J. Phys. Chem. B 2000, 104, 10253-10257
10253
Transition from Normal to Flowerlike Micelles† Souvik Maiti and Prabha R. Chatterji* Speciality Polymers Group, Indian Institute of Chemical Technology (IICT), Hyderabad 500 007, India ReceiVed: January 20, 2000; In Final Form: August 9, 2000
A systematic investigation of the interfacial and solution properties of a series of triblock amphiphiles with the general formula C16EO20Cn′, n′ ) 0-16, indicates that flowerlike micelles are formed only when n′ g 8. This is consistent with the thermodynamic calculations of the free energy of looping.
1. Introduction “Flowerlike” micelles have been postulated as one of the multifarious aggregates of triblock amphiphiles of the type CnEOmCn, where m . n.1 Generally known as associative thickeners, the aggregation behavior of this class of compounds has been of great academic and technological interest. The long PEG chains with relatively short alkyl terminals aggregate in an aqueous medium, altering the rheological characteristics. This property has been advantageously exploited in the paints and coating industry.2 Techniques such as light scattering,3-5 fluorescence,3,6-8 surface tension,5 and NMR spectroscopy9,10 have been used to probe the size and shape of these aggregates. Several theoretical studies have also addressed the same issue.10,11 There are suggestions that flowerlike micelles could be a transient phase prior to secondary aggregation into the network structure.3 Our earlier studies revealed that low molecular weight symmetric triblock amphiphiles (CnEmCn, m ) 4-35 and n ) 22) and their corresponding diblocks form two separate classes of compounds.12,13 If their hydrophilic-lipophilic balance (HLB > 10) is favorable, the triblocks show well-defined critical micellar concentration (cmc). We have also noticed that the difference in the cmc between a given diblock and its triblock analogue depends on the relative length of the fatty acid and PEG chains13 (the cmc of C7E26 is 8.7 mM, and that of C7E26C7 is 1.2 mM) and is less for the amphiphiles with comparable segmental lengths (the cmc of C22E26 is 0.064 mM, and that of C22E26C22 is 0.048 mM). We have explained this behavior taking into account the wrapping tendencies of the solvophilic segment to protect the solvophobic segment. In the diblock, the flexible PEG segment with longer length than the fatty acid chain can form a protective wrap around the hydrophobic chain, blocking the hydrophobic interaction and pushing micelle formation to a higher concentration threshold. Formations of flowerlike micelles have been postulated for triblock amphiphiles of the type CnEmCn′ with sufficiently high values of n, m, and n′. We take a critical look at this possibility for the system C16E20Cn′. What will be the minimum value of n′ above which only flowerlike micelles are formed? To answer this question, we first chose Brij58 (C16E20) and synthesized its alkyl derivatives (C16E20Cn′, where n′ ) 0-15). We felt evaluation of the cmc, area per molecule, Amin, maximum * To whom correspondence should be addressed. Present address: GEIndia Technology Center, Innovator Building, International Technology Park, Whitefield Road, Bangalore 560 066, India. † IICT Communication Number 4482.
densities, Γmax, aggregation number, Nagg, and thermodynamic parameters of micellization (∆Gm°, and ∆Sm°) would help us to answer this question. 2. Experimental Section 2.1. Materials. All aliphatic acids, N,N-dibutylaniline (DBA), p-toluenesulfonic acid (PTSA), and eicosaethylene glycol hexadecyl ether (Brij58), from Fluka, were used as received. Pyrene (Py), also from Fluka, was recrystallized from ethanol before use. 2.2. Synthesis. Synthesis of Alkyl-End-Capped Brij Compound (C16E20Cn′). A 0.1 mol sample of Brij compound, 0.12 mol of aliphatic acid, and a catalytic amount of PTSA were dissolved in 50 mL of benzene and refluxed in a Dean-Stark apparatus for 6 h. Later, benzene was removed under reduced pressure. Unreacted acid was removed by passing the product mixture through alkaline silica gel with a chloroform-ether (1:1) mixture as eluent. The product was isolated and characterized as follows. The IR spectra of the above samples provide qualitative proof of esterification. The peak at ∼1730 cm-1 is indicative of the formation of an ester linkage. The absence of a peak at 1690 cm-1 confirms the absence of free acid in the product. The bifurcated peak between 2800 and 2900 cm-1 is due to the alkyl group (stretching) of PEG. The broad peak at 1100 cm-1 is that of the ether linkage (-CH2-O-CH2-) of PEG. The absence of a broad peak between 3200 and 3600 cm-1 confirms the esterification of Brij compounds. In NMR, resonances at δ 3.5-3.8, 0.9, and 1.3 are due to the ethoxy protons and aliphatic methylene and terminal methyl protons, respectively. 2.3. Methods. 1H NMR spectra were produced on a Gemini 200 MHz spectrometer. IR spectra were run on a Perkin-Elmer model 882 instrument. GPC was run on a Shimadzu unit, fitted with an RI detector using a Waters 100 Å styragel column at a flow rate of 0.5 mL/min. The concentration was 50 mg/mL. Surface tension measurements were taken at 27 °C on a Du Nouy tensiometer using a platinum ring having a circumference of 4 cm. For steady-state fluorescence measurements, the concentration of Py was held constant at 5.0 µM and the surfactant concentration varied over a wide range. A 3 mL sample of each solution was placed in a 10 mm rectangular quartz cell, and the spectra were run on a SPEX fluorolog spectrophotometer in right angle geometry using slit openings of 2 mm. Py was excited at 339 nm, and emissions at 374 and 386 nm were taken as first and third vibronic peaks. The emission spectra were accumulated with an integration time of 1 s/0.5 nm. An OMEGA, ITC, microcalorimeter of Microcal,
10.1021/jp000248a CCC: $19.00 © 2000 American Chemical Society Published on Web 10/19/2000
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Northampton, MA, was used for the thermometric measurements. A concentrated solution of a surfactant was taken in a microsyringe of capacity 250 or 500 µL and was added in multiple stages to water in the calorimeter cell of capacity 1.325 mL under constant stirring, and the stepwise thermograms of the heats of dilution of the surfactant solution were recorded at 30 ( 0.01 °C. Enthalpy calculations were performed with the help of ITC software. The fluorescence decay curves were recorded using an IBH single-photon-counting spectrofluorimeter (model 5000u). This instrument was operated with a thyraton-gated flush lamp filled with hydrogen at a pressure of 0.5 atm. The lamp was operated at a frequency of 40 kHz, and the pulse width of the lamp under the operation conditions was ∼1.2 ns. To determine the aggregation number, self-quenching of pyrene fluorescence spectra was analyzed. The fluorescence decay curves were recorded using a single-photon-counting apparatus. The general decay equation14,15
I(t) ) I(0) exp{-A2t - A3[1 - exp(-A4t)]}
(1)
was fitted to the decay curves using a weighted least-squares procedure. I(t) and I(0) are the fluorescence intensities at time t and zero, following excitation. A2, A3, and A4 are three constants which are determined as adjustable parameters. It follows that
A2 ) k
(2)
A3 ) [Py]/{Ct(cmc)/N}
(3)
A4 ) kq
(4)
where k is the rate constant for intramicellar fluorescence decay, Ct is the total surfactant concentration, N is the aggregation number, and kq is the rate constant for intramicellar quenching. From A3 we have calculated N. Due to the poor value of the kinetic ratio (kq/k) for the quencher dimethylbenzophenone (DMBP), dibutylaniline (DBA) in this nonionic system, we took advantage of pyrene self-quenching, which has a better kq/k value.16 In this case, eq 1 is still valid as long as the number of pyrenes per micelle is approximately 1 or less, and deactivation is much faster than the dissociation of the excimer, which is observed in analogous systems.16 Very recently Hakansoon et al. have followed the same procedure to determine the aggregation number of the C17E84 surfactant.17 In our experiment, Py concentration was adjusted in such a way that the number of pyrenes per micelle was less than 1 but enough to form the excimer, which was checked by steady-state fluorescence having an extra broad peak at around 500 nm other than the monomer peak in Py fluorescence spectra. The fluorescence decay curve was analyzed at an emission wavelength of 387 nm exciting Py at 339 nm. To begin with, the lifetime of pyrene was measured keeping Py at 0.5 µM and the surfactant at ∼10 × cmc as aggregation of nonionic amphiphiles is very much concentration dependent. To determine N, experiments were done at least twice. [Py] ) 5 µM, and [surfactant] ) 10 × cmc. Here A2 ) k ) 1/τ0 (s-1) and almost the same when [Py] ) 0.5 µM. From A3 we calculate N. A4 ) kex (s-1). In each case a minimum of three experiments were done. 3. Results and Discussion 3.1. Critical Micellar Concentration. The critical micellar concentrations of the amphiphiles C16E20Cn′, where n′ varied
Figure 1. (a) Determination of the cmc by surface tension for the C16E20Cn′ system. (b) Determination of the cmc by fluorescence methods for the C16E20Cn′ system.
TABLE 1: Cmc, Γmax, and Amin of Different Amphiphiles C16E20Cn′a at 300 K cmc/µM n′
surface tensionb
fluorescencec
Γmaxd × 106/ (mol/cm2)
Amine/ nm2
0 3 4 5 6 8 10 12 16
50.0 21.9 19.9 19.0 18.2 13.2 12.0 10.0 4.17
51.5 31.5 28.0 26.5 25.0 16.0 14.0 12.5 7.5
3.60 2.65 2.49 2.54 2.54 2.44 2.09 2.17 2.17
0.46 0.63 0.67 0.65 0.65 0.68 0.79 0.76 0.76
aC E C )C E b c 16 20 n′ 16 20-O-CO-[CH2]n′-2-CH3. Within (3%. Within (8%. d Within (5%. e Within (5%.
from 0 to 16 (the number of carbon atoms present in the fatty acid is taken as n′) are determined by both surface tension and fluorescence methods (Figure 1). To avoid overcrowding, we have shown only two typical cases, for n′ ) 4 and 10. The complete data are presented in Table 1. The cmc obtained by the fluorescence method is higher than that obtained by the surface tension method. This is a common feature for nonionic amphiphiles with a low cmc.18 This is because the two methods focus on two different events. While surface tension measures the process leading to micellization, fluorescence detects the solubilization of the probe after micellization has occurred. When the cmc is low, at an amphiphile concentration just above the cmc, the micellar population is low and the amount of the probe solubilized in micelles is too low to be detected. Initially there is an abrupt drop in the cmc from 50 to 21.9 µM when the free -OH group is capped by the hydrophobic ethyl group.
Transition from Normal to Flowerlike Micelles
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10255 TABLE 2: ∆Gm°/(kJ/mol), ∆Sm°/(J/mol), and ∆Gad°/(kJ/ mol) of the Amphiphiles C16E20Cn′ at 300 Ka n′
∆Gad°/ (kJ/mol)
∆Gm°/ (kJ/mol)
∆Sm°/ (J/mol)
0 3 4 5 6 8 10 12 16
-31.94 -37.33 -38.24 -37.73 -38.24 -39.08 -40.70 -40.46 -41.97
-24.46 -26.76 -27.00 -27.11 -27.22 -28.02 -28.26 -28.71 -30.91
81.53 89.20 90.00 90.36 90.77 93.40 94.20 95.70 103.03
a ∆H °/(J/mol) was taken as zero. The ∆G ° values are within (5%, m ad ∆Gm° within (3%, and ∆Sm° within (3%.
Figure 2. Dependence of the cmc on n′ for the C16E20Cn′ system.
Figure 3. Variation of Γmax with n′ for the C16E20Cn′ system.
Thereafter the cmc decreases steadily as n′ increases. Figure 2, which depicts the dependence of the cmc on n′ marks two distinct regions, n′ < 8 and > 8. 3.2. Interfacial Properties. The maximum densities, Γmax (mol/cm2), i.e., the amount of surfactant adsorbed per unit area at the air/water interface after complete monolayer formation and the minimum area per molecule [Amin (nm2)] were calculated from the equations19,20
Γmax ) (1/2.303RT) lim (dΠ/d log C)
(5)
Amin ) 1018/NΓmax
(6)
Cfcmc
where R, T, and N are the gas constant, temperature on an absolute scale, and Avogadro number, respectively. The values of Γmax and Amin are presented in the Table 1. The values indicate that introduction of an extra carbon atom to the ω terminal of the amphiphile makes the monolayer less dense. Variation of Γmax with n′ is shown in the Figure 3. 3.3. Thermodynamics of Adsorption and Micellization. The standard free energy of adsorption at the air/water interface (∆Gad°) was obtained from the relationship19,20
∆Gad° ) ∆Gm° - (Πcmc/Γmax)
(7)
where Πcmc is the surface pressure at the cmc and Γmax is the surface excess at the maximum adsorption level. The free energy of micelle formation per mole of monomer unit (∆Gm°) was evaluated according to the equation19,20
∆Gm° ) ∆RT ln cmc
(8)
The values of ∆Gm° and ∆Gad° have been computed using the cmc values measured by surface tension methods and are presented in Table 2. ∆Gm° and ∆Gad° are both negative. Their magnitudes reveal that the latter is more spontaneous than the former. The hydrophobicity primarily leads them toward the air/water interface, and the formation of micelles occurs above a critical concentration (cmc). For a complete thermodynamic analysis, along with ∆G0m and ∆Gad°, ∆Hm° is required. The thermodynamic titration method for determination of the cmc and enthalpy of micellization (∆Hm°) was introduced by Kreshek and Hergraves21 and pursued in other laboratories.22-29 Very recently Majhi et al.28 have reassessed the basis of micellization with the help of this unique microcalorimetric method. The isothermal titration experiments were conducted by step-by-step injections of concentrated amphiphile solutions into a cell containing water. We noticed (Figure 4) no measurable change in enthalpy per injection throughout the concentration range from below the cmc to well above the cmc. A similar observation has been made by Ghosh et al. for Tween20 and Brij35 and their mixtures with SDS.30 We suspect at the concentrations employed the heat change associated with the micellization process would have been below the detection limit of the calorimeter. Theoretically evaluation of the cmc as a function of temperature could also help to estimate ∆Hm°. But two issues complicate this approach and lead to an overestimation of ∆Hm°: one, the aggregation number itself varies with temperature,31 and two, the dehydration and consequent increased hydrophobicity of the PEG segment with an increase in temperature. Assuming ∆Hm° is zero, from the Gibbs equation the entropy of micellization (∆Sm°) can be determined:
∆Sm° ) -∆Gm°/T
(9)
In every case ∆Sm° was fairly positive. The randomness produced by a melting “iceberg” or “flickering cluster” around the nonpolar tail of the amphiphile monomers during micellization and the localization of the nonpolar ends in similar or like environments in the micellar interior is corroborated by the positive ∆Sm°. The T∆Sm° > ∆Hm° results indicate the micellization process is entropically controlled in every case. 3.4. Micropolarity. The ratio of the first and third vibrational peaks, i.e., I1/I3, of the pyrene fluorescence emission spectrum can be a measure of the polarity of the micellar interface because pyrene is assumed to be located at the hydrophobic-hydrophilic interface.32-34 Fluorescence experiments give the magnitude of I1/I3, which is a measure of the polarity of the ambience in which Py resides. This ratio shows a sharp change from 1.13 to 1.06 when n′ changes from 6 to 8, suggesting the ambience becoming
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Figure 5. Variation of micropolarity with n′ for the C16E20Cn′ system.
Figure 6. Variation of the aggregation number for the C16E20Cn′ system.
Figure 4. (a) Microcalorimetric trace for Brij58. (b) Microcalorimetric trace for C6. (c) Microcalorimetric trace for C16. Experimental details are in the text.
TABLE 3: Micropolarity (I1/I3) and Aggregation Number (Nagg) of the Amphiphiles C16E20Cn′ n′
I1/I3
Nagg
n′
I1/I3
Nagg
0 3 4 5 6
1.15 ( 0.01 1.13 ( 0.01 1.13 ( 0.01 1.12 ( 0.01 1.13 ( 0.01
62 ( 3
8 10 12 16
1.07 ( 0.01 1.08 ( 0.01 1.07 ( 0.01 1.06 ( 0.01
27 ( 2 25 ( 3 21 ( 2 22 ( 2
58 ( 3 58 ( 4
increasingly nonpolar (Table 3). Taking C16E20 and C16E20C15 as the two extreme cases, we can conclude that, at n′ ) 8, the system undergoes a definite change (Figure 5). 3.5. Micellar Aggregation Number. The aggregation numbers of these amphiphiles are presented in Table 3. We obtained the N value for parent C16E20 amphiphiles of 62, which is in agreement with the value reported by Abe et al.35 The results show that the aggregation number does not vary much when a diblock nonionic is end-capped with a small hydrophobe. The aggregation number varies marginally from 62 to 58 as the terminal group changes from OH to C6. At n′ ) 8 the aggregation number drops to 27 (Figure 6). The essence of the data presented in Figures 2, 3, 5, and 6 and Tables 2 and 3 is the quantification of the effect of n′ on cmc, Γmax (mol/cm2), Nagg, and I1/I3 values. Without any
ambiguity in all cases it turns out that, as n′ progressively increases, the system C16E20Cn′ undergoes a definite change at n′ g 8. On the strength of the experimental evidence we draw the conclusion that n′ ) 8 is the threshold value at and above which flowerlike micelles are formed. We reinforce this inference with free energy calculations on the looping probability of the PEG chain and the free energy gain as the hydrophobic terminal is inserted into the micellar core. 3.6. Free Energy Calculations of Looping. In a recent study, as a kind of minimum stability criterion for a flowerlike micelle Raspaud et al.11 proposed a comparison between a flowerlike micelle anda “stripped flower micelle”. The series being studied by us is more suited to verify the model where the micellar core will be formed by the fixed hydrophobic chain and another ω-hydrophobic chain will share the same core depending on its hydrophobicity. The free energy connected with formation of a loop of an end-modified polymer has been discussed in the literature. There are two opposing thermodynamic parameters that determine the formation of flowerlike micelled: loss of entropy due to looping of the middle block and energy gain of transferring the hydrophobic chain from the solvent to the micellar core. If the former is less than the latter, the flowerlike micelle will be stable. The free energy of looping can be estimated by combining the loss of entropy when the PEG chain is back-folded and the gain in hydrophobic free energy when the alkyl chains at the second terminal get inserted into the core.
∆Glooping ) ∆Gbending + ∆Ghydrophobic
(10)
Alami et al.3 have made estimations for the C12E460C12 system. They calculated the probability of PEG chain bending from the end distribution probability of a Gaussian chain and the gain in
Transition from Normal to Flowerlike Micelles
J. Phys. Chem. B, Vol. 104, No. 44, 2000 10257
TABLE 4: Calculated Values for ∆Ghydrophobic (∆Gbending for C16E20C20 ) 1.89 RT) no. of CH2 groups in the ∆Ghydrophobic, hydrophobic chain in RT -0.3 -0.6 -0.9 -1.2
1 2 3 4
no. of CH2 groups in the ∆Ghydrophobic, hydrophobic chain in RT 5 6 7 8
-1.5 -1.8 -2.1 -2.4
free energy as the second alkyl terminal associates into the micellar core.3
∆Gbending ) -2.6RT + 1.5RT ln NE
(11)
∆Ghydrophobic ) (-0.3 to -0.5)RTNCH2
(12)
For our system with NE ) 20, ∆Gbending works out to be 1.89 kT. To calculate ∆Ghydrophobic, we used the prefactor 0.3, because the alkyl chain is linked to the EO chain through a hydrophilic ester group. The values shown in Table 4 indicate that an EO chain of 20 units requires a minimum of 7 units to make the looping energetically profitable. This authenticates our results. 4. Conclusions For the CnEmCn′ system where n ) m ) ∼16, flowerlike micelles are formed only when n′ g 8. We also provide ample evidence for long-chain PEG amphiphiles (EO > 20). Acknowledgment. S.M. acknowledges financial help from UGC, New Delhi, in the form of a Senior Research Fellowship. Supporting Information Available: 1H NMR and FTIR spectra of C16E20, C16E20C2, and C16E20C6. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Polymer in Aqueous Media; Glass, G. E., Ed.; Advances in Chemistry Series 223; American Chemical Society: Washington, DC, 1989. (2) For a recent review, see: Winnik, M. A.; Yekta, A. Curr. Opin. Colloid Interface Sci. 1997, 2, 242. (3) Alami, E.; Almgren, M.; Brown, W.; Francois, J. Macromolecules 1996, 29, 2229.
(4) Maechling-Strasser, C.; Francois, J.; Clouet, F.; Tripette, C. Polymer 1992, 33, 627. (5) Maechling-Strasser, C.; Clouet, F.; Francois, J. Polymer 1992, 33, 1021. (6) Hansson, P.; Almgren, M. Langmuir 1994, 10, 2115. (7) Binan-Limbele, W.; Clouet, F.; Francois, J. Colloid Polym. Sci. 1993, 271, 748. (8) Yekta, A.; Duhamel, J.; Adiwidjaja, H.; Brochard, P.; Winnik, M. A. Langmuir 1993, 9, 881. (9) Nystrom, B.; Walderhaug, H.; Hansen, F. K. J. Phys. Chem. 1993, 97, 7743. (10) Walderhaug, H.; Hansen, F. K.; Abrahmsen, S.; Persson, K.; Stilbs, P. Phys. Chem. 1993, 97, 8336. (11) Raspaud, E.; Lairez, D.; Adam, M.; Carton, J. P. Macromolecules 1994, 27, 2956. (12) Padmavathi, N. Ch.; Chatterji, P. R. Langmuir 1995, 11, 767. (13) Maiti, S.; Chatterji, P. R. Macromolecules, submitted for publication. (14) Almgren, M.; Lofroth, J. E.; van Stam, J. J. Phys. Chem. 1996, 90, 4431. (15) Almgren, M.; van Stam, J.; Swarup, S.; Lofroth, J. E. Langmuir 1986, 2, 432. (16) Almgren, M. In Kinetics and Catalysis in Microheterogeneous Systems; Gratzel, M., Kalyanasundaram, K., Eds.; Marcel Dekker: New York, 1991. (17) Hakansson, B.; Hansson, P.; Regev, O.; Soderman, O. Langmuir 1998, 14, 5730. (18) Xu, B.; Li, L.; Yekta, A.; Masoumi, S.; Kanagaligam, S.; Winnik, M.; Zhang, K.; Macdonald, M. P.; Menchen, S. Langmuir 1997, 13, 2447. (19) Rosen, M. J.; Cohen, A. W.; Dahanayake, M.; Hua, X. Y. J. Phys. Chem. 1982, 86, 541. (20) Dahanayake, M.; Cohen, A. W.; Rosen, M. J. J. Phys. Chem. 1986, 90, 2413. (21) Kresheek, G. C.; Hargraves, W. A. J. Colloid Interface Sci. 1974, 48, 481. (22) Jana P. K.; Moulik, S. P. J Phys. Chem. 1991, 95, 9523. (23) Mukherji, K. Mukherji, D. C.; Moulik, S. P. J Phys. Chem. 1994, 98, 4713. (24) Moulik, S. P.; Huque, Md. E.; Das, A. R. J. Phys. Chem. 1996, 100, 701. (25) Huque, Md. E.; Das, A. R.; Moulik, S. P. J. Phys. Chem. 1995, 99, 14032. (26) Jha, R.; Ahluwalia, J. C. J. Phys. Chem. 1991, 95, 7782. (27) Mazer, N. A.; Olofsson, G. J. Phys. Chem. 1982, 86, 4584. (28) Birdi, K. S. Colloid Polym. Sci. 1983, 261, 45. (29) Majhi, P. R.; Moulik, S. P. Langmuir 1998, 14, 3986. (30) Ghosh, S.; Moulik S. P. J. Colloid Interface Sci. 1998, 208, 357. (31) Schick, M. J. Nonionic surfactants: Physical chemistry; Marcel Dekker: New York, 1987; Vol. 23. (32) Kalyansundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (33) Hanson, P.; Almgren, M. J. Phys. Chem. 1995 99, 16684. (34) Hu, Y.; Zhao, C.; Winnik, M. A.; Sundarajan, P. R. Langmuir 1990, 6, 880. (35) Abe, M.; Uchiyama, H.; Yamaguchi, T.; Suzuki, T.; Ogino, K.; Scamenorn, J. F.; Christain, S. D. Langmuir 1993, 8, 2147.
