Enhanced aqueous solubility of naphthalene and pyrene by binary and ternary Gemini cationic and conventional nonionic surfactants

Chemosphere 89 (2012) 1347–1353

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Enhanced aqueous solubility of naphthalene and pyrene by binary and ternary Gemini cationic and conventional nonionic surfactants Jia Wei a, Guohe Huang a,⇑, Lei Zhu b, Shan Zhao a, Chunjiang An a, Yurui Fan a a b

Environmental Systems Engineering Program, Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan, Canada S4S 0A2 Beijing Institute of Graphic Communication, Xinghua North Road, Daxing District, Beijing 102600, China

h i g h l i g h t s " We evaluated micellar behavior of selected surfactant systems by several models. " The CMCexps were less than CMCideals for all multi-component mixtures. " The mixing effect of ternary system is not the highest among all mixtures. " The effectiveness of solubilization is in tune with b values. " There is a difference of solubilization between naphthalene and pyrene.

a r t i c l e

i n f o

Article history: Received 15 February 2012 Received in revised form 22 May 2012 Accepted 24 May 2012 Available online 25 June 2012 Keywords: Solubilization Gemini Multi-component surfactant systems Polycyclic aromatic hydrocarbon

a b s t r a c t A systematic study has been carried out to get insight into the micellar behavior of Gemini cationic and conventional nonionic in their single as well as equimolar bi and ternary mixed state using the technique of tensiometry. The models proposed by Clint, Rubingh and Motomura et al. have been employed to interpret the formation of mixed micelles and find out synergism. The obtained experimental CMCs are lower than the ideal CMCs, indicating negative deviation from ideal behavior for all multi-component mixed micelles formation. The solubilization capacities of selected equimolar bi and ternary surfactant systems towards polycyclic aromatic hydrocarbons (PAHs), naphthalene and pyrene, have been evaluated from measurements of the molar solubilization ratio (MSR), the micelle–water partition coefficient (Km), the deviation ratio (R) and the free energy of solubilization (DG0s ) of PAHs. The results show that the solubility of naphthalene and pyrene over that in water in case of Gemini cationic surfactant is dramatically enhanced by adding equimolar nonionic surfactant in both bi and ternary mixed surfactant systems. The studied equimolar ternary surfactant system shows higher solubilizing efficiency than Gemini cationic binary system but lower than their cationic–nonionic counterpart. In addition, the solubilizing power of multi-component mixed surfactants towards naphthalene and pyrene increases with increasing log Kow of PAHs. Certainly, the solubilization abilities of the selected surfactants not only depend on their structure and mixing effect but also associate with solubilizing microenvironment and chemical nature of organic solutes. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous pollutant formed by natural and anthropogenic pyrolysis of organic matter from chemical manufacture, fossil fuel utilization and forest fires (Atkins et al., 2010). They are strongly absorbed into soils or sediments, thereby making them persist in the soil for long periods of time and less variable bioavailability. Poor solubility and toxicity of such compounds has motivated the development of various

⇑ Corresponding author. Tel.: +1 306 585 4095; fax: +1 306 585 4855. E-mail address: [email protected] (G. Huang). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.05.091

technologies to overcome the obstacles to remedy the contaminated soil and water (Wu et al., 2008; Gan et al., 2009; Petitgirard et al., 2009). In situ surfactant-enhanced remediation (SER) process (Mulligan et al., 2001) is one of the most promising techniques for the removal of sorbed contaminant in subsurface involving the use of aqueous surfactant solutions above their critical micelle concentrations (CMCs) which are able to amplify the aqueous phase concentrations of sparingly water-soluble substances efficiently. Numerous efforts (Menger and Littau, 1993; Din et al., 2010; Chavda et al., 2011; Guo et al., 2011) have been taken on synthesizing and selecting efficient surfactant to facilitate the solubilization of PAHs. A few studies (Zhou and Zhu, 2008; Madni et al., 2010; Vilasau et al., 2011) of mixed micelle formation of known

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composition are of great interest in scientific and industrial applications since the surfactants are almost always prepared commercially and used as mixtures rather than pure forms. In view of this, investigation on the behavior of multi-component surfactant systems with two or more species, which are much more efficient in a working scenario, has generally been performed. Although considerable work (Zhou and Zhu, 2005; Mir et al., 2011; Sales et al., 2011) has been carried out to date on examining the enhanced solubility of PAHs by mixed conventional surfactant systems, there are sporadic reports of the same in Gemini and its mixed systems. Geminis are the third generation surfactants which consist of two hydrophobic chains and two hydrophilic headgroups connected through a relatively short (rigid or flexible) spacer group at or near the headgroups (Hu et al., 2011). They have been drawing increased attention resulted from their unique properties that are superior to those of corresponding conventional surfactants with equal chain length, such as enhanced surface activity, much lower critical micelle concentration (CMC), better wetting properties and mildness to skin (Zana, 2002). The alkanediyl-a,x-bis(alkyl-dimethylammonium bromide) type Gemini surfactants have been the most investigated quaternary ammonium surfactant systems. The quaternary ammonium surfactants have been widely reported and used in pharmaceutical/cosmetic/household products preparations, owing to their excellent cell membrane penetration properties, low toxicity, good environmental stability, non-irritation, low corrosivity and extended residence time and biological activity (Tan and Xiao, 2008). This type of Gemini surfactant is often referred to as m-s-n surfactant, where m and n are the carbon numbers of alkyl chains and s represents the carbon numbers present in the polymethylene group in spacer, respectively (Sharma et al., 2003). These surfactants show a very high bactericidal activity and have find manifold application in the detergent and cosmetic industries (Khan et al., 2011). Previous research (Wei et al., 2011) has observed that this type of Gemini surfactant has better solubilization capacity for phenanthrene than conventional cationic and nonionic surfactants with the same carbon chain length. It is found (Mehta et al., 2009; Din et al., 2010; Sheikh et al., 2011; Wei et al., 2011) that the mixed systems involving Gemini and conventional surfactants often exhibit superior characteristic properties on the application front when contrasted to the pure system due to the presence of two charged sites in a Gemini which proposes strong interaction with other surfactants (Gemini or conventional). Practically, the detailed fundamental studies regarding the mixed micellization of Gemini and conventional surfactants are limited though these mixtures are extensively applied in a wide range. Moreover, to our knowledge, there is no study available on the solubilization of PAHs by mixed ternary Gemini and conventional surfactant combinations. The current work attempts to carry out experimental and theoretical investigations to study the effect of single, binary and ternary Gemini cationics C12-2-n (n = 12, 16) and conventional nonionic (C12E23) mixtures on the solubilization aspects of PAHs. More specifically, the detailed investigation has been undertaken: (a) to correlate the interaction parameter in the respect of equimolar Gemini cationic–cationic, Gemini cationic–conventional nonionic and Gemini cationic–cationic–conventional nonionic micelles formation and to elucidate the cause of their mixing effect for finding synergism, and (b) to evaluate solubilization capabilities of the selected equimolar multi-component surfactant systems and compare with that predicted according to the ideal mixing rule. The analysis gave sufficient valuable information to understand the mixing and solubilizing behaviors of Gemini cationic and conventional nonionic multi-component surfactant systems, which can contribute to their potential application in the selection of surfactant mixtures to enhance the recovery of soils and aquifers contaminated by hydrophobic organic compounds (HOC’s).

2. Experimental section 2.1. Materials Naphthalene and pyrene, the two and four-ring PAHs, were selected as the hydrophobic solute in this study (all >98%, Aldrich products) for their varying physicochemical properties and their widespread presence in contaminated sites. Gemini cationic surfactants C12-2-12 and C12-2-16 were obtained from Chengdo Organic Chemicals Co., Ltd., Chinese Academy of Science, with a purity of 98%. The nonionic surfactant C12E23 with purity >99% was purchased from Sigma–Aldrich. The structure and properties of chosen surfactants and organic compounds are all included in Table 1. The stock solutions of Gemini and nonionic surfactants were prepared by dissolving the relevant surfactants in deionized water. Then desired mole fractions were obtained by mixing precalculated volumes of the stock solutions and the following experimental procedures performed.

2.2. Tensiometric measurements The tensiometric measurements were made with a Model K11MK3 surface tensiometer, manufactured by Krüss. The total surfactant concentration of the stock solutions was kept constant at 5.0 mM. In an experimental run, different single, equimolar binary and ternary dilution series were prepared with the help of a microsyringe in small installments and every dilution was allowed to equilibrate for approximately 3 h before CMC measurement. Then the surface tension values were measured until constant values indicating that equilibrium had been reached. The accuracy of measurements was within ±0.1 mNm1. A digital thermostated bath from Polyscience was used for maintaining the temperature within 25 ± 0.01 °C. The CMC values were determined by noting distinct breaks in the plot of surface tension (c) versus the logarithm values of surfactant solutions (log Ct) over a wide concentration range, as Fig. 1a and b shown.

2.3. Solubilization measurements Batch tests for solubilization of naphthalene and pyrene were performed subsequently in the single and equimolar bi and ternary combinations of symmetric Gemini surfactant C12-2-12, dissymmetric Gemini surfactant C12-2-16 and conventional nonionic surfactant C12E23. Each contaminant–surfactant system involved five to seven batch experiments with surfactant solutions having a range of concentrations above the CMC. For each batch test, excess amounts of naphthalene and pyrene were separately added to 10 mL of surfactant solutions with different concentrations and composition, which were placed in borosilicate glass vials of 20 mL capacity to saturate the solution. These samples were then agitated for 48 h on a thermostated shaker maintained at a temperature of 25 ± 0.5 °C. (Previous experimental results showed 48 h mixing time to be sufficient for reaching solubilizing and partitioning equilibria) After this, subsequent centrifugation for 30 min at 5000 rpm was performed to remove the undissolved solid solute. An appropriate aliquot of the supernatant was then carefully withdrawn with a volumetric pipette and was diluted to 10 mL in flasks with 1 mL methanol and rest with the corresponding surfactant solution. The concentration of dissolved naphthalene and pyrene was detected spectrophotometrically with a Varian spectrophotometer (Cary 300) at the wavelength of 220 nm and 334 nm. The concentration of surfactant was kept the same in both the reference and the measurement cells to eliminate the effect of surfactant on determining solubility. All data reported are the average

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J. Wei et al. / Chemosphere 89 (2012) 1347–1353 Table 1 The structure and physicochemical properties of the selected surfactants and solutesa in this study. Material C12-2-12 C12-2-16 C12E23 Naphthalene

Structure +

+

C12H25N (CH3)2 (CH2)2–N (CH3)2 C12H252Br C12H25N+(CH3)2 (CH2)2–N+(CH3)2 C16H332Br C12H25(OCH2CH2)23OH

Pyrene

a b



MW (g mol1)

CMCexpb (mM)

614.7 670.8 1198 128.2

0.8 0.2 0.061

202.3

Water solubility (M)

Log Kow

2.5  104

3.36

7

5.18

6.7  10

Data for PAHs is reported by Verschueren (1996). Error limits of CMCexps are ±4%.

(a)

(b)

(Berthod et al., 2001; Sun et al., 2007). Apparently, the chain length of Gemini surfactant is a major driving factor for micellization and hydrophobic interactions as same as conventional surfactant (i.e., the larger the chain length, the lower the CMC values). The lowest CMC value of nonionic surfactant C12E23 owning to a larger number of oxyethylene (OE) units in its hydrophilic head (Dar et al., 2007). Mixed systems comprising of the equimolar combinations of C12-2-12/C12E23, C12-2-16/C12E23, C12-2-12/C12-2-16 and C12-2-12/C12-2-16/ C12E23 in water undergo presumably physicochemical changes due to interactions and invariably yield enhanced micellar properties. Usually the CMCs of mixed surfactants determined from the surface tension (c)–log[Ct] plots (Fig. 1) fall between those of individual pure components (Dar et al., 2007; Hu et al., 2011), as are observed here (Table 2) which are different from the earlier findings where Gemini–nonionic mixtures have lower CMC values than both of the constituent surfactants (Sharma et al., 2006). The difference might be attributed to the shorter spacer and hydrophobic tails of the Gemini surfactant, and the longer polarized oxygen atoms (PEOs) chain length of the nonionic surfactant of our systems. Moreover, the reduced CMC values demonstrate that nonionic monomers can easily intercalate among the Gemini monomers, and PEO chains may coil around the charged headgroups of Gemini surfactants (Hu et al., 2011), so the electrostatic repulsion is reduced, which will reinforce the hydrophobic environment in the mixed media in comparison to pure Gemini’s state. Additionally, the mixed CMC (CMCmix) obtained by mixing the constituents is given by the following equation based on the Rubingh’s regular solution theory (Rubingh, 1979): n X 1 ai ¼ CMCmix f CMCi i¼1 i

ð1Þ

where ai, CMCi and fi are the stoichiometric mole fraction, CMC value and activity coefficient of ith component in mixed micelle. In case of ideal behavior, fi = 1 (i = 1, 2,   , n) and hence Eq. (1) reduces to the form:

Fig. 1. Variations of c values of selected surfactant systems over a wide concentration range at 25 °C: (a) single surfactant systems and (b) equimolar binary and ternary mixed surfactant systems.

of at least three independent samples and the typical error in the measurement was less than 5%. 3. Results and discussion

n X 1 ai ¼ CMCideal CMC i i¼1

ð2Þ

as proposed by Clint (1974) for ideal mixed. The obtained experimental CMC values (CMCexp) are observed to be less than ideal critical micelle concentration values (CMCideal) for all equimolar bi and ternary systems included in Table 2, demonstrating the existence of interactions leading to distinctions between the constituent surfactants in nonideality of the multi-component systems (i.e., micellization takes place at lower concentration with respect to the ideal state).

3.1. Critical micelle concentration properties of all surfactant systems 3.2. Surfactant–surfactant interactions in mixed micelles The CMC values of single and their equimolar bi and ternary surfactant systems are presented in Tables 1 and 2. The values for pure surfactants are complementary with literature values

As discussed above, CMCexps are less than CMCideals indicating negative deviation from ideal behavior for all studied mixed

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Table 2 Experimental critical micelle concentration (CMCexp), ideal critical micelle concentration (CMCmi), interaction parameter (bm), micellar mole fraction (X m i ) and activity coefficients (fim ) values of equimolar surfactant mixtures at 25 °C.a

a

Surfactant system

CMCexp (mM)

CMCmi (mM)

bm

m m Xm 1 =X 2 =X 3

f1m =f2m =f3m

C12-2-12/C12E23 C12-2-16/C12E23 C12-2-12/C12-2-16 C12-2-12/C12-2-16/C12E23 C12-2-12/(C12-2-16/C12E23) C12-2-16/(C12-2-12/C12E23) C12E23/(C12-2-12/C12-2-16)

0.10 0.07 0.30 0.0994 0.0994 0.0994 0.0994

0.113 0.093 0.320 0.1325 0.1025 0.1674 0.1447

1.2708 1.4163 0.3792 1.1495 2.0478 1.2637 0.1371

0.1549/0.8451 0.3303/0.6697 0.2342/0.7658 0.0192/0.1593/0.8215 0.2176/0.7823 0.6052/0.3947 0.8184/0.1816

0.4034/0.9644 0.5298/0.8568 0.8005/0.9744 0.0729/0.1456/0.8986 0.2855/0.9076 0.6296/0.8214 0.9955/0.9133

Error limits of parameters are ±2%.

micelle formations. The degree of the negative deviation and hence the extent of nonideality of binary surfactant systems could be estimated by the interaction parameter (bm) as follows (Rubingh, 1979):

bm ¼

ln



CMC12 a1 CMC1 X m 1



2 ð1  X m 1Þ

¼

  12 a2 ln CMC CMC X m 2 2

2 m ðX m 1 Þ lnða1 CMC12 =X 1 CMC1 Þ

ð1 

2 Xm 1Þ

ð3Þ

2 ð1  X m 2Þ

ln½ð1  a1 ÞCMC12 =ð1  X m 1 ÞCMC2 

¼1

ð4Þ

m where X m 1 and X 2 are the micellar mole fractions of components in the mixed micelle; a1 and a2 are their corresponding bulk mole fractions; CMC1, CMC2 and CMC12 are critical micelle concentrations for two component surfactants and their mixture respectively. Eq. (4) is solved iteratively for X m 1 , which is then substituted into Eq. (3) to obtain bm. The negative value of bm implies synergism demonstrating reduction in free energy of micellization over that predicted by the ideal solution theory (Rao and Paria, 2008) whereas the positive value signifies incompatible surfactants and repulsion while zero value states no interaction, hence ideal mixing (Sheikh et al., 2011). Furthermore, the regular solution theory yields expressions for activity coefficient, fim , of ith surfactant within the mixed micelles related to the interaction parameter of bm and X m 1 through the relation (Rubingh, 1979):

2 lnðf1m Þ ¼ bm ð1  X m 1Þ

ð5Þ

2 lnðf2m Þ ¼ bm ðX m 1Þ

ð6Þ

The comprehensive set of results for all binary surfactant systems is tabularized in Table 2. The values of bm in the present case are all negative indicating a positive effect for all mixed surfactant systems which not only depend on the interaction but also have relevant association with properties of component surfactants of mixture (Tanford, 1980). From data it is also observed that the activity coefficients, obtained from the Eqs. (5) and (6), are all less than unity, indicating synergistic nonideal interaction between surfactant monomers in the micelles, which are comparable with the earlier findings for conventional cationic–cationic and cationic–nonionic mixed surfactant systems (Mehta et al., 2009). The minor value of bm and small deviation of fim values with each other in the case of Gemini cationic–cationic system imply less attractive interactions of individual surfactant owing to the conjoint effect of hydrophobic interaction among their carbon tails and intensive electrostatic repulsion among their hydrophilic groups. Whereas, the stronger synergism effect of Gemini cationic and conventional nonionic surfactants would result in part from the momentous electrostatic self-repulsion of cationics decreased by intercalating the nonionic monomers into cationic micelles (i.e., the attractive ion-dipole interaction between the positively charged head groups of cationic monomers and the –O– groups

of PEO chains). Another reason is that the electrostatic selfrepulsion of cationic surfactant and weak steric self-repulsion of nonionic surfactant are weakened by dilution effects (Kabir ud et al., 2009). Analysis shows that mixed micelles of Gemini cationic–conventional nonionic contain large fraction of nonionics as indicated by X m 1 values in Table 2 due to its stronger tendency to micellize as also experimentally verified by other studies on different cationic–nonionic mixed micelles (Dar et al., 2007). Because the nonionic C12E23 has large numbers of PEO chains containing lone pair of electrons, which will have stronger tendency to interact with the hydrophilic groups of Gemini cationics. Furthermore, more negative bm value of C12-2-16/C12E23 mixed system over C12-2-12/C12E23 system is thus the result of stronger charge–dipole interaction and also may be ascribed to the additional chain–chain interactions which play a major role in the stability of mixed micelles, while in case of C12-2-12/C12E23, chain–chain interactions are negligible because of similar chain lengths. The bm value of ternary mixture C12-2-12/C12-2-16/C12E23 has been calculated using pseudobinary treatment (Burman et al., 2000) which equals to the average value of three possible combinations (i.e., one is regarded as the first component and the other two components are treated as the second component). In this study, X m i and bm obviously depend on the way of selection of the pairing m of three components (Moulik et al., 1996). The calculated b values of different combinations are various which are not concordant with the previous report (Dar et al., 2007) for conventional cationic and nonionic ternary surfactant systems. This might because of the large CMC distinction of Gemini cationic surfactant studied herein. Nevertheless, all bm values of ternary mixture and its three pseudobinary combinations are negative indicating mutual synergistic interaction of three components in the system, which is higher than its corresponding cationic/cationic part and lower than cationic/nonionic part. Analysis also presents the predominance of nonionic surfactant in the mixed ternary surfactant system demonstrating its intense inclination to micellize regardless of the constitutes of surfactant mixtures. The activity coefficients and micellar composition for ternary mixture have been addressed using a generalized nonideal multicomponent micelle model on the basis of pseudo-phase separation approach developed by Holland and Rubingh (1983). The model makes a good use of interaction parameters determined from CMC measurements on binary system. The basic equation is present below:

lnfim ¼

n X

i¼1 ði–jÞ

m2 bm þ ij X j

n X

j1 X m m m m ðbm ij þ bik  bjk ÞX j X k

ð7Þ

j ¼ 1 k¼1 ði–j–kÞ

where bm ij is the pairwise interaction between component i and j which can be obtained independently from binary mixtures, and Xm is the mole fraction of jth component in the n-component j mixture system. The results of the activity coefficients for a

J. Wei et al. / Chemosphere 89 (2012) 1347–1353

three-component system (i.e., f1m , f2m and f3m ), can be obtained by using the method of successive substitutions subject to the constraint that the sum of X m i ’s equals to unity (Dar et al., 2007). These calculations were made using MATLAB. As evident from the results, the ternary system also has attracted interaction suggesting strong synergism as denoted by bm. The CMCmix value is found to be lower than the CMCideal, demonstrating the synergistic nonideality of ternary micellar system. The mole fractions of individual surfactants in the ternary mixed micelles are different from stoichiometric composition (e.g., Xm value is much lower than a value for cationic component, but is fairly higher than a value and is close to unity for nonionic component) (Table 2). The activity coefficients of the individual surfactants within the ternary mixture are in the order of C12-2-12 < C12-2-16 < C12E23 which is in conformity with their mole fractions showing the predominance of the nonionic surfactant and its active effect in the mixed ternary micelles. 3.3. Solubilization of naphthalene and pyrene The variations of solubilization capabilities are plotted as a function of single and equimolar mixed surfactant solution concentrations for each data set as shown in Figs. 2a and b, respectively. The apparent solubility of naphthalene and pyrene elevated linearly with increased concentrations of single surfactant systems

above CMC. Similar trends can be observed in all bi and ternary surfactant systems studied, indicating the formation of mixed micelles and their potential abilities to improve the solubility of PAHs in water. The behavior is generally associated with the incorporation or partitioning of organic solutes within multi-component micelles (Wang et al., 2005). For both naphthalene and pyrene, their solubility enrichment in single surfactants over that in water is in conformity with surfactants’ surface tension characters (i.e., the lower CMC value of surfactant, the superior solubilization ability towards PAHs). The noted difference between C12-2-12 and C12-2-16 could be attributed to the hydrocarbon chain length of different Gemini surfactants. According to Rosen (1989), solubilizing power increased with increasing hydrocarbon chain length. After all, Gemini surfactant possessing longer chain length would process stronger solubilization ability. The strongest solubilizing power of C12E23 is might be interrelated to its lower CMC and larger micellar size as compared to that of C12-2-12 and C12-2-16, which states that a large fraction of C12E23 is present in micellar form, thus creating more locations within the solution at which PAHs may be solubilized. It is also obviously to find in Fig. 2 that the solubilization capabilities of Gemini cationic surfactants towards naphthalene and pyrene are all enhanced by adding equimolar nonionic surfactant in both bi and ternary mixed systems. The nonionic surfactant with large POE groups that has a stronger tendency to intercalate into the Gemini cationics making large size micelles within the solution at PAHs may be solubilized. Among multi-component surfactant systems, Gemini cationic–cationic system has the least solubilizing power for both naphthalene and pyrene, however, being significantly improved on adding nonionic surfactant in the ternary system as observed. What is worth noting that C12-2-12/C12-2-16 is found to be a superior solubilization aid for pyrene relative to that by C12-2-12 Gemini surfactant, which is on the contrary for naphthalene. This phenomenon is presumably attributed to the major role of solubilizate’s chemical nature. It was suggested by Moroi (1992) that the solubilization of naphthalene is believed to occur at the micelle–water interface. Therefore, in case of the present study, specific surface area of micelle decreases with the formation of large micelle through intercalating surfactant bearing relatively long carbon chain length, bringing about the depressed interact area of solute and micelle–water interface, thus the solubility of naphthalene becomes less. On the contrary, pyrene is more hydrophobic in comparison with relatively polar naphthalene, and therefore preferable to incorporate into the palisade layers of the micelles and interact with the hydrophilic group of Gemini cationic monomers through the interaction between p-electrons of arenes and the positive charges. Therefore, C12-2-12 monomers are incorporated by long carbon chain length C12-2-16 monomers generating larger dimension palisade layers within micelles, which conduce to more pyrene molecule solubilized. A measure of the solubilization capacity of a particular surfactant for a given substance is known as the molar solubilization ratio (MSR), which is characterized as the number of moles of compound solubilized by one mole of micellized surfactant (Edwards et al., 1991). MSR is expressed via equation:

MSR ¼ ðSac  Scmc Þ=ðC ac  CMCÞ

Fig. 2. Variations of solubility of (a) naphthalene and (b) pyrene with total surfactant concentrations of single, equimolar binary and ternary mixed surfactant systems.

1351

ð8Þ

which can be obtained from the slope of the linearly fitted line that is plotted when the concentration of solute is against surfactant concentration above the CMC in the presence of excess solute. Sac is the total apparent solubility of solute in given surfactant solutions at a specified surfactant concentration Cac (the surfactant concentration greater than the CMC at which Sac is evaluated), and Scmc is the apparent solubility of solute at the CMC, which is taken as solute’s water solubility, since it changes only very mildly up to the CMC of the surfactant. Earlier researches gave the solubility of

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Table 3 Molar solubilization ratio (MSR), micelle–water partition coefficient (Km), deviation ratio (R) and free energy of solubilization (DG0s ) of naphthalene and pyrene in various single and equimolar bi and ternary micellar systems at 25°Ca. Compound

Surfactant system

MSR

Log Km

DG0s (kJ mol1)

R

Naphthalene

C12-2-12 C12-2-16 C12E23 C12-2-12/C12-2-16 C12-2-12/C12E23 C12-2-16/C12E23 C12-2-12/C12-2-16/C12E23

0.1677 0.2907 0.3953 0.1427 0.3065 0.3844 0.3043

5.997 6.193 6.293 5.937 6.211 6.284 6.208

34.24 35.35 35.92 33.89 35.45 35.87 35.44

0.8193 0.8310 1.1798 0.8197

C12-2-12 C12-2-16 C12E23 C12-2-12/C12-2-16 C12-2-12/C12E23 C12-2-16/C12E23 C12-2-12/C12-2-16/C12E23

0.0065 0.0178 0.0304 0.0184 0.0264 0.0297 0.0252

4.651 5.083 5.310 5.097 5.251 5.300 5.231

26.55 29.02 30.31 29.10 29.97 30.26 29.86

1.9114 0.9675 1.2596 0.9130

Pyrene

a

Error limits of parameters are ±2%.

naphthalene and pyrene in pure water as recorded in Table 1. All concentrations are expressed in millimoles per liter. The effectiveness of solubilization can also be quantified in terms of the micelle–water partition coefficient (Km) which is defined as distribution of the mole fraction of solute between surfactant micelles and aqueous phases and has been calculated by using the following formula (Edwards et al., 1991):

MSR Km ¼ Scmc V w ð1 þ MSRÞ

ð9Þ

where Vw is the molar volume of water equal to 1.805  102 M1 at 25 °C. Solubility data of two PAHs by all surfactant systems studied herein are tabulated in Table 3. The difference of hydrophobic chain length of Gemini cationic surfactant produces a distinction in MSR and Km values, especially for pyrene, significant ones are observed. Meanwhile, the two kind values of nonionic are found to be higher than Gemini cationic surfactants for both naphthalene and pyrene indicating that with the same hydrophobic chain length, nonionic bears higher solubilizing power for the given solutes due to its larger micellar size assisting in more micellar core solubilization (Mehta et al., 2009). Here, the solubilizing efficiencies of mixed systems are mostly in between that of single surfactant, which is in tune with CMC analysis. Comparison of C12-2-12/ C12E23 and C12-2-16/C12E23 shows that the later exhibits larger values of MSR and Km in contrast with the former. The behavior is attributed to the lower CMC as well as the larger micellar size of C12E23 intercalating into Gemini C12-2-16 with long hydrophobic chain length resulting in larger effective solubilization area in mixed micelles. Simultaneously, the two values of C12-2-12/C12-2-16 for studied PAHs are appreciably increased as adding C12E23 in the case of ternary system. This is directly related to the higher micellar core solubilization of nonionic surfactant as well as the micelle–water interface adsorption. Therefore, MSR and Km of multi-component surfactant systems for both naphthalene and pyrene follow the order of C12-2-12/C12-2-16 < C12-2-12/C12-2-16/ C12E23 < C12-2-12/C12E23 < C12-2-16/C12E23, which is the same as that of bm values for various surfactant systems. Meanwhile, their rise is coupled with an increase log Kow of PAHs (naphthalene < pyrene), in accord with previous findings (Li and Chen, 2002). Nevertheless, there are substantial difference of MSR values between naphthalene and pyrene, yet relatively less deviation of Km values between them are displayed. Presumably, the palisade layer of micelle is accountable for solubilizing pyrene, thus Km might be approximately comparative to the nonpolar part of the micelle. This has been found support by Kile and Chiou (1989) that Km values could

be better moderately related to nonpolar part of cationic, anionic and nonionic surfactant. To further evaluate the mixing effect of selected multi-component surfactant systems on solubilization of naphthalene and pyrene and seeing the nature of deviation, the parameter R could be proposed through the following equation (Zhou and Zhu, 2004):

R ¼ MSRexp =MSRideal

ð10Þ

MSRideal is the MSR for solute in mixed surfactant system at the ideal mixed state and can be determined using the MSR of single surfactant solutions based on the ideal mixing rule:

MSRideal ¼

X MSRi X m i

ð11Þ

i

where MSRi and X m i are the ith experimental MSR value of solubilizate and the mole fraction in multi-component surfactant solutions, respectively. For most of the mixed surfactant systems, it is visibly observed from Table 3 that R is close to 1 demonstrating solubilization follows almost ideal behavior with small deviation. The R values of C12-2-16/C12E23 system are greater than 1 signify its favorable mixing effect on solubilization and positive deviation of MSRexps from ideal mixing rule regarding both naphthalene and pyrene. Additionally, the largest R value (1.9114) shows evidence of positive mixing effect of C12-2-12/C12-2-16 in case of solubilizing pyrene, further demonstrating that the solubilization abilities depend not only on the changed surfactant micellar microstructures (packing of surfactant monomers in the mixed micelle cores as well as at the micelle–water interfaces) when mixed micelles formed, but also on the chemical nature of solubilizate and their interaction with mixed micelles. In the present study, the standard free energy change of solubilization, DG0s (Rangel-Yagui et al., 2005) during the process of PAH molecules incorporating into chosen mixed micelles are all negative demonstrating the solubilization capacities of studied mixed micelles can be regarded as a normal partitioning of the PAHs between the micellar and aqueous phases (i.e., spontaneous solubilization). The larger DG0s values of the surfactant systems designate that the behaviors of solubilizing naphthalene and pyrene are energetically more complementary owing to the combined effects discussed above. 4. Conclusions The micellar behavior of equimolar bi and ternary Gemini cationic and conventional nonionic surfactant systems were studied for finding synergism to gain better insight into the mixing effect

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of multi-component surfactants in aqueous solution. Then the results of solubilization assays show that the solubility of naphthalene and pyrene increases linearly over the range of multi-component surfactant concentrations above the CMC, which illustrates the potential capacities of Gemini cationic–conventional nonionic mixed surfactants to enhance the solubility of PAHs in water. Meanwhile, the tensiometric and solubilization data manifest that the greater synergistic interaction among each component of bi and ternary mixed surfactants results in the larger negative deviation of the CMC from ideal mixture, and then the mixing effect of multi-component surfactant systems on solubilizing power towards naphthalene and pyrene becomes stronger. However, MSR and Km values exhibit notable difference between the two different PAHs, demonstrating the solubilization behavior not only depends on the structure and mixing effect of surfactant but also associates with solubilizing microenvironment and the chemical nature of organic solutes. The experimental results of this study make a clear understanding of solubilization capabilities of equimolar bi and ternary Gemini cationic and conventional nonionic surfactant systems and extend the surfactant selection used in solubilization for soil and water contamination treatment. Acknowledgments This research was supported by the Major Project Program of the Natural Sciences Foundation (51190095) and the Natural Science and Engineering Research Council of Canada. References Atkins, A., Bignal, K.L., Zhou, J.L., Cazier, F., 2010. Profiles of polycyclic aromatic hydrocarbons and polychlorinated biphenyls from the combustion of biomass pellets. Chemosphere 78, 1385–1392. Berthod, A., Tomer, S., Dorsey, J.G., 2001. Polyoxyethylene alkyl ether nonionic surfactants: physicochemical properties and use for cholesterol determination in food. Talanta 55, 69–83. Burman, A.D., Dey, T., Mukherjee, B., Das, A.R., 2000. Solution properties of the binary and ternary combination of sodium dodecyl benzene sulfonate, polyoxyethylene sorbitan monlaurate, and polyoxyethylene lauryl ether. Langmuir 16, 10020–10027. Rangel-Yagui, C.O., Hsu, H.W.L., Pessoa Jr., A., Tavares, L.C., 2005. Micellar solubilization of ibuprofen: influence of surfactant head groups on the extent of solubilization. Braz. J. Pharm. Sci. 41, 237–246. Chavda, S., Kuperkar, K., Bahadur, P., 2011. Formation and growth of gemini surfactant (12-s-12) micelles as a modulate by spacers: a thermodynamic and small-angle neutron scattering (SANS) study. J. Chem. Eng. Data 56, 2647–2654. Clint, J.H., 1974. Micellization of mixed nonionic surface active agents. J. Chem. Soc. Faraday Trans. 71, 1327–1334. Dar, A.A., Rather, G.M., Das, A.R., 2007. Mixed micelle formation and solubilization behavior toward polycyclic aromatic hydrocarbons of binary and ternary cationicnonionic surfactant mixtures. J. Phys. Chem. B 111, 3122–3132. Din, K.U., Sheikh, M.S., Dar, A.A., 2010. 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, 6023–6032. Edwards, D.A., Luthy, R.G., Liu, Z., 1991. Solubilization of polycyclic aromatic hydrocarbons in micellar nonionic surfactant solutions. Environ. Sci. Technol. 25, 127–133. Gan, S., Lau, E.V., Ng, H.K., 2009. Remediation of soils contaminated with polycyclic aromatic hydrocarbons (PAHs). J. Hazard. Mater. 172, 532–549. Guo, C., Zhou, P., Shao, J., Yang, X., Shang, Z., 2011. Integrating statistical and experimental protocols to model and design novel Gemini surfactants with promising critical micelle concentration and low environmental risk. Chemosphere 84, 1608–1616. Holland, P.M., Rubingh, D.N., 1983. Nonideal multicomponent mixed micelle model. J. Phys. Chem. 87, 1984–1990. Hu, C., Li, R., Yang, H., Wang, J., 2011. Properties of binary surfactant systems of nonionic surfactants C12E10, C12E23, and C12E42 with a cationic gemini surfactant in aqueous solutions. J. Colloid Interface Sci. 356, 605–613. Kabir ud, D., Shafi, M., Bhat, P.A., Dar, A.A., 2009. Solubilization capabilities of mixtures of cationic Gemini surfactant with conventional cationic, nonionic and

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