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Study of surface and solution properties of gemini-conventional surfactant mixtures and their effects on solubilization of polycyclic aromatic hydrocarbons
Journal of Molecular Liquids 163 (2011) 93–98
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Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q
Study of surface and solution properties of gemini-conventional surfactant mixtures and their effects on solubilization of polycyclic aromatic hydrocarbons Manorama Panda, Kabir-ud-Din ⁎ Department of Chemistry, Aligarh Muslim University, Aligarh-202 002, India
a r t i c l e
i n f o
Article history: Received 19 April 2011 Received in revised form 22 July 2011 Accepted 3 August 2011 Available online 17 August 2011 Keywords: Gemini surfactant Polycyclic aromatic hydrocarbon Synergism Mixed micelle Solubilization
a b s t r a c t The aqueous solubility enhancement of the polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene and pyrene by micellar solutions of single gemini surfactant hexanediyl-1,6-bis(dimethylcetylammonium bromide) (G6) and its mixtures with cationic cetyltrimethylammonium bromide (CTAB), anionic sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and nonioinic polyoxyethylene (20) cetyl ether (Brij 58) have been investigated. Above the cmc, maximum solubilization occurs in the Brij 58 surfactant micelles whereas the solubilization is least in presence of AOT. The PAHs are solubilized synergistically in mixed gemini-conventional surfactant solutions, which is attributed to the formation of mixed micelles, their lower cmc values, and the increase of the solvents' molar solubilization ratios or micellar partition coefficients because of the lower polarity of the mixed micelles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Organic pollutants like polycyclic aromatic hydrocarbons (PAHs) are hydrophobic and can be removed from the contaminated soils and ground water by surfactants mainly due to the solubilization or mobilization of the pollutants inside micelles [1–5]. Solubility enhancements are, however, closely related to the properties of organic compounds and surfactants. The micelle-solubilization effects for various organic chemicals have been investigated by many workers to quantify the efficiency of surfactant-enhanced remediation (SER). In the remediation of hydrophobic organic polluted environment with high surfactant concentrations, the surfactants applied to remediate a site could again contaminate the soil and ground water significantly. To get a better system, mixed micellar systems have already been used for the significant enhancement of water solubility of poorly soluble organic compounds [6–8]. Mixed surfactants improve the performance of surfactant-enhanced remediation of soils and sediments by decreasing the applied surfactant level and thus the remediation cost [8–12]. Gemini surfactants are better solubilizers as they have better surface-active properties than the corresponding conventional surfactants of equal chain length. Differently from conventional surfactants, gemini surfactants consist of two hydrophobic groups, two hydrophilic groups and a spacer linked at or near head groups [13,14]. As a result, gemini surfactants form larger micelles than the conventional surfactants [15] and thus should have a better solubilizing capacity. In their study of the solubilization of pyrene in the micellar solutions of gemini
⁎ Corresponding author. Tel.: + 91 571 2700920x3353. E-mail address: [email protected] (Kabir-ud-Din). 0167-7322/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2011.08.002
surfactants of different spacer length, Zheng and Zhao [16] have shown that such micelles show stronger ability for pyrene solubilization than the conventional surfactant micelles. Although the gemini-conventional mixed micellar systems can increase the water-solubility of PAHs and other organic compounds significantly, those have not been explored extensively. To our knowledge, only limited reports of solubilization of PAHs in gemini-conventional mixed surfactant systems are available [17,18]. The objectives of the present study are: (i) to evaluate and compare the efficiency of some gemini-conventional mixed surfactants in enhancing the water solubility of PAHs, and (ii) to have a clear idea about the synergistic solubilization of PAHs by mixed surfactant systems. The experiments are aimed to ascertain if a mixed surfactant solution with a lesser total surfactant amount for reducing the surfactant quantities may be used in the SER of organic contaminants. In this study, we have examined mixed micellar systems of gemini-cationic, gemini-anionic, and gemini-nonionic conventional surfactants and intercomparison has been made for their abilities to solubilize the PAHs. 2. Experimental section 2.1. Chemicals Anthracene (99.5%) was purchased from Koch-Light Laboratories Ltd., England. Naphthalene (99.7%) and pyrene (99%) were obtained from Fluka, Switzerland. The amphiphile AOT was procured from S. D. Fine, India with a purity of 98.5%. CTAB (99%) and Brij 58 were purchased from Merck, Germany. All the chemicals were used without further purification. Freshly prepared distilled water was used throughout. The PAHs have two or more benzene rings in their
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mixture was stirred using a magnetic stirring bar for 24 h at 30 °C. An aliquot of the sample was collected and then centrifuged at 12,000 rpm to remove the undissolved PAH. The concentration of the solubilized PAH was determined spectrophotometrically with a Shimadzu spectrophotometer (Model UV mini–1240) following appropriate dilution of an aliquot of the supernatant with the corresponding surfactant solution. The surfactant concentration was kept the same in both the reference and the measurement cells to eliminate its effect on the UVabsorbance.
structures (their properties and structures along with the structure of the surfactants are given in Scheme 1). 2.2. Synthesis of gemini surfactant The dimeric gemini hexanediyl-1,6-bis(dimethylcetylammonium bromide) C16H33(CH3)2–N +-(CH2)6–N +(CH3)2C16H33,2Br − surfactant (16-6-16, G6) was synthesized in the laboratory by refluxing 1,6-dibromohexane with N, N-dimethylcetylamine (molar ratio 1:2.1) in dry ethanol with continuous stirring at 80 °C for 48 h to ensure as much as possible a complete bisquaternization.
3. Results and discussions 3.1. Surface and micellar properties
Reflux; dry ethanol
Br ðCH2 Þ6 Br + 2CH3 −ðCH2 Þ15 −NðCH3 Þ2 → 48h; 80 C
þ
3.1.1. cmc The concentration of surfactants in aqueous medium above which the surfactant monomers start assembling together to form the micelles is known as critical micelle concentration (cmc). The cmc values of pure as well as of binary surfactant mixtures (cmcexp) were evaluated on the basis of tensiometric measurements. Surface tension decreases as the concentration of the surfactant increases. Surfactant molecules at low concentrations adsorb at the liquid/air interface until the surface of the solution is completely occupied. Then the excess molecules tend to self-associate in the solution to form micelles, and surface tension becomes constant. Two opposite effects control micellization: the effect of the hydrophobic group is an important driving force in micellization and the effect of the hydrophilic group opposing it. The cmc values were determined by noting inflections in the surface tension (γ) versus logarithm of surfactant concentration isotherms and are given in Table 1. The gemini surfactant has remarkably low cmc value as compared to the conventional surfactants because of its two polar head groups and two hydrophobic chains which transfer at the same time from the aqueous phase to micellar phase.
þ
C16 H33 ðCH3 Þ2 N −ðCH2 Þ6 −N ðCH3 Þ2 C16 H33⋅ 2Br−
The progress of the reaction was monitored by using TLC technique. After the completion of the reaction the solvent was removed under vacuum. After crystallization, the surfactant was characterized by 1H NMR and FT-IR [19]. All the values obtained were satisfying, which indicated that the surfactant was well purified. 2.3. cmc determination by surface tension measurements Tensiometric experiments were performed using a platinum ring by the ring detachment method with a Kruss (Germany, Model K11) tensiometer equipped with a constant temperature water circulating device. The surfactant concentration was varied by adding concentrated surfactant solution in small installments, and the readings were noted after thorough mixing and temperature equilibration. 2.4. Solubilization experiments The solubility of naphthalene, anthracene and pyrene were measured in different surfactant solutions between ranges of concentration above the cmc. Excess PAH was added to screw-capped vials containing a fixed volume of micellar solution to ensure maximum solubility. This
a
Naphthalene
3.1.2. Γmax and Amin The liquid/air interface of a surfactant solution is well populated by the adsorbed molecules. The surfactant concentration is always more
Anthracene
Pyrene
b
O +
+
O
–
C16H33 (CH3)2N – (CH2)6– N (CH3)2C16H33. 2Br
S
O O
(G6)
O Na +
O
O
(AOT) +
-
CH3-–(CH2)15-–N -–(CH3)3.Br (CTAB)
CH3–(CH2)15–(OCH2CH2)20–OH (Brij 58)
Scheme 1. (a) Structures and physicochemical properties, viz., molecular weight (MW), aqueous solubility (sol.), log KOW (KOW = octanol − water partition coefficient), and molecular volume (MV) of the polycyclic aromatic hydrocarbons used in this study. (b) Structures of surfactant molecules used in this study: hexanediyl-1,6-bis (dimethylcetylammonium bromide) (G6), cetyltrimethylammonium bromide (CTAB), sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and polyoxyethylene (20) cetyl ether (Brij 58).
M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 163 (2011) 93–98
95
Table 1 Critical micelle concentration (cmcexp) (literature values are given in the parentheses) of single surfactants, cmcideal of binary surfactant mixtures (1:1), micellar composition (X1m), interaction parameter (βm) and activity coefficients (fim) of binary surfactant mixtures using Rubingh's method at 30 °C. Systems
cmcexp (mM)
cmcideal (mM)
X1m
X
G6
0.001 (0.003)a 0.776 (0.815)b 0.004 (0.0039)c 0.638 (0.640)d 0.0015 0.0014 0.005
0.002 0.0016 0.002
0.859 0.737 –
0.999 0.800 –
CTAB Brij 58 AOT G6-CTAB G6-Brij 58 G6-AOT a b c d
βm
f1m
f2m
ΔGex (kJ mol− 1)
− 6.764 − 0.750 –
0.874 0.950 –
0.007 0.665 –
− 2.069 − 0.366 –
Ref. [20]. Ref. [21]. Ref. [22]. Ref. [23].
at the interface due to adsorption as compared to the concentration of the surfactant in the bulk. The maximum surface excess concentration at the air/water interface, Γmax, was evaluated by the Gibbs adsorption equation (Eq. (1)) [24]
Γmax = ð−1 = 2:303nRT Þðdγ =d log C ÞT;P :
ð1Þ
Here, R is the universal gas constant, T is the absolute temperature, n is introduced to allow for the simultaneous adsorption of cations and anions. The value of n was calculated by taking into consideration the type of surfactants/surfactant mixtures [24]. The values of Γmax (and also of Amin) given in Table 2 are based upon n = 4 or 5 for surfactant mixtures (4 for G6 + Brij 58 and 5 for G6 + CTAB/AOT). For single surfactants, the values of n are 1, 2 and 3 for Brij 58, CTAB/AOT and G6, respectively. 1 The Γmax values of the pure surfactants are in the order: G6 N CTAB N Brij 58 N AOT and for binary systems the order is G6– CTAB N G6–AOT N G6-Brij 58. The difference in Γmax values for the surfactant systems is due to the intermolecular head group distance. Although both G6 and CTAB are cationic in nature, their mixed interface is most highly populated. The reason lies in their structures. Both contain flexible hydrophobic chains and it is easy for them to get accommodated in small space. Hence, the interface is highly populated. There should be attraction between the oppositely charged head groups of G6 and AOT and their interface should be most tightly packed. However, this is not the case. As compared to Brij 58, AOT is a less flexible molecule with a short hydrophilic and a bulky hydrophobic part. Therefore, it is difficult for AOT to adjust itself at the interface. For G6-Brij 58, the large non-ionic molecule is responsible for the least value of Γmax. Hence, G6-CTAB is a better mixed system than G6-AOT and G6-Brij 58. The minimum area per head group, Amin, can be evaluated by Eq. (2)
16
Amin = 1 × 10
= ðNA Γmax Þ
ð2Þ
where NA is Avogadro's number. The Amin value of gemini is the smallest among all the surfactants. The lower values of G6 and G6-CTAB systems are due to the electrostatic repulsion between the head groups. As the flexibility of AOT is very less, the Amin value is not high although there is attraction 1
ideal
We are thankful to the Reviewer for drawing attention to this point.
between the head groups in the mixed system. The G6 surfactant molecules are more tightly packed in both single and binary systems. 3.1.3. Surfactant–surfactant interactions The Clint equation (Eq. (3)) [25], which differentiates the ideal and nonideal behaviors of surfactants in their mixtures, was used to obtain the ideal cmc (cmcideal) values.
i
1 = cmcideal = ∑ αi = cmci
ð3Þ
i=1
Here αi is the bulk mole fraction of the ith component in mixed surfactant solution and cmci its critical micelle concentration in pure form. We see that the obtained cmc values (cmcexp) of G6-Brij58 and G6-CTAB surfactant mixtures are lower than the cmcideal (Table 1) indicating synergistic interaction, whereas for G6-AOT system the cmcexp is higher than the cmcideal value showing antagonistic interaction. We can say that a mixed micelle is formed between the G6 and AOT surfactants as the cmcexp value of the mixed surfactant system is less than that of AOT. Micellar mole fraction of the gemini surfactant can be evaluated with the help of Rubingh's equation (Eq. (4)) [26] m 2 m X1 1n cmc exp α1 = cmc1 X1 n o = 1: 1−X1m 2 1n cmc exp ð1−α1 Þ = cmc2 1−X1m
ð4Þ
Here cmcexp denotes the experimental cmc value of the binary mixture and X1m is the micellar mole fraction of surfactant 1 (G6) in the mixed micelle. Table 2 The maximum surface excess concentration at the air/water interface (Γmax), minimum area per head group (Amin), surface composition (X1σ), interaction parameter (βσ) and activity coefficients (fiσ) at 30 °Ca. Systems
Γmax × 1011 (mol cm− 2)
Amin (Å2)
X1σ
βσ
f1σ
f2σ
G6 CTAB Brij 58 AOT G6-CTAB G6-Brij 58 G6-AOT
48.39 23.43 17.64 9.80 21.80 9.91 11.02
34.31 70.86 94.12 169.51 76.17 167.58 150.64
0.914 0.747 –
− 4.912 − 1.016 –
0.964 0.937 –
0.017 0.567 –
a
The values of X1σ, βσ, f1σ, f2σ were calculated using Rosen's equation [24].
M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 163 (2011) 93–98
Analogous to Rubingh's equation, Rosen [24] gave an equation for mole fraction of surfactant at the interface (X1σ) in which concentrations of individual components and binary mixtures to produce a particular surface tension are used (instead of cmc values). These equations were non-convergent for G6-AOT systems. For G6-CTAB system, mixed micelles contain more CTAB than the gemini as compared to the mixed interface. Two hydrophobic chains present in gemini surfactant makes it difficult to adjust in a curved micellar surface than at the planar interface. The attractive interaction between the two surfactants in the mixed micelle/mixed interface is accompanied by the decrease of energy, which is measured in terms of interaction parameter β m/β σ: 1n cmc exp α1 = cmc1 X1m β = 1−X1m 2 m
0.5
0.4
[Naphthalene](mM)
96
G6 CTAB Brij58 AOT
0.3
0.2
0.1
0.0 0.0
ð5Þ
0.4
0.8
1.2
1.6
2.0
and
0.010
σ 1n cmc α = cmc X 1 exp 1 1 σ β = : 1−X1σ 2
ð6Þ
n o m m m 2 f1 = exp β 1−X1
0.008
[Anthracene](mM)
β m/β σ is an indicator of the degree of interaction between the two surfactants in mixed micelles/mixed interface relative to the selfinteraction of the two surfactants under similar conditions before mixing, and accounts for deviation from ideality. The larger the negative value of β m/β σ, the stronger is the attractive interaction between the two surfactant molecules. The activity coefficients, fim, of the two surfactants within the mixed micelle are related to the interaction parameter through Eqs. (7a) and (7b):
G6 CTAB Brij58 AOT
0.006
0.004
0.002
0.000 0.0
ð7aÞ
0.4
0.8
1.2
1.6
2.0
2.4
σ f1
σ
f2
and
G6 CTAB Brij58 AOT
σ
are related to β through Eqs. (8a) and (8b):
n o σ σ 2 = exp β 1−X1
ð8aÞ
σ σ2 : = exp β X1
ð8bÞ
The values of β m/β σ for the surfactant mixtures G6-Brij58 and G6CTAB are negative (Tables 1 and 2) indicating that the attractive interaction between the two surfactants after mixing is more than that before mixing (i.e., synergistic interaction). Synergism in surfactant mixtures depends not only on the strength of interactions but also on the relevant properties of surfactants. The synergism in G6-CTAB is more than G6-Brij 58 system. Brij 58, with polyoxyethylene (POE) groups, has a large number of oxygen atoms with lone pair of electrons. Thus it should have a tendency to react coulombically with the cationic gemini surfactant; but the presence of long POE head group imposes some steric constraint due to thermal vibrations, restricting the effective head group interactions, hence showing lower values of interaction parameter. Further, the values of β m and X1m find support from the low values of activity coefficients of conventional surfactants (f2m), which may be due to less partitioning of conventional surfactants in the gemini micelles. The values of the activity coefficients obtained from Eqs. (7a), (7b) and (8a), (8b) are less than unity indicating nonideal behavior. The values of excess free energy of micellization, ΔGex (=[X1m.lnf1m + (1= X1m).lnf2m])RT are negative (Table 1) indicating the higher stability of the mixed micelles than the single surfactant micelles. G6-CTAB form
0.05
[Pyrene](mM)
Similarly,
3.2
ð7bÞ
0.06 f2σ
2.8
[Surfactant](mM)
m m m2 f2 = exp β X1 f1σ
2.4
[Surfactant](mM)
0.04 0.03 0.02 0.01 0.00 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[Surfactant](mM) Fig. 1. Variation of solubility of PAHs with surfactant concentration.
more stable mixed micelles than G6-Brij 58 as can be seen from its higher value of ΔGex. 3.2. Micellar solubilization 3.2.1. Solubilization behavior of different surfactants PAH solubilizations plotted as a function of surfactant concentration are presented in Figs. 1 and 2. The aqueous solubility of PAHs increased linearly in the concentration range above the cmc both in the single and mixed surfactant systems showing the potential of the surfactants to enhance the solubilization. This increased solubilization
M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 163 (2011) 93–98
0.3
Here, Scmc is apparent solubility of PAH at cmc, which is taken as their water solubility S, as it changes slightly up to cmc of surfactant. Edwards et al. [27] have suggested the use of micelle-water partition coefficient (Km) for describing the solubilization by aqueous phase given by Km = Xm/Xa (ratio of mole fraction of the organic compound in the micellar phase Xm, to that in the aqueous phase Xa). The value of Xm in terms of MSR can be written as Xm = MSR/(1 + MSR) and Xa = [Scmc] Vm, Vm is the volume of water equal to 0.01807 LM − 1 at 30 °C. With these expressions, Km becomes
0.2
Km = MSR = f½Scmc Vm ð1 + MSRÞg:
0.7
[Naphthalene](mM)
0.6
G6CTAB G6Brij58 G6AOT
0.5 0.4
0.1 0.0 0.0
0.4
0.8
1.2
1.6
2.0
1.6
2.0
[Surfactant](mM) 0.020
[Anthracene](mM)
0.016
G6CTAB G6Brij58 G6AOT
0.012
0.008
0.004
0.000 0.0
0.4
0.8
1.2
[Surfactant](mM) 0.07 0.06 0.05
[Pyrene](mM)
97
G6CTAB G6Brij58 G6AOT
0.04 0.03 0.02 0.01 0.00 0.0
0.4
0.8
1.2
1.6
[Surfactant](mM) Fig. 2. Variation of solubility of PAHs with G6 concentration in 1:1 binary surfactant combinations.
is due to the incorporation or partitioning of organic solutes within the micelles. A useful way to evaluate the effectiveness of a surfactant in solubilizing a given solubilizate is molar solubilization ratio (MSR) which is defined as the number of moles of organic compound solubilized per mole of surfactant added to the solution [9,11]. It is obtained from the slope that results when solubilizate concentration is plotted against the surfactant concentration MSR = ðSt −Scmc Þ = ðCt −Ccmc Þ:
ð9Þ
ð10Þ
The results (Table 3) reveal that MSR and Km values are the highest for nonionic surfactant. It is assumed that the inner nonpolar core of the micelles is responsible for the solubilization and Km should be approximately proportional to the nonpolar content of the surfactant [28]. The difference in the solubilizing ability among the surfactants could be attributed to their molecular structure. Brij58 facilitates solubilization due to weak interaction of oxygen atoms of POEs with π-electrons of arenes. The lower values of MSR and Km in case of cationic surfactants are due to limited solubilization at micelle–water interface and micellar core. For naphthalene, cationic gemini (G6) shows higher solubilization power than the corresponding conventional CTAB of the same chain length whereas for anthracene and pyrene the solubilization power of CTAB is more than that of G6. AOT presents least MSR and Km values due to repulsive interaction between the π-electrons of the solutes and the negative charge of the surfactant, in addition to the smaller micellar size which causes difficulty in packing within the micelle (due to the double tails in AOT, see Scheme 1). The order of solubilizing power for organic solutes by inner nonpolar core of micelles has been reported to be nonionic N cationic N anionic surfactants having the same nonpolar chain length [29], which is similar to our findings. The order of the MSR and Km of the surfactant mixtures is G6CTAB N G6-Brij58 N G6-AOT for naphthalene and G6-AOT N G6CTAB ~ G6-Brij58 for anthracene and pyrene. The greater MSR and Km values for binary systems than the single surfactant systems indicate synergism in mixed surfactant systems for PAH solubility enhancement. This may be due to the larger effective solubilization area in mixed micelles including the electric dipole as reported by Tokouta et al. [30]. This enhancement in solubility is attributed to the formation of mixed micelles, lower cmc of the mixed surfactant systems, and the increase of solute's molar solubilization ratio or micellar partition coefficient due to low polarity of mixed micelles. Hydrophilic–hydrophilic interactions occur at the mixed micelle– water interface and affect naphthalene solubilization whereas hydrophobic–hydrophobic interactions occurring in the micellar core affect the solubilization of more hydrophobic PAHs, i.e., anthracene and pyrene. Actually, these interpretations ascribed the mixing effect to the changes of structure and properties of mixed micelles, which are the results of the interaction between the components of the mixed surfactant, attractive or repulsive. The micelle–water partition coefficient and the cmc are two important factors influencing the solubilization of the solute in the mixed surfactants. In mixed surfactant systems with more negative value of β m, the micelle becomes densely packed with reduced solubilization capability. Thus, enhancement of solubilization in G6-AOT mixed micelle is maximum for anthracene and pyrene than the other two systems. The ideal molar solubilization ratio (MSRideal) was calculated using Eq. (11) MSRideal = Σi MSRi αi
ð11Þ
where MSRi is the experimental MSR value of the solubilizate in the pure ith surfactant solution whose bulk mole fraction in the mixture is
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Table 3 Molar solubilization ratio (MSR), ln Km and the free energy of solubilization (ΔGs0) for the selected surfactant systems at 30 °C. System
Naphthalene MSR
Anthracene lnKm
(MSRideal) G6 CTAB Brij 58 AOT G6 + CTAB G6 + Brij 58 G6 + AOT
0.2110 0.1236 0.2112 0.0605 0.3711 (0.1673) 0.3345 (0.2111) 0.1798 (0.1358)
ΔGs0 (kJ mol
Pyrene
MSR −1
10.58 10.12 10.59 9.47
− 26.68 − 25.52 − 26.68 − 23.86
11.02
− 27.79
10.95
− 27.59
10.45
− 26.34
)
lnKm
(MSRideal)
(kJ mol
0.0024 0.0043 0.0044 0.0007 0.0085 (0 .0034) 0.0085 (0.0034) 0.0103 (0.0016)
αi. The experimental MSR values of the solutes in the mixed micelle are higher than the ideal value implying positive mixing effect of surfactants on solubilization.
ΔGs0
MSR −1
6.30 6.88 6.90 5.07
− 15.87 − 17.33 − 17.39 − 12.77
7.56
− 19.04
7.56
− 19.04
7.75
− 19.52
)
lnKm
(kJ mol− 1)
(MSRideal) 0.0095 0.0139 0.0401 0.0019 0.0403 (0.0117) 0.0399 (0.0248) 0.0526 (0.0057)
ΔGs0
7.67 8.04 9.08 6.06
− 19.32 − 20.27 − 22.88 − 15.28
9.08
− 22.89
9.07
− 22.86
9.34
− 23.53
the mixing effect on solubilization capabilities. Understanding the water-solubility enhancements of organic pollutants by mixed surfactants can extend the scope of contaminant remediation.
3.3. Thermodynamics of solubilization Acknowledgements The knowledge of the thermodynamic parameters controlling solubilization is helpful for better understanding of the mechanism involved in the process. From the thermodynamic point of view, solubilization can be considered as normal partitioning of the PAH between two phases, micellar and aqueous, and the standard free energy of solubilization, ΔGS0, can be represented by the expression [31] 0
ΔGS = −RTlnKm
ð12Þ
The ΔGS0 values thus calculated are presented in Table 3. For all the systems, the ΔGS0 values come out to be negative showing spontaneity of the solubilization process. 4. Conclusion In an attempt to find a useful method to increase the water solubility of poorly soluble organic compounds, we have examined the mixed micellar systems. The present study investigates the solubilization behavior of naphthalene, anthracene and pyrene by single and mixed gemini-conventional surfactant systems. Solubility of PAHs in water is greatly enhanced in a linear fashion by the single and binary systems. In the binary combinations of gemini with conventional surfactants, the enhancement of solubilization of PAHs in G6-AOT system is lowest for naphthalene, and highest for anthracene and pyrene. For naphthalene the extent of solubilization was more enhanced in G6-CTAB than the other two systems whereas for anthracene and pyrene the enhancement of solubilization in G6CTAB and G6-Brij 58 are the same. On mixing of surfactants, the interaction parameters such as ΔGex, β m are found to be negative for G6-CTAB and G6-Brij 58 systems, and synergism is observed in properties like cmc, surface tension, solubilization, etc. The synergism in the mixed micelles would make them more efficient and economic in surfactant enhanced remediation of contaminated sites. The experimental results of the present study can be used to understand
This study was financially supported by the Department of Science and Technology, Government of India (Project No. SR/WOS-A/CS46/2007).
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Contents lists available at ScienceDirect
Journal of Molecular Liquids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m o l l i q
Study of surface and solution properties of gemini-conventional surfactant mixtures and their effects on solubilization of polycyclic aromatic hydrocarbons Manorama Panda, Kabir-ud-Din ⁎ Department of Chemistry, Aligarh Muslim University, Aligarh-202 002, India
a r t i c l e
i n f o
Article history: Received 19 April 2011 Received in revised form 22 July 2011 Accepted 3 August 2011 Available online 17 August 2011 Keywords: Gemini surfactant Polycyclic aromatic hydrocarbon Synergism Mixed micelle Solubilization
a b s t r a c t The aqueous solubility enhancement of the polycyclic aromatic hydrocarbons (PAHs) naphthalene, anthracene and pyrene by micellar solutions of single gemini surfactant hexanediyl-1,6-bis(dimethylcetylammonium bromide) (G6) and its mixtures with cationic cetyltrimethylammonium bromide (CTAB), anionic sodium bis(2-ethylhexyl)sulfosuccinate (AOT) and nonioinic polyoxyethylene (20) cetyl ether (Brij 58) have been investigated. Above the cmc, maximum solubilization occurs in the Brij 58 surfactant micelles whereas the solubilization is least in presence of AOT. The PAHs are solubilized synergistically in mixed gemini-conventional surfactant solutions, which is attributed to the formation of mixed micelles, their lower cmc values, and the increase of the solvents' molar solubilization ratios or micellar partition coefficients because of the lower polarity of the mixed micelles. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Organic pollutants like polycyclic aromatic hydrocarbons (PAHs) are hydrophobic and can be removed from the contaminated soils and ground water by surfactants mainly due to the solubilization or mobilization of the pollutants inside micelles [1–5]. Solubility enhancements are, however, closely related to the properties of organic compounds and surfactants. The micelle-solubilization effects for various organic chemicals have been investigated by many workers to quantify the efficiency of surfactant-enhanced remediation (SER). In the remediation of hydrophobic organic polluted environment with high surfactant concentrations, the surfactants applied to remediate a site could again contaminate the soil and ground water significantly. To get a better system, mixed micellar systems have already been used for the significant enhancement of water solubility of poorly soluble organic compounds [6–8]. Mixed surfactants improve the performance of surfactant-enhanced remediation of soils and sediments by decreasing the applied surfactant level and thus the remediation cost [8–12]. Gemini surfactants are better solubilizers as they have better surface-active properties than the corresponding conventional surfactants of equal chain length. Differently from conventional surfactants, gemini surfactants consist of two hydrophobic groups, two hydrophilic groups and a spacer linked at or near head groups [13,14]. As a result, gemini surfactants form larger micelles than the conventional surfactants [15] and thus should have a better solubilizing capacity. In their study of the solubilization of pyrene in the micellar solutions of gemini
⁎ Corresponding author. Tel.: + 91 571 2700920x3353. E-mail address: [email protected] (Kabir-ud-Din). 0167-7322/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.molliq.2011.08.002
surfactants of different spacer length, Zheng and Zhao [16] have shown that such micelles show stronger ability for pyrene solubilization than the conventional surfactant micelles. Although the gemini-conventional mixed micellar systems can increase the water-solubility of PAHs and other organic compounds significantly, those have not been explored extensively. To our knowledge, only limited reports of solubilization of PAHs in gemini-conventional mixed surfactant systems are available [17,18]. The objectives of the present study are: (i) to evaluate and compare the efficiency of some gemini-conventional mixed surfactants in enhancing the water solubility of PAHs, and (ii) to have a clear idea about the synergistic solubilization of PAHs by mixed surfactant systems. The experiments are aimed to ascertain if a mixed surfactant solution with a lesser total surfactant amount for reducing the surfactant quantities may be used in the SER of organic contaminants. In this study, we have examined mixed micellar systems of gemini-cationic, gemini-anionic, and gemini-nonionic conventional surfactants and intercomparison has been made for their abilities to solubilize the PAHs. 2. Experimental section 2.1. Chemicals Anthracene (99.5%) was purchased from Koch-Light Laboratories Ltd., England. Naphthalene (99.7%) and pyrene (99%) were obtained from Fluka, Switzerland. The amphiphile AOT was procured from S. D. Fine, India with a purity of 98.5%. CTAB (99%) and Brij 58 were purchased from Merck, Germany. All the chemicals were used without further purification. Freshly prepared distilled water was used throughout. The PAHs have two or more benzene rings in their
94
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mixture was stirred using a magnetic stirring bar for 24 h at 30 °C. An aliquot of the sample was collected and then centrifuged at 12,000 rpm to remove the undissolved PAH. The concentration of the solubilized PAH was determined spectrophotometrically with a Shimadzu spectrophotometer (Model UV mini–1240) following appropriate dilution of an aliquot of the supernatant with the corresponding surfactant solution. The surfactant concentration was kept the same in both the reference and the measurement cells to eliminate its effect on the UVabsorbance.
structures (their properties and structures along with the structure of the surfactants are given in Scheme 1). 2.2. Synthesis of gemini surfactant The dimeric gemini hexanediyl-1,6-bis(dimethylcetylammonium bromide) C16H33(CH3)2–N +-(CH2)6–N +(CH3)2C16H33,2Br − surfactant (16-6-16, G6) was synthesized in the laboratory by refluxing 1,6-dibromohexane with N, N-dimethylcetylamine (molar ratio 1:2.1) in dry ethanol with continuous stirring at 80 °C for 48 h to ensure as much as possible a complete bisquaternization.
3. Results and discussions 3.1. Surface and micellar properties
Reflux; dry ethanol
Br ðCH2 Þ6 Br + 2CH3 −ðCH2 Þ15 −NðCH3 Þ2 → 48h; 80 C
þ
3.1.1. cmc The concentration of surfactants in aqueous medium above which the surfactant monomers start assembling together to form the micelles is known as critical micelle concentration (cmc). The cmc values of pure as well as of binary surfactant mixtures (cmcexp) were evaluated on the basis of tensiometric measurements. Surface tension decreases as the concentration of the surfactant increases. Surfactant molecules at low concentrations adsorb at the liquid/air interface until the surface of the solution is completely occupied. Then the excess molecules tend to self-associate in the solution to form micelles, and surface tension becomes constant. Two opposite effects control micellization: the effect of the hydrophobic group is an important driving force in micellization and the effect of the hydrophilic group opposing it. The cmc values were determined by noting inflections in the surface tension (γ) versus logarithm of surfactant concentration isotherms and are given in Table 1. The gemini surfactant has remarkably low cmc value as compared to the conventional surfactants because of its two polar head groups and two hydrophobic chains which transfer at the same time from the aqueous phase to micellar phase.
þ
C16 H33 ðCH3 Þ2 N −ðCH2 Þ6 −N ðCH3 Þ2 C16 H33⋅ 2Br−
The progress of the reaction was monitored by using TLC technique. After the completion of the reaction the solvent was removed under vacuum. After crystallization, the surfactant was characterized by 1H NMR and FT-IR [19]. All the values obtained were satisfying, which indicated that the surfactant was well purified. 2.3. cmc determination by surface tension measurements Tensiometric experiments were performed using a platinum ring by the ring detachment method with a Kruss (Germany, Model K11) tensiometer equipped with a constant temperature water circulating device. The surfactant concentration was varied by adding concentrated surfactant solution in small installments, and the readings were noted after thorough mixing and temperature equilibration. 2.4. Solubilization experiments The solubility of naphthalene, anthracene and pyrene were measured in different surfactant solutions between ranges of concentration above the cmc. Excess PAH was added to screw-capped vials containing a fixed volume of micellar solution to ensure maximum solubility. This
a
Naphthalene
3.1.2. Γmax and Amin The liquid/air interface of a surfactant solution is well populated by the adsorbed molecules. The surfactant concentration is always more
Anthracene
Pyrene
b
O +
+
O
–
C16H33 (CH3)2N – (CH2)6– N (CH3)2C16H33. 2Br
S
O O
(G6)
O Na +
O
O
(AOT) +
-
CH3-–(CH2)15-–N -–(CH3)3.Br (CTAB)
CH3–(CH2)15–(OCH2CH2)20–OH (Brij 58)
Scheme 1. (a) Structures and physicochemical properties, viz., molecular weight (MW), aqueous solubility (sol.), log KOW (KOW = octanol − water partition coefficient), and molecular volume (MV) of the polycyclic aromatic hydrocarbons used in this study. (b) Structures of surfactant molecules used in this study: hexanediyl-1,6-bis (dimethylcetylammonium bromide) (G6), cetyltrimethylammonium bromide (CTAB), sodium bis(2-ethylhexyl) sulfosuccinate (AOT), and polyoxyethylene (20) cetyl ether (Brij 58).
M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 163 (2011) 93–98
95
Table 1 Critical micelle concentration (cmcexp) (literature values are given in the parentheses) of single surfactants, cmcideal of binary surfactant mixtures (1:1), micellar composition (X1m), interaction parameter (βm) and activity coefficients (fim) of binary surfactant mixtures using Rubingh's method at 30 °C. Systems
cmcexp (mM)
cmcideal (mM)
X1m
X
G6
0.001 (0.003)a 0.776 (0.815)b 0.004 (0.0039)c 0.638 (0.640)d 0.0015 0.0014 0.005
0.002 0.0016 0.002
0.859 0.737 –
0.999 0.800 –
CTAB Brij 58 AOT G6-CTAB G6-Brij 58 G6-AOT a b c d
βm
f1m
f2m
ΔGex (kJ mol− 1)
− 6.764 − 0.750 –
0.874 0.950 –
0.007 0.665 –
− 2.069 − 0.366 –
Ref. [20]. Ref. [21]. Ref. [22]. Ref. [23].
at the interface due to adsorption as compared to the concentration of the surfactant in the bulk. The maximum surface excess concentration at the air/water interface, Γmax, was evaluated by the Gibbs adsorption equation (Eq. (1)) [24]
Γmax = ð−1 = 2:303nRT Þðdγ =d log C ÞT;P :
ð1Þ
Here, R is the universal gas constant, T is the absolute temperature, n is introduced to allow for the simultaneous adsorption of cations and anions. The value of n was calculated by taking into consideration the type of surfactants/surfactant mixtures [24]. The values of Γmax (and also of Amin) given in Table 2 are based upon n = 4 or 5 for surfactant mixtures (4 for G6 + Brij 58 and 5 for G6 + CTAB/AOT). For single surfactants, the values of n are 1, 2 and 3 for Brij 58, CTAB/AOT and G6, respectively. 1 The Γmax values of the pure surfactants are in the order: G6 N CTAB N Brij 58 N AOT and for binary systems the order is G6– CTAB N G6–AOT N G6-Brij 58. The difference in Γmax values for the surfactant systems is due to the intermolecular head group distance. Although both G6 and CTAB are cationic in nature, their mixed interface is most highly populated. The reason lies in their structures. Both contain flexible hydrophobic chains and it is easy for them to get accommodated in small space. Hence, the interface is highly populated. There should be attraction between the oppositely charged head groups of G6 and AOT and their interface should be most tightly packed. However, this is not the case. As compared to Brij 58, AOT is a less flexible molecule with a short hydrophilic and a bulky hydrophobic part. Therefore, it is difficult for AOT to adjust itself at the interface. For G6-Brij 58, the large non-ionic molecule is responsible for the least value of Γmax. Hence, G6-CTAB is a better mixed system than G6-AOT and G6-Brij 58. The minimum area per head group, Amin, can be evaluated by Eq. (2)
16
Amin = 1 × 10
= ðNA Γmax Þ
ð2Þ
where NA is Avogadro's number. The Amin value of gemini is the smallest among all the surfactants. The lower values of G6 and G6-CTAB systems are due to the electrostatic repulsion between the head groups. As the flexibility of AOT is very less, the Amin value is not high although there is attraction 1
ideal
We are thankful to the Reviewer for drawing attention to this point.
between the head groups in the mixed system. The G6 surfactant molecules are more tightly packed in both single and binary systems. 3.1.3. Surfactant–surfactant interactions The Clint equation (Eq. (3)) [25], which differentiates the ideal and nonideal behaviors of surfactants in their mixtures, was used to obtain the ideal cmc (cmcideal) values.
i
1 = cmcideal = ∑ αi = cmci
ð3Þ
i=1
Here αi is the bulk mole fraction of the ith component in mixed surfactant solution and cmci its critical micelle concentration in pure form. We see that the obtained cmc values (cmcexp) of G6-Brij58 and G6-CTAB surfactant mixtures are lower than the cmcideal (Table 1) indicating synergistic interaction, whereas for G6-AOT system the cmcexp is higher than the cmcideal value showing antagonistic interaction. We can say that a mixed micelle is formed between the G6 and AOT surfactants as the cmcexp value of the mixed surfactant system is less than that of AOT. Micellar mole fraction of the gemini surfactant can be evaluated with the help of Rubingh's equation (Eq. (4)) [26] m 2 m X1 1n cmc exp α1 = cmc1 X1 n o = 1: 1−X1m 2 1n cmc exp ð1−α1 Þ = cmc2 1−X1m
ð4Þ
Here cmcexp denotes the experimental cmc value of the binary mixture and X1m is the micellar mole fraction of surfactant 1 (G6) in the mixed micelle. Table 2 The maximum surface excess concentration at the air/water interface (Γmax), minimum area per head group (Amin), surface composition (X1σ), interaction parameter (βσ) and activity coefficients (fiσ) at 30 °Ca. Systems
Γmax × 1011 (mol cm− 2)
Amin (Å2)
X1σ
βσ
f1σ
f2σ
G6 CTAB Brij 58 AOT G6-CTAB G6-Brij 58 G6-AOT
48.39 23.43 17.64 9.80 21.80 9.91 11.02
34.31 70.86 94.12 169.51 76.17 167.58 150.64
0.914 0.747 –
− 4.912 − 1.016 –
0.964 0.937 –
0.017 0.567 –
a
The values of X1σ, βσ, f1σ, f2σ were calculated using Rosen's equation [24].
M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 163 (2011) 93–98
Analogous to Rubingh's equation, Rosen [24] gave an equation for mole fraction of surfactant at the interface (X1σ) in which concentrations of individual components and binary mixtures to produce a particular surface tension are used (instead of cmc values). These equations were non-convergent for G6-AOT systems. For G6-CTAB system, mixed micelles contain more CTAB than the gemini as compared to the mixed interface. Two hydrophobic chains present in gemini surfactant makes it difficult to adjust in a curved micellar surface than at the planar interface. The attractive interaction between the two surfactants in the mixed micelle/mixed interface is accompanied by the decrease of energy, which is measured in terms of interaction parameter β m/β σ: 1n cmc exp α1 = cmc1 X1m β = 1−X1m 2 m
0.5
0.4
[Naphthalene](mM)
96
G6 CTAB Brij58 AOT
0.3
0.2
0.1
0.0 0.0
ð5Þ
0.4
0.8
1.2
1.6
2.0
and
0.010
σ 1n cmc α = cmc X 1 exp 1 1 σ β = : 1−X1σ 2
ð6Þ
n o m m m 2 f1 = exp β 1−X1
0.008
[Anthracene](mM)
β m/β σ is an indicator of the degree of interaction between the two surfactants in mixed micelles/mixed interface relative to the selfinteraction of the two surfactants under similar conditions before mixing, and accounts for deviation from ideality. The larger the negative value of β m/β σ, the stronger is the attractive interaction between the two surfactant molecules. The activity coefficients, fim, of the two surfactants within the mixed micelle are related to the interaction parameter through Eqs. (7a) and (7b):
G6 CTAB Brij58 AOT
0.006
0.004
0.002
0.000 0.0
ð7aÞ
0.4
0.8
1.2
1.6
2.0
2.4
σ f1
σ
f2
and
G6 CTAB Brij58 AOT
σ
are related to β through Eqs. (8a) and (8b):
n o σ σ 2 = exp β 1−X1
ð8aÞ
σ σ2 : = exp β X1
ð8bÞ
The values of β m/β σ for the surfactant mixtures G6-Brij58 and G6CTAB are negative (Tables 1 and 2) indicating that the attractive interaction between the two surfactants after mixing is more than that before mixing (i.e., synergistic interaction). Synergism in surfactant mixtures depends not only on the strength of interactions but also on the relevant properties of surfactants. The synergism in G6-CTAB is more than G6-Brij 58 system. Brij 58, with polyoxyethylene (POE) groups, has a large number of oxygen atoms with lone pair of electrons. Thus it should have a tendency to react coulombically with the cationic gemini surfactant; but the presence of long POE head group imposes some steric constraint due to thermal vibrations, restricting the effective head group interactions, hence showing lower values of interaction parameter. Further, the values of β m and X1m find support from the low values of activity coefficients of conventional surfactants (f2m), which may be due to less partitioning of conventional surfactants in the gemini micelles. The values of the activity coefficients obtained from Eqs. (7a), (7b) and (8a), (8b) are less than unity indicating nonideal behavior. The values of excess free energy of micellization, ΔGex (=[X1m.lnf1m + (1= X1m).lnf2m])RT are negative (Table 1) indicating the higher stability of the mixed micelles than the single surfactant micelles. G6-CTAB form
0.05
[Pyrene](mM)
Similarly,
3.2
ð7bÞ
0.06 f2σ
2.8
[Surfactant](mM)
m m m2 f2 = exp β X1 f1σ
2.4
[Surfactant](mM)
0.04 0.03 0.02 0.01 0.00 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
[Surfactant](mM) Fig. 1. Variation of solubility of PAHs with surfactant concentration.
more stable mixed micelles than G6-Brij 58 as can be seen from its higher value of ΔGex. 3.2. Micellar solubilization 3.2.1. Solubilization behavior of different surfactants PAH solubilizations plotted as a function of surfactant concentration are presented in Figs. 1 and 2. The aqueous solubility of PAHs increased linearly in the concentration range above the cmc both in the single and mixed surfactant systems showing the potential of the surfactants to enhance the solubilization. This increased solubilization
M. Panda, Kabir-ud-Din / Journal of Molecular Liquids 163 (2011) 93–98
0.3
Here, Scmc is apparent solubility of PAH at cmc, which is taken as their water solubility S, as it changes slightly up to cmc of surfactant. Edwards et al. [27] have suggested the use of micelle-water partition coefficient (Km) for describing the solubilization by aqueous phase given by Km = Xm/Xa (ratio of mole fraction of the organic compound in the micellar phase Xm, to that in the aqueous phase Xa). The value of Xm in terms of MSR can be written as Xm = MSR/(1 + MSR) and Xa = [Scmc] Vm, Vm is the volume of water equal to 0.01807 LM − 1 at 30 °C. With these expressions, Km becomes
0.2
Km = MSR = f½Scmc Vm ð1 + MSRÞg:
0.7
[Naphthalene](mM)
0.6
G6CTAB G6Brij58 G6AOT
0.5 0.4
0.1 0.0 0.0
0.4
0.8
1.2
1.6
2.0
1.6
2.0
[Surfactant](mM) 0.020
[Anthracene](mM)
0.016
G6CTAB G6Brij58 G6AOT
0.012
0.008
0.004
0.000 0.0
0.4
0.8
1.2
[Surfactant](mM) 0.07 0.06 0.05
[Pyrene](mM)
97
G6CTAB G6Brij58 G6AOT
0.04 0.03 0.02 0.01 0.00 0.0
0.4
0.8
1.2
1.6
[Surfactant](mM) Fig. 2. Variation of solubility of PAHs with G6 concentration in 1:1 binary surfactant combinations.
is due to the incorporation or partitioning of organic solutes within the micelles. A useful way to evaluate the effectiveness of a surfactant in solubilizing a given solubilizate is molar solubilization ratio (MSR) which is defined as the number of moles of organic compound solubilized per mole of surfactant added to the solution [9,11]. It is obtained from the slope that results when solubilizate concentration is plotted against the surfactant concentration MSR = ðSt −Scmc Þ = ðCt −Ccmc Þ:
ð9Þ
ð10Þ
The results (Table 3) reveal that MSR and Km values are the highest for nonionic surfactant. It is assumed that the inner nonpolar core of the micelles is responsible for the solubilization and Km should be approximately proportional to the nonpolar content of the surfactant [28]. The difference in the solubilizing ability among the surfactants could be attributed to their molecular structure. Brij58 facilitates solubilization due to weak interaction of oxygen atoms of POEs with π-electrons of arenes. The lower values of MSR and Km in case of cationic surfactants are due to limited solubilization at micelle–water interface and micellar core. For naphthalene, cationic gemini (G6) shows higher solubilization power than the corresponding conventional CTAB of the same chain length whereas for anthracene and pyrene the solubilization power of CTAB is more than that of G6. AOT presents least MSR and Km values due to repulsive interaction between the π-electrons of the solutes and the negative charge of the surfactant, in addition to the smaller micellar size which causes difficulty in packing within the micelle (due to the double tails in AOT, see Scheme 1). The order of solubilizing power for organic solutes by inner nonpolar core of micelles has been reported to be nonionic N cationic N anionic surfactants having the same nonpolar chain length [29], which is similar to our findings. The order of the MSR and Km of the surfactant mixtures is G6CTAB N G6-Brij58 N G6-AOT for naphthalene and G6-AOT N G6CTAB ~ G6-Brij58 for anthracene and pyrene. The greater MSR and Km values for binary systems than the single surfactant systems indicate synergism in mixed surfactant systems for PAH solubility enhancement. This may be due to the larger effective solubilization area in mixed micelles including the electric dipole as reported by Tokouta et al. [30]. This enhancement in solubility is attributed to the formation of mixed micelles, lower cmc of the mixed surfactant systems, and the increase of solute's molar solubilization ratio or micellar partition coefficient due to low polarity of mixed micelles. Hydrophilic–hydrophilic interactions occur at the mixed micelle– water interface and affect naphthalene solubilization whereas hydrophobic–hydrophobic interactions occurring in the micellar core affect the solubilization of more hydrophobic PAHs, i.e., anthracene and pyrene. Actually, these interpretations ascribed the mixing effect to the changes of structure and properties of mixed micelles, which are the results of the interaction between the components of the mixed surfactant, attractive or repulsive. The micelle–water partition coefficient and the cmc are two important factors influencing the solubilization of the solute in the mixed surfactants. In mixed surfactant systems with more negative value of β m, the micelle becomes densely packed with reduced solubilization capability. Thus, enhancement of solubilization in G6-AOT mixed micelle is maximum for anthracene and pyrene than the other two systems. The ideal molar solubilization ratio (MSRideal) was calculated using Eq. (11) MSRideal = Σi MSRi αi
ð11Þ
where MSRi is the experimental MSR value of the solubilizate in the pure ith surfactant solution whose bulk mole fraction in the mixture is
98
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Table 3 Molar solubilization ratio (MSR), ln Km and the free energy of solubilization (ΔGs0) for the selected surfactant systems at 30 °C. System
Naphthalene MSR
Anthracene lnKm
(MSRideal) G6 CTAB Brij 58 AOT G6 + CTAB G6 + Brij 58 G6 + AOT
0.2110 0.1236 0.2112 0.0605 0.3711 (0.1673) 0.3345 (0.2111) 0.1798 (0.1358)
ΔGs0 (kJ mol
Pyrene
MSR −1
10.58 10.12 10.59 9.47
− 26.68 − 25.52 − 26.68 − 23.86
11.02
− 27.79
10.95
− 27.59
10.45
− 26.34
)
lnKm
(MSRideal)
(kJ mol
0.0024 0.0043 0.0044 0.0007 0.0085 (0 .0034) 0.0085 (0.0034) 0.0103 (0.0016)
αi. The experimental MSR values of the solutes in the mixed micelle are higher than the ideal value implying positive mixing effect of surfactants on solubilization.
ΔGs0
MSR −1
6.30 6.88 6.90 5.07
− 15.87 − 17.33 − 17.39 − 12.77
7.56
− 19.04
7.56
− 19.04
7.75
− 19.52
)
lnKm
(kJ mol− 1)
(MSRideal) 0.0095 0.0139 0.0401 0.0019 0.0403 (0.0117) 0.0399 (0.0248) 0.0526 (0.0057)
ΔGs0
7.67 8.04 9.08 6.06
− 19.32 − 20.27 − 22.88 − 15.28
9.08
− 22.89
9.07
− 22.86
9.34
− 23.53
the mixing effect on solubilization capabilities. Understanding the water-solubility enhancements of organic pollutants by mixed surfactants can extend the scope of contaminant remediation.
3.3. Thermodynamics of solubilization Acknowledgements The knowledge of the thermodynamic parameters controlling solubilization is helpful for better understanding of the mechanism involved in the process. From the thermodynamic point of view, solubilization can be considered as normal partitioning of the PAH between two phases, micellar and aqueous, and the standard free energy of solubilization, ΔGS0, can be represented by the expression [31] 0
ΔGS = −RTlnKm
ð12Þ
The ΔGS0 values thus calculated are presented in Table 3. For all the systems, the ΔGS0 values come out to be negative showing spontaneity of the solubilization process. 4. Conclusion In an attempt to find a useful method to increase the water solubility of poorly soluble organic compounds, we have examined the mixed micellar systems. The present study investigates the solubilization behavior of naphthalene, anthracene and pyrene by single and mixed gemini-conventional surfactant systems. Solubility of PAHs in water is greatly enhanced in a linear fashion by the single and binary systems. In the binary combinations of gemini with conventional surfactants, the enhancement of solubilization of PAHs in G6-AOT system is lowest for naphthalene, and highest for anthracene and pyrene. For naphthalene the extent of solubilization was more enhanced in G6-CTAB than the other two systems whereas for anthracene and pyrene the enhancement of solubilization in G6CTAB and G6-Brij 58 are the same. On mixing of surfactants, the interaction parameters such as ΔGex, β m are found to be negative for G6-CTAB and G6-Brij 58 systems, and synergism is observed in properties like cmc, surface tension, solubilization, etc. The synergism in the mixed micelles would make them more efficient and economic in surfactant enhanced remediation of contaminated sites. The experimental results of the present study can be used to understand
This study was financially supported by the Department of Science and Technology, Government of India (Project No. SR/WOS-A/CS46/2007).
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