J. Chem. Thermodynamics 71 (2014) 205–211

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J. Chem. Thermodynamics

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Effect of temperature and additives on the critical micelle concentrationand thermodynamics of micelle formation of sodium dodecyl benzenesulfonate and dodecyltrimethylammonium bromide in aqueoussolution: A conductometric study

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⇑ Corresponding author. Tel.: +91 177 2830803; fax: +91 177 2830775.E-mail address: chauhansuvarcha@rediffmail.com (S. Chauhan).

S. Chauhan ⇑, Kundan SharmaDepartment of Chemistry, Himachal Pradesh University, Shimla 171005, India

a r t i c l e i n f o

Article history:Received 24 September 2013Received in revised form 28 November 2013Accepted 17 December 2013Available online 24 December 2013

Keywords:Amino acidsCritical micelle concentrationHydrophobic interactionsThermodynamic parameters

a b s t r a c t

Specific conductance of (0.3 to 3.0) mmol � kg�1 sodium dodecyl benzene sulfonate (SDBS) and (3.0 to30.0) mmol � kg�1 dodecyltrimethylammonium bromide (DTAB) has been determined in water and inthe presence of (0.01, 0.05 and 0.10) mol � kg�1 aqueous solution of glutamine/histidine/methionine atT = (293.15, 298.15, 303.15, 308.15 and 313.15) K. From the conductivity data, the critical micellarconcentration (CMC) and thermodynamic parameters of micellization (DGo

m; DHom and DSo

m) have beencomputed by applying the mass action model. Enthalpy–entropy compensation effect has also beenobserved. The effect of amino acid on the micellar properties of SDBS and DTAB depends upon their nat-ure, concentration, as well as on temperature and has been used to study the interactions present in themicellar systems. There occurs a gradual increase in the value of CMC with temperature in case of SDBSwhile in case of DTAB, it passes through a broad minimum and then tends to increase with increase intemperature. Increase of amino acid concentration is found to decrease CMC in both the surfactants.The DGo

m values are negative and the feasibility of the micellization is found to increase with rise intemperature. The magnitude of hydrophilic and hydrophobic dehydration determines whether theCMC values increase or decrease with rise in temperature.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The present study has been undertaken to understand howmicelles get affected in an aqueous environment containing aminoacids. The addition of amino acids to the solvent may affect themicellization process of a surfactant as a result of changes insolvent characteristics like hydrogen bond formation capacity,dielectric constant, density, viscosity and degree of ionization [1].The properties of amino acids themselves may also affect micelli-zation including surface lattice aggregation, polar/non-polar orzwitterions character, dimerization and hydrophobicity, etc. [2].These properties determine the tendency of non-polar groups toassociate in aqueous solutions.

Among amino acids, glutamine (Gln), a polar uncharged aminoacid serves as ammonia transporter to the liver and kidney for ureasynthesis which is a small, non-toxic compound excreted via urine[3,4]. Methionine (Met), is sulfur containing non-polar amino acid.Its necessity is to provide the methyl group (CH3) to acceptormolecules in one-carbon metabolism which is important in the

production of red blood cells, white blood cells and platelets. His-tidine (His) is positively charged basic amino acid at a pH ofapproximately 6 or below. It is the precursor molecule to hista-mine, the compound that causes many allergic reactions and whichmay be blocked by the use of anti-histamines. Because of this,many people who have itching-related health problems may beprescribed a drug like doxepin which has both histamineantagonistic properties and anxiolytic properties. On the otherhand, the surfactants have been used in a similar way in biologicalsystem as are employed in technical systems e.g. to overcomesolubility problems, as emulsifiers, as dispersants and to modifysurfaces, etc. The ionic surfactants, SDBS and DTAB have been cho-sen because of their strong interaction with protein and a regularingredient used in industry [3,4].

In recent years, there has been a growing interest in theinteractions present between amino acid and surfactant due totheir many applications in biosciences, foods and cosmetics, drugdelivery, detergency, and biotechnological processes [5,6]. Usingvarious numbers of tools and techniques, these interactions havebeen studied and published in the past few years [7–12]. Takinginto consideration, the diversity of such molecules (surfactantsand amino acids), we intend to design such system which could

(Gln) (His)

(Met)

FIGURE 2. Chemical structure of glutamine (Gln), histidine (His) and methionine(Met).

206 S. Chauhan, K. Sharma / J. Chem. Thermodynamics 71 (2014) 205–211

prove its efficacy in every field including food, pharmaceutical andbiological industry, etc.

Although a number of studies on the interaction of surfactantswith amino acid molecules have been reported in literature [13–19], but to the best of our knowledge, very little is known aboutthe intriguingly of the present system containing amino acidsand surfactants. Keeping these considerations in mind, we em-ployed simple and promising technique, in particular, conductanceto substantiate the interactions present between the anionic sur-factant (SDBS) and cationic surfactant (DTAB) with amino acids(glutamine, histidine and methionine) at different compositionsand temperatures. The resulting data have been discussed in termsof the interactions operating in surfactant–amino acid–H2O sys-tems including the effect of amino acids on the micellization ofthese surfactants. The structure of both, surfactants and aminoacids, which have been attempted in this work, have been shownin figures 1 and 2, respectively.

2. Experimental

2.1. Materials and method

Glutamine and histidine were obtained from MERCK (Germany)and methionine from S.D. Fine-Chem Ltd. (India), all were of A.R.grade and were used as received. SDBS and DTAB were also ofA.R. grade obtained from HIMEDIA (India) and S.D. Fine-ChemLtd., respectively. However, a pure sample of SDBS and DTAB wasobtained by giving the additional treatment as reported in litera-ture [20,21]. Aqueous solution of surfactants (SDBS and DTAB) ofdifferent molal concentration in the range (0.3 to 3.0) mmol � kg�1

for SDBS and (3.0 to 30.0) mmol � kg�1 for DTAB were prepared bythe addition of small aliquots of concentrated solution of the sur-factant to 10 mL of (0.01, 0.05 and 0.10) mol � kg�1 amino acidsolution prepared as a solvent medium. The solutions so obtainedwere gently stirred on magnetic stirrer before subjecting to mea-surements. A sample of distilled water was collected from the Mil-lipore Elix distillation unit which was subjected to furtherdistillation on acidified KMnO4 over a long fractionating columnoperating at 750 torr. Different fractions of distilled water werecollected having j and pH values in the range (1 to 3) � 10�6 -S � cm�1 and (6.75 to 6.95), respectively. The sample of purifiedwater so obtained was not used after two days. A high precisionwater thermostat fitted with a digital temperature controlled de-vice used for all experimental measurements supplied by NSW–New Delhi. The temperature of thermostat was maintained within(0.1 K over the entire temperature range studied. Conductivitymeasurements were carried out with digital conductivity meterCyberscan CON-510. The temperature of the solution was

FIGURE 1. Chemical structure of SDBS and DTAB.

maintained to ±0.1 K by circulating water from thermostat througha double walled vessel containing the solution. The accuracy of theconductance measurement was well within ±0.4%. The provenanceand purity of the sample used have been provided in table 1.

3. Results and discussion

3.1. Critical micelle concentration and micellization

To design and interpret the amino acid–surfactant interactionstudies, it is necessary to know the critical micelle concentration(CMC) of the two studied surfactants (SDBS and DTAB) in aqueoussolutions in the absence and in the presence of amino acids. Theconductivity data for aqueous solution of SDBS and DTAB atT = (293.15,298.15,303.15,308.15,313.15) K have been summa-rized in table SM1 (Supplementary material) and the correspond-ing plots have been shown in figure 3. Each plot showed a linearvariation in the j values with respect to increased surfactant con-centration both in the pre-micellar and post-micellar regions. Theabrupt change in conductivity (j) at a certain concentration of sur-factant produces sharp break point in the plots. This break pointbetween the two straight lines gives the value of critical micelleconcentration (CMC) which has been converted into their molefraction unit, XCMC before subjecting them to determine the ther-modynamic parameters of micellization. The CMC and correspond-ing, XCMC values for aqueous SDBS and DTAB have been reported intable 2 which reveal that the CMC values for the surfactant, SDBSand DTAB in water are not very large as compared to the values re-ported in literature; as 1.3 mmol � kg�1 for SDBS [22] and15.6 mmol � kg�1 in case of DTAB [23,24], respectively. The temper-ature dependence of XCMC values for both SDBS and DTAB providesinformation about the inhibitory effect of temperature.

Since, the variation in the specific conductance of a surfactant isquite linear before and after the break, a comparison among themonomeric and the micellar species over the whole concentrationrange can be made by computing the pre-micellar (S1) and post-micellar (S2) slopes. Both S1 and S2 values were determined fromthe linear regression analysis of the conductivity data with a corre-lation factor always much better than 0.997. The slope in the pre-micellar region has always been found greater than that in thepost-micellar region. Also, there are some examples [25,26] of sim-ilar binary ionic surfactant combinations in which the breakpointin the conductivity curve is not sharp. This can be generally ex-plained on the basis of two situations: first, when instead of aninstantaneous micelle formation process, stepwise micellizationoccurs as in the case of all bile salts [27] or in the presence of or-ganic additives [28]; second, when apart from the ordinary spher-ical micelles, bilayer assembly [29] or insoluble salt formationtakes place, for example, as in the case of binary combinations ofoppositely charged ionic surfactants.

TABLE 1Specification and mass fraction purity of chemical samples.

Chemical name Source Purification method Mass fraction purity

Glutamine (Gln) Merck None 0.99a

Histidine (His) Merck None 0.99a

Methionine (Met) S.D. Fine None 0.98a

Sodium dodecyl benzene sulfonate (SDBS) HIMEDIA Recrystalized 0.97Dodecyltrimethylammonium bromide (DTAB) S.D. Fine Recrystalized 0.98

a Declared by supplier.

0.0 0.5 1.0 1.5 2.0 2.5 3.00

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

κ / μ

S cm

−1

103 · mSDBS / mol · kg−1

(a)

0 5 10 15 20 25 300

200

400

600

800

1000

1200

1400

1600

1800

κ / μ

S · c

m−1

103 · mDTAB / mol · kg−1

(b)

FIGURE 3. Conductivity vs. concentration plots for aqueous solution of (a) SDBS and(b) DTAB at T = 293.15 K: j, 298.15 K: N, 303.15 K: d, 308.15 K: ., 313.15 K:temperatures.

TABLE 2CMC and corresponding, XCMC, values in aqueous solution of SDBS and DTAB.

T/K SDBS DTAB

103 � CMC 104 � XCMC 103 � CMC 104 � XCMC

293.15 1.20 0.216 15.1 2.72298.15 1.28 0.230 15.3 2.75303.15 1.36 0.245 15.5 2.79308.15 1.42 0.256 15.8 2.84313.15 1.51 0.272 16.1 2.90

The uncertainty in the CMC and corresponding XCMC measurements are:±(0.01 � 10�3 and 0.002 � 10�4) in case of SDBS and ±(0.1 � 10�3 and 0.02 � 10�4) incase of DTAB, respectively.

S. Chauhan, K. Sharma / J. Chem. Thermodynamics 71 (2014) 205–211 207

3.1.1. Effect of additivesIn this section, we studied the effect of additives i.e., amino

acids on the critical micelle concentration (CMC) of SDBS and DTAB

in order to examine more closely the manner in which amino acidsaffect the micellization of surfactant in their aqueous solutions.The conductivity data for SDBS and DTAB in aqueous amino acidshave been summarized in tables SM2–SM7. The dependence of jon surfactant concentration is shown in figures SM1–SM6 andthe corresponding XCMC values have been presented in table SM8.

Additives, on the basis of their influence on the micellizationprocess, can be classified in two main categories: electrolytes andnon-electrolytes [30]. Electrolytes generally facilitate the forma-tion of ionic micelles, primarily by lowering the coulombic Gibbsenergy of the interface, resulting in decreased CMC, so that at highionic strength, huge surfactant aggregates are formed [31]. On theother hand, non-electrolyte organic additives, which can be furtherclassified as polar and non-polar, affect micellization in differentways depending on the nature of the additives as well as its con-centration [32,33].

Furthermore, if we compare the XCMC values of these surfac-tants, as presented in table SM8, it is clear that at the given concen-tration of amino acid, it decreases in the order: His > Gln > Met incase of SDBS while in case of DTAB, the order is: Gln > Met > His.This trend can be explained on the basis of ion-pair formation be-tween oppositely charged amino acid and head group of surfactantleading to solubilization of respective amino acid. The effect occursto maximum extent in case of histidine (positively charged) withSDBS which has negatively charged head group, thus delaying mic-ellization. However, in case of DTAB, the head group is positivelycharged, hence micellization in case of histidine seems to be morefacilitated [20]. Thus, it can be suggested that the nature of aminoacid has a special bearing on the XCMC value of both the surfactants.

3.1.2. Temperature dependence of XCMC (or CMC)The effect of temperature on XCMC values of SDBS and DTAB

have been presented in figures 4–6 indicating a linear increasewith rise in temperature for SDBS, while in case of DTAB, XCMC val-ues pass through a broad minimum at around T = (298.15 to308.15) K, thus both the surfactants behave differently. In general,the effect of temperature on the XCMC value of surfactant in aque-ous medium is complex [34] and is analyzed in terms of hydropho-bic and hydrophilic hydrations. In monomeric form of surfactant,both the hydrophobic as well as hydrophilic hydrations are possi-ble whereas only hydrophilic hydration is possible for micellizedsurfactant system. Both types of hydrations are known to decreasewith increase in temperature [35]. At lower temperature, a hydro-philic dehydration favors the micelle formation while with the in-crease in temperature; hydrophobic dehydration disfavours themicelle formation [36,37]. Thus the magnitude of these two factorsdetermines whether the CMC (XCMC) values increase or decreaseover a particular temperature range.

In case of micellization of DTAB, the gradual decrease of XCMC

values at lower temperature and gradual increase of XCMC valuesat higher temperature may be due to the dominating effect of firstand second factors, respectively. However, in case of SDBS micelli-zation, the gradual increase of XCMC values with temperature maybe due to the dominance of second factor only. Therefore, as thetemperature increases, the effect of hydrophobic groups begins

290 295 300 305 310 3152.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.0

3.1

3.2

105 ·

X cmc

T / K

(a)

290 295 300 305 310 315

2.7

2.8

2.9

3.0

104 ·

X cmc

T / K

(b)

FIGURE 4. Plots of XCMC vs. temperature in aqueous solution for (a) SDBS and (b)DTAB containing 0.01 mol � kg�1: j, 0.05 mol � kg�1: N, 0.10 mol � kg�1: d, concen-trations of glutamine.

290 295 300 305 310 315

2.4

2.6

2.8

3.0

3.2

3.4

105 ·

X cmc

T / K

(a)

290 295 300 305 310 3152.5

2.6

2.7

2.8

2.9

104 ·

X cmc

T / K

(b)

FIGURE 5. Plots of XCMC vs. temperature in aqueous solution for (a) SDBS and(b) DTAB containing 0.01 mol � kg�1: j, 0.05 mol � kg�1: N, 0.10 mol � kg�1: d,concentrations of histidine.

208 S. Chauhan, K. Sharma / J. Chem. Thermodynamics 71 (2014) 205–211

to exert its influence and finally predominates as the XCMC reachesa minimum value and finally increases with temperature. The exis-tence of a minimum CMC in the XCMC temperature curve is thus anoutcome of these two opposing effects. In most ionic and severalnon-ionic surfactants, minimum in CMC–temperature profile hasbeen the usual trend [38,39] and the factors affected by the changeof temperature like surfactant solubility, de-solvation, changed sol-vent structure, etc. play important role in this respect [40].

From another point of view, it has been observed that with in-crease in temperature, the thermal motions of surfactant and sol-vent molecules enhance so that the formation of ordered micellestructures becomes difficult, i.e., the thermal motions may be moreimportant than the breakage of water structure at high tempera-tures. The increase of temperature further makes the kinetic ener-gies enhance and the ordered micellar structures destroy, causingdecrease in the micelle aggregation number but the XCMC value in-creases. Therefore, higher the temperature, greater is the disaggre-gation degree of micelle, consequently higher is the XCMC.

3.2. Thermodynamics of micelle formation

The temperature dependence of XCMC can be employed to com-pute the thermodynamic parameters of micellization for amphi-philes in aqueous solution. For ionic surfactants, the standardenthalpy of micellization DHo

m is given by the equation [41,42];

DHom ¼ �RT2ð2� aÞ½dðln XCMCÞ=dT�; ð1Þ

where, dðln XCMC=dTÞ was determined as the slope of the straightline obtained by plotting ln XCMC against T and subjecting the datato a least-squares treatment. Here, a is the degree of counter-iondissociation, which was calculated from equation (2) [43];

a ¼ S2=S1; ð2Þ

where, S1 and S2 are the slopes in pre- and post-micellar regionsdetermined from the conductivity plots (table 3 and table SM9).The standard Gibbs energy of micellization, DGo

m and entropy ofmicellization, (DSo

m) have been estimated from the following equa-tions [44,45];

DGom ¼ ð2� aÞRT lnðXCMCÞ; ð3Þ

DSom ¼ ðDHo

m � DGomÞ=T: ð4Þ

The values of DHom, DGo

m and DSom for aqueous solution of SDBS

and DTAB have been summarized in table 3. On investigating thedata, we found that DHo

m values for aqueous solution of SDBS andDTAB are negative over the entire temperature range studied. Thisobservation appears to suggest that the micellization of thesesurfactants, which is found to be spontaneous process, over the en-tire temperature range, is energy driven. However, since DHo

m incase of DTAB is less negative than found in SDBS, this differencein behavior can be attributed to the fact that micellization of DTABis relatively more entropy driven than SDBS. This may beinterpreted to mean that transferring of DTAB molecule to micellarregion is accompanied with a greater disruption of the solvent

290 295 300 305 310 3152.2

2.3

2.4

2.5

2.6

2.7

2.8

2.9

3.0

3.1

3.210

5 , Xcm

c

T / K

(a)

290 295 300 305 310 3152.6

2.7

2.8

2.9

3.0

104 ·

X cmc

T / K

(b)

FIGURE 6. Plots of XCMC vs. temperature in aqueous solution for (a) SDBS and(b) DTAB containing 0.01 mol � kg�1: j, 0.05 mol � kg�1: N, 0.10 mol � kg�1: d,concentrations of methionine.

TABLE 3Standard thermodynamic parameters of micellization (DGo

m , DHom and DSo

m) foraqueous solution of SDBS and DTAB at different temperatures.

T/K DGom/kJ �mol�1 DHo

m/kJ �mol�1 DGom/J � K�1 �mol�1 a

SDBS293.15 �30.71 �9.33 73.0 0.812298.15 �30.98 �9.62 71.7 0.815303.15 �31.24 �9.92 70.4 0.818308.15 �31.53 �10.23 69.2 0.821313.15 �31.78 �10.54 67.9 0.824

DTAB293.15 �34.76 �8.79 88.6 0.241298.15 �35.25 �9.08 87.8 0.244303.15 �35.73 �9.38 87.0 0.245308.15 �36.25 �9.68 86.3 0.246313.15 �36.71 �9.99 85.4 0.248

The uncertainty in the temperature and thermodynamic measurements for SDBSand DTAB are: 0.01 K in temperature, ±0.03 kJ �mol�1 in DHo

m , ±0.02 kJ �mol�1 inDGo

m and ±2 J � K�1 �mol�1 in DSom , respectively. The uncertainty w.r.t. a in case of

SDBS and DTAB are ±0.02.

290 295 300 305 310 315

-10.6

-10.4

-10.2

-10.0

-9.8

-9.6

-9.4

-9.2

-9.0

-8.8

-8.6

ΔHo m

/ kJ

· m

ol−1

(a)

T / K

290 295 300 305 310 31565

70

75

80

85

90

ΔSo m

/ kJ

· mol

− 1

(b)

T / K

FIGURE 7. Plot of (a) DHom and (b) DSo

m vs. temperature for aqueous SDBS: j, DTAB:N.

S. Chauhan, K. Sharma / J. Chem. Thermodynamics 71 (2014) 205–211 209

structure, explaining DSom > 0. As depicted in figure 7, we find that

DHom and DSo

m both decrease with rise in temperature indicatingthat micellization tend to be energy driven at higher temperature,and thus compensate the contribution due to enthalpy and entropymaking DGo

m < 0 practically independent of temperature.Further, the XCMC data reported in table SM8 in respect of DTAB

were subjected to the treatment of a second degree polynomial[46] in the form as described by equation (5) as

ln XCMC ¼ aþ bT þ cT2; ð5Þ

where, the coefficients a, b and c are determined by a least-squaresregression analysis. The DHo

m values were then calculated bysubstituting equation (5) into equation (1). However, DGo

m has beencalculated as mentioned below;

DHom ¼ �RT2ð2� aÞðbþ 2cTÞ; ð6Þ

DGom ¼ RTð2� aÞðaþ bT þ cT2Þ: ð7Þ

The thermodynamic parameters of micellization of SDBS andDTAB determined using the above formulations have been summa-rized in tables SM10–SM12, and their temperature dependencebehavior in case of glutamine has been presented in representativefigure 8. The most interesting trend that we observe in the data re-ported in tables SM10–SM12, is that in case of DTAB, DHo

m is posi-tive up to T = 298.15 K, and becomes negative over and above303.15 K in all compositions of amino acids irrespective of the nat-ure and the concentration of amino acid. This sequence is similar tothat observed by Chen et al. [47].

For different amino acid-containing solutions, the standardenthalpy and entropy of micellization decrease with increase intemperature. This behavior can be justified as follows: at lowtemperatures, the reduction of the hydrophobic hydration isresponsible for the observed increase in the value of DSo

m. However,with rise in temperature, the structure and size of water moleculeaggregates decreases, consequently, DHo

m becomes more exother-mic, and this effect becomes predominant [48]. Positive, DHo

m

290 295 300 305 310 315-14.0

-13.6

-13.2

-12.8

-12.4

-12.0

-11.6

-11.2

-10.8

-10.4

-10.0

T / K

ΔHo m

/ kJ

· m

ol− 1

)

(a)

290 295 300 305 310 315

56

58

60

62

64

66

68

70

72

74

ΔSo m

/ J ·

K−1 ·

mol

−1

T / K

(b)

FIGURE 8. Sample plot for (a) DHom and (b) DSo

m of SDBS vs. temperature in aqueoussolution of 0.01 mol � kg�1: j, 0.05 mol � kg�1: N, 0.10 mol � kg�1: d glutamine.

210 S. Chauhan, K. Sharma / J. Chem. Thermodynamics 71 (2014) 205–211

values can demonstrate the importance of hydrophobic interac-tions, whereas negative DHo

m values can taken as evidence thatLondon – dispersion interactions represent the major attractiveforce for micellization [30]. With increase in temperature, theenthalpic contribution to the Gibbs energy increases, meaningthereby, the hydrogen bond between water molecules startdiminishing and therefore less energy is required to break up thewater cluster. Thus, DHo

m becomes more significant at highertemperatures [49].

The entropy change in all cases is positive which confirms thataggregation of surfactant is favored entropically. Since micelle for-mation is a structure formation from monomeric surfactant mole-cules; hence, the entropy change is expected to be negative.However, its positive value indicates the melting of iceberg clus-ters around the hydrocarbon tails of the surfactant monomer andthe increased randomness of the hydrocarbon chains in the micel-lar core [50]. The values of DSo

m are decreasing with increasing tem-perature as seen from tables which may be that self – aggregationbecomes poorer at higher temperatures because of enhancedmolecular motion at higher temperature [51].

The DGom value is the sum of the enthalpic (DHo

m) and entropic(�TDSo

m) contributions. The result in tables SM10–SM12 show thatnegative values of DGo

m are mainly due to the large positive value ofDSo

m especially at low temperatures; become more negative athigher temperatures, indicating a larger driving force formicellization [52]. Different arrangements of solvent moleculesare expected to differ in enthalpy and entropy in a mutually com-pensating manner, so that DGo

m value is not significantly affected.

For amphoteric and ionic surfactants, DGom has been reported to

be in between �(23 and 42) kJ �mol�1 at T = 298.15 K [48].We now turn to DHo

m and DSom values of SDBS reported in tables

SM10–SM12. Interestingly, the magnitude of both these parame-ters is found to be practically independent of temperature as wellas the amino acid concentration, presumably suggesting that thehydrophobic interaction is augmented in the presence of aminoacid. Therefore, we can treat the micellization of SDBS in aqueoussolutions of amino acid as a normal micellization process. Theeffective compensation obtained between DHo

m and DSom values of

SDBS are reflected in the DGom < 0 values which remain constant

over the entire concentration and temperature range studied. Thismeans that the London–dispersion interactions as an alternativeforce contribution for SDBS micellization [53].

3.3. Enthalpy–entropy compensation for SDBS and DTAB micellization

According to the viewpoints of Lumry and Rajender [54] for thecompensation phenomenon, the micellization can be described asconsisting of two-part process: (a) the ‘de-solvation’ part, i.e., thedehydration of the hydrocarbon tail of surfactant molecules, and(b) the ‘chemical’ part, i.e., aggregation of the hydrocarbon tailsof surfactant molecules in the formation of micelle. In general,the compensation phenomenon between DHo

m and DSom in the var-

ious processes can be described as follows;

DHom ¼ DH�m þ TcDSo

m; ð8Þ

where Tc in DHom vs. DSo

m curve, known as compensation tempera-ture, can be interpreted as a characteristic of solute–solvent interac-tions, i.e., proposed as a measure of the ‘de-solvation’ part of theprocess of micellization. The intercept DH�m characterizes the sol-ute–solute interaction, i.e., considered as an index of the ‘chemical’part of the process of micellization. Tc value generally lies in therange T = (270 to 300) K has been used as a diagnostic test for theparticipation of water in the solution [55,56]. Note that the DHo

m

stands for the enthalpy effect under the condition DSom = 0. The in-

crease in the DHom thus corresponds to a decrease in the stability

of the structure of micelles. In the present study, we found that inall cases there exist a good correlation between DHo

m and DSom values

of SDBS and DTAB with the correlation coefficient lying near 0.999and Tc of magnitude in the range �(290 to 300) K. Similar enthalpy–entropy compensation have been observed in case of SDS in aque-ous solutions of various amino acids [20].

4. Conclusions

On examining the results, we found that XCMC values of bothSDBS and DTAB decrease with increase in concentration of aminoacid in the solution. This decrease can be interpreted in terms ofinteractions of the amino acid with surfactant molecules. On onehand, it may be proposed that increase in amino acid concentrationmay cause partial destruction of the hydration shell around thealkyl chain of the surfactant monomer, and on the other hand,addition of amino acid molecules may result in decreased thick-ness of the solvation layer around the ionic heads of surfactant.So, the hydrophilicity of the surfactants is decreased, that is, itssurface activity is enhanced, with the result, molecules aggregateeasily on the surface and in the solution; consequently, the valueof CMC decreases. Further, the result indicates the presence of bothelectrostatic and hydrophobic interactions at lower surfactant con-centration and temperature but the contribution of hydrophobicinteraction becomes dominant at higher temperature.

S. Chauhan, K. Sharma / J. Chem. Thermodynamics 71 (2014) 205–211 211

Acknowledgements

Kundan Sharma thanks, UGC, New Delhi for the award of BasicScientific Research fellowship (No. F. 4-1/2006 (BSR)/7-75/2007(BSR)).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jct.2013.12.019.

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JCT 13-566