J Surfact Deterg (2011) 14:353–362 DOI 10.1007/s11743-011-1263-6
ORIGINAL ARTICLE
Interaction Between Nonionic and Gemini (Cationic) Surfactants: Effect of Spacer Chain Length Suresh Chavda • Pratap Bahadur • Vinod K. Aswal
Received: 7 October 2010 / Accepted: 24 February 2011 / Published online: 24 March 2011 Ó AOCS 2011
Abstract The interaction between mixtures of nonionic surfactant polyethylene glycol p-(1,1,3,3-tetramethyl butyl)-phenyl ether and cationic gemini surfactants alkanediyl-a,x-bis(dimethyldodecylammonium bromide) (12s-12, where s = 2, 4 and 6) was studied using surface tension and small-angle neutron scattering measurements. Marked interaction was observed for the investigated surfactants mixtures which depend upon the hydrophobic spacer length of the gemini surfactant and also on the fraction of nonionic surfactant in the mixed systems. The results are discussed in terms of interaction parameters calculated according to the theory of regular solutions which uses the critical micelle concentration determined tensiometrically to calculate the molecular interaction parameter and the mole fractions of the two components in the mixed micelles. A relatively high negative molecular interaction parameter value (up to -3.40) obtained for mixtures of nonionic and cationic gemini surfactant indicates a presence of strong attractive interaction in the mixed system that increases with the spacer length of the gemini surfactant. Micellar parameters deduced from small-angle neutron scattering measurements also compliment the surface tension results. Keywords Gemini surfactant Nonionic surfactant Mixed micelle SANS
S. Chavda (&) P. Bahadur Department of Chemistry, Veer Narmad South Gujarat University, Surat 395 007, India e-mail:
[email protected] V. K. Aswal Solid State Physics Division, Bhabha Atomic Research Center, Mumbai 400 085, India
Introduction Surfactant micellization is driven by hydrophobic interactions but opposed by the repulsion of charged head groups (for ionic surfactants) and hydration (for nonionic surfactants). Mixed surfactants are used in many applications such as detergents, food, emulsions, cosmetics, paints, pharmaceuticals, adhesives and many household products. In practical applications, the mixtures of surfactants are often used because they are usually more effective than a single surfactant. On the other hand, some effects that are not expected in single surfactant systems can take place in aqueous solution containing mixed surfactants [1–5]. The performance superiority exhibited by the mixtures of surfactants is attributed to the synergistic or to the antagonistic interaction, depending on the properties of the surfactants [6–12]. Decreased critical micelle concentration (CMC), high interfacial activity, altered micellar morphology as compared to unmixed surfactant alone, reduce the total amount of surfactant and hence the cost and environmental impact [13, 14]. To get information on the extent of interaction between two surfactants in a micelle, regular solution theory has often been applied to calculate the molecular interaction parameter (b) from the CMC value of the surfactant mixture in aqueous solution [15–23]. Gemini or dimeric amphiphiles consist of two amphiphilic monomers having a covalent linking at the head group level. Compared to their monomeric analogs, gemini surfactants have a much lower CMC. Due to low Krafft temperatures and an excellent ability to reduce surface tension in aqueous solution, gemini surfactants have a high potential for practical applications. Due to their higher molecular weight, the skin penetration of the gemini surfactant is expected to be low, which is one of the desirable properties of a surfactant to be used in body care products
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such as soaps, shampoos, and cosmetics. However, the main factor preventing the use of gemini surfactants in practical applications is their high cost. Gemini surfactants, especially cationic ones, have been synthesized for more than 50 years, and they have drawn considerable research interest in recent years and been the topics of several studies [24–27]. Most of the gemini surfactants studied so far have a spacer chain between the two hydrophilic groups in the molecule, usually represented by m–s–m, where m and s are the carbon number in the alkyl chains and in the alkanediyl spacer. It is known that the spacer chain strongly influences the physicochemical properties of the surfactants [24]. Keeping the fact in mind that the use of a pure gemini surfactant in a practical application is not attractive due to its high cost, we studied the mixture of a gemini surfactant with a conventional nonionic surfactant. In order to get a clear picture of the micelle formation and micellar characteristics, we report a systematic study of the interaction between nonionic surfactant polyethylene glycol p-(1,1,3,3-tetramethylbutyl)phenyl ether (symbolized as PEG-TMBPE) and a cationic type gemini surfactant alkanediyl-a,x-bis(dimethyldodecylammonium bromide) (abbreviated as 12-s-12, where s = 2, 4 and 6) at different mole fractions of the mixture using surface tension and small-angle neutron scattering (SANS) measurements. In this paper, first we present the surface tension data of the aqueous solution of mixed gemini surfactants with PEG-TMBPE. Then we discuss the results from SANS.
Materials and Methods Gemini surfactants ethanediyl-1,2-bis(dodecyldimethylammonium bromide)(12-2-12), butanediyl-1,4-bis(dodecyldimethylammonium bromide)(12-4-12), hexanediyl1,6-bis(dodecyldimethylammonium bromide)(12-6-12), were synthesized by refluxing 1-bromododecane withe N,N0 -bis (dimethyl ethane)-a,x-diamine, N,N0 -bis (dimethyl butane)-a,x-diamine, N,N0 -bis (dimethyl hexane)-a, x-diamine, respectively, in dry acetone according to the method reported by Zana et al. [28]. All three Gemini surfactants were recrystallized at least three times from hexane—ethyl acetate mixtures. Polyethylene glycol p-(1,1,3,3-tetramethylbutyl)-phenyl ether (PEG-TMBPE) (commercialy known as Triton X-100) was from the Sigma Chemical Company (USA) and used as received. Structural formulas of the gemini surfactants and PEG-TMBPE are shown in Fig. 1 Deionized and triply distilled water (surface tension 72 ± 0.2 m Nm-1, specific conductivity 10-3 mScm-1) was used throughout the solution preparation for surface tension measurements. D2O (99.9% D) obtained from the Sigma Chemical Company (USA)
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Fig. 1 Structural formula of the surfactants
was used for solution preparation during the SANS measurements. Surface Tension Surface tension measurements were performed using a Kru¨ss (Model K10T) tensiometer using a sand-blasted platinum plate of ca. 5 cm perimeter. The plate was cleaned with distilled water and flamed before each measurement. The mixture of the desired composition of surfactant was made by dissolving the weighed amount of the respective surfactant in water. All solutions were allowed to remain for 1 day, before measurements for proper equilibration. The sample solution was added with the help of a Hamilton micro syringe and surface tension was measured after proper mixing and equilibration at a constant temperature. Surface tension measurements were carried out in order to investigate the CMC for pure and mixed surfactant systems in aqueous solution. Surface tension was measured (±0.2 mNm-1) and plotted as a function of the log of surfactant concentration. The break point in the plot corresponded to the CMC value. Small Angle Neutron Scattering (SANS) SANS experiments were performed at the Dhruva reactor, BARC, India [29]. The solutions in D2O were kept at 30 ± 0.1 °C for 1 h to attain thermal equilibrium. The ˚ mean wavelength of the incident neutron beam was 5.2 A with a wavelength resolution of approximately 15% obtained by a SANS diffractometer with a polycrystalline beryllium oxide (BeO) filter as monochromator. The data were collected in the wave-vector transfer magnitude Q ˚ -1. All the measured SANS distrirange of 0.017–0.35 A butions were corrected for the background and solvent contributions and were normalized to the unit of cross section using standard procedures [29]. The coherent
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differential scattering cross section (dR/dX), per unit volume of solution for interacting micelles is given by [30]. n o dR=dX ¼ nm Vm2 ðqm qs Þ hF 2 ðQÞi þ hF ðQÞi2 ½SðQÞ 1 þB ð1Þ where nm denotes the number density of the micelles of volume Vm, qm and qs are scattering length densities of the micelle and solvent, respectively. F(Q) is the single particle (intraparticle) form factor and S(Q) is the interparticle structure factor, and B is a constant term that represents the incoherent scattering background, which is mainly due to hydrogen in the sample. The details of the analysis are described elsewhere [31].
Results and Discussion Surface Tension for Mixed Surfactant Systems The surface tensions of aqueous solutions of PEG-TMBPE and cationic gemini surfactants 12-2-12, 12-4-12 and 12-612 and their mixtures at mole fractions of PEGTMBPE = 0.10, 0.25, 0.35, 0.50, 0.65, 0.75 and 0.90 were measured over a wide concentration range to obtain CMCs for pure surfactants and their mixtures. Figure 2 shows a surface tension-log concentration plot for 12-2-12 ? PEGTMBPE similar plots were also obtained for 12-4-12 ? PEG-TMBPE and 12-6-12 ? PEG-TMBPE. The surface tensions of single and mixed surfactant systems showed a remarkable decrease with a concentration typical of surfactants without any minimum close to the CMC. The measured values of CMCs for pure surfactants are recorded in Table 1 and are in good agreement with the reported values. The data also reveal that, for all three mixed systems, CMC values are lower than those from pure gemini surfactant solutions. The CMCs decrease with an increase in the PEG-TMBPE mole fraction. Gemini surfactant molecules adopt different conformations depending on the spacer length. Before micellization 12-2-12 monomers are in the trans conformation. While in the case of 12-4-12 and 12-6-12, they are in the cis conformation [15]. In the case of the trans conformation, the free energy of transfer for the surfactant monomer from the aqueous phase to the pseudo micellar phase is relatively lower as compared to the cis conformation case. Hence, micellization is relatively more easily facilitated for 12-212 (lower CMC) and vice versa for 12-4-12 and 12-6-12. Further comparison between 12-4-12 and 12-6-12 (both have cis conformation) shows that the CMC is lower in the case of 12-6-12. It is due to a more hydrophobic spacer for 12-6-12. However, when PEG-TMBPE is mixed with 12-6-
Fig. 2 Variation of the surface tension versus logarithm of surfactant concentration of pure12-2-12 (open triangles), pure PEG-TMBPE (inverted triangles) and their mixtures in water at various mole fraction of PEGTMBPE (aPEG-TMBPE) 0.10 (filled circles), 0.25 (open circles), 0.35 (open diamonds), 0.50 (filled stars), 0.65 (open stars), 0.75 (open squares) and 0.90 (filled squares) at 35 °C Table 1 CMCs of the pure surfactants Surfactant
CMC, mM Measured
Reported
12-2-12
1.04
0.89a
12-4-12
1.22
1.17a
12-6-12
1.13
1.09a
PEG-TMBPE
0.20
0.20b
a b
Wettig and Verrall [32] Ruiz et al. [33]
12 and 12-4-12 some gemini molecules probably change their conformation from a cis position to a trans position, being affected by their hydrophobic interaction with hydrophobic chains of PEG-TMBPE molecules [15]. To obtain ideal CMC values for the mixed surfactant systems, Rubingh’s regular solution theory for mixed micelles was applied and the CMC for the mixture (CMC12) was calculated using Eq. 2 [15–19, 34–37]. 1 a1 ð1 a1 Þ ¼ þ CMC12 f1 CMC1 f2 CMC2
ð2Þ
Here 1 and 2 refers to the gemini surfactant and PEGTMBPE respectively, e.g. a1 is the mole fraction of the gemini surfactant in the total mixed solution. Similarly, f1 and f2 are the activity coefficients of the gemini surfactant
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and PEG-TMBPE, respectively in the mixed micelle. In the case of ideal behavior f1 = f2 = 1 and hence Eq. 2 takes the form of Eq. 3 as proposed by Clint [38] 1 a1 ð1 a1 Þ ¼ þ CMC12 CMC1 CMC2
ð3Þ
Rubingh’s theory also provides the mole fraction of the surfactant in the micelle as well as the interaction parameter, b, by using Eqs. 4 and 5, respectively. ðX1 Þ2 ln ½ða1 CMC12 =X1 CMC1 Þ 2
ð1 X1 Þ ln ½ð1 a1 ÞCMC12 =ð1 X1 ÞCMC2 Þ b¼
½ða1 CMC12 =X1 CMC1 Þ ð1 X1 Þ2
¼1
ð4Þ ð5Þ
ln f2 ¼ bðX1 Þ
2
ð6Þ ð7Þ
where f1 and f2 are activity coefficients of the gemini surfactant and PEG-TMBPE, respectively in the micelle. The ideal and experimental CMCs are plotted in Fig. 2a, b and c as a function of the mole fraction of PEG-TMBPE for all three mixed systems. The Fig. 2 clearly demonstrates that these mixed systems exhibit a negative deviation from ideal behavior indicating an attractive interaction. This synergistic behavior is also supported by negative values of b as discussed later. Figure 3 also shows that upon increasing the amount of PEG-TMBPE, the CMC is decreased; initially up to aPEGTMBPE = 0.35, the decrease in CMC is more abrupt for each system but when aPEG-TMBPE C 0.50 only a little decrease in CMC was found. In water, PEG-TMBPE interact with the gemini surfactant that reduces the electrostatic repulsion between cationic head groups and promotes micellization. Initially, when aPEG-TMBPE \ 0.5, the reduction in the electrostatic repulsion induced by the addition of PEG-TMBPE would therefore reduce the CMC to a more significant extent. However, when aPEG-TMBPE C 0.5, the electrostatic repulsion is significantly lowered, and any further increase in PEGTMBPE does not significantly affect micellization; this results in a slow decrease in CMC [15].
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aPEG-TMBPE
XPEG-TMBPE
b
f1
f2
PEG-TMBPE ? 12-2-12 0.10
0.38
-0.32
0.95
0.88
0.25
0.58
-1.32
0.64
0.79
0.35
0.65
-1.42
0.55
0.84
0.50
0.73
-1.48
0.45
0.90
0.65
0.80
-1.53
0.38
0.94
0.75
0.83
-1.77
0.29
0.95
0.90
0.91
-1.82
0.22
0.99
bave = -1.38
where X1 is the mole fraction of the gemini surfactant in the mixed micelle. Eq. 4 can be solved iteratively to obtain X1, from which the interaction parameter b is evaluated using the relationship in Eq. 5. Values obtained from Eq. 4 are used to obtain the mole fraction of PEG-TMBPE in the mixed micelle (XPEG-TMBPE = 1 – X1). The b parameters and XPEG-TMBPE are listed in Table 2 for all three mixed systems. The interaction parameter (b) is related to the activity coefficient of the surfactant within the micelle by the following relations: ln f1 ¼ bð1 X1 Þ2
Table 2 Mole fractions of PEG-TMBPE in solution (aPEG-TMBPE), in the mixed micelle (XPEG-TMBPE), the activity coefficient of the surfactant (f1 and f2), the interaction parameter (b) and its average values (bave) for mixed systems at 35 °C
PEG-TMBPE ? 12-4-12 0.10
0.42
-0.48
0.92
0.85
0.25
0.59
-2.06
0.49
0.71
0.35
0.64
-2.07
0.43
0.76
0.50 0.65
0.71 0.77
-2.10 -2.26
0.35 0.26
0.84 0.89
0.75
0.79
-2.62
0.19
0.89
0.90
0.86
-3.04
0.11
0.94
bave = -2.09 PEG-TMBPE ? 12-6-12 0.10
0.41
-0.53
0.91
0.83
0.25
0.57
-2.73
0.41
0.60
0.35
0.62
-2.48
0.39
0.70
0.50
0.70
-2.31
0.32
0.81
0.65
0.77
-2.16
0.28
0.89
0.75
0.78
-2.86
0.18
0.87
0.90
0.86
-3.40
0.08
0.94
bave = -2.35
Negative b values indicate strong interactions. The interaction from b is observed as being higher for cationic gemini surfactants with a longer spacer. Though b did not remain constant with the change in the mole fraction, a marginal increase was noticed with an increase in PEGTMBPE in the mixed system. As shown in Table 2 the negative values of the average interaction parameter (bave) for all the three systems support synergism. Further, as suggested by Fig. 4, bave becomes more negative with increased spacer length. For the short spacer gemini surfactant (12-2-12), two long alkyl hydrophobic chains are relatively closer, a situation allowing for more interaction between themselves and restricting the interaction with other surfactants [39]. However, with increased spacer length (s) (i.e. distance between two alkyl chain of the gemini surfactant studied here) an interaction decrease between hydrophobic alkyl tails facilitates an interaction with PEG-TMBPE. Further, the difference in the bave value
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Fig. 4 Average interaction parameter bave as a function of the spacer length (s)
Fig. 3 Variation in experimental (empty circles) and ideal (filled circles) CMC12 values for mixed systems as a function of the mole fraction of PEG-TMBPE in the mixed solutions (aPEG-TMBPE). a PEGTMBPE ? 12-2-12. b PEG-TMBPE ? 12-4-12 and c PEGTMBPE ? 12-6-12
between PEG-TMBPE ? 12-2-12 and PEG-TMBPE ? 12-4-12 is relatively high as compared to the differences for PEG-TMBPE ? 12-4-12 and PEG-TMBPE ? 12-6-12 which may be due to the similar spacer conformation (cis) for 12-4-12 and 12-6-12 as compared to the trans conformation for 12-2-12 before micellization. Thus, with an increase in s the interaction of the gemini surfactant with PEG-TMBPE increases and results in a more negative b. The activity coefficients reflect the effect and contribution of individual components in mixed micelles [40]. From Table 2, one can see a lower value of activity coefficient and mole fraction in the mixed micelle (X1 = 1-XPEG-TMBPE) for all gemini surfactants, as compared to PEG-TMBPE which suggests that the gemini surfactant is far away from its standard state in a mixed micelle. On the other hand, PEGTMBPE is in a standard state and mixed micelles are rich in PEG-TMBPE in all three systems studied, and these increase with the content of PEG-TMBPE in the mixed system. According to Ruiz and Aguiar [41] and Maeda [42] chain-chain and head-head interactions are present for mixed surfactant systems. The Rubingh’s interaction
parameter (b) includes only head-head interaction. However, when the chain length of the surfactant in the mixed system is different, chain-chain interaction plays an important part. To understand this interaction, Maeda [42] suggested a new approach which is based on the phase separation model and describes the thermodynamic stability of mixed micelle using Gibbs energy of micellization (DGm) given by relation; DGm ¼ RT B0 þ B1 X1 þ B2 X12 ð8Þ where, B0 ¼ ln CMC2 B1 þ B2 ¼ ln CMC1=CMC2
ð9Þ ð10Þ
B2 ¼ b
ð11Þ
B0 is an independent term related with the CMC of the nonionic surfactant, B1 is connected with the change in free energy when an ionic surfactant replaces a non ionic surfactant molecule in a nonionic pure micelle, and negative values of B1 indicate strong chain-chain interaction between the two surfactants [43]. B2 is the regular solution theory interaction parameter but with opposite sign; values for these parameters are listed in Table 3. The Gibbs energy of micellization for pure cationic gemini as well as for non ionic PEG-TMBPE are calculated using Eqs. 12 and 13, respectively. DGm ¼ 2RT ð1:5 2aÞ ln XCMC
ð12Þ
DGm ¼ RT ln XCMC
ð13Þ
Here a is the degree of counterion dissociation and XCMC is the CMC of the surfactant on the mole fraction scale. R and T are the gas constant and the absolute temperature, respectively.
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J Surfact Deterg (2011) 14:353–362 surfactant systems at varying mole fractions of PEG-TMBPE in mixed solution (aPEG-TMBPE)
Table 3 Interaction Parameters B1, B2 and values of the Gibbs energy of micellization for the mixed micelle (DGm) According to Maeda’s approach for PEG-TMBPE ? 12-2-12/12-4-12/12-6-12 mix aPEG-TMBPE
PEG-TMBPE ? 12-2-12 B1
PEG-TMBPE ? 12-4-12 -1
B2
DGm (kJ mol ) -70.54* (-72.10)b
PEG-TMBPE ? 12-6-12 -1
B2
DGm (kJ mol )
–
–
-65.20* (-60.95)c
B1
B1 –
B2
DGm (kJ mol-1)
–
-57.61* (-63.10)b
0.00
–
–
0.10
1.33
0.32
-1.70
1.33
0.48
-1.74
1.20
0.53
-1.83
0.25
0.33
1.32
-3.17
-0.25
2.06
-3.50
-1.00
2.73
-3.93
0.35
0.23
1.42
-3.47
-0.26
2.07
-3.68
-0.75
2.48
-3.93
0.50
0.17
1.48
-3.73
-0.29
2.1
-3.89
-0.58
2.31
-4.03
0.65
0.12
1.53
-3.90
-0.45
2.26
-4.08
-0.43
2.16
-4.08
0.75
-0.12
1.77
-4.04
-0.81
2.62
-4.26
-1.13
2.86
-4.40
0.90
-0.17
1.82
-4.12
-1.23
3.04
-4.41
-1.67
3.40
1.00
–
–
-32.10# (-32.10)a
–
–
-32.10 (-32.10)a
* Values are obtained using conductance measurements at 35 °C, compared values are at c 25 °C and # a b c
– b
–
-4.55 -32.10 (-32.10)a
30 °C
Values are obtained from surface tension measurements Ruiz et al. [33] Rodrı´guez et al. [44] Sohrabi et al. [45]
Values of B1 (see Table 3) indicate that with an increasing mole fraction of PEG-TMBPE in the mixture, the chain-chain interaction become stronger. It also reveals from the DGm values that mixed micelles become more stable with increasing mole fractions of PEG-TMBPE and is relatively lower for the mixed system of gemini with a shorter spacer. For 12-2-12, due to the smaller spacer interaction between themself, is relatively higher, while for 12-4-12 and 12-6-12 it is lower, which leads to a cooperatively lower value of B1 for the 12-2-12 system and higher in the cases of 12-4-12 and 12-6-12. The maximum value of B1 for 12-6-12 is accounted by its more hydrophobic spacer. Size and Aggregation Number of Single and Mixed Surfactant Systems The SANS measurements (Fig. 5a, b, c) on 100 mM gemini surfactant in D2O reveal measured scattering profiles with close resemblance to each other. The SANS distribution data shows typical peaks for the ionic surfactant. It is found that intra-micellar P(Q) interactions as well as inter-micellar interactions S(Q) are present. The SANS data for three geminis are recorded in Table 3 The correlation peak occurs at around Qm = 2p/d, where d is the distance between the micelles and Qm is the value of Q at the peak position [46]. 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 (a) are the parameters obtained after analyzing the SANS data. From the relation Nagg = 4pab2/3v,
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where v is the volume of the surfactant monomer aggregation number (Nagg) was calculated. The parameters in the analysis were optimized by means of a nonlinear leastsquare fitting program [47]. The SANS results for PEGTMBPE in D2O at 30 °C is shown in Fig. 5 along with three gemini surfactants and their corresponding equimolar mixtures with PEG-TMBPE. Solid lines in figures are the fitted curves to the experimental data. It can be seen from Fig. 5a, b and c for 100 mM pure gemini surfactants as we move from 12-2-12 to 12-6-12, there is a peak shift towards higher Q values with a decrease in intensity attributed to the effect of spacer length. From Table 4 for 12-2-12, 12-4-12 and 12-6-12 the aggregation numbers are 318, 78 and 36 and the fractional charges are 0.12, 0.20 and 0.30, respectively. These reverse trends in the aggregation number and the fractional charge for cationic gemini surfactant also support the decrease in micelle size with increasing spacer length [48]. For the 12-2-12 gemini surfactant, the fractional charge (a) value is found to be a minimum which demonstrate that the net charge at two head groups of the gemini surfactant is less, hence the coulombic repulsive forces are also a minimum (decrease in head group polarity) and facilitate the surfactant molecule aggregation and results in a higher Nagg value. However, in the case of 12-4-12 and 12-6-12, the aggregation number is relatively lower due to the increased distance between the charged heads. This can also be understood by considering relatively higher a values for 12-4-12 (a = 0.20) and 12-6-12 (a = 0.30) that indicate an increase in the head group polarity, i.e. a relatively higher repulsion, which results in a Nagg decrease.
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Fig. 5 Plot of normalized neutron scattering cross-section (dR/dX) versus the scattering vector Q for pure 12-2-12 (open circles), 12-4-12 (open squares), 12-6-12 (open triangles), PEG-TMBPE (open stars) and with 1:1 mixture a PEG-TMBPE ? 12-2-12 (filled circles), b PEG-TMBPE ? 12-4-12 (filled squares) and c PEGTMBPE ? 12-6-12 (filled triangles) at 30 °C
Goyal et al. [49] have shown that 1% (0.12 M) PEGTMBPE micelles were oblate and the hydrophilic shell of the micelle has about 20 water molecules per surfactant molecule. This micellar model can explain the data over the whole range of concentrations at ambient temperature without invoking further growth. Close examination of Fig. 5 reveals that the peak for an equimolar mixed system shifts towards the higher Q region along with the increase in intensity in each case, but for the 12-2-12 ? PEGTMBPE system, the peak intensity is reduced as compared with pure 12-2-12 (100 mM). However, for PEGTMBPE ? 12-4-12 and PEG-TMBPE ? 12-6-12, the peak intensities are higher as compared to the corresponding
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pure gemini surfactant. Thus, for 12-2-12 in the presence of equimolar PEG-TMBPE, the micelle size is lower as compared to pure 12-2-12 and is attributed to the decrease in the inter micelle distance (Qm = 2pd) The mixed systems of PEG-TMBPE ? 12-4-12 and PEG-TMBPE ? 12-6-12 exhibit an increase in the micellar size as compared to 100 mM pure gemini. These observation are also supported by a less negative b value for 12-2-12 (b = -1.48) as compared to more negative ones for 12-4-12 (b = -2.10) and 12-6-12 (b = -2.31) for a mole fraction of PEG-TMBPE (aPEG-TMBPE) = 0.5 in each system. A relatively high hydrophobic interaction between 12-2-12 molecules restrict the replacement of more 12-2-12 molecules by PEG-TMBPE, hence a relatively lower aggregation number of PEG-TMBPE (NPEGTMBPE = 135) is found in the case of PEG-TMBPE ? 122-12 system. However, the size of this micelle is still bigger due to a higher aggregation number (Nagg = 90) of 12-2-12 in mixed micelles as compared to the other two system. While in the case of 12-4-12 and 12-6-12, less intramolecular interaction facilitates the replacement of gemini molecules by PEG-TMBPE, thus mixed micelles are composed of a higher number of PEG-TMBPE (NPEGTMBPE = 150 and 168, respectively for PEG-TMBPE ? 12-4-12 and PEG-TMBPE ? 12-6-12 systems) as compared to the PEG-TMBPE ? 12-2-12 system. Due to the double alkyl chain of a gemini surfactant as compared to the single chain of PEG-TMBPE, the size of a mixed micelle depends more on the gemini surfactant. Thus although, the extent of interaction of PEG-TMBPE with the gemini surfactants follows the order PEG-TMBPE ? 12-612 (bave = -1.38) [ PEG-TMBPE ? 12-4-12 (bave = -2.05) [ PEG-TMBPE ? 12-2-12 (bave = -2.35), due to the decrease in the aggregation number of gemini in the mixed micelle systems (Ngemini) as a function of spacer length, a totally reverse order is found for size and the total micellar aggregation number of mixed systems (NTotal). For mixed systems, the aggregation number depends on two kinds of repulsions (1) electrostatic between cationic head groups and (2) steric between hydrophilic oxyethylene groups of the nonionic surfactants. The presence of PEG-TMBPE molecules with the gemini surfactant reduces the first kind of repulsion, but at a higher PEG-TMBPE proportion (equimolar), steric repulsion becomes preponderant and it increases the area required per surfactant head group, which leads to the formation of a mixed micelle with a higher curvature, resulting in the reduction of the NTotal [36]. The number of PEG-TMBPE molecules in a mixed system is found to be a minimum for the PEGTMBPE ? 12-2-12 system, because the 12-2-12 molecules have relatively high hydrophobic intramolecular interactions between themselves, hence the mixed micelles of PEG-TMBPE ? 12-12-12 are quite larger in size with the
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Table 4 Semi major axis (a), semi minor axis (b), fractional charge (a) and aggregation number of gemini surfactant (Ngemini), PEGTMBPE (NPEG-TMBPE) and total micellar aggregation number for an Surfactant PEG-TMBPE
(a) ˚ A
(b) ˚ A
equimolar mixture of surfactants (NTotal) (total surfactant concentration 100 mM) obtained from SANS at 30 °C (a)
Ngemini
42.0 ± 1.5
20.0 ± 1.0
185 ± 9
–
169.8 ± 1.8
19.8 ± 0.5
0.12 ± 0.01
318 ± 10.0
–
–
12-4-12
69.4 ± 1.6
15.8 ± 0.5
0.20 ± 0.01
78 ± 7.0
–
–
12-6-12
34.7 ± 1.6
15.7 ± 0.5
0.30 ± 0.02
36 ± 3.0
PEG-TMBPE ?12-2-12
76.0 ± 1.8
20.4 ± 0.8
0.15 ± 0.01
90 ± 8.0
135 ± 8.0
225 ± 8.0
PEG-TMBPE ?12-4-12
55.5 ± 2.0
19.0 ± 0.7
0.26 ± 0.02
50 ± 9.0
150 ± 9.0
200 ± 9.0
PEG-TMBPE ?12-6-12
37.1 ± 1.9
17.3 ± 0.9
0.41 ± 0.05
25 ± 6.0
168 ± 6.0
193 ± 6.0
Conclusion In the present paper we studied the binary systems formed by the nonionic surfactant polyethylene glycol p-(1,1,3,3tetramethylbutyl)-phenyl ether (PEG-TMBPE) and three gemini surfactants alkanediyl-a,x-bis (dimethyldodecylammonium bromide) having similar tail length but varying spacer length. The mixtures show strong synergism in their micellar properties viz. CMC, size, and aggregation number. According to Rubing’s regular solution theory, the interaction parameter, b at all mole fractions for each mixed system was negative, thus indicating nonideality and synergism. The b becomes more negative with the mole fraction of PEG-TMBPE and the gemini surfactant spacer length. Maeda’s approach suggests negative values of DGm which indicate that mixed micellization is thermodynamically favored and mixed micelles are stable in the order
123
–
NTotal
12-2-12
higher aggregation of the 12-2-12 molecules. These intramolecular interactions reduce the amount of water in the mixed micelle, which facilitate a high aggregation of surfactant monomer in mixed micelles. However, these hydrophobic intramolecular interactions reduce with spacer length (for the gemini surfactant studied here), hence NTotal decreases with the increase in spacer length. Thus, for PEG-TMBPE ? 12-2-12 system, the NTotal is found to be at a maximum and then decreases with the spacer length of the gemini surfactant. In the cases of 12-4-12 and 12-6-12, the increase in hydrophobicity at the level of the head group and the modification in the distance between the head groups support synergism [13], allow for more interaction with nonionic PEG-TMBPE. Hence, the number of PEG-TMBPE molecules in mixed micelles (NPEGTMBPE) is relatively higher in mixed systems of 12-4-12 and 12-6-12, obviously at a maximum for the PEGTMBPE ? 12-6-12 system due to the maximum hydrophobicity (relatively longer spacer) at the level of the head group and higher head group polarity of 12-6-12.
–
NPEG-TMBPE
–
–
PEG-TMBPE ? 12-2-12 \ PEG-TMBPE ? 12-4-12 \ PEG-TMBPE ? 12-6-12. Mixed micelle size and the aggregation number from SANS also indicates the effect of the spacer length. The synergism and nonideality originate from the reduction in the electrostatic repulsion between the head groups of cationic gemini surfactants and the increase in steric repulsion of hydrophilic head groups of PEG-TMBPE as a function of the content of PEG-TMBPE, and depend on the hydrophobicity of spacers and the polarity of the gemini surfactant head groups. Acknowledgment SC thanks UGC, New Delhi for financial assistance in the form of a Rajiv Gandhi National Fellowship [F.16-1228(SC)/2008(SA-III)].
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Author Biographies Suresh Chavda M.Sc., is a Ph.D. student at Veer Narmad South Gujarat University, Surat, India. Pratap Bahadur Ph.D., D.Sc. has been Professor of Chemistry at Veer Narmad South Gujarat University Surat, India since 1988. His research interests are in colloidal systems and polymer solutions.
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Vinod K. Aswal Ph. D., is a Scientific Officer G at the Solid State Physics Division of the Bhabha Atomic Research Centre, Mumbai, India. He has been working in the field of small-angle neutron scattering (SANS) for the instrument development and its applications to the soft condensed matter, i.e. amphiphilic system, block copolymers, magnetic fluids and biological system.
