Journal of Molecular Liquids 196 (2014) 395–403

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Journal of Molecular Liquids

j ourna l homepage: www.e lsev ie r .com/ locate /mol l iq

Study of interaction between cationic surfactants and cresol red dye byelectrical conductivity and spectroscopy methods

Anwar Ali ⁎, Sahar Uzair, Nisar Ahmad Malik, Maroof AliDepartment of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, India

⁎ Corresponding author. Tel.: +91 11 26981717x3257;E-mail addresses: anwarali.chem@gmail.com, anwar_j

http://dx.doi.org/10.1016/j.molliq.2014.04.0130167-7322/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 27 December 2013Received in revised form 11 April 2014Accepted 15 April 2014Available online 2 May 2014

Keywords:Cationic surfactantCresol redElectrical conductivityUV–visible spectroscopyInteractionCritical micelle concentration

Interaction of anionic dye, cresol red (CR), with cationic surfactants, dodecyltrimethylammonium bromide(DTAB) and cetyltrimethylammonium bromide (CTAB), in aqueous solution in the submicellar andmicellar con-centration ranges has been investigated by using thermodynamic and spectroscopic methods. The equilibriummodel has been used to calculate the standard free energy (ΔGm

o ), enthalpy (ΔHmo ), and entropy (ΔSmo ) of micelle

formation. The increase in the critical micelle concentration (cmc) with rise in temperature is attributed to thedisruption of the structured water surrounding the hydrophobic groups of the surfactants. A marked decreasein the cmc is observed as the number of carbon atoms in the hydrophobic group increases from DTAB to CTAB.Higher values of the degree of ionization (α) of the micelles of DTAB and CTAB are obtained in the presence ofCR than in its absence. Negative values of ΔGm

o and ΔHmo for the surfactants in aqueous and in aqueous CR solu-

tions show that the process of micellization is thermodynamically spontaneous and exothermic. The values of−TΔSmo are much higher than the ΔHm

o values, indicating that the micellization process is governed primarilyby the entropy gain associated with it. The heat capacity of micellization (ΔCp,mο ) is negative which is mainlydue to the change in the exposure of the hydrophobic groups to thewater molecules. UV–visible spectra suggestinteraction between dye and surfactant monomers forming ion-pair complex in the pre-micellar region whereasabove the cmc solubilization of dye in the micelle dominates over ion-pair formation.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The study of interactions between surfactants and dyes in aqueoussolutions has attracted significant interest in the recent years becauseof their widespread applications and relatively complex behavior. Thisinvestigation is important from the point of view of technology ofdyeing processes as well as for chemical research, such as biochemistry,analytical chemistry, and photosensitization. Interactions of dyes withsurfactants in aqueous solutions can provide useful information aboutthe mechanism according to which surfactants operate as levelingagents and information on the thermodynamics and kinetics of dyeing[1,2]. Moreover, understanding of the electrostatic and hydrophobicinteractions between surfactants and dyes can add to our existingknowledge because similar interactions prevail in biologically impor-tant processes [3–5].

Surfactants are composed of a polar hydrophilic group and a nonpo-lar hydrophobic chain. This unique structural feature makes them toestablish interactions with both the hydrophilic as well as hydrophobicmolecules [6,7]. They form aggregates (micelles) in aqueous solutionsover a narrow concentration range, known as the critical micelle

fax: +91 11 26981232.mi@yahoo.co.in (A. Ali).

concentration (cmc), below which the surfactant molecules are pre-dominantly dispersed asmonomers [8]. For many practical applicationsof detergent micelles play important roles. For instance, micelles inaqueous medium solubilize the organic compounds which are poorlysoluble in water by incorporating them in the micellar phase; micellesare conveniently exploited to act as catalysts for many reactions dueto their large surface area; they alter the reaction pathways, rates andequilibria [9,10]. Moreover, micelle systems are convenient to use be-cause they are optically transparent, stable, and relatively non-toxic[9,10]. Cationic surfactants possess valuable characteristics such asemulsification, wetting, water proofing, repellence and spreading.They are widely used in food industry, pharmaceuticals, and solubiliza-tion of water insoluble dyes and have a great bearing on day-to-day life[8].

Dyes are aromatic compounds containing chromophores, delocalizedelectron systems with conjugated double bonds, and auxochromes, elec-tron withdrawing substituents that cause or intensify the color of thechromophore [11]. Cresol red is a triphenylmethane dye. It is extensivelyused in textile industries for dyeing nylon, polyacrylonitrile modifiednylon, wool, silk, and cotton. Some of the triphenylmethane dyes areused as medicines and biological stains. Paper and leather industries arealso major consumers of triphenylmethane dyes [12]. Electrostatic inter-actions, hydrophobic interactions, hydrogen bonds, pi-stacking, cation–piinteractions, and van der Waals forces are the typical examples of the

396 A. Ali et al. / Journal of Molecular Liquids 196 (2014) 395–403

intermolecular forces that dominate the interaction of dye moleculeswith the surfactant aggregates [13]. However, the nature of interactionmay vary with the types of dye–surfactant systems. Karukstis et al. [14],Yamamoto and Motomizu [15], and Mukerjee and Mysels [16] have re-ported the formation of insoluble or poorly soluble salts or ion-pairs be-tween oppositely charged dye and surfactant in the pre-micellar regionof the surfactant. Above cmc, the dye may revert to its monomericstate, bound to the surface of themicelle or may be solubilized in themi-celle [10,17]. It is, therefore, interesting to investigate the species formedas a result of dye–surfactant interaction in the pre- as well as in the post-micellar concentration ranges of the surfactants, DTAB and CTAB.Although many researchers have reported dye–surfactant interactionsusing different techniques [17–22], thermodynamic study of such inter-actions has received little alteration. Therefore, in the present work, inorder to have fine details of dye–surfactant interactions at molecularlevel, thermodynamic study is complemented with the spectroscopicstudy.

In this paper, we report on the interactions of the cationic surfac-tants, DTAB and CTAB with anionic dye, CR (Fig. 1) using conductomet-ric and UV–visible spectroscopic methods in the pre- as well as in thepost-micellar regions of the two surfactants. The effect of increasingalkyl chain length from DTAB to CTAB, having the same polar groupbut different hydrophobic groups, on the dye–surfactant interactionswill also be explored in order to understand the importance of thehydrophobic forces. Literature survey indicates that the study onCR–DTAB/CTAB interactions in aqueous medium involving boththermodynamic as well as spectroscopic methods is rare. Theseconsiderations led us to undertake the present study.

2. Experimental

2.1. Materials

Cresol redwas purchased fromAlfa Aesar, India. It was recrystallizedfrom pure water and dried before use. Dodecyltrimethylammoniumbromide and cetyltrimethylammonium bromide were extra-pure, pur-chased from Acros Organics, Belgium, and were used after recrystalliza-tion from ethanol, dried in vacuum over P2O5 at room temperature forabout 72 h. The specifications of the chemicals used are given in theTable 1. All the solutions were prepared in doubly distilled deionized

a)

c)

b)

Fig. 1.Molecular structures of (a) cetyltrimethylammoniumbromide, (b) dodecyltrimethyl-ammonium bromide, and (c) cresol red.

and degassed water, with electrical conductivity less than 1.05 × 10−4

S m−1 at 298.15 K.

2.2. Methods

2.2.1. Electrical conductivity measurementSolutions of DTAB in the concentration range from 0.008 to 0.024 m

(mol kg−1) and of CTAB in the concentration range from 0.0005to 0.0013 mol kg−1 were prepared in pure water. Stock solution of3.0 × 10−5 mol kg−1 cresol red in pure water was prepared and wasused as solvent for the preparation of surfactant solutions in aqueouscresol red. Surfactant solutions in the concentration range from 0.008to 0.024 mol kg−1 for DTAB and from 0.0001 to 0.0013 mol kg−1 forCTAB in 3.0 × 10−5 mol kg−1 aqueous cresol red were prepared tocover the pre- as well as post-micellar concentration ranges. Theweighings were done on Precisa XB-220 A (Swiss-make) electronic bal-ancewith a precision of ±0.1mg. All necessary precautions were takento prepare the solutions. All the solutions were prepared afresh. Electri-cal conductivities of surfactants in purewater and in aqueous cresol redwere measured with a digital conductivity meter, PICO + (LabindiaInstruments Pvt. Ltd.). Prior to use, the conductivitymeter was calibrat-ed bymeasuring the electrical conductivities of 0.01 and 0.1 N solutionsof potassiumchloride (Merck, purity N 99%). The cell constant of the cellused was 1.00 cm−1. The glass cell with two platinum electrodes wasdipped in the sample solution contained in a corning glass tube whichwas properly covered and immersed in an electronically controlledthermostated water bath (Julabo, Model MD, Germany), maintainingthe temperature within ±0.02 K. Electrical conductivity was recordedwhen the solution attained thermal equilibrium. The measurementswere taken in triplicate and mean values were used in all thecalculations. The accuracy in the electrical conductivity measurementwas up to ±1.5%.

2.2.2. UV–visible spectral measurementThe spectra of the dye (CR) in pure water and in the presence of

different concentrations of surfactants were recorded using a PerkinElmer Lambda-40, double-beam UV–visible spectrophotometer with amatched pair of glass cuvettes, 1.0 cm in optical path length, at298.15 K. The stock solutions of the surfactants were prepared by dis-solving the required amounts of DTAB and CTAB in aqueous cresol red.All the test solutions were prepared by diluting the respective stock so-lutions. While the concentrations of both the surfactants were varied,the concentration of CR used throughout this study was kept constantat 3.0 × 10−5 mol kg−1.

3. Results and discussion

In the present work, two sets of experiments were carried out inorder to describe the effect of cresol red dye on the thermodynamicproperties of DTAB and CTAB micellization. For the first set of experi-ment, the electrical conductivity measurements for DTAB and CTAB inpure water and in 3.0 × 10−5 mol kg−1 aqueous solutions of cresolred at different surfactant concentrations covering the pre- and post-micellar concentration ranges were made at 298.15, 303.15, 308.15,and 313.15 K (Table 2). The conductometric study of the present sys-tems is complemented by the second set of experiment — UV–visiblespectra of cresol red dye in purewater and in the presence of the surfac-tants, DTAB and CTAB.

3.1. Conductometric study

Electrical conductivity technique has been found to be highly usefulfor studying the association behavior of various systems [8,10,22–25].Plots of specific conductivity (κ) as a function of [DTAB]/[CTAB] andtemperature in aqueous and in aqueous CR are shown in Fig. 2. The crit-icalmicelle concentration, cmc, values of the two surfactants in aqueous

Table 1Specification of the chemicals used.

Compound CAS number Suppliers Purity

(Stated by suppliers)

Cresol red 1733-12-6 Alfa Aesar, UK N95%Dodecyltrimethylammonium bromide 1119-94-4 Acros Organics, Belgium 99%Cetyltrimethylammonium bromide 57-09-0 Acros Organics, Belgium 99+%

397A. Ali et al. / Journal of Molecular Liquids 196 (2014) 395–403

and in aqueous CR solutions were obtained from the intersection of thestraight lines of κ vs. surfactant concentration plots (Fig. 2) above andbelow the break points at the investigated temperatures, and aregiven in Table 3. The experimental cmc values of DTAB and CTAB inwater were found to be 15.03, 15.43, 15.62 and 15.74 and 0.95, 0.99,1.00, and 1.03 × 10−3 mol kg−1 at 298.15, 303.15, 308.15, and313.15 K, respectively, which are in good agreement with the reportedvalues as shown in Table 3 at the corresponding temperatures. More-over, it has been reported that a minimum in the cmc vs temperaturecurve appears around 298.15 K for most of the ionic surfactants [8,26].The effect of temperature on the cmc of the surfactants in aqueous solu-tion is usually analyzed in terms of two opposing factors. First, in the

Table 2Values of electrical conductivities (κ) of different concentrations of DTAB and CTAB inwater and in 3.0 × 10−5 mol kg−1 aqueous cresol red solutions at different temperatures.

m(mol kg−1)

T

298.15 K 303.15 K 308.15 K 313.15 K

κ (10−3 S cm−1)DTAB + water0.008 0.67 0.68 0.71 0.730.010 0.87 0.87 0.91 0.910.012 1.06 1.07 1.09 1.110.014 1.23 1.25 1.27 1.280.016 1.38 1.41 1.43 1.460.018 1.43 1.47 1.50 1.540.020 1.49 1.53 1.56 1.590.022 1.54 1.58 1.62 1.650.024 1.59 1.63 1.67 1.72

DTAB + aq. CR0.008 0.69 0.72 0.73 0.740.010 0.87 0.90 0.92 0.940.012 1.06 1.08 1.10 1.120.014 1.22 1.25 1.27 1.290.016 1.33 1.36 1.37 1.400.018 1.39 1.42 1.45 1.480.020 1.46 1.50 1.52 1.550.022 1.52 1.55 1.58 1.620.024 1.57 1.60 1.64 1.67

κ (10−6 S cm−1)CTAB + water0.0005 22.72 24.69 26.30 30.700.0006 29.68 32.08 34.02 37.270.0007 36.17 38.44 40.70 43.430.0008 42.51 45.06 47.43 49.570.0009 48.93 51.54 53.75 55.830.0010 53.17 56.22 59.07 61.240.0011 54.98 59.03 61.77 63.820.0012 56.72 60.46 64.20 65.980.0013 58.31 61.84 65.54 67.49

CTAB + aq. CR0.0001 14.91 15.16 15.21 15.330.0003 33.20 33.93 34.40 35.100.0005 49.77 49.96 51.76 51.920.0006 57.45 57.93 59.80 59.970.0007 63.41 64.53 64.93 67.020.0008 67.62 69.23 70.68 72.480.0009 71.60 73.10 75.30 77.450.0010 75.13 76.87 78.96 82.100.0011 78.98 80.58 83.10 85.750.0012 82.12 84.08 86.60 89.430.0013 84.99 87.42 90.19 93.10

lower temperature range, below ≈298.15 K, increase in temperaturecauses decreased hydration of the hydrophilic groups, which favorsmicellization, however, at relatively higher temperature, beyond≈298.15 K, an increase in temperature also causes disruption of thestructured-water surrounding the hydrophobic groups of the surfac-tants and this is unfavorable for the micellization [8,30]. It seems fromthe data in Table 3 that the second effect is predominant over the firstone in the studied temperature range, resulting in an increase in thecmc of DTAB andCTAB. At higher temperatures increase in the solubilityof hydrocarbon stabilizes the surfactant monomers, hindering them toform micelles and, hence, the increase in the cmc values is observed.Stabilization of the surfactant monomers due to increased solubility ofthe hydrocarbon chain of the surfactant monomers in aqueous solutionhas also been reported in the literature [8,31]. It is also observed(Table 3) that, as expected, there is a marked decrease in the cmc asthe number of carbon atoms in the hydrophobic group increases [8]from DTAB to CTAB in aqueous as well as in aqueous CR solutions ateach investigated temperature. This is because of the fact that polarhead group\Nþ(CH3)3 remaining the same, increase in the hydropho-bic group from (\C12H25) DTAB to (\C16H33) CTAB, favors micelle for-mation due to stronger hydrophobic–hydrophobic interaction betweennonpolar hydrophobic groups of CTAB than DTAB, yielding lower cmcvalues for the former than for the latter surfactant. Similar decrease inthe cmc of the surfactants DTAB, tetradecyltrimethylammonium bro-mide (TTAB), and CTAB with increase in the number of carbon atomsin thehydrophobic group in the sequence:DTAB b TTAB b CTAB in aque-ous and in aqueous phenyl red (PR) dye solution [23] endorses theabove finding. Moreover, it is clear from Table 3 that there is a pro-nounced decrease in the cmc values of both the surfactants DTAB andCTAB in the presence of CR than in its absence. Depression in thecmcs' of the surfactants in aqueous CR than in purewatermay be attrib-uted to the solubilization of the additive CR molecules in the outer por-tion of the micellar core between the surfactant molecules. This wouldresult in decrease in the mutual repulsion of the ionic head groups inthe micelle which, in turn, decreases the work required for the forma-tion of the micelles, thereby, resulting in lower cmc values of DTABand CTAB in the presence of CR than in its absence. Reduction in thecmcs' of the ionic surfactants in the presence of additives which are sol-ubilized in the outer portion of the micelle has also been reported else-where [8,31,32]. Decrease in the cmc of DTAB and CTAB in the presenceof dye CR can also be explained by considering the formation of close-packed ion pairs between the anionic group,\SO3

− of CR and the oppo-sitely charged cationic group,\Nþ(CH3)3, of the surfactants due to thecombined electrostatic and hydrophobic interactions in the submicellarconcentration ranges of the surfactants [3,23,33]. The formation of theclose-packed ion pairs between DTAB/CTAB and CR is treated as the for-mation of new nonionic surfactant with larger head group [23,34]which, in turn, facilitates the micellization of the surfactants, loweringthe cmc values [8] in the presence of dye CR than in its absence [23]. Itwould be interesting to compare the cmc values 14.52 × 10−3 and6.08 × 10−4 mol·kg−1 of DTAB and CTAB, respectively, at 298.15 K(Table 3) in aqueous CR observed here with the corresponding values6.0 × 10−3 [23] and 6.0 × 10−4 [3] mol·dm−3 in aqueous PR. Thepresence of the two electron-donating groups \CH3 [35] attached tothe two benzene rings of CR molecule enhances theП-electron densityof the molecule, making CR molecule more polar; the donation ofП-electrons toward electron seeking proton of water molecule

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

298.15 K 303.15 K308.15 K313.15 K

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

298.15 K 303.15 K 308.15 K 313.15 K

0.005 0.010 0.015 0.020 0.025 0.030

0.0000 0.0003 0.0006 0.0009 0.0012 0.0015 0.0000 0.0003 0.0006 0.0009 0.0012 0.0015

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

298.15 K 303.15 K 308.15 K 313.15 K

κ / (

mS

cm

-1)

κ / (

mS

cm

-1)

κ / (

mS

cm

-1)

[DTAB] / (mol kg-1)

a)

[CTAB] / (mol kg-1)

b)

c)

[CTAB] / (mol kg-1)

d)

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

298.15 K 303.15 K 308.15 K 313.15 K

0.005 0.010 0.015 0.020 0.025 0.030

κ / (

mS

cm

-1)

[DTAB] / (mol kg-1)

Fig. 2.Variation of specific conductivity (κ) with [DTAB] (a) inwater and (b) in 3.0 × 10−5 mol kg−1 aqueous cresol red;with [CTAB] (c) inwater and (d) in 3.0 × 10−5 mol kg−1 aqueouscresol red at 298.15 K (■), 303.15 K ( ), 308.15 K ( ), and 313.15 K ( ).

398 A. Ali et al. / Journal of Molecular Liquids 196 (2014) 395–403

becomes easier, thereby, accounting for increased donor-acceptortype interaction between CR and water molecules. In addition,due to the increased hydrophilic character of CR molecule, interac-tion between\SO3

− group of the dye and proton of watermolecule alsobecomes stronger. On the other hand, in the absence of electron-donating groups in PR molecule there would be weaker interactionbetween PR and water molecules, resulting in relatively stronger

Table 3Values of critical micelle concentration (cmc) of DTAB and CTAB in water and in 3.0 × 10−5 m

T

298.15 K 30

cmc (10−3 mol kg−1)

DTAB + water 15.03, 16.00a, 15.00b, 14.80c 15DTAB + aq. CR 14.52 14CTAB + water 0.95, 0.95f, 0.96e 0.CTAB + aq. CR 0.61 0.

a Reference [23].b Reference [24].c Reference [26].d Reference [27].e Reference [28].f Reference [29].

interaction between \Nþ (CH3)3 group of DTAB/CTAB and \SO3−

group of PR. Consequently, stronger interaction between CR and watermolecules partially hinders the interaction of \Nþ (CH3)3 group ofDTAB/CTAB with \SO3

− group of CR, this, in turn, exposes the polarhead groups of the surfactants, thus, increasing the mutual electrostaticrepulsion between similar charged head groups of DTAB and CTAB onthe surface of the micelle. This causes delay in the micellization and,

ol kg−1 aqueous cresol red solutions at different temperatures.

3.15 K 308.15 K 313.15 K

.43, 15.63d 15.62, 15.30c 15.74, 16.12d

.77 14.95 15.1099, 0.99e 1.00, 1.01e, 1.16f 1.03, 1.09e

63 0.63 0.64

295.150.00

0.10

0.20

0.30

0.40

0.50

0.60

α

T / (K)

aq. DTABaq. CTABDTAB + aq. CRCTAB + aq. CR

300.15 305.15 310.15 315.15

Fig. 3. Variation of degree of ionization (α) with temperature (T) of aqueous DTAB (■),aqueous CTAB ( ), DTAB in 3.0 × 10−5 mol kg−1 aqueous cresol red ( ), and CTAB in3.0 × 10−5 mol kg−1 aqueous cresol red ( ) respectively.

399A. Ali et al. / Journal of Molecular Liquids 196 (2014) 395–403

hence, higher cmc values of DTAB/CTAB in aqueous CR than in aqueousPR are observed. Hydration of the hydrophilic group \SO3

− of the dyehas also been reported by Garcia and Sanz-Medel [10]. Furthermore, itis worthmentioning that the cmc value of DTAB is much higher where-as that of CTAB is slightly higher in aqueous CR than in aqueous PR so-lution. As stated above, the combined effect of both hydrophobic andhydrophilic (electrostatic) interactions operating between surfactantand dye in aqueous medium controls the micellization and, hence, thecmc of the surfactant. It seems that the increased electrostatic repulsionbetween relatively more exposed polar head groups of DTAB dimin-ishes the tendency of the surfactant monomers to form micelles in thepresence of CR than in the presence of PR, leading to higher cmc valuesof DTAB in CR than in PR solution. On the other hand, large hydrophobicgroups of CTAB molecules cause strong hydrophobic–hydrophobic in-teraction which seems to be almost equally effective in overcomingthe electrostatic repulsion between polar head groups of the micelles,thereby, yielding almost equal cmc values of CTAB in aqueous CR aswell as in aqueous PR solutions. At this point, it is worth mentioningthat among the various factors which affect the cmc such as the struc-ture of the surfactant, the presence of additives (electrolyte and organiccompounds), temperature, etc. hydrophobic character of the surfactantexhibits the most profound effect on the cmc in aqueous solution. Thecmc decreases as the number of carbon atoms in the hydrophobicgroup increases to about 16, and a general rule is that for ionic surfac-tants the cmc is halved by the addition of one methylene group(\CH2) to the straight-chain hydrophobic group attached to a singleterminal polar group [8], as in the case of CTAB. Thus, it is interestingto note that in spite of the different natures of additives CR and PR,the cmc values of CTAB in aqueous CR and in aqueous PR are almostsame. This reinforces the view that hydrophobic–hydrophobic interac-tion between the nonpolar (\C16H33) groups of CTAB molecules is thedeciding factor and equally reduces the cmcs of the surfactant in boththe aqueous CR and aqueous PR solutions.

The degree of counterion dissociation (α) of themicelle near its cmcwas obtained from the ratio (S2/S1) of the slopes of the post-micelle (S2)to the pre-micelle (S1) regions [8]. The observed values of α for DTABand CTAB in aqueous medium (Table 4) are 0.28 and 0.26 at 298.15 K,respectively, which compare well with the corresponding literaturevalues 0.29 [36] and 0.27 [37]. It is important to assess the counterioneffect by comparing the α values of DTAB (0.28, 0.30, 0.31, this work)with those of dodecyltrimethylammonium chloride (DTAC) (0.389,0.421, 0.450) at 298.15, 303.15, and 308.15 K reported in the literature[26]. It is observed that when the counterion Br− (DTAB) is replacedby Cl− (DTAC), α increases. This is attributed to the smaller size of Cl−

ion than Br− ion, which makes the former ion strongly hydrated thanthe latter one. Weakly hydrated Br− ions are more readily absorbed inthe micellar surface, increase the surface area per head group (i.e., thesurface charge density is decreased) in the ionic micelle [8,26]. Thishinders the ionization of Br− ions compared to Cl− ions from themicel-lar surface of the surfactants and, therefore, smaller values of α are ob-served for DTAB than for DTAC. The aforementioned explanation isstrongly supported by the fact that the larger the hydrated radius ofthe counterion, the greater the degree of dissociation [8,38,39]. Table 4

Table 4Values of degree of ionization (α) of DTAB and CTAB inwater and in 3.0 × 10−5 mol kg−1

aqueous cresol red solutions at different temperatures.

T

298.15 K 303.15 K 308.15 K 313.15 K

α

DTAB + water 0.28 0.30 0.31 0.34DTAB + aq. CR 0.34 0.35 0.36 0.37CTAB + water 0.26 0.27 0.29 0.30CTAB + aq. CR 0.42 0.44 0.46 0.48

shows that the degree of counterion dissociation increases regularlywith temperature. The variations of α with T in the absence and in thepresence of CR are shown in Fig. 3. Similar increase inαwith rise in tem-perature has also been observed for alkyltrimethylammonium bromide(DTAB, TTAB, CTAB) [37], CTAB [40] and dodecyl dimethybenzyl ammo-nium chloride [41] in aqueous solutions. The increase in cmc and αwithincrease in temperature is in good agreement with the results reportedfor the ionic surfactants in aqueousmedium [8]. The increase inα can beattributed to the combined effect due to the columbic and thermalforces [42]. The former force attracts the counterions toward the polarsurfactant head groups while the latter one induces the dissociation ofcounterions from the surfactant head groups. It seems that the thermalforces predominate over the columbic forces, causing ionization of thesurfactants DTAB and CTAB, leading to the increased α value with tem-perature. It is to be noted that the values of α of DTAB and CTAB arehigher in the presence of CR than in its absence. This may be due tothe presence of the additive CR in the outer portion of the micelle and,thus, causing steric hindrance to the binding of counterions to the mi-celle, facilitating the dissociation of the counterions, which yields higherα values in the presence of CR than in its absence. The effect of the addi-tive CR on α can also be explained by considering the surface area perhead group, i.e., the surface charge density [39]. The increase in the de-gree of counterion dissociation α of DTAB and CTAB in the presence ofCR than in pure water (Table 4) is attributed to the solubilization ofCR in the palisade layer of the micelle. This increases the surface areaper ionic head group (or decreases the surface charge density), facilitat-ing the ionization of the counterions, Br−, from themicellar headgroupsof these surfactants, and, thereby, yielding higher α values in the pres-ence of CR than in its absence. This is in accordance with the reportedresults [8,39] for a number of cationic surfactants, as in the presentstudy.

The knowledge of the thermodynamic parameters ofmicellization isfound to be very useful for a clear understanding of the micellizationprocess and the factors that affect the micelle formation [8]. A widelyused model is the equilibrium model [43,44], according to which anequilibrium between counterions, surfactant monomers, and monodis-perse micelles, can be represented as:

n−pð Þ C− þ nSþ↔Mpþ: ð1Þ

where for a cationic surfactant, S+ represents the surfactant ions, C−

the corresponding counterions, and MP+ the aggregate of n monomerswith an effective charge p. The equilibrium constant for Eq. (1) can be

400 A. Ali et al. / Journal of Molecular Liquids 196 (2014) 395–403

related to the standard free energy of micelle formation per monomerunit by:

ΔGοm=RT ¼ − 1=nð Þ lnCMPþ þ lnCSþ þ 1−p=nð Þ lnCC− : ð2Þ

In the case of typical micelles, as DTAB and CTAB, n lies in the rangeof 50–100 in aqueousmedium, the value ofMP+ is small and insensitiveto the large errors in the evaluated CM

P+ value, and CC− and CS

+ can bereplaced by the value of cmc in the second and third terms in theabove equation to give [31,42,43]:

ΔGοm ¼ 2−αð ÞRT lnXcmc ð3Þ

where Xcmc is the cmc expressed in mole fraction unit, α (=p/n) is thedegree of counterion dissociation from themicelle, R is the gas constant,and T is the temperature in Kelvin scale. The enthalpy of micellizationcan be obtained by applying the Gibbs–Helmholtz equation:

ΔHοm ¼ −RT2 2−αð Þ ∂ lnXcmc=∂Tð ÞP− lnXcmc ∂α=∂Tð ÞP

� �: ð4Þ

The term (∂ ln Xcmc/∂T)P was calculated from the slope of the plot ofln Xcmc versus temperature and neglecting the term (∂α/∂T)P which isvery small over the investigated temperature range.

The entropy of micellization (ΔSmo ) can be estimated from theknown values of ΔGm

o and ΔHmo using the relation:

ΔSοm ¼ ΔHοm−ΔGο

m� �

=T: ð5Þ

The values of ΔGmo , ΔHm

o , and ΔSmo are listed in Table 5. It shows thatthe values of ΔGm

o (−47.26 and−35.02 kJ mol−1) of CTAB and DTAB at298.15 K compare well with the literature values (−47.19 and−35.58 kJmol−1) [36]. The free energy ofmicellization (ΔGm

o ) is amea-sure of the readiness with which the micelles are formed. ΔGm

o valuesare negative (Table 5) in both aqueous and aqueous CR solutions ateach investigated temperature and become increasingly negative withincrease in temperature. This indicates that the micellization of DTABand CTAB is thermodynamically spontaneous and that the decrease inΔGm

o with rise in temperature is ascribed to the desolvation of the

Table 5Values of standard free energy (ΔGm

0 ), enthalpy (ΔHm0 ), and entropy (ΔSm0 ) of micellization

of DTAB and CTAB in water and in 3.0 × 10−5 mol kg−1 aqueous cresol red solutions atdifferent temperatures.

T

298.15 K 303.15 K 308.15 K 313.15 K

DTAB + waterΔGm

0 (kJ mol−1) −35.02 −35.18 −35.31 −35.32ΔHm

0 (kJ mol−1) −3.80 −3.90 −3.98 −4.05ΔSm0 (kJ mol−1 K−1) 0.11 0.10 0.10 0.10TΔSm0 (kJ mol−1) 31.22 31.29 31.33 31.27

DTAB + aq. CRΔGm

0 (kJ mol−1) −33.93 −34.19 −34.45 −34.77ΔHm

0 (kJ mol−1) −3.05 −3.14 −3.22 −3.30ΔSm0 (kJ mol−1 K−1) 0.11 0.10 0.10 0.10TΔSm0 (kJ mol−1) 30.87 31.05 31.23 31.47

CTAB + waterΔGm

0 (kJ mol−1) −47.26 −47.56 −47.94 −48.24ΔHm

0 (kJ mol−1) −6.42 −6.59 −6.76 −6.93ΔSm0 (kJ mol−1 K−1) 0.14 0.14 0.13 0.13TΔSm0 (kJ mol−1) 40.84 40.97 41.18 41.31

CTAB + aq. CRΔGm

0 (kJ mol−1) −44.62 −44.68 −44.83 −44.88ΔHm

0 (kJ mol−1) −4.31 −4.40 −4.49 −4.64ΔSm0 (kJ mol−1 K−1) 0.14 0.13 0.13 0.12TΔSm0 (kJ mol−1) 40.31 40.29 40.34 40.25

hydrophilic groups of these surfactants [45]. Also, in light of Eq. (3),the increasing negative ΔGm

o value with rise in temperature is primarilydue to the combined effect of the increase of the coefficient RT and anincrease in the negative value of lnXcmc. Again, as expected, at a giventemperature the standard free energy ofmicellization becomes increas-ingly negative primarily due to the decrease in the cmc as the alkylchain length increases from DTAB to CTAB. It can be seen fromTables 3 and 5 that the cmc and ΔGm

o both decrease with increase inthe carbon chain length thus indicating the easewithwhich themicellesare formed as the hydrophobicity increases from DTAB to CTAB. At agiven temperatureΔGm

o becomesmore negative for both the surfactantsstudied in aqueous solution than in the presence of CR (Table 5). It isevident from Eq. (3) that at a given temperature the value of ΔGm

o isdue to the combined effect of (2 − α) and lnXcmc; the former quan-tity is found to be positive and higher while the latter one is morenegative in aqueous than in the presence of CR, making ΔGm

o morenegative in the absence of CR than in its presence. Moreover, it isworth mentioning that a decrease of 3.06 and 2.67 kJ per mol ofmethylene group (\CH2\) at 298.15 K in ΔGm

o as the number of car-bon atoms increases from 12 (DTAB) to 16 (CTAB) in the hydropho-bic groups of the surfactants in aqueous and aqueous CR, respectively,is observed. This is in close agreement with the reported decrease ofabout 3 kJ per mole of \CH2\ group of the ionic surfactants in aque-ous medium [8,41].

The values ofΔHmo , likeΔGm

o , of DTAB and CTAB in aqueous aswell asin aqueous CR are negative and becomemore negative with rise in tem-perature, suggesting that the micellization of the studied surfactants isexothermic. Moreover, at each investigated temperature, ΔHm

o valuesof CTAB are found to bemore negative than those of DTAB in both aque-ous and aqueous cresol red solutions (Table 5). This indicates that thehydrophobic–hydrophobic interaction is stronger between the largernonpolar group of CTAB and the non-ionic moiety of CR than betweenrelatively smaller nonpolar group of DTAB and the non-ionic moiety ofCR, makingΔHm

o more negative, hence, more exothermic for the formersurfactant than for the latter one. In addition to this, negative ΔHm

o

values can also be due to the electrostatic interaction between the cat-ionic head, \Nþ(CH3)3, of the surfactants and negative, \SO3

−, site ofCR. It is worth noting that ΔHm

o is not always negative, often becomespositive, depending upon the structural and solution aspects prevailingin the surfactant solutions in the presence of additive. For example, ΔHm

o

has positive values 42.27 and 12.24 kJ mol−1 for sodium N-dodecanoylsarcosinate, an anionic surfactant, in aqueous medium at 293.15 and298.15 K, respectively [46]; 4.98 and 2.47 kJ mol−1 for sodiumdodecylsulfate in the presence of dipeptide (Tyr–Phe) and tripeptide(Val–Tyr–Val) in aqueous medium at 298.15 K [47]; 19.5, 17.7, and18.9 kJ mol−1 for CTAB in the presence of the drug amitriptyline hydro-chloride in aqueous medium at 308.15 K, the value decreased with in-creasing concentration of the drug [38]. The behavior ofpolyoxyethylene (POE) non-ionic surfactants in aqueous medium isquite interesting. ΔHm

o becomes increasingly positive with increase inthe number of polar oxyethylene units in the hydrophilic head of thesurfactant, whereas, it remains almost constant with increase/decreasein the length of the non-polar hydrophobic alkyl tail [8,48]. Consequent-ly, the micellization of the abovementioned surfactants is an endother-mic process, contrary to the micellization of DTAB and CTAB in aqueousaswell as in aqueous CR solutions. Based upon the results cited above, itis observed that increase in ΔHm

o values is accompanied by large valuesof ΔSmo . The possible explanation of such trends in ΔHm

o and ΔSmo is thatduring micelle formation the transfer of the nonpolar hydrophobic tailfrom the bulk to the interior of the micelle disrupts the structuredwater molecules surrounding the hydrophobic tail which, in turn, in-creases the entropy of the system, associated with positive ΔHm

o value.This is in accordance with the view suggested by others [38,46]. It is in-teresting to examine the enthalpy of micellization (ΔHm

o ) calculatedfrom the temperature dependence of the cmc and α using Eq. (4),which considerably differs from that determined calorimetrically [40,

401A. Ali et al. / Journal of Molecular Liquids 196 (2014) 395–403

45–48]. For instance, the value of ΔHmo for DTAB in aqueous solution,

calculated using Eq. (4) is−3.90 kJmol−1 (thiswork) and that obtainedby calorimetric method is −3.56 kJ mol−1 [49] at 303.15 K. In the caseof CTAB, in aqueous solution, the calculated value of ΔHm

o is found to be−6.59 kJ mol−1 (this work) and that obtained calorimetrically is−5.82 kJ mol−1 [50] at 303.15 K. The difference between the calculatedΔHm

o (Eq. (4)) values, based on the mass action model assuming associ-ation–dissociation equilibrium between the surfactant monomers andmicelles and those obtained by direct calorimetric method may be dueto the fact that the overall enthalpy change of the solution in the calori-metric measurement has contributions from the solvation–desolvationof species involved, their mixing, ionization, new structural arrange-ments as a result of interspecies interactions, etc., whereas such contri-butions are not considered in the mass action equilibrium model [40,43]. The entropy of micellization (ΔSmo ) (Table 5) for both the surfac-tants, DTAB and CTAB, in aqueous as well as in aqueous CR solutions ispositive, indicating that the micellization process is favored by entropygain, a pre-requisite condition for the micelle formation [8]. It isobserved that there is small decrease in ΔSmo with rise in temperature.Increase in temperature enhances the ionization of the surfactants, asis evident from the increased value of αwith temperature, making cat-ionic head groups of DTAB and CTAB easily available for the interactionwith the anionic\SO3

− groups of the dye, CR. Such surfactant–dye inter-action causes decrease in randomness, hence, results in decreased ΔSmovalue with increase in temperature. Further, the observed value40.84 kJ mol−1 of TΔSm0 (Table 5) of CTAB in pure water compareswell with the reported value 39.21 kJmol−1 [28] at 298.15K. It is impor-tant to investigate the enthalpic and entropic components of ΔGm

o forthe studied surfactants. Large values of −TΔSm0 than those of ΔHm

0 inaqueous and in aqueous cresol red indicate that the process ofmicelliza-tion is governed mainly by entropy gain and that the driving force forthe micellization is the tendency of the hydrophobic groups of the sur-factants in transferring from the bulk solvent to the interior of the mi-celle [8,30]. This may be due to the breaking up of the structuredwater molecules surrounding the hydrophobic alkyl groups of the sur-factants when it is transferred from the solvent environment to the in-terior of the micelle and also due to the increased freedom of thesehydrophobic groups in the non-polar interior of the micelle than inthe aqueous environment [8].

The change in the heat capacity of the surfactants upon micelliza-tion (ΔCp,m

ο (=(∂ΔHmο /∂T)P)) can be estimated from the slope of ΔHm

ο

vs. T plot. It is clear from Table 5 that (∂ΔHmο /∂T)P is negative, making

ΔCp,mο values negative: −0.017 kJ mol−1 K−1 for DTAB and

−0.034 kJ mol−1 K−1 for CTAB in pure water and −0.016 and−0.022 kJ mol−1 K−1 in the presence of CR. It is interesting to notethat there are large variations in the reported literature values of ΔCp,mο. For instance, ΔCp,m

ο values for DTAC in water, in 0.01 M NaCl, andin 0.001 M sodium salicylate (NaSal) are −0.418, −0.412, and−0.077 kJ mol−1 K−1, while it shows two distinct values−0.407 kJ mol−1 K−1 at lower temperatures and−0.088 kJ mol−1 K−1

at higher temperatures (greater than 305 K) [49]; −0.471 and−0.334 kJ mol−1 K−1 for dodecyldimethylethylammonium bromideand DTAB, respectively, in water [50]. All these values of ΔCp,mο are ob-tained from the slopes of the linear plots of ΔHm

o against T, but, surpris-ingly, the values clearly differ to a large extent. Such unexpectedvariations in ΔCp,mο values may be attributed to the controversial dataon ΔHm

o reported in the literature [50]. For example, considering thatDTAB is one of the most studied cationic surfactants, the values ofΔHm

o range from −8 [51] to −1.5 kJ mol−1 [52] at 298.15 K. Moulikand co-worker [53] determined ΔHm

o value as −1.77 kJ mol−1 at303.15 K; this value has also been quoted by Khatua et al. [54] in theirwork. Therefore, it may be concluded that the large variations reportedin the values of ΔCp,mο for surfactants are due mainly to the large varia-tions in ΔHm

o values. Therefore, a more clear understanding of the solu-tion properties which affect the magnitude of ΔHm

o and, hence, of ΔCp,mο

is required. Negative ΔCp,mο values are generally observed for the

aggregation of the amphiphilicmolecules in aqueousmediumprimarilydue to the removal of the non-polar group of the surfactant from thecontact of the water molecules on micellization [49]. It has been sug-gested [49,55] that although the hydrophilic head-groups of the ionicsurfactants remain hydrated on micellization, ΔCp,mο is assumed to besolely due to the change in the exposure of the hydrophobic tails tothe water molecules. Chen et al. [56] and Sorac and Bester-Rogac [57]have successfully used this approach in describing the change inΔCp,m

ο on the micellization of ionic as well as non-ionic surfactantsin aqueous medium. This is truly in consistent with the fact thatthe micellization occurs mainly due to transfer of the hydrophobictail of the surfactant from the solvent to the interior of the micelle[8].

3.2. UV–visible spectroscopic study

The absorption spectrum of 3.0 × 10−5 mol kg−1 cresol red in purewater (Fig. 4(a)) shows absorption band with λmax at 434 nmwhich isin excellent agreement with the value reported elsewhere [3]. Fig. 4(b)displays the spectral changes observed for 3.0 × 10−5 mol kg−1 CR inthe presence of different concentrations of CTAB in the pre-micellarand above the cmc of the surfactant. It is observed that this absorptionband (λmax = 434 nm) starts decreasing on the addition of CTAB withthe appearance of a new band with λmax at 584 nm and that theabsorption band increased up to 6.0 × 10−4 mol kg−1 of the surfac-tant (i to v), then starts decreasing above this concentration ofCTAB (vi, dotted line). In the presence of CTAB, the absorbance ofCR increases up to 6.0 × 10−4 mol kg−1 of the surfactant which istaken as the cmc of CTAB which is very close to the observed cmcvalue 6.08 × 10−4 mol kg−1 at 298.15 K (Table 3) by electrical con-ductivity method. The increase of the absorbance band below cmcclearly suggests that the band at 584 nm arises due to the interac-tions (electrostatic and hydrophobic) between the dye, CR andmonomers of the surfactant forming CR–CTAB ion-pairs. However,when the concentration of CTAB is increased beyond its cmc, i.e., inthe post-micellar region, the absorption band starts decreasingFig. 4(b) (vi, dotted line). This confirms the solubilization of the dyeCR in the micelles of the surfactants, CTAB and DTAB. As expected, thecmc (6.0 × 10−4 mol kg−1) of the surfactant in the presence of cresolred is lower than in pure water. This can be explained by consideringthat the negatively charged sulfonate groups, \SO3

−, of CR prefer tocluster with the oppositely charged cationic head groups, \Nþ(CH3)3,of the surfactant, thereby, reducing the electrostatic repulsion, withinthe polar cationic head groups of CTAB which, in turn, lowers the cmcvalue of the surfactant in the presence of dye CR than in pure water. De-pression in the cmc values of CTAB and TTAB in the presence of thedye congo red than in pure water has been ascribed to the electro-static interaction between \SO3

− group of the dye and cationichead groups of the surfactants [17,58]. The ion-pair which wasformed in the pre-micellar region breaks down as the micelles areformed with increasing concentration of the surfactant above thecmc and solubilization of dye dominates over the ion-pair formation.Not much spectral variation was observed on changing the surfac-tants from CTAB to DTAB; λmax remains almost unchanged(Fig. 4(c)). The cmc of DTAB in the presence of CR was observed tobe 14.0 × 10−3 mol kg−1 from the spectroscopic method. The resultsobtained here are in good agreement with those reported by others[3,23] for the UV–visible spectroscopic study of cresol red in thepresence of CTAB and DTAB. An interesting aspect emerges fromthe present spectroscopic study of dye–surfactant interaction inaqueous medium that the more the hydrophobic dye, the strongerthe dye–surfactant interaction, resulting in lower cmc values. Theabove result is supported by the fact that the dyes, PR [3,23] and C.I. Reactive Orange 16 (RO16) [59] being more hydrophobic comparedto CR interactmore stronglywith the surfactants DTAB, TTAB, and CTAB,yielding lower cmc values in the presence of PR and RO16 compared to

400-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6434

Ab

sorb

ance

Wavelength / (nm)

a)

400-0.1

0.0

0.1

0.2

0.3

0.4

0.5

Ab

sorb

ance

Wavelength / (nm)

(vi)

b)

(i)

(v)(vi)

(i)

(v)

600500400

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Ab

sorb

ance

Wavelength / (nm)

(i)

(v)

(i)

(vi)

(ii)

c)

500 600

500 600

Fig. 4. (a) Absorption spectrum of 3.0 × 10−5 mol kg−1 cresol red in aqueous solution at298.15 K. (b) Absorption spectra of 3.0 × 10−5 mol kg−1 aqueous cresol red in the pres-ence of different concentrations of CTAB (i) 2.0 × 10−4, (ii) 3.0 × 10−4, (iii) 4.0 × 10−4,(iv) 5.0 × 10−4, (v) 6.0 × 10−4, and (vi) 7.0 × 10−4 mol kg−1 at 298.15 K. (c) Absorptionspectra of 3.0 × 10−5 mol kg−1 cresol red in the presence of different concentra-tions of DTAB (i) 1.81 × 10−3, (ii) 5.09 × 10−3, (iii) 1.08 × 10−2, (iv) 1.2 × 10−2 m, (v)1.4 × 10−2, and (vi) 1.6 × 10−2 mol kg−1 at 298.15 K.

402 A. Ali et al. / Journal of Molecular Liquids 196 (2014) 395–403

those in the presence of CR. Thus, it is concluded that the results obtain-ed by thermodynamic considerations are satisfactorily supported by thespectroscopic study of the present systems.

4. Conclusions

The increase in the cmc values of DTAB and CTAB in the studied tem-perature range is attributed to the breaking up of the structured watersurrounding the hydrophobic groups of the surfactants. A marked de-crease in the cmc of CTAB than those of DTAB in aqueous as well as inaqueous cresol red is due to the larger hydrophobic chain of CTAB mol-ecule than of DTABmolecule, favoring easy micellization for the formerthan the latter surfactant. A pronounced decrease in the cmc of both thesurfactants in the presence of CR than in its absence is attributed to thesolubilization of the additive CRmolecule in the palisade layer of themi-celle, resulting in decreased mutual repulsion of the ionic groups in themicelle which requires less work for the formation of the micelles,thereby, resulting in lower cmc values of DTAB and CTAB in the pres-ence of CR than in its absence. Also, the values of α of both the surfac-tants are higher in the presence of CR than in its absence. Thepresence of CR in the palisade layer of the micelle causes steric hin-drance to the binding of the counterions (Br−) to the cationic polarhead groups, \Nþ(CH3)3, of the surfactants, favoring dissociation ofthe counterions and, hence, higherα values are obtained in thepresenceof CR than in its absence. The values ofΔGm

ο andΔHmο for both DTAB and

CTAB are negative and tend to becomemore negative as the alkyl chainlength increases from DTAB to CTAB, indicating that themicellization ofthe surfactants in the studied solvents is thermodynamically spontane-ous and exothermic in nature and that themicelle formation is more fa-vorable for CTAB than for DTAB due to large non-polar group in theformer than in the latter surfactant. Furthermore, a decrease of about3 kJ per mole of methylene group (\CH2\) at 298.15 K in ΔGm

ο as thenumber of carbon atoms increases from 12 (DTAB) to 16 (CTAB) inthe non-polar hydrophobic groups of the surfactants is in good agree-ment with the reported decrease of about 3 kJ per mole of (\CH2\)group of the ionic surfactants. The values of −TΔSmο are much largerthan those ofΔHm

ο for both the surfactants, suggesting thatmicellizationis primarily governed by the entropy gain due to the breaking up of thestructured water molecules surrounding the hydrophobic groups of thesurfactants when these groups are transferred from the bulk solvent tothe interior of the micelle where they feel more freedom because of thenon-polar environment of the interior of themicelle. Negative values ofΔCp,mο (=(∂ΔHm

ο /∂T)P) further support the aggregation of the amphiphil-ic molecules in the studied media. UV–visible absorption spectra indi-cate that there is ion-pair formation as a result of the interactionbetween dye, CR and monomers of the surfactants CTAB/DTAB in thepre-micellar region, but as the concentration of the surfactants isincreased above their cmcs' values the ion-pair complex breaks downand dye is solubilized in the micelles. Thus, the spectroscopic results ofthe investigated systems support the results obtained conductometrically.

The present study emphasizes on the importance of both electro-static and hydrophobic interactions between the cationic surfactantsDTAB/CTAB and anionic dye CR. As the thermodynamics of dye–surfactantinteractions has received little attention, these results in conjunction withthe spectroscopic results would be of great value in understanding themechanisms of the chemical equilibria and kinetics of surfactant sensi-tized color and fluorescence reactions.

Acknowledgments

Sahar Uzair and Nisar Ahmad Malik are thankful to UGC for provid-ing scholarships in the form of BSR (Basic Scientific Research) andMaroof Ali is also thankful to UGC for DSKPDF.

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