Indian Journal of Chemical Technology Vol. 22, September 2015, pp. 194-200
Surface and solution properties of single and mixed gemini/conventional micelles on solubilization of polycyclic aromatic hydrocarbons Huma Siddiqui1, M Kamil1,* & Nazish Fatima2 1
Department of Petroleum Studies, AMU Aligarh 202 002, UP, India 2 Department of Chemistry AMU Aligarh 202 002, UP, India E-mail:
[email protected] Received 22 May 2013; accepted 17 January 2014
The surface properties and mixed micellization behaviour of single gemini bis(hexadecyldimethylammonium) pentane dibromide (G5) surfactant, three conventional surfactants (Cationic cetyltrimethylammonium bromide (CTAB), anionic sodium dodecyl sulfate (SDS) and nonioinic Polyethylene glycol dodecyl ether (Brij35)) and their equimolar binary mixtures have been investigated by surface tension and spectrophotometry methods at 300K. The critical micelle concentration (CMC), surface properties, Γmax (maximum surface excess), Amin (minimum surface area per molecule) and πCMC (surface pressure at the CMC) have been determined. The deviation from the ideal of the mixed micelles has been discussed on the basis of Clint theory. For mixed systems, regular solution theory (Rubingh) and Rosen’s theories are used, which helps to evaluate the composition of the mixed micelle, the activity coefficient, and interaction parameters (β and βσ). Negativity of interaction parameters (β and βσ) for mixed micelle formation indicates attractive interaction for all the system. Keywords: Gemini surfactants, PAHs, Solubilization, Mixed micelle, Solution property.
Polycyclic aromatic hydrocarbons (PAHs) are compounds which contains two or more fused benzene rings. The occurrence of PAHs in soils, sediments, aerosols, water, fuels, animals and plants is of increasing environmental concern because of their toxic effects1,2. Coal tar and other coal processing wastes, petroleum sludge, asphalt, creosote, and other wood preservative wastes, chemical waste, and soot3 are also some major PAH sources and these PAHs and their degradation products in environment may cause serious harm to mankind as they are identified as poisonous, carcinogenic and mutagenic4-7. Among physical, chemical and biological methods surfactant washing is identified as most promising technique for PAH removal8,9. In recent years investigation on interfacial properties, micellization properties as well as solubilization capacities of mixed surfactant (binary and ternary combinations) have been in focus10. Number of investigations on cationic-cationic, cationic-nonionic, cationic-anionic systems11-13 have been reported, but studies on gemini surfactants are limited14. Thus for the present work a cationic gemini surfactant is chosen along with conventional surfactants. Surfactants used in the present work are classified in two classes: conventional surfactant which are composed of a long hydrophobic hydrocarbon tail
with an ionic or polar hydrophilic head, and gemini monomer (the name gemini was created by Menger15), an amphiphile made up of two hydrocarbon tails and two ionic head groups connected by a ‘spacer’ in the sequence: hydrocarbon tail /ionic group /spacer /ionic group /hydrocarbon tail16. Surface active properties of geminis are superior to those of corresponding conventional surfactants with one hydrophilic and one hydrophobic group. Thus, they have lower CMC values and hence more efficient in lowering the surface tension of water. In this work, we have studied interfacial properties and molecular interactions of individual as well as equimolar binary mixtures of cationic Gemini and conventional surfactants. Experimental Section Materials
Cetyltrimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS), and polyethylene glycol dodecyl ether (Brij 35) were purchased from Sigma Aldrich Chemical Co. Gemini surfactant pentanediyl-1, 5-bis(dimethylcetylammonium romide), or G5 was synthesized in the research laboratory of the Department of Chemistry, AMU, Aligarh. Synthesis of geminis entail refluxing of 1, 6-dibromohexane with N, N-dimethylcetylamine (molar ratio 1:2.1) in dry
SIDDIQUI et al.: SURFACE AND SOLUTION PROPERTIES OF GEMINI/CONVENTIONAL MICELLES
ethanol. For maximum bisquaternization continuous stirring at 80°C is done for 48 h. Br(CH2)6 Br + 2CH3 -(CH2)15 -N(CH3)2 Reflux; dry ethanol → C16H33(CH3)2N+ – (CH2)6 – N+ 48h, 80°C
(CH3)2 C16H33⋅2BrPhenantherene and fluorene were used as polycyclic aromatic hydrocarbons in the present work. PAH compounds were also procured by Sigma Aldrich Chemical Co. Surfactant solutions were prepared in double distilled water Methods Critical micelle concentration determination by surface tension measurements
For CMC determination tensiometric experiments were performed for single as well as for mixed surfactant systems. The apparatus used for the purpose was Hardson tensiometer (Hardson make Kolkata, India), which works on ring detachment method. The
Fig. 1 — Surface tension versus log [surfactant] plots for pure (a) CTAB and Brij35, and (b) SDS and G5.
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vertically hung ring was initially dipped in the surfactant solution to measure its surface tension. It was then forced to pull out from the solution. The required force which was applied to pull out the ring from the solution is its surface tension. Experiments were repeated twice for each surfactant to ensure the reproducibility of the results. The surface tension vs. log [surfactant] plots for individual and mixed surfactant systems are shown in Figs. 1(a), 1(b) and 2 respectively. The concentration at which inflexion in curve is obtained is the CMC of that substance. Solubilization experiments
Solubility of PAH in surfactant system was determined by solubilization experiments. Solutions of surfactants of concentration higher than their corresponding CMC were mixed with an excess amount of PAH. Extra amount of PAH was added to ensure maximum solubility in surfactant solution. These samples are then agitated on magnetic stirrer for a period of 24 h at 30°C. To ensure good agitation magnetic Teflon pieces were also dropped in each vial. Stirred samples are then collected in eppendorf tubes and centrifuged at 12000 rpm, using a high speed micro centrifuge (REMI, RM-12C) to settle down the undissolved PAH. The concentration of the solubilized PAH of centrifuged sample was determined spectrophotometrically using Shimadzu spectrophotometer (model UV mini-1240), following by appropriate dilution of a sample of the supernatant with the corresponding surfactant solution. Before taking spectra baseline correction was done with the surfactant solution of same concentration.
Fig. 2 — Surface tension versus log [surfactant] plots for G5/conventional mixed surfactant systems.
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evaluated with the help of the Gibbs adsorption equation17-19.
Results and Discussion Critical micelle concentration
The surfactant concentration at which monomers begin to assemble in ordered, colloidal aggregates or micelle is termed as critical micelle concentration (CMC). Micelle formation is a process of compromising of two opposite effects: hydrophobic effect, to remove alkyl chains from the contact with water and the effect of columbic repulsion of head groups. The CMC values of pure as well as of equimolar binary surfactant mixtures (CMCexp) were evaluated on the basis of tensiometric measurements. Surface tension is reduced as the concentration of the surfactant increases in the aqueous solution. From Figs 1(a), 1(b) and 2, it is observed that on initial addition of surfactant or at low concentrations, surfactant molecules adsorb at the liquid/air interface, and thus surface tension of system reduces with each increment of surfactant concentration. On further addition, the surface becomes saturate and excess molecules starts to form aggregate in the solution to form micelles, and thus surface tension becomes constant. This behavioural change is easily visible in plots at the point where sudden change in slope of the curve is observed. The CMC values were determined by observing these inflections in the surface tension (γ) versus logarithm of surfactant concentration plots. In Table 1 the observed values of CMCs of used pure surfactants are compared with literature values, and that of mixed surfactants are compared with ideal values. Surface properties
The surface properties Γmax (the maximum surface excess) and Amin (the minimum surface area per molecule) of individual as well as equimolar binssary surfactant systems were determined. The adsorption efficacy of selected surfactants and their mixtures at the air/solution interface were
Γmax =
1 2.303 × n × R × T
dy d log x Γ, P
… (1)
where, γ is surface tension of the solution, R is the universal gas constant (8.314 JK−1 mol−1), T is temperature in the absolute scale, x is concentration of the surfactant in solution, and ‘n’ is a constant, which depends on the number of species constituting the surfactant. dγ The factor was obtained from the slopes of d log x the plots of surface tension vs. log [surfactant] (Figs. 1(a), 1(b) and 2). Γmax values were used to calculate the minimum area per surfactant molecule (Amin) at the air/water interface using the equation: Amin =
1 N × Γ max
… (2)
where, N = Avogadro’s Number As observed from the Table 1, the minimum area per surfactant molecule was found to be minimum for Brij 35, and maximum for SDS. Surfactant – surfactant interaction
To determine, whether the binary systems follows ideal or nonideal behavior, the experimental CMC values of equimolar binary surfactant systems were compared with ideal CMC values. The CMC ideal values were calculated using Clint equation24:
α1 α2 1 = + CMCideal CMC1 CMC2
… (3)
where CMC1, CMC2, α1 and α2 are the critical micelle concentrations and the mole fractions of component 1 and 2 in mixed surfactant solutions. In Table 1, it is
Table 1 ― Experimental CMC (literature values are given in parenthesis) and ideal CMC values of surfactants, the maximum surface excess (Γ max ) , the minimum surface area per molecule (Amin) values at 30°C.
System CTAB Brij35 SDS G5 G5-CTAB G5-Brij35 G5-SDS
CMCexp (mM) 0.67600 (0.776 20) 0.00363 (0.030 21) 7.92000 (8.000 22 ) 0.03160 (0.036 23) 0.01380 0.00446 0.02750
CMCideal (mM)
Γ max * 107 (mol/m2)
Amin (m2/mol)
0.03018 0.00325 0.03147
27.6260 31.9085 22.58 13.8091 7.9760 7.5595 12.7730
60.0991 52.0331 73.36 120.2319 208.1597 219.6291 129.9847
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observed that all CMCexp values were less than CMCideal, as predicted by above equation which shows that the formation of mixed micelles exhibits a negative deviation with respect to ideal mixture. It was also observed that CMCs of ionic surfactants are much higher than nonionic surfactant. This fact can be justified as nonionic surfactant molecules show hydrophobic interaction among hydrocarbon chains, which are easily separated from the aqueous environment, whereas ionic surfactants requires higher concentrations to overcome the electrostatic repulsion between ionic head groups while aggregating25. Moreover it was also observed that the CMCexp values of binary systems are lower than their corresponding ideal values, which indicate synergistic interaction in all mixed systems. The deviation of CMCexp values for mixed surfactant systems from CMCideal can be measured by evaluating the interaction parameter, β. It is a parameter that informs whether the attractive interaction of micelles is synergistic or antagonistic. When the interaction of mixed micelle is concerned Rubingh’s model26 was used, and to analyze the interaction between the amphiphiles in a mixed surfactant system at air/water interface, Rosen model27 analogous to Rubingh’s model, was applied. Interaction parameter can be calculated as β (Rubingh’s equation26) / β σ (Rosen model27):
β=
ln(CMC12α1 / CMC1 X 1m ) ln(CMC12α 2 / CMC2 X 2m ) = (1 − X 1m ) 2 (1 − X 2m ) 2
… (4)
βσ =
ln(C12α1 / C1 X 1σ ) ln(C12α 2 / C2 X 2σ = (1 − X 1σ ) 2 (1 − X 2σ ) 2
… (5)
where X 1m , X 2m are the micellar mole fraction of surfactant 1 and 2 in the mixed micelles, X 1σ , and X 2σ are mole fraction of surfactant at interface, CMC1,
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CMC2 and CMC12 are critical micelle concentrations of surfactants 1 and 2, and CMC of mixed surfactant system, consisting of surfactant 1 and 2, both respectively, C12, C1 and C2 are the concentration of mixture at a fixed surface tension value and the concentrations of individual surfactants at a fixed surface tension value and, α1, and α2 are their corresponding bulk mole fractions. The values of β , β σ , X m and X σ are presented in Table 2. Positive value of β signifies antagonism between components of surfactant combination whereas a negative value of β shows negative deviation of CMCexp from CMCideal, which indicates a reduction in free energy of micellization over that predicted by the ideal solution theory28, which implies good interaction (i.e. synergistic effect). β values for all systems as negative, which indicate good interaction between the components of mixed system and demonstrate synergistic effect for all binary equimolar mixed surfactant systems. The order of deviation exhibited through β is G5-SDS > G5-CTAB > G5-Brij35. The results were supported by the reported results of Sheikh, et al.29. The strongest synergism effect is found between cationic gemini and anionic conventional surfactant. The reason behind this might be the attractive forces between oppositely charged head groups. The least value was for cationic gemini and n onionic conventional surfactant, as Brij35 has polyoxyethylene (POE) groups with large number of oxygen atoms and a lone pair of electron, thus it may have a tendency to react coulombically with cationic gemini surfactant, but the existence of long polyoxyethylene head group imposes some steric constraints due to thermal vibrations, which causes control on effective head group interactions and give reason to reduce the value of β20. The micellar mole fraction X 1m was calculated with the help of following equation for non-ideal binary mixture of surfactants by solving iteratively26:
Table 2 ― Micellar mole fraction X 1m , interaction parameter (β), activity coefficients (f1 and f2) valuesa, Surface composition at air/water interface ( X 1σ ) , interaction parameter (βσ ), activity coefficients (ƒ1σ and ƒ2σ ) valuesb for gemini/conventional mixed surfactant systems at 30°C. System
X 1m
β
ƒ1
ƒ2
∆Gex (KJ/mol)
G5-CTAB 0.6475 – 8.3776 0.3531 0.0298 –4.8169 G5-Brij35 0.2672 – 2.4793 0.2641 0.8377 –1.2229 G5-SDS 0.7515 – 8.8488 0.5790 0.0067 –4.1628 a Values of X 1m , β, ƒ1, ƒ2 were evaluated using Rubingh’s model. b Values of X 1σ , β , f1σ , f1σ were evaluated using Rosen model.
X 1σ
βσ
ƒ1σ
ƒ2σ
∆Gex (KJ/mol)
0.5 0.5 0.5
8.0075 5.0531 11.314
7.4030 3.5370 16.9212
7.4030 3.5370 16.9212
5.0430 3.1823 16.9212
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CMC12α1 X 1m ln m CMC1 X 1 2 ln[CMC12 (1 − α1 ) / CMC2 (1 − X 1m )]
(1 − X ) m 1
… (6)
Comparable to above equation Rosen model offer calculation of mole fraction of surfactant 1 at the mixed adsorbed film as27: 2 C α X1σ ln 12 σ1 C X 1 1
Similarly activity coefficients calculated by Rosen Model is : f1σ = exp{β σ (1 − X σ ) 2 }& f 2σ = exp{β σ ( X σ ) 2 } ... (10 & 11) Table 2 shows values of excess free energy of micellization, ∆Gex as negative, which represents higher stability of mixed micelles. Solubilization by surfactants
Values of f1 & f2 lower than unity indicates non-ideal behavior of system.
To determine the extent of solubilization of PAH in surfactant, absorbance of PAH in surfactant solution (of known concentration) is checked with the help of spectrophotometer. Graphs of the solubility of a poorly soluble PHE and FLR as a function of the concentration of surfactant are plotted in Figs. 3(a), 3(b) and 4, both plots show that on increasing surfactant concentration, concentration of dissolved PAH is also increasing, or solubility of PAH increases linearly with the increasing surfactant concentrations above CMC30. This behavior indicates that solubilization is related to micellization. Though, the reduced CMC value does not absolutely represent the
Fig. 3 ― Variation of solubility of (a) PHE and (b) FLR with pure surfactant concentration
Fig. 4 ― Variation of solubility of (a) PHE and (b) FLR with G5 concentration in 1:1 binary surfactant systems
( )
2 C (1 − α1 ) 1 − X1σ ln 12 C2 1 − X σ 1
(
)
(
=1
… (7)
)
Another parameter, activity coefficients f1 and f2 within the mixed micelles derived from Rubingh equations was equated as26: f1 = exp{β (1 − X m ) 2 } & f 2 = exp{β ( X m ) 2 } … (8 & 9)
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Table 3 ― Molar solubilization ratio (MSR), log Km , and the free energy of solubilization (∆G0s) values for PHE and FLR solubilized in individual and mixed surfactant systems at 30°C PHE System CTAB Brij35 SDS G5 G5-CTAB G5-Brij35 G5-SDS
MSR * 105 12.00 2.60 0.30 4.20 5.50 5.0 25.00
Log Km 3.069 2.4107 1.4737 2.6169 2.7327 2.7235 3.3873
FLR ∆Gso (KJ/mol) –17.81 –13.99 –0.855 –15.18 –15.85 –15.81 –19.65
increased solubilization ability. Thus, water solubility enhancement of PHE and FLR by selected single and equimolar binary surfactant systems was further to evaluate and compare. Solubilization by single surfactants
A measure of the effectiveness of a surfactant in solubilizing a given solubilizate is the Molar Solubilization Ratio, (MSR) which is given by20, 31- 34. MSR = {[St] – [Scmc]} / {Ct – CMC} … (12) where Scmc and St are the solubilities at CMC and at total surfactant concentration Ct respectively. Since (Ct – CMC) was the concentration of the surfactant in the micellar form, MSR was equal to the ratio of solubilizate concentration in the micelles to the concentration of surfactant in the form of micelles. Value of MSR is obtained from the slope of solubilizate concentration versus surfactant concentration plot. In the presence of excess PAH, MSR values of both single and mixed surfactants can be obtained from the slope of the linearly fitted line in\which the concentration of PAH was plotted against surfactant concentration above the CMC (both the concentrations were in mM) as given in Figs 3(a), 3(b) and 4. On increasing surfactant concentration, PAH concentration is also risen up, which provides positive value of MSR. The effectiveness of solubilization can also be expressed with the help of the partition coefficient Km17,35 which is defined as distribution of the mole fraction of PHE between surfactant micelles and the aqueous phase. It may be calculated as35: Km= Xm / Xa
… (13)
where Xm and Xa are the mole fraction PAH in micelle phase and mole fraction PAH in aqueous phase. Substituting the values of (= MSR/ (1 + MSR)) and Xa (=SCMC VW ) Km expression can be rearranged as:
of of Xm the
Km =
MSR * 105 13.8 38.2 1.07 15.6 2.01 10.57 22.44
MSR (1 + MSR )VW SCMC
Log Km 2.8539 3.2965 1.7423 2.94 2.0174 2.7384 3.0652
∆Gso (KJ/mol) –16.56 –19.13 –10.11 –16.86 –11.70 –15.89 –17.78
… (14)
where SCMC is the total apparent solubility of the solute at CMC and VW is the molar volume of water (1.807 × 10-2 L/mol at 30°C). The order of solubilizing power for organic solutes by inner nonpolar core of micelles has been reported as nonionic > cationic > anionic surfactant having same nonpolar chain length29,36,37. For the case of FLR the presented data support these findings. The difference in solubilization capabilities of surfactant is because of their different structures. Higher solubilization power of Brij35 than G5 and SDS may be due to its larger micellar size helping in more micellar core solubilization26. The cationic surfactant exhibited lower MSR than nonionic due to limited solubilization at micelle/water interface and core of micelle as given in Table 3. Solubilization by equimolar binary mixed surfactant systems
MSR and log Km values of cationic-nonionic were found higher than for cationic-cationic mixed surfactant solutions as previously reported by Wei et al.37. This is because nonionic micelle processing higher micellar core solubilization characteristic interpolates to the micelle-water interface producing greater solubilization power towards PAHs. In addition, the solubilization of PHE and FLR by mixed gemini/nonionic surfactant solutions is also higher than those in single nonionic surfactant as given in Table 3. Conclusion The experimental results obtained in the present study may be useful for the selection of appropriate mixed surfactant systems. This study would also facilitate the design and optimization of new surfactant systems for their better performance. The results are as follows;
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2
3
The mixed micelles of gemini surfactant G5 with all the conventional surfactants i.e. Brij35, CTAB and SDS are studied. Highest attracting interaction in mixed micelle formation is observed in G5-SDS and lowest attracting interaction for G5-Brij35, which indicates good synergism in mixed micelles. Rosen’s approach tells antagonistic effect in the mixed monolayer in comparison to the mixed micelles in all systems. The values of ∆G ex are negative for all mixed systems, demonstrating the stability of the micelles.
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