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Unique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdfUnique role of ionic liquid bminBF4 during curcumin–surfactant association and micellization of cationic, anionic and nonionic surfactant solutions.pdf
Spectrochimica Acta Part A 79 (2011) 1823– 1828 Contents lists available at ScienceDirect Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy j ourna l ho me page: www.elsevier.com/locate/saa Unique role of ionic liquid [bmin][BF 4 ] during curcumin–surfactant association and micellization of cationic, anionic and non-ionic surfactant solutions Digambara Patra ∗ , Christelle Barakat Department of Chemistry, Faculty of Arts and Sciences, American University of Beirut, P.O. Box: 11-0236, Riad El Solh, Beirut, 1107-2020, Lebanon a r t i c l e i n f o Article history: Received 30 September 2010 Received in revised form 17 May 2011 Accepted 24 May 2011 Keywords: Curcumin Hydrophilic ionic liquid Micelle Surfactant Spectroscopy a b s t r a c t Hydrophilic ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroburate, modified the properties of aqueous surfactant solutions associated with curcumin. Because of potential pharmaceutical applications as an antioxidant, anti-inflammatory and anti-carcinogenic agent, curcumin has received ample attention as potential drug. The interaction of curcumin with various charged aqueous surfactant solutions showed it exists in deprotonated enol form in surfactant solutions. The nitro and hydroxyl groups of o-nitrophenol interact with the carbonyl and hydroxyl groups of the enol form of curcumin by forming ground state complex through hydrogen bonds and offered interesting information about the nature of the interac- tions between the aqueous surfactant solutions and curcumin depending on charge of head group of the surfactant. IL[bmin][BF 4 ] encouraged early formation of micelle in case of cationic and anionic aqueous surfactant solutions, but slightly prolonged micelle formation in the case of neutral aqueous surfactant solution. However, for curcumin IL [bmin][BF 4 ] favored strong association (7-fold increase) with neutral surfactant solution, marginally supported association with anionic surfactant solution and discouraged (∼2-fold decrease) association with cationic surfactant solution. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Micellar systems of aqueous origin have immense technological applications as flow field regulators, solubilizing and emulsify- ing agents, membrane mimetic media, nanoreactors for enzymatic reaction and drug delivery system [1–8]. It is anticipated that curcumin, 1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene- 3,5-dione, may find applications as a novel drug in the near future to control various diseases, including inflammatory dis- orders, carcinogenesis and oxidative stress-induced pathogenesis [9–12]. Curcumin has drawn intense interest recently due to its potential pharmaceutical importance [13–24]. However, curcumin is very poorly soluble in water by reducing its effectiveness as a drug. Therefore, various methods are being developed to make cur- cumin better soluble and enhance effectiveness of the drug during its delivery [16]. Physiochemical properties of an aqueous surfactant solution depend on the identity of the surfactant. The aqueous solution of a surfactant at a given concentration posses more or less fixed physiochemical properties that are difficult to modulate. Other than changing temperature and pressure, the usual way to mod- ify the physiochemical properties of a given surfactant solution is to use external additives, such as cosolvents, cosurfactants, ∗ Corresponding author. Tel.: +961 1350 000x3985; fax: +961 1365217. E-mail address: dp03@aub.edu.lb (D. Patra). electrolytes, non-polar organics, polar organics, etc. Ionic liquids (ILs) are solvents composed entirely of ions and composed of poorly coordinating ions and can therefore be highly polar yet non-coordinating [25–27]. These are immiscible with a number of organic solvents and provide non-aqueous polar alternatives for two phase systems. They are of particular interest because of their environmentally friendly nature, their exciting features and their economical convenience [28–35]. The unusual properties of ILs demonstrate a unique role in altering the properties of aqueous surfactant solutions such as aggregation number [3,4]. The effec- tiveness of this modification of aqueous surfactant solutions by IL may largely depend on the kind and extent of interaction/s between cation/anion of the IL and the head group of the surfactant [4]. How- ever, hydrophobic effect of IL with surfactant molecule might play a role. In addition we hypothesize that IL may drive the associa- tion of the drug molecule towards better solubilization in micellar system (which is very important during drug delivery) as per the head group of the surfactant charge and physiochemical properties of the drug molecule. In order to understand the better insight of the role of these interactions of IL during solubilization of poorly water soluble drug such as curcumin in micellar systems and micellization, we extend the study of interaction of IL and surfactant solutions [4] further to systems composed of various (positive and negative) charged and uncharged surfactant solutions, curcumin and an IL (1-butyl-3-methylimidazolium tetrafluoroburate, [bmin][BF 4 ]). The association of curcumin with various charged surfactant 1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2011.05.064 1824 D. Patra, C. Barakat / Spectrochimica Acta Part A 79 (2011) 1823– 1828 solutions and fluorescence quenching of curcumin by o- nitrophenol in different surfactant solutions may explore the kind of interaction between curcumin and various charged/uncharged surfactant solutions without IL. Due to cation/anion of the IL, it may remarkably alter the interaction of curcumin and surfactant solu- tions based on the charge of the head group of the surfactant and deprotonated form of curcumin, therefore impact drug–surfactant association. Comparative study of various charged/uncharged sur- factant molecules may conclude importance of hydrophobic effect of IL during micellization. 2. Materials and methods 2.1. Materials The surfactants cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulfate (SDS) and Triton X-100 (TX100) were obtained from Acros Organics and were dissolved in different volumes of double distilled water for the preparation of several con- centrations of surfactant solutions. The stock solutions consisted of 10 mM CTAB, 100 mM SDS and 10 mM TX100. Curcumin was also obtained from Acros Organics and was used without further purifi- cation. To prepare the stock solution, curcumin was dissolved in spectroscopic grade acetonitrile (Acros Organics) so that the final concentration of acetonitrile in the surfactant solutions remained less than 1% (v/v). 1-Butyl-3-methylimidazolium tetrafluoroburate, [bmin][BF4] was obtained from Fluka and o-nitrophenol was a Merck Schuchardt product. The solvents were used without further purification. 2.2. Spectroscopic measurements The absorption spectra in various solvents and in cationic CTAB, anionic SDS, and neutral TX100 were recorded at room temperature using a JASCO V-570 UV–VIS–NIR Spectrophotometer. Fluores- cence measurements were done on a JOBIN YVON Horiba Fluorolog 3 spectrofluorometer. The excitation source was a 100 W Xenon lamp. The detector used was R-928 operating at a voltage of 950 V. The excitation and emission slits width were 5 nm. The spectral data were collected using Fluorescence software and data analysis was made using OrginPro 6.0 software. 3. Results and discussion 3.1. Curcumin–surfactant interaction in absence of IL Generally, curcumin showed a strong and intense absorption band in the 350–480 nm wavelength region in all the investi- gated surfactant solutions. Representative absorption spectra of curcumin in various concentrations of TX100 solutions are depicted in Fig. 1. The interaction between curcumin and micelles can be described as: C + S K b CS where C is curcumin; S is the surfactant (CTAB, SDS or TX100); CS is the curcumin–surfactant complex; and K b is the association constant. The concentration of the micellized surfactant is given by: S m = S s − cmc where S s is the surfactant concentration. 700600500400300 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 7-9 6 5 4 3 2 1 Curcu min with [TX100] Absorbance Wavelength (nm ) (1) 0.0 2 mM (2) 0.0 4 mM (3) 0.1 mM (4) 0.2 mM (5) 0.4 mM (6) 0.6 mM (7) 1.0 mM (8) 1.2 mM (9) 1.4 mM Fig. 1. Absorption spectra of curcumin in various aqueous TX100 concentrations. Table 1 Association rate constants of curcumin with various aqueous surfactant solutions in the absence and presence of ionic liquid. Sample cmc used for calculation (mM) K b SDS 7.3 6193 M −1 CTAB 0.8 20,467 M −1 TX100 0.2 11,555 M −1 SDS + IL (1%, v/v) 0.95 6315 M −1 CTAB + IL (1%, v/v) 0.1 10,227 M −1 TX100 + IL (1%, v/v) 0.4 82,737 M −1 The association constants can be determined [6,36–39] as: C T S m l A = S m ε s − ε 0 + 1 K gb (ε s − ε 0 ) where l is the optical path length, ε m is the molar excitation coeffi- cient of curcumin fully bound to micelles, ε 0 is the molar excitation coefficient of curcumin in the solvent, C T is the total curcumin con- centration and A = A − A 0 where A is the absorbance of curcumin in the presence of surfactant solution and A 0 is the absorbance of curcumin in the absence of micelle/surfactant. Using Scott’s plots [6,36–39], the association constants of CTAB, SDS and TX100 were determined to be 20,467 M −1 , 6193 M −1 and 11,555 M −1 (Table 1), respectively. It should be noted that the crtical micellar concentration (cmc) for the calculation of association con- stants for various micelle was estimated by fluorescence method as explained later on. It is observed that, K b CTAB > K b TX100 > K b SDS . These results implied that the different micelles have different affinities for curcumin. Cationic CTAB is bound to curcumin with the highest affinity, followed by neutral TX100 and then anionic SDS. This could be due to the electrostatic interactions between cur- cumin and the positive charge on the head group of CTAB present in the Stern layer of the micelle, thus indicating that curcumin at the given conditions is mainly found in its deprotonated anionic forms [40] (see Supplement 1). In the case of SDS, the repulsion between deprotonated enol (anionic) forms of curcumin and the negative charge on the head group of SDS present in the Stern layer of the micelle make a weaker interaction, hence decreasing the associa- tion rate constant. However, given that the head group of TX100 is nonionic, the value of the association rate constant for TX100 was in between that of CTAB and SDS. 3.2. Critical micellar concentration determination Fluorescence excitation and emission spectra of curcumin with various concentrations of surfactant noted that the fluorescence D. Patra, C. Barakat / Spectrochimica Acta Part A 79 (2011) 1823– 1828 1825 300 35 040 045 0 500 55 060 065 070 0 0.0 4.0x10 6 8.0x10 6 1.2x10 7 1.6x10 7 2.0x10 7 10 8-9 Curc umin with [TX100 ] (1) No TX10 0 (2) 0.02 mM (3) 0.04 mM (4) 0.06 mM (5) 0.1 mM (6) 0.2 mM (7) 0.6 mM (8) 0.8 mM (9) 1.0 mM (10 ) 1.2 mM (11 ) 1.4 mM (12 ) 1.6 mM Fluorescence Intensity (a.u) Wavelength (nm ) 0.0 4.0x10 6 8.0x10 6 1.2x10 7 1.6x10 7 2.0x10 7 2.4x10 7 2.8x10 7 11-12 11-12 10 8-9 7 7 6 6 1-5 1-5 Fig. 2. Fluorescence excitation and emission spectra of curcumin in various aqueous TX100 concentrations. intensity of the emission and excitation spectra of curcumin in TX100 (shown in Fig. 2) and SDS (not shown) increased as the concentration of the surfactant was increased. However, the flu- orescence spectra of CTAB exhibited a different behavior (not shown). The fluorescence intensity initially decreased until it reached 0.5 mM of CTAB and once the cmc was reached, the intensity started increasing with concentration. A red shift was also observed after the cmc for CTAB. The Stokes’ shift of curcumin in various concen- trations of CTAB, SDS and TX100 was determined as the difference between absorption and emission maxima obtained from the cor- rected spectra on the wavenumber scale [41,42]. The plot of Stokes’ shift versus surfactant concentration offered three different kinds of change, respectively, for cationic (CTAB), anionic (SDS) and neu- tral (TX100) surfactant solutions. In the case of CTAB, the value of Stokes’ shift rarely changed before the cmc. A big jump of 5000 cm −1 was observed around the cmc and after the cmc it remained more or less unaltered. The cmc of CTAB was estimated by finding the midpoint of the tangent joining the two lines, as shown in Fig. 3A. For SDS, Stokes’ shift of curcumin for different surfactant con- centrations varied differently, it initially decreased till the cmc was reached. Above the cmc, it marginally increased. By extrapolating these two linear equations, before and after the cmc, with respec- tive negative and positive slopes, a minimum intersecting point was obtained to calculate the cmc (Fig. 3B). Stokes’ shift of curucmin increased with TX100 concentration until cmc was attained and then it decreased dramatically. In this case the maximum value of Stokes’s shift was used to estimate cmc as marked in Fig. 3C. The cmc values estimated using Stokes’ shift of curcumin is summarized in Table 2, the values obtained without IL are similar to the reported values [4,5,43] establishing the reliability of the method. The differ- Table 2 cmc values of aqueous CTAB, SDS and TX100 solutions in the presence and absence of ionic liquid. Sample cmc Curcumin (cm −1 ) Pyrene I I /I III a Reported b SDS 7.3 mM 7.0 mM 6.0–8.0 mM CTAB 0.8 mM – 0.26 mM TX100 0.2 mM 0.25–0.5 mM 0.9 mM SDS + IL (1%, v/v) 0.95 mM 1 mM (2%, v/v) – CTAB + IL (1%, v/v) 0.1 mM – – TX100 + IL (1%, v/v) 0.4 mM 0.5–1.0 mM (2%, v/v) – a From Refs. [3,4]. b From Ref. [43]. 0.0000 0.000 5 0.0010 0.001 5 0.0020 5000 6000 7000 8000 9000 1000 0 1100 0 1200 0 A Stokes' shift (cm -1 ) [CTAB] CTAB 0.0000 0.0005 0.0010 0.0015 0.0020 35000 40000 45000 50000 55000 60000 65000 70000 cmc of CTAB + IL cmc of CT AB CTAB + IL 0.000 0.005 0.010 0.015 0.020 4000 4200 4400 4600 4800 5000 5200 5400 B cmc of SDS + IL Stokes' shift (cm -1 ) [SDS] SDS 0.00 0 0.005 0.010 0.015 0.020 18000 18200 18400 18600 18800 19000 19200 19400 19600 19800 cmc of S DS SDS + IL 0.000 0 0.000 3 0.0006 0.000 9 0.00 12 0.001 5 0.00 18 3000 3500 4000 4500 5000 5500 6000 C Stokes' shift (cm -1 ) [TX10 0] TX100 0.00 00 0.000 3 0.00 06 0.000 9 0.0012 0.001 5 0.0018 0 1000 2000 3000 4000 5000 cmc of Tx10 0 + I L cmc of TX100 TX100 + IL Fig. 3. Variation of Stokes’ shift of curcumin in different concentrations of aqueous CTAB (A), SDS (B) and TX100 (C) in the absence and presence of IL. ent trends of Stokes’s shift for various surfactants could be due to the various kinds of interactions between the charged/uncharged head groups of the surfactants and the deprotonated forms of cur- cumin. 3.3. Quenching study by o-nitrophenol o-Nitrophenol can strongly quench the fluorescence of cur- cumin by forming a ground state complex through hydrogen bonding [24] as given in Scheme 1. However, the extent to which it quenches may highly depend on the conditions of the medium in which curcumin and o-nitrophenol 1826 D. Patra, C. Barakat / Spectrochimica Acta Part A 79 (2011) 1823– 1828 HO O O H 3 CO OCH 3 OH H H O O - N O Formation cyclic groun d stat e compl ex of curc umin with o-nitroph enol Scheme 1. Ground state complex formation of curcumin with o-nitrophenol causing fluorescence quenching of curcumin by o-nitrophenol. can interact and hence, on the nature of the surfactants. The position of the functional groups in o-nitrophenol and the geom- etry of the molecule predict the location of o-nitrophenol in the micelle [44]. The benzene ring of the phenol is pushed towards the hydrocarbon core and the polar functional groups remain in the hydrophilic layer of the micelle [44]. Given that the stoichiometric ratio of o-nitrophenol to curcumin is 1:1, the nitro and hydroxyl groups of the quencher interact with the carbonyl and hydroxyl groups of the enol form of curcumin by means of strong hydrogen bonds [24]. This associated complex, which is formed in the ground state, greatly quenches the fluorescence of curcumin through the following process: curcumin* + o-nitrophe nol [curcumin- o-nitrophenol ]* [curcumin- o-nitrophenol ] curcumin + o-nitrophenol [curcumin- o-nitrophenol ] [curcumin- o-nitrophenol ]* hν a hν a hν fl hν fl Using the Stern Volmer equation [45] the quenching rate constant K sv of curcumin and the quencher, o-nitrophenol, was determined as I 0 f I f = 1 + K sv [oNP] I 0 f I f = 1 + k q 0 [oNP] where K sv is the Stern Volmer rate constant, I 0 f is the fluorescence intensity without the quencher, I f is the fluorescence intensity with the quencher, k q is the quencher rate coefficient, 0 is the fluores- cence lifetime of curcumin without the presence of the quencher and [oNP] is the concentration of o-nitrophenol. Fig. 4 illustrates the fluorescence spectra of curcumin in the presence of SDS with- out and with various concentrations of o-nitrophenol. The insert in Fig. 4 presents the Stern Volmer plot [45] for curcumin in presence of various concentration of o-nitrophenol. The fluorescence spectra of curcumin in water, CTAB and TX100 without and with various concentrations of o-nitrophenol along with their respective Stern Volmer plots showed similar trends (not shown). The estimated values of K sv and k q for fluorescence quench- ing of curcumin by o-nitrophenol in water and various micellar media is determined as per the Stern Volmer equation [45] and given in Table 3. The quenching rate constant of curcumin by o- nitrophenol in water was determined to be 449 M −1 in comparison to 3973 M −1 in cationic CTAB. The high quenching rate of CTAB is due to the stabilizing electrostatic interactions between the pos- itively charged head groups of the micelles and the negatively charged enolic curcumin (see Supplement 1). This attractive inter- action facilitates the penetration of curcumin in the Stern layer of the micelle and hence the formation of the complex [CUR–NP]. In the case of anionic SDS, a decrease in the quenching rate constant was found relative to that of water. This change can be linked to Fig. 4. Fluorescence emission spectra of curcumin in SDS in the presence of various concentration of o-nitrophenol. The fluorescence intensity decreases with increase in o-nitrophenol concentration. Insert shows Stern Volmer plot for the determina- tion of the quenching rate constant K sv . D. Patra, C. Barakat / Spectrochimica Acta Part A 79 (2011) 1823– 1828 1827 Table 3 Quenching rate constants of curcumin by o-nitrophenol in water, CTAB, SDS and TX100 surfactant solutions. Sample K sv (M −1 ) k q (( 0av = 2.366 ns) Water 449 1.9 × 10 11 M −1 s −1 CTAB 3973 1.7 × 10 12 M −1 s −1 SDS 367 1.6 × 10 11 M −1 s −1 TX100 550 2.3 × 10 11 M −1 s −1 the repulsion between the negatively charged head groups of the micelle and the negative charge on the deprotonated curcumin, thus destabilizing the complex [CUR–NP]. In the case of neutral TX100, a slight increase in the quenching rate was observed relative to that of water. The neutrality of this surfactant does not change the physical properties of the solvent but helps in bringing together o-nitrophenol and curcumin due to hydrophobic interactions. 3.4. Effect of ionic liquid [bmin][BF 4 ] on drug–surfactant association The properties of various aqueous surfactant solutions were modified by a common and popular hydrophilic 1-butyl-3- methylimidilazolium tetrafluoroborate, [bmin][BF4]. For modify- ing properties of aqueous surfactant solution, the IL concentration 1% (v/v) was chosen from the literature [4,5]. The absorption (see Fig. 5) and fluorescence excitation and emission (see Fig. 5) spec- tra of curcumin in various surfactant concentrations in presence of IL showed the absorbance or fluorescence intensity of curcumin 700600500400300 0.0 0.5 1.0 1.5 2.0 2.5 3.0 7-8 6 5 4 3 2 Absorbance Wavelength (nm) (1 ) NO TX100 (2 ) 0.02 mM (3 ) 0.06 mM (4 ) 0.2 mM (5 ) 0.4 mM (6 ) 0.6 mM (7 ) 0.8 mM (8 ) 1.0 mM Curcum in plus IL with [T X10 0] 1 700650600550500450400350300 0.0 4.0x10 6 8.0x10 6 1.2x10 7 1.6x10 7 2.0x10 7 8 8 7 7 6 6 4-5 Fluorescence Intensity (a.u) Wave leng th (nm ) 0 1x10 6 2x10 6 3x10 6 4x10 6 5x10 6 6x10 6 7x10 6 8x10 6 9x10 6 Curcumin plus IL with TX100 10 10 9 9 4-5 1-3 1-3 (1) No TX100 (2) 0.02 mM (3) 0.04 mM (4) 0.06 mM (5) 0.1 mM (6) 0.2 mM (7) 0.4 mM (8) 0.8 mM (9) 1.0 mM (10 ) 1.6 mM Fig. 5. Absorption and fluorescence (excitation and emission) spectra of curcumin in various aqueous TX100 concentrations in the presence of IL. in CTAB, SDS and TX100, increased with surfactant concentration. The association constants for the three surfactants with curcumin in the presence of IL were determined as explained earlier and given in Table 1. The association constant of CTAB in the presence of IL decreased significantly relative to CTAB without IL. Though the short hydrophobic effect of the tail may encourage the IL to locate around the Stern layer of the micelle, the positive charged head group would repulse with the similar charged head groups of CTAB. Finally both CTAB and IL will compete to bind with deprotonated form of curcumin. This competition could account for the decrease in the associa- tion constant of curcumin with CTAB. However, in the case of SDS in the presence of IL, an increase of the association rate constant was observed compared to SDS without IL. In the absence of IL, there is repulsion between the negative charge of the head group (sul- fate ion) of SDS and the negative charge of the deprotonated form of curcumin. When IL is added, its positive charge head group will act as a stabilizer between negatively charged SDS and negatively charged curcumin (deprotonated form), thus facilitating the asso- ciation of curcumin with SDS. On the other hand, the association rate constant of curcumin with TX100 increased significantly in the presence of IL. A possible explanation would be the induction of hydrogen bonding and dipole–dipole forces by the positive charge of the head group of the IL with TX100 [4], assisting interaction or strong association of curcumin with neutral surfactant solution. 3.5. Effect of ionic liquid [bmin][BF 4 ] on micellization As discussed earlier, the cmc of various aqueous surfactant solu- tions was evaluated based on the change in Stokes’ shift (see Fig. 3) of curcumin in the presence of 1% (v/v) IL. Variation of Stokes’ shift with surfactant concentration for CTAB with and without IL showed similar trends. It could therefore be implied that there is no new kind of favorable interaction between the IL and CTAB. However, similar plots for SDS with and without IL gave two different trends indicating that the interaction of curcumin with SDS in the pres- ence and absence of IL are not similar. As shown earlier, in the absence of IL, the Stokes’ shift of curcumin increased with increase in SDS concentrations until cmc was reached. However, when IL was present, Stokes’ shift continued to decrease, but at a much smaller rate, with increasing SDS concentration. This trend could imply that in the case of SDS, there could be a favorable interac- tion that stabilizes the micelles in the presence of IL. For TX100, variation of Stokes’ shift with surfactant concentration showed dif- ferent trends in the presence and absence of IL. Without IL, there was a big increase in Stokes’ shift of curcumin after the cmc was reached whereas in the presence of IL, there was a notable decrease of Stokes’ shift after the cmc. This implies that the interactions of TX100 solutions in the presence and absence of IL are of different nature. It was found that cmc of CTAB decreased when 1% (v/v) IL was added (Table 2). This decrease indicates that in the pres- ence of the hydrophilic IL, the formation of micelles is favored at relatively lower concentrations. A possible reason for this observa- tion would be the favorable hydrophobic interaction of the carbon chains of both CTAB and [bmin][BF4] as well as the cumulative electrostatic interaction among CTAB, curcumin and [bmin][BF4]. Thus, both the electrostatic interaction and the tendency of the hydrophobic chains to come together further encourage the for- mation of micelles and hence lowers the cmc. Similarly, the cmc of SDS decreased significantly in the presence of IL (Table 3). The lowering of the cmc of SDS in the presence of IL was also reported earlier [3] and this could be attributed to both the hydrophobic effect and the attraction between the anionic SDS and the positively charged IL. The cmc of TX100 in IL increases from 0.2 mM to 0.4 mM by Stokes’ shift measurement. Along with an aryl and an eight car- bon hydrophobic chain (C 8 H 17 ), TX100 has 100 monomoric units 1828 D. Patra, C. Barakat / Spectrochimica Acta Part A 79 (2011) 1823– 1828 containing an oxygen atom (ether group). The head of TX100 con- tains a –OH group that interacts directly with the head group of IL via hydrogen bonding and dipole–dipole interactions [4]. If the micellar formation of TX100 had to be favorable in the presence of IL, then the immediately available etheric monomeric group of TX100 (after the –OH group) must interact with the immediately available hydrophobic tail of IL (after the polar head group). How- ever, the short hydrophobic tail of IL and the polar monomeric chain of TX100 make this interaction unfavorable at low concentrations. Thus, to form micelles, the etheric chains of TX100 must overcome the hydrophobic effect induced by the tail of the IL. This causes the cmc of TX100 to increase in the presence of IL. 4. Conclusion The association of dye/drug molecule with surfactant solutions depends on the charge of the head group of the surfactant and physiochemical properties of the dye [36–39]. The present binding study of curcumin with various surfactant solutions and quenching of curcumin by o-nitrophenol clearly predict electrostatic inter- action of head group of surfactant molecule and deprotonated form of curcumin, while curcumin having greatest affinity for cationic than non-ionic and finally anionic surfactant solution. The observation that the changes of association of drug like cur- cumin with surfactant solutions are dramatic in the presence of IL [bmin][BF 4 ] compared to without IL [bmin][BF 4 ] presents clear evi- dence the importance of IL [bmin][BF 4 ] in modulating association of curcumin with surfactant solutions. The interaction involving non-ionic TX100 surfactant appear to have more dramatic effect on the association of curcumin-surfactant solutions compared to that involving cationic CTAB and then anionic SDS surfactant due to interactions of IL [bmin][BF 4 ], curcumin and head group of the surfactant. Though the major reason for alternation of aggregation number by IL [bmin][BF 4 ] [3,4] is due to electrostatic interactions between head group of the surfactant and anion [46] or cation [47] of the IL [bmin][BF 4 ], our results showing early formation of micelle irrespective of cationic or anionic aqueous surfactant solutions and delay in micelle formation in the case of neutral aqueous surfactant solution suggest hydrophobic interaction of IL [bmin][BF 4 ] do play a crucial role. These findings will further enhance potential appli- cation of IL as a modulator in solubilization in the micellar system, association of drug–surfactant during drug delivery, micellization and chemistry. Acknowledgements Financial support provided by Lebanese National Council for Scientific Research (LNCSR) and American University of Beirut, Lebanon through the University Research Board (URB) and Long- term Faculty Development grant to carry out this work is greatly acknowledged. Appendix A. 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Colloid Interface Sci. 331 (2009) 196. [47] K. Behera, H. Om, S. Pandey, J. Phys. Chem. B 113 (2009) 786. . association and micellization of cationic, anionic and non -ionic surfactant solutions Digambara Patra ∗ , Christelle Barakat Department of Chemistry, Faculty of Arts and . Molecular and Biomolecular Spectroscopy j ourna l ho me page: www.elsevier.com/locate/saa Unique role of ionic liquid [bmin][BF 4 ] during curcumin surfactant association and . of head group of the surfactant. IL[bmin][BF 4 ] encouraged early formation of micelle in case of cationic and anionic aqueous surfactant solutions, but slightly