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Langmuir 2000, 16, 980-987
Solution Properties of Nonionic Surfactants and Their Mixtures: Polyoxyethylene (10) Alkyl Ether [CnE10] and MEGA-10† Shireen B. Sulthana,‡ P. V. C. Rao,§ S. G. T. Bhat,§ T. Y. Nakano,| G. Sugihara,| and A. K. Rakshit*,‡ Department of Chemistry, Faculty of Science, M.S. University of Baroda, Baroda 390 002, India, Research and Development Centre, Indian Petrochemicals Corporation Limited, Baroda 391 346, India, and Department of Chemistry, Fukuoka University, Fukuoka 814-0180, Japan Received June 8, 1999. In Final Form: October 27, 1999 The interfacial and thermodynamic properties of nonionic surfactants and their mixtures are of both theoretical and practical interest. The nonionic surfactants used in this study are polyoxyethylene (10) alkyl ether [CnE10; m ) 10 and n ) 12, 16] and N-decanoyl-N-methylglucamine (MEGA-10). The critical micelle concentrations of pure surfactants and their mixtures were determined by surface tension measurements at different mixed ratios and temperatures. Interfacial parameters such as the maximum surface excess (Γmax) and the minimum area per molecule (Amin) at the air/water interface were also determined from surface tension data. Standard thermodynamic parameters of micellization and adsorption were also computed and discussed. Steady-state fluorescence studies were also carried out to determine the micellar aggregation number (Nagg) and the microenvironment/polarity in the mixed micelle from the I1/I3 ratio. The interaction parameters that measure the interaction between the surfactant molecules in the mixed micelle were computed by Rubingh’s approach. 1H NMR was also used to investigate the interaction between the surfactants.
Introduction Nonionic surfactants of the alkyl polyethylene oxide type, generally abbreviated as CnEm, are widely used as detergents, solubilizers, and emulsifiers. Their unique chemical structure offers a model system to study the systematic variations in the hydrophobic/hydrophilic character that affect the micellar solution properties.1 Hence, they are largely exploited in both chemical and biochemical research. Surfactants used in various practical and commercial applications are invariably mixtures. Studies involving the investigation of various physicochemical properties of mixed surfactants have proved to be a powerful technique to optimize their properties to desired ranges by just changing the solution composition. Therefore, a thorough understanding of the underlying physics and chemistry of such systems is highly desirable.2 Alkyl-N-methylglucamines (MEGA-n) nonionic surfactants have been utilized to solubilize membrane proteins since they were synthesized by Hildreth in 1982.3 These MEGA-n surfactants have a characteristic feature, as observed by earlier workers:4 they do not show a cloud point even if their solutions are heated to boiling and a large amount of sodium chloride is added. This unique property is due to the difference in the molecular structure † Part of the Special Issue “Clifford A. Bunton: From Reaction Mechanisms to Association Colloids; Crucial Contributions to Physical Organic Chemistry”. * Corresponding author. ‡ M.S. University of Baroda. § Indian Petrochemicals Corporation Limited. | Fukuoka University.
(1) Briganti, G.; Puvvada, S.; Blankschtein, D. J. Phys. Chem. 1991, 95, 8989. (2) Ogino, K.; Abe, M. Mixed Surfactant Systems; Surfactant Science Series; Marcel Dekker: New York, 1992; Vol. 46. (3) Hildreth, J. E. K. Biochem. J. 1982, 363, 207. (4) Okawauchi, M.; Hagio, M.; Ikawa, Y.; Sugihara, G.; Murata, Y.; Tanaka, M. Bull. Chem. Soc. Jpn. 1987, 60, 2718.
Figure 1. Structure of MEGA-10.
of MEGA-n with the polyoxyethylene ether type surfactants.4 Because of the present day importance of nonionic surfactants in industry and in understanding the micellization process as well as our own intense interest in these types of compounds,5-7 we decided to study the solution properties of polyoxyethylene type surfactants and MEGA-10 (Figure 1) in pure states and in mixtures. Literature reveals mixed surfactant studies of MEGA-n with different kinds of surfactants such as sodium perfluorooctanoate (SPFO) and bile salts (sodium deoxycholate and sodium chenodeoxycholate).8 However, we failed to find any such study with conventional polyoxyethylene ether type surfactants. Also, we know that the micelle structure and properties can vary substantially with variation in the hydrophilic or hydrophobic chain length in POE type surfactants. On the basis of these facts, we chose to study C12E10/MEGA-10 and Brij 56, that is, C16E10/MEGA-10 mixed surfactant systems. This article deals with the physicochemical characterization of polyoxyethylene alkyl ethers (CnEm; m ) 10 and n ) 12, 16) with MEGA-10 (N-decanoyl-N-methylglu(5) Sulthana, S. B.; Bhat, S. G. T.; Rakshit, A. K. Colloids Surf. 1996, 111, 57. (6) Sulthana, S. B.; Bhat, S. G. T.; Rakshit, A. K. Langmuir 1997, 13, 4562. (7) Sulthana, S. B.; Rao, P. V. C.; Bhat, S. G. T.; Rakshit, A. K. J. Phys. Chem. B 1998, 102, 9653. (8) (a) Sugihara, G.; Yamamoto, M.; Wada, Y.; Murata, Y.; Ikawa, Y. J. Solution Chem. 1988, 17, 225. (b) Sugihara, G. In Surfactants in Solutions; Mittal, K. L., Ed.; Plenum: New York, 1989; Vol. 7, p 397.
10.1021/la990730o CCC: $19.00 © 2000 American Chemical Society Published on Web 01/04/2000
Nonionic Surfactants and Their Mixtures
Langmuir, Vol. 16, No. 3, 2000 981 monomers in the surfactant solution were obtained with a fluorescence spectrophotometer (F-4010 Hitachi Fluorescence Spectrophotometer) at the excitation wavelength 335 nm. Each spectrum had one to five vibronic peaks from shorter to longer wavelength. The fluorescence intensities were monitored at 385 nm. All measurements were carried out at room temperature. An aliquot of a stock solution of pyrene in ethanol was transferred into a flask, and the solvent was evaporated with nitrogen. The surfactant solution was added, and the concentrations of pyrene and surfactant were kept constant at 1 × 10-6 M and 1 × 10-2 M, respectively. The quencher concentration was varied from 0 to 2 × 10-5 M. The aggregation number (Nagg) was determined from the equation9,10
ln I ) ln I0 - [Q]/[M] ) ln I0 - Nagg[Q]/[S] - cmc
Figure 2. Representative plots of surface tension (γ) versus log C for different systems: (.) 1:9 Brij 56/MEGA-10 at 303 K; (b) 3:7 C12E10/MEGA-10 at 308 K; (2) 3:7 Brij 56/MEGA-10 at 303 K.
camine) surfactant mixtures. The study involves the determination of the critical micelle concentration (cmc) by a surface tension method at various mole ratios of the surfactant mixtures. Interfacial parameters like the maximum surface excess (Γmax) and the minimum area per molecule (Amin) were determined from surface tension data. Standard thermodynamic parameters of micellization and adsorption were also computed. Steady-state fluorescence quenching was used to determine the micellar aggregation number (Nagg) and the microenvironment of the mixed micelles. Also, lH NMR was used to investigate the interaction between the two surfactants. Materials and Experimental Method Nonionic surfactants, polyoxyethylene (10) lauryl ether [C12H25(OCH2CH2)10OH] (C12E10, Sigma), and polyoxyethylene (10) cetyl ether [C16H33(OCH2CH2)10OH] (Brij 56, Sigma) were used without any further purification. MEGA-10 (purchased from Dojindo Laboratories, Japan) was recrystallized three times from a 9:1 diethyl ether-ethanol mixture. Cetyl pyridinium chloride (CPyCl) (Loba Chemie, Baroda, India) was recrystallized twice from benzene. Pyrene (Fluka, Germany) was used as received. All solutions were prepared in doubly distilled water. Surface Tension Measurements. The surface tension (γ) was measured by a ring method using a duNouy ring tensiometer (S. C. Dey and Co., Calcutta, India) at different temperatures of 30, 35, 40, and 45 °C. The temperatures were maintained constant by circulating thermostated water through a jacketed vessel containing the solution. Representative plots of surface tension (γ) against log concentration (log C) are shown in Figure 2. The reproducibility of the surface tension-concentration curves was checked by duplicate runs. The reproducibility (standard deviation of the mean) in the cmc was found to be (1.0%, calculated from the experimental cmc data from at least two runs. Cloud Points. The cloud points of polyoxyethylene (10) lauryl ether in all experimental solutions were determined. The experimental procedure has been described earlier.11 The total surfactant concentration was 1% (w/v), and the cloud point was measured at different ratios in the presence of 1 M and 2 M NaCl for both systems. The cloud points presented here are the average of the temperatures of the appearance and disappearance of clouds, the maximum difference being no greater than 0.4 °C under constant stirring. Fluorescence Measurements. The micellar aggregation number (Nagg) was determined by a steady-state fluorescence quenching method. The fluorescence emission spectra of pyrene
where [Q], [M], and [S] are the concentrations of quencher, micelle, and total surfactant, respectively. I0 and I are the fluorescence intensities in the absence and in the presence of the quencher (Figure 3). NMR Experiments. Proton NMR measurements were carried out in deuterium oxide (D2O, 99.9 atom % D, Aldrich) solvent at room temperature (∼25 °C). The mixed solutions of C12E10 and MEGA-10 as well as solutions of Brij 56 and MEGA-10 at a total concentration of 0.1 M were prepared for MEGA-10 mole fractions of 0.0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0, respectively. Proton NMR spectra were recorded with a JEOL JNM FX-100 FTNMR spectrometer operating at 100 MHz and using a deuterium field frequency locked on the solvent D2O.
Results and Discussion The critical micelle concentrations (cmc’s) of single and mixed surfactants were determined by a surface tension method. The cmc values are presented in Table 1. The cmc values of polyoxyethylene alkyl ethers, that is, C12E10 and Brij 56 (C16E10), as well as MEGA-10 were found to decrease with an increase in temperature. Such behavior is a typical characteristic of a nonionic surfactant within the limited range of temperature studied. The experimental results of other workers4 suggest a minimum in the cmc-temperature profile for the MEGA-n series; however, surprisingly, we did not observe any such effect. In comparison to the case of polyoxyethylene alkyl ether, it has been proposed4 that, due to the hydrophilicity, there arise two hydrogen bonds per oxyethylene group with water whereas a hydroxyl group of the hydrophilic part of MEGA-n possibly forms three hydrogen bonds with water. This can be one of the reasons for the higher cmc in MEGA-10 relative to the polyoxyethylene type nonionics studied. It should be mentioned here that the cmc value of MEGA-10 at 30 °C was found to be dependent on the method used (Table 1). The cmc’s obtained by the surface tension measurement given in the literature match well with our data (Table 1). The formation of the micelle is the result of hydrophobic interaction.12 It is also known that the London dispersion force is the main attractive force helping in the formation of the micelle.13 In the case of nonionic surfactants, the cmc decreases with increasing temperature due to the dehydration of the hydrophilic moiety of the surfactant molecule. As shown in Table 1, as the hydrocarbon chain (9) Turro, N. J.; Yekta, A. J. Am. Chem. Soc. 1978, 100, 5951. (10) Abe, M.; Uchiyama, H.; Yamaguchi, T.; Suzuki, T.; Ogino, K.; Scamehorn, J. F.; Christian, S. D. Langmuir 1992, 8, 2147. (11) Koshy, L.; Saiyad, A. H.; Rakshit, A. K. Colloid Polym. Sci. 1989, 274, 582. (12) Saito, S. In Nonionic Surfactants, Physical Chemistry; Schick, M. J., Ed.; Marcel Dekker: New York, 1987; Vol. 23, p 885. (13) del Rio, J. M.; Pombo, C.; Prieto, G.; Sarmiento, F.; Mosquera, V.; Jones, M. N. J. Chem. Thermodyn. 1994, 26, 879.
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Sulthana et al. Table 1. Critical Micelle Concentration (cmc) of C12E10/ MEGA-10 and Brij 56/MEGA-10 Mixed Systems at Different Temperatures C12E10/ MEGA-10
303 K
cmc (µM) 308 K
313 K
318 K
10:0 9:1 7:3 5:5 3:7 1:9 0:10b
12.7 10.2 13.1 18.6 30.1 70.5 4.68, 5.0,c,d 6.6c,e
11.8a 9.8 12.8 18.3 29.8 70.2 4.57
10.0a 9.4 12.4 18.0 29.4 69.9 4.46
8.9a 9.0 12.0 17.8 29.0 69.6 4.33
Brij 56/ MEGA-10
303 K
10:0 9:1 7:3 5:5 3:7 1:9
8.0 7.8 8.5 10.1 18.7 46.5
cmc, (µM) 308 K 313 K 7.8 7.5 8.2 9.9 18.4 46.1
7.3 7.2 8.0 9.7 18.0 45.9
318 K 6.9 7.0 7.7 9.5 17.8 45.7
a Reference 6. b The cmc values for pure MEGA-10 are in mM. Reference 25. d By surface tension study. e By light-scattering study.
c
Figure 3. (a) Representative emission fluorescence spectra of a 10-6 M solution of pyrene in an aqueous micellar solution of C12E10/MEGA-10 (5:5) at 25 °C in the presence and absence (1) of different concentrations of cetyl pyridinium chloride (CPyCl). (b) Representative plots of ln I0/I versus CPyCl concentration for different systems: (b) C12E10/MEGA-10 (7:3); (9) C12E10/ MEGA-10 (9:1); (2) C12E10.
length increases from dodecyl to cetyl in POE type surfactant, a decrease in the cmc was observed, as expected. The cmc values at all mole fractions for both C12E10/ MEGA-10 and Brij 56/MEGA-10 systems were found to be lower than the ideal values calculated from Clint’s equation14
R1 (1 - R1) 1 ) + cmcmix cmc1 cmc2 where cmcmix, cmc1, and cmc2 are the cmc values of the mixture, surfactant 1, and surfactant 2, respectively. R1 is the mole fraction of surfactant 1, and R2 ()1 - R1) is the mole fraction of surfactant 2, respectively. The cmcmix (14) Clint, J. J. Chem. Soc., Faraday Trans. 1 1975, 71, 1327.
values thus obtained experimentally were found to be lower than the values calculated from the above equation, which indicates nonideality. This is indicative of some interaction between the surfactant molecules, which can possibly be due to (i) the interaction between the head groups of these surfactants through hydration, (ii) a small repulsive interaction between the oxonium ion of polyoxyethylene and the slightly positive nitrogen atom of glucamine, (iii) incorporation of MEGA-10 molecules lowering the steric repulsions between the large oxyethylene head groups, and (iv) decreased hydration of the hydrophilic moiety of MEGA-10 due to the addition of C12E10/Brij 56. The overall experimentally observed results can be attributed to the above factors. As mentioned earlier, MEGA-10 does not show a cloud point even if its solutions is heated to boiling and large amounts of NaCl are added to its solution. However, the addition of NaCl depresses the cloud point of polyoxyethylene alkyl ethers. This indicates that there is a considerable difference between the hydration of hydroxyl groups of MEGA-10 with water and that of the oxyethylene groups of polyoxyethylene alkyl ether with water. Hence, we studied the effect of addition of MEGA-10 on the cloud points of both C12E10 and Brij 56. The cloud points of both C12E10 and Brij 56 increased as MEGA-10 was added. Since the cloud point was above 90 °C, we measured the cloud point of C12E10 and Brij 56 mixed with MEGA-10 at various ratios (Figure 4) in the presence of 1 M and 2 M NaCl. It can be observed from Figure 4 that although the cloud point decreased in the presence of NaCl for pure C12E10 and Brij 56, MEGA-10 caused an increase in C.P. for both C12E10/MEGA-10 and Brij 56/MEGA-10 systems. The lowering of the cloud point in the case of pure C12E10 and Brij 56 indicates that NaCl removes water from near the micelle and thereby helps the micelles to approach each other easily. According to Kjellander et al.,15 the formation of clouds is entropy-dominated. The ethylene oxide group is highly hydrated, and the nonionic micelles are expected to have water deep inside. As NaCl is added, the amount of water available for the hydration of the polyoxyethylene moiety of the surfactant is reduced. Now, (15) Kjellender, R.; Floriu, E. J. Chem. Soc., Faraday Trans. 1981, 77, 2053.
Nonionic Surfactants and Their Mixtures
Langmuir, Vol. 16, No. 3, 2000 983 Table 2. Standard Thermodynamic Parametersa of Micellization for Pure and Mixed C12E10/MEGA-10 and Brij 56/MEGA-10 Systems -∆G°m (kJ mol-1) C12E10/ MEGA-10
303 K
308 K
313 K
318 K
10:0 9:1 7:3 5:5 3:7 1:9 0:10
38.5 39.1 38.4 37.5 36.3 34.2 23.6
39.3 39.8 39.1 38.2 36.9 34.8 24.1
40.4 40.6 39.8 38.9 37.6 35.3 24.5
41.4 41.3 40.6 39.5 38.2 35.9 25.0
∆H°m ∆S°m (kJ mol-1) (kJ mol-1 K-1) 19.6 6.6 4.4 2.4 2.0 1.7 4.1
0.19 0.15 0.14 0.13 0.13 0.12 0.09
-∆G°m (kJ mol-1)
Figure 4. Cloud point of 1% (w/v) surfactant solutions of C12E10/ MEGA-10 and Brij 56/MEGA-10 mixtures in the presence of NaCl: (b) 1 M NaCl, C12E10; (2) 1 M NaCl, Brij 56; (×) 2 M NaCl, C12E10; (.) 2 M NaCl, Brij 56.
with two relatively less hydrated micelles approaching each other, the hydration spheres overlap and some water molecules are freed to increase the entropy of the system. At the cloud point the water molecules get totally detached from the micelles. The overall entropy is high, and hence the free energy change is relatively more negative11 and the appearance of the cloud point is facile. The increases in the cloud points of C12E10 and Brij 56 on addition of MEGA-10 are due to increased hydrophilicity offered to the system by MEGA-10. And because of that, even in presence of 2 M NaCl, the cloud point was not obtained at higher mole fractions of MEGA-10 in the mixture. As the cmc can serve as a measure of micelle stability in a given state, the standard thermodynamic parameters of micellization can be determined from the temperature dependence of the cmc.16 The standard free energy of micellization (∆G°m) for a nonionic surfactant is directly proportional to the cmc (cmc on the mole fraction scale), following the relation17
∆G°m ) RT ln cmc The standard state here is a hypothetical system with a unit mole fraction of the surfactant solution at the cmc. The ∆G°m values are shown in Table 2. It is observed that the ∆G°m values are all negative and the first addition of MEGA-10 to both POE type surfactants made the formation of mixed micelles more facile. However, further addition of surfactants made the ∆G°m values relatively less negative, indicating that the mixed micelle formation is easy at the first addition but then, though there is mixed micelle formation, the tendency is relatively less spontaneous. With an increase in temperature, the micellization process becomes relatively more spontaneous. A plot of (∆G°m/T) versus T-1 is not a very good straight line. However the slopes of the line at various temperatures were determined and ∆H°m values were calculated. The error in ∆H°m was large ((9%), and hence only an average value of ∆H°m is given for each system in Table 2. The average corresponding values of ∆S°m are also hence given (16) del Rio, J. M.; Prieto, G.; Sarmiento, F.; Mosquera, V. Langmuir 1995, 11, 1511. (17) Attwood, D.; Florence, A. T. Surfactant Systems, Their Chemistry, Pharmacy and Biology; Chapman and Hall: London, 1988.
Brij 56/ MEGA-10
303 K
308 K
313 K
318 K
∆H°m (kJ mol-1)
∆S°m (J mol-1 K-1)
10:0 9:1 7:3 5:5 3:7 1:9 0:10
39.7 39.8 39.5 39.1 37.5 35.2 23.6
40.4 40.5 40.3 39.8 38.2 35.8 24.1
41.2 41.2 41.0 40.5 38.9 36.4 24.5
42.0 42.0 41.7 41.2 39.5 37.0 25.0
7.9 5.4 5.3 3.3 3.1 1.2 4.1
0.16 0.15 0.15 0.14 0.13 0.12 0.09
a The maximum error in ∆G° is 0.1%; that in ∆H° is 9%; and m m that in ∆S°m is 8%.
in Table 2. The overall micellization process for both C12E10/ MEGA-10 and Brij 56/MEGA-10 systems is endothermic. It is observed from Table 2 that ∆H°m values are lowered as the mole fraction of MEGA-10 in the mixed systems increases. However, for pure-MEGA-10 the ∆H°m was relatively higher. This implies that a remarkable change in the environment surrounding the hydrocarbon chain of the surfactant molecules takes place. The effect of temperature on the environment surrounding the hydrocarbon chain is difficult to determine because of the high error in the ∆H°m values. The ∆S°m values were all positive, indicating that the micellization process is entropy-dominated. The relative positive magnitudes of ∆S°m in C12E10 and Brij 56 (C16E10) are counterintuitive, as the values are 0.19 and 0.16 kJ mol-1 K-1, respectively. However, with the error of (8% in ∆S°m, the values can be assumed to be almost the same. Moreover, the ∆S°m values are the overall result of the following factors: (i) micelle formation by the surfactants, (ii) water structure breaking, (iii) formation of some structure around the micelle, (iv) breaking of “icebergs” or “flickering clusters” around the hydrocarbon tails of the surfactant monomers, and increased randomness of the hydrocarbon chains in the micellar core.7 These values will also depend upon the size of the micelles. All of these factors together finally determine the ∆S°m values, and we are not in a position to determine the effect of each of these factors individually. It can be observed that the magnitude of ∆S°m is lowered as the mole fraction of MEGA-10 in the mixed system increases for both C12E10/ MEGA-10 and Brij 56/MEGA-10. Such lowering of ∆S°m can be attributed to a relatively more ordered structure in the case of MEGA-10. Such lowering can also be attributed to the shortened hydrophobic group, in comparison to C12 or C16, in MEGA-10. The ∆H°m and ∆S°m calculated for each temperature (for each system) were plotted. A reasonably good straight line with a compensation temperature of 310 K was obtained for almost all systems. However, the average ∆H°m and ∆S°m values (as given in Table 2) for all systems together did not yield a very good straight line. This compensation phenomenon indicates that the structured water molecules are now more free, thereby increasing
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Sulthana et al.
Table 3. Minimum Area per Molecule (Amin) for C16E10/ MEGA-10 and C12E10/MEGA-10 at Different Temperatures 102Amin (nm2 molecule-1) n ) 16 (i.e. Brij 56)
n ) 12
CnE10/ MEGA-10
303 K
308 K
313 K
318 K
303 K
308 K
313 K
318 K
10:0 9:1 7:3 5:5 3:7 1:9 0:10
69.2 66.4 63.8 61.5 59.3 55.3 53.6
66.4 63.8 59.3 55.3 51.9 48.8 47.4
59.3 57.2 55.3 47.4 44.9 43.7 42.6
47.4 46.1 44.9 43.7 42.6 39.5 37.7
75.5 69.2 63.8 61.5 59.3 57.2 53.6
72.2 66.4 61.5 57.2 51.9 48.8 47.4
69.2 63.8 59.3 53.6 48.8 46.1 42.6
50.3 47.4 44.9 43.7 42.6 40.5 37.7
Table 4. Standard Thermodynamic Parameters of Adsorption of C12E10/MEGA-10 and Brij 56/MEGA-10 Systems -∆G°ad (kJ mol-1) C12E10/ MEGA-10
303 K
308 K
313 K
318 K
-∆H°ad (kJ mol-1)
∆S°ad (J mol-1 K-1)
10:0 9:1 7:3 5:5 3:7 1:9 0:10
52.1 50.5 48.9 47.8 46.3 43.9 33.1
54.5 50.9 49.4 48.2 46.8 44.3 33.5
51.2 51.3 49.9 48.5 47.3 44.9 33.8
47.3 51.6 50.2 48.9 48.1 45.6 34.0
27.3 22.6 26.5 9.9 9.5 15.2
77 87 70 119 113 59
-∆G°ad (kJ mol-1) Brij 56/ MEGA-10
303 K
308 K
313 K
318 K
10:0 9:1 7:3 5:5 3:7 1:9 0:10
51.1 51.1 50.5 49.8 47.8 45.0 33.1 .
51.7 51.6 51.1 50.1 48.1 45.4 33.5
52.2 52.0 51.7 50.3 48.4 45.7 33.8
52.8 52.4 52.3' 50.6 48.7 46.1 34.0
-∆H°ad ∆S°ad (kJ mol-1) (J mol-1 K-1) 17.7 23.9 14.6 33.9 30.5 23.6 15.2
110 90 118 52 57 71 59
∆S°m. To free water molecules from various hydrogen bonds or clusters, more and more energy is needed, that is, an increase in positive ∆H°m values. That is how this compensation phenomenon observed in many systems can be explained. The surface excess concentration under the conditions of surface saturation Γmax can conveniently be used as a measure of the maximum extent of adsorption of surfactants using the well-known Gibb’s adsorption equation18
Γmax ) -
1 dγ mol cm-2 2.303RT d log C
where γ is in ergs cm-2, R is in ergs K-1 mol-1, and T is in K. Also, the limiting surface area per molecule of the surfactant Amin at the surface was obtained at different temperatures using the relation16
Amin )
Figure 5. I1/I3 ratio versus mole fraction of MEGA-10 for (a) C12E10/MEGA-10 and (b) Brij 56/MEGA-10 mixed system.
tendency to locate at the air/water interface and hence a decrease in Amin. However, it can also be observed that a similar trend in Γmax with respect to temperature is seen in the case of MEGA-10. The Γmax values at all the mole fractions of the mixtures are found to lie within those for pure surfactants for both the C12E10/MEGA-10 and Brij 56/MEGA-10 mixed systems. This also indicates that, because of dehydration of the molecules, the hydrophobicity increases, and hence more molecules are at the interface. An increase in the hydrocarbon chain length from C12H25 to C16H33 does not show much effect in Amin at higher temperature. However, a significant effect in Amin is generally observed only when the hydrocarbon chain length exceeds 16.19 It is said that the most pronounced structural influence on Amin comes from the nature of the hydrophilic group.19 The values observed in the mixtures can be attributed to the presence of both types of surfactants at the interface. In Table 4 the standard thermodynamic parameters of adsorption at the air/water interface for both C12E10/ MEGA-10 and Brij 56/MEGA-10 are presented. The standard free energy of adsorption (∆G°ad) was calculated using the relation20
∆G°ad ) RT ln cmc - NΠcmcAcmc
14
10 nm2 molecule-1 NΓmax
The values thus calculated for both the mixed systems are shown in Table 3. It is obvious from the tabulated data that Amin values decrease (i.e. Γmax increases) with increasing temperature not only in pure compounds but also in the mixtures. This is because as the temperature increases, the hydration of the ethoxy segment of the nonionic surfactant decreases, leading to a greater (18) Chattoraj, D. K.; Birdi, K. S. Adsorption and The Gibb’s Surface Excess; Plenum: New York, 1984; p 22.
where Πcmc and Acmc are the surface pressure and the area per molecule at cmc. The standard state for the adsorbed surfactant here is a hypothetical monolayer at its minimum surface area per molecule but at zero surface pressure. The second term in the equation represents the surface work involved in going from zero surface pressure to Πcmc at constant minimum area per molecule Amin ()Acmc). (19) Rosen, M. J. In Surfactants and Interfacial Phenomenon; John Wiley: New York, 1988. (20) Rosen, M. J.; Cohen, W.; Dahanayake, M.; Hua, X. Y. J. Phys. Chem. 1982, 86, 541.
Nonionic Surfactants and Their Mixtures
Langmuir, Vol. 16, No. 3, 2000 985 Table 5. Activity Coefficient Values of Surfactants in the Mixed Micelle for C12E10/MEGA-10 and Brij 56/MEGA-10 Mixed Systems f1 308 K
f2
C12E10/ MEGA-10
303 K
313 K
9:1 7:3 5:5 3:7 1:9
0.0015 0.007 0.013 0.03 0.05
Brij 56/ MEGA-10
303 K
308 K
313 K
9:1 7:3 5:5 3:7 1:9
0.002 0.004 0.006 0.017 0.04
0.002 0.004 0.006 0.017 0.04
0.001 0.004 0.006 0.018 0.04
318 K
0.0016 0.003 0.002 0.02 0.015 0.009 0.03 0.02 0.02 0.06 0.05 0.09
303 K
308 K
0.84 0.98 0.86 0.86 0.73
0.86 0.97 0.12 0.99 0.89 0.73 0.89 0.53 0.99 0.77 0.70 0.92
f1
Figure 6. Micellar aggregation number (Nagg) versus mole fraction of MEGA-10 for (a) C12E10/MEGA-10 and (b) Brij 56/ MEGA-10 mixed systems.
∆G°ad values were all found to be negative for both the systems, which suggests a spontaneous adsorption. Moreover, ∆G°ad values indicate that when a micelle is formed, work has to be done to transfer the surfactant molecules in the monomeric form at the surface to the micellar stage through the aqueous medium.6,7 Also, as temperature increases, ∆G°ad values become more and more negative, suggesting that the adsorption process is relatively more favorable at higher temperatures. This can be ascribed to the fact that dehydration of the hydrophilic group is required for the adsorption to take place at the air/water interface and that, since at higher temperature the surfactant is comparatively less hydrated, adsorption at the air/water interface becomes easier. The standard enthalpy ∆H°ad and entropy ∆S°ad of adsorption were computed from ∆G°ad-T plots, the intercept being ∆H°ad and the slope being ∆S°ad. The ∆H°ad values indicate an exothermic contribution to adsorption at the air/water interface. The endothermic/exothermic nature of adsorption can generally be ascribed to whether bond breaking or bond making predominates during the adsorption process. The ∆G°ad-T plot in the adsorption process is not a good straight line for C12E10 systems. Therefore, ∆H°ad and ∆S°ad for these systems are not given in Table 4. However, it must be pointed out that the experimental results were reproducible. The inversion in the magnitude of ∆G°ad with an increase in temperature is difficult to explain, and we refrain from doing so at this
313 K
318 K
f2 318 K
303 K
308 K
313 K
318 K
0.002 0.95 0.95 0.004 0.87 0.86 0.006 1 0.77 0.78 0.020 0.84 0.85 0.05 0.75 0.76
0.96: 0.88 0.80 0.87 0.79
0.97 0.89 0.82 0.90 0.82
moment. It can however be noted from the Brij 56/MEGA10 system that a synergetic effect is present in the ∆H°ad values. This probably indicates the molecular exothermic interaction between Brij 56 and MEGA-10 at the air/water interface. The positive ∆S°ad values can be attributed to larger freedom of motion of hydrocarbon chains at the interface and also to the possible interaction between the surfactant molecules. A steady-state fluorescence technique was used to determine the micelle aggregation number for C12E10, Brij 56, and MEGA-10 and their various mixtures. It is wellknown that the ratio I1/I3, that is, the ratio of the intensities of the first (375 nm) to the third (385 nm) peaks, is almost proportional to the polarity in the region near the pyrene molecule solubilized in the micelles.21 The ratio I1/I3 (a measure of micropolarity) decreases with an increase in the hydrophobic environment. The micropolarity in the micelle was monitored by measuring the ratio I1/I3. From Figure 5, it can be observed that the ratio I1/I3 decreases with increasing MEGA-10 concentration. In other words, the micropolarity decreases with increasing amount of MEGA-10. It is observed that the I1/I3 values are in general much lower than the ideal line; that is, the polarity is significantly lower than expected for an ideal mixed micellar system. This decrease in the polarity is not easy to explain, as MEGA-10 is more hydrophilic (and hence relatively more polar). However, due to steric factors, the geometry of the mixed micelle and hence the compactness may be such that the micropolarity decreases with an increase in the MEGA-10 concentration. The micellar aggregation number (Nagg) was found to be lower for the pure components compared to the mixtures at all ratios (Figure 6). As observed from the I1/I3 ratio, it can be suggested that the structure of the mixed micelle is apparently rigid enough that the penetration of water molecules from the bulk into the mixed micelle is considerably reduced. Rubingh’s approach22 to the thermodynamics of micellization in mixed system is capable of reproducing the vast majority of the available experimental cmc trends. This treatment, for a solution containing two different surfactants 1 and 2 uses regular solution theory (RST), and the pseudophase separation for micellization βm is the interaction parameter depending on cmc1, cmc2, and (21) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (22) Rubingh, D. N. In Solution Chemistry of Surfactants; Mittal, K. L., Ed.; Plenum: New York, 1979; Vol. 1.
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Sulthana et al.
Table 6. Interaction Parameter (βm) Values for C12E10/ MEGA-10 and Brij 56/MEGA-10 Mixed Systems at Different Temperatures and Mixed Ratios βm C12E10/ 303 MEGA-10 K 9:1 7:3 5:5 3:7 1:9
-8.7 -7.2 -6.1 -5.3 -5.1
308 K
βm 313 K
318 Brij 56/ 303 K MEGA-10 K
-8.5 -6.5 -5.8 -4.4 -5.7 -7.3 -4.9 -7.9 -1.6 -4.8' -5.3 -3.3
-7.3 -7.4 -7.7 -5.9 -5.4
9:1 7:3 5:5 3:7 1:9
308 K
313 K
318 K
-7.4 -7.5 -7.6 -5.8 -5.3
-7.1 -7.3 -7.4 -5.6 -5.0
-6.8 -7.1 -7.2 -5.3 -4.8
Figure 8. Plot of the change in the upfield shift of the hydroxyl proton signal (∆δ, ppm) versus mole fraction of MEGA-10 for (a) C12E10/MEGA-10 and (b) Brij 56/MEGA-10 mixed systems.
Figure 7. Plot of the change in the downfield shift of the oxyethylene proton signal (∆δ, ppm) versus mole fraction of MEGA-10 for (a) C12E10/MEGA-10 and (b) Brij 56/MEGA-10 mixed systems.
cmcmix given by the relation
βm )
ln(cmcmixR1/cmc1x1) (1 - x1)2
where x1 is the mole fraction of surfactant 1 in the micelle and
x12 ln(cmcmixR1/cmc1x1) (1 - x1)2 ln[cmcmix(1 - R1)/(1 - x1)cmc2]
)1
By using the experimentally determined cmc values in the above equation and by using an iteration procedure, we can obtain x1 and hence βm. From the x1 values, it was convenient to calculate the quantities f1 and f2 using the following relations22
f1 ) exp[βm(1 - x1)2] m
2
f2 ) exp(β x1 )
where f1 and f2 are the activity coefficients of surfactants 1 and 2, respectively, in the mixed micelles. The values of f1 and f2 are presented in Table 5 for both C12E10/MEGA10 and Brij 56/MEGA-10 mixed systems. The βm value was found to vary depending upon the composition of the mixture, though it is generally expected to be constant. Such variation was found earlier also and has been discussed.23 To calculate f1 and f2, we computed βm at a particular composition and did not use an average value. The very low activity coefficient values of C12E10 and C16E10 in the mixed micelle probably indicate that these components in the mixed micelle are far away from their standard states, where the activity coefficients should be unity. The activity coefficients of MEGA-10 are much higher (near unity) in both the mixtures, indicating that these are very near their standard state, and with an increase in temperature the values increase. In general, a positive βm value means repulsion between the mixed species of surfactants while a negative βm value implies attraction.23 In our case, for both C12E10/MEGA-10 and Brij 56/MEGA-10, negative βm values were obtained, which can be ascribed to an attractive interaction between the surfactants in the mixed micelle (Table 6). Proton NMR spectroscopy was also used for studies of C12E10/MEGA-10 and Brij 56/MEGA-10 mixed system. The peak assignments for CnEm type surfactants were done as reported earlier.24 The changes in the chemical shifts on the gradual addition of MEGA-10 to C12E10 and Brij 56 at (23) Haque, M. E.; Das, A. R.; Rakshit, A. K.; Moulik, S. P. Langmuir 1996, 12, 4084. (24) Ribeiro, A. A.; Dennis, E. A. J. Phys. Chem. 1977, 81, 957.
Nonionic Surfactants and Their Mixtures
different mole fractions were monitored. Parts a and b of Figure 7 show the changes in the chemical shift of the ethylene oxide signal against mole fraction of MEGA-10 for C12E10 and Brij 56 systems, respectively. Also, the changes in the chemical shifts of the hydroxyl protons of MEGA-10 were monitored at various mole fractions of C12E10 and Brij 56 (Figure 8). It can be observed from the Figures 7 and 8 that the ethylene oxide signals of both C12E10 and Brij 56 show a downfield shift whereas the hydroxyl protons of MEGA-10 show an upfield shift on addition of POE type surfactants. These changes in the chemical shifts observed can be ascribed to an interaction between the head groups of both the surfactants, as mentioned earlier. Conclusion
Langmuir, Vol. 16, No. 3, 2000 987
MEGA-10 mixed surfactant systems were determined. The overall micellization was spontaneous not only in the pure compounds but also in the mixtures. Both the mixed systems showed nonideal behavior. The micellization process was endothermic. The ∆S°m values were all positive, and the values were lowered as the mole fraction of MEGA-10 increased in the mixture. The minimum areas per molecule (Amin) for various mixed systems were found to lie between those of pure surfactants. The ∆G°ad values indicate that the adsorption process is relatively more spontaneous than micellization. The I1/I3 ratio was lower for the mixtures, and the micelle aggregation numbers (Nagg) were higher than those for the pure components. The interaction parameter βm was negative, indicating an attractive interaction, as seen from 1H NMR studies.
The surface and thermodynamic properties of micellization and adsorption of C12E10/MEGA-10 and Brij 56/
Acknowledgment. Dr. T. Mukherjee of BARC, Mumbai, is gratefully acknowledged for fluorescence measurements. The M.S. University authorities are acknowledged for a temporary lectureship to S.B.S.
(25) Sugihara, G.; Hagio, M.; Tanaka, M.; Ikawa, Y. J. Colloid Interface Sci. 1988, 123, 544.
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