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Thermodynamic Study of a Cationic Surfactant inAqueous Solution Containing Ethylene Glycol and ItsOligomers: Effect of Number of Ethereal Oxygen Atomsin Glycols MoleculeSuresh Chavda a & Pratap Bahadur aa Department of Chemistry , Veer Narmad South Gujarat University , Surat , IndiaAccepted author version posted online: 30 Jan 2012.Published online: 21 Dec 2012.

To cite this article: Suresh Chavda & Pratap Bahadur (2013) Thermodynamic Study of a Cationic Surfactant in AqueousSolution Containing Ethylene Glycol and Its Oligomers: Effect of Number of Ethereal Oxygen Atoms in Glycols Molecule,Journal of Dispersion Science and Technology, 34:1, 84-91, DOI: 10.1080/01932691.2011.648496

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Thermodynamic Study of a Cationic Surfactant inAqueous Solution Containing Ethylene Glycol and ItsOligomers: Effect of Number of Ethereal Oxygen Atomsin Glycols Molecule

Suresh Chavda and Pratap BahadurDepartment of Chemistry, Veer Narmad South Gujarat University, Surat, India

GRAPHICAL ABSTRACT

Effect of glycols viz., ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG) andtetraethylene glycol (TeEG) on micellization parameter of cationic surfactant tetradecyletrimethylammonium bromide (TTAB) was examined by means electrical conductivity measurements. Thecritical micelle concentration (CMC), degree of counterion dissociation (a) and Gibbs energy ofmicellization (DGm) as well as Gibbs energy of transfer (DGt) of surfactant tail from bulk phaseto micellar phase as a functions glycol concentration at 303.15K were evaluated. In ordered toobtain enthalpy (DHm) and entropy (DSm) of micellization, CMCs were determined in 10.0% ofglycols in temperature range from 303.15K to 323.15K. Thermodynamic parameters of the micel-lization reveal that effectiveness of glycols to reduce micellization increases with concentration andwith number of ethereal oxygen, [-O-]n in glycol molecule.

Keywords Glycol, micellization, surfactant, thermodynamic

Received 17 November 2011; accepted 1 December 2011.Author thanks UGC, New Delhi, for providing financial support in form of Rajiv Gandhi National Fellowship. Letter no.

(F.16-1228(SC)=2008(SA-III)).Address correspondence to Suresh Chavda, Department of Chemistry, Veer Narmad South Gujarat University, Surat 395007,

India. E-mail: sureshchavda1983@gmail.com

Journal of Dispersion Science and Technology, 34:84–91, 2013

Copyright # Taylor & Francis Group, LLC

ISSN: 0193-2691 print=1532-2351 online

DOI: 10.1080/01932691.2011.648496

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INTRODUCTION

Applications of surfactants in fields like lubrication oroil-wetting cleaners and detergency require the use of sur-factant in either a water-free or water-poor medium. Thishas stimulated a significant amount of interest towardsthe micellization of various surfactants in non-aqueous,polar organic media[1,2] and in mixed solvent having a sig-nificant amount of a polar solvent mixed with water.[3–12]

Glycols are well known for their use as coolants and anti-freezes and find application in pharmaceutical, cosmetics,and food industries.[13] In most these applications, surfac-tant is also present and, therefore, it is important to studythe effect of glycol on micellization of surfactants. Theproperties of surfactant micelles can change significantlyas the polar glycol solvent replaces the water in the system.Also, the addition of glycols to aqueous micellar surfactantsolutions provides an opportunity to study the so called sol-vophobic effect (as opposed to the hydrophobic effect),which is used to describe micellization in polar solvent[3,14]

The micellization in these solvents is similar in many aspectsto the micellization in water although micellization is not asfavorable as in water for given surfactant.[15]

There is enormous literature that deals with effect ofaqueous ethylene glycol (EG) on micellization of cationic,anionic and nonionic surfactants.[3–8,16–30] From thermo-dynamic studies of micellization in presence of EG Ruiz[5,6]

observed that due to lower cohesive energy of EG þwatermixed solvent system, aggregation number of micellesdecreased. For alkyltrimethylammonium bromide surfac-tant in presence of aqueous EG and glycerol from electro-chemical studies using surfactant selective electrode,Palepu and coworkers[3] concluded that logarithm of thecritical micellar concentration (CMC) of surfactant isdirectly proportional to the solvent=water ratio expressedin weight percent. They have also investigate the effect ofEG on binary mixtures of alkyltriphenylphosphonium bro-mides[18]; the excess free energy and the other related ther-modynamic parameters of mixing were evaluated anddiscussed in terms of the stability of the mixed micelles.Bakshi[21,22] explored the effect of glycols on mixed micelleof ionic surfactants and suggested that non-ideality ofmixed system increases with glycol content due to solvationof surfactant hydrophobic tail by glycol via hydrocarboninteraction. Moya[23–28] comprehensively studied theternary mix system (cationic surfactants-water-EG) toinvestigate micellization of surfactant and examined thesesystems as reaction media for kinetic studies. These studiesconclusively suggest that progressive increase in glycolconcentration makes surfactant aggregation process lessfavorable and reduces rates of reaction.

From the literature survey, it is clear that effect of EGon micellization have been widely studies but, there areonly few reports on oligomers of EG viz., diethylene glycol

(DEG), triethylene glycol (TEG) and tetraethylene glycol(TeEG) which, are equally important.[13,31–35] Further,fewer reports deal with effect of EG oligomers on thermo-dynamic parameters of micellization of cationic surfactant(except Gibbs energy of micellization). This motivated usto investigate thermodynamic of micellization process ofcationic surfactant in the presence of EG and its oligomers.

The characteristic property of glycol and its oligomers isthe presence of two hydroxyl (-OH) groups and increasingnumber of ether groups (-CH2CH2O-) or ethereal oxygen(-O-) with molecular weight. Hydroxyl and ether groupsin glycol enable to form intra- and intermolecular hydro-gen bonds (H-bond), which is also the cause of high vis-cosity of these solvents. Glycols can also form H-bondwith water and, thus, behave as water structure breaker.[36]

For these solvents with increase in number of etherealoxygen ([-O-]n) dielectric constant (e) decreases whereasdipole moments (l) increases (see Table 1). Thus, polarityof the glycols increases with number of ethereal oxygenin their molecules.[37] Glycols=glycol oligomers used in thisstudy viz EG, DEG, TEG, and TeEG have high cohesiveforce.[38] Due to these characteristic features of glycols,their presence greatly influences the micellar behaviourof surfactant in aqueous solutions by changing waterstructure.

Here we report new results for critical micelle concen-tration (CMC), degree of counterion dissociation (a), Gibbsenergy (DGm), enthalpy (DHm), entropy (DSm) micellizationof a cationic surfactant tetradecyltrimethylammonium bro-mide (TTAB) in water containing 10% of EG, DEG, TEG,and TeEG. Also, Gibbs energy (DGt) for transfer of hydro-phobic tail of surfactant from bulk phase (water-glycol mix-ture) to pseudo micellar phase. Apart from this, CMC,degree of counterion dissociation (d), DGm and DGt at303.15K in presence of these additives in concentrationrange from 5.0 to 20.0% are investigated. To evaluateCMC, a and thermodynamic parameters electrical conduc-tivity measurements has been carried out because it con-sidered as a sensitive technique for ionic surfactant study.Obtained data are correlated with [-O-]n in glycol molecule.These thermodynamics data extend the understanding ofaggregation process of surfactant in water-glycol oligomermixtures.

EXPERIMENTAL

Materials

The cationic surfactant tetradecyltrimethylammoniumbromide (TTAB) of 99.9% purity was obtained from SigmaChemical Company (USA) and was used as received. Theglycols viz., ethylene glycol (EG), diethylene glycol(DEG), triethylene glycol (TEG) and tetraethylene glycol

EFFECT OF GLYCOLS ON TTAB 85

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(TeEG) were all analytical grade reagents (99% pure) fromMerck and used as received. Table 1 represents the struc-ture, molecular weight, dielectric constant (e), and dipolemoment (l) values of these glycols. Triply distilled water(specific conductivity order of 10�3 mS cm�1) was usedby preparation of the solutions.

Method

Conductometric measurements were done using anESICO microprocessor-based conductivity bridge, Model1601, and a dip-type cell made of platinum black havingunit cell-constant. The instrument was calibrated usingKCl solutions of known concentrations. Temperature equi-librium was maintained after thorough mixing via Equitron(An automatic thermostat bath from Medica Mgf. Co,India) to �0.05K during the mixing and measurement.For conductance measurement solvent systems wereprepared by mixing different amount of glycol then thesesystem were use to prepare surfactant solution of properconcentration and solution were kept overnight to attainequilibrium. Measurements were done by successivedilution method in which TTAB solution of appropriateconcentration in solvent system was gradually diluted bysame solvent system and for each increment of dilution spe-cific conductance was measured as function of surfactantconcentration. In each experiment, about 70 experimentalvalues were registered.

RESULTS AND DISCUSSION

Effect of Glycol Concentration on CMC, a, DGm,and DGt

CMC is the fundamental parameter of surfactant in sol-ution since surfactant behaves differently before and afterCMC. When the CMC is reached, large changes occur insurface properties including its wetting ability, emulsionformation, dissolution, foaming ability, etc. Anotherimportant parameter is the degree of counterion dis-sociation a. The value of a reflects hydrophobic effect ofthe alkyl chains and the electrostatic interactions both

between head groups and between head-groups and thesurrounding water molecules.[29]

Figure 1 demonstrates a representative plot of specificconductivity (k) versus concentration (c) of TTAB in pres-ence of different concentration of DEG. The two straightline plots having different slope intersect at point, whichcorresponds to the CMC values. The data above (postmi-cellar) and below (premicellar) the CMC were linearly fit-ted, with correlation coefficients >0.998 and the ratio ofslope of lines is considered as a. The CMC and a valuesin presence of different concentration of EG, DEG,TEG, and TeEG at 303.15K are listed in Table 2. It isclearly seen that both the CMC and the a increase in thepresence of EG and its oligomers and follows the orderTeEG>TEG>DEG>EG which is in accordance to theirhydrophilicity and has been reported and discussed.[34]

From Figure 1 it can be seen that with increase glycolconcentration, specific conductance decreases which isdue to change in dielectric constant (e) and increase inviscosity of solvent system. This leads to decreased

TABLE 1Structural formula, molecular weight (M), dielectric constant (e), and dipole moment (l) of glycols

Glycol Structure M gmol�1 e l Cmol�1

EG 62.07 40.80a 2.38a

DEG 106.12 30.70a 2.69a

TEG 150.17 23.40a 2.99a

TeEG 194.23 19.50a 3.35a

aRef.[37]

FIG. 1. Specific conductivity (k) versus concentration of TTAB (C) in

DEG þ water mixture. (�) 0.0%, (.) 5.0%, (()7.5%, (&) 10.0%, (4)

0.15.0% and (~) 0.20.0% DEG at 303.15K.

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mobility of surfactant monomer and micelle.[39] Accordingto Ruiz,[5] the presence of glycol decreases the cohesiveenergy density of the aqueous phase that results inreduction of solubility parameter of water, thus, increasingthe solubility of surfactant and hence the CMC. Further,decrease of e for mixed solvent (as compare to pure water)by the presence of glycol pushes away the bromide counter-ion and favors mutual repulsion of the trimethylammo-nium head groups in the micelle, thus opposingmicellization and increasing the CMC and a. This effectincreases with increase in glycol concentration. Dependingupon the explanations, it is obvious that at common con-centration of different glycols trend in CMC and a of sur-factant can be linked with glycols e value or with [-O-]n.Lower the e, higher will be CMC and a. From Table 1,order of decreasing e is as follows TeEG (e¼ 19.50)<TEGTEG (e¼ 23.40)<DEG (e¼ 30.70)<EG (e¼ 40.80) henceCMC and a values should have opposite trend to this. Thisis in accordance with our results (Table 2).

Glycols are water structure breakers and have a signifi-cant impact on the structure of the water molecules that

surround the hydrophobic chains of the surfactant.[40]

Water structure breaking ability (or the ability to formintermolecular H-bonds with water) of glycols is morepronounced in TeEG than for TEG, DEG, and EGrespectively. One of the apparent reasons for this trend isthe greater number ethereal oxygen atoms, [-O-]n on TeEG(as depicted in Table 1 five oxygen atoms) available to par-ticipate in H-bonding as compared to TEG (four), DEG(three) and EG (two).Consideration of this effect clearlysupports the results of CMC and a in Table 2 andFigures 2a and 2b.

EG is a dihydric alcohol (diol) with two hydrophilic -OHend groups attached to hydrophobic (-CH2CH2-) group,whereas, in DEG,TEG, and TeEG two, three and four ethergroups (-CH2CH2O-) are there between two -OH groups,respectively (see Table 1). The (-CH2CH2O-) group con-tains both the hydrophobic hydrocarbon part (-CH2CH2-)and hydrophilic oxygen having lone pair of electrons cap-able of forming hydrogen bonds with water molecules.[41]

This (-CH2CH2O-) group thus would not be truly hydro-phobic in nature. Moving from EG to DEG, TEG, andTeEG as the number of (-CH2CH2O-) group increases,the hydrophobic contact between (-CH2CH2-) groupsbecome more favorable that results in the reduction of the(-CH2CH2-) area which is accessible to water. Due to

TABLE 2Critical micelle concentration (CMC), degree of counteriondissociation (a), Gibbs energy of micellization (DGm), andGibbs energy of transfer (DGt) in Presence of different% of

glycol at 303.15K

[Glycol]%

CMCmM a

�DGm

kJmol�1�DGt

kJmol�1

0.0 3.70(3.74)a 0.24 42.66(42.30)b —EG 5.0 3.81 0.26 41.89 0.76

7.5 4.15 0.27 41.20 1.4510.0 4.45 0.29 40.35 2.2815.0 4.83 0.31 39.37 3.2920.0 5.78 0.34 37.76 4.90

DEG 5.0 4.04 0.28 41.13 1.537.5 4.38 0.30 40.21 2.45

10.0 4.70 0.31 39.58 3.0815.0 5.29 0.35 37.97 4.6920.0 6.15 0.38 36.47 6.19

TEG 5.0 4.30 0.30 40.37 2.297.5 4.71 0.32 39.41 3.24

10.0 5.13 0.35 38.26 4.4015.0 5.81 0.37 37.08 5.5820.0 6.31 0.41 35.64 7.02

TeEG 5.0 4.35 0.32 39.84 2.827.5 4.65 0.34 38.99 3.67

10.0 5.28 0.38 37.43 5.2315.0 5.93 0.39 36.52 6.1320.0 6.42 0.42 35.32 7.34

aRef.[30]bRef.[13]

FIG. 2. a) Critical micelle concentration (CMC), b) degree of counter-

ion dissociation, c) Gibbs energy of micellization (DGm), and d) Gibbs

energy of transfer (DGt) in presence of 10.0% glycols as a function of

number of ethereal oxygen ([-O-]n) at 303.15K.

EFFECT OF GLYCOLS ON TTAB 87

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reduction of (-CH2CH2-) area hydrophobic interactiondecreases[42] that makes glycols more hydrophilic withincreases in number of (-CH2CH2O-) groups. This is alsosupported by trend of e values of these glycols. Above dis-cussion clearly explain stronger CMC and a increasingeffect showing the trend TeEG>TEG>DEG>EG.

The thermodynamic tendency of micellization of cationicsurfactant as function of glycol concentration in mixed sol-vent system at fixed temperature can be visualized by Gibbsenergy of micellization (DGm); for monomeric ionic surfac-tant calculated using Equation (1).[5,13,29,30,34]

DGm ¼ ð2� aÞRTlnXCMC; ½1�

where a is degree of counterion dissociation, R is gas constant,T is temperature in Kelvin scale, and XCMC is CMC in molfraction scale. DGm values for TTAB in pure water and inaqueous solution of different glycols at 5.0–20.0% are listedin Table 2. The value in pure water is in good agreement withpreviously reported.[5,13,30] In all water-glycol systems, DGm

becomes less negative with increasing glycol content in themixture. This manifests that micellization process becomesless spontaneous with glycol concentration. Further (asshown in Figure 2c), a comparison of DGm for differentwater-glycol mixed systems suggests that as the number ofethereal oxygen, [-O-]n in glycol molecules increases, sponta-neity of micellization decreases (as DGm values increase).

According to Mehta et al.[34] in water-glycol mixed sol-vent system, there are many factors that contribute toDGm. (i) Surfactant tail transfer energy (responsible for sol-vophobic effect); contribution by this factor increases withconcentration of glycol and delayed the micellization (i.e.,increases CMC). (ii) Associated with interfacial tensionbetween micelle core (hydrocarbon) and solvent interface.Contribution results from this factor are decreased with gly-col content because glycol-hydrocarbon interfacial tensionis relatively smaller as compared to water-hydrocarbon.[43]

Smaller glycol-hydrocarbon interfacial tension may causedecrease in CMC but since contribution of first factor is pro-nounced. An increase in CMCwas always found in presenceof glycol. (iii) Increase in electrostatic repulsion betweencharge head group in presence of water-glycol mixed solventas compared to that in pure water. Further, lower dielectricconstant of water-glycol mixed solvent system enhances thenumber of surfactant monomers in bulk phase (increasesolubility of surfactant consequently increase CMC anddecrease aggregation number). Thus, due to higher numberof ionic species (surfactant monomer) ionic strength of sol-vent system is increased which increases a and DGm. (seeEquation (1)). As explain in case of CMC and a it is clearthat decreasing tendency of micellization (i.e., increasingvalues of DGm) in presence of water-glycol mixed solventsystems follows the order TeEG>TEG>DEG>EG.Further, this tendency also decrease with concentration of

particular glycol in mixed solvent system. Mehta et al.[34]

observed similar trend for 1-hexadecylpyridinium chloride.Another parameter that describes the effectiveness of

glycols to suppress micellization is Gibbs energy of transfer(DGt) of surfactant hydrophobic tail from hydrophilic bulkphase (binary mixture of water þglycol) to hydrophobicmicellar phase. The DGt is the difference between DGm inwater-glycol (DGm(H2O-glycol)) mixed solvent system tothat in pure water (DGmðH2OÞ). DGt values are calculatedusing Equation (2)[5,13,29,30,34] and are recorded in Table 2.

DGt ¼ DGmðH2O�glycolÞ � DGmðH2OÞ: ½2�

From the DGt values, it is apparent that transfer of surfac-tant hydrophobic tail from bulk phase to micellar phasebecomes difficult with increase in glycols concentration(becauseDGt increases). As illustrated in Figure 2d, compari-son of DGt for all glycol studied manifests that micellizationis more difficult as number of ethereal oxygen, [-O-]n increasein glycol structure which is in accordance with our results ofCMC and a. From Table 2, it is found that for each glycolstudied DGt is positive and increases with concentrationand increase with H-bonding ability of glycols (or decreasewith e values of glycols). This increase=decrease is under-stood based on reduction in the solvophobic interactionscaused by the improved solvation. A solvophobic interactionleads to an increase in the solubility of the hydrocarbon tailsof surfactant (due to lower value e for water-glycol mixture ascompare to that of pure water) in the presence of glycols andconsequently an increase in the CMC and a.[5] Thus,DGt alsofollows the similar trend as CMC and a at common concen-tration of glycols (i.e., TeEG>TEG>DEG>EG), which issimilar order of H-bonding ability (or solvent power ofwater-glycol mixture).

Thermodynamic Parameters of Micellization in Presenceof Fixed Concentration of Glycols.

A representative plot (Figure 3) shows the temperaturedependence of the specific conductance (k) of TTAB sol-ution in 10.0% TEG. Similar plots were obtained in othercases. The CMC of TTAB in pure water as well in 10.0%aqueous glycol solution increases with temperature. Forionic surfactant, increase in temperature can affect CMCtwo ways (i) by dehydrating of the hydrophilic head group,which decreases CMC. (ii) By gradually destroy of the waterstructure surrounded to hydrophobic tail, which is unfavor-able for micellization and increase the CMC. These twoeffects oppose each other. From the data in Table 3 it wasclear that the second effect predominates. Additionally,an increase in the temperature causes a decrease in thecharge density at micellar surface by lowering the aggre-gation number of micelle which leads to increase in a.[44,45]

Information of temperature dependence of CMC and avalue facilitate the calculation of the enthalpy of micellization

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(DHm) for ionic surfactant using Equation (3) (Gibbs–Helmholtz equation).[5,13,29,30,34]

DHm ¼ �2RT2ð2� aÞ d lnXCMC

dT: ½3�

The values of lnXCMC in the presence of 10.0% glycolwere plotted against the temperature, T and the slopevalue determined from the linear plot gives the value ofdlnXCMC=dT.

Accordingly, the entropy of micellization (DSm) is calcu-lated by Equation (4)[5,13,29,30,34]

DSm ¼ ðDHm � DGmÞT

: ½4�

DGm and DGt values in presence of fixed glycol concen-tration as a function of temperature also evaluated usingEquations (1) and (2), respectively. Here, determinationof DHm was done using Equation (3), which does not con-sider the change in size and shape of micelles with tempera-ture. Hence, values of DHm and TDSm reported here are

FIG. 3. Specific conductivity (k) versus concentration of TTAB (C) in

10.0% TEG at (�) 303.15K, (.) 308.15K, (() 313.15K, (&) 318.15K, and

(4) 323.15K.

TABLE 3Critical micelle concentration (CMC), degree of counterion dissociation (a), thermodynamic parameters of micellization

(DGm, DHm, TDSm) at different temperature (T) in presence of 10.0% glycols

[Glycol] % T K CMC mM a �DGm kJmol�1 �DGt kJmol�1 �DHm kJmol�1 TDSm kJmol�1

0.0 303.15 3.70 0.24 42.66 – 8.46(7.51)d 34.20(29.2)d

308.15 3.87 0.26 42.74 – 8.64 34.10313.15 4.08 0.27 43.04 – 8.87 34.17318.15 4.27 0.29 43.09 – 9.05 34.04323.15 4.43 0.31 43.09 – 9.23 33.86

10.0 EG 303.15 4.45 0.29 40.35 2.31 13.27 27.07308.15 4.69 0.31 40.31 2.43 13.56 26.75313.15 4.90 0.32 40.53 2.51 13.92 26.61318.15 5.14 0.34 40.47 2.62 14.19 26.28323.15 5.48 0.36 40.33 2.51 14.47 25.87

10.0 DEG 303.15 4.70 0.31 39.58 3.08 14.38 25.20308.15 5.01 0.33 39.49 3.25 14.69 24.80313.15 5.31 0.35 39.40 3.64 14.99 24.41318.15 5.48 0.37 39.40 3.69 15.28 24.12323.15 5.92 0.38 39.44 3.65 15.67 23.77

10.0 TEG 303.15 5.13 0.35 38.26 4.40 14.93 23.33308.15 5.45 0.37 38.16 4.58 15.24 22.93313.15 5.66 0.39 38.15 4.89 15.54 22.61318.15 6.11 0.40 38.19 4.90 15.94 22.25323.15 6.53 0.41 38.27 4.82 16.34 21.92

0.100 TeEG 303.15 5.28 0.38 37.43 5.23 15.82 21.61308.15 5.73 0.39 37.48 5.26 16.24 21.23313.15 6.13 0.41 37.33 5.71 16.57 20.76318.15 6.45 0.42 37.48 5.61 16.99 20.48323.15 6.85 0.44 37.33 5.76 17.31 20.02

aRef.[5]

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different than that measured by means of calorimetry.[46]

The DGm, DGt, DHm, and TDSm at five different tempera-tures ranging from 303.15 to 323.15K are listed in Table 3.

As shown in Table 3, values of DGm in 10.0% glycol andin water remain negative and almost constant as a functionof temperature attributes that Gibbs energy of micelliza-tion is more or less independent of temperature. Similarly,DGt is independent of temperature.

Negative values of DHm (Table 3) reflect micellization asan exothermic process. The process becomes even more exo-thermic with increasing temperature in pure water as well asin 10.0% glycols. Further, from Figure 4a, it is apparentthat at fixed temperature (e.g., 303.15K) with increase[-O-]n in glycol structure, micellization becomes more exo-thermic (as DHm become more negative). For any ionic sur-factant, DHm depends on many factors. One of thefundamental and important factors that contributes nega-tively (i.e., makes DHm more negative) is the release of morearranged water molecules (than bulk water) surrounded tohydrophobic surfactant tail; during transformation of tailfrom bulk phase to micellar phase.[44,47] Other importantinteraction that contributes negatively to DHm is repulsiveelectrostatic interaction between head groups. From theabove facts, and as discussed earlier; the presence of glycolsdue to their hydrogen bonding ability increases repulsionbetween the ionic heads. The effect of glycols to decreaseDHm should follows the order TeEG>TEG>DEG>EG.

EG. Results shown in Table 3 and trend in Figure 4asupport these explanations.

It can be seen from Table 3 that the DSm (or TDSm)values are positive in each solvent system and decrease inthe studied temperature range. From these results it isapparent that presence of glycols induces randomness inaqueous surfactant solution (generated by breaking ofarranged water molecules). This facilitates reduction inhydrophobic interaction between water and surfactanthydrophobic tail. Comparison of TDSm values forwater-glycol systems at 10.0% glycol with respect to [-O-]nin glycols molecule suggests decrease in hydrophobic inter-action and it follows the order TeEG<TEG<DEG<EG(Figure 4b). These attribute that the efficiency of studiedglycol to decrease hydrophobic effect increases with [-O-]n(or ability of glycol to form H-bond) and decrease with erespectively. This is in accordance with positive valuesof DGt. From Figures 4a and 4b, it is apparent thatTDSm>�DHm, that is, micellization is entropy driven.[47]

Further, the DHm is negative and becomes more negative,where, as DSm is positive and becomes less positive withthe temperature attributes that there would be anenthalpy-entropy compensation for the micellization in allthe studied systems.[46]

CONCLUSIONS

This paper provides information on the effect of concen-tration of ethylene glycol and its oligomers as additives onmicellization of a cationic surfactant tetradecyltrimethylam-monium bromide (TTAB) in aqueous solutions and newdata for enthalpy (DHm) and entropy (DSm) of micellizationof cationic surfactant TTAB in presence of tetrathylene gly-col (TeEG), triethylene glycol (TEG), and diethylene glycol(DEG). For complete comparison data of ethylene glycol(EG) also provided. The CMC, degree of counterion dis-sociation, Gibbs energy of micellization (DGm), and Gibbsenergy of transfer (DGt) of alkyl chain from bulk phase tomicellar phase of TTAB shows linear increase with additiveconcentration. These observations lead to conclude that thepresence of glycols reduces the micellization tendency ofcationic surfactant. Comparison of thermodynamic para-meters of micellization viz.,DGm, DHm, and DSm in the pres-ence 10.0% glycol oligomers suggests that thermodynamictendency of micellization go in line with the presence ofnumber of ethereal oxygen [-O-]n in glycol oligomers, hydro-gen bonding ability or water structure breaking ability ofglycols and follows the order TeTG>TEG>DEG>EG.DGm and DGt are almost constant over the temperaturerange studied shows that Gibbs energy of micellization ismore or less independent of temperature. DHm is becomesmore negative and DSm becomes less positive with increasein temperature resulting an enthalpy-entropy compensationfor the micellization in all the studied systems.

FIG. 4. a) Enthalpy of micellization (DHm)and b) entropy of micelli-

zation in from of TDSm in presence of 10.0% glycols as a function of num-

ber of ethereal oxygen ([-O-]n) at 303.15K.

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