However in a broad sense, the magnitude of DGomic decreases with the addition of MB suggesting a decreased spontaneity of micellization. 3.2

Spectroscopic measurements

Next, to have a detailed picture of the physical state of MB in conjunction with BAILs, spectroscopic monitoring of these interactions was carried out, as MB exhibits interesting photophysical behavior with the formation of aggregates that can be easily characterized through UV-visible absorbance and steady-state fluorescence spectroscopy. Hence any change in the local environment of MB on interacting with BAILs can be best predicted spectroscopically as BAILs themselves showed no absorption or emission background in the regions being investigated. 3.2.1 UV-visible spectroscopy. The UV-visible absorption spectra of MB in water were recorded in the concentration range of 103 M to 107 M (Fig. S2, ESI†), where the bands at 664 nm, 612 nm and 556 nm could be assigned to the presence of monomers (MB+), dimers [(MB+)2] and higher aggregates of MB [(MB+)n] respectively.52 These dimers, trimers and higher aggregates are held together by virtue of hydrophobic interactions, hydrogen bonding, van der Waals forces, London dispersion forces and other short range forces with charges maintained as far as possible. An increase in the concentration of MB saw a linear increase in the dimer as well as the monomer absorbance till the concentration reaches 104 M (Fig. S3, ESI†). But at higher concentrations, MB failed to follow the Beer–Lambert law due to the aggregation of MB to form dimers and higher aggregates.53 Therefore, further measurements were carried out at a fixed concentration of MB i.e. 0.05 mM (where the aggregation was mainly limited to dimerisation) and the concentration of BAILs was varied from the very dilute region where BAILs behave as monomers to post-micellar regions as revealed from conductivity measurements. The absorption spectra of MB in the presence of varying amounts of [C4mim][C12OSO3] and [C6mim][C12OSO3] are shown in Fig. 2(a) and Fig. S4 (ESI†) respectively. For both MB–BAIL systems, the gradual addition of BAILs is seen to bring about a marked decrease in the absorbance of MB without shifting the wavelength of maximum intensity (l664). This provides a clue to the existence of some sort of binding/interactions between the MB and BAIL molecules. So, to have an idea about the physical state of MB in binding with the BAIL molecules the ratio of dimer to monomer absorbance (A612/A664) was plotted as a function of molar concentration of BAILs for both [C4mim][C12OSO3] and [C6mim][C12OSO3] as shown in Fig. 2(b). Here, it was observed that A612/A664 ratio varied differently with the concentration of the BAILs, again showing two prominent break points corresponding to C1 and C2 with regions I, II and III differentiated in concordance with conductivity measurements. In region I, there occurs an overall decrease in the absorption intensities of all the peaks leading to a very small decrease in the A612/A664 ratio. This suggests that both the MB species (whether monomeric or dimeric) being positively charged are involved in ion-pair formation with the negatively charged BAIL ions. It is emphasized that

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Fig. 2 (a) Absorption spectrum of MB (0.05 mM) in the presence of increasing amounts of [C4mim][C12OSO3]. (b) Plot of ratio of dimer to monomer absorbance (A612/A664) vs. molar concentration of [C4mim][C12OSO3] and [C6mim][C12OSO3].

besides the anion of BAIL, the alkyl imidazolium cation must be interacting with MB via hydrophobic and weak van der Waals interactions which adds to the higher extent of interactions of [C6mim][C12OSO3] with MB as compared to [C4mim][C12OSO3]. Therefore this region actually represents the complex formation between the MB and BAIL molecules stabilized by a combination of electrostatic and hydrophobic forces. In region II, the peak intensity at 612 nm starts increasing at the expense of the peak at 664 nm manifesting a conspicuous increase in A612/A664 ratio indicating that the dimer formation/ aggregation of MB has been induced by the presence of BAIL molecules. This monomer–dimer transition is also revealed by the appearance of an isosbestic point around 640 nm which becomes blue shifted with increasing concentration of MB as per Fig. 2(a). It can be anticipated that the formation of ion-pair complexes of MB with BAILs leads to the reduction of electrostatic forces among MB monomers which now tend to approach each other more closely.54 Moreover, the increase in the concentration of BAIL molecules after the ion-pair formation leads to an increased hydrophobicity of the medium eventually leading to the dimerisation of MB molecules due to the structural effects of water.10 Some earlier reports have also suggested the existence of premicellar aggregates in regions just below the cmc to be the driving force for inducing the aggregation in dyes.46 However the reverse is observed for the region after C2 (region III) evidencing the increase in relative abundance of monomers of MB at concentrations >cmc of BAILs. It has been pointed earlier that after cmc, as there is a large excess of micelles in relation to MB molecules, the number of binding sites is more and hence the formation of dimers is no longer favored.48 Thus the monomers are distributed to different micelles and may be solubilised within the micelles of BAILs. 3.2.2 Fluorescence spectroscopy. The emission spectrum of a molecule is known to be highly dependent on the polarity of the surrounding medium as compared to absorption measurements since the fluorophore stays for a longer time in the excited state and exposed to the relaxed environment with solvent

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molecules oriented around the dipole moment of the excited state.55,56 Thus fluorescence spectroscopy can provide valuable information regarding the interactional behavior of MB and BAILs. The fluorescence emission spectrum of MB in aqueous solutions was found to exhibit a red shift in peak maximum with the increase in concentration (Fig. S5, ESI†). This was accompanied by an increase in fluorescence intensity till 0.05 mM of MB after which the intensity began to decrease. This decrease in fluorescence intensity can be assigned to the aggregation of the dye molecules as the dimers and higher aggregates of MB are known to be non fluorescent.52 Similar to that of UV-visible absorption measurements, the amount of BAILs was varied at a fixed concentration of MB (0.05 mM) and the corresponding changes in the fluorescence emission spectra for the peak maxima at 696 nm were noted and analyzed. Fig. 3(a) and Fig. S6 (ESI†) depict the variation in the fluorescence emission spectrum of MB in its aqueous solutions on addition of [C4mim][C12OSO3] and [C6mim][C12OSO3] respectively. The variation in the maximum emission intensity (at 696 nm) as a function of concentration of BAILs is shown in Fig. 3(b). In line with the results observed from conductivity and UV-visible absorption measurements, initially in region I, a small increase in fluorescence intensity up to C1, with no change in the peak maxima is observed. On interacting with the constituent ions of BAILs, the change in solvation environment seems to be responsible for increase in fluorescence intensity. After C1, in region II, a decrease in the fluorescence emission accompanied by a hypsochromic shift of about 8 nm was observed. Here the quenching of MB fluorescence suggests the reduced availability of the fluorophores i.e. MB monomers, which is thought to arise due to the formation of MB–BAIL complexes where MB remains mainly in dimeric form induced by BAILs as also indicated by UV-visible absorption measurements. However, in region III, as the cmc of the respective BAILs is reached, the fluorescence intensity begins to increase with a corresponding increase in wavelength for the peak maximum which does not go beyond 696 nm as observed for MB monomers in the absence of BAILs

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Fig. 3 (a) Normalised fluorescence emission spectrum of MB (0.05 mM) in the presence of increasing amounts of [C4mim][C12OSO3]. (b) Plot of fluorescence emission intensity at 696 nm vs. molar concentration of [C4mim][C12OSO3] and [C6mim][C12OSO3].

and is shown in Fig. S7 (ESI†). As here no shift in the peak maximum is observed irrespective of the concentration of respective BAIL above cmc, it can be anticipated that the MB molecules are not moved from the polar aqueous phase to the less polar micellar region.57 Therefore it is inferred that the solubilisation of MB monomers does not occur within the core of the micelles of BAILs; rather they are present at the micellar interface or the palisade layer of the micelles. In different regions, the concentrations corresponding to different breakpoints i.e. C1 and C2 as observed from different measurements are in excellent agreement with each other (Table S1, ESI†), further supporting the results obtained from complementary techniques i.e. emission and absorption spectroscopy. 3.3

Voltammetric measurements

As MB is known to exhibit redox activity, advantage was taken of voltammetric measurements to gather more information about the interaction of MB with BAILs. It is important to mention here that due to their broad electrochemical window, BAILs do not exhibit any redox behavior in the potential region being investigated. The cyclic voltammetric (CV) measurements of 0.05 mM MB in 0.1 M HCl solution (Fig. S8, ESI†) depict a pair of reversible peaks at the platinum electrode, signifying the reduction of MB to leuco methylene blue (LMB) as per Scheme 1. The separation of the anodic and cathodic peak potentials was 33 mV, indicating that the process is a reversible, 2e redox process. On altering the scan rate, the change in the peak currents was found to be linear with no shift in peak potentials suggesting that the electrochemical process is adsorption controlled and the electron transfer rate is very fast under these conditions. Hence the Randles–Sevick equation58 was employed to calculate the diffusion coefficient for MB from the slope of the plots of peak current (Ip) vs. square root of the scan rate (n1/2). As shown in Fig. 4, the diffusion coefficient for pure MB was evaluated to be 4.08 mcm2 s1. However, the addition of BAILs markedly affects the values of the diffusion coefficient for MB. The corresponding values in the presence of 0.04 mM, 1.0 mM and 2.5 mM of

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Scheme 1 Proposed mode of interactions between MB and BAILs at the platinum electrode.

[C4mim][C12OSO3] were found to be 4.29 mcm2 s1, 6.00 mcm2 s1 and 4.44 mcm2 s1 respectively. In the presence of 0.04 mM, 0.5 mM and 1.5 mM of [C6mim][C12OSO3], the diffusion coefficients of MB were calculated to be 6.01 mcm2 s1, 8.45 mcm2 s1 and 7.58 mcm2 s1 respectively. This showed that the presence of BAILs below cmc increased the diffusion of MB to the electrode surface. Above cmc, the diffusion however lessened because of the solubilisation of MB in the palisade layer of micellar aggregates. At similar concentration of BAILs, the diffusion of MB in the presence of [C6mim][C12OSO3] is more as compared to [C4mim][C12OSO3] due to the greater extent of interactions in the former case. Further, the complex formation between MB and the BAILs (region I as predicted from spectroscopic measurements) was investigated by successive additions of small aliquots of the respective BAILs to 0.05 mM solution of MB prepared in 0.1 M HCl. A representative plot showing the corresponding CVs for the MB + [C4mim][C12OSO3] and MB + [C6mim][C12OSO3] mixtures is shown in Fig. 5 and Fig. S9 (ESI†). It was observed that the addition of BAILs led to an increase in both the anodic and cathodic peak currents signifying that

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On the other hand, MB+ is capable of interacting electrostatically as well as hydrophobically with the negatively charged ions of the BAIL, favoring its adsorption too at the electrode surface (Scheme 1). In addition to these interactions, we cannot rule out the possibility of cation–p and p–p interactions existing between the imidazolium moieties of the BAIL and the aromatic ring system of MB.40 Hence as a result, both cathodic and anodic peak currents grow on addition of ILs. This enhancement of electrochemical responses was also confirmed through differential pulse voltammetric (DPV) measurements for both MB–BAIL systems as depicted in Fig. S10(a) and (b) (ESI†). 3.4 Quantitative evaluation of the interactions between MB and BAILs

Fig. 4 Plot of Ip vs. square root of scan rate (n1/2) for MB (0.05 mM) in the presence of varying amounts of [C4mim][C12OSO3].

3.4.1 Formation of ion-pair complexes in region I. In view of the conductivity and spectroscopic measurements, region I suggests the formation of MB–BAIL ion-pair complexes because of the electrostatic as well as hydrophobic interactions between the MB and BAIL molecules. The quantitative evaluation of these interactions in terms of binding constant (ka) can be made by the use of eqn (3),40 as: 1 1 1 ¼ þ DA DAmax ka DAmax ½BAILn

Fig. 5 Cyclic voltammograms of 0.05 mM MB in the presence of varying amounts of [C4mim][C12OSO3] in 0.1 M HCl and at a scan rate of 0.1 mV s1.

where DA = A0  A; A0 and A represent the absorbance of 0.05 mM aqueous MB solution at 664 nm in the absence and presence of BAILs respectively, [BAIL] denotes the molar concentration of the added BAILs viz. [C4mim][C12OSO3] and [C6mim][C12OSO3] and n is the stoichiometric coefficient. For MB–BAIL systems, an appraisal of the absorbance by MB in region I as per eqn (3) shows that a double reciprocal plot of 1/(A0  A) vs. 1/[BAIL] gives an excellent linearity [Fig. 6(a)] confirming the formation of a 1 : 1 ion-pair complex between MB and BAILs in corroboration with conductivity measurements. Hence ka was evaluated from the ratio of intercept to slope and is given in Table 2. Similarly from CV measurements, on the basis of the increase in the peak current of MB in the presence of BAILs, double reciprocal plots of 1/DIp vs. 1/[BAIL] were constructed in accordance with eqn (4):40 1 1 1 ¼ þ DIp DIp max ka DIp max ½BAIL

the presence of BAILs affects the adsorption of MB at the electrode surface. Similar results have been reported earlier,59,60 where the aggregation of surfactants at the electrode surface is shown to alter the adsorption and e transfer rate of MB. Hu and coworkers61,62 have proposed a synergistic adsorption mechanism, according to which surfactants firstly get adsorbed at the electrode surface and then through association with the substrates strengthen their adsorption on the electrode surfaces too, in turn leading to the enlargement of electrochemical responses. Here in this case too, BAILs are capable of interacting with both the MB+ (oxidized form) and LMB (reduced form) equally well because of their biamphiphilic nature. LMB is basically neutral and hydrophobic in nature, so it is expected that the hydrophobic interaction with the BAILs mainly plays a role in holding it on the electrode surface.

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(3)

(4)

where DIp represents the change in peak current of MB on the addition of BAIL with respect to MB solutions at zero concentration of BAIL. Again, a good linear relationship was obtained for these double reciprocal plots of MB in the presence of both the BAILs as can be seen from Fig. 6(b). The ka values as depicted from CV measurements have been found to be in excellent agreement with those from UV-visible measurements. Using the values of ka, the free energy change for the ion-pair formation between MB and the BAILs can be evaluated from the following equation: DG = RT ln ka

(5)

The values of DG as listed in Table 2 have been found to be negative indicating the feasibility of formation of an ion-pair

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Fig. 6 Binding constant determination for the MB–BAIL systems using (a) changes in the absorbance spectra of MB and (b) changes in the peak current of MB.

Table 2 Binding constants (ka), correlation coefficients (Rc) and corresponding free energy change of binding (DG) for ion-pair complex formation between MB and BAILs ([C4mim][C12OSO3] and [C6mim][C12OSO3]) evaluated from UV-visible and CV measurementsa

System

ka (103 M1)

Rc

DG (kJ mol1)

Absorbance data MB + [C4mim][C12OSO3] MB + [C6mim][C12OSO3]

3.88 14.85

0.9961 0.9989

20.5 23.8

Voltammetric data MB + [C4mim][C12OSO3] MB + [C6mim][C12OSO3]

3.38 14.97

0.9970 0.9981

20.1 23.8

a

Maximum uncertainty limit in ka is 0.09  103 M1.

complex between the MB and BAILs under the given conditions. Both the ka and DG are higher in magnitude in the MB–[C6mim][C12OSO3] system as compared to MB–[C4mim][C12OSO3] which suggests the greater feasibility and stability of ion-pair formation in the case of the former. This was also verified from the percentage enhancement of the fluorescence intensity of MB in region I, which was found to be 4.15% and 6.76% for MB–[C4mim][C12OSO3] and MB–[C6mim][C12OSO3] respectively. 3.4.2 BAIL induced dimerisation of the MB in region II. In region II, similar double reciprocal plots as mentioned in the earlier section (from absorbance data) were found to be curvilinear even for n = 1, signifying some change in the mode of interaction between the components as shown in Fig. S11 (ESI†). As indicated earlier from conductivity, UV-visible absorption and fluorescence measurements, the presence of BAIL is seen to induce the dimerisation of MB. MB is known to form dimers and higher aggregates and the nature of interacting pairs in these dimers can be discussed using molecular exciton theory.63,64 According to the theory, the dye molecules may aggregate in a parallel configuration (plane to plane stacking) to form a sandwich type arrangement (H-aggregates) or in a head to tail arrangement (end to end stacking) to form J-aggregates. A transition to the upper state in parallel

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aggregates having parallel transition moments (in H-type) and to a lower state in a head to tail arrangement (in J-type) with perpendicular transition moments leads to hypsochromic (blue) and bathochromic (red) shifts, respectively (Fig. S12, ESI†). The angle between the line of centers of a column of dye molecules and the long axis of any one of the parallel molecules is called the angle of slippage (y). Hence, in order to evaluate the dimerisation of MB in region II, UV-visible absorption spectra of MB aqueous solutions at some selected concentrations of added BAILs were broken down into Gaussian components utilizing Origin 6.1 as described elsewhere (Fig. S13, ESI†).65 Different Gaussian bands were deconvoluted simultaneously using standard algorithms based on minimizing w2, with all fit parameters having a 95% confidence interval. The Gaussian fitting could well resolve different bands corresponding to monomers (lmax 664 nm), dimers (lmax 612 nm) and higher aggregates (lmax 556 nm) (see Supplementary Figure, ESI†). With the addition of both the BAILs, the area for the band corresponding to higher aggregates increased only marginally which indicates the ineffectiveness of BAILs to induce formation of higher aggregates of MB. Earlier studies have also pointed to the formation of higher aggregates of dyes in the presence of surfactants.46,63 However, here the lower cmc of BAILs as compared to conventional ionic surfactants seems to be the probable reason for this different behavior. The induction of dimerisation by the BAIL molecules at such lower concentrations is also in contrast with its conventional counterpart, SDS, where the dimer formation in 8 mM MB solutions is induced at concentrations (B5–10  cmc) reverting to monomers at still higher concentrations (B20–30  cmc).48 However, a similar behavior to ours has been shown by SDS for another dye i.e. Thiazole Orange, where the formation of dimers and higher aggregates occurs at concentrations below the cmc of SDS.66 This is quite likely as the dimerisation/aggregation of the dyes is greatly influenced by the structure of the dye, nature of the environment, temperature and additives. Assuming that the distance between the dipoles remain unchanged, the angle y existing between the monomers in

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Table 3 Excitonic parameters of MB dimers in the presence of varying amounts of BAILs (C(BAIL)); wavelengths corresponding to dimer (ldimer) and monomer (lmonomer) peaks, angle of slippage (y) and the interaction energy (U)

y (1)

U (cm1)

C(BAIL) (mM)

ldimer

lmonomer

0.00

612

664

94.4

639.8

MB + [C4mim][C12OSO3] 0.12 610 0.25 608 0.54 606 1.00 601 1.61 602

664 665 666 667 665

92.3 90.1 86.5 81.4 86.2

666.6 704.9 743.3 823.2 786.8

MB + [C6mim][C12OSO3] 0.14 612 0.29 610 0.42 608 0.69 602 0.85 602

664 665 666 668 664

90.9 96.2 97.8 105.6 101.2

639.8 677.5 716.0 820.5 775.5

the dimer was calculated from the ratio of the areas of the long wavelength (S1 = 664 nm) and the short wavelength (S2 = 612 nm) of the Gaussian bands as per eqn (6):53 y S1 tan ¼ 2 S2 2

(6)

Large values of y for BAIL induced MB dimers indicate the possibility of formation of H-type dimers which is also consistent with the blue shift of the band at 612 nm with the addition of BAILs (Table 3). The aggregation of these dyes leads to a strong coupling of the molecular transition dipoles causing the splitting of energy levels of the excited states of molecules whereas the ground state of the dimer remains doubly degenerate. The difference in the energy of these split levels is a function of the interaction energy (U) between the dye molecules in the dimer species. Taking into account the frequency difference between the maxima of neighbouring bands, the interaction energy can be computed as follows: 1 U ¼ ðndimer  nmonomer Þ 2

(7)

As per Table 3, the interaction energy for pure MB in water is found to be in good agreement with literature values.53 On increasing the concentration of the added BAILs, the interaction energy increases suggesting the enhancement in dimerization of MB. Furthermore after concentrations near to the cmc of respective BAILs, a decrease in U has been observed which supports the disruption of dimers into monomeric forms as also qualitatively revealed by conductivity and spectroscopic measurements. Further, as in region II, quenching of MB fluorescence takes place on the addition of BAILs, the Stern–Volmer plots could provide us some insights in the nature of quenching. The quenching process can be dynamic or static resulting from the collisions or the complexation between the fluorophore and the quencher molecules respectively. Hence the fluorescence intensity data of MB both in the absence (I0) and presence (I) of

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Fig. 7 Stern–Volmer plots of fluorescence quenching of MB by [C4mim][C12OSO3] and [C6mim][C12OSO3].

varying amounts of BAILs [Q] at 696 nm was fitted to the following Stern–Volmer equation,67,68 I0 ¼ 1 þ kq t0 ½Q ¼ 1 þ kSV ½Q I

(8)

A good linear relationship as depicted in Fig. 7 was obtained for the plots of I0/I vs. [Q] from where the Stern–Volmer quenching constant (kSV) was calculated and is given in Table 4. As per Table 4, a higher value of kSV for quenching of MB by [C6mim][C12OSO3] as compared to [C4mim][C12OSO3] supports a greater extent of interactions of MB with the former. Taking into account an average fluorescence lifetime (t0) of MB to be 108 s,69 the collisional quenching rate constants (kq) were evaluated. The values of kq have been found to be greater than the maximum diffusion collision quenching rate constant (2.0  1010 M1 s1) which signifies that the quenching was being mainly controlled by a static process rather than being a dynamic one. The corresponding free energies of quenching were evaluated employing the equation: DGq = RT ln kSV

(9)

and were found to be negative for the systems under study suggesting that the quenching of MB fluorescence by BAILs is a spontaneous process. Again, the quenching phenomenon is found

Table 4 Stern–Volmer quenching constants (kSV, kq) and corresponding free energy change (DGq) for quenching of MB fluorescence emission in the presence of [C4mim][C12OSO3] and [C6mim][C12OSO3]a

System

kSV ( 103 M1) Rc

4.48 MB + [C4mim][C12OSO3] MB + 5.04 [C6mim][C12OSO3] a

DGq kq ( 1010 M1 s1) (kJ mol1)

0.9980 44.8

20.8

0.9891 50.4

21.1

Maximum uncertainty limit in kSV is 0.07  103 M1.

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to be comparatively more spontaneous for MB + [C6mim][C12OSO3] system as indicated by more negative DGq values. Hence, an attempt has been made to investigate the interactions between MB and BAILs both qualitatively and quantitatively in aqueous medium. It has been established that the constituent ions of BAILs interact with MB via electrostatic and hydrophobic interactions where the extent of hydrophobic interactions with MB is evidently large for [C6mim][C12OSO3] as compared to [C4mim][C12OSO3]. Although [C6mim][C12OSO3] possesses marginally longer alkyl chain length as compared to its counterpart yet it possesses threefold lower cmc than [C4mim][C12OSO3]. This reflects how a small increase in alkyl chain length of cation modifies the hydrophobicity and hence micellization process of BAILs. The hydrophobic interactions between the 6-carbon alkyl chain of cation of BAIL is producing a more hydrophobic environment by interacting with the alkyl chains of the anion similar to that of mixed micelles where surfactants possessing alkyl chains equal to or longer than 6 carbon atoms can participate in formation of mixed micelles with other surfactants.70

4. Conclusions The interactional behavior of a redox active dye, methylene blue (MB), with biamphiphilic surface active ILs (BAILs): 1-butyl-3methylimidazolium dodecylsulfate, [C4mim][C12OSO3] and 1-hexyl-3-methylimidazolium dodecylsulfate [C6mim][C12OSO3] has been explored using a multi-technique approach both qualitatively and quantitatively in aqueous medium. Using UV-visible spectroscopy, fluorescence spectroscopy and cyclic voltammetric measurements, it is established that MB forms ion-pair complexes of 1 : 1 stoichiometry by interacting with the ions of BAILs via electrostatic and hydrophobic interactions in very low concentration range typically below the concentration of MB used. At higher concentrations, BAIL induced dimerization of MB occurs forming H-type dimers by reducing electrostatic repulsion among the MB monomers as a consequence of adsorption of constituent ions of BAILs on to the MB monomers up to the cmc of BAILs. After cmc, MB was dissolved in the palisade layer of micelles formed by BAILs in the monomeric form in equilibrium with dimers present in the bulk solution. In the presence of MB, the increase in cmc of [C6mim][C12OSO3] with a corresponding decrease in DGomic is greater in magnitude as compared to that of [C4mim][C12OSO3] indicating a higher extent of interaction between MB and [C6mim][C12OSO3] which was also established through evaluation of the binding constants. Very interestingly a marginal increase in the alkyl chain length of the cation in [C6mim][C12OSO3] produced many changes both qualitatively and quantitatively. We conclude the hydrophobic interactions also play a very important role in the interactions of BAILs with MB along with electrostatic interactions where the alkyl chain of the cation in case of [C6mim][C12OSO3] interacts with the alkyl chain of the anion in a fashion similar to mixed micelles of long chain and short chain surfactants in a synergetic manner producing more hydrophobic environment.

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Acknowledgements Reshu Sanan thanks the Council of Scientific and Industrial Research (CSIR), New Delhi, India, for the award of Senior Research Fellowship.

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