Solubilization of Phenanthrene and Fluorene in Equimolar Binary Mixtures of Gemini/Conventional Surfactants

Huma Siddiqui,Mohammad Kamil,*,Manorama Panda,Kabir-ud-Din

Chemical Engineering Thermodynamics

Solubilization of Phenanthrene and Fluorene in Equimolar Binary Mixtures of Gemini/Conventional Surfactants

Huma Siddiqui1,Mohammad Kamil1,*,Manorama Panda2,Kabir-ud-Din2

1Department of Petroleum Studies,Aligarh Muslim University,Aligarh 202002,India2Department of Chemistry,Aligarh Muslim University,Aligarh 202002,India

A R T I C L EI N F O

Article history:

Gemini surfactant

Mixed micelles

Critical micelle concentration

Solubilization

Molar solubilization ratio

This study deals with the enhanced solubilization of polycyclic aromatic hydrocarbons(PAHs)such as phenanthrene(PHE)andf l uorene(FLR)ina purecationicgemini(G6)andthreeconventionalsurfactants[polyethylene glycol dodecyl ether(Brij35),cetyltrimethyl ammonium bromide(CTAB)and sodium lauryl sulfate(SDS)]as well as in their equimolar binary combinations(G6-Brij35,G6-CTAB and G6-SDS).Their solubilization eff i ciency toward PHE and FLR has been quantif i ed in terms of the molar solubilization ratio(MSR)and the micelle-water partition coeff i cient(Km).The ideality/nonideality of the mixed micelles is discussed with the help of Clint, RubinghandRosen'sapproaches.Thesetheoriesdeterminethedeviationofexperimentalcriticalmicelleconcentration(CMC)valuesfromidealcriticalmicelleconcentration,whichwasmeasuredby evaluatingtheinteraction parameters(βmand βσ).Negative values of βmwere observed in all the equimolar binary systems,which show synergisminthemixedmicelles.Whereasatair/liquidinterfacesynergismwasobservedinthesystemsG6-CTAB and G6-Brij35;G6-SDS exhibited an antagonistic effect.The order of MSR and Kmwas G6-CTAB＞G6-Brij35＞G6-SDS for phenanthrene as well as for f l uorene.

?2014TheChemicalIndustry andEngineeringSocietyofChina,andChemicalIndustryPress.Allrightsreserved.

1.Introduction

Contamination of environment by petroleum hydrocarbons has been a subject of concern.These compounds are only sparingly soluble in water and form separate phases in the soil matrix,which also becomes a route to enter in water cycle and causes water pollution [1].Due to their toxic,carcinogenic and mutagenic effects,and also because of their ubiquity in the environment(water,air and soils)[2], strict legal controls are imposed to regulate production,use and emission of polycyclic aromatic hydrocarbons(PAHs)[3].US environmental protection agency(EPA),in 1984,recognized surfactant washingasone of the most capable techniques for the removal of PAH compounds [4].Thus application of surfactants forremediation of petroleumhydrocarbons has become a topic of interest[5-7].PAHs,also known as polynuclear aromatic hydrocarbons,consist of fused aromatic rings and do not contain heteroatoms.These compounds are found in oil,coal tar, soil and water in objectionable quantities.PAHs are hydrophobic and can be removed from the contaminated soils,ground water and hydrocarbonfuelbysurfactantsmainlyduetothesolubilizationormovement of the chemicals inside micelles.The solubility of organic substances rises with their merger into micelles of surfactant aqueous solutions. This phenomenon is called micellar solubilization[8,9].

In petroleum industry,surfactants may be applied at all stages in the petroleum recovery and processing,from oil well drilling,reservoir injection,oil well production to pipeline and seagoing transportation of petroleum emulsion[10].Moreover,petroleum fuels emit a mixture of toxicandhazardoussubstancesoncombustion,whichmayhavecarbon monoxide,lightvolatileorganiccompounds,semi-volatileorganiccompounds,particulate emissions,oxides of sulfur,oxides of nitrogen and PAHs.

In the present work the solubility enhancement of two PAH compounds[phenanthrene(PHE)and f l uorene(FLR)]in different single and mixed surfactant systems was investigated.Phenanthrene(PHE) is def i ned as the simplest PAH by International Union of Pure and Applied Chemistry(IUPAC).Being as the nearest neighbor of phenanthrene,f l uorene is chosen as another PAH to understand the competitive effect between the two PAHs.The surfactants selected were of differentpolarities(cationic,anionic and non-ionic).A mixedsurfactant system represents here an equimolar binary mixture of conventional surfactant with a cationic gemini surfactant.The theories of Clint[11], Rubingh[12]and Rosen[13]were used to analyze and compare the experimental results for formation and characterization of mixed micelles.These models allow predicting and realizing the synergism or antagonism of mixed surfactant systems.Apart from single tailsingle head conventional surfactants,another class of surfactants used in the present work is the gemini surfactant.These surfactants have a quite interesting structure having two hydrophilic heads,which are joined with two hydrophobic tails,separated by a covalently bondedrigid or f l exible spacer[14-16].This class of surfactants exhibits low criticalmicelleconcentration(CMC)values,whichmakethemaleading member of the surfactant family and show a good potential for micellar solubilization.A schematic representation of the gemini surfactants is given in Fig.1.

Gemini surfactants are signif i cantly more surface-active than the conventional surfactants.Properties of gemini surfactants may change according to change in its constituents,i.e.hydrophilic group,hydrophobic group and linkage[17].It has been found that the solubilizing capacity of gemini surfactants is much better than that of the conventional ones[18].They possess relatively lower CMC values than the conventional surfactants.

A gemini surfactant molecule with two Cm(m is the number of alkyl carbon atoms)tails and a Cs(s is the number of alkyl carbon atoms)spacer separating the quaternary nitrogen atoms is represented as m-s-m.For example,the dimeric gemini butane diyl-α,ω-bis (dimethylcetylammonium bromide),i.e.C16H33-(CH3)2N+-(CH2)4-N+(CH3)2-C16H33·2Br?is 16-4-16.Some specif i c characteristics of the gemini surfactants are:

(1)Their CMC values are remarkably low than corresponding conventional surfactants having an equivalent chain length.

(2)For geminis having a short spacer(2-8 atoms),the CMC values are not affected by polarity.

(3)A long spacer of gemini molecule provides more hydrophobicity to the surfactant,which reduces the molecule's solubility and thus enhances its tendency of aggregation.

(4)ExtentofreductionofCMCvaluesduetoanincreaseintaillength is more visible in geminis than in conventional surfactants.

(5)Anionic geminis exhibit lower CMC values than corresponding cationic geminis.

Fig.1.Schematic representation of a gemini surfactant.

2.Material Used and Methods

2.1.Materials used

For thepresentstudy,equimolarbinarysurfactantblends of cationic gemini surfactants with cationic,anionic and non-ionic conventional surfactants were examined.Here,cetyltrimethyl ammonium bromide (CTAB)wasemployed asa cationic surfactant,whereas sodium dodecyl sulfateorsodiumlaurylsulfate(SDS)wasselectedfromtheanionicsurfactant family and polyethylene glycol dodecyl ether(Brij35)as a nonionic surfactant.All surfactants used in the present study were supplied by Sigma Aldrich Chemicals Co.except the gemini,i.e.,hexanediyl-1,6-bis(dimethylcetylammonium bromide),or G6.This gemini surfactant was synthesized in the research laboratory of the Department of Chemistry,AMU,Aligarh.The structures of surfactants and PAHs used are given in Fig.2.

Fig.2.Structuresofchemicalsusedinthestudy:polyethyleneglycoldodecylether(Brij35),cetyltrimethylammoniumbromide(CTAB),sodiumlaurylsulfate(SDS),geminisurfactant(16-6-16),phenanthrene(PHE)and f l uorine(FLR).

Table 1Physical properties of substances used in the present work

Phenanthreneandf l uorenewereusedasthepolycyclicaromatichydrocarbons in the presentwork,which were procured by SigmaAldrich Chemicals Co.Surfactant solutions were prepared in double-distilled water.Physical properties of substances used in the present work are given in Table 1.

2.2.Methods

2.2.1.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),whichworksonringdetachmentmethod.Thesolutionunderinvestigation was taken in a clean 100 ml borosil dish and the ring was made to submerge completely into the surfactant solution.

Whenthesystembecamestable,theknobstartedtorotateslowlyto apply a uniform force on the ring.The force on the ring was increased slightly to raise the ring toward the solution surface by rotating the knob.When the liquid f i lm tears from the ring,the knob rotation was stopped,and the reading on the scale was noted.Unit of this value is in mN?m?1with accuracy of±0.05 mN·m?1.The surfactant concentrations were increased slowly by adding concentrated surfactant stock solution in small installments using a Hamilton Microsyringe of capacity 0.1 ml.Readings were noted after thorough mixing and temperature equilibration with a time interval of about 15 min.The values of CMC were determined as the concentration at sharp breaks in the plot of surface tension(γ)versus the log[surfactant]over a wide concentration range(Figs.3 and 4).The experiments were repeated twice for each surfactant to ensure reproducibility of the results.

2.2.2.Solubilization experiments

After determining CMC,the solubility of PAH compounds was measured in different surfactant solutions between ranges of concentrations above the CMC.The solubility of PAHs in surfactant system was determined by solubilization experiments as detailed herein.The solutions of surfactants and their 1:1 mixtures were prepared of concentrations higher than their corresponding CMC.3 ml of these micellar solutions was then f i lled in borosilicate screwcapped,glass vials having a capacity of 5 ml.Then PAH was added in excess amount to these screw-capped vials(the extra amount of PAH was added to ensure maximum solubility in each surfactant solution).To ensure a good mixing magnetic Tef l on pieces were dropped in each vial,which were then agitated using magnetic stirrer for a period of 24 h at 30°C.After this,a portion of the samples was collected in Eppendorf tubes and centrifuged at 12,000 rpm, using a high speed microcentrifuge(Remi centrifuge,RM-12C)to remove or settle down the undissolved PAH.Appropriate dilution of the sample of the supernatant was made with the corresponding surfactant solution and then the concentration of the solubilized PAH of centrifuged sample was determined spectrophotometrically using Shimadzu spectrophotometer(model UV mini-1240).Baseline correction was done with the surfactant solution of the same concentration.

Themolarextinctioncoeff i cientofPHEandFLRwasdeterminedwith the use of Lambert-Beer law,taking the absorbance of their solutions at the desired wavelength in methanol.From the slope of absorbance versus surfactant concentration plots the molar extinction coeff i cients of PHE and FLR were found to be 61,380 L·mol?1cm?1and 11,490 L·mol?1cm?1,respectively.The solubility of PHE and FLR at each surfactant concentration was determined at their characteristic wavelengths of 250 nm and 262 nm,respectively.

Fig.3.(a)Surface tension versus lg[surfactant]plots for pure(a)CTAB and SDS,and(b)Brij35 and G6.

Fig.4.Surface tension versus lg[surfactant]plots for G6/conventional mixed surfactant systems.

3.Results and Discussion

3.1.Critical micelle concentration

In the present study,the main focus has been on the solubilization of PHE and FLR in single surfactants(cationic gemini and conventional surfactants)and equimolar mixtures of cationic gemini and conventional surfactants.Solubilization is closely related to the solution properties of the surfactant micelles.Moreover,the performances of the mixed surfactant systems are complex in nature and mostly follow a non-ideal path.

Hence,micellar properties of the selected single surfactants and their equimolar combinations were studied to understand their solubilization abilities.The experimentally obtained CMC values(CMCexp),evaluated fromtheplotsofsurfacetension(γ)versuslogarithmvaluesofsurfactant concentration(Table 2),are in good agreement with the respective reportedvalues(CMClit).FromTable2,itisalsoclearthatCMCsofionicsurfactants are much higher than the nonionic surfactant.The nonionic surfactantmoleculesshowonlyahydrophobicinteractionamonghydrocarbonchains,whichareeasilyseparatedfromtheaqueousenvironment, whereas ionic surfactants require,in addition,higher concentrations to overcome the electrostatic repulsion between ionic head groups while aggregating[19].Moreover,it was also observed that the CMCexpvalues of the binary systems are lower than the corresponding ideal values, which indicates synergistic interaction in all the mixed systems.

To determine whether the binary systems follow ideal or nonideal behavior,the experimental CMC values of equimolar binary surfactant systems were compared with ideal CMC values,which were calculated using the Clint equation[11]:

Table 2Experimental and literature CMC values of surfactants

where CMC1,CMC2,α1and α2are the critical micelle concentrations and the mole fractions of components 1 and 2 in the pure surfactant solutions.In Table 2,it is observed that all the CMCexpvalues are less than CMCideal,as predicted by the above equation,which shows that the formation of mixed micelles exhibits a negative deviation with respect to the ideal mixture.

3.2.Surfactant-surfactant interaction

3.2.1.Rubingh model

Inthelightoftheregularsolutiontheory,deviationofCMCexpvalues, for mixed surfactant systems,from CMCidealcan be measured by evaluating the interaction parameter,βm.This parameter can be calculated with the help of Rubingh's equation[12]:

where X1mand X2mare the micellar mole fractions of surfactants 1 and 2 in the mixed micelles,CMC12is a critical micelle concentration of a mixed surfactant system,consisting of surfactants 1 and 2.The micellar mole fraction X1mwascalculated with the help of thefollowingequation for nonideal binary mixture of surfactants by solving iteratively.

A negative value of βmshows a negative deviation of CMCexpfrom CMCideal,which indicates a reduction in free energy of micellization over that predicted by the ideal solution theory[12].This implies a good interaction between the surfactants in mixed systems.A positive value of βmsignif i es antagonism between components of surfactant combination.The parameter activity coeff i cients(f1mand f2m)within the mixed micelles,derived from Rubingh equations,were equated as [12]:

The values of βm,along with X1m,f1mand f2m,for the selected surfactants are given in Table 3.

The negative βmvalues in Table 3 indicate a good interaction between the components of mixed systems and demonstrate a synergistic effect for all the binary equimolar mixed surfactant systems.The larger negative value of βmdenotes the greater negative deviation of CMCexp's from CMCideal.The order of deviation exhibited through of βmis G6-SDS＞G6-CTAB＞G6-Brij35.The results are supported by the reported values of Kabir-ud-Din et al.[19]and Rao and Paria[24].The strongest synergism is found between cationic gemini and anionic conventionalsurfactant.ThereasonbehindthismightbetheCoulombic attractive forces between theoppositely charged headgroups.Theleast value was for the mixture of the cationic gemini and nonionic conventional surfactant,as Brij35 has polyoxyethylene(POE)groups with alarge number of oxygen atoms and a lone pair of electron,thus it may have a tendency to react Coulombically with thecationic gemini surfactant,but the existence of long polyoxyethylene head group imposes somestericconstraintsduetothermalvibrations,whichcausesthecontrol on effective head group interactions and gives a reason to reduce the value of βm[20].

Table 3Micellar mole fraction(X1m),interaction parameter(βm),and activity coeff i cient(f1mand f2m)values for gemini/conventional mixed surfactant systems at 30°C

3.2.2.Rosen model

Rubingh's model deals with the interaction in the mixed micelle formation.To analyze the interaction between the amphiphiles in a mixed surfactant system at air/water interface,Rosen model[25]was used.According to this model the mole fraction of surfactant 1 at the mixed adsorbed f i lm can be calculated iteratively as:

where C12,C1and C2are the concentrations of the mixture and of individual surfactants at a f i xed surface tension value.From this expression thevalueofXσwasobtained,whichwasthenusedtoevaluatetheinteraction parameter βσat air/water interface,with the help of the following equation: The activity coeff i cients within the mixed micelles(f1σand f2σ), whereσsignif i escorrespondingvaluesforRosenmodel,werecalculated through the Rosen approach with the help of interaction parameters as given below

From Table 4,it is observed that G6-Brij35 and G6-CTAB exhibit a negative value of the interaction parameter,which shows a synergistic effect between components of mixed surfactant systems.A negative value of ΔGexσshows spontaneity of the systems.The G6-SDS system possesses an antagonistic effect between the surfactants at air/solution interface.The strength of the interaction of mixtures containing two surfactants not only depends on the variation of their CMC values,but also relies on the relevant properties of their structure.

3.3.Solubilization by surfactants

Beforeexaminingthesolubilizationpowerofbinarymixtures,single systems were f i rst studied to get an idea about the eff i ciency of gemini in comparison with conventional surfactants.Plots of the solubility of poorly soluble PHE and FLR,as a function of the concentration of surfactant(Figs.5,6 and 7)show that the solubility increases linearly with the increasingsurfactant concentrationsabove CMC.This behavior indicatesthatsolubilizationisrelatedtomicellization.ThereducedCMC value does not absolutely represent the increased solubilization ability. The water solubilityenhancementof PHEand FLRby the selectedsingle and equimolar binary surfactant systems was evaluated and compared which are detailed below.

3.3.1.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 by [18,20,26-29].

Fig.5.Variation of solubility of PHE with surfactant concentration.

Table 4Surface composition at air/water interface(X1σ),interaction parameter(βσ)and activity coeff i cient(f1σand f2σ)values for gemini-conventional mixed surfactant systems at 30°C

Fig.6.Variation of solubility of FLR with surfactant concentration.

where SCMCand Stare the solubilities at CMC and at total surfactant concentration(Ct),respectively.Since(Ct?CCMC)is the concentration of the surfactant in the micellar form,MSR is equal to the ratio of the solubilizateconcentrationin themicelles totheconcentration of surfactantintheformofmicelles.ValuesofMSRwereobtainedfromtheslope of solubilizate concentration versus surfactant concentration plots.

In the presence of excess PAH,MSR values of both single and mixed surfactants can be obtained from the slope of the linearly f i tted line in which the concentration of PAH was plotted against the surfactantconcentration above theCMC(both theconcentrationswere in mmol·L?1)as given in Figs.5,6 and 7.The effectiveness of solubilization can also be expressed with the help of the partition coeff i cient Km[30,31], which is def i ned as distribution of the mole fraction of PAH between surfactant micelles and the aqueous phase.It may be calculated as[30]:

where Xmand Xaare the mole fractions of PAH in the micelle phase and mole fraction of PAH in the aqueous phase.The quantity Xmcan be expressed in terms of MSR as: Mole fraction of the solute in the aqueous phase was approximated for dilute solution by:

whereSCMCisthetotalapparentsolubilityofthesoluteatCMCandVWis the molar volume of water(1.807×10?2L·mol?1at 30°C).Thus,the Kmexpression can be rearranged as:

As can be observed from Tables 5 and 6,the MSR and Kmvalues are highest for cationic surfactant and lowest for anionic and follow the order CTAB＞Brij35＞G6＞SDS for PHE,whereas for FLR the order is found to be as Brij35＞G6＞CTAB＞SDS.The order of solubilizing power for organic solutes by inner nonpolar core of micelles has been reported to be a nonionic＞cationic＞anionic surfactant having the samenonpolarchainlength[19,31,32].ForthecaseofFLR,ourobserved data support these f i ndings.The difference in solubilization capabilities of the surfactants is because of their different structures.Higher solubilization power of Brij35 than G6 and SDS may be due to its larger micellar size helping in more micellar core solubilization[12].

3.3.2.Solubilization by equimolar binary mixed surfactant systems

When MSR values were compared for all the mixed systems,the order was found as G6-CTAB＞G6-Brij35＞G6-SDS for both the PAHs. In the interest of ascertaining the mixing effect of surfactants on solubilization of PAHs and seeing the nature of deviation,the deviation ratio (R)between MSRexpand MSRidealcan be determined by the following equation[31,33]:

Fig.7.Variation of solubility of PAHs with G6 concentration in 1:1 binary surfactant solutions.

Table 5Molar solubilization ratio(MSR),lg Km,the free energy of solubilization(ΔG0s),and R and B values for PHE solubilized in individual and mixed surfactant systems at 30°C

MSRidealis the MSR for organic compounds in mixed surfactant system at the ideal mixed state and can be estimated using the MSR of single surfactant solutions based on the ideal mixing rule:

Table 6Molar solubilization ratio(MSR),lg Km,the free energy of solubilization(ΔG0s),and R and B values for FLR solubilized in individual and mixed surfactant systems at 30°C

where α1,α2,MSR1and MSR2are the mole fraction and the molar solubilization ratio for solute of components 1 and 2 in mixed surfactant solutions,respectively.The data of parameter R from Tables 5 and 6 obviously indicate that the MSRexpvalues have a positive deviation from an ideal mixture for the gemini/nonionic and gemini/anionic surfactant systems meaning that they have a positive mixing effect on the solubilization of PHE.However,the opposite results were found in the gemini/cationic surfactant systems.For the case of FLR,only gemini/anionic system exhibits a positive deviation from the ideal mixture. Another parameter,Km12,the partition coeff i cient of a neutral organicsolute between the micelles and aqueous phase in a mixed surfactant, has been used by Treiner et al.[34].This parameter provides better understanding of the mixing effect of mixed surfactant systems on solubilization of solutes.This partition coeff i cient's expression is based on the regular solution approximation as follows:

where Km1and Km2are the micelle-water partition coeff i cients of the individual surfactants constituting the mixed micelles,and X1mrepresents the micellar mole fraction of a surfactant having the value of Km1.B is an empirical parameter involving both the surfactantsurfactant and surfactant-solute interactions.If the value of B becomes 0 it means that there would be no mixing effect on partitioning of a solute between the aqueous and micellar phases[31,35]whereas for B＞0(or＜0),it implies that Km12in the mixed surfactant system is larger(or smaller)than that predicted by the ideal mixing rule[31, 35].As presented in Tables 5 and 6,the B values are foundto be positive for all the equimolar binary surfactant solutions except for FLR in the G6-Brij35 mixed systems.

3.4.Thermodynamics of solubilization

From the thermodynamic point of view,solubilization can be considered asnormal partitioningof thePAH betweentwophases,micellar and aqueous,and the standard free energy of solubilization,ΔGS0,can be represented by the expression[36] where R,T and Kmare the universal gas constant,the absolute temperature and the molar partition coeff i cient between the micellar and the aqueous phases,respectively.The ΔGS0values thus calculated are presented in Tables 5 and 6.For all the systems,the ΔGS0values come out to be negative indicating spontaneity of the solubilization process.

4.Conclusions

For all the equimolar binary surfactant solutions the interaction parameter βmis found to be negative and synergism is observed in properties like CMC,surface tension,and solubilization.The interfacial parameters like ΔGex,βσshow synergism for the systems G6-CTAB and G6-Brij58.Micellar solubilization is a good method of choice for the dissolution of hydrophobic organic contaminants in aqueous environments which depends on the hydrophobicity,hydrophilicity and charge of the surfactant.The gemini surfactant(G6),used in this study,has a lower CMC than the conventional ones and shows an excellent solubilization toward PHE and FLR due to a more micellar core solubilization.The results obtained during the investigation show that irrespective of the surfactant type the solubility of PAHs increases linearly with the increase of surfactant concentration,for the pure as well as the mixed surfactant systems.For the binary combinations of gemini with conventional surfactants,the enhancement of solubilization of PAHs in G6-SDS system is the lowest for both the PAHs;the order of MSR(or Km)values is G6-CTAB＞G6-Brij35＞G6-SDS.

[1]J.D.Rouse,T.Morita,K.Furukawa,B.-J.Shiau,Solubilization of mixed polycyclic aromatichydrocarbonsystemsusingananionicsurfactant,ColloidsSurf.APhysicochem. Eng.Asp.325(2008)180-185.

[2]J.Santodonato,Review of the estrogenic and antiestrogenic activity of polycyclic aromatic hydrocarbons:Relationship to carcinogenicity,Chemosphere 34(1997) 835-848.

[3]F.P.Koltalo,K.Oukebdane,L.Robin,F.Dionnet,P.L.Desbene,Quantif i cation of volatile PAHs present at trace levels in air f l ow by aqueous trapping—SPE and HPLC analysis wit f l uorometric detection,Talanta 71(2007)1825-1833.

[4]D.Grasso,K.Subramaniam,J.J.Pignatello,Y.Yang,D.Ratte,Micellar desorption of polynuclear aromatic hydrocarbons from contaminated soil,Colloids Surf.A Physicochem.Eng.Asp.194(2001)65-74.

[5]J.H.Harwell,in:D.A.Sabatini,R.C.Knox(Eds.),Transport and Remediation of Subsurface Contaminants,ACS Symposium Series,vol.491,American Chemical Society, Washington,DC,1992,pp.124-132.

[6]J.C.Fountain,in:D.A.Sabatini,R.C.Knox(Eds.),Transport and Remediation of Subsurface Contaminants,ACS Symposium Series,vol.491,American Chemical Society,Washington,DC,1992,pp.182-191.

[7]B.-J.Shiau,J.D.Rouse,D.A.Sabatini,J.H.Harwell,in:D.A.Sabatini,R.C.Knox,J.H. Harwell(Eds.),Surfactant-Enhanced Subsurface Remediation:Emerging Technologies,ACS Symposium Series,vol.594,American Chemical Society,Washington,DC, 1995,pp.65-79.

[8]M.Almgren,F.Grieser,J.K.Thomas,Dynamic and static aspects of solubilization of neutral arenes in ionic micellar solutions,J.Am.Chem.Soc.101(1979)279-291.

[9]N.Matubayasi,K.K.Liang,M.Nakahara,Free-energy analysis of solubilization in micelle,J.Chem.Phys.124(2006)154908-1-154908-13.

[10]L.L.Schramm,Surfactants:FundamentalsandApplicationsinthePetroleumIndustry, Cambridge University Press,Cambridge,2000.

[11]J.H.Clint,Micellization of mixed nonionic surface active agents,J.Chem.Soc.Faraday Trans.I 71(1975)1327-1334.

[12]D.N.Rubingh,in:K.L.Mittal(Ed.),Solution Chemistry of Surfactants,vol.1,Plenum Press,New York,1979,pp.337-354.

[13]Q.Zhou,M.J.Rosen,Molecular interactions of surfactants in mixed monolayers at the air/aqueous solution interface and in mixed micelles in aqueous media:The regular solution approach,Langmuir 19(2003)4555-4562.

[14]F.M.Menger,C.A.Littau,Gemini surfactants:A new class of self-assembling molecules,J.Am.Chem.Soc.115(1993)10083-10090.

[15]M.J.Rosen,Geminis:A new generation of surfactants,ChemTech 23(1993)30-33.

[16]R.Zana,Dimeric and oligomeric surfactants.Behavior at interfaces and in aqueous solution:A review,Adv.Colloid Interface Sci.97(2002)205-253.

[17]T.A.Camesano,R.Nagarajan,Micelle formation and CMC of gemini surfactants:A thermodynamic model,Colloids Surf.A Physicochem.Eng.Asp.167(2000)165-177.

[18]Kabir-ud-Din,M.Shaf i,P.A.Bhat,A.A.Dar,Solubilization capabilities of mixtures of cationic Gemini surfactant with conventional cationic,nonionic and anionic surfactants towards polycyclic aromatic hydrocarbons,J.Hazard.Mater.167 (2009)575-581.

[19]Kabir-ud-Din,M.S.Sheikh,A.A.Dar,Analysis of mixed micellar and interfacial behavior of cationic gemini hexanediyl-1,6-bis(dimethylcetylammonium bromide) with conventional ionic and nonionic surfactants in aqueous medium,J.Phys.Chem.B 114(2010)6023-6032.

[20]M.Panda,M.S.Sheikh,Kabir-ud-Din,Solubility enhancement of polycyclic aromatic hydrocarbons(PAHs)using synergistically interacting gemini-conventional surfactant systems,Z.Phys.Chem.225(2011)427-439.

[21]A.Patist,S.S.Bhagwat,K.W.Penf i eld,P.Aikens,D.O.Shah,On the measurement of critical micelle concentrations of pure and technical grade non-ionic surfactants, J.Surfactant Deterg.3(2000)53-57.

[22]P.K.Misra,S.Panigrahi,U.Dash,A.B.Mandal,Organization of amphiphiles.Part XI: physico-chemical aspects of mixed micellization involving normal conventional surfactant and a non-ionic gemini surfactant,J.Colloid Interface Sci.345(2010) 392-401.

[23]W.Jiang,B.Xu,Q.Lin,J.Li,F.Liu,X.Zeng,H.Chen,Metal promoted hydrolysis of bis(p-nitrophenyl)phosphate by trivalent manganese complexes with Schiff base ligands in gemini micellar solution,Colloids Surf.A Physicochem.Eng.Asp.315 (2008)103-109.

[24]K.J.Rao,S.Paria,Solubilization of naphthalene in the presence of plant-synthetic mixed surfactant systems,J.Phys.Chem.B 113(2009)474-481.

[25]M.J.Rosen,Surfactant and Interfacial Phenomena,third ed.John Wiley&Sons,2004.

[

26]J.L.Li,B.H.Chen,Solubilization of model polycyclic aromatic hydrocarbons by nonionic surfactants,Chem.Eng.Sci.118(2002)2825-2835.

[27]L.Zhu,S.Feng,Synergistic solubilization of polycyclic aromatic hydrocarbons by mixed anionic-nonionic surfactants,Chemosphere 53(2003)459-467.

[28]O.Zheng,J.-X.Zhao,Solubilization of pyrene in aqueous micellar solutions of gemini surfactants C12-s-C12.2Br,J.Colloid Interface Sci.300(2006)749-754.

[29]S.Paria,P.K.Yuet,Solubilization of naphthalene by pure and mixed surfactants,Ind. Eng.Chem.Res.45(2006)3552-3558.

[30]D.A.Edwards,R.G.Luthy,Z.Liu,Solubilization of polycyclic aromatic hydrocarbons in micellar nonionic surfactant solutions,Environ.Sci.Technol.25(1991)127-133.

[31]J.Wei,G.Huang,H.Yu,C.An,Eff i ciency of single and mixed gemini/conventional micelles on solubilization of phenanthrene,Chem.Eng.J.168(2011)201-207.

[32]S.Saito,Solubilization properties of polymer-surfactant complexes,J.Colloid Interface Sci.24(1967)227-234.

[33]W.Zhou,L.Zhu,Solubilization of polycyclic aromatic hydrocarbons by anionicnonionicmixedsurfactant,ColloidsSurf.APhysicochem.Eng.Asp.255(2005)145-152.

[34]C.Triener,M.Nortz,C.Vaution,Micellar solubilization in strongly interacting binary surfactant systems,Langmuir 6(1990)1211-1216.

[35]A.A.Dar,G.M.Rather,A.R.Das,Mixed micelle formation and solubilization behavior toward polycyclic aromatic hydrocarbons of binary and ternary cationic-nonionic surfactant mixtures,J.Phys.Chem.B 111(2007)3122-3132.

[36]C.O.Rangel-Yagui,A.Pessoa Jr.,L.C.Tavares,Micellar solubilization of drugs,J. Pharm.Pharm.Sci.8(2005)147-165.

11 March 2013

*Corresponding author.

E-mail address:sm_kamil@rediffmail.com(M.Kamil).

http://dx.doi.org/10.1016/j.cjche.2014.06.028

1004-9541/?2014 The Chemical Industry and Engineering Society of China,and Chemical Industry Press.All rights reserved.

Received in revised form 29 October 2013

Accepted 23 December 2013

Available online 30 June 2014