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

Journal of Molecular Liquids 196 (2014) 395–403

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Study of interaction between cationic surfactants and cresol red dye by electrical conductivity and spectroscopy methods Anwar Ali ⁎, Sahar Uzair, Nisar Ahmad Malik, Maroof Ali Department of Chemistry, Jamia Millia Islamia (Central University), New Delhi 110025, India

a r t i c l e

i n f o

Article history: Received 27 December 2013 Received in revised form 11 April 2014 Accepted 15 April 2014 Available online 2 May 2014 Keywords: Cationic surfactant Cresol red Electrical conductivity UV–visible spectroscopy Interaction Critical micelle concentration

a b s t r a c t Interaction of anionic dye, cresol red (CR), with cationic surfactants, dodecyltrimethylammonium bromide (DTAB) and cetyltrimethylammonium bromide (CTAB), in aqueous solution in the submicellar and micellar concentration ranges has been investigated by using thermodynamic and spectroscopic methods. The equilibrium model has been used to calculate the standard free energy (ΔGom), enthalpy (ΔHom), and entropy (ΔSom) of micelle formation. The increase in the critical micelle concentration (cmc) with rise in temperature is attributed to the disruption of the structured water surrounding the hydrophobic groups of the surfactants. A marked decrease in 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 of CR than in its absence. Negative values of ΔGom and ΔHom for the surfactants in aqueous and in aqueous CR solutions show that the process of micellization is thermodynamically spontaneous and exothermic. The values of −TΔSom are much higher than the ΔHom values, indicating that the micellization process is governed primarily by the entropy gain associated with it. The heat capacity of micellization (ΔCοp,m) is negative which is mainly due to the change in the exposure of the hydrophobic groups to the water molecules. UV–visible spectra suggest interaction between dye and surfactant monomers forming ion-pair complex in the pre-micellar region whereas above 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 aqueous solutions has attracted significant interest in the recent years because of their widespread applications and relatively complex behavior. This investigation is important from the point of view of technology of dyeing processes as well as for chemical research, such as biochemistry, analytical chemistry, and photosensitization. Interactions of dyes with surfactants in aqueous solutions can provide useful information about the mechanism according to which surfactants operate as leveling agents and information on the thermodynamics and kinetics of dyeing [1,2]. Moreover, understanding of the electrostatic and hydrophobic interactions between surfactants and dyes can add to our existing knowledge because similar interactions prevail in biologically important processes [3–5]. Surfactants are composed of a polar hydrophilic group and a nonpolar hydrophobic chain. This unique structural feature makes them to establish interactions with both the hydrophilic as well as hydrophobic molecules [6,7]. They form aggregates (micelles) in aqueous solutions over a narrow concentration range, known as the critical micelle

⁎ Corresponding author. Tel.: +91 11 26981717x3257; fax: +91 11 26981232. E-mail addresses: [email protected], [email protected] (A. Ali).

http://dx.doi.org/10.1016/j.molliq.2014.04.013 0167-7322/© 2014 Elsevier B.V. All rights reserved.

concentration (cmc), below which the surfactant molecules are predominantly dispersed as monomers [8]. For many practical applications of detergent micelles play important roles. For instance, micelles in aqueous medium solubilize the organic compounds which are poorly soluble in water by incorporating them in the micellar phase; micelles are conveniently exploited to act as catalysts for many reactions due to their large surface area; they alter the reaction pathways, rates and equilibria [9,10]. Moreover, micelle systems are convenient to use because they are optically transparent, stable, and relatively non-toxic [9,10]. Cationic surfactants possess valuable characteristics such as emulsification, wetting, water proofing, repellence and spreading. They are widely used in food industry, pharmaceuticals, and solubilization of water insoluble dyes and have a great bearing on day-to-day life [8]. Dyes are aromatic compounds containing chromophores, delocalized electron systems with conjugated double bonds, and auxochromes, electron withdrawing substituents that cause or intensify the color of the chromophore [11]. Cresol red is a triphenylmethane dye. It is extensively used in textile industries for dyeing nylon, polyacrylonitrile modified nylon, wool, silk, and cotton. Some of the triphenylmethane dyes are used as medicines and biological stains. Paper and leather industries are also major consumers of triphenylmethane dyes [12]. Electrostatic interactions, hydrophobic interactions, hydrogen bonds, pi-stacking, cation–pi interactions, and van der Waals forces are the typical examples of the

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intermolecular forces that dominate the interaction of dye molecules with the surfactant aggregates [13]. However, the nature of interaction may vary with the types of dye–surfactant systems. Karukstis et al. [14], Yamamoto and Motomizu [15], and Mukerjee and Mysels [16] have reported the formation of insoluble or poorly soluble salts or ion-pairs between oppositely charged dye and surfactant in the pre-micellar region of the surfactant. Above cmc, the dye may revert to its monomeric state, bound to the surface of the micelle or may be solubilized in the micelle [10,17]. It is, therefore, interesting to investigate the species formed as a result of dye–surfactant interaction in the pre- as well as in the postmicellar concentration ranges of the surfactants, DTAB and CTAB. Although many researchers have reported dye–surfactant interactions using different techniques [17–22], thermodynamic study of such interactions has received little alteration. Therefore, in the present work, in order to have fine details of dye–surfactant interactions at molecular level, thermodynamic study is complemented with the spectroscopic study. In this paper, we report on the interactions of the cationic surfactants, DTAB and CTAB with anionic dye, CR (Fig. 1) using conductometric and UV–visible spectroscopic methods in the pre- as well as in the post-micellar regions of the two surfactants. The effect of increasing alkyl chain length from DTAB to CTAB, having the same polar group but different hydrophobic groups, on the dye–surfactant interactions will also be explored in order to understand the importance of the hydrophobic forces. Literature survey indicates that the study on CR–DTAB/CTAB interactions in aqueous medium involving both thermodynamic as well as spectroscopic methods is rare. These considerations led us to undertake the present study. 2. Experimental 2.1. Materials Cresol red was purchased from Alfa Aesar, India. It was recrystallized from pure water and dried before use. Dodecyltrimethylammonium bromide and cetyltrimethylammonium bromide were extra-pure, purchased from Acros Organics, Belgium, and were used after recrystallization from ethanol, dried in vacuum over P2O5 at room temperature for about 72 h. The specifications of the chemicals used are given in the Table 1. All the solutions were prepared in doubly distilled deionized

a)

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 measurement Solutions of DTAB in the concentration range from 0.008 to 0.024 m (mol kg− 1) and of CTAB in the concentration range from 0.0005 to 0.0013 mol kg− 1 were prepared in pure water. Stock solution of 3.0 × 10−5 mol kg−1 cresol red in pure water was prepared and was used as solvent for the preparation of surfactant solutions in aqueous cresol red. Surfactant solutions in the concentration range from 0.008 to 0.024 mol kg−1 for DTAB and from 0.0001 to 0.0013 mol kg−1 for CTAB in 3.0 × 10− 5 mol kg− 1 aqueous cresol red were prepared to cover the pre- as well as post-micellar concentration ranges. The weighings were done on Precisa XB-220 A (Swiss-make) electronic balance with a precision of ±0.1 mg. All necessary precautions were taken to prepare the solutions. All the solutions were prepared afresh. Electrical conductivities of surfactants in pure water and in aqueous cresol red were measured with a digital conductivity meter, PICO + (Labindia Instruments Pvt. Ltd.). Prior to use, the conductivity meter was calibrated by measuring the electrical conductivities of 0.01 and 0.1 N solutions of potassium chloride (Merck, purity N 99%). The cell constant of the cell used was 1.00 cm−1. The glass cell with two platinum electrodes was dipped in the sample solution contained in a corning glass tube which was properly covered and immersed in an electronically controlled thermostated water bath (Julabo, Model MD, Germany), maintaining the temperature within ±0.02 K. Electrical conductivity was recorded when the solution attained thermal equilibrium. The measurements were taken in triplicate and mean values were used in all the calculations. The accuracy in the electrical conductivity measurement was up to ±1.5%. 2.2.2. UV–visible spectral measurement The spectra of the dye (CR) in pure water and in the presence of different concentrations of surfactants were recorded using a Perkin Elmer Lambda-40, double-beam UV–visible spectrophotometer with a matched pair of glass cuvettes, 1.0 cm in optical path length, at 298.15 K. The stock solutions of the surfactants were prepared by dissolving the required amounts of DTAB and CTAB in aqueous cresol red. All the test solutions were prepared by diluting the respective stock solutions. While the concentrations of both the surfactants were varied, the concentration of CR used throughout this study was kept constant at 3.0 × 10−5 mol kg−1. 3. Results and discussion

b)

c)

In the present work, two sets of experiments were carried out in order to describe the effect of cresol red dye on the thermodynamic properties of DTAB and CTAB micellization. For the first set of experiment, the electrical conductivity measurements for DTAB and CTAB in pure water and in 3.0 × 10− 5 mol kg−1 aqueous solutions of cresol red at different surfactant concentrations covering the pre- and postmicellar concentration ranges were made at 298.15, 303.15, 308.15, and 313.15 K (Table 2). The conductometric study of the present systems is complemented by the second set of experiment — UV–visible spectra of cresol red dye in pure water and in the presence of the surfactants, DTAB and CTAB. 3.1. Conductometric study

Fig. 1. Molecular structures of (a) cetyltrimethylammonium bromide, (b) dodecyltrimethylammonium bromide, and (c) cresol red.

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

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

397

Table 1 Specification of the chemicals used. Compound

CAS number

Suppliers

Purity (Stated by suppliers)

Cresol red Dodecyltrimethylammonium bromide Cetyltrimethylammonium bromide

1733-12-6 1119-94-4 57-09-0

Alfa Aesar, UK Acros Organics, Belgium Acros Organics, Belgium

and in aqueous CR solutions were obtained from the intersection of the straight lines of κ vs. surfactant concentration plots (Fig. 2) above and below the break points at the investigated temperatures, and are given in Table 3. The experimental cmc values of DTAB and CTAB in water 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, and 313.15 K, respectively, which are in good agreement with the reported values as shown in Table 3 at the corresponding temperatures. Moreover, it has been reported that a minimum in the cmc vs temperature curve 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 solution is usually analyzed in terms of two opposing factors. First, in the

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

m (mol kg−1) −3

298.15 K

303.15 K

308.15 K

313.15 K

−1

κ (10 S cm ) DTAB + water 0.008 0.010 0.012 0.014 0.016 0.018 0.020 0.022 0.024 DTAB + aq. CR 0.008 0.010 0.012 0.014 0.016 0.018 0.020 0.022 0.024 κ (10−6 S cm−1) CTAB + water 0.0005 0.0006 0.0007 0.0008 0.0009 0.0010 0.0011 0.0012 0.0013 CTAB + aq. CR 0.0001 0.0003 0.0005 0.0006 0.0007 0.0008 0.0009 0.0010 0.0011 0.0012 0.0013

0.67 0.87 1.06 1.23 1.38 1.43 1.49 1.54 1.59

0.68 0.87 1.07 1.25 1.41 1.47 1.53 1.58 1.63

0.71 0.91 1.09 1.27 1.43 1.50 1.56 1.62 1.67

0.73 0.91 1.11 1.28 1.46 1.54 1.59 1.65 1.72

0.69 0.87 1.06 1.22 1.33 1.39 1.46 1.52 1.57

0.72 0.90 1.08 1.25 1.36 1.42 1.50 1.55 1.60

0.73 0.92 1.10 1.27 1.37 1.45 1.52 1.58 1.64

0.74 0.94 1.12 1.29 1.40 1.48 1.55 1.62 1.67

22.72 29.68 36.17 42.51 48.93 53.17 54.98 56.72 58.31

24.69 32.08 38.44 45.06 51.54 56.22 59.03 60.46 61.84

26.30 34.02 40.70 47.43 53.75 59.07 61.77 64.20 65.54

30.70 37.27 43.43 49.57 55.83 61.24 63.82 65.98 67.49

14.91 33.20 49.77 57.45 63.41 67.62 71.60 75.13 78.98 82.12 84.99

15.16 33.93 49.96 57.93 64.53 69.23 73.10 76.87 80.58 84.08 87.42

15.21 34.40 51.76 59.80 64.93 70.68 75.30 78.96 83.10 86.60 90.19

15.33 35.10 51.92 59.97 67.02 72.48 77.45 82.10 85.75 89.43 93.10

N95% 99% 99+%

lower temperature range, below ≈298.15 K, increase in temperature causes decreased hydration of the hydrophilic groups, which favors micellization, however, at relatively higher temperature, beyond ≈ 298.15 K, an increase in temperature also causes disruption of the structured-water surrounding the hydrophobic groups of the surfactants and this is unfavorable for the micellization [8,30]. It seems from the data in Table 3 that the second effect is predominant over the first one in the studied temperature range, resulting in an increase in the cmc of DTAB and CTAB. At higher temperatures increase in the solubility of hydrocarbon stabilizes the surfactant monomers, hindering them to form micelles and, hence, the increase in the cmc values is observed. Stabilization of the surfactant monomers due to increased solubility of the hydrocarbon chain of the surfactant monomers in aqueous solution has 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 as the number of carbon atoms in the hydrophobic group increases [8] from DTAB to CTAB in aqueous as well as in aqueous CR solutions at each investigated temperature. This is because of the fact that polar head group \N þ (CH3)3 remaining the same, increase in the hydrophobic group from (\C12H25) DTAB to (\C16H33) CTAB, favors micelle formation due to stronger hydrophobic–hydrophobic interaction between nonpolar hydrophobic groups of CTAB than DTAB, yielding lower cmc values for the former than for the latter surfactant. Similar decrease in the cmc of the surfactants DTAB, tetradecyltrimethylammonium bromide (TTAB), and CTAB with increase in the number of carbon atoms in the hydrophobic group in the sequence: DTAB b TTAB b CTAB in aqueous and in aqueous phenyl red (PR) dye solution [23] endorses the above finding. Moreover, it is clear from Table 3 that there is a pronounced decrease in the cmc values of both the surfactants DTAB and CTAB in the presence of CR than in its absence. Depression in the cmcs' of the surfactants in aqueous CR than in pure water may be attributed to the solubilization of the additive CR molecules in the outer portion of the micellar core between the surfactant molecules. This would result in decrease in the mutual repulsion of the ionic head groups in the micelle which, in turn, decreases the work required for the formation of the micelles, thereby, resulting in lower cmc values of DTAB and CTAB in the presence of CR than in its absence. Reduction in the cmcs' of the ionic surfactants in the presence of additives which are solubilized in the outer portion of the micelle has also been reported elsewhere [8,31,32]. Decrease in the cmc of DTAB and CTAB in the presence of dye CR can also be explained by considering the formation of closepacked ion pairs between the anionic group, \SO− 3 of CR and the oppositely charged cationic group, \Nþ (CH3)3, of the surfactants due to the combined electrostatic and hydrophobic interactions in the submicellar concentration ranges of the surfactants [3,23,33]. The formation of the close-packed ion pairs between DTAB/CTAB and CR is treated as the formation of new nonionic surfactant with larger head group [23,34] which, in turn, facilitates the micellization of the surfactants, lowering the cmc values [8] in the presence of dye CR than in its absence [23]. It would be interesting to compare the cmc values 14.52 × 10− 3 and 6.08 × 10− 4 mol·kg− 1 of DTAB and CTAB, respectively, at 298.15 K (Table 3) in aqueous CR observed here with the corresponding values 6.0 × 10−3 [23] and 6.0 × 10−4 [3] mol·dm−3 in aqueous PR. The presence of the two electron-donating groups \CH3 [35] attached to the two benzene rings of CR molecule enhances the П-electron density of the molecule, making CR molecule more polar; the donation of П-electrons toward electron seeking proton of water molecule

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b)

1.80

1.80

1.60

1.60

1.40

1.40

κ / (mS cm-1)

κ / (mS cm-1)

a)

1.20 1.00

298.15 303.15 308.15 313.15

0.80 0.60 0.40 0.005

0.010

0.015

0.020

[DTAB] / (mol

c)

K K K K

1.00

298.15 303.15 308.15 313.15

0.80 0.60 0.40 0.005

0.030

0.025

1.20

0.010

0.015

0.020

0.025

K K K K

0.030

[DTAB] / (mol kg-1)

kg-1)

d)

0.08

0.10 0.09

0.07

0.08 0.07

κ / (mS cm-1)

κ / (mS cm-1)

0.06 0.05 0.04

298.15 303.15 308.15 313.15

0.03

K K K K

0.06 0.05 0.04

298.15 303.15 308.15 313.15

0.03 0.02

0.02

K K K K

0.01 0.01 0.0000

0.0003

0.0006

0.0009

[CTAB] / (mol

0.0012

0.0000

0.0015

kg-1)

0.0003

0.0006

0.0009

[CTAB] / (mol

0.0012

0.0015

kg-1)

Fig. 2. Variation of specific conductivity (κ) with [DTAB] (a) in water and (b) in 3.0 × 10−5 mol kg−1 aqueous cresol red; with [CTAB] (c) in water and (d) in 3.0 × 10−5 mol kg−1 aqueous cresol red at 298.15 K (■), 303.15 K ( ), 308.15 K ( ), and 313.15 K ( ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

becomes easier, thereby, accounting for increased donor-acceptor type interaction between CR and water molecules. In addition, due to the increased hydrophilic character of CR molecule, interaction between \SO− 3 group of the dye and proton of water molecule also becomes stronger. On the other hand, in the absence of electrondonating groups in PR molecule there would be weaker interaction between PR and water molecules, resulting in relatively stronger

interaction between \ N þ (CH3)3 group of DTAB/CTAB and \SO− 3 group of PR. Consequently, stronger interaction between CR and water molecules partially hinders the interaction of \N þ (CH3)3 group of DTAB/CTAB with \SO− 3 group of CR, this, in turn, exposes the polar head groups of the surfactants, thus, increasing the mutual electrostatic repulsion between similar charged head groups of DTAB and CTAB on the surface of the micelle. This causes delay in the micellization and,

Table 3 Values of critical micelle concentration (cmc) of DTAB and CTAB in water 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

15.43, 15.63d 14.77 0.99, 0.99e 0.63

15.62, 15.30c 14.95 1.00, 1.01e, 1.16f 0.63

15.74, 16.12d 15.10 1.03, 1.09e 0.64

cmc (10−3 mol kg−1) DTAB + water DTAB + aq. CR CTAB + water CTAB + aq. CR a b c d e f

Reference [23]. Reference [24]. Reference [26]. Reference [27]. Reference [28]. Reference [29].

15.03, 16.00a, 15.00b, 14.80c 14.52 0.95, 0.95f, 0.96e 0.61

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

Table 4 Values of degree of ionization (α) of DTAB and CTAB in water 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

0.28 0.34 0.26 0.42

0.50

0.40

0.30

0.10

0.00 295.15

0.31 0.36 0.29 0.46

300.15

305.15

310.15

315.15

T / (K) 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 in 3.0 × 10−5 mol kg−1 aqueous cresol red ( ) respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

shows that the degree of counterion dissociation increases regularly with temperature. The variations of α with T in the absence and in the presence of CR are shown in Fig. 3. Similar increase in α with rise in temperature has also been observed for alkyltrimethylammonium bromide (DTAB, TTAB, CTAB) [37], CTAB [40] and dodecyl dimethy benzyl ammonium chloride [41] in aqueous solutions. The increase in cmc and α with increase in temperature is in good agreement with the results reported for the ionic surfactants in aqueous medium [8]. The increase in α can be attributed to the combined effect due to the columbic and thermal forces [42]. The former force attracts the counterions toward the polar surfactant head groups while the latter one induces the dissociation of counterions from the surfactant head groups. It seems that the thermal forces predominate over the columbic forces, causing ionization of the surfactants DTAB and CTAB, leading to the increased α value with temperature. It is to be noted that the values of α of DTAB and CTAB are higher in the presence of CR than in its absence. This may be due to the presence of the additive CR in the outer portion of the micelle and, thus, causing steric hindrance to the binding of counterions to the micelle, facilitating the dissociation of the counterions, which yields higher α values in the presence of CR than in its absence. The effect of the additive CR on α can also be explained by considering the surface area per head group, i.e., the surface charge density [39]. The increase in the degree of counterion dissociation α of DTAB and CTAB in the presence of CR than in pure water (Table 4) is attributed to the solubilization of CR in the palisade layer of the micelle. This increases the surface area per ionic head group (or decreases the surface charge density), facilitating the ionization of the counterions, Br−, from the micellar head groups of these surfactants, and, thereby, yielding higher α values in the presence of CR than in its absence. This is in accordance with the reported results [8,39] for a number of cationic surfactants, as in the present study. The knowledge of the thermodynamic parameters of micellization is found to be very useful for a clear understanding of the micellization process and the factors that affect the micelle formation [8]. A widely used model is the equilibrium model [43,44], according to which an equilibrium between counterions, surfactant monomers, and monodisperse micelles, can be represented as: ðn−pÞ C

0.30 0.35 0.27 0.44

aq. DTAB aq. CTAB DTAB + aq. CR CTAB + aq. CR

0.20

313.15 K

α DTAB + water DTAB + aq. CR CTAB + water CTAB + aq. CR

0.60

α

hence, higher cmc values of DTAB/CTAB in aqueous CR than in aqueous PR are observed. Hydration of the hydrophilic group \SO− 3 of the dye has also been reported by Garcia and Sanz-Medel [10]. Furthermore, it is worth mentioning that the cmc value of DTAB is much higher whereas that of CTAB is slightly higher in aqueous CR than in aqueous PR solution. As stated above, the combined effect of both hydrophobic and hydrophilic (electrostatic) interactions operating between surfactant and dye in aqueous medium controls the micellization and, hence, the cmc of the surfactant. It seems that the increased electrostatic repulsion between relatively more exposed polar head groups of DTAB diminishes the tendency of the surfactant monomers to form micelles in the presence of CR than in the presence of PR, leading to higher cmc values of DTAB in CR than in PR solution. On the other hand, large hydrophobic groups of CTAB molecules cause strong hydrophobic–hydrophobic interaction which seems to be almost equally effective in overcoming the electrostatic repulsion between polar head groups of the micelles, thereby, yielding almost equal cmc values of CTAB in aqueous CR as well as in aqueous PR solutions. At this point, it is worth mentioning that among the various factors which affect the cmc such as the structure of the surfactant, the presence of additives (electrolyte and organic compounds), temperature, etc. hydrophobic character of the surfactant exhibits the most profound effect on the cmc in aqueous solution. The cmc decreases as the number of carbon atoms in the hydrophobic group increases to about 16, and a general rule is that for ionic surfactants the cmc is halved by the addition of one methylene group (\CH2) to the straight-chain hydrophobic group attached to a single terminal polar group [8], as in the case of CTAB. Thus, it is interesting to 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 almost same. This reinforces the view that hydrophobic–hydrophobic interaction between the nonpolar (\C16H33) groups of CTAB molecules is the deciding factor and equally reduces the cmcs of the surfactant in both the aqueous CR and aqueous PR solutions. The degree of counterion dissociation (α) of the micelle near its cmc was 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 DTAB and CTAB in aqueous medium (Table 4) are 0.28 and 0.26 at 298.15 K, respectively, which compare well with the corresponding literature values 0.29 [36] and 0.27 [37]. It is important to assess the counterion effect 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 replaced by Cl− (DTAC), α increases. This is attributed to the smaller size of Cl− ion than Br− ion, which makes the former ion strongly hydrated than the latter one. Weakly hydrated Br− ions are more readily absorbed in the micellar surface, increase the surface area per head group (i.e., the surface charge density is decreased) in the ionic micelle [8,26]. This hinders the ionization of Br− ions compared to Cl− ions from the micellar surface of the surfactants and, therefore, smaller values of α are observed for DTAB than for DTAC. The aforementioned explanation is strongly supported by the fact that the larger the hydrated radius of the counterion, the greater the degree of dissociation [8,38,39]. Table 4

399

0.34 0.37 0.30 0.48

−

þ

þ nS ↔M

pþ

:

ð1Þ

where for a cationic surfactant, S+ represents the surfactant ions, C− the corresponding counterions, and MP+ the aggregate of n monomers with an effective charge p. The equilibrium constant for Eq. (1) can be

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

related to the standard free energy of micelle formation per monomer unit by: ο

ΔGm =RT ¼ −ð1=nÞ lnC MPþ þ lnC Sþ þ ð1−p=nÞ ln C C − :

ð2Þ

In the case of typical micelles, as DTAB and CTAB, n lies in the range of 50–100 in aqueous medium, the value of MP+ is small and insensitive + to the large errors in the evaluated CMP+ value, and C− C and CS can be replaced by the value of cmc in the second and third terms in the above equation to give [31,42,43]: ο

ΔGm ¼ ð2−α ÞRT ln X cmc

ð3Þ

where Xcmc is the cmc expressed in mole fraction unit, α (=p/n) is the degree of counterion dissociation from the micelle, R is the gas constant, and T is the temperature in Kelvin scale. The enthalpy of micellization can be obtained by applying the Gibbs–Helmholtz equation:  ο 2 ΔH m ¼ −RT ð2−α Þð∂ ln X cmc =∂T ÞP − lnX cmc ð∂α=∂T ÞP :

ð4Þ

The term (∂ ln Xcmc/∂T)P was calculated from the slope of the plot of ln Xcmc versus temperature and neglecting the term (∂α/∂T)P which is very small over the investigated temperature range. The entropy of micellization (ΔSom) can be estimated from the known values of ΔGom and ΔHom using the relation: ο ο ο
ΔSm ¼ ΔH m −ΔGm =T:

ð5Þ

The values of ΔGom, ΔHom, and ΔSom are listed in Table 5. It shows that the values of ΔGom(−47.26 and −35.02 kJ mol−1) of CTAB and DTAB at 298.15 K compare well with the literature values (− 47.19 and −35.58 kJ mol−1) [36]. The free energy of micellization (ΔGom) is a measure of the readiness with which the micelles are formed. ΔGom values are negative (Table 5) in both aqueous and aqueous CR solutions at each investigated temperature and become increasingly negative with increase in temperature. This indicates that the micellization of DTAB and CTAB is thermodynamically spontaneous and that the decrease in ΔGom with rise in temperature is ascribed to the desolvation of the

Table 5 Values of standard free energy (ΔG0m), enthalpy (ΔH0m), and entropy (ΔS0m) of micellization of DTAB and CTAB in water 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

DTAB + water ΔG0m (kJ mol−1) ΔH0m (kJ mol−1) ΔS0m (kJ mol−1 K−1) TΔS0m (kJ mol−1)

−35.02 −3.80 0.11 31.22

−35.18 −3.90 0.10 31.29

−35.31 −3.98 0.10 31.33

−35.32 −4.05 0.10 31.27

DTAB + aq. CR ΔG0m (kJ mol−1) ΔH0m (kJ mol−1) ΔS0m (kJ mol−1 K−1) TΔS0m (kJ mol−1)

−33.93 −3.05 0.11 30.87

−34.19 −3.14 0.10 31.05

−34.45 −3.22 0.10 31.23

−34.77 −3.30 0.10 31.47

CTAB + water ΔG0m (kJ mol−1) ΔH0m (kJ mol−1) ΔS0m (kJ mol−1 K−1) TΔS0m (kJ mol−1)

−47.26 −6.42 0.14 40.84

−47.56 −6.59 0.14 40.97

−47.94 −6.76 0.13 41.18

−48.24 −6.93 0.13 41.31

CTAB + aq. CR ΔG0m (kJ mol−1) ΔH0m (kJ mol−1) ΔS0m (kJ mol−1 K−1) TΔS0m (kJ mol−1)

−44.62 −4.31 0.14 40.31

−44.68 −4.40 0.13 40.29

−44.83 −4.49 0.13 40.34

−44.88 −4.64 0.12 40.25

hydrophilic groups of these surfactants [45]. Also, in light of Eq. (3), the increasing negative ΔGom value with rise in temperature is primarily due to the combined effect of the increase of the coefficient RT and an increase in the negative value of lnXcmc. Again, as expected, at a given temperature the standard free energy of micellization becomes increasingly negative primarily due to the decrease in the cmc as the alkyl chain length increases from DTAB to CTAB. It can be seen from Tables 3 and 5 that the cmc and ΔGom both decrease with increase in the carbon chain length thus indicating the ease with which the micelles are formed as the hydrophobicity increases from DTAB to CTAB. At a given temperature ΔGom becomes more negative for both the surfactants studied in aqueous solution than in the presence of CR (Table 5). It is evident from Eq. (3) that at a given temperature the value of ΔGom is due to the combined effect of (2 − α) and lnXcmc; the former quantity is found to be positive and higher while the latter one is more negative in aqueous than in the presence of CR, making ΔGom more negative in the absence of CR than in its presence. Moreover, it is worth mentioning that a decrease of 3.06 and 2.67 kJ per mol of methylene group (\CH2\) at 298.15 K in ΔGom as the number of carbon atoms increases from 12 (DTAB) to 16 (CTAB) in the hydrophobic groups of the surfactants in aqueous and aqueous CR, respectively, is observed. This is in close agreement with the reported decrease of about 3 kJ per mole of \CH2\ group of the ionic surfactants in aqueous medium [8,41]. The values of ΔHom, like ΔGom, of DTAB and CTAB in aqueous as well as in aqueous CR are negative and become more negative with rise in temperature, suggesting that the micellization of the studied surfactants is exothermic. Moreover, at each investigated temperature, ΔHom values of CTAB are found to be more negative than those of DTAB in both aqueous and aqueous cresol red solutions (Table 5). This indicates that the hydrophobic–hydrophobic interaction is stronger between the larger nonpolar group of CTAB and the non-ionic moiety of CR than between relatively smaller nonpolar group of DTAB and the non-ionic moiety of CR, making ΔHom more negative, hence, more exothermic for the former surfactant than for the latter one. In addition to this, negative ΔHom values can also be due to the electrostatic interaction between the cationic head, \Nþ (CH3)3, of the surfactants and negative, \SO− 3 , site of CR. It is worth noting that ΔHom is not always negative, often becomes positive, depending upon the structural and solution aspects prevailing in the surfactant solutions in the presence of additive. For example, ΔHom has positive values 42.27 and 12.24 kJ mol−1 for sodium N-dodecanoyl sarcosinate, an anionic surfactant, in aqueous medium at 293.15 and 298.15 K, respectively [46]; 4.98 and 2.47 kJ mol−1 for sodium dodecylsulfate in the presence of dipeptide (Tyr–Phe) and tripeptide (Val–Tyr–Val) in aqueous medium at 298.15 K [47]; 19.5, 17.7, and 18.9 kJ mol−1 for CTAB in the presence of the drug amitriptyline hydrochloride in aqueous medium at 308.15 K, the value decreased with increasing concentration of the drug [38]. The behavior of polyoxyethylene (POE) non-ionic surfactants in aqueous medium is quite interesting. ΔHom becomes increasingly positive with increase in the number of polar oxyethylene units in the hydrophilic head of the surfactant, whereas, it remains almost constant with increase/decrease in the length of the non-polar hydrophobic alkyl tail [8,48]. Consequently, the micellization of the abovementioned surfactants is an endothermic process, contrary to the micellization of DTAB and CTAB in aqueous as well as in aqueous CR solutions. Based upon the results cited above, it is observed that increase in ΔHom values is accompanied by large values of ΔSom. The possible explanation of such trends in ΔHom and ΔSom is that during micelle formation the transfer of the nonpolar hydrophobic tail from the bulk to the interior of the micelle disrupts the structured water molecules surrounding the hydrophobic tail which, in turn, increases the entropy of the system, associated with positive ΔHom value. This is in accordance with the view suggested by others [38,46]. It is interesting to examine the enthalpy of micellization (ΔHom) calculated from the temperature dependence of the cmc and α using Eq. (4), which considerably differs from that determined calorimetrically [40,

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

45–48]. For instance, the value of ΔHom for DTAB in aqueous solution, calculated using Eq. (4) is −3.90 kJ mol−1 (this work) and that obtained by calorimetric method is −3.56 kJ mol−1 [49] at 303.15 K. In the case of CTAB, in aqueous solution, the calculated value of ΔHom 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 calculated ΔHom(Eq. (4)) values, based on the mass action model assuming association–dissociation equilibrium between the surfactant monomers and micelles and those obtained by direct calorimetric method may be due to the fact that the overall enthalpy change of the solution in the calorimetric measurement has contributions from the solvation–desolvation of species involved, their mixing, ionization, new structural arrangements as a result of interspecies interactions, etc., whereas such contributions are not considered in the mass action equilibrium model [40, 43]. The entropy of micellization (ΔSom) (Table 5) for both the surfactants, DTAB and CTAB, in aqueous as well as in aqueous CR solutions is positive, indicating that the micellization process is favored by entropy gain, a pre-requisite condition for the micelle formation [8]. It is observed that there is small decrease in ΔSom with rise in temperature. Increase in temperature enhances the ionization of the surfactants, as is evident from the increased value of α with temperature, making cationic head groups of DTAB and CTAB easily available for the interaction with the anionic \SO− 3 groups of the dye, CR. Such surfactant–dye interaction causes decrease in randomness, hence, results in decreased ΔSom value with increase in temperature. Further, the observed value 40.84 kJ mol− 1 of TΔS0m (Table 5) of CTAB in pure water compares well with the reported value 39.21 kJ mol−1 [28] at 298.15 K. It is important to investigate the enthalpic and entropic components of ΔGom for the studied surfactants. Large values of −TΔS0m than those of ΔH0m in aqueous and in aqueous cresol red indicate that the process of micellization is governed mainly by entropy gain and that the driving force for the micellization is the tendency of the hydrophobic groups of the surfactants in transferring from the bulk solvent to the interior of the micelle [8,30]. This may be due to the breaking up of the structured water molecules surrounding the hydrophobic alkyl groups of the surfactants when it is transferred from the solvent environment to the interior of the micelle and also due to the increased freedom of these hydrophobic groups in the non-polar interior of the micelle than in the aqueous environment [8]. The change in the heat capacity of the surfactants upon micellizaο (=(∂ΔHοm/∂T)P)) can be estimated from the slope of ΔHοm tion (ΔCp,m vs. T plot. It is clear from Table 5 that (∂ΔHοm/∂T)P is negative, making ΔCοp,m 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 note that there are large variations in the reported literature values of ΔCp, ο ο m . For instance, ΔCp,m values for DTAC in water, in 0.01 M NaCl, and in 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 bromide and DTAB, respectively, in water [50]. All these values of ΔCοp,m are obtained from the slopes of the linear plots of ΔHom against T, but, surprisingly, the values clearly differ to a large extent. Such unexpected variations in ΔCοp,m values may be attributed to the controversial data on ΔHom reported in the literature [50]. For example, considering that DTAB is one of the most studied cationic surfactants, the values of ΔHom range from − 8 [51] to − 1.5 kJ mol−1 [52] at 298.15 K. Moulik and co-worker [53] determined ΔHomvalue as − 1.77 kJ mol− 1 at 303.15 K; this value has also been quoted by Khatua et al. [54] in their work. Therefore, it may be concluded that the large variations reported in the values of ΔCοp,m for surfactants are due mainly to the large variations in ΔHom values. Therefore, a more clear understanding of the soluο tion properties which affect the magnitude of ΔHom and, hence, of ΔCp,m is required. Negative ΔCοp,m values are generally observed for the

401

aggregation of the amphiphilic molecules in aqueous medium primarily due to the removal of the non-polar group of the surfactant from the contact of the water molecules on micellization [49]. It has been suggested [49,55] that although the hydrophilic head-groups of the ionic ο is assumed to be surfactants remain hydrated on micellization, ΔCp,m solely due to the change in the exposure of the hydrophobic tails to the water molecules. Chen et al. [56] and Sorac and Bester-Rogac [57] have successfully used this approach in describing the change in ΔCοp,m on the micellization of ionic as well as non-ionic surfactants in aqueous medium. This is truly in consistent with the fact that the micellization occurs mainly due to transfer of the hydrophobic tail 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 pure water (Fig. 4(a)) shows absorption band with λmax at 434 nm which is in 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 in the presence of different concentrations of CTAB in the pre-micellar and above the cmc of the surfactant. It is observed that this absorption band (λmax = 434 nm) starts decreasing on the addition of CTAB with the appearance of a new band with λmax at 584 nm and that the absorption band increased up to 6.0 × 10− 4 mol kg− 1 of the surfactant (i to v), then starts decreasing above this concentration of CTAB (vi, dotted line). In the presence of CTAB, the absorbance of CR increases up to 6.0 × 10− 4 mol kg− 1 of the surfactant which is taken as the cmc of CTAB which is very close to the observed cmc value 6.08 × 10− 4 mol kg− 1 at 298.15 K (Table 3) by electrical conductivity method. The increase of the absorbance band below cmc clearly suggests that the band at 584 nm arises due to the interactions (electrostatic and hydrophobic) between the dye, CR and monomers of the surfactant forming CR–CTAB ion-pairs. However, when the concentration of CTAB is increased beyond its cmc, i.e., in the post-micellar region, the absorption band starts decreasing Fig. 4(b) (vi, dotted line). This confirms the solubilization of the dye CR in the micelles of the surfactants, CTAB and DTAB. As expected, the cmc (6.0 × 10−4 mol kg−1) of the surfactant in the presence of cresol red is lower than in pure water. This can be explained by considering that the negatively charged sulfonate groups, \SO− 3 , of CR prefer to cluster with the oppositely charged cationic head groups, \Nþ (CH3)3, of the surfactant, thereby, reducing the electrostatic repulsion, within the polar cationic head groups of CTAB which, in turn, lowers the cmc value of the surfactant in the presence of dye CR than in pure water. Depression in the cmc values of CTAB and TTAB in the presence of the dye congo red than in pure water has been ascribed to the electrostatic interaction between \SO − 3 group of the dye and cationic head groups of the surfactants [17,58]. The ion-pair which was formed in the pre-micellar region breaks down as the micelles are formed with increasing concentration of the surfactant above the cmc and solubilization of dye dominates over the ion-pair formation. Not much spectral variation was observed on changing the surfactants from CTAB to DTAB; λmax remains almost unchanged (Fig. 4(c)). The cmc of DTAB in the presence of CR was observed to be 14.0 × 10− 3 mol kg− 1 from the spectroscopic method. The results obtained here are in good agreement with those reported by others [3,23] for the UV–visible spectroscopic study of cresol red in the presence of CTAB and DTAB. An interesting aspect emerges from the present spectroscopic study of dye–surfactant interaction in aqueous medium that the more the hydrophobic dye, the stronger the dye–surfactant interaction, resulting in lower cmc values. The above result is supported by the fact that the dyes, PR [3,23] and C. I. Reactive Orange 16 (RO16) [59] being more hydrophobic compared to CR interact more strongly with the surfactants DTAB, TTAB, and CTAB, yielding lower cmc values in the presence of PR and RO16 compared to

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

a)

4. Conclusions 0.6 434

0.5

Absorbance

0.4 0.3 0.2 0.1 0.0 -0.1

400

500

600

Wavelength / (nm)

b) 0.5

(i) (vi) (v)

Absorbance

0.4

0.3

0.2

0.1 (vi)

0.0

(v)

(i)

-0.1

500

400

600

Wavelength / (nm)

c) 0.6 (i)

Absorbance

0.5

(ii) (vi)

(v)

0.4 0.3 0.2 0.1 0.0

(i)

400

500

600

The increase in the cmc values of DTAB and CTAB in the studied temperature range is attributed to the breaking up of the structured water surrounding the hydrophobic groups of the surfactants. A marked decrease in the cmc of CTAB than those of DTAB in aqueous as well as in aqueous cresol red is due to the larger hydrophobic chain of CTAB molecule than of DTAB molecule, favoring easy micellization for the former than the latter surfactant. A pronounced decrease in the cmc of both the surfactants in the presence of CR than in its absence is attributed to the solubilization of the additive CR molecule in the palisade layer of the micelle, resulting in decreased mutual repulsion of the ionic groups in the micelle which requires less work for the formation of the micelles, thereby, resulting in lower cmc values of DTAB and CTAB in the presence of CR than in its absence. Also, the values of α of both the surfactants are higher in the presence of CR than in its absence. The presence of CR in the palisade layer of the micelle causes steric hindrance to the binding of the counterions (Br−) to the cationic polar head groups, \N þ (CH3)3, of the surfactants, favoring dissociation of the counterions and, hence, higher α values are obtained in the presence of CR than in its absence. The values of ΔGοm and ΔHοm for both DTAB and CTAB are negative and tend to become more negative as the alkyl chain length increases from DTAB to CTAB, indicating that the micellization of the surfactants in the studied solvents is thermodynamically spontaneous and exothermic in nature and that the micelle formation is more favorable for CTAB than for DTAB due to large non-polar group in the former than in the latter surfactant. Furthermore, a decrease of about 3 kJ per mole of methylene group (\CH2\) at 298.15 K in ΔGοm as the number of carbon atoms increases from 12 (DTAB) to 16 (CTAB) in the non-polar hydrophobic groups of the surfactants is in good agreement with the reported decrease of about 3 kJ per mole of (\CH2\) group of the ionic surfactants. The values of − TΔSοm are much larger than those of ΔHοm for both the surfactants, suggesting that micellization is primarily governed by the entropy gain due to the breaking up of the structured water molecules surrounding the hydrophobic groups of the surfactants when these groups are transferred from the bulk solvent to the interior of the micelle where they feel more freedom because of the non-polar environment of the interior of the micelle. Negative values of ΔCοp,m(=(∂ΔHοm/∂T)P) further support the aggregation of the amphiphilic molecules in the studied media. UV–visible absorption spectra indicate that there is ion-pair formation as a result of the interaction between dye, CR and monomers of the surfactants CTAB/DTAB in the pre-micellar region, but as the concentration of the surfactants is increased above their cmcs' values the ion-pair complex breaks down and dye is solubilized in the micelles. Thus, the spectroscopic results of the investigated systems support the results obtained conductometrically. The present study emphasizes on the importance of both electrostatic and hydrophobic interactions between the cationic surfactants DTAB/CTAB and anionic dye CR. As the thermodynamics of dye–surfactant interactions has received little attention, these results in conjunction with the spectroscopic results would be of great value in understanding the mechanisms of the chemical equilibria and kinetics of surfactant sensitized color and fluorescence reactions.

Wavelength / (nm) Acknowledgments Fig. 4. (a) Absorption spectrum of 3.0 × 10−5 mol kg−1 cresol red in aqueous solution at 298.15 K. (b) Absorption spectra of 3.0 × 10−5 mol kg−1 aqueous cresol red in the presence 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) Absorption spectra of 3.0 × 10− 5 mol kg−1 cresol red in the presence of different concentrations 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.

those in the presence of CR. Thus, it is concluded that the results obtained by thermodynamic considerations are satisfactorily supported by the spectroscopic study of the present systems.

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