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Thermodynamics of non-ionic surfactant Triton X-100-cationic surfactants mixtures at the cloud point
J. Chem. Thermodynamics 43 (2011) 1800–1803
Contents lists available at ScienceDirect
J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct
Thermodynamics of non-ionic surfactant Triton X-100-cationic surfactants mixtures at the cloud point Çig˘dem Batıgöç, Halide Akbasß ⇑, Mesut Boz Department of Chemistry, Faculty of Sciences, Trakya University, 22030 Edirne, Turkey
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 24 May 2011 Accepted 8 June 2011 Available online 21 June 2011 Keywords: Cloud point Gemini surfactant Triton X-100 Thermodynamic parameter
a b s t r a c t This study investigates the effects of gemini and conventional cationic surfactants on the cloud point (CP) of the non-ionic surfactant Triton X-100 (TX-100) in aqueous solutions. Instead of visual observation, a spectrophotometer was used for measurement of the cloud point temperatures. The thermodynamic parameters of these mixtures were calculated at different cationic surfactant concentrations. The gemini surfactants of the alkanediyl-a-x-bis (alkyldimethylammonium) dibromide type, on the one hand, with different alkyl groups containing m carbon atoms and an ethanediyl spacer, referred to as ‘‘m-2-m’’ (m = 10, 12, and 16) and, on the other hand, with –C16 alkyl groups and different spacers containing s carbon atoms, referred to as ‘‘16-s-16’’ (s = 6 and 10) were synthesized, purified and characterized. Additions of the cationic surfactants to the TX-100 solution increased the cloud point temperature of the TX-100 solution. It was accepted that the solubility of non-ionic surfactant containing polyoxyethylene (POE) hydrophilic chain was a maximum at the cloud point so that the thermodynamic parameters were calculated at this temperature. The results showed that the standard Gibbs free energy ðDGcp Þ, the enthalpy ðDHcp Þ and the entropy ðDScp Þ of the clouding phenomenon were found positive in all cases. The standard free energy ðDGcp Þ increased with increasing hydrophobic alkyl chain for both gemini and conventional cationic surfactants; however, it decreased with increasing surfactant concentration. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Non-ionic surfactants are widely used as an agent to solubilize and emulsify processes in the textile, detergent and cosmetic industries. The solubility of non-ionic surfactants in aqueous solution is due to the high hydration of oxyethylene chains [1,2]. Since hydrogen bonding is a temperature sensitive phenomenon, heating of aqueous surfactant solutions to a certain temperature causes the appearance of clouds due to incomplete solubility. This temperature is called as ‘Cloud Point’ and is an important character of surfactant [3,4]. At this temperature, the surfactant is no longer soluble in water and phase separation (a surfactant-rich phase and a surfactant-poor phase in which surfactant concentration is approximately critical micelle concentration) occurs in solution [2,5]. The phase separation is reversible, and on cooling of the mixture to a temperature below the cloud point, the two phases merge to form once again a clear solution. This separation is probably due to the sharp increase in the aggregate number of the micelles and the non-ionic surfactant loss of some or all of the surfactant functions above the CP [6,7]. Currently, new classes of surfactants, referred to as dimeric surfactants (also known as gemini surfactants), have attracted ⇑ Corresponding author. Tel.: +90 284 235 9592; fax: +90 284 235 1544. E-mail address: [email protected] (H. Akbasß). 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.06.005
increasing attention due to their superior surface activity compared to that of the conventional (monomeric) surfactants. These surfactants have two hydrophobic tails and two polar heads connected by a spacer and tend towards a much lower critical micelle concentration, which can produce lower surface tension than conventional surfactants at the same molar concentration. Gemini surfactants have better solubilizing, wetting, and foaming ability, as well as show stronger biological activity and lime-soap dispersing properties compared to conventional surfactants [8–10]. The CP is very sensitive to the presence of additives even at very low concentrations. The additives modify the surfactant solvent interaction. As a consequence, they change the critical micelle concentration (cmc), the size of the micelles and the phase behavior in the surfactant solutions [11]. Many authors have investigated the effect of additives on the cloud point to include inorganic electrolytes [12–14], organic compounds [15,16] and other surfactants [17–19]. In our previous studies, we have investigated the influence of inorganic additives on the cloud points of Triton X-405 and Brij35 [13,20] and on the cloud points of Triton X-100-gemini and conventional cationic surfactants mixtures by the spectrophotometric method [21]. In this paper, we investigated the effects of gemini and conventional cationic surfactants on the cloud point (CP) of the non-ionic surfactant Triton X-100 (TX-100) in aqueous solutions. The thermodynamic parameters of these mixtures were calculated at different cationic surfactant concentrations.
Ç. Batıgöç et al. / J. Chem. Thermodynamics 43 (2011) 1800–1803
2. Experimental 2.1. Material Triton X-100 has the chemical formula R–C6H4–(OC2H4)p–OH, where R is a branched octyl group and p is the average number of oxyethylene groups (p 9.5). The non-ionic surfactant (Triton X-100) was supplied by Aldrich–Sigma. All supplied material was of reagent grade and were used without further purification. The cationic surfactants, dodecyltrimethylammonium bromide (DTAB), hexadecyltrimethylammonium bromide (CTAB), dodecylpyridinium chloride (DPC) were supplied by Merck. The hexadecylpyridinium chloride (CPC) and hexadecylpyridinium bromide (CPB) supplied from Fluka and Aldrich. The water used to prepare the sample solutions was double distilled in a distillation apparatus (GFL 2102). The specific conductivity of this water was in the range of (1 to 2) lS cm1. The cationic gemini surfactants, alkanediyl-ax-bis (alkyldimethylammonium bromide), were synthesized. For these, 1-bromodecane, 1-bromododecane, 1-bromohexa decane, N,N,N0 ,N0 -tetramethylethylenediamine, 1,6-dibromohexane, 1,10dibromodecane, N,N-dimethylhexa decylamine, and acetone were supplied by Fluka and Merck. 2.2. Methods 2.2.1. Synthesis of gemini surfactants The structures of cationic gemini surfactants are shown in figure 1. A mixture of N,N,N0 ,N0 -tetramethylethylenediamine and a corresponding alkyl bromide was refluxed in acetone for 24 h to synthesize 10-2-10, 12-2-12, and 16-2-16 type gemini or dimeric cationic surfactants. For the synthesizes of 16-6-16 and 16-10-16 type gemini cationic surfactants, a mixture of N,N-dimethylhexadecylamine and a corresponding alkyl dibromide was refluxed in acetone for 24 h. The white solids obtained were filtered after cooling and then recrystallized from acetone at least two times [22–25]. The yield of the reaction was almost quantitative, 97%. The purities were checked via nuclear magnetic resonance (NMR). The 1H NMR spectra were recorded in deuteriochloroform solution with a Varian Mercury Plus 300 MHz spectrometer. 13C spectra were recorded at 75 MHz.
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C NMR (75 MHz, CDCl3): d = 14.38, 22.93, 23.27, 26.47, 28.40, 29.00, 29.43, 29.60, 29.62, 29.67, 29.79, 29.83, 29.92, 29.98, 32.16,51.26, 57.07, 66.02. 16-6-16; 1H NMR (300 MHz, CDCl3): d = 0.86 (t, 6H), 1.23–1.33 (m, 52H), 1.57 (m, 4H), 1.70 (m, 4H), 2.0 (m, 4H), 3.37 (s, 12H), 3.40–3.47 (m, 4H), 3.69–3.72 (m, 4H). 13C NMR (75 MHz, CDCl3): d = 14.36, 21.77, 22.92, 23.10, 24.42, 26.54, 27.73, 27.94, 29.50, 29.59, 29.62, 29.70, 29.83, 29.85, 29.88, 29.91, 32.15, 51.26, 64.40, 65.06. 16-10-16; 1H NMR (300 MHz, CDCl3): d = 0.88 (t, 6H), 1.25 (m, 48H), 1.35 (m, 8H), 1.44 (m, 8H), 1.74 (m, 8H), 3.35 (s, 12H), 3.43–3.49 (m, 4H), 3.75–3.80 (m, 4H). 13C NMR (75 MHz, CDCl3): d = 14.35, 22.70, 22.90, 23.07, 26.07, 26.52, 28.40, 28.65, 29.49, 29.58, 29.64, 29.71, 29.83, 29.87, 29.91, 32.13, 51.24, 64.42, 64.54. 2.3. Determination of cloud point temperatures The UV–vis spectra were recorded with a Shimadzu model UV-1700 spectrophotometer. A Shimadzu CPS 240 temperature controller (0 to 110) °C including a Peltier system was used as the heater in the cloud point determinations to heat the cell holder of the spectrophotometer. The ultraviolet spectrum of 1% (w per v) TX-100 at different temperatures is shown in figure 2. As seen from figure 2, the absorbance values taken at (65.2 to 65.9) °C over the range of (350 to 200) nm wavelength are approximately same. When the temperature was increased by 0.1 °C (at 66.0 °C), the absorbance values at (350 to 300) nm and 250 nm interval wavelength significantly increased. This temperature was accepted as the CP value of the TX-100 solution. All cloud points given were the average of two measurements (the values of CP were reproducible within ±0.1 °C). The CP results for TX-100 solutions (0.2, 0.5, 1, 2, 3, and 4) wt% in water are presented in figure 3. As seen in figure 3, a significant change in CP is observed, that is to say the CP becomes almost constant with increasing concentration [25,26]. Therefore, the concentration of non-ionic surfactant used for CP measurements was maintained at 1% (w per v). 3. Results and discussion We observed the variations of cloud point of TX-100-cationic surfactant mixtures at different surfactant concentrations. The
2.2.2. Spectral characteristics of gemini surfactants 10-2-10; 1H NMR (300 MHz, CDCl3): d = 0.87 (t, 6H), 1.22–1.41 (m, 28H), 1.80 (m, 4H), 3.48 (s, 12H), 3.70 (m, 4H), 4.72 (s, 4H). 13 C NMR (75 MHz, CDCl3): d = 14.37, 22.91, 23.28, 26.45, 29.53, 29.72, 32.09, 51.32, 57.02, 66.12. 12-2-12; 1H NMR (300 MHz, CDCl3): d = 0.86 (t, 6H), 1.17–1.30 (m, 36H), 1.81 (m, 4H), 3.47 (s, 12H), 3.70 (m, 4H), 4.70 (s, 4H). 13 C NMR (75 MHz, CDCl3): d = 14.38, 22.92, 23.28, 26.48, 29.42, 29.60, 29.77, 29.80, 29.88, 32.15, 51.30, 57.02, 66.09. 16-2-16; 1H NMR (300 MHz, CDCl3): d = 0.86 (t, 6H), 1.23–1.34 (m, 52H), 1.82 (m, 4H), 3.46 (s, 12H), 3.70 (m, 4H), 4.69 (m, 4H).
FIGURE 1. Structures of gemini cationic surfactants types 10-2-10, 12-2-12, 16-216, 16-6-16, and 16-10-16.
FIGURE 2. The absorption spectra for 1% (w per v) Triton X-100 in water at different temperatures.
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FIGURE 3. Plot of the cloud point temperature against surfactant concentration for Triton X-100.
thermodynamic parameters of the non-ionic surfactant were calculated using the CP values at phase separation. The values of the standard Gibbs free energy ðDGcp Þ, the enthalpy ðDHcp Þ and the entropy ðDScp Þ of the clouding phenomenon were calculated considering the solubility limit at the clouding temperature [27,28] by the phase separation model from the relationships
DGcp ¼ RT ln X cs ; DHcp ¼ T 2
dDGcp =T dT
ð1Þ ð2Þ
where Xcs is the mole fraction of cationic surfactant in surfactant mixture; R is the gas constant, and T is taken as Kelvin temperature (CP + 273.15) [28,29]. The enthalpy of the cloud point was calculated from the slope of a plot of DGcp =T versus T at a particular temperature using figure 4. The entropy of the micellization ðDSm Þ was estimated from the values of calculated enthalpy and Gibbs free energy.
T DScp ¼ DHcp DGcp :
ð3Þ
Ionic surfactants increase the CP value of the non-ionic surfactant leading to charged mixed micelles which cause electrostatic repulsions between micelles [26]. The addition of cationic surfactants on the TX-100 solution raised the CP of the non-ionic surfactant forming positive charged mixed micelles. Thermodynamic parameters of TX-100 at the clouding temperatures have positive values in the presence of cationic surfactants. These parameters are given in table 1. As shown in table 1, the standard Gibbs free energy ðDGcp Þ increases with increasing hydrophobic alkyl chain for both gemini and conventional cationic surfactants. However, they decrease with increasing concentration of surfactant. The positive values of DGcp indicate that the process proceeds non-spontaneously. The DHcp and T DScp values decrease with increasing hydrophobic alkyl chain of cationic surfactants. However, they increase with increasing cationic surfactant concentration. Because TX-100 forms mixed micelles with cationic surfactants, inter-micellar repulsions occur in the solution. Hence the values of DHcp and T DScp are positive [27]. Also, the positive values of DHcp can be attributed to a disruption
FIGURE 4. Plots of DGcp =K versus temperature for different Triton X-100-cationic surfactant mixtures. (a) 10-2-12 (d), 12-2-12 (j), 16-2-16 (N), 16-6-16 (), and 1610-16 (+). (b) DPC (d), CPC (j), CPB (N), DTAB (+), and CTAB ().
of water structure around the hydrophobic alkyl tails of non-ionic surfactant molecules [15]. The results in table 1 showed that the values of CP and DGcp increased less with increased spacer length by comparison to the change of alkyl chain length. The studies show that the cmc of surfactant typed m-s-m with a hydrophobic polymethylene spacer is a maximum at s P 6 irrespective of the value of m and then the cmc values again decrease [8,22,30,31]. The similar state was observed in the values of DHcp and T DScp . This case is attributed to a conformational change of the surfactant ion at low s and a progressive penetration of the spacer group in to the micelle hydrophobic core [32]. In the cases of CPC and CPB, the effect of Br ions on the CP value of TX-100 is more than that by Cl- ions (the increase of CP values for CPB is 32.4 °C and for CPC is 29.9 °C at 0.4 mM). The Br ion is capable of connecting to more than Cl ions and the aggregation number of CPB is greater than that of the CP. Romsted showed that the ionization degree of Br is greater than for Cl [33]. Thus the CPB further increased the CP values creating larger micelles. On the other hand, there are not significant changes in
Ç. Batıgöç et al. / J. Chem. Thermodynamics 43 (2011) 1800–1803 TABLE 1 Thermodynamic parameters of Triton X-100 and cationic gemini surfactant mixtures at cloud point. Additive Concentration (mM)
Cloud point (K)
DH / T DS / DG / (kJ mol1) (kJ mol1) (kJ mol1)
10-2-10
0.1 0.2 0.3 0.4 0.5 0.6
345.95 350.15 353.45 356.15 356.85 357.75
38.02 36.47 35.61 35.04 34.44 33.99
144.02 147.64 150.36 152.66 153.26 154.04
106.00 111.17 114.75 117.62 118.82 120.05
12-2-12
0.1 0.2 0.3 0.4 0.5 0.6
351.15 359.15 362.75 367.25 368.65 371.65
38.59 37.40 36.55 36.13 35.58 35.31
90.74 94.92 96.82 99.25 100.01 101.65
52.15 57.52 60.27 63.12 64.43 66.34
16-2-16
0.1 0.2 0.3 0.4 0.5 0.6
351.35 359.35 364.35 369.15 372.15 374.45
38.61 37.42 36.73 36.31 35.92 35.57
78.31 81.92 84.31 86.45 87.86 88.95
39.70 44.50 47.58 50.14 51.94 53.38
16-6-16
0.1 0.2 0.3 0.4 0.5 0.6
352.65 360.55 365.75 370.75 372.85 375.35
38.75 37.55 36.85 36.47 35.99 35.66
80.03 83.65 86.08 88.45 89.46 90.66
41.28 46.10 49.23 51.98 53.47 55.00
16-1016
0.1 0.2 0.3 0.4 0.5 0.6
352.65 361.55 366.75 371.25 374.15 376.65
38.75 37.65 36.95 36.52 36.11 35.78
79.50 83.56 85.98 88.10 89.49 90.69
40.75 45.91 49.03 51.58 53.38 54.91
DTAB
0.1 0.2 0.3 0.4 0.5 0.6
344.65 349.65 352.15 355.15 356.35 359.15
37.88 36.41 35.48 34.94 34.39 34.12
125.16 128.81 130.66 132.90 133.80 135.91
87.28 92.40 95.18 97.96 99.41 101.79
0.1 0.2 0.3 0.4 0.5 0.6
349.55 358.35 364.55 368.15 370.25 372.15
38.40 37.32 36.73 36.22 35.74 35.35
78.60 82.61 85.49 87.19 88.19 89.09
40.20 45.29 48.76 50.97 52.45 53.74
DPC
0.1 0.2 0.3 0.4
344.85 350.85 355.55 358.15
37.90 36.54 35.82 35.23
101.71 105.28 108.12 109.70
63.81 68.74 72.30 74.47
CPC
0.1 0.2 0.3 0.4
348.35 356.05 365.35 369.05
38.28 37.08 36.81 36.30
64.20 67.07 70.62 72.06
25.92 29.99 33.81 35.76
CPB
0.1 0.2 0.3 0.4
349.85 360.75 368.05 371.55
38.46 37.57 37.08 36.55
64.13 68.15 70.93 72.29
25.67 30.58 33.05 35.74
CTAB
of DGcp , DHcp , and T DScp values were investigated, it was found that the Gibbs free energy of the clouding phenomenon is controlled by both the enthalpic and entropic contributions. 4. Conclusions Cationic surfactants formed charged mixed micelles with TX100 and the electrostatic repulsions between micelles occurred in solution. Thus the CP of TX-100 increased with the addition of ionic surfactants. It was observed that the effect of alkyl chain length was greater than that of the spacer group on the CP for the gemini surfactants. The thermodynamic parameters were calculated at the cloud point for all the mixtures studied. The clouding phenomenon was found to be a non-spontaneous process. Also, the values of DGcp , DHcp , and T DScp were found to be positive in all cases. This is due to the fact that the mixed micelles are non-ideal because of the repulsive interactions between charged mixed micelles. Acknowledgement The authors gratefully acknowledge the financial supports of the Trakya University Research Fund (TUBAP-922 and TUBAP-783). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
thermodynamic parameters for these surfactants. When CTAB and CPB are compared, the effect of CPB is greater than that of CTAB on the CP of TX-100. For these surfactants having different hydrophilic groups, the CPB has a larger polar head group than CTAB [34] and the electrostatic repulsions between its mixed micelles formed with TX-100 are greater than CTAB. Consequently, when the values
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[31] [32] [33] [34]
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JCT 11-150
Contents lists available at ScienceDirect
J. Chem. Thermodynamics journal homepage: www.elsevier.com/locate/jct
Thermodynamics of non-ionic surfactant Triton X-100-cationic surfactants mixtures at the cloud point Çig˘dem Batıgöç, Halide Akbasß ⇑, Mesut Boz Department of Chemistry, Faculty of Sciences, Trakya University, 22030 Edirne, Turkey
a r t i c l e
i n f o
Article history: Received 27 April 2011 Received in revised form 24 May 2011 Accepted 8 June 2011 Available online 21 June 2011 Keywords: Cloud point Gemini surfactant Triton X-100 Thermodynamic parameter
a b s t r a c t This study investigates the effects of gemini and conventional cationic surfactants on the cloud point (CP) of the non-ionic surfactant Triton X-100 (TX-100) in aqueous solutions. Instead of visual observation, a spectrophotometer was used for measurement of the cloud point temperatures. The thermodynamic parameters of these mixtures were calculated at different cationic surfactant concentrations. The gemini surfactants of the alkanediyl-a-x-bis (alkyldimethylammonium) dibromide type, on the one hand, with different alkyl groups containing m carbon atoms and an ethanediyl spacer, referred to as ‘‘m-2-m’’ (m = 10, 12, and 16) and, on the other hand, with –C16 alkyl groups and different spacers containing s carbon atoms, referred to as ‘‘16-s-16’’ (s = 6 and 10) were synthesized, purified and characterized. Additions of the cationic surfactants to the TX-100 solution increased the cloud point temperature of the TX-100 solution. It was accepted that the solubility of non-ionic surfactant containing polyoxyethylene (POE) hydrophilic chain was a maximum at the cloud point so that the thermodynamic parameters were calculated at this temperature. The results showed that the standard Gibbs free energy ðDGcp Þ, the enthalpy ðDHcp Þ and the entropy ðDScp Þ of the clouding phenomenon were found positive in all cases. The standard free energy ðDGcp Þ increased with increasing hydrophobic alkyl chain for both gemini and conventional cationic surfactants; however, it decreased with increasing surfactant concentration. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Non-ionic surfactants are widely used as an agent to solubilize and emulsify processes in the textile, detergent and cosmetic industries. The solubility of non-ionic surfactants in aqueous solution is due to the high hydration of oxyethylene chains [1,2]. Since hydrogen bonding is a temperature sensitive phenomenon, heating of aqueous surfactant solutions to a certain temperature causes the appearance of clouds due to incomplete solubility. This temperature is called as ‘Cloud Point’ and is an important character of surfactant [3,4]. At this temperature, the surfactant is no longer soluble in water and phase separation (a surfactant-rich phase and a surfactant-poor phase in which surfactant concentration is approximately critical micelle concentration) occurs in solution [2,5]. The phase separation is reversible, and on cooling of the mixture to a temperature below the cloud point, the two phases merge to form once again a clear solution. This separation is probably due to the sharp increase in the aggregate number of the micelles and the non-ionic surfactant loss of some or all of the surfactant functions above the CP [6,7]. Currently, new classes of surfactants, referred to as dimeric surfactants (also known as gemini surfactants), have attracted ⇑ Corresponding author. Tel.: +90 284 235 9592; fax: +90 284 235 1544. E-mail address: [email protected] (H. Akbasß). 0021-9614/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jct.2011.06.005
increasing attention due to their superior surface activity compared to that of the conventional (monomeric) surfactants. These surfactants have two hydrophobic tails and two polar heads connected by a spacer and tend towards a much lower critical micelle concentration, which can produce lower surface tension than conventional surfactants at the same molar concentration. Gemini surfactants have better solubilizing, wetting, and foaming ability, as well as show stronger biological activity and lime-soap dispersing properties compared to conventional surfactants [8–10]. The CP is very sensitive to the presence of additives even at very low concentrations. The additives modify the surfactant solvent interaction. As a consequence, they change the critical micelle concentration (cmc), the size of the micelles and the phase behavior in the surfactant solutions [11]. Many authors have investigated the effect of additives on the cloud point to include inorganic electrolytes [12–14], organic compounds [15,16] and other surfactants [17–19]. In our previous studies, we have investigated the influence of inorganic additives on the cloud points of Triton X-405 and Brij35 [13,20] and on the cloud points of Triton X-100-gemini and conventional cationic surfactants mixtures by the spectrophotometric method [21]. In this paper, we investigated the effects of gemini and conventional cationic surfactants on the cloud point (CP) of the non-ionic surfactant Triton X-100 (TX-100) in aqueous solutions. The thermodynamic parameters of these mixtures were calculated at different cationic surfactant concentrations.
Ç. Batıgöç et al. / J. Chem. Thermodynamics 43 (2011) 1800–1803
2. Experimental 2.1. Material Triton X-100 has the chemical formula R–C6H4–(OC2H4)p–OH, where R is a branched octyl group and p is the average number of oxyethylene groups (p 9.5). The non-ionic surfactant (Triton X-100) was supplied by Aldrich–Sigma. All supplied material was of reagent grade and were used without further purification. The cationic surfactants, dodecyltrimethylammonium bromide (DTAB), hexadecyltrimethylammonium bromide (CTAB), dodecylpyridinium chloride (DPC) were supplied by Merck. The hexadecylpyridinium chloride (CPC) and hexadecylpyridinium bromide (CPB) supplied from Fluka and Aldrich. The water used to prepare the sample solutions was double distilled in a distillation apparatus (GFL 2102). The specific conductivity of this water was in the range of (1 to 2) lS cm1. The cationic gemini surfactants, alkanediyl-ax-bis (alkyldimethylammonium bromide), were synthesized. For these, 1-bromodecane, 1-bromododecane, 1-bromohexa decane, N,N,N0 ,N0 -tetramethylethylenediamine, 1,6-dibromohexane, 1,10dibromodecane, N,N-dimethylhexa decylamine, and acetone were supplied by Fluka and Merck. 2.2. Methods 2.2.1. Synthesis of gemini surfactants The structures of cationic gemini surfactants are shown in figure 1. A mixture of N,N,N0 ,N0 -tetramethylethylenediamine and a corresponding alkyl bromide was refluxed in acetone for 24 h to synthesize 10-2-10, 12-2-12, and 16-2-16 type gemini or dimeric cationic surfactants. For the synthesizes of 16-6-16 and 16-10-16 type gemini cationic surfactants, a mixture of N,N-dimethylhexadecylamine and a corresponding alkyl dibromide was refluxed in acetone for 24 h. The white solids obtained were filtered after cooling and then recrystallized from acetone at least two times [22–25]. The yield of the reaction was almost quantitative, 97%. The purities were checked via nuclear magnetic resonance (NMR). The 1H NMR spectra were recorded in deuteriochloroform solution with a Varian Mercury Plus 300 MHz spectrometer. 13C spectra were recorded at 75 MHz.
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C NMR (75 MHz, CDCl3): d = 14.38, 22.93, 23.27, 26.47, 28.40, 29.00, 29.43, 29.60, 29.62, 29.67, 29.79, 29.83, 29.92, 29.98, 32.16,51.26, 57.07, 66.02. 16-6-16; 1H NMR (300 MHz, CDCl3): d = 0.86 (t, 6H), 1.23–1.33 (m, 52H), 1.57 (m, 4H), 1.70 (m, 4H), 2.0 (m, 4H), 3.37 (s, 12H), 3.40–3.47 (m, 4H), 3.69–3.72 (m, 4H). 13C NMR (75 MHz, CDCl3): d = 14.36, 21.77, 22.92, 23.10, 24.42, 26.54, 27.73, 27.94, 29.50, 29.59, 29.62, 29.70, 29.83, 29.85, 29.88, 29.91, 32.15, 51.26, 64.40, 65.06. 16-10-16; 1H NMR (300 MHz, CDCl3): d = 0.88 (t, 6H), 1.25 (m, 48H), 1.35 (m, 8H), 1.44 (m, 8H), 1.74 (m, 8H), 3.35 (s, 12H), 3.43–3.49 (m, 4H), 3.75–3.80 (m, 4H). 13C NMR (75 MHz, CDCl3): d = 14.35, 22.70, 22.90, 23.07, 26.07, 26.52, 28.40, 28.65, 29.49, 29.58, 29.64, 29.71, 29.83, 29.87, 29.91, 32.13, 51.24, 64.42, 64.54. 2.3. Determination of cloud point temperatures The UV–vis spectra were recorded with a Shimadzu model UV-1700 spectrophotometer. A Shimadzu CPS 240 temperature controller (0 to 110) °C including a Peltier system was used as the heater in the cloud point determinations to heat the cell holder of the spectrophotometer. The ultraviolet spectrum of 1% (w per v) TX-100 at different temperatures is shown in figure 2. As seen from figure 2, the absorbance values taken at (65.2 to 65.9) °C over the range of (350 to 200) nm wavelength are approximately same. When the temperature was increased by 0.1 °C (at 66.0 °C), the absorbance values at (350 to 300) nm and 250 nm interval wavelength significantly increased. This temperature was accepted as the CP value of the TX-100 solution. All cloud points given were the average of two measurements (the values of CP were reproducible within ±0.1 °C). The CP results for TX-100 solutions (0.2, 0.5, 1, 2, 3, and 4) wt% in water are presented in figure 3. As seen in figure 3, a significant change in CP is observed, that is to say the CP becomes almost constant with increasing concentration [25,26]. Therefore, the concentration of non-ionic surfactant used for CP measurements was maintained at 1% (w per v). 3. Results and discussion We observed the variations of cloud point of TX-100-cationic surfactant mixtures at different surfactant concentrations. The
2.2.2. Spectral characteristics of gemini surfactants 10-2-10; 1H NMR (300 MHz, CDCl3): d = 0.87 (t, 6H), 1.22–1.41 (m, 28H), 1.80 (m, 4H), 3.48 (s, 12H), 3.70 (m, 4H), 4.72 (s, 4H). 13 C NMR (75 MHz, CDCl3): d = 14.37, 22.91, 23.28, 26.45, 29.53, 29.72, 32.09, 51.32, 57.02, 66.12. 12-2-12; 1H NMR (300 MHz, CDCl3): d = 0.86 (t, 6H), 1.17–1.30 (m, 36H), 1.81 (m, 4H), 3.47 (s, 12H), 3.70 (m, 4H), 4.70 (s, 4H). 13 C NMR (75 MHz, CDCl3): d = 14.38, 22.92, 23.28, 26.48, 29.42, 29.60, 29.77, 29.80, 29.88, 32.15, 51.30, 57.02, 66.09. 16-2-16; 1H NMR (300 MHz, CDCl3): d = 0.86 (t, 6H), 1.23–1.34 (m, 52H), 1.82 (m, 4H), 3.46 (s, 12H), 3.70 (m, 4H), 4.69 (m, 4H).
FIGURE 1. Structures of gemini cationic surfactants types 10-2-10, 12-2-12, 16-216, 16-6-16, and 16-10-16.
FIGURE 2. The absorption spectra for 1% (w per v) Triton X-100 in water at different temperatures.
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FIGURE 3. Plot of the cloud point temperature against surfactant concentration for Triton X-100.
thermodynamic parameters of the non-ionic surfactant were calculated using the CP values at phase separation. The values of the standard Gibbs free energy ðDGcp Þ, the enthalpy ðDHcp Þ and the entropy ðDScp Þ of the clouding phenomenon were calculated considering the solubility limit at the clouding temperature [27,28] by the phase separation model from the relationships
DGcp ¼ RT ln X cs ; DHcp ¼ T 2
dDGcp =T dT
ð1Þ ð2Þ
where Xcs is the mole fraction of cationic surfactant in surfactant mixture; R is the gas constant, and T is taken as Kelvin temperature (CP + 273.15) [28,29]. The enthalpy of the cloud point was calculated from the slope of a plot of DGcp =T versus T at a particular temperature using figure 4. The entropy of the micellization ðDSm Þ was estimated from the values of calculated enthalpy and Gibbs free energy.
T DScp ¼ DHcp DGcp :
ð3Þ
Ionic surfactants increase the CP value of the non-ionic surfactant leading to charged mixed micelles which cause electrostatic repulsions between micelles [26]. The addition of cationic surfactants on the TX-100 solution raised the CP of the non-ionic surfactant forming positive charged mixed micelles. Thermodynamic parameters of TX-100 at the clouding temperatures have positive values in the presence of cationic surfactants. These parameters are given in table 1. As shown in table 1, the standard Gibbs free energy ðDGcp Þ increases with increasing hydrophobic alkyl chain for both gemini and conventional cationic surfactants. However, they decrease with increasing concentration of surfactant. The positive values of DGcp indicate that the process proceeds non-spontaneously. The DHcp and T DScp values decrease with increasing hydrophobic alkyl chain of cationic surfactants. However, they increase with increasing cationic surfactant concentration. Because TX-100 forms mixed micelles with cationic surfactants, inter-micellar repulsions occur in the solution. Hence the values of DHcp and T DScp are positive [27]. Also, the positive values of DHcp can be attributed to a disruption
FIGURE 4. Plots of DGcp =K versus temperature for different Triton X-100-cationic surfactant mixtures. (a) 10-2-12 (d), 12-2-12 (j), 16-2-16 (N), 16-6-16 (), and 1610-16 (+). (b) DPC (d), CPC (j), CPB (N), DTAB (+), and CTAB ().
of water structure around the hydrophobic alkyl tails of non-ionic surfactant molecules [15]. The results in table 1 showed that the values of CP and DGcp increased less with increased spacer length by comparison to the change of alkyl chain length. The studies show that the cmc of surfactant typed m-s-m with a hydrophobic polymethylene spacer is a maximum at s P 6 irrespective of the value of m and then the cmc values again decrease [8,22,30,31]. The similar state was observed in the values of DHcp and T DScp . This case is attributed to a conformational change of the surfactant ion at low s and a progressive penetration of the spacer group in to the micelle hydrophobic core [32]. In the cases of CPC and CPB, the effect of Br ions on the CP value of TX-100 is more than that by Cl- ions (the increase of CP values for CPB is 32.4 °C and for CPC is 29.9 °C at 0.4 mM). The Br ion is capable of connecting to more than Cl ions and the aggregation number of CPB is greater than that of the CP. Romsted showed that the ionization degree of Br is greater than for Cl [33]. Thus the CPB further increased the CP values creating larger micelles. On the other hand, there are not significant changes in
Ç. Batıgöç et al. / J. Chem. Thermodynamics 43 (2011) 1800–1803 TABLE 1 Thermodynamic parameters of Triton X-100 and cationic gemini surfactant mixtures at cloud point. Additive Concentration (mM)
Cloud point (K)
DH / T DS / DG / (kJ mol1) (kJ mol1) (kJ mol1)
10-2-10
0.1 0.2 0.3 0.4 0.5 0.6
345.95 350.15 353.45 356.15 356.85 357.75
38.02 36.47 35.61 35.04 34.44 33.99
144.02 147.64 150.36 152.66 153.26 154.04
106.00 111.17 114.75 117.62 118.82 120.05
12-2-12
0.1 0.2 0.3 0.4 0.5 0.6
351.15 359.15 362.75 367.25 368.65 371.65
38.59 37.40 36.55 36.13 35.58 35.31
90.74 94.92 96.82 99.25 100.01 101.65
52.15 57.52 60.27 63.12 64.43 66.34
16-2-16
0.1 0.2 0.3 0.4 0.5 0.6
351.35 359.35 364.35 369.15 372.15 374.45
38.61 37.42 36.73 36.31 35.92 35.57
78.31 81.92 84.31 86.45 87.86 88.95
39.70 44.50 47.58 50.14 51.94 53.38
16-6-16
0.1 0.2 0.3 0.4 0.5 0.6
352.65 360.55 365.75 370.75 372.85 375.35
38.75 37.55 36.85 36.47 35.99 35.66
80.03 83.65 86.08 88.45 89.46 90.66
41.28 46.10 49.23 51.98 53.47 55.00
16-1016
0.1 0.2 0.3 0.4 0.5 0.6
352.65 361.55 366.75 371.25 374.15 376.65
38.75 37.65 36.95 36.52 36.11 35.78
79.50 83.56 85.98 88.10 89.49 90.69
40.75 45.91 49.03 51.58 53.38 54.91
DTAB
0.1 0.2 0.3 0.4 0.5 0.6
344.65 349.65 352.15 355.15 356.35 359.15
37.88 36.41 35.48 34.94 34.39 34.12
125.16 128.81 130.66 132.90 133.80 135.91
87.28 92.40 95.18 97.96 99.41 101.79
0.1 0.2 0.3 0.4 0.5 0.6
349.55 358.35 364.55 368.15 370.25 372.15
38.40 37.32 36.73 36.22 35.74 35.35
78.60 82.61 85.49 87.19 88.19 89.09
40.20 45.29 48.76 50.97 52.45 53.74
DPC
0.1 0.2 0.3 0.4
344.85 350.85 355.55 358.15
37.90 36.54 35.82 35.23
101.71 105.28 108.12 109.70
63.81 68.74 72.30 74.47
CPC
0.1 0.2 0.3 0.4
348.35 356.05 365.35 369.05
38.28 37.08 36.81 36.30
64.20 67.07 70.62 72.06
25.92 29.99 33.81 35.76
CPB
0.1 0.2 0.3 0.4
349.85 360.75 368.05 371.55
38.46 37.57 37.08 36.55
64.13 68.15 70.93 72.29
25.67 30.58 33.05 35.74
CTAB
of DGcp , DHcp , and T DScp values were investigated, it was found that the Gibbs free energy of the clouding phenomenon is controlled by both the enthalpic and entropic contributions. 4. Conclusions Cationic surfactants formed charged mixed micelles with TX100 and the electrostatic repulsions between micelles occurred in solution. Thus the CP of TX-100 increased with the addition of ionic surfactants. It was observed that the effect of alkyl chain length was greater than that of the spacer group on the CP for the gemini surfactants. The thermodynamic parameters were calculated at the cloud point for all the mixtures studied. The clouding phenomenon was found to be a non-spontaneous process. Also, the values of DGcp , DHcp , and T DScp were found to be positive in all cases. This is due to the fact that the mixed micelles are non-ideal because of the repulsive interactions between charged mixed micelles. Acknowledgement The authors gratefully acknowledge the financial supports of the Trakya University Research Fund (TUBAP-922 and TUBAP-783). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
thermodynamic parameters for these surfactants. When CTAB and CPB are compared, the effect of CPB is greater than that of CTAB on the CP of TX-100. For these surfactants having different hydrophilic groups, the CPB has a larger polar head group than CTAB [34] and the electrostatic repulsions between its mixed micelles formed with TX-100 are greater than CTAB. Consequently, when the values
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[31] [32] [33] [34]
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JCT 11-150