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J. Phys. Chem. B 2005, 109, 16070-16074
Microcalorimetric Study on Micellization of Nonionic Surfactants with a Benzene Ring or Adamantane in Their Hydrophobic Chains Yajuan Li,† James Reeve,‡ Yilin Wang,*,† Robert K. Thomas,‡ Jinben Wang,† and Haike Yan† Key Laboratory of Colloid and Interface Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, and Physical and Theoretical Chemistry Laboratory, Oxford UniVersity, South Parks Road, Oxford OXI 3QZ, United Kingdom ReceiVed: May 9, 2005; In Final Form: June 30, 2005
The micellization of a novel family of nonionic surfactants poly(oxyethylene) glycol alkyl ethers has been studied by microcalorimetry. One of the surfactants has adamantane, and the other nonionic surfactants have a benzene ring in their hydrophobic chains, which moves from the terminal of the hydrophobic chain toward the headgroup. Moreover, the alkyl chain of the nonionic surfactants is straight or branched. Both the critical micelle concentration (cmc) and the thermodynamic parameters associated with the micelle formation have been obtained. The cmc decreases and the enthalpy of micelle formation (∆Hmic) becomes less positive gradually as the length of hydrophobic chain increases, whereas the values of cmc and ∆Hmic tend to increase for the surfactants with a longer ethylene oxide chain. However, the ∆Hmic value of the surfactant with seven carbon atoms in a hydrophobic chain is more positive than that of the surfactant with six carbon atoms in a hydrophobic chain. Comparing with the nonionic surfactant with a methylene hydrophobic chain, the surfactants with benzene rings and adamantane groups have larger cmc values and the cmc values increase with the size of the groups. Furthermore, moving the phenyl group from the terminal of the hydrophobic chain to the neighbor of the hydrophilic headgroup leads to the decreased cmc. Both the variation of hydrophobic interaction from the movement of phenyl group and π-π interaction among adjacent phenyl groups affect ∆Hmic values.
Introduction Nonionic surfactants have many unique properties that are superior to those of ionic surfactants with comparable hydrophobic groups, such as remarkably low critical micelle concentrations (cmc’s), high efficiency in reducing surface tensions, and better solubilizing properties, which make them potentially useful in a wide variety of industrial applications.1-4 Many papers have shown that hydrophobic chains play a principal role in the micellization of nonionic surfactants.5-8 The effect of the hydrophobic chain has been attributed to the longer hydrophobic chain aiding closer packing of surfactant molecules and then decreasing the surface tension and cmc. However, to our knowledge, systematic information regarding the influence of the architecture of hydrophobic chains on the surfactant micellization is not available in the literature. The present work is aimed at investigating how variations of the hydrophobic chain architecture affect the micellization of nonionic surfactants. For this purpose, we studied a novel series of nonionic surfactants by microcalorimetry. The structures and nomenclature of the surfactants are given in Figure 1. A benzene ring or adamantane is introduced to the hydrophobic chains of these nonionic surfactants. Moreover, the hydrophobic chains are all different from each other, involving variations in length and branching as well as the location of phenyl group. Experimental Section Materials. The ΦCnEm series were synthesized as follows. A range of ω-phenyl alcohols (n ) 6, 7, 8, 10) (Aldrich) were tosylated and then ethoxylated with the appropriate poly-
Figure 1. Chemical structures and nomenclature of the studied nonionic surfactants.
(ethylene glycol) to produce ω-phenyl alkyl pentaethylene (or hexaethylene) glycol following the general procedure:
* To whom correspondence should be addressed. E-mail: yilinwang@ iccas.ac.cn. † Chinese Academy of Sciences. ‡ Oxford University.
10.1021/jp0523874 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/04/2005
Micellization of Nonionic Surfactants
J. Phys. Chem. B, Vol. 109, No. 33, 2005 16071
Figure 2. Representative calorimetric signals for dilution of concentrated Adam-C2E5 solution in water at 303.15 K.
t-C8ΦE5, t-C8ΦE6, and Adam-C2E5 were synthesized using the same procedure except with the use of t-octylphenol and 1-adamantaneethanol (Aldrich) as the starting materials. n-C4ΦC4E5 and t-C4ΦC4E6 were synthesized in the following three steps. First, anhydrous AlCl3 was carefully added to a mixture of butylbenzene and succinic anhydride in 1,2-dichloroethane and maintained at about 13 °C. The mixed solution was stirred for about 12 h, then poured into ice-cold HCl, extracted with ethyl acetate, washed with brine, and dried over MgSO4. The solvent was evaporated to leave a white solid, which was recrystallized from ethyl acetate. Second, the product 4-(4-butylphenyl)-4-oxobutanoic acid was hydrogenated under a pressure of 50 psi H2 with 5% Pd/C catalyst, H2SO4, and ethyl acetate in a bomb reaction vessel until uptake ceased (approximately 12 h). The catalyst was filtered off and the filtrate was washed with brine, dried over MgSO4, and evaporated to leave a white solid, crude 4-(4-butylphenyl)butanoic acid. Then, a solution of the acid in sodium-dry diethyl ether was gradually transferred under argon pressure to a flask containing LiAlH4 also in sodium-dry diethyl ether. The flask was submersed in an ice bath and fitted with a condenser and an air-driven stirring rod. Once addition was complete, the mixture was further stirred for 2 h at ambient temperature. The reaction mixture was then cooled again and quenched with water, acidified with HCl, extracted with diethyl ether, dried over MgSO4, and evaporated to leave 4-(4-butylphenyl)-1-butanol as a colorless oil. Finally, 4-(4-butylphenyl)-1-butanol was ethoxylated via the tosylate as described above. All the crude surfactants were column purified using a d ) 36 mm chromatography column, packed with ∼120 g of silica gel for flash chromatography, with ether as the eluting solvent. The composition and the purity of all the surfactants were confirmed by NMR and elemental analysis. High-purity C12E5 was purchased from TCI and used without further purification. Triply distilled water was used for all experiments. Isothermal Titration Microcalorimetry. The TAM 2277201 isothermal titration microcalorimeter (Thermometric AB, Ja¨rfa¨lla, Sweden) was used to obtain the critical micelle concentration and enthalpy changes for micelle formation of the surfactants. Both the sample cell and reference cell of the microcalorimeter are 1 mL, and they were initially loaded with 0.6 and 0.7 mL of pure water, respectively. Each aliquot of 3-20 µL of concentrated surfactant solutions was added consecutively to the stirred sample cell using a 250-µL Hamilton syringe controlled by a Thermometric 612 Lund pump until the
Figure 3. Variations of observed enthalpy (∆Hobs) for nonionic surfactants in water with the final concentration (C) at 303.15 K.
desired concentration range had been covered. The system was stirred at 50 rpm with a gold propeller. For each surfactant, the experiments were repeated at least twice. The accuracy of the calorimeter was periodically calibrated electrically and verified by measuring the dilution enthalpies of concentrated sucrose solution.9 All experiments were performed at 303.15 ( 0.01 K. Figure 2 is a representative enthalpogram of titration of Adam-C2E5 aqueous solution into pure water, which shows heat flow P as a function of time t. The observed enthalpies (∆Hobs) of the titration were obtained by integrating the area of the peaks. Results and Discussion The variation of ∆Hobs is plotted against the final concentration (C) of the studied nonionic surfactants at 303.15 K in Figure 3, where the data points are the experimentally observed enthalpies per mole of surfactant. The dilution curves of these surfactants in pure water are all approximately sigmoidal in
16072 J. Phys. Chem. B, Vol. 109, No. 33, 2005
Li et al.
Figure 4. Determination of cmc and ∆Hmic from observed enthalpy curve of Adam-C2E5 diluted in water at 303.15 K.
TABLE 1: Critical Micelle Concentrations and Thermodynamic Parameters of the Nonionic Surfactants in Aqueous Solution at 303.15 K surfactants
cmc (mM)
∆Hmic (kJ/mol)
∆Gmic (kJ/mol)
T∆Smic (kJ/mol)
C12E5 ΦC6E6 ΦC7E6 ΦC8E6 ΦC10E6 ΦC6E5 ΦC7E5 ΦC8E5 ΦC10E5 t-C8ΦE6 t-C8ΦE5 t-C4ΦC4E6 n-C4ΦC4E5 Adam-C2E5
0.06 ( 0.01 5.38 ( 0.20 1.57 ( 0.10 0.63 ( 0.10 0.05 ( 0.01 3.27 ( 0.20 2.60 ( 0.20 0.33 ( 0.04 0.04 ( 0.01 0.50 ( 0.04 0.41 ( 0.02 1.27 ( 0.13 0.11 ( 0.03 5.64 ( 0.10
9.8 ( 0.3 6.8 ( 0.2 7.6 ( 0.3 6.6 ( 0.2 6.3 ( 0.2 3.1 ( 0.2 4.6 ( 0.1 2.8 ( 0.1 2.7 ( 0.1 1.7 ( 0.1 1.6 ( 0.1 8.5 ( 0.4 3.9 ( 0.2 10.0 ( 0.3
-24.50 -13.17 -16.27 -18.57 -24.96 -14.42 -15.00 -20.20 -25.52 -19.16 -19.66 -16.81 -22.97 -13.05
34.3 20.0 23.9 25.2 31.3 17.5 19.6 23.0 28.2 20.9 21.3 25.3 26.8 23.1
shape, and each can be subdivided into two concentration regions separated by a transition region associated with micelle formation, corresponding to the critical micelle concentration (cmc). When C lies below cmc, all added micelles are demicellized into monomers and the monomers are further diluted. When C is above cmc, only the micellar solution is diluted and ∆Hobs drops toward zero. From such titration curves, both cmc and the enthalpy changes for micellization (∆Hmic) can be derived. As illustrated in Figure 4, the fitted curves are differentiated with respect to C and the position of the extremum is taken as cmc;10 the values of ∆Hmic are obtained from the titration curves by taking the enthalpy difference between the two linear segments of the enthalpy curves extrapolated to the cmc.11 Then the Gibbs free energy of micellization (∆Gmic) can be calculated using the expression ∆Gmic ) RT ln cmc, where cmc is expressed in molarity.12-14 The entropy change of micellization (∆Smic) can be derived from the Gibbs-Helmholtz equation: ∆Gmic ) ∆Hmic - T∆Smic. All the obtained cmc values and the corresponding thermodynamic parameters expressed by per mole of surfactant are listed in Table 1. For all the surfactants, it is noted that the values of ∆Hmic and T∆Smic are all positive and T∆Smic is much larger than ∆Hmic. This indicates that the gain of entropy is the main contributor to micellization and overrides the unfavorable endothermic enthalpy term. The positive entropy of micellization for nonionic surfactants is mainly related to the dehydration of ethylene oxide units.15,16 In the following section, we will endeavor to present a detailed and systematic account of the effect of various structure factors on the cmc and enthalpy of micellization.
Length of Hydrophobic Chain. The effect of the length of the hydrophobic chain on micellization is illustrated by comparing the experimental results of ΦCnE5 and ΦCnE6 in parts a and b, respectively, of Figure 3. The number of methylene groups in the hydrophobic chain varies from 6 to 10. As expected, for a given hydrophilic group, the cmc moves to lower concentration with increasing hydrophobic chain length, while the ∆Hmic values are all positive and decrease gradually except those for ΦC7E6 and ΦC7E5, which are out of the decreasing trends of ∆Hmic. Generally, the gradual decreasing of cmc and ∆Hmic upon an increase in the hydrophobic chain length can be attributed to the increased hydrophobicity of the hydrophobic chain.17 Nevertheless, it should be noted that the ∆Hmic values decrease from 3.1 to 2.7 kJ/mol for C6-C10 surfactants in the ΦCnE5 series and decrease from 6.8 to 6.3 kJ/mol for C6-C10 surfactants in the ΦCnE6 series. However, ∆Hmic for C7 surfactant is 0.8 kJ/mol larger than that for C6 surfactant in the ΦCnE6 series and 1.5 kJ/mol larger in the ΦCnE5 series. Therefore, the ∆Hmic values of the two C7 surfactants are out of the decreasing trends of ∆Hmic. This discrepancy is probably caused by the difference between the hydrophobic chains with odd and even numbered carbon atoms.18,19 For the surfactant with an even numbered hydrophobic chain, the orientation of the C-C bond attached to the phenyl group may be along the major molecular axes, which facilitates the aggregation of the hydrophobic chains. However, the orientation of the C-C bond attached to the phenyl group for the surfactant with an odd numbered hydrophobic chain is probably deviated from the major molecular axes, which is against the aggregation of the hydrophobic chain.20,21 In addition, as reported before,20 the large phenyl group attached at the end of the hydrophobic chain could amplify the odd/even effect on the molecular aggregation. Thus, hydrophobic interactions among the hydrophobic chains of the surfactant with odd numbered carbon atoms are anticipated to be weaker than those of the surfactants with even numbered carbon atoms, eventually resulting in the ∆Hmic value of C7 being unusually more positive than that of C6. Bulky Group in Hydrophobic Chain. The aggregation behavior of nonionic surfactants is very sensitive to introducing a bulky group into a hydrophobic chain. The obtained cmc values tend to increase following the order of C12E5, ΦC8E5, and Adam-C2E5, whereas the positive ∆Hmic value is largest for Adam-C2E5 and smallest for ΦC8E5. These three surfactants have the same hydrophilic group and almost the same carbon numbers in their hydrophobic chains. However, a benzene ring or an adamantane group is introduced to the ends of hydrophobic chains for ΦC8E5 and Adam-C2E5, respectively. These bulky groups indeed yield steric hindrance to the aggregation of surfactants. Especially for the cage architecture of adamantane, it brings about a much higher hindrance to the aggregation, which provides an extreme illustration of how introducing a bulky group can affect the aggregation. Instead, different from C12E5 and Adam-C2E5, there is a benzene ring in the ΦC8E5 molecule. So, the existence of π-π interaction among the adjacent phenyl groups of ΦC8E5 molecules is possible, which should be a favorable factor to the aggregation of surfactant. This phenomenon is consistent with our previous work,22 where comparative studies on the micellization of sodium bis(4phenylbutyl) sulfosuccinate (SBPBS) and sodium bis(2-ethylheyl) sulfosuccinate (AOT) were conducted. The π-π interaction among the adjacent phenyl groups of SBPBS molecules may be the main reason for the higher surface tension, larger micropolarity inside the SBPBS aggregates, more compact
Micellization of Nonionic Surfactants aggregate structure, and the exothermic effects of micellization for SBPBS in comparison with AOT. In the present work, comparing with C12E5 and Adam-C2E5, ΦC8E5 has a benzene ring in the end of hydrophobic chains. The ∆Hmic value for ΦC8E5 is found to be much less positive than those of C12E5 and Adam-C2E5. This is also possibly attributed to the π-π interaction among the adjacent phenyl groups of ΦC8E5 molecules. Consequently, the cmc values increase with an introduction of bulky groups to the surfactant molecules, while the ∆Hmic value for the micellization of ΦC8E5 terminated by a phenyl group becomes much less endothermic, even less endothermic than that of C12E5. Location of Phenyl Group at Hydrophobic Chain. Interestingly, it is noted in Table 1 that cmc of ΦC8E5 is larger, but ∆Hmic is slightly less positive, than those of n-C4ΦC4E5. Meanwhile, the cmc and ∆Hmic of t-C4ΦC4E6 are larger and more positive than those of t-C8ΦE6, respectively. For the twogroup surfactants ΦC8E5 and n-C4ΦC4E5, and t-C4ΦC4E6 and t-C8ΦE6, the total carbon numbers of hydrophobic chains are equal to each other, respectively. However, the location of the phenyl group moves from the terminal position toward the headgroup. The location of the phenyl group should, therefore, be the principal factor determining the above differences in the micellization. The results imply that the effective hydrophobicity of the phenyl group increases as it moves from the terminal position toward the headgroup. For ΦC8E5, the flexible C8 alkyl chain may allow the terminal phenyl group to affect the packing of hydrophobic chains efficiently. In this case, the larger volume of the phenyl group tends to disfavor the formation of micelle considerably. For n-C4ΦC4E5, however, the phenyl group locates in the middle of the two linear C4 chains. Then the effect of the phenyl group on the formation of micelle may be reduced by the two linear C4 chains to a certain extent. As a result, the cmc of ΦC8E5 is larger than that of n-C4ΦC4E5. The slightly less positive ∆Hmic of ΦC8E5 is presumed to be primarily originated from its π-π interaction. An electron-donating substituent has been demonstrated to increase the repulsion between the π system and thus reduce the π-π interaction.23,24 As for n-C4ΦC4E5, a benzene ring locates between two electrondonating straight C4 chains, which could be expected to enhance the repulsion between the π electrons on the benzene ring. Thus, π-π interaction may be also reduced. Thus, the ∆Hmic of n-C4ΦC4E5 is a little more positive than ΦC8E5. Comparing t-C4ΦC4E6 with t-C8ΦE6, a similar effect from the movement of the phenyl group has also been observed. For t-C8ΦE6, the aggregation of the alkyl chains would be less hindered by the phenyl group because the phenyl group is connected with the headgroup. Accordingly, the cmc of t-C4ΦC4E6 is larger than that of t-C8ΦE6. Meanwhile, there may exist a more effective π-π interaction for t-C8ΦE6. However, comparing with a linear C4 chain, a branched C4 chain holds much stronger ability to donate electrons. Then the π-π interaction for t-C4ΦC4E6 could be markedly reduced because the phenyl group locates between one electron-donating straight alkyl chain and one stronger electron-donating branched alkyl chain. Hence, the ∆Hmic value of t-C4ΦC4E6 is much more positive than that of t-C8ΦE6. Number of Ethylene Oxide Groups. The effect of ethylene oxide chain length on the cmc and ∆Hmic values of the nonionic surfactants can be seen from Table 1. As expected, the cmc and ∆Hmic values are all increasing functions of the number of ethylene oxide groups. A similar observation has been previously reported.15,25,26 With increasing ethylene oxide chain length, a
J. Phys. Chem. B, Vol. 109, No. 33, 2005 16073 greater number of oxygen atoms will contribute to hydrogen bonding, which is responsible for the hydrophilicity of the nonionic surfactant molecules. Such an inherent stronger hydrophilicity leads to the formation of micelles at higher concentration, resulting in larger cmc and more positive ∆Hmic values. In particular, the number of hydrophilic groups has a very significant effect while the surfactants have less effective hydrophobicity among their hydrophobic chains. Conclusions A series of nonionic surfactants with different hydrophobic chains, involving variations in length and branching, introducing bulky groups, and considering the location of the phenyl group, have been investigated. The cmc decreases as the alkyl chain length increases or the phenyl group moves from the end of the hydrophobic chain toward the headgroup. This is attributed to increased effective hydrophobicity. For the surfactant with a benzene ring or an adamantane in the hydrophobic chain, the surfactant molecule has more difficulty in aggregation because of the steric inhibition among the hydrophobic chains. Additionally, the different situations of π-π interaction among phenyl groups in hydrophobic chains also affects the final ∆Hmic values. In conclusion, changing the architecture of the hydrophobic chain offers the possibility to tune the micellization properties of nonionic surfactants. Acknowledgment. We are grateful for financial support from the National Natural Science Foundation of China, the Chinese Academy of Sciences, and the Royal Society (Grants 20233010 and 20473101). References and Notes (1) Schick, M. J. In Nonionic Surfactant: Physical Chemistry; Marcel Dekker: New York, 1986. (2) Degiorgio, V. In Physics of Amphiphiles: Micelles, Vesical and Microemulsions; Degiorgio, V., Corti, M., Eds.; North-Holland: Amsterdam, 1985. (3) Kahlweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 38813894. (4) Kunieda, H.; Nakano, A.; Akimaru, M. J. Colloid Interface Sci. 1995, 170, 78-84. (5) Islam, M. N.; Kato, T. J. Phys. Chem. B 2003, 107, 965-971. (6) Wongwailikhit, K.; Ohta, A.; Seno, K.; Nomura, A.; Shinozuka, T.; Takiue, T.; Aratono, M. J. Phys. Chem. B 2001, 105, 11462-11467. (7) Ohta, A.; Murakami, R.; Takiue, T.; Ikeda, N.; Aratono, M. J. Phys. Chem. B 2000, 104, 8592-8597. (8) Sulthana, S. B.; Rao, P. V. C.; Bhat, S. G. T.; Nakano, T. Y.; Sugihara, G.; Rakshit, A. K. Langmuir 2000, 16, 980-987. (9) Gucker, F. T., Jr.; Pickard, H. B.; Planck, R. W. J. Am. Chem. Soc. 1939, 61, 459-470. (10) Kira´ly, Z.; Deka´ny, I. J. Colloid Interface Sci. 2001, 242, 214219. (11) Johnson, I.; Olofsson, G.; Jo¨nsson, B. J. Chem. Soc., Faraday Trans. 1 1987, 83 (11), 3331-3344. (12) Rosen, M. J.; Cohen, A. W.; Dahanayake, M.; Hua, X. Y. J. Phys. Chem. 1982, 86, 541-545. (13) Islam, M. N.; Kato, T. Langmuir 2003, 19, 7201-7205. (14) Olofsson, G. J. Phys. Chem. 1985, 89, 1473-1477. (15) Crook, E. H.; Trebbi, G. F.; Fordyce, D. B. J. Phys. Chem. 1964, 68, 3592-3599. (16) Schick, M. J. J. Phys. Chem. 1963, 67, 1796-1799. (17) Blokzijl, W.; Engberts, J. B. F. N. Angew. Chem., Int. Ed. Engl. 1993, 32, 1545-1579. (18) Gutmann, V. In The Donor-Acceptor-Approach to Molecular Interaction; Plenum Press: New York and London, 1978. (19) Cuy, E. J. J. Am. Chem. Soc. 1920, 42, 503-514. (20) Mare`elja, S. J. Chem. Phys. 1974, 60, 3599-3604. (21) Li, Y. J.; Li, P. X.; Wang, J. B.; Wang, Y. L.; Yan, H. K.; Thomas, R. K. Langmuir 2005, 21, 6703-6706. (22) Fan, Y. R.; Li, Y. J.; Yuan, G. C.; Wang, Y. L.; Wang, J. B.; Han, C. C.; Yan, H. K.; Li, Z. X.; Thomas, R. K. Langmuir 2005, 21, 38143820.
16074 J. Phys. Chem. B, Vol. 109, No. 33, 2005 (23) Cozzi, F.; Ponzini, F.; Annunziata, R.; Cinquini, M.; Siegel, J. S. Angew. Chem., Int. Ed. Engl. 1995, 34, 1019-1020. (24) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112, 5525-5534.
Li et al. (25) Crook, E. H.; Fordyce, D. B.; Trebbi, G. F. J. Phys. Chem. 1963, 67, 1987-1994. (26) Hsiao, L.; Dunning, H. N.; Lorenz, P. B. J. Phys. Chem. 1956, 60, 657-660.
