Ionic liquid induced alterations in the physicochemical properties of aqueous solutions of sodium dodecylsulfate (SDS)

Colloids and Surfaces A: Physicochem. Eng. Aspects 430 (2013) 58–64

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Ionic liquid induced alterations in the physicochemical properties of aqueous solutions of sodium dodecylsulfate (SDS) Amalendu Pal ∗ , Sheena Chaudhary Department of Chemistry, Kurukshetra University, Kurukshetra 136119, India

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Micellization behaviour of anionic surfactant in presence of pentyl appended imidazolium based ionic liquid has been investigated systematically. • Various thermodynamics parameters have been evaluated using conductivity data. • An increase in cmc on addition of ionic liquid has been observed.

a r t i c l e

i n f o

Article history: Received 23 January 2013 Received in revised form 31 March 2013 Accepted 1 April 2013 Available online 11 April 2013 Keywords: Anionic surfactants Ionic liquids Micelles Pyrene fluorescence Conductivity Cmc

a b s t r a c t Changes in the micellization behavior of an anionic surfactant sodium dodecylsulfate (SDS) in aqueous solutions have been investigated at varying concentration of hydrophobic ionic liquid 3methy-1-pentylimidazolium hexafluorophosphate [C5 mim][PF6 ] using techniques like conductometry, densiometry and speed of sound at a temperature range of 298.15–318.15 K. Critical micelle concentration (cmc) of SDS increases upon addition of [C5 mim][PF6 ], which is also evidenced from fluorescence spectra of pyrene probe. Presence of solvophobic interactions is proposed to be the reason for these observations. © 2013 Elsevier B.V. All rights reserved.

1. Introduction. Favorable alterations of the physicochemical properties of aqueous micellar solutions of surfactants are important due to their applications in colloidal formulation. Ionic liquids (ILs) have emerged as one of the most interesting chemical both in fundamental and applicative research during last three decades [1–6]. ILs are special because of their properties such as low vapor pressure, tunebility [7], large liquid ranges and unique solvating ability which can be utilized in synthesis, catalysis and extraction

∗ Corresponding author. Tel.: +91 1744 239765; fax: +91 1744 238277. E-mail address: [email protected] (A. Pal). 0927-7757/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2013.04.001

processes for the reduction of the amount of volatile organic solvents (VOSs) used in industry [8]. Disubstituted imidazolium cation based ionic liquids have been the most extensively studied in the open literature with anions like [BF4 ]− [PF6 ]− , [N(CF3 SO2 )2 ]− , [Br]− , [Cl]− , [CF3 SO3 ]− , [MeSO4 − ], [SCN]− , [MDEGSO4 ]− , [FeCl4 ]− and [CoBr4 ]− for various applications [9–33]. The utilization of ILs to alter the solution properties of surfactant solutions is an interesting and appealing concept both from environmental and applicative point of view. Armstrong et al. and Fletcher and Pandey have demonstrated the effectiveness of ILs in altering the key physicochemical properties of the aqueous solution of zwitterionic, nonionic and anionic surfactants [33–40]. The ability of IL in changing the properties of aqueous solution of surfactant is dependent on extent of interactions between the cation/anion of

A. Pal, S. Chaudhary / Colloids and Surfaces A: Physicochem. Eng. Aspects 430 (2013) 58–64

PF6 N

N

[C5mim][PF6] O O

S

O Na

O

SDS Scheme 1. Chemical structure of ionic liquid 3-methy-1-pentylimidazolium hexafluorophosphate [C5 mim][PF6 ] and surfactant sodium dodecylsulfate (SDS).

the IL and surfactant head group. Most of these investigations are based on the formation of IL aggregates in a variety of surfactants with imidazolium cation appended with even number of carbon atoms in alkyl chains [34,36,40–43]. It is well reported in literature that cmc of ionic surfactants in aqueous solutions decreases with the increase in ionic strength of the solution [44]. The presence of salt in water screens the electrostatic repulsion between charged head groups facilitating aggregation between surfactants and thereby lowering the cmc. Considering the lesser hydrophilic nature of IL ions compared to common inorganic ions the cmc of ionic surfactants is expected to be less because of better counter ion binding as water produces lesser resistance to the IL ions [34,35]. There are reports in literature that cmc of the surfactants is higher in ILs compared to pure water which is attributed to lower solvatophobicity of the surfactant in ionic liquids [45]. Herein we report the changes occurring in micellar behavior of anionic surfactant sodium dodecylsulfate (SDS) with the addition of hydrophobic ionic liquid 3-methy-1-pentylimidazolium hexafluorophosphate [C5 mim][PF6 ]. Due to the solubility problems of [C5 mim][PF6 ] in aqueous media the present study is limited only up to lower concentration. The outcomes of the results have been compared with interaction behavior of 1-butyl-3-methylimidazolium hexafluorophosphate with the surfactant solution [39]. There has been no report so far regarding the micelles formed by ions having pentyl side chain. Since there is always continued search for formulations with specific properties for various applications, the present results will be useful in designing formulations consisting of SDS and ionic liquid. The present study can prove to be useful from both practical and academic points of view. Chemical structures of the IL [C5 mim][PF6 ] and sodium dodecylsulfate (SDS) studied in this system are presented in Scheme 1. 2. Materials SDS (purity 99%, AR) was obtained from Himedia and was used as received. IL [C5 mim][PF6 ] was prepared in our laboratory and structure was confirmed by IR and NMR technique. IL was dried under reduced pressure prior to the experiment and moisture content was checked by using Karl Fischer analysis. Deuterium oxide (SD Fine Chemicals) having isotopic purity ≥ 99.9% was used as solvent in 1 H NMR studies. Pyrene (purity 99.9%) from Sigma–Aldrich, cetylpyridinium chloride (purity 99%) from Loba Chemie and methanol (99%) from Rankem were used as received. All molar quantities were based on the International Union of Pure and Applied Chemistry (IUPAC) relative atomic mass table [46]. 2.1. Synthesis of [C5 mim][PF6 ] 1-Bromopentane added dropwise to 1-methylimidazole in a round bottom flask. The solution heated to reflux around 70–80 ◦ C

59

for 24 h under nitrogen, and then cooled to room temperature for 12 h. The resulting compound was washed with ether several times to yield a viscous liquid, which was dried in vacuo to give 3methyl-1-pentylimidazolium bromide ([C5 mim][Br]) with a yield of approximately 82%. KPF6 was added to a solution of [C5 mim][Br] in dichloromethane and stirred for 24 h. The suspension was filtered to remove the precipitated bromide salt. The organic phase was repeatedly washed with distilled water (4 × 30 mL) until no precipitation of AgBr occurred in the aqueous phase upon the addition of a concentrated AgNO3 solution. The organic phase then washed two more times with water to ensure the complete removal of the bromide salt, dried over MgSO4 . The solvent removed in vacuo and the resulting IL stirred with activated charcoal for 12 h. The IL was then passed through a short alumina column(s) (acidic and/or neutral) to give a colorless IL, which was dried at 100 C in vacuo for 24 h or until no visible signs of water were present in the IR spectrum (ABB MB3000) [47]. Yield: 80%. Karl Fischer coulometric titration (Metrohm) was used to measure the residual water content. The analysis was conducted as a function of time over 3 days, under ambient conditions, until no more water was detected. 1 H NMR (D O, 300 MHz): ı = 0.95(3H,t), 1.31(2H,m), 3.72(2H,t), 2 2.46(1H,d), 3.11(1H,s), 1.96(3H,s), 1.74(2H,m) 3. Methods Required amounts of materials were weighed using an A&D Co. limited electronic balance (Japan, model GR-202) with a precision of ±1 × 10−2 mg. All the experiments were carried out in doubly distilled de-ionized water obtained from a Millipore, Milli-Q Academic water purification system having resistivity ≥18 M cm. 3.1. Density and speed of sound measurements Densities and speeds of sound were measured with an Anton Paar DSA 5000 (oscillating U-tube density and speed of sound analyzer) instrument and the temperature was controlled to ±1 × 10−2 K by a built-in solid-state thermostat. Before each series of measurements, the densimeter was calibrated with doubly distilled, degassed water and with dry air at atmospheric pressure. The maximal error in the measurements of density and speed of sound relative to water (997.050 kg m−3 and 1496.687 m s−1 ) [48,49] is estimated to be less than 5 × 10−3 kg m−3 and 5 × 10−2 m s−1 . 3.2. Conductometry Electrical conductivities were measured at different temperatures (298.15–318.15 K) with an uncertainty of ±1 × 10−2 K in a water jacketed flow dilution cell, by using a digital conductivity meter CM-183 microprocessor based EC-TDS analyzer with ATC probe and conductivity cell with platinized platinum electrodes purchased from Elico Ltd., India. Prior to measurements, cell was calibrated with the aqueous KCl solutions in the concentration range of 0.01–1.0 mol kg−1 . At least five measurements made for each concentration and only the mean values were taken into consideration. Uncertainty of the measurements was less than 0.3%. 3.3. Fluorescence measurements Fluorescence spectra were taken on model RF-5301PC with blazed holographic grating excitation and emission monochromators having 150 W xenon lamp purchased from Shimadzu. SDS and IL solutions at various mole fractions in water were freshly prepared in doubly distilled de-ionized degassed water. Stock solution of fluorescence probe, pyrene was prepared in methanol and stored in pre-cleaned amber glass vial. Aqueous SDS solutions

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900 800

(b) 308.15 K

(a) 298.15 K

800

700

700

500 0 0.02 0.05 0.07 0.1

400 300 200

4

8

12

16

κ / (μS)

κ / (μS)

600

600 500 0 0.02 0.05 0.07 0.1

400 300 200

20

4

6

8

10

12

14

16

18

20

[SDS] / (mM)

[SDS] / (mM) 2.3

(c) 318.15 K

2.2 4

800

κ / (μS)

700 600 500 0 0.02 0.05 0.07 0.1

400 300 200

4

6

8

10

12

14

16

18

20

CMC / mol frac x 10

900

2.1 2.0 1.9 1.8 1.7 298.15 K 308.15 K 318.15 K

1.6 1.5 0.00

0.02

0.04

0.06

0.08

0.10

[C5mim][PF6] / (w/w%)

[SDS] / (mM)

Fig. 1. Specific conductance () of aqueous SDS at different wt% (() 0, (䊉) 0.02, () 0.05, () 0.07, () 0.1 wt%) of [C5 mim][PF6 ] at different temperature (a) 298.15 K, (b) 308.15 K, (c) 318.15 K, (d) temperature dependence of micellization of SDS as a function of different wt% of [C5 mim][PF6 ].

of the probes were prepared taking appropriate aliquots of the probes from the stock and evaporating methanol using a gentle stream of high purity nitrogen gas. Aqueous SDS and IL of desirable concentration were added to achieve required final probe concentration. 3.4. Spectral analysis The Fourier transform infrared (FT-IR) spectra of aqueous SDS with fixed wt% IL were recorded using ABB Horizon (MB 3000) spectrometer. The NMR chemical shifts for 1 H were observed with a Bruker FT-NMR spectrometer operating at 300 MHz. In order to determine chemical shift ı for [C5 mim][PF6 ]-SDS solution in water, deuterium oxide (D2 O) was used as an external solvent for all the NMR measurements. All data analysis performed using Microsoft Excel and Origin 6.1 software’s. 4. Results and discussion Micellization of SDS in aqueous solution: The micellization behavior of SDS aqueous solution at varying amount of [C5 mim][PF6 ] has been studied using densiometry, conductometry and fluoremetry techniques. 4.1. Density and speed of sound measurements Experimental data of density () and speed of sound (u) of the solutions of SDS measured over the concentration range 5–14 mM

in IL/water mixtures have been given in Tables S1–S3 in supporting information. At lower concentrations,  and u remain almost constant, then changes rapidly, and levels off at higher concentrations. This behavior is a consequence of micellization, the sudden decrease in  corresponds to micelle formation. The critical micelle concentration thus obtained of aqueous SDS by adding varying amounts of [C5 mim][PF6 ] are in reasonable agreement with those obtained from other methods reported below. However, speed of sound measurements gave tentative observations. 4.2. Conductometry Specific conductance () measurements are usually used to study the micellization behavior of surfactants in aqueous solution [50–52]. The plots of specific conductance as a function of concentration of SDS upon addition of 0–0.1 wt% of [C5 mim][PF6 ] at a temperature range varying from 298.15 to 318.15 K, are given in Fig. 1. As can be seen from Fig. 1 (a)–(c) all the systems studied shows similar behavior. Specific conductance increases with a higher slope before cmc due to the increase in concentration of SDS ions in the bulk. The slope of  versus [SDS] decreases after cmc as the micelles are usually less conducting due to decrease in ionic mobility and screening of the charge by counter ions. Precise values of cmc of SDS at different wt% of [C5 mim][PF6 ] were obtained by conductivity data using the Philips definition [53] of the cmc as the “point corresponding to the maximum change in the gradient of a physical property of solution against concentration”. Distinct break points in the specific conductance

A. Pal, S. Chaudhary / Colloids and Surfaces A: Physicochem. Eng. Aspects 430 (2013) 58–64

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Table 1 Critical micelle concentration (cmc) of SD at different temperatures and values of standard free energy of micellization (Gm ◦ ), standard entropy of micellization (Sm ◦ ) and degree of ionization (˛) at 298.15 K, in varying amount of [C5 mim][PF6 ]. [C5 mim][PF6 ]

cmc (mol fraction) × 104

wt%

298.15 K

308.15 K

0 0.02 0.05 0.07 0.1

1.53 1.66 1.77 1.84 1.93

1.59 1.73 1.84 1.91 2.05

Gm ◦

Sm ◦

318.15 K

(kJ mol−1 )

(J K−1 mol−1 )

1.64 1.82 1.95 2.04 2.22

−29.9 −31.2 −30.7 −30.4 −30.2

91.6 93.9 93.0 92.4 85.2

versus concentration profiles of the aqueous solutions of SDS with different wt% IL were also observed at the cmc’s. Based on Onsager theory of electrolyte conductivity [51], one expects two linear fragments for conductance in pre and post micellar regions of an aqueous ionic surfactant solution and the slope becomes smaller, once the micelle is formed. The degree of counter ion dissociation (˛) has been evaluated from the ratio of the slopes of post to pre-micellar region. Experimental points of linear fragments were fitted to first-degree polynomial with a correlation coefficient greater than 0.998. It is well documented in literature that the cmc of surfactants in aqueous solution of ILs tend to be higher than that of aqueous solution of surfactants due to ‘solvophobicity’ of the ionic liquid ions [14,34]. The obtained cmc of SDS in pure water is in good agreement with the literature report [54]. Fig. 1(d) shows the effect of temperature on cmc of SDS at varying amount of [C5 mim][PF6 ]. The cmc of SDS increases with the increase in temperature. The increase in cmc both with the increase in additive concentration and temperature is attributed to disruption of the palisade layer of the micelles. Micellization of the surfactant in aqueous solution is generally considered to be feasible with the increase in entropy. As water does not hydrogen bond with the surfactant hydrocarbon chains, the water molecules would form a structure surrounding the hydrophobic groups, which produces cavities in the water structure which is driven by decrease in entropy due to ordering of water molecule around surfactant ion. At cmc surfactant hydrophobic cores are removed from the water and forms micellar cores with their hydrophilic parts directed toward water causing disruption in the water ordering and hence increase in entropy [55]. According to general picture of the structure of aqueous solutions, insertion of hydrophobic IL into the inter-cluster space of water gives rise to hydrophobic interactions [56]. The disruption of water structure around the IL makes the solution more hydrophobic in character and consequently solubilize greater amount of surfactant, which in turn leads to an increase in cmc. From the temperature dependence of the cmc, the enthalpy of micellization (Hm ◦ ) obtained using van’t Hoff relation [52]: Hm ◦ = −RT 2

d (ln cmc) dT

(1)

Standard free energies of micellization (Gm ◦ ), of SDS in the solutions were derived by applying the mass action model using the equation [57]: Gm ◦ = (2 − ˛)RT ln cmc Entropy of micellization (Sm tion [52]: Gm ◦ = Hm ◦ − TSm ◦

(2) ◦)

was calculated using the rela(3)

where R is the gas constant, T is the absolute temperature, cmc is expressed in mol fraction and ˛ is the degree of ionization of the micelles which is computed from the ratio of slopes of the post-micellar and pre-micellar regions of the specific conductance versus concentration profiles. The thermodynamic parameters calculated have been summarized in Table 1. The negative sign of Gm ◦ indicates the feasibility of surfactant micellization for

˛

0.626 0.569 0.568 0.577 0.574

every system studied. It also indicates the onset of solvophobic interactions (analogous to hydrophobic interactions in aqueous surfactant solution) which governs the micelle formation of surfactant molecules in [C5 mim][PF6 ]. The Gm ◦ of SDS is greater for the system containing different wt% of the IL when compared pure water, indicating that micellization process is more feasible in water + IL system and the feasibility is maximum in 0.02% of IL + water system. The large increase in entropy also supports the spontaneous micellization as micellization is driven by increase in entropy. Contribution of entropy term toward micellization is significant which a characteristic nature of the solvophobic interactions is in the system [58]. ˛ for water + IL system is less compared to the pure water and remains almost constant with the increase in IL concentration. This implies that addition of [C5 mim][PF6 ] do not hinder the structure of micelle rather it only delays the process of micellization. As the ionic liquid ions contains a pentyl chain which also posses the property of aggregation although micellization is not feasible in the pentyl chain system. So the delay in micellization could be due to hydrophobic interaction between the SDS and IL cation alkyl chain at low concentration of SDS. But as the concentration of the SDS is increased dodecyl chain of SDS anion displaces IL cation from the interior of the micelle leading to spontaneous micellization. The possibility of IL cation to sit on the palisade layer of the dodecyl sulfate micelle can also not be ruled out owing to the electrostatic interaction between PF6 − and Na+ in the solution. However, due to lack of any literature precedence on cmc determination of pentyl chain appended IL-surfactant solutions, we decided to confirm the cmc by well-established fluorescence probe method. Toward this, we have used most common and popular fluorescence probe pyrene.

4.3. Fluoremetry The fluorescence of the polarity probe such as pyrene can be exploited to find the polarity of the cybotactic region of the fluorophore in the surfactant solution undergoing micellization. Pyrene is one of the few condensed aromatic hydrocarbons that show significant fine structure (vibrational bands) in their monomer fluorescence spectra in solution. The Pyrene polarity scale is defined as emission intensity ratio ‘II /IIII ’ where band I (ca. 376 nm) corresponds to S1 ( = 0) → S0 ( = 0) transition and band III (ca. 387 nm) is a S1 ( = 0) → S0 ( = 1) [35]. Pyrene polarity scale is a function of solvent dielectric constant (␧) and refractive index (n) via the dielectric cross term [f (␧, n2 )]. Pyrene fluorescence spectra recorded as a function of SDS at two different wt% of [C5 mim][PF6 ] are given in Fig. 2. The curves are the fit of the data to a simplistic sigmoidal equation. The II /IIII decrease very slowly initially because of the adsorption of surfactant molecule at the air/water interface indication a little change in polarity in the bulk. A decrease in II /IIII ratio with the increase in SDS concentration indicates the movement of pyrene to the non-polar region in the bulk which arises due to the aggregation of the SDS monomers in the bulk. The platue formation after cmc indicates that pyrene resides in the hydrophobic core of the micelles. It is observed that the value of II /IIII is less

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1.28 1.26 1.24

II/IIII

Table 3 Chemical shift (ı) of protons close to the tail of SDS in the presence of [C5 mim][PF6 ] (0.1 and 0.05 wt%).

0 0.05 0.1

IL (wt%)

SDS (mM)

1.22

Terminal CH3

Bulk CH2

0.1

0.09 0.07 0.05

0.835 0.841 0.835

1.257 1.264 1.257

0.05

0.09 0.07 0.05

0.864 0.867 0.857

1.288 1.286 1.275

1.20 1.18

Proton peaks in SDS micellar solutions (ppm)

1.16 1.14 0.7

0.8

0.9

1.0

1.1

dissociation and reduction in hydrophobic effect that is usually the major contributor to micelle formation.

1.2

log10[SDS] Fig. 2. Pyrene II /IIII as a function of SDS (mM) at different wt% of [C5 mim][PF6 ]; 0 (), 0.05 (䊉) and 0.1 (). ex = 334 nm and slit widths = 3 and 3 nm.

for the native IL + water system indicating the residence of pyrene in the hydrophobic solution. Also the cmc obtained for the SDS in water + IL is higher than pure water and values are in good agreement with those determined by other methods. The cmc of SDS in the pure water and in the presence of [C5 mim][PF6 ] obtained is provided in Table 2. The increase in the cmc can be due to hydrophobic interactions between the dodecyl sulfate chain and alkyl chain of IL cation as explained in conductivity section. 4.4. Micellar aggregation number from fluorescence quenching The aggregation number (Nagg ) of aqueous SDS micelles in the presence of two different concentrations of [C5 mim][PF6 ] is obtained by fluorescence quenching of pyrene by a cosurfactant cetylpyridinium chloride (CPC) according to the equation [59–61]:

 

ln

IO IQ

=

[CPC]micelle Qmicelle = [micelle] [micelle]SDS

= [CPC]micelle



Nagg [SDS] − cmcSDS

 (4)

where IO and IQ are the fluorescence intensities of pyrene in the absence and presence of quencher CPC, respectively. Qmicelle (or [CPC]micelle ), [micelle]SDS , and [SDS] are the concentrations of quencher CPC within the micellar pseudo-phase, SDS micelles, and SDS surfactant, respectively. The Nagg of 20 mM aqueous SDS is calculated from Eq. (4) using the slope and the cmc from pyrene II /IIII variation. The Nagg thus obtained are presented in Table 3 along with thermodynamic parameters. Fair-to-good linear relationship is observed between ln (IO /IQ ) and [CPC]micelle (0.9953 ≤ r2 ≤ 0.9996). The Nagg in absence of IL is in fair agreement with that reported in the literature [62]. It is observed that addition of IL to 20 mM aqueous SDS results in decreased Nagg . A gradual decrease in Nagg indicates a decreased degree of [C5 mim][PF6 ] Table 2 The cmc (mol fraction) and aggregation number (Nagg ) of aqueous solution of SDS in presence of two different concentrations of [C5 mim][PF6 ] at ambient conditions using different techniques. Conc. of [C5 mim][PF6 ]

cmc (mol fraction × 104 )

wt%

mM

Conductance

Pyrene I1 /I3

Density

0 0.05 0.1

0 1.7 3.4

1.53 1.77 1.93

1.46 1.68 1.84

1.48 1.59 1.80

Nagg

60 58 57

4.5. Spectral studies To obtain further insight into the interaction between the IL and the surfactant, the system was further investigated by FTIR and NMR studies. FTIR analysis shows a slight change in O-H stretching around the cmc from 3356 to 3364 cm−1 whereas C–C stretching frequency remained almost unaffected in [C5 mim][PF6 ]-SDS micellar solutions. The above observations show that the added IL interacts with the surfactant hydrophilic moiety in the solution. The 1 H NMR chemical shift studies of solutions containing (0.05 and 0.1) wt% [C5 mim][PF6 ] and different concentrations of SDS are reported in Table 3. The 1 H NMR spectra of SDS display four characteristics peaks, terminal methyl protons appear at 0.842 ppm, bulk (–CH2 ) protons at 1.264 ppm, ␤-CH2 protons at 1.642 ppm while –CH2 group in ˛ position near sulfate group resonates at 3.985 ppm. The 1 H NMR spectra of different concentrations of SDS in the presence of 0.05 and 0.1 wt% [C5 mim][PF6 ] was recorded. The presence of IL affects the environment of surfactant protons. The effect is more pronounced on terminal methyl protons which clearly indicate that tail of surfactant is interacting with the IL and head is in contact with aqueous solution. As we move from 0.05 wt% to 0.1 wt% of IL, the terminal methyl group shifts from downfield to upfield. The above observations reveal the surfactant-IL interactions and upfield shift at higher concentrations can be rationalized by assuming the progressive dense packing of surfactant anion in the aggregates with the IL cation adhering to the micellar cavity [63]. 5. Conclusions Micellization behavior of anionic surfactant (SDS) at different wt% of [C5 mim][PF6 ] was investigated. The presence of [C5 mim][PF6 ] appears to severely alter the micellization behavior of SDS in the solution. The alterations were successfully analyzed via specific conductance and spectroscopic measurements at varying amount of [C5 mim][PF6 ]. The cmc of SDS increases both with increasing the concentration of [C5 mim][PF6 ] and temperature suggesting the solvophobic interactions around the surfactant hydrocarbon chains caused by [C5 mim] alkyl chain which were further rationalized by calculating the changes in thermodynamic param0 values indicated that micellization is more eters [53,59]. Gm feasible in water + IL system and is driven by increase in entropy. The observed results are contrary with to that reported for the micellization of SDS with the addition of IL [C4 mim][PF6 ] [39]. There are several reports on cmc modulation of aqueous SDS on IL addition. Addition of [C2 mim][I] and [C4 mim][Cl] increases the cmc of aqueous SDS to 170 mM and 70 mM from mM respectively, the decrease in cmc to 2.8, 1.9, 1.9 and 0.9 were observed when [C6 mim][Cl], [C8 mim][Cl], [C8 mim][BF4 ] and [C4 mim][PF6 ] respectively were

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added [39,42]. The higher value of cmc in our system as compared to [C4 mim][PF6 ] indicates the importance of chain length of the IL cation in delaying the micellization of SDS in the solution. The solvophobic interactions were considered to be the sole reason in delaying the micellization. The decrease in magnitude of ˛ on addition of 0.05 wt% IL could be due to decreasing micellar size. On further addition of IL, ˛ remains constant which implies the structure of micelle remains the same. These observations are strongly supported by Nagg which could be further attributed to more efficient interactions between [C5 mim]+ and anionic micellar surface of SDS due to relatively less hydration of [C5 mim]+ [39,41]. FTIR and NMR results provide an insight into the dynamics of [C5 mim][PF6 ] association with surfactant moiety. These inferences indicate that surfactant micellization in [C5 mim][PF6 ] is governed primarily by combination of cation-anion interactions with the hydrophobic effect which results in the formation of mixed micelles. [53,59,64]. The present study, along with the literature reports shows that by judicious choice of ILs we can alter the physicochemical properties of surfactant the understanding of which is essential to apply micellar formulations in pharmaceuticals and industrial applications. Acknowledgement One of the authors, S.C. would like to thank UGC, India for a JRF fellowship.

[14] [15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa. 2013.04.001.

[23]

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