Journal of Molecular Liquids 187 (2013) 106–113

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Journal of Molecular Liquids

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Solubilization of polycyclic aromatic hydrocarbons bygemini–conventional mixed surfactant systems

Manorama Panda, Kabir-ud-Din ⁎Department of Chemistry, Aligarh Muslim University, Aligarh 202002 India

⁎ Corresponding author. Tel.: +91 571 2703515.E-mail address: kabir7@rediffmail.com (Kabir-ud-Di

0167-7322/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.molliq.2013.06.008

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 May 2013Received in revised form 15 June 2013Accepted 18 June 2013Available online 1 July 2013

Keywords:Gemini surfactantSynergistic interactionMixed micelleSolubilization

The effects ofmixed gemini–conventional surfactants on the aqueous solubility of polycyclic aromatic hydrocarbons(PAHs—naphthalene, anthracene, pyrene)were investigated. The criticalmicelle concentration (cmc) of the cationicgemini pentanediyl-1,5-bis(dimethylcetylammonium bromide) (G5) and its 1:1 mixtures with conventionalsurfactants cetyltrimethylammonium bromide (CTAB), sodium bis(2-ethylhexyl) sulfosuccinate (AOT) andpolyoxyethylene (20) cetyl ether (Brij 58) were evaluated using the surface tensionmeasurements. The maximumsurface excess concentration at the air/water interface (Γmax), minimum area per head group (Amin), interactionparameters (βm, βσ) as well as other thermodynamic and micellar parameters were evaluated. Above cmc, foreach of the single or binary system, the solubility of PAH in water is greatly enhanced in a linear fashion; theenhancement being more in the mixed surfactants. For the binary surfactant systems synergism in G5–CTAB,G5–Brij 58 and antagonism in G5–AOT are observed. The gemini–conventional mixed surfactant systems cansolubilize PAHs synergistically due to the increase in molar solubilization ratio (MSR) and partition coefficient(Km) of the solute in the micellar phase.Results from this study could be exploited for the use of surfactant mixtures for environmental remediationas the mixed surfactants are anticipated to improve the performance of surfactant-enhanced remediation ofsoils and sediments contaminated by hydrophobic organic compounds by decreasing the applied surfactantlevel for the needed solubility of the PAHs in aqueous solutions.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Geminis, the double-chained surfactants composed of twohydrophilichead groups, two long hydrophobic tails and a spacer [1–6], are widelyused in various industrial and commercial applications. Cationic geminisare the most studied dimeric surfactants because of their easier methodsof synthesis [7–10]. The main advantages of geminis as compared tothe corresponding conventional surfactants are due to their unusualphysicochemical properties like higher surface activity, lower cmc,better solubilizing power, low Krafft point, and better viscoelasticproperties [4,5,10–12]. Most of their fundamental properties are stronglyaffected by the number of ethylene groups in the spacer [3,11,13,14].Thermodynamic parameters for micellization of geminis with varyingspacer lengths have been compared with conventional surfactants byseveral researchers [8,14,15].

Surfactant-based remediation technologies for organic contaminatedsites are of increasing importance as the surfactants reduce the treatmenttime of a site compared to the use ofwater alone because of their capacityto solubilize slightly soluble organic compounds in aqueous solution [16].To quantify the efficiency of surfactant-enhanced remediation (SER) ofthe organic-polluted environment, several researchers have measured

n).

rights reserved.

the micelle-solubilization effects for various hydrophobic organiccompounds (HOCs). The organic pollutants can be removed from thecontaminated soils and ground water by the surfactants by solubilizingor mobilizing these in the micelles. As the solubility enhancements areclosely related to the properties of organic compounds and surfactants,the amount and type of surfactants applied to remediate a site couldinfluence the fate of other pollutants in surface water and ground water.The amount of surfactants required for remediation can be reduced byusing the mixture of two or more surfactants which can formmixed micellar aggregates that exhibit superior properties than theindividual components. Although most of the solubilization studieshave been performed in single-surfactant solutions [17–28], mixedmicelles have also been used to increase the water solubility of poorlysoluble organic compounds [29–34]. Mixed surfactants have mostlybeen used for the study of interfacial and bulk properties of surfactantsand to increase the water solubility of poorly soluble organiccompounds. Although the gemini–conventional mixed micelles canalso increase the water-solubility of organic compounds, thosehave been sparsely explored towards their scope of contaminantremediation [35–37]. For the present study, the objectives are,therefore, to compare the capabilities of the gemini surfactantpentanediyl-1,5-bis(dimethylcetylammonium bromide) (G5) in itssingle as well as binary mixed systems (G5–conventional) withsimilar systems using G4 (butanediyl-1,4-bis(dimethylcetylammonium

Table 1Structures and physicochemical properties of the polycyclic aromatic hydrocarbonsand surfactants used in this study (the literature cmc values are given in theparentheses).

Structure Mwa Solubility (M)b log KOWc MV (Å3)d

(Naphthalene, NAP)

128.2 2.44 × 10−4 3.36 126.9

(Anthracene, ANT)

178.2 2.53 × 10−7 4.54 157.6

(Pyrene, PYR)

202.3 6.57 × 10−7 5.18 161.9

cmc (mM)CH3–(CH2)15–N+–(CH3)3Br−

(CTAB)0.776 (0.815)e

CH3–(CH2)15–(OCH2CH2)20–OH(Brij 58)

0.004 (0.0039)f

(AOT)

0.638 (0.640)g

C16H33 (CH3)2 N+–(CH2)4–+N(CH3)2C16H33·2Br−

(G4)0.006h

C16H33 (CH3)2 N+–(CH2)5–+N(CH3)2C16H33·2Br−

(G5)0.003

C16H33 (CH3)2 N+–(CH2)6–+N(CH3)2C16H33·2Br−

(G6)0.001i

a Molecular weight.b Aqueous solubility.c Octanol–water partition coefficient.d Molecular volume.e Ref. [45].f Ref. [46].g Ref. [35].h Ref. [36].i Ref. [37].

107M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 187 (2013) 106–113

bromide)) [36] and G6 (hexanediyl-1,6-bis(dimethylcetylammoniumbromide)) [37] in enhancing the water-solubility of PAHs, and to findout the cause for the enhancement in solubilization, if any. In additionto finding the effect of the spacer of gemini surfactants, the experimentsare aimed to ascertain whether a mixed surfactant solution with a lesseramount of surfactant can be used in the SER of organic contaminants. Assolubilization of HOCs commences after micellization, the use of geminisurfactants (singly or in combination) is important from a greenchemistry view point because of their quite low cmc values. Of course,this demands the use of lesser amounts for application purposes.

2. Experimental section

2.1. Chemicals

The surfactant AOT (98.5%) was procured from s. d. fine, India. CTAB(99%) and Brij 58 (Mw = 1124) were from Merck, Germany. N,N-Dimethylhexadecylamine (≥95%) and 1,5-dibromopentane(≥98%)were fromFluka anddry ethanol (≥99.9%)was fromMerck. Naphthalene(NAP, 99.7%) and pyrene (PYR, 99%) were obtained from Fluka,Switzerland. Anthracene (ANT, 99.5%) was from Koch-Light LaboratoriesLtd., England. All the chemicals were used as received. Freshly prepareddistilled water was used throughout the study. The structure andproperties of the PAHs and surfactants are recorded in Table 1.

2.2. Synthesis of gemini surfactant

The dicationic gemini pentanediyl-1,5-bis (dimethylcetylammoniumbromide) (G5) was synthesized in the laboratory by refluxing 1,5-dibromopentane with N,N-dimethylcetylamine (molar ratio 1:2.1) indry ethanol with continuous stirring at 80 °C for 48 h.

Br CH2ð Þ5Brþ 2CH3− CH2ð Þ15

−N CH3ð Þ2→48; 80 �C

C16H33 CH3ð Þ2Nþ− CH2ð Þ5−þN CH3ð Þ2C16H33⋅2Br−

Reflux; dry ethanol:

Progress of the reaction was monitored by the TLC technique.After the completion of the reaction the solvent was removed undervacuum from the reaction mixture and the obtained solid wascrystallized several times from hexane/ethyl acetate mixture toobtain the compounds in pure form. After crystallization, the surfactantwas characterized by 1H NMR. All of the values obtained were inagreement with the literature values [38]. The purity of the geminisurfactant was further ensured by the absence of minimum in surfacetension versus log [surfactant] plot [39].

2.3. Determination of cmc by surface tension measurements

The critical micelle concentration (cmc) values were determinedby plotting the surface tension versus the surfactant concentrationsby using a Kruss tensiometer (Model K11, Germany) at a temperatureof 30 °C. The tensiometric experiments were performed by the ringdetachment method. The surfactant concentration was varied byadding concentrated surfactant solution in small installments, andthe readings were noted after thorough mixing and temperatureequilibration.

2.4. Solubilization experiments

The experiments for the aqueous solubility of PAHs wereperformed in screw-capped vials at different concentrations of surfac-tants (above cmc) in the presence of excess quantities of PAH crystals.The mixture was stirred using a magnetic stirring bar for 24 h. An al-iquot of the sample was collected and then centrifuged for 15 min at

12,000 rpm to separate the undissolved portions of PAHs. Theconcentration of PAH in the solution was measured by a Shimadzuspectrophotometer (Model UV mini-1240) after the dilution ofan aliquot of the supernatant with the corresponding surfactantsolution. The surfactant concentration was kept the same in boththe reference and the measurement cells to eliminate its effect onthe UV-absorbance.

3. Results and discussion

Since the solubilization of hydrophobic organic compoundsdepends primarily on a system's micellar properties, we have, firstof all, made studies on the concerned systems from this view point.The results are detailed below.

3.1. Surface and micellar properties

3.1.1. cmcThe cmc values of the single and binary surfactant systems, as

given in Tables 1 and 2, were obtained by noting inflections in thesurface tension (γ) versus logarithm of surfactant concentrationisotherms (Fig. 1). Surfactants can reduce the surface and interfacialtensions by accumulating at the liquid/air interface, and the surfacetension becomes constant at the cmc. As expected, the cmc of thepure nonionic surfactant Brij 58 is lower than that of the ionic

Table 2Experimental cmc (cmc12), ideal cmc (cmcideal), micellar composition (X1

m), interaction parameter (βm) and activity coefficient (f1m, f2m) values of binary surfactant mixtures (1:1) at30 °C.

Systems cmc12(mM)

cmcideal(mM)

X1m Xideal βm f1

m f2m ΔGex

(kJ mol−1)

G4–CTABa 0.0044 0.0123 0.7186 0.9921 −8.91 0.494 0.010 −4.5G4–Brij 58a 0.0025 0.0049 0.4585 0.3922 −3.24 0.387 0.506 −2.0G4–AOTa 0.0101 0.0123 – – – – – –

G5–CTAB 0.0034 0.0058 0.7945 0.9963 −7.20 0.738 0.011 −2.9G5–Brij 58 0.0012 0.0033 0.5262 0.5797 −4.16 0.393 0.316 −2.6G5–AOT 0.0093 0.0058 – – – – – –

G6–CTABb 0.0015 0.0020 0.859 0.999 −6.76 0.874 0.007 −2.1G6–Brij 58b 0.0014 0.0016 0.737 0.800 −0.75 0.950 0.665 −0.37G6–AOTb 0.0050 0.0020 – – – – – –

a Ref. [36]b Ref. [37]

108 M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 187 (2013) 106–113

surfactants CTAB or AOT. The gemini surfactants have remarkably lowcmc values as compared to the conventional surfactants becausethese possess two polar head groups and two hydrophobic chainswhich transfer at the same time from the aqueous phase to the micel-lar phase.

3.1.2. Interfacial properties of the single and mixed micellar systemsThe surfactant concentration is always more at the air/water

interface due to adsorption as compared to its concentration in thebulk. The maximum surface excess concentration at the interface,Γmax, was evaluated by the Gibbs adsorption Eq. (1).

Γmax ¼ −1=2:303nRTð Þ dγ=d logCð ÞT;P : ð1Þ

Here, R is the universal gas constant, T is the absolute temperature,and n is introduced to allow for the simultaneous adsorption ofcations and anions. The value of n was calculated using the equationn = 1 + C / (C + Cs), where Cs is the concentration of the additive.The values of Γmax (and also of Amin), given in Table 3, are basedupon n = 4 or 5 for surfactant mixtures (4 for gemini + Brij 58 and5 for gemini + CTAB/AOT). For single surfactants, the values of n

30

35

40

45

50

55

60

65

70

γ /

mN

m-1

Log C

G5

G5CTAB

G5Brij58

G5AOT

-4.0 -3.6 -3.2 -2.8 -2.4 -2.0 -1.6

Fig. 1. Variation of surface tension (γ) with the concentration for gemini surfactant G5and its mixtures.

are 1, 2 and 3 for Brij 58, CTAB/AOT and gemini, respectively. Theminimum area per head group, Amin, was evaluated using Eq. (2)

Amin ¼ 1� 1016= NAΓmaxð Þ ð2Þ

where NA is Avogadro's number.The Γmax values of the pure surfactants are in the order:

G6 N CTAB N Brij 58 N G4 N AOT N G5 and for binary systems theorder is G6–CTAB N G6–AOT N G6–Brij 58 N G5–CTAB N G4–Brij58 N G4–CTAB N G5–AOT N G5–Brij 58 N G4–AOT. The difference inΓmax values for the surfactant systems is due to the intermolecularhead groupdistance. Considering the nature of surfactants (i.e., cationic,anionic and nonionic), Γmax values for gemini–CTAB were expected tobe the lowest while for gemini–AOT the highest. However, the obtainedtrend is different. Although both G6 and CTAB are cationic in nature,their mixed interface is most highly populated as both contain flexiblehydrophobic chains of equal length and it is easy for them to get accom-modated in a small space. Hence, the interface is highly populated.There should be an attraction between the oppositely charged headgroups of gemini and AOT, and their interface should be most tightlypacked. AOT is a less flexible molecule with a short hydrophilic and abulky hydrophobic part. Therefore, it is difficult for AOT to adjust itselfat the interface and the G4–AOT system has the least Γmax valueamongst all the binary systems. G5–AOT and G6–AOT also have lowerΓmax values as compared to the mixtures of the gemini with CTAB.Gemini–CTAB is a better mixed system than gemini–AOT or gemini–Brij 58. The lower value of Amin for the G6–CTAB system is due to theequal chain length of the two components. For G4–CTAB and G4–Brij58 systems the values are almost equal. The Amin values of gemini–AOT systems are in the order G4–AOT N G5–AOT N G6–AOT. This canbe explained by the formation of a loop in G6 and thus the lesser flexi-bility of both G6 and AOT. As there is attraction between the oppositelycharged head groups, the Amin value of G4–AOT is the highest. The G6surfactant molecules are more tightly packed in both single and binarysystems.

3.1.3. Surfactant–surfactant interactions in mixed micelles/mixedmonolayers

The Clint equation (Eq. (3)) [40], which differentiates the idealand nonideal behaviors of surfactants in their mixtures, was used toobtain the ideal cmc (cmcideal) values.

1=cmcideal ¼Xi

i¼1

αi=cmci: ð3Þ

Here αi is the bulk mole fraction of the ith component in a mixedsurfactant solution and cmci is its cmc in pure form. We see that theobtained cmc values (cmc12) of the gemini–Brij 58 and gemini–CTAB surfactant mixtures are lower than the cmcideal (Table 2),

109M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 187 (2013) 106–113

indicating synergistic interaction. For G5–AOT and G6–AOT systemsthe cmc12 values are higher than the cmcideal value showing antago-nistic interaction whereas synergistic interaction was noticed forthe G4–AOT mixed system. We can say that in all the three casesmixed micelles are formed between the gemini and AOT as thecmc12 value of the mixed systems are less than that of AOT.

Micellar mole fraction of the gemini surfactant can be evaluatedwith the help of Rubingh's equation (Eq. (4)) [41].

Xm1

� �2 ln cmc12α1=cmc1Xm1

� �1−Xm

1

� �2 ln cmc12 1−α1ð Þ=cmc2 1−Xm1

� �� � ¼ 1: ð4Þ

Here cmc12 denotes the experimental cmc value of the binarymixture, X1

m being the micellar mole fraction of surfactant 1 (gemini)in the mixed micelle.

Analogous to Rubingh's equation, Rosen [39] gave an equation forthe mole fraction of a surfactant at the interface (X1

σ) in whichconcentrations of individual components and binary mixtures(conc1, conc2, conc12) to produce a particular surface tension areused (instead of cmc values). These equations were non-convergentfor the mixed gemini–anionic AOT. As can be seen from Tables 2and 3, both the mixed micelles and mixed interface contain moregemini than the conventional one. Except for the G4–Brij 58 system,for all the other binary mixtures, the values of X2 and f2 are lowerthan the X1 and f1 values whichmay be due to less partitioning of con-ventional surfactants into gemini micelles.

The attractive interaction between two surfactants in the mixedmicelle/mixed interface is accompanied by the decrease of energy,which is measured in terms of the interaction parameter βm/βσ:

βm ¼ ln cmc12α1=cmc1Xm1

� �1−Xm

1

� �2 ð5Þ

and

βσ ¼ ln conc12α1=conc1Xσ1

� �1−Xσ

1

� �2 : ð6Þ

βm/βσ is an indicator of the degree of interaction between the twosurfactants in mixed micelles/mixed interface relative to theself-interaction of the two surfactants under similar conditions before

Table 3The maximum surface excess concentration at the air/water interface (Γmax), minimumarea per head group (Amin), surface composition (X1

σ), interaction parameter (βσ), andactivity coefficient (f1σ, f2σ) values at 30 °C.a

Systems Γmax × 1011

(mol cm−2)Amin

(Å2)X1σ βσ f1

σ f2σ

CTABb 23.23 70.86 – – – –

Brij 58b 17.64 94.12 – – – –

AOTb 9.80 169.51 – – – –

G4 15.03 110.47 – – – –

G4–CTAB 7.27 228.50 0.708 −9.350 0.451 0.009G4–Brij 58 7.79 213.13 0.495 −3.140 0.449 0.463G4–AOT 3.50 474.37 – – – –

G5 8.97 185.10 – – – –

G5–CTAB 8.71 190.62 0.845 −6.206 0.861 0.012G5–Brij 58 4.85 341.33 0.592 −4.166 0.499 0.233G5–AOT 5.08 326.70 – – – –

G6b 48.39 34.31 – – – –

G6–CTABb 21.80 76.17 0.914 −4.912 0.964 0.017G6–Brij 58b 9.91 167.58 0.747 −1.016 0.937 0.567G6–AOTb 11.02 150.64 – – – –

a The values of X1σ, βσ, f1σ, f2σ were calculated using Rosen's equation.

b Ref. [37]

mixing, and accounts for deviation from ideality. The larger thenegative value of βm/βσ, the stronger is the attractive interactionbetween the two surfactant molecules. The activity coefficients, f1m,f2m of the two surfactants within the mixed micelle are related to

the interaction parameter through Eqs. (7a) and (7b). The activitycoefficients measure the ideality of the mixed systems.

f m1 ¼ exp βm 1−Xm1

� �2n oð7aÞ

f m2 ¼ exp βmXm2

1

� �: ð7bÞ

Similarly, f1σ and f2σ are related to βσ through Eqs. (8a) and (8b)

f σ1 ¼ exp βσ 1−Xσ1

� �2n oð8aÞ

f σ2 ¼ exp βσXσ2

1

� �: ð8bÞ

The values of the interaction parameter between the surfactantmolecules in the mixed micelles evaluated using Rubingh's equation(Eq. (4)), are presented in Table 2. The values of βm/βσ for gemini–Brij 58 and gemini–CTAB mixtures are negative (Tables 2 and 3)indicating the presence of synergistic interactions between the twosurfactants, which means that the attraction between the twocomponents after mixing is more than before mixing. Synergism insurfactant mixtures depends not only on the strength of interactionsbut also on the relevant properties of the surfactants. The synergismin mixed gemini–CTAB systems is more than that of gemini–Brij 58systems. Brij 58, with polyoxyethylene (POE) groups, has a largenumber of oxygen atoms with a lone pair of electrons. Thus, it shouldhave a tendency to react Coulombically with the cationic geminisurfactant; but the presence of a long POE head group imposessome steric constraint due to thermal vibrations, restricting theeffective head group interactions, hence showing a lower value ofthe interaction parameter. This explanation supports the lowestvalue of Γmax, the highest value of Amin for Brij 58 among the singlesurfactant systems, and the comparatively higher Amin values forgemini–Brij 58 systems than the other binary systems. Further, thevalues of βm and X1

mfind support from the low values of activity

coefficients of conventional surfactants (f2m) (except for gemini–Brij58), which may be due to less partitioning of conventional surfactantsin the gemini micelles. The values of the activity coefficients obtainedfrom Eqs. (7a), (7b), (8a) and (8b) are less than the unity indicatingnonideal behavior.

The values of excess free energy of micellization, ΔGex =[X1 · lnf1 + (1 − X1) · lnf2] RT are negative (Table 2) indicating thehigher stability of the mixed micelles than the single surfactantmicelles. The stability of the geminis as well as the mixed micellesformed by the geminis are in the order: G4 N G5 N G6 (as can beseen from their ΔGex values).

3.2. Micellar solubilization

3.2.1. Solubilization behavior of different surfactant/surfactant mixturesPAH solubilizations plotted as a function of surfactant concentration

are presented in Figs. 2–4. The aqueous solubility of PAHs increasedlinearly in the concentration range above the cmc both in the singleand mixed surfactant systems showing the potential of the surfactantsto enhance the solubilization. This increased solubilization is due tothe incorporation or partitioning of the organic solutes within themicelles. A useful way to evaluate the effectiveness of a surfactant insolubilizing a given solubilizate is molar solubilization ratio (MSR)which is defined as the number of moles of organic compound solubi-lized per mole of surfactant added to the solution [42]. It is obtained

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

[Nap

htha

lene

](m

M)

[Surfactant](mM)

G4 G5 G6 CTAB Brij58 AOT

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

[Nap

htha

lene

](m

M)

[Surfactant](mM)

G4CTABG4Brij58G4AOT

0.0

0.1

0.2

0.3

0.4

0.5

0.6

[Nap

htha

lene

](m

M)

[Surfactant](mM)

G5CTAB G5Brij58 G5AOT

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

[Nap

htha

lene

](m

M)

[Surfactant](mM)

G6CTAB G6Brij58 G6AOT

0.0 0.4 0.8 1.2 1.6 2.0 2.4 2.8 0.0 0.5 1.0 1.5 2.0

0.0 0.5 1.0 1.5 2.0 0.4 0.8 1.2 1.6 2.00.0

Fig. 2. Variation of the solubility of naphthalene in single and binary surfactant solutions.

110 M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 187 (2013) 106–113

from the slope that results when solubilizate concentration is plottedagainst the surfactant concentration.

MSR ¼ St−Scmcð Þ= Ct−Ccmcð Þ: ð9Þ

Here St is the total apparent solubility of PAH in single/mixedsurfactant solutions at a particular total surfactant concentration Ct. Scmc

is the apparent solubility of PAH at cmc, which is taken as their watersolubility S, as it changes slightly up to the cmc of the surfactant. Analternate way of describing the solubilization is in terms of the use ofmicelle–water partition coefficient (Km), given by Km = Xm / Xa (ratioof mole fraction of the organic compound in the micellar phase, Xm, tothat in the aqueous phase Xa). The value of Xm in terms of MSR can bewritten as Xm = MSR / (1 + MSR) and Xa = ScmcVm, Vm being thevolume of water (=0.01807 L mol−1 at 30 °C). With these expressions,Km comes out to be

Km ¼ MSR= ScmcVm 1þMSRð Þf g: ð10Þ

The partition coefficient (Km) of PAH between micelles and waterhas been determined in both the single and mixed-surfactant (1:1)solutions (Table 4). The inner nonpolar core of the micelles is respon-sible for the solubilization and Km should be approximately propor-tional to the nonpolar content of the surfactant. The difference inthe solubilizing ability among the surfactants could be attributed totheir molecular structure. Brij 58 facilitates solubilization due to theweak interaction of oxygen atoms of POEs with π-electrons of arenes.The lower values of MSR and Km in the case of cationic surfactants aredue to limited solubilization at the micelle–water interface and

micellar core. For NAP, the cationic geminis G5 and G6 show a highersolubilization power than the corresponding conventional CTAB ofthe same chain length, whereas CTAB solubilizes more ANT or PYRas compared to these two geminis. G4 has lesser MSR and Km valuesthan CTAB for NAP, whereas for ANT and PRY, G6 has a lower MSRvalue. AOT presents the least MSR and Km values due to the repulsiveinteraction between the π-electrons of the solutes and the negativecharge of the surfactant, in addition to the smaller micellar sizewhich causes difficulty in packing within the micelle (due to thedouble tails in AOT, see Table 1). The order of solubilizing power fororganic solutes by the inner nonpolar core of micelles has beenreported to be nonionic N cationic N anionic surfactants having thesame nonpolar chain length [43], which is similar to our findings. Inthe gemini G6, solubilization is the highest for NAP and the lowestfor ANT and PYR. The order of the MSR and Km values of NAP in thesurfactant mixtures containing G4 is G4–Brij 58 N G4–CTAB N G4–AOT.In G5–AOT and G6–AOT systems, the solubilization is the lowest forNAP and the highest for ANT or PYR; this can be due to the bigger spaceavailable in the mixed micelles for solubilization. The greater MSR andKm values for the binary systems than the single surfactant systemsindicate synergism in mixed surfactant systems for PAH solubilityenhancement. This may be due to the larger effective solubilization areain mixed micelles [29]. The enhancement in PAH solubility is attributedto the formation of mixed micelles, lower cmc of the mixed surfactantsystems, increase of solutes' molar solubilization ratio and increase inmicellar partition coefficient, which is due to lower polarity of mixedmicelles. These are the results of the interaction between the componentsof the mixed surfactants, attractive or repulsive. The micelle–waterpartition coefficient and the cmc are two important factors influencingthe solubilization of the solute in the mixed surfactants. In mixed

0.000

0.004

0.008

0.012

0.016

0.020

[Ant

hrac

ene]

(mM

)

[Surfactant](mM)

G4 G5 G6 CTAB Brij58 AOT

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

[Ant

hrac

ene]

(mM

)

[Surfactant](mM)

G4CTAB G4Brij58 G4AOT

0.000

0.004

0.008

0.012

0.016

0.020

[Ant

hrac

ene]

(mM

)

[Surfactant](mM)

G5CTAB G5Brij58 G5AOT

0.000

0.004

0.008

0.012

0.016

0.020

[Ant

hrac

ene]

(mM

)

[Surfactant](mM)

G6CTAB G6Brij58 G6AOT

0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.4 0.8 1.2 1.6

0.0 0.5 1.0 1.5 2.0 0.0 0.4 0.8 1.2 1.6 2.0

Fig. 3. Variation of the solubility of anthracene in single and binary surfactant solutions.

111M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 187 (2013) 106–113

surfactant systemswith amore negative value of βm, themicelle becomesdensely packed with reduced solubilization capability. Thus, enhance-ment of solubilization in gemini–AOT mixed micelles is the maximumfor ANT and PYR than the other two systems.

3.2.2. Partitioning of PAHs in micelle/aqueous phaseThe partition coefficient of the PAHs between the micelle and

aqueous phase in the mixed surfactant systems is represented bythe relationship [44]

lnKm12 ¼ Xm1 lnKm1 þ 1−Xm

1� �

lnKm2 þ BXm1 1−Xm

1� �

: ð11Þ

Here Km1, Km2 and Km12 are the micelle–water partition coefficientsof the solute in the single and mixed surfactant systems. B is anempirical parameter [31] which incorporates both the surfactant–surfactant (as in β) and surfactant–solute interactions. WhenB = 0, mixing has no effect on the partitioning of the PAH, butwhen B N 1 or B b 1, Km in the mixed surfactant system is largerthan that predicted by the ideal mixing rule. As can be seen inTable 4, B N 0 for all the binary surfactant mixtures. G5–CTAB andG6–CTAB have larger B values than their corresponding Brij 58mixtures, whereas G4–Brij 58 shows larger positive values ofB than its CTAB and AOT mixtures. Because of their non-convergent nature, for G5–AOT (or G6–AOT) B values couldnot be calculated. As there is remarkable enhancement insolubilization of PAHs in the G5–AOT and G6–AOT systems (ascan be seen from the MSR and Km values), we can say that the

surfactant–surfactant interaction is not the sole criteria forsolubilization and the stabilization of PAHs by gemini–conventionalsurfactant mixtures plays a greater role. The ideal molar solubilizationratio (MSRideal) was calculated using Eq. (12).

MSRideal ¼ ΣiMSRiai ð12Þ

whereMSRi is the experimentalMSR value of the solubilizate in the pureith surfactant solutionwhose bulkmole fraction in themixture isαi. Theexperimental molar solubilization ratio (MSRexp) values (Table 4) of thesolutes in the mixed micelle are higher than the ideal value implyinga positive mixing effect of surfactants on solubilization. A betterunderstanding of the mixing effect of the gemini–conventionalsurfactants on solubilization of PAHs is made on the basis of the devia-tion ratio, R (R = MSRexp / MSRideal). The R N 1 values imply a positivemixing effect of the surfactants on solubilization. In the case of cationic–nonionic surfactant systems, a slight positive charge developed on themixed micellar surface facilitating micelle–water interface adsorption inaddition to micellar core solubilization, resulting in the value of R greaterthan unity.

3.2.3. Thermodynamics of solubilizationThe knowledge of the thermodynamic parameters controlling

solubilization is helpful for a better understanding of the mechanisminvolved in the process. From the thermodynamic point of view,solubilization can be considered as a normal partitioning of PAHbetween the two phases, micellar and aqueous. The standard free

[Pyr

ene]

(mM

)

[Surfactant](mM)

G4 G5 G6 CTAB Brij58 AOT

0.00

0.01

0.02

0.03

0.04

0.05

[Pyr

ene]

(mM

)

[Surfactant](mM)

G4CTABG4Brij58G4AOT

[Pyr

ene]

(mM

)

[Surfactant](mM)

G5CTAB G5Brij58 G5AOT

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

[Pyr

ene]

(mM

)

[Surfactant](mM)

G6CTAB G6Brij58 G6AOT

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 0.0 0.2 0.4 0.6 0.8 1.0

0.0 0.4 0.8 1.2 1.6 2.0 0.0 0.4 0.8 1.2 1.60.00

0.01

0.02

0.03

0.04

0.05

0.06

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

Fig. 4. Variation of the solubility of pyrene in single and binary surfactant solutions.

Table 4Molar solubilization ratio (MSR), lnKm and free energy of solubilization (ΔGs

0) for selected surfactant systems at 30 °C.

System Naphthalene Anthracene Pyrene

MSR(MSRideal)

lnKm ΔGs0

(kJ mol−1)B MSR

(MSRideal)lnKm ΔGs

0

(kJ mol−1)B MSR (MSRideal) lnKm ΔGs

(kJ mol−1)B

CTABa 0.1236 10.12 −25.52 – 0.0043 6.88 −17.33 – 0.0139 8.04 −20.27 –

Brij 58a 0.2112 10.59 −26.68 – 0.0044 6.90 −17.39 – 0.0401 9.08 −22.88 –

AOTa 0.0605 9.47 −23.86 – 0.0007 5.07 −12.77 – 0.0019 6.06 −15.28 –

G4a 0.112 10.75 −27.00 – 0.013 15.38 −38.63 – 0.035 15.37 −38.60 –

G4–CTABa 0.215(0.118)

11.11 −27.91 1.04 0.023(0.0086)

14.89 −37.40 0.45 0.056(0.0244)

15.65 −39.32 2.73

G4–Brij 58a 0.219(0.162)

11.13 −27.94 2.97 0.028(0.0087)

15.53 −38.99 2.65 0.071(0.0376)

16.04 −40.29 2.57

G4–AOTa 0.149(0.086)

10.81 −27.14 3.46 0.016(0.0068)

14.61 −36.67 0.80 0.047(0.0185)

15.82 −39.73 7.55

G5 0.2694 10.78 −27.17 – 0.0059 7.19 −18.13 – 0.0224 8.51 −21.45 –

G5–CTAB 0.2932(0.1965)

10.85 −27.34 1.26 0.0076(0.0051)

7.44 −18.76 1.92 0.0256(0.0182)

8.64 −21.78 1.39

G5–Brij 58 0.3125(0.2403)

10.90 −27.46 1.05 0.0075(0.0052)

7.43 −18.73 1.51 0.0281(0.0313)

8.83 −22.01 0.20

G5–AOT 0.1777(0.1650)

10.44 −26.31 – 0.0119(0.0033)

7.89 −19.88 – 0.0323(0.0122)

8.87 −22.35 –

G6b 0.2110 10.58 −26.68 – 0.0024 6.30 −15.87 – 0.0095 7.67 −19.32 –

G6 + CTABb 0.3711(0.1673)

11.02 −27.79 4.17 0.0085(0.0034)

7.56 −19.04 9.73 0.0403(0.0117)

9.08 −22.89 11.21

G6 + Brij 58b 0.3345(0.2111)

10.95 −27.59 1.90 0.0085(0.0034)

7.56 −19.04 5.69 0.0399(0.0248)

9.07 −22.86 5.31

G6 + AOTb 0.1798(0.1358)

10.45 −26.34 – 0.0103(0.0016)

7.75 −19.52 – 0.0526(0.0057)

9.34 −23.53 –

a Ref. [36].b Ref. [37].

112 M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 187 (2013) 106–113

113M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 187 (2013) 106–113

energy of solubilization, ΔGS0, can then be represented by the

expression [23]

ΔG0S ¼ −RT lnKm: ð13Þ

The ΔGS0 values obtained using Eq. (13) are given in Table 4. For

all the systems, the ΔGS0 values come out to be negative showing

spontaneity of the solubilization process.

4. Conclusion

The aqueous solubility enhancement of polycyclic aromatichydrocarbons (PAHs) naphthalene, anthracene and pyrene by micellarsolutions of a series of gemini surfactants and their mixtures withconventional surfactants has been investigated. The physicochemicalparameters of micellization were computed. The interaction parametersbetween surfactant molecules in mixed micelles were evaluated usingRubingh's approach. The negative interaction parameter (βm) valuesindicate attractive interactions between head groups of the geminiand conventional surfactants leading to the stabilization of the mixedmicelles. The solubility of PAHs in water is greatly enhanced in a linearfashion by each system, the enhancement being more in the mixedsurfactants than the single surfactant systems. Thus the PAHs aresolubilized synergistically in mixed gemini–conventional surfactantsolutions. The higher solubilization capacity of geminis when mixedwith conventional surfactants make them cost effective. The mix-tures surfactants could thus decrease the amount of surfactantsrequired for the surfactant-enhanced remediation of soils andsediments. The synergistic behavior of mixed surfactant systemscan reduce the total amount of surfactant used in particular appli-cations resulting in a reduction of cost and environmental impact.

Acknowledgements

This work was financially supported by the Department of Scienceand Technology, Government of India (Project No. SR/WOS-A/CS-46/2007).

References

[1] C.A. Bunton, L. Robinson, J. Schaak, M.F. Stam, Journal of Organic Chemistry 36(1971) 2346–2350.

[2] F.M. Menger, C.A. Littau, Journal of the American Chemical Society 113 (1991)1451–1452.

[3] R. Zana, M. Benrraou, R. Rueff, Langmuir 7 (1991) 1072–1075.[4] M.J. Rosen, D.J. Tracy, Journal of Surfactants and Detergents 1 (1998) 547–554.[5] S.K. Hait, S.P. Moulik, Current Science 82 (2002) 1101–1111.[6] D. Shukla, V.K. Tyagi, Journal of Oleo Science 55 (2006) 381–390.[7] J. Zhao, S.D. Christian, B.M. Fung, The Journal of Physical Chemistry. B 102 (1998)

7613–7618.[8] G. Bai, J. Wang, H. Yan, Z. Li, R.K. Thomas, The Journal of Physical Chemistry. B 105

(2001) 3105–3108.

[9] R. Zana, Advances in Colloid and Interface Science 97 (2002) 205–253.[10] A.R. Tehrani-Bagha, H. Oskarsson, C.G. van Ginkel, K. Holmberg, Journal of Colloid

and Interface Science 312 (2007) 444–452.[11] S. De, V.K. Aswal, P.S. Goyal, S. Bhattacharya, Journal of Physical Chemistry 100

(1996) 11664–11671.[12] K.S. Sharma, C. Rodgers, R.M. Palepu, A.K. Rakshit, Journal of Colloid and Interface

Science 268 (2003) 482–488.[13] S. De, V.K. Aswal, P.S. Goyal, S. Bhattacharya, The Journal of Physical Chemistry. B

101 (1997) 5639–5645.[14] X. Wang, J. Wang, Y. Wang, J. Ye, H. Yan, R.K. Thomas, Journal of Colloid and

Interface Science 286 (2005) 739–746.[15] Kabir-ud-Din, M.S. Sheikh, M.A. Mir, A.A. Dar, Journal of Colloid and Interface

Science 344 (2010) 75–80.[16] S. Paria, Advances in Colloid and Interface Science 138 (2008) 24–58.[17] P. Lianos, M.L. Viriot, R. Zana, Journal of Physical Chemistry 88 (1984)

1098–1101.[18] T. Morisue, Y. Moroi, O. Shibata, Journal of Physical Chemistry 98 (1994)

12995–13000.[19] A. Tiehm, Applied and Environmental Microbiology 60 (1994) 258–263.[20] S.J. Grimberg, J. Nagel, M.D. Aitken, Environmental Science and Technology 29

(1995) 1480–1487.[21] Y. Moroi, K. Mitsunobu, T. Morisue, Y. Kadobayashi, M. Sakai, Journal of Physical

Chemistry 99 (1995) 2372–2376.[22] T. Dam, J.B.F.N. Engberts, J. Karthauser, S. Karaborni, N.M. Van Os, Colloids and

Surfaces A: Physicochemical and Engineering Aspects 118 (1996) 41–49.[23] T.-S. Choi, Y. Shimazu, H. Shirai, K. Hamada, Dyes and Pigments 45 (2000)

145–152.[24] D. Neupane, J.-W. Park, Chemosphere 41 (2000) 787–792.[25] Y.-J. An, E.R. Carraway, M.A. Schlautman, Water Research 36 (2002) 300–308.[26] C.O. Rangel-Yagui, A. Pessoa-Jr, L.C. Tavares, Journal of Pharmacy and Pharmaceutical

Sciences 8 (2005) 147–163.[27] S. Paria, P.K. Yuet, Industrial and Engineering Chemistry Research 45 (2006)

712–718.[28] O. Zheng, J-Xi Zhao, Journal of Colloid and Interface Science 300 (2006) 749–754.[29] Y. Tokuota, H. Uchiyama, M. Abe, Journal of Physical Chemistry 98 (1994)

6167–6171.[30] W. Zhou, L. Zhu, Journal of Hazardous Materials B109 (2004) 213–220.[31] W. Zhou, L. Zhu, Colloids and Surfaces A: Physicochemical and Engineering

Aspects 255 (2005) 145–152.[32] S. Yamaguchi, Journal of Colloid and Interface Science 286 (2005) 355–359.[33] A. Mohamed, A.M. Mahfoodh, Colloids and Surfaces A: Physicochemical and

Engineering Aspects 287 (2006) 44–50.[34] K.J. Rao, S. Paria, The Journal of Physical Chemistry. B 113 (2009) 474–481.[35] Kabir-ud-Din, M.S. Sheikh, P.A. Bhat, A.A. Dar, Journal of Hazardous Materials 167

(2009) 575–581.[36] M. Panda, M.S. Sheikh, Kabir-ud-Din, Zeitschrift für Physikalische Chemie 225

(2011) 427–439.[37] M. Panda, Kabir-ud-Din, Journal of Molecular Liquids 163 (2011) 93–98.[38] Kabir-ud-Din, W. Fatma, Z.A. Khan, A.A. Dar, The Journal of Physical Chemistry. B

111 (2007) 8860–8867.[39] M.J. Rosen, Surfactants and Interfacial Phenomena, Wiley-Interscience, New Jersey,

2004.[40] J.H. Clint, Journal of the Chemical Society, Faraday Transactions 71 (1975)

1327–1334.[41] D.N. Rubingh, in: K.L. Mittal (Ed.), Solution Chemistry of Surfactants, vol. 1,

Plenum Press, New York, 1979, p. 337.[42] D.A. Edwards, R.G. Luthy, Z. Liu, Environmental Science and Technology 25

(1991) 127–133.[43] S. Saito, Journal of Colloid and Interface Science 24 (1967) 227–234.[44] C. Treiner, Chemical Society Reviews 23 (1994) 349–356.[45] T. Chakraborty, S. Ghosh, S.P. Moulik, The Journal of Physical Chemistry. B 109

(2005) 14813–14823.[46] M.S. Kalekar, S.S. Bhagwat, Journal of Dispersion Science and Technology 27

(2006) 1027–1034.