Short communication

Determination of cmc of imidazolium based surface active ionic liquidsthrough probe-less UV–vis spectrophotometry

Mudasir Ahmad Rather, Ghulam Mohammad Rather, Sarwar Ahmad Pandit,Sajad Ahmad Bhat, Mohsin Ahmad Bhat n

Department of Chemistry, University of Kashmir, Srinagar-190006, J&K, India

a r t i c l e i n f o

Article history:Received 29 May 2014Received in revised form15 July 2014Accepted 18 July 2014Available online 29 July 2014

Keywords:Surface active ionic liquidsProbe-less UV–visible spectrophotometryTensiometryMicellization

a b s t r a c t

In the first of its kind we herein report the results of our studies undertaken on the micellizationbehaviour of imidazolium based surface active ionic liquids (SAILs) to prove that their critical micelleconcentration (cmc) can be estimated through ultraviolet–visible (UV–vis) spectroscopy without usingany external probe. Tensiometric and spectrophotometric investigations of a series of freshly preparedSAILs viz. 1-octyl-3-methylimidazolium chloride ([OMIM][Cl]), 1-octyl-3-methylimidazolium dodecyl-sulphate ([OMIM][DS]), 1-octyl-3-methylimidazolium benzoate ([OMIM][Bz]), 1-octyl-3-methylimi-dazolium salicylate ([OMIM][Sc]), 1-octyl-3-methylimidazolium acetate ([OMIM][Ac]) are presented asa case study in support of the said claim. The cmcs estimated through spectrophotometric method werefound to be close to the values estimated through tensiometry for the said SAILs. The cmcs for theinvestigated SAILS were found to vary in order of [OMIM][Cl]4[OMIM][Ac]4[OMIM][Bz]4[OMIM][Sc]4[OMIM][DS]. To the best of our knowledge the present communication will be the first report aboutthe synthesis, characterization and micellization behaviour of [OMIM][Bz] and [OMIM][Sc].

& 2014 Elsevier B.V. All rights reserved.

1. Introduction

Ionic liquids (ILs) are a class of organic molten electrolytes withmelting point close to or below 100 1C [1,2]. In addition to beingenvironmentally benign, inherent physico-chemical properties ofILs like high thermal stability, wide liquidus range, high conduc-tivities, large electrochemical window, tuneable solubility andmixing properties make them promising materials for diverseapplications like catalysis [3,4], electrochemistry [5,6] and separa-tions [7–9] etc.

Very recently ILs bearing long alkyl chains have been reportedto exhibit amphiphilic properties. This new category of ILs namedas surface active ionic liquids (SAILs) which are supposed to offercombined desired features of ILs and surfactants [10], have startedattracting considerable attention from solution chemists. On accountof their eco-friendly and amphiphilic nature, the SAILs can beexplored as promising substitutes of conventional surfactants inapplications like micellar catalysis [11], solubilisation [12,13,14],protein folding [15,16] and drug delivery [17] etc. An importantadvantage of working with SAILs is that their hydrophobic and

hydrophilic character can be fine-tuned through easily achievablestructural/functional alterations in their cationic and anionic sub-stituent groups [18,19]. SAILs based on 1-alkyl-3-methyl imidazo-lium cation have become a focus of recent investigations in colloidand interface science [20–24].

In solution chemistry the critical micelle concentration (cmc) isconsidered as one of the important parameters used for evalua-tion/comparison of efficiency of surface active agents for theirdesired applications. Hence the designing of safe, cost effective,rapid and reliable methods for estimation of cmc has been anattractive area of research in colloidal chemistry. The presentlyused methods for estimation of cmc rely on changes in the natureof functional dependence of some physical properties with con-centration on account of micelle formation by a surfactant in itssolution [25]. The simplest and quickest among these are thespectroscopic methods which rely on the change in absorptionbehaviour upon aggregation. However, a disadvantage attributedto spectrophotometric cmc estimation of conventional surfactantsis that the method requires the use of an organic dye or someother species, with a characteristic UV–vis absorption behaviour,as a probe to reflect the onset of micellization [26,27]. Theseadditives in turn may alter the micellization characteristics of thesurfactants and hence lead to estimation of an apparent and notthe actual cmc. Moreover, these probes may alter the structure and

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Talanta

http://dx.doi.org/10.1016/j.talanta.2014.07.0460039-9140/& 2014 Elsevier B.V. All rights reserved.

n Corresponding author. Tel./fax: þ91 194 2414049.E-mail address: mohsinch@kashmiruniversity.ac.in (M.A. Bhat).

Talanta 131 (2015) 55–58

the stability of micelles in an undeterminable way, thereby makingspectrophotometry a less reliable and less preferred method forCMC estimations in surface chemistry laboratories.

The peculiar inherent structural features in certain class ofsurfactants abrogate the need for use of any foreign probe for theestimation of their cmc through spectrophotometry [28]. After akeen assessment of available literature about spectroscopic studieson imidazolium based ILs, it was concluded that the presence ofimidazolium ring makes them to absorb significantly in the entireUV region [29–31]. In view of the presence of UV–vis activeimidazolium in SAILs and expected variations in the environmentaround this ring during micellization, some questions do ariseabout the solution behaviour of this new class of amphiphillicionic liquids: Should UV–vis spectra of such SAILs show a devia-tion or transition from usual behaviour near CMC? How sharpshall this deviation be and how close match the range represent-ing the said transition in absorption behavior will be to the cmcestimated through other reliable methods for such SAILs? Can theimidazolium ring of SAILs act as an intrinsic chromophore forestimation of their cmc spectrophotometrically? In search for ananswer to these questions we investigated the micellization beha-viour of a series of freshly prepared SAILs viz 1-octyl-3-methy-limidazolium chloride ([OMIM][Cl]), 1-octyl-3-methylimidazoliumdodecyl sulphate ([OMIM][DS]), 1-octyl-3-methylimidazoliumbenzoate ([OMIM][Bz]), 1-octyl-3-methylimidazolium salicylate([OMIM][Sc]) and 1-octyl-3-methylimidazolium acetate ([OMIM][Ac]). To the best of our knowledge the synthesis of [OMIM][Bz]and [OMIM][Sc] is yet to be reported in the literature. Althoughmicellization behaviour of [OMIM][Cl] has been reported [32]there is no such report about [OMIM][DS], [OMIM][Bz], [OMIM][Sc] and [OMIM][Ac]. The cmcs of aforesaid SAILs were determinedusing probe-less UV–vis spectrophotometry and tensiometry.

2. Experimental section

Spectroscopic grade imidazolium based surface active ionicliquids (SAILs) were synthesised through the reported procedures[31,33–37] with slight modifications for obtaining better yields. Thedetails of the synthetic procedures are given in the supplementaryinformation. For the estimation of the CMCs of these SAILs, tensio-metric and spectrophotometric techniques were employed. Tesnio-metric measurements were carried out on a Kruss K9 Tensiometerconnected to a HAAKE GH bath that maintained the temperatureat the desired value (within70.1 1C). The spectrophotometricmeasurements were carried on Evolution 201 Thermo scientificUV–visible Spectrophotometer equipped with a Peltier system fortemperature control from 0 to 100 1C (within 70.1 1C).

3. Results and discussion

In our spectrophotometric study, the concentration effect onabsorbance behaviour of each SAIL in UV–visible range (200–360 nm) was monitored. Fig. 1 depicts sets of UV–vis spectrarecorded at different concentrations of [OMIM][DS] in water at298 K. The absorption behaviour is typical of imidazolium basedILs. The inset in Fig. 1, represents a magnification of the recordedspectra in the wave length range of 240–360 nm. The absorptionbehaviour in this range is attributed to aggregates of imidazoliumions [31,38]. A typical absorption versus concentration plot forwavelengths in this range reveals that upto a certain SAIL con-centration the absorbance varies linearly with concentration inaccordance with Beer–Lambert law. But above this SAIL specificconcentration the rate of the increase, though linear, is verydifferent from the rate before. Absorbance versus concentration

plots (Fig. 2) as recorded in the present study fit to two straightline segments making a sharp intersect. The concentration corre-sponding to the point of intersection was found to be specific toeach SAIL and seems to represent the onset of micellizationprocess. The concentrations corresponding to the break pointsobserved in the Beer plots are tabulated in Table 1. Surface tensionmeasurements were also performed to evaluate the cmc of theinvestigated SAILS in their aqueous solutions. Fig. 3 presentssurface tension versus log C profiles at 298 K showing a lineardecrease of surface tension with log C for all the investigatedsolutions up to a SAIL specific concentration beyond which thesurface tension remains almost constant. The absence of minimaaround the breakpoints is a clear indication of absence of impu-rities in the SAILs. The breakpoint concentrations representing thecmc values of the SAILs are presented in Table 1.

As seen from the entries in Table 1, the break point concentrationsobserved in the Beer plots are a close match to those observed in thetensiometric plots for the investigated SAILs, clearly establishing thatthe former correspond to their cmcs. Hence the cmc of imidazoliumbased SAILs can be estimated through spectrospcopic methodswithout involving use of any probe as required for conventionalsurfactants. Needless to mention that the cmc obtained for [OMIM][Cl] through both the techniques in the present study is found to bein good agreement with the value already reported for the said SAILthrough conductometry and tensiometry [32].

The cmcs obtained through tensiometry as well as spectro-photometry follow the same trend and vary in the order of[OMIM][Cl]4[OMIM][Ac]4[OMIM][Bz]4[OMIM][Sc]4[OMIM][DS]. However, as evident from Fig. 2, the Beer plots of [OMIM][Cl]and [OMIM][Sc] exhibit different behaviour after cmc with smallincrease in absorbance in contrast to a sharp increase in others.Both the observations can be explained on the basis of the extentto which the presence of counter ions at micellar surface stabilizesor screens the electrostatic repulsion between head groups andhow the resulting stabilization alters the aggregation behaviour ofthe particular SAIL. The micelle stabilization ability of counterionshas been observed to be a collective result of factors like theirhydrated radius, polarizibility, hydrophobicity, bulkiness and posi-tion in Hofmeister series [15,39–41]. Generally it is observed thatthe average aggregation number of micelles increases withincreasing hydrophobicity of counterions. This implies that ionswith stronger hydrophobicity can bind their counterions morestrongly and hence reduce the repulsive interaction betweensimilarly charged head groups thereby stabilizing the micelle withconsequent reduction of cmc. In line with this argument and as

Fig. 1. UV–vis spectra recorded for changing concentrations of [OMIM][DS] inwater at 298 K. The inset shows magnified version of the spectra in the wavelengthrange 240–360 nm.

M.A. Rather et al. / Talanta 131 (2015) 55–5856

reasoned by Jiao et al. [42] for the lower cmc of [BMIM][DS], thecmc of [OMIM][DS] with long alkyl chains associated with both thecation and anion was observed to be the lowest among theinvestigated SAILs. The low cmcs of [OMIM][Sc] and [OMIM][Bz]in comparison to [OMIM][Cl] and [OMIM][Ac] can be attributed topresence of hydrophobic phenyl ring in the counter anions in boththe cases. Presence of these hydrophobic rings which intersperseinto the palisade layer stabilizes the micelle leading to lowering ofcmc. However, the cmc of [OMIM][Sc] is observed to be less than

that of [OMIM][Bz] which can be attributed to the presence of anextra –OH group in salicylate which might stabilize the micelle toa greater extent. Similar observation has been reported by Akramet al. [43] in their studies related to impact of counter anions onthe micellization behaviour of Gemini surfactants. In the observedcmc sequence, [OMIM][Bz] is followed by [OMIM][Ac], with[OMIM][Cl] having the highest cmc value, perhaps due to higherhydrated size of chloride than acetate ion, owing to which thecounter ion binding shall be less effective in the former case.

Besides cmc, parameters like surface excess and minimum areaoccupied per head group were calculated from the tensiometricplots (Fig. 3) and the estimated values are included in Table 1.Surface excess together with minimum area occupied by a headgroup decide the extent of packing at the air/water interface.Higher surface excess and hence lower Amin indicate close packingat air/water interface [42,43]. As clear from the entries in Table 1,the surface excess values are in the order [OMIM][Bz]4[OMIM][DS]4[OMIM][Cl]4[OMIM][Ac]4[OMIM][Sc], and expectedlythe Amin values follow the reverse trend. The observed trendreflects the extent of stabilization of monolayer through variouspossible interactions, at air/water interface, of the surface activeions with their respective counter ions. The possible interactionsinclude π–π, cation–π, hydrogen bonding and electrostatic inter-actions. The highest surface excess of [OMIM][Bz] may be due tothe extra stabilization of monolayer by hydrophobic benzoate ions,which might reside closer to the interface. Comparatively thelower surface excess of [OMIM][DS] may be attributed to thecompetitive adsorption between its cation and anion, both beingamphiphilic and hence expected to lead to steric crowding at theinterface. Next to [OMIM][DS] in the observed sequence is [OMIM][Cl]

Fig. 2. Variation of absorbance with concentration of Imidazolium based surface active ionic liquids [SAIL] in water in the wavelength range of 240–360 nm.

Table 1cmcs Of various of Imidazolium based surface active ionic liquids [SAILs] in water as estimated through tensiometry and UV–visible spectrophotometry at 298 K. Surfaceexcess (Гmax) and minimum surface area per head group (Amin) estimated from tensiometric plots (Fig. 3) are included.

[SAIL] cmc (mM) cmc (mM) Гmax Amin

Tensiometry UV–visible spectrophotometry (�103 mol m�2) (Å2)

[OMIM][DS] 0.23 0.26 2.072 0.080[OMIM][Sc] 52.50 49.50 1.723 0.096[OMIM][Bz] 69.40 62.10 2.243 0.074[OMIM][Cl] 247.20 248.10 1.927 0.086[OMIM][Ac] 87.30 80.80 1.866 0.089

Fig. 3. Variation of surface tension with concentration of Imidazolium basedsurface active ionic liquids ([SAIL]) in water as recorded at 298 K.

M.A. Rather et al. / Talanta 131 (2015) 55–58 57

which has higher surface excess than [OMIM][Ac] followed by[OMIM][Sc]. The negative charge on acetate as well as salicylate isdelocalized over their large sizes, the result being a weakerinteractions with the imidazolium cation and hence their lowersurface excess compared to [OMIM][Cl] [38].

4. Conclusions

Tensiometric and UV–visible spectrophtometric measurementsof the aqueous solutions of imidazolium based surface active ionicliquids (SAILs) were carried out. These studies establish that thecmc of imidazolium based SAILs can be estimated spectrophoto-metrically from Beer plots without involving any external probewhich is required in case of conventional surfactants. The resultsclearly demonstrate that SAILs show sensitivity to counter ionslike other conventional surfactants for their micellization andsurface activity in aqueous media.

Acknowledgement

MAB thanks Department of Science and Technology, New Delhi,India, for the research grant no. SR/S1/PC-11/2009. MAR thanksCSIR for the financial assistance. Authors would like to thank Prof.Khaliquz Zaman Khan for his support in correcting the grammarand editing of long sentences for the revised MS.

Appendix A. Supporting information

Supplementary data associated with this article can be found inthe online version at http://dx.doi.org/10.1016/j.talanta.2014.07.046.

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