Surface activity, micellization and solubilization of cationic gemini surfactant-conventional surfactants mixed systems

Journal of Molecular Liquids 225 (2017) 888–896

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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq

Surface activity, micellization and solubilization of cationic gemini surfactant-conventional surfactants mixed systems U. Patel a, P. Parekh a,⁎, N.V. Sastry b, V.K. Aswal c, P. Bahadur a a b c

Department of Chemistry, Veer Narmad South Gujarat University, Surat 395007, India Department of Chemistry, Sardar Patel University, Vallabh Vidhyanagar, Gujarat, India. Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e

i n f o

Article history: Received 15 April 2016 Received in revised form 28 October 2016 Accepted 10 November 2016 Available online 11 November 2016

a b s t r a c t In this paper, we have studied the mixed surfactant systems containing a cationic gemini surfactant N,N′bis(dimethyldodecyl)-1,2-pentanediammonium dibromide (12-5-12) with several differently charged surfactants (cationic/anionic/non-ionic/zwitterionic) by using surface tension, small angle neutron scattering (SANS) under standard condition. All the surfactants have same dodecyl chain but different in polar head group viz. Sodium dodecyl trioxyethylene sulphate (SDES)(anionic), dodecyl trimethyl ammonium bromide (DTAB) (cationic), 3-[dodecyldimethyl ammonio] propane sulphonate (C12-PS)(zwitterionic), hexaethylene glycol monododecyl ether (C12E6)(non-ionic) and dimethyl dodecyl amine oxide (DMDAO) (cationic-non-ionic). The Clint, Rubingh, Maeda and Rosen approaches were employed to determine various parameters like critical micelles concentration (CMC), surface excess concentration (Гmax), surface pressure at CMC (πCMC), minimum area per molecule (Amin) etc. as well as thermodynamic parameters. Solubilization of some polycyclic aromatic hydrocarbons (PAHs) have done in single as well as in mixed surfactant systems. Order of solubilization capacity of PAHs compounds are different in pure/mixed surfactant micelles. The solubilization efficiency towards naphthalene, anthracene, and pyrene were measured and the molar solubilization ratio (MSR), deviation ratio (R) and micelle-water partition coefficient (km) were evaluated for single and mixed systems. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Mixed surfactants are often important due to synergistic interactions that improves their performance by increasing interfacial activity and micellar behaviour [1]. Several mixed systems containing similarly charged surfactants cationic-cationic [2–7] anionic-anionic [8], nonionic-non-ionic [9], and dissimilar charged e.g. ionic-non-ionic and anionic-cationic have been investigated in detail [10–16]. In most of these studies adsorption and micellization parameters were evaluated to obtain information of the mixed surfactant micellar system and results were discussed using interaction parameter. Very weak interaction has been observed for similarly charged surfactant systems, while oppositely charged surfactants interacted strongly [17] and often formed coacervates/precipitates under different molecular composition and concentration. Gemini surfactants have emerged as a new class of surfactants. These are dimeric surfactants where two alkyl chains with polar head group covalently joined together through a spacer. Like mixed systems, Gemini's show enhanced surface activity and micellar characteristics of monomeric surfactants [18]. Different types of gemini surfactants viz. ⁎ Corresponding author. E-mail address: [email protected] (P. Parekh).

http://dx.doi.org/10.1016/j.molliq.2016.11.017 0167-7322/© 2016 Elsevier B.V. All rights reserved.

anionic [19–22], cationic [23–27], non-ionic [28] have been synthesized and their solutions behaviour were investigated [18]. Dissimilar chain gemini surfactants [29], and those with different types of spacers rigid or flexible [30,31] have also been synthesized and their solution behaviour also investigated. Wetting/foaming/emulsify activity, solubilization/microemulsion/viscoelasticity and anti-microbial/antistatic/ corrosion inhibition activity of geminis have been investigated using several instrumentation techniques viz., surface tension, viscosity, conductance, thermal, spectral and scattering were used [32–35]. The most frequently examined or commonly used geminis were quaternary ammonium salts based cationics formed together by dodecyl chain (m) of the type m-s-m, where s is spacer with polymethylene chain of 2-12 carbon. These geminis have also been examined in mixed solvents by many authors. Chavda et al. [36].examined the effect of different alkanols and alkanediols on the cmc of cationic gemini surfactant and concluded that short chain alcohols with structure breaking ability result in increase in cmc of gemini, while long chain alcohols and alkanediols decreases the cmc. Mixed systems using m-s-m type cationic gemini, with Sodium dodecyl trioxyethylene sulphate (SDES) have shown that gemini surfactant binds tightly with SDES by electrostatic, hydrophobic, and ion-dipole interactions [17]. Rosen et al. [37]. revealed strong interaction of cationic gemini with sugar surfactants [37]. Bakshi and coworkers [38] studied on non-ionic-gemini mixed systems by

U. Patel et al. / Journal of Molecular Liquids 225 (2017) 888–896

fluorescence and cloud point measurements and reported synergistic interaction. Esumi et al. [39] using light scattering and NMR studies reported a weak interaction between cationic gemini and non-ionic surfactant. There are also few reports in the literature showing the influence of spacer chain length on the micellar morphology using SANS [30,40,41]. PAHs are organic pollutants with fused aromatic rings and are highly toxic showing strong mutagenic, teratogenic and carcinogenic properties. Several chemical, physical, and biological procedure have been adopted for the removal of the PAHs by increasing their solubility [42, 43]. But these methods have some limitations and side effects. Micellar solubilization is quite suitable and a best technique to dissolve the hydrophobic organic contaminants in aqueous environment [44–46]. Park et al. [47] examined the solubilization of naphthalene and phenanthrene in three different anionic surfactants and concluded that the hydrophobic chains in micellar core play more important role for the solubilization of PAHs than the benzene ring in palisade layer of micelle. Bernardez and his group [48,49] studied the solubilization of two PAHs in five non-ionic surfactants and determined that the solubilization rate depends on chemical structure of PAHs and type of surfactants. Panda et al. [44] investigated solubilization of anthracene and pyrene in micellar solution of gemini and its equimolar binary mixtures of conventional cationic, anionic, and non-ionic surfactants in water, and concluded that solubilization capacity of pure/mixed surfactant micelles, for anthracene and pyrene are not same. Many techniques such as NMR [50], light scattering [12,51], and SANS [52–54] have also been used for the study of mixed micellar interaction of gemini and conventional surfactants [55]. In this paper we used 12-5-12 and examined its mixed systems with ionic, nonionic, zwitterionic surfactants i.e. 12-5-12 + dodecyl trimethyl ammonium bromide (DTAB (cationic), 12-5-12 + SDES (anionic), 125-12 + 3-[dodecyldimethyl ammonio] propane sulphonate (C12PS)(zwitterionic), 12-5-12 + hexaethylene glycol monododecyl ether (C12E6)(non-ionic), 12-5-12 + dimethyl dodecyl amine oxide (DMDAO) (cationic-nonionic). There are two purpose of selecting these surfactants: (a) their similar hydrophobic chain lengths, a factor expected to lead ideal mixing, and (b) a large difference in their cmc values. Hence, a systematic study of surface and micellar properties of the above mentioned surfactants was performed using tensiometry. The analysis of data has been carried out using various theoretical models including Rubingh, Rosen, Clint, and Maeda to expose the comparative performance of these models. We have also studied the interaction of mixed system and find the effect of gemini with differently charge surfactants on the size and shape of micelles by SANS measurements. Here we have used PAHs like naphthalene; anthracene; pyrene. The extent of solubility of these compounds was also determine in single and equimolar mixed surfactant system.

889 CH3

H3C Br H3C + HC N

H3C Br

CH3 Br N+ CH 3

3

O

HO

CH3

SDES

N

CH3 -

O

DMDAO CH3

O

O O-

CH3

3

H3C

H3C

CH3

-

DTAB O O S O O

12-5-12

CH3

+

N

+

CH3

C12 E6 CH3

N

S O

H3 C

C12 PS

Scheme 1. Structural formula of surfactants

were kept for at least 30 min for equilibration. Surface tension was measured at several different surfactant concentration (in mole/liter), and the cmc were determined from the inflections in γ versus logarithm of surfactant concentration plot. The break point in the plot show the CMC value. The surface tension was accurate within ±0.1 mN m−1. 2.2. Solubilization of PAHs The PAHs were added in excess amount in the vials containing different concentration of surfactant solution, so that the PAHs are solubilized at maximum extent. The sample vials were sealed and surfactant solution containing PAHs stirred for 24-h in the shaker at 30 °C, and then filtered with 0.45 μm cellulose filter to remove excess amount of undissolved PAHs. Absorbance of naphthalene, anthracene, and pyrene was measured by UV–visible spectrophotometer. Concentration of the solubilized PAHs was calculated by the Lambert-Beer's law [56]. 2.3. Small angle neutron scattering The SANS results were obtained at the SANS-I facility, Swiss Spallation Neutron source SINQ, Paul Scherrer Institute, Switzerland by the courtesy of Dr. V. K. Aswal, BARC, Mumbai. For the SANS measurements, all the solution were prepared in D2O. The data were collected in the wave vector transfer magnitude Q range of 0.008–0.3 Å−1. All the measurements were carried out at a fixed concentration (50 mM) of surfactants. Samples were held in Hellma quartz cells having a thickness of 2 nm, and the temperature was kept constant at 30 °C during the measurements. SANS distributions were corrected for the background and solvent contributions and were normalized to the unit of cross section using standard procedures. The detail procedure of experiments can be found elsewhere [57,58]. 3. Results and discussion

2. Materials and method 3.1. cmc of pure and mixed gemini-conventional (1:1) surfactants system Gemini surfactant N,N′-bis(dimethyldodecyl)-1,2pentanediammonium dibromide (12-5-12) was gift sample from Prof. D.G. Marangoni, St. Francis Xavier University, Antigonish, Canada. SDES, DMDAO, C12-PS, and DTAB were obtained from Sigma-Aldrich. The structures of surfactants used are shown in Scheme 1. 2.1. Surface tension measurements Surface tension was measured by Wilhelmy plate method and using Kruss K100 tensiometer. The plate was cleaned with millipore water and flamed before each measurement. The temperature was maintained at 30 °C by circulating water. The pure surfactants solutions were prepared by diluting the concentrated stock solutions. Millipore water (surface tension 72 ± 0.2 mM m−1), was used throughout the solution preparation for surface tension measurements. The mixed solutions were prepared by mixing solutions of individual surfactants and

The cmc values of pure surfactants and their mixed systems of 12-512 with DTAB, DMDAO, SDES, C12PS, and C12E6 were measured. The surfactant chain length and hydrophobic interaction are a major driving factor for micellization [59]. With increase in chain length of conventional surfactant usually there is marked decrease in cmc but no particular trend was observed in gemini surfactant because it has two head groups connected through spacer which effect uncommon way on CMC [41]. Gemini surfactant monomers in solution exist in cis conformation to allow for the intermolecular interaction between two alkyl chains of the molecule therefore cmc is higher in gemini with spacer length range ~5–6 [60]. Fig. 1 shows the surface tension vs log concentration plots of all mixtures and pure compounds. The surface tension of single and mixed surfactant systems shows decrease in cmc. The cmc were calculated from the breakpoint. The cmc of single surfactants are in good agreement to the reported values [61,62].

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U. Patel et al. / Journal of Molecular Liquids 225 (2017) 888–896

Fig. 1. Show surface tension vs log[surfactant] of mixed system contain (Ο) 12-5-12 (Δ) pure DTAB, DMDAO, C12E6, SDES and C12 PS (⧠) 1:1 ratio of mixed respective surfactant with the 12-5-12.

3.2. Interaction of 12-5-12 with conventional surfactants in mixed micelles: The cmc values (cmcexp) of different combination of two surfactants 12-5-12-DTAB, 12-5-12-DMDAO, 12-5-12-C12E6, 12-5-12-C12PS and 12-5-12-SDES were determined from the surface tension (γ)log[Surfactant] plot lower than the components cmc's and from ideal one. Here, mixing behaviour of surfactant used in this study is expected to be non-ideal on the basis of difference in their head group. For ideal mixing behaviour, cmcideal was calculated by Clint's equation for a binary system [63].

1 α 1 ð1−α 1 Þ ¼ þ C 12 C 1 C2

ð1Þ

Two tails of 12-5-12 make the molecule more hydrophobic and mixing is more favourable. The approximation of negative deviation of experimental cmc from ideal cmc and non ideality of mixed micellar system can be calculated by the Rubingh's equation, ðX 1 Þ2 ln ðα 1 C 12 =X 1 C 1 Þ ð1−X 1 Þ2 ln ½ð1−α 1 ÞC 12 =ð1−X 1 ÞC 2 

¼1

ð2Þ

where, X1 is the mole fraction of the first surfactant in the total mixed solution. According to Rubingh's equation, it is noticed that mixed micelles of 12-5-12-convetional surfactants; the decreasing values of Xm 1 of gemini in the order: the DTAB b DMDAO b C12PS b SDES b C12E6.

U. Patel et al. / Journal of Molecular Liquids 225 (2017) 888–896

891

Table 1 PAHs, their chemical structures, aqueous solubility and ln Po/w values. Compounds

Formula

Molecular weight

Solubility (mol/L)

Po/w

Naphthalene

Structure

C10H8

128.4

0.25 × 10−3

3.35

Anthracene

C14H10

178.2

0.253 × 10−6

4.40

Pyrene

C16H10

202.5

6.52 × 10−4

5.20

Hence, the mole fraction of individual surfactants in the mixed micelles depends on stoichiometric composition of micelles. The interaction parameter βm, calculated by given equation,

βm ¼

ln ðα 1 C 12 =X 1 C 1 Þ ð1−X 1 Þ2

ð3Þ

Negative values of βm indicate synergism or attractive interaction, positive value of βm indicate antagonism or repulsion and zero value of βm indicates ideal mixing of two surfactant. Results of all mixtures in this paper show a negative value of interaction parameter suggest stronger synergism and non-ideality. m The activity coefficients fm 1 and f2 of the individual surfactants within the mixed micelles are related to the interaction parameter by this equation,

(ΔGMaeda), as a function of ionic component in the mixed micelle (Xm 1) is, therefore, given by   ΔGMaeda ¼ RT B0 þ B1 X 1 þ B2 X 21

ð6Þ

where B0 is an independent first term related to the CMC of second surfactant within the mixture and is given by B0 ¼ ln C2

ð6aÞ

The second parameter, B1 in Eq. (7) is related to the standard free energy change, while the last coefficient, B2, is equivalent to β in the Regular Solution Theory (RST), specifically as B2 ¼ −β

ð6bÞ

m

ð4Þ

Finally, the parameters B1 and B2 are related to the cmc values of pure systems by

m

ð5Þ

B1 þ B2 ¼ ln

ln f 1 ¼ β ð1−X 1 Þ2 ln f 2 ¼ β ðX 1 Þ2

m where f m 1 and f 2 are the activity coefficients of 12-5-12 and SDES, C12E6, DMDAO, C12PS and DTAB, respectively. The values of βm are negative for all the systems indicating that the synergistic interaction between the two surfactants after mixing. The results show that βm is negative in all the binary systems with a value of − 4.9, − 12.0, − 2.2, − 5.7 and − 17.6 for 12-5-12-DTAB, 12-5-12-C12PS, 12-5-12-DMDAO, 12-5-12C12E6 and 12-5-12-SDES respectively, suggesting strong synergism and the formation of mixed micelle. The existence of synergism is not only due to the strength of interactions between them but also depends on relevant properties of individual surfactants forming the mixtures [64]. Higher βm value for 12-5-12 + SDES system is due to the strong electrostatic interaction between the oppositely charged head group of the surfactants. m Higher the value of activity coefficient (fm 1 and f2 ) higher the contribution of gemini than the conventional surfactants in the mixed micelles. The low values of activity coefficient of SDES and C12E6 indicating non-ideal behaviour and attractive interaction between surfactants in the micelle.

3.3. Interfacial properties of pure/mixed systems: Maeda [65] and Ruiz et al. [66] suggested that both head group–head group and chain–chain interactions are present in mixed systems which are neglected in Rubingh approach. According to Maeda, the values cover only head group–head group interactions but no hydrocarbon chain–chain interactions. The mixed cmc of ionic–nonionic surfactant systems is lower than pure due to decrease in ionic head group–head group repulsion caused by the presence of nonionic surfactant molecules between the ionic head groups (Table 1). As per Maeda, the stability of micelles is contributed not only on βm interaction parameter but also on another parameter B1. The free energy of micellization

C1 C2

ð6cÞ

Thus once B2 is calculated, we can obtain B1 from Eq. (6c). The evaluated parameters are shown in Table 3. If the value of B1 is negative, it indicates that the chain-chain interaction play important role in the stability of mixed micelles. The amount of surfactant absorbed per unit area of the surface can be calculated with the help of Gibbs adsorption equation. The surface excess Γmax (mol m−2) and minimum area per molecule at solution/ air interface (Amin, Å2) were calculated by below equation, πcmc¼ γwater – γcmc Γmax ¼ −

ð7Þ

  1 ∂γ nRT ∂ ln c

ð8Þ

Table 2 Micellar parameters (ideal and experimental average CMC (CMCideal and CMCexp) interaction parameter (β), activity coefficients of gemini surfactants with conventional surfactants at 30 °C. Compounds

cmcideal (mM) cmcexp (mM) Xm 1

12-5-12 12-5-12 + DTAB DTAB 12-5-12 + C12 PS C12 PS 12-5-12 + DMDAO DMDAO 12-5-12 + C12E6 C12E6 12-5-12 + SDES SDES

1.12 0.033 15 0.25 2.5 0.37 1 7.14 0.09 1.31 0.38

0.933 0.870 15.13 0.724 2.34 0.645 1.380 0.051 0.075 0.007 0.38

– 0.704 – 0.532 – 0.546 – 0.340 – 0.477 –

βm

fm 1

fm 2

– −4.69 – −12.00 – −2.21 – −5.77 – −17.65 –

– 0.66 – 0.07 – 0.63 – 0.08 – 0.007 –

– 0.09 – 0.03 – 0.51 – 0.51 – 0.018 –

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U. Patel et al. / Journal of Molecular Liquids 225 (2017) 888–896

Table 3 Surface excess (Гmax), minimum area per molecule (Amin), packing parameter (P) and surface pressure (πcmc) of Gemini with conventional surfactant at 30 °C. γcmc mN m−1 Гmax (*106mol m−2) Amin (A2) P

Compounds DTAB 12-5-12 + DTAB C12 PS 12-5-12 + C12 PS DMDAO 12-5-12 + DMDAO C12E6 12-5-12 + C12E6 SDES 12-5-12 + SDES

 2 Amin Å ¼

39.04 40.11 39.89 33.45 30.11

9.25 0.85 1.13 0.79 1.70 0.85 0.56 0.53 1.51 0.35

17.94 195.44 146.38 210.13 97.82 194.25 293.88 313.39 109.85 470.18

2.33 0.15 0.28 0.14 0.42 0.15 0.14 0.09 0.38 0.07

πcmc

Systems

32.16

31.3 37.75 41.09

ð9Þ

where ∂γ/∂ ln c is the maximum slope, R = 8.314 J mol−1 K−1, T is the absolute temperature in kelvin, NA is Avogadro's number. Here, we have taken n = 3 based on previous reports on mixed surfactant studies of gemini [67,68]. The values of cmc (mixed cmc), γcmc (the surface tension at the cmc) for the mixtures are listed in Table 3. The minimum area per molecule Amin indicates that the surfactant molecules are closely packed or loosely packed at air-water interface. The surface excess concentration (Γmax) defined surface saturation using the Gibbs isotherm (Eq. (8)). The non-ionic surfactants which contain poly(ethylene oxide) chain have a large number of oxygen atoms with lonepaired electrons. This will have a tendency to react coulombically with cationic Gemini surfactant. The degree of Amin is low, suggesting that the air/ water interface is closely packed [69] The surface area of surfactants in mixed micelle is use to find out the packing parameter P, P¼

Vo lc Amin

ð10Þ

where Vo is the volume of hydrocarbon chain, given by Tanford formula [64]. Vo = [27.4 + 26.9(nc-1)] 2 Å3, lc = [1.54 + 1.26(nc-1)] Å is maximum chain length and nc is the number of carbon atoms in hydrocarbon chains. The packing parameter gives information about the size of micelle. If packing parameter lies in the range of 0–0.33 then micelle is in ellipsoidal or spherical shape, 0.33–0.5 then cylindrical shape, and 0.5–1 then vesicle in shape. The packing parameter values for all the five systems lie in the range of 0–0.33 (Table 2) which implies that all the micelles are ellipsoidal in nature. Also the packing parameter values suggest loose arrangement of molecules at the air/water interface and possibly mixed micelles formation may take place [70]. The same geometry has also been established from SANS results. For evaluation of synergism in mixing, another thermodynamic parameter described as free energy of a surface at equilibrium, Gsmin ¼ Amin γcmc NA

Naphthalene

12-5-12 DTAB DMDAO C12EO6 C12PS SDES 12-5-12 + DTAB 12-5-12 + DMDAO 12-5-12 + C12EO6 12-5-12 + C12PS 12-5-12 + SDES

31.08

1020 NA Γmax

Table 5 Solubilization parameter MSR, lnkm, and R (deviation ratio) for the pure and mixtures (1:1) of surfactants.

ð11Þ

MSR

logkm R

0.139 0.040 0.057 0.032 0.009 0.025 0.045 0.074 0.075 0.027 0.080

1.43 0.94 1.08 0.84 0.33 0.74 0.98 1.18 1.19 0.77 1.21

– – – – – – 0.50 0.75 0.87 0.37 0.96

Anthracene

Pyrene

MSR

logkm R

MSR

logkm R

0.018 0.006 0.004 0.005 0.005 0.002 0.006 0.007 0.011 0.012 0.013

3.60 3.13 3.01 3.07 3.08 2.79 3.17 3.23 3.42 3.44 3.49

0.019 0.029 0.017 0.089 0.013 0.024 0.016 0.024 0.073 0.065 0.077

3.20 3.37 3.15 3.83 3.04 3.29 3.11 3.30 3.75 3.71 3.77

– – – – – – 0.54 0.66 1.00 1.04 1.31

– – – – – – 0.66 1.33 1.35 3.99 3.55

where γcmc is surface tension at cmc, NA is the Avogadro's number. The extent of lowering the free energy may be measured for evolution of synergism in mixed surfactant system [71]. The lower the value of free energy, the more thermodynamically stable surface is formed or the more surface activity is attained, which is a measure of evolution of synergism. The excess free energy of micellization, ΔGex ΔGex ¼ ½X 1 ln f 1 þ ð1−X 1 Þ ln f 2  RT

ð12Þ

The negative value of excess free energy of micellization indicates the energetic stabilization of the mixed micelles. As the value of excess free energy increases indicating the formation of more stable micelle; this is also supported by interaction parameters (Table 2). The free energy of micellization can be calculated by ΔGM ¼ ½X 1 ln X 1 f 1 þ X 2 ln X 2 f 2 RT

ð13Þ

Regular solution theory (RST) assumes that excess entropy of mixing and ideal enthalpy of mixing are zero. According to RST, the relation between excess free energy of micellization, excess enthalpy of micellization can be calculated as follow [72]. ΔGex ¼ ΔHex ¼ ΔH M ¼ ½X 1 ln f 1 þ ð1−X 1 Þ ln f 2 RT

ð14Þ

Using Eqs. (13) and (14), the entropy of micellization can be calculated as, ΔSM ¼

ΔH M −ΔGM T

ð15Þ

The free energy of micellization is negative, which indicates that mixed micelle formation is favourable and the micelles are thermodynamically stable. The positive entropy contribution must be the driving force of micellization. Hence, it can be assumed that mixed micelle formation is an entropically favourable process Table 4.

Table 4 Chain–chain interaction parameters (B0, B1, B2) and thermodynamic parameters (free energy micellization by Maeda's approach (ΔGMaeda), surface free energy (ΔGmin), free energy of micellization (ΔGM), excess free energy (ΔGex), entropy of micellization (△SM) of surfactant mixtures at 30 °C. Compounds

B0

B1

B2

ΔGmaeda (kJ mol−1)

ΔGmin (kJ mol−1)

ΔGM (kJ mol−1)

ΔGex = ΔHM (kJ mol−1)

ΔSM (J mol−1 K−1)

12-5-12 + DTAB 12-5-12 + C12 PS 12-5-12 + DMDAO 12-5-12 + C12E6 12-5-12 + SDES

2.71 0.85 0.32 −2.57 −0.96

−7.48 −12.9 −2.60 −3.27 −16.76

4.69 12.00 2.21 5.77 17.65

−0.56 −6.61 −1.11 −7.61 −12.45

45.95 50.77 46.68 63.14 85.27

−3.99 −9.26 −3.11 −4.88 −12.84

−2.46 −7.52 −1.38 −3.26 −11.09

5.04 5.74 5.72 5.33 5.75

U. Patel et al. / Journal of Molecular Liquids 225 (2017) 888–896

893

Table 6 Micellar parameter of Pure and mixed micellar systems. Additives

Ra (Å)

Rb (Å)

Ra/Rb

Nagg.

Α

Shape of micelle

50 mM 12-5-12 50 mM DTAB 50 mM C12E6 50 mM DMDAO 50 mM C12PS 50 mM 12-5-12 + 50 mM DTAB 50 mM 12-5-12 + 50 mM C12E6 50 mM12-5-12 + 50 mM DMDAO 50 mM 12-5-12 + 50 mM C12PS

30.16 19.3 500 30.78 16.7 27.37 27.87 23.60 24.15

17.8 16.6 22.88 17.78 – 18.5 17.46 16.25 19.15

1.70 1.16

48 74

1.73 – 1.47 1.60 1.45 1.26

115 35 33 30 22 31

0.2 ± 0.01 0.2 ± 0.01 – – – ~0.9 0.3 ± 0.01 0.5 ± 0.01 0.2 ± 0.01

Ellipsoidal Ellipsoidal Short rod-like Ellipsoidal Spherical Ellipsoidal Ellipsoidal Ellipsoidal Ellipsoidal

Where Rh, hydrodynamic radius; Ra, semimajor axis; Rb, semiminor axis; Ra/Rb, axial ratio (ellipticity); α, fractional charge; Nagg, aggregation number.

electrostatic and hydrophobic forces to each other so the higher attraction leads to more solubilization of PAHs.

3.4. Solubilization of PAHs Molar solubilization ratio (MSR) is the number of moles of organic compound solubilized per mole of the surfactant solution and can be calculated from the slope of the absorbance versus concentration of surfactant [43]. Above cmc, in case of both pure and mixed surfactant systems, the PAH solubility was increased. MSR can also be calculated from the equation as follows, MSR ¼

St −Scmc Ct −Ccmc

The ratio of experimental molar solubilization (MSRexp) and the ideal value of (MSRideal) is known as the deviation ratio (R). The deviation ratio is the mixing effect of mixed surfactants on solubilization of PAHs. It can be calculated by the below equation:

ð16Þ R¼

where St is the total apparent solubility of the PAH in pure/mixed surfactant solution at a particular surfactant concentration Ct. Scmc is the solubility of the PAH at cmc. The solubilizing capacity of the organic compound in the micellar system can also expressed in terms of the partition coefficient km. The value of km is depends mainly on temperature, nature of surfactant and PAHs. The partition coefficient can be mathematically calculated by the following equation, Xm km ¼ Xa

3.5. Effect of surfactants on solublization of PAHs

ð17Þ

where Xm and Xa are the mole fraction of solute in micelles and the aqueous phase. The Xm value can be calculated by Xm = MSR/ (1 + MSR) and Xa = ScmcVw equation, here Vw is the molar volume of water (18.03 ml/mol). The curve of PAHs solubility versus surfactant concentration are shown in Fig. 2. The solubility of PAHs was increases linearly with increase in concentration of pure surfactant and their mixtures above their cmc. The MSR values and the km values of single and mixed system are shown in Table 5. The MSR and km values of PAHs in single surfactants are lower than binary system because of single surfactant have limited solubilizing capacity. The order of solubilization of naphthalene, anthracene and pyrene is different. The MSR values obtained for naphthalene and pyrene were higher than anthracene for all the surfactant systems. The more hydrophobic pyrene solubilize in surfactants solution more than the anthracene because it depends upon its molecular arrangement. In anthracene three benzene rings are linearly fused and in pyrene four fused benzene resulting in flat aromatic system. Hydrophilic-hydrophilic interactions occur at the mixed micelle-water interface and disturb the solubilization of PAHs whereas hydrophobic-hydrophobic interactions taking place in the micellar core affect the solubilization of PAHs. Higher value of MSR and km in binary system indicate synergism in mixed system. In mixed system of 12-5-12 and SDES, C12PS there was a less value of βm show the higher solubilization of PAHs. But the difference in the solubilizing ability in all surfactants depends on their molecular structure properties like polarity, hydrophobicity, solute concentration etc. In the mixture of 12-512 and SDES, there was strong electrostatic interaction between two oppositely charged head groups. These surfactants bind tightly, via

MSRexp MSRideal

ð18Þ

MSRideal can be calculated by using the MSRexp of single surfactant solution based on the ideal mixing rule. MSRideal was calculated by the, MSRideal ¼ MSR1 α1 þ MSR2 α2

ð19Þ

where α1, α2, MSR1 and MSR2 are the mole fraction and the molar solubilization ratio of individual components 1 and 2 in mixed surfactant solution respectively. The deviation ratio (R) indicate MSRexp values have the positive deviation from the ideal mixing of all surfactants. 3.6. Small angle neutron scattering analysis The SANS data of pure gemini and pure conventional surfactants and their mixtures are shown in Fig. 3 and solid lines in the figures show the fitted curves and the value of extracted parameters are given in Table 6. The micellar systems are assumed to be monodisperse for the simplicity of the calculation and to limit the number of unknown parameters in the analysis. The semi major axis (a), semi minor axis (b) and the fractional charge (α) are the parameters obtained by the analysis of the SANS data. The aggregation number (Nagg) can be calculated by this formula Nagg = 4πab2/3ν, where ν is the volume of the surfactant monomer. The figures show SANS measurements of 50 mM 12-5-12 gemini surfactant in D2O expose measured scattering profiles with close resemblance to each other. The SANS distribution data of pure gemini shows peaks because of ionic nature. It is found that both intra micellar P(Q) interaction and inter micellar S(Q) interaction are present. From the figure it can be seen that in the mixture of gemini and conventional surfactants, there is a peak shifts to lower Q value and the peak height increases. This suggests the micellar growth. The formation of bigger micelles indicates that there is an increase in inter-micellar interaction and hence the peak shifts to lower Q values. The correlation peak occurs around Qm = 2π/d where d is the distance between the micelles and the Qm is the value of Q at the peak position. The shape of mixed micelle remained ellipsoidal at the equal ratio because gemini surfactant restrict the growth of conventional surfactant in a mixture.

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Fig. 2. Variation of the solubility of naphthalne, anthracene, and pyrene with surfactant concentration in single and binary (1:1) surfactant solution.

4. Conclusion The study of mixed micelle formation from the surfactant mixtures of known composition and known structures are studied in this paper. In these binary system of 12-5-12 gemini and all differently charged surfactants show strong synergism. The strong interaction in SDES and C12E6 show very low cmc and large negative interaction parameter (beta) were due to weakening of electrostatic head repulsion which favours the mixed micelle formation. The values of packing parameter (p b 1/3) indicate that the micelles are in ellipsoidal shape. ΔGex conforms the stability of micelles which is supported by interaction parameter. From the thermodynamic point of view micelle formation is

spontaneous, stable, and entropically favourable process. The interaction parameter show strong synergism. Based on Δ Gmin value, it can be concluded that thermo dynamically more surface activity is attained with synergistic interaction not only with SDES, C12E6 but also with C12PS, DMDAO and DTAB. The solubilization of three PAHs in single and mixed surfactant systems was studied. The single and mixed surfactant system increases the solubility of PAHs. Among all the mixture, 12-5-12 and SDES binds tightly due to opposite charge head groups. Therefore, they have relatively more solubilization capacity towards PAHs. These mixed systems have potential capacity to facilitate the solubilization in water. p-Electron ring of PAHs. The binary surfactant solutions

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Fig. 3. SANS data for 50 mM 12-5-12 gemini surfactant in absence (⧠), the pure surfactant 50 mM of DTAB, 50 mM C12PS, DMDAO, C12E6 (Ο) and their (1:1) mixture (Δ).

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