Langmuir 1997, 13, 5011-5015
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Comparison of the Aggregation Behavior of Di- and Triblock Nonionic Amphiphiles with Linear and Cyclic Hydrophobic Tails† S. Maiti and P. R. Chatterji* Division of Organic Coatings and Polymers, Indian Institute of Chemical Technology, Hyderabad-500 007, India Received February 28, 1997. In Final Form: June 18, 1997X The micellization characteristics of nonionic surfactants of the type C7E26 and symmetric C7E26C7 where C7 ) the heptanoic acid (C7H14O2) or the cyclohexanecarboxylic acid (C7H12O2) group is investigated. Structural formulas being nearly the same, the linear and cyclic analogues possess roughly the same molecular weights and hydrophilic-lipophilic balance (HLB) values. Surface tension and fluorescence experiments provide information on the thermodynamics of micellization, interfacial adsorption, mean aggregation number, etc. Comparisons are made between linear and cyclic analogues and between diblocks and triblocks. The results indicate that, compared to their linear counterparts, amphiphiles with cyclic hydrophobes possess higher CMC values and the micelles formed are less compact with lower aggregation number. A qualitatively similar trend is seen between diblock and triblock surfactants. The results for triblocks are consistent with a micellar structure in which the PEG chain folds to insert both the terminal hydrophobic segments into the micellar core.
1.0. Introduction Amphiphilic compounds efficiently lower the interfacial tension by virtue of their preferential adsorption at the gas/liquid, liquid/solid or gas/solid interface. In aqueous medium they aggregate into micelles above a critical concentration called the critical micellar concentration (CMC). The driving force for micellization is hydrophobic in nature. Consequently the amphiphilic character of these molecules is generally quantified as hydrophiliclipophilic balance (HLB) which reflects their micellization, dispersion, and emulsification tendencies.1 HLB values are calculated according to the following equation
HLB ) 20MH/(MH + ML)
(1)
where MH and ML are the molecular weights of the hydrophilic and hydrophobic components in the surfactant, respectively. Monoalkyl ethers and -esters of poly(oxyethylene glycol)s with the general formula CnEm, where n denotes the number of carbons in the hydrocarbon segment and m the number of ethylene glycol units in the PEG segment, are well-known nonionic surfactants. They can as well be classified as di- and triblock amphiphiles. The influence of the number of methylene groups in the linear hydrophobic part2-5 and the number of ethylene oxide groups in the hydrophilic part6-8 on the micelle organization are well established. The interfacial area per molecule, the CMC, chain geometry, and tendency to self coil have been reported for surfactants with polar or aromatic groups * Author to whom correspondence should be sent. † IICT Communication number 3792. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Schick, M. J. Nonionic surfactants: Physical chemistry, Marcel Dekker, Inc.: New York, 1987; Vol. 23. (2) Kucharski, S.; Burczyk, B. Tenside Deterg. 1971, 8, 69. (3) Kuwamura, T.; Takahashi, H. Bull. Chem. Soc. Jpn. 1973, 45, 617. (4) Takahashi, H.; Kuwamura, T. Bull. Chem. Soc. Jpn. 1973, 46, 623. (5) Yeates, S. G.; Craven, J. R.; Mobbs, R. H.; Booth, C. J. Chem. Soc., Faraday Trans. 1986, 82, 1865. (6) Schick, M. J. J. Coll. Sci. 1962, 17, 801. (7) Lange, H.; Koll, Z. Z. Polymer. 1965, 131, 201. (8) Barry, B. W.; El.Eini, J. Colloid Interface Sci. 1975, 54, 339.
S0743-7463(97)00230-8 CCC: $14.00
either at the alkyl terminus or along the hydrocarbon chain.9 The functionalized amphiphiles display larger molecular areas than their saturated analogues.10 However relatively less information is available regarding the influence of the molecular architecture of the hydrocarbon segment on the surface and micellar properties of PEG.10-18 There are some reports on the influence of branching in the hydrocarbon segments. Varadaraj et al.15-18 have recorded that the branching of the hydrocarbon chain changes the interfacial and bulk properties of sodium dodecylsulfate surfactants. Their studies include ethoxylates and ethoxy sulfates which exhibit the same trend. That branching in the hydrophobic and/or hydrophilic segments of a surfactant leads to properties and performance qualitatively different from those of their linear counterparts in the process of micellization, critical micellar concentration, equilibrium surface tension, and area per molecule have been reiterated very recently by Kratazat et al.10 Their investigations indicate that hydrocarbon branching can affect the liquid crystalline phases of surfactants.11-13 We focus our attention here on the introduction of a cyclic group into the hydrocarbon chain. For a systematic interpretation we evaluate the properties of di- and triblock surfactants with cyclic hydrocarbon tail against their linear analogues possessing nearly the same structural formula and HLB value. Table 1 lists the essential structural and physical features of the compounds in(9) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1976, 80, 1462. (10) Kratzat, K.; Finkelmann, H. L. Langmuir 1996, 12, 1765. (11) Kratzat, K.; Finkelmann, H. L. Colloid Polym. Sci. 1994, 272, 400. (12) Kratzat, K.; Schmidit, C.; Finkelmann, H. J. Coll. Interface Sci. 1994, 163, 190. (13) Kratzat, K.; Schmidit, C.; Finkelmann, H. Colloid Polym. Sci. 1995, 327, 257. (14) Varadaraj, R.; Bock, J.; Valint, P.; Zushma, S.; Thomas, R. J. Phys. Chem. 1991, 95, 1671. (15) Varadaraj, R.; Bock, J.; Valint, P.; Zushma, S.; Thomas, R. J. Phys. Chem. 1991, 95, 1677. (16) Varadaraj, R.; Bock, J.; Valint, P.; Zushma, S.; Thomas, R. J. Phys. Chem. 1991, 95, 1679. (17) Varadaraj, R.; Bock, J.; Valint, P.; Zushma, S.; Thomas, R. J. Phys. Chem. 1991, 95, 1682. (18) Varadaraj, R.; Bock, J.; Zushma, S.; Brons, N. Langmuir 1992, 8, 14.
© 1997 American Chemical Society
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Table 1. Structural and Physical Characteristics of the Surfactants surfactants
codea
HLB value
MW
LE
18.24
1288
CE
18.27
1286
LEL
16.77
1400
CEC
16.80
1396
a Key: L ) linear hydrocarbon; C ) cyclic hydrocarbon; E ) ethylene oxide chain.
vestigated. Thermodynamics of micellization and adsorption, aggregation number, and behavior at the air/ water interface of these surfactants have been investigated by means of surface tension and fluorescence probing techniques. An attempt is made to fit the results to a suitable model. 2.0. Experimental Section 2.1. Materials. Poly(ethylene glycol) (PEG 1000), cyclohexanecarboxylic acid, heptanoic acid, N,N-dibutylaniline (DBA), and p-toluenesulfonic acid (PTSA) from Fluka were used as received. Pyrene (Py) also from Fluka, was recrystallized from ethanol before use. PEG was characterized by gel permeation chromatography (GPC) and NMR. The GPC profile gave a dispersity index of 1.08 and NMR gave molecular weight as 1176, equivalent to 26 EO units. The solvent tetrahydrofuran (THF) for the GPC experiment was of HPLC grade. Double-distilled water was used for all experiments. 2.2. Synthesis. (A) Preparation of Diblock Surfactants (LE and CE Systems). In order to avoid esterification on both sides of PEG molecules, the hydroxyl group on one side was blocked by complexing it with boric acid.19,20 For this 1 mol of PEG was treated with 0.33 mol of boric acid under continuous vacuum at 80-90 °C until the evolution of water ceased. To this reaction mixture were added aliphatic acid (1 mol) and PTSA as catalyst. A vacuum of 7 mmHg at 110 °C was maintained throughout. The reaction mixture was then refluxed with the minimum amount of water for 1 h to hydrolyze the borate complex and then neutralized with sodium acetate. The underivatized PEG was washed away with saturated brine. The washings were done untill the product responded negatively to the presence of free PEG with Dragendroff reagent. The unreacted acid was removed by passing the reaction mixture through alkaline silica gel with a chloroform-ether mixture (1:1) as eluent. (B) Preparation of Triblock Surfactants (LEL and CEC). Aliphatic acid (2 mol), PEG (1 mol), and a catalytic amount of PTSA were dissolved in benzene and refluxed in a Dean-Stark apparatus for 6 h. Benzene was removed under reduced pressure. The diester of PEG was extracted from the reaction mixture as described for monoesters. 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 concentrations were 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 fluorescence measurements, the concentration of Py was held constant at 5.0 µM and surfactant concentration varied over a wide range. Then 3 mL 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. For fluorescence quenching studies, the (19) Hartman, L. J. Chem. Soc. Part II 1957, 1918. (20) Rao, T. C.; Sastry, Y. S. R.; Rao, R. S. J. Am. Oil Chem. Soc. 1977, 54, 15.
Figure 1. Infrared spectra: (a) monoester; (b) diester. desired amount of stock solution of DBA in acetone was taken in a volumetric flask. After evaporation of acetone, surfactant solution was added into these flasks and DBA dissolved by sonication. All solutions were aged for 24 h before the experiment.
3.0. Results and Discussion 3.1. Characterization of Di- and Triblock Surfactants. The IR spectra of the above samples provided qualitative proof of esterification (Figure 1). The peak at ∼1730 cm-1 is indicative of the formation of an ester linkage. The absence of peak at 1690 cm-1 confirmed 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 broad peak between 3200 and 3600 cm-1 suggests the presence of hydroxyl group in the diblock at one terminal (Figure 1a), and the total disappearance of the same in the spectra of the triblocks confirms the esterification of PEG on both sides of (Figure 1b). From the NMR spectra, the molecular weight of the compounds were calculated based on the integration of the four equivalent protons responsible for resonance at 4.2-4.4 ppm marked as a in Figure 2. The same procedure was followed for the monoesters. Resonances at 3.5-3.8 ppm and at 1.2-2.5 ppm are due to the ethoxy protons and aliphatic protons, respectively. 3.2. Interfacial Properties. Variation of surface pressure (Π ) γsolvent - γsolution) as a function of the logarithms of individual surfactant concentration is shown in Figure 3. All the surfactants were effective in reducing the surface tension of aqueous solution with minor variations in the numerical value. This is consistent with the efficient positive adsorption at air/water interface. The maximum densities, Γmax, (mol/cm2) i.e., the amounts of surfactant adsorbed per unit area at the air/water interface after complete monolayer formation and minimum area per molecule (Amin (nm2) were calculated from the equations21,22 (21) Rosen, M. J.; Cohen, A. W.; Dahanayake, M.; Hua, X. Y. J. Phys. Chem. 1982, 86, 541.
Aggregation Behavior of Nonionic Amphiphiles
Figure 2.
1H
Langmuir, Vol. 13, No. 19, 1997 5013
NMR spectrum of diester of PEG. Table 2. The Cmc, Πcmc, Γmax, and Amin of Different Surfactants at 300 K CMC, mM amphiphiles
STa
Fb
Πcmc, erg/cm2
Γmax, mol/cm2 × 106
Amin, nm2
LE CE LEL CEC
8.71 12.0 1.20 3.98
7.7 15.0 1.0 4.0
35.5 31.0 35.0 33.0
2.68 2.09 1.65 1.49
0.62 0.79 1.00 1.14
a
Surface tension. b Pyrene fluorescence.
Figure 3. Surface pressure (Π) vs log C plots: (a) O, LE; b, LEL; (b) O, CE; b, CEC.
Γmax ) (1/2.303RT) lim (dΠ/d(log C))
(2)
Amin ) 1018/NΓmax
(3)
c f cmc
where R, T, and N are gas constant, temperature on the absolute scale, and Avogadro’s number, respectively. Values of Π, Γmax, and Amin are presented in Table 2. Amin for the surfactant containing the linear chain is marginally lower than that for the corresponding cyclic analogue. Between the diblocks and triblocks, the triblock occupies a larger area than the diblock. In the triblocks, the terminal hydrophobic groups would force the PEG chain to loop up at the interface. We have already alluded to this possibility in an earlier instance while discussing the solution properties of hydrophobically endcapped PEGs.23 This would possibly explain the smaller difference in Amin between LE/CE and between LEL/CEC systems and the relatively larger difference between LE/LEL and between CE/CEC. (22) Dahanayake, M.; Cohen, A. W.; Rosen, M. J. J. Phys. Chem. 1986, 90, 2413. (23) Padmavathi, N. Ch.; Chatterji, P. R. Langmuir 1995, 11, 767.
Figure 4. Influence of surfactant concentration on the I1/I3 for pyrene in water. Pyrene concentration was held constant at 5 µM: (0) LE; (2) CE; (O) LEL; (+) CEC.
To sum up, in conformity with the essential sterical requirements of the molecules the area occupied by a single surfactant chain at the interface increase in the following order:
CEC > LEL > CE > LE 3.3. CMCs and Gibbs Free Energy for Micellization and Adsorption. The critical micelle concentration for the surfactants in aqueous solution was determined by surface tension and fluorescence methods (Table 2; Figures 3 and 4). The values obtained by both methods show good agreement. The CMC of the diblock surfactant is higher than that of the corresponding triblock surfactant. We have observed the same behavior for a series of mono- and dibehenyl esters of PEG.23 Surfactants aggregate in aqueous medium due to hydrophobic interactions among the hydrocarbon chains. All the four surfactants studied are predominantly hydrophilic in
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Table 3. ∆G0m and ∆G0ad of Different Surfactants at 300 K amphiphile
-∆Gm0, kJ/mol
-∆Gad0, kJ/mol
LE CE LEL CEC
11.93 11.03 16.77 13.78
24.44 23.75 37.94 36.85
character due to the long PEG chain. However the additional terminal hydrophobic group in the triblock renders it less hydrophilic that the corresponding diblock. This is evident from their HLB values (Table 1). The standard free energy of adsorption at air/water (∆Gad0) was obtained from the relationship21,22 0
0
∆Gad ) ∆Gm - (Πcmc/Γmax)
(4)
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 (∆Gm0) was evaluated according to the equation21,22
∆Gm0 ) RT ln CMC
(5)
The values of ∆Gm0 and ∆Gad0 for linear surfactants are more negative that those of their cyclic analogues (Table 3). This indicates that these two processes are more spontaneous for linear surfactants than for the cyclic analogues. Between the diblock and triblock amphiphiles, the values for both processes are substantially more favorable for triblocks. Varadaraj et al.14-18 have extensively investigated the effect of branching of the hydrocarbon chain on the solution properties of a nonionic surfactant. They observed that branched surfactants exhibit higher CMCs than their linear analogues. For example CMC of branched dodecyl sodium sulfate was twice that of SDS.18 The surface tension at the air-water interface drops at a faster rate for branched C16 surfactant than its linear counterpart. All these have been attributed to an unfavorable steric factor introduced by the branching of the hydrophobe. The process of micellization is indeed a process of organising the hydrocarbon tails into a compact core. It is argued that branching of the tails causes steric hindrance and interferes with micellization. It is possible that cyclic group induces a similar effect. In the present study the CMCs of cyclic systems are slightly higher than the corresponding linear systems. For example, the CMC of CE is 12.00 mM whereas that of LE is 8.7 mM; between the triblocks the CMC of CEC is 3.98 mM while the linear analogue has a value of 1.2 mM. Structurally the cyclohexyl group is not so sterically destabilising as the Guebert hydrophobe,14-18 and this could be the reason for the relatively small differences in the CMCs between linear and cyclic systems. However the difference in CMCs between the di and triblocks are of much larger magnitude. There could very well be an additional reason for the higher CMCs of diblocks in comparison to that of the triblocks. This has to do with the solubilization of the hydrophobic tail. Using a two dimensional self-consistent field theory, Singh et al.24,25 report that diblock copolymers could self-organize into complex geometries such as “pinned micelles”, or “onion; garlic; dumbbell; or flowerlike” structures under special circumstances. The essential feature of all these structures is that the solvophilic (24) Singh, C.; Balazs, A. C. Macromolecules 1996, 29, 8904. (25) Zhulina, E. B.; Singh, C.; Balazs, A. C. Macromolecules 1996, 29, 8254.
Figure 5. Probable looping of PEG chain around the hydrocarbon segment: (a) short linear hydrocarbon segment; (b) cyclic hydrocarbon segment. Table 4. Cmc and ∆G0m of Di- and Triblock Surfactants (Behenyl Derivatives of PEG) at 300 K amphiphile
CMC, mM
-∆Gom, kJ/mol
C22E26 C22E26C22 C22E35 C22E35C22
0.065 0.048 0.150 0.070
24.04 24.80 21.96 23.86
segment effectively protects the solvophobic block by wrapping around it. We envisage a simpler model for the amphiphiles being discussed here. In the diblocks, the long flexible PEG chain can form a protective wrap around the hydrocarbon segment blocking hydrophobic interactions (Figure 5). This would improve the solubility of the diblocks in the aqueous phase, pushing micelle formation to a higher concentration threshold. Such an arrangement would be thermodynamically consistent only if the PEG chain is sufficiently longer than the hydrocarbon tail. If the PEG chain is long, then entropy losses would be minimal because the EO chain would still be in the relaxed form. While this situation is conceivable for the diblocks (C7E26) in the present study, both steric and thermodynamic factors would prevent this in triblocks (C7E26C7). Our earlier investigations23 on the solution properties of hydrophobically end-capped diblock and triblock poly(ethylene glycol)s lend support to this hypothesis (Table 4). Between C22E26 and C22E35, the CMC goes up from 0.065 to 0.150 mM while the difference is less dramatic. 3.4. Aggregation Number. Aggregation number has been determined by the fluorescence quenching of fluorescence emission (I3 peak) of pyrene by DBA quencher using the relation26,27
ln(I0/I) ) (〈N〉[Q])/([S] - CMC)
(6)
where I0 and I are fluorescence intensities in the absence and presence of quencher, 〈N〉 is the average aggregation number, and [Q] and [S] are the quencher and total surfactant concentration, respectively. For each experiment Py was held constant at 8 µM and surfactant concentration of ten times the corresponding CMC was employed. For the calculation a pseudophase micellization process has been assumed. The graphs ln(I0/I) vs [Q] are represented in Figure 6, and 〈N〉 values are listed in Table 5. The aggregation number of triblock surfactants are lower than the diblock surfactants. That is, diblock micelles are more compact than the triblock micelles. It is not surprising that among the two classes, the cyclics have lower aggregation number for obvious steric reasons. 3.5. The Proposed Model. Extensive studies on the size and shape of micelles formed by CnEm surfactants with different values of n and m have been carried out. The results indicate possibilities of both spherical and disk-shaped micelles depending on the structural parameters of the surfactant such as the area and volume (26) Turro, N. J.; Yekya, Y. J. Am. Chem. Soc. 1978, 18, 5951. (27) Turro, N. J.; Gratzel, M.; Braun, A. M. Angrew. Chem., Int. Ed. Engl. 1980, 19, 675.
Aggregation Behavior of Nonionic Amphiphiles
Langmuir, Vol. 13, No. 19, 1997 5015
Figure 6. ln I0/I vs concentration of quenchher plots: (0) LE, (4) CE; (O) LEL, (*) CEC. Table 5. Micropolarity at Micelle Interface and Mean Aggregation Number, 〈N〉, of Different Surfactants at 300 K amphiphiles
I1/I3
〈N〉a
LE CL LEL CEC
1.27 1.29 1.21 1.25
54 44 45 41
a 〈N〉 is determined by quenching method in which the surfactant concentration is equal to 10 × CMC and Py concentration is 8 µM.
occupied by the hydrophobic chain.28,29 At present we are unable to choose between the two options because our data from viscosity and static and dynamic light-scattering studies on C22Em and C22EmC22 systems where m varies from 9 to 35 are not conclusive.30 While there is no dispute about how the diblocks organize into micelles, our results indicate the possibility in triblocks that the PEG chain folds back and inserts the hydrophobic terminals into the micellar core. This model through energetically favorable would incur heavy losses in entropy if the PEG chain is relatively shorter. In fact such systems would have unfavorable HLB values not conductive for dissolution and micellization at all. Indeed we have observed23 that while the dibehenyl derivative of PEG 400 (C22E9C22) with an HLB value of 7.2, is totally insoluble in water, that of PEG 600 (C22E15C22) with an HLB value of 9.2 is a borderline case, and C22E26C22 with an HLB value of 11.8 (28) Tanford, C. J. Phys. Chem. 1974, 78, 2469. (29) Tanford, C.; Nozaki, Y.; Rohde, M. F. J. Phys. Chem. 1977, 81, 1555. (30) Padmavathi, N. Ch.; Chatterji, P. R. Unpublished results.
Figure 7. Postulate model for the micelles of (a) LE, (b) CE, (c) LEL, and (d) CEC.
is water soluble with a well-defined CMC of 48 µM. In the present case (C7E26C7), the PEG chain with 26 ethylene oxide units will have ample segmental flexibility to assume a conformation which will balance the entropic losses against energy gain during micellization. This model also explains the reduced micropolarity at the interface of the triblocks. The ratio of the first and third vibrational peaks, i.e., I1/I3, in the pyrene fluorescence emission spectrum is a measure of the polarity of the micellar interface.31-34 Normally low I1/I3 ratios (0.50.8) indicate the environment to be nonpolar. The data in Table 4 establish that the interfaces in all instances are distinctly polar. The introduction of a cyclic group does not change the polarity of the interface. Nevertheless it is noteworthy that the interface in triblocks is relatively less polar that in the diblocks. While water can freely enter the interchain space in diblocks, the looping of the PEG chain could restrict a similar situation in triblocks. Acknowledgment. S.M. thanks the University Grants Commission, New Delhi, for financial assistance in the form of a junior research fellowship. He also thanks Kanak K. Majumdar for the help with the synthesis. LA9702300 (31) Chen, M.; Gratzel, M.; Thomas, J. K. J. Am. Chem. Soc. 1975, 97, 2052. (32) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039. (33) Nakajima, A. J. Mol. Spectrosc. 1976, 61, 467. (34) Nakajima, A. Luminescence 1976, 11, 429.
