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Langmuir 1997, 13, 3284-3288
Energetics of Micellization of Aerosol-OT in Binary Mixtures of Eucalyptus Oil with Butanol and Cinnamic Alcohol Pinaki R. Majhi, Kallol Mukherjee, and Satya P. Moulik* Centre for Surface Science, Department of Chemistry, Jadavpur University, Calcutta-700 032, India Received September 27, 1996. In Final Form: March 14, 1997X The critical miceller concentrations (cmc) of Aerosol-OT (AOT) in binary mixtures of eucalyptus oil (EO) with butanol (Bu) and cinnamic alcohol (CA) are determined by the methods of calorimetry and fluorometry and are found to be in good agreement with each other. The mixed EO and Bu medium has shown a minimum and a maximum in cmc at 10% and 50% (w/w) of oil, respectively. On the other hand, for the EO/CA mixture, only a maximum is obtained at 50% (w/w) of EO. Enthalpy of micellization of AOT in the former has shown two maxima, one higher at 10% EO and the other lower at 50% EO, whereas in the latter only one maximum at 50% EO is obtained. The entropies of micellization are all positive having fluctuations with composition and do not linearly correlate with the enthalpies. The heat capacities of the resulting binary mixtures have indicated nonideal mixing behavior.
Aerosol-OT (sodium bis(2-ethylhexyl) sulfosuccinate, AOT) is a versatile ionic surfactant that is widely used in different chemical and biophysical studies.1-4 It is soluble in polar and nonpolar solvents and can easily form both normal and reverse micelles in the media, respectively. In ternary mixtures, with water and oil, it may exhibit formation of microemulsion and lameller and liquid crystalline phases under specific environmental conditions.5-11 The surfactant is nontoxic and, therefore, can be used in pharmaceutical and medicinal preparations. The formation, stability, and characterization of “biological microemulsions” using AOT and edible oils have been limitedly studied in the recent past.12,13 In miceller enzymology, employment of reverse micelles of AOT has been found to be of great significance, and a number of studies in this area have appeared in the literature.14-17 Although the basic understanding of the surface chemical properties of AOT in aqueous medium is fairly advanced, the knowledge of the thermodynamics of its micellization in nonaqueous media (both polar and apolar) is limited. In continuation of our studies on the energetics of X
Abstract published in Advance ACS Abstracts, May 15, 1997.
(1) Eastoe, J.; Robinson, B. H.; Visser, A. J. W. G.; Steyfler, D. C. J. Chem. Soc., Faraday Trans. 1991, 87, 1899. (2) Candau, F.; Leong, Y. S.; Pouyet, G.; Candu, S. J. Colloid Interface Sci. 1984, 101, 167. (3) Surfactants by Cyanamid; American Cynamid Co.: Wayne, NJ, 1983. (4) Tamamushi, B.; Watanabe, N. Colloid Polym. Sci. 1980, 258, 174. (5) Huang, J. S.; Kim, M. W. Soc. Pet. Eng. J. 1984, 24, 197. (6) Kotlarchyk, M.; Chem, S. H.; Huang, J. S. Phys. Rev. A 1983, 28, 508; 1984, 29, 2054. (7) Huang, J. S.; Safran, S. A.; Kim, M. W.; Grest, G. S.; Kotlarchyk, M.; Quink, N. Phys. Rev. Lett. 1984, 53, 592. (8) Mukhopadhyay, L.; Bhattacharya, P. K.; Moulik, S. P. Colloids Surf. 1990, 50, 295. (9) Friberg, S. E.; Liang, Y. C. Colloids Surf. 1987, 24, 325. (10) Huang, J. S. J. Surf. Sci. Technol. 1989, 5, 83. (11) Pal, B. K.; Moulik, S. P.; Mukherjee, D. C. Indian J. Chem. 1991, 28, 174. (12) Mitra, N.; Mukhopadhyay, L.; Bhattacharyay, P. K.; Moulik, S. P. Indian J. Biochem. Biophys. 1994, 31, 115. (13) Mitra, N.; Mukhopadhyay, L.; Bhattacharyay, P. K.; Moulik, S. P. Indian J. Biochem. Biophys. 1996, 33, 206. (14) Luethi, P.; Luisi, P. L. J. Am. Chem. Soc. 1984, 106, 7285. (15) Malokhova, E. A.; Chebotareva, N. A.; Kurganov, B. I.; Simonyan, A. L. Biol. Membr. 1991, 8, 453. (16) Bonavelli, C. D.; Cosa, J. J.; Previtali, C. M. Langmuir 1992, 8, 1070. (17) Gupta, S.; Mukhopadhyay, L.; Moulik, S. P. Colloids Surf. B 1994, 3, 191.
S0743-7463(96)00940-7 CCC: $14.00
micellization of AOT in both polar and apolar media as well as their binary mixtures,18 we have herein used eucalyptus oil (EO) in binary combinations with butanol (Bu) as well as cinnamic alcohol (CA). The biological importance of EO is well-known.19-21 It is a volatile, pale yellow liquid having the characteristic odor and taste of punel. It contains not less than 70% (w/w) eucalyptol19-21 (or cineole) of chemical name 1,3,3trimethyl-2-oxabicyclo[2,2,2]octane. It is used in pharmaceutical and industrial preparations as solvent. It has also medicinal uses.19-21 EO has good potential for the preparation of emulsion and microemulsion.22 To achieve this, amphiphiles are added in oil/water mixture. Mixed amphiphiles (viz., a surfactant and a cosurfactant) are often used for better stabilizing performance. AOT and Bu are commonly used surfactant and cosurfactant in microemulsion formation. We have found that CA can perform the dual function of oil and cosurfactant.23 The efficacy of the amphiphile combination of polyoxyethylene sorbitan monolaurate (Tween 20) and CA in the mutual dispersion of water EO has been recently investigated.22 In this presenation, a fundamental study of the selforganization of the surfactant AOT leading to micelle formation in binary mixtures of EO and Bu as well as EO and CA has been described. The results would enrich the existing knowledge of the basic surface-chemical properties of AOT in mixed nonaqueous polar/apolar media as well as help in the understanding of their combined role in the formulations of emulsion/microemulsion. Experimental Section Materials and Methods. AOT (99% pure) was obtained from Sigma. It’s water content determined by Karl Fischer titration was 3% (w/w). EO (density of 0.9516 at 25 °C) was procured from B. D. Pharmaceuticals, India. It contained 70% (w/w) (18) Mukherjee, K.; Mukherjee, D. C.; Moulik, S. P. Langmuir 1993, 9, 1727. (19) Gennavo, A. R. Remington’s Pharmaceutical Sciences, 18th ed.; Mack Publishers: Easton, PA, 1990. (20) Atal, C. K.; Kapur, B. M. Cultiv. Util. Med. Aromat. Plants 1982, 47. (21) The Wealth of India: A Dictionary of Indian Raw Materials & Industrial Products; CSIR: New Delhi, 1952; Vol. III D-E. (22) Majhi, P. R.; Moulik, S. P. Submitted for publication in J. Dispersion Sci. Technol. (23) Mitra, N.; Mukherjee, L.; Bhattacharya, P. K.; Moulik, S. P. Indian J. Biochem. Biophys. 1994, 31, 115.
© 1997 American Chemical Society
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cineole, and the remaining 30% contained essentially R-pinene, p-cymene, R-terpineole, geraniol, citral, and eudesmol of nonspecified composition. The cineole content of EO is between 20 and 80% depending on the source.21 Bu and CA were obtained from E. Merck, Germany. Their purities were checked by determination of density, refractive index, boiling point, and specific heat. The probe pyrene used for fluorescence measurements was the same sample used in other studies.24,25 A Tronac-458 Isoperibol calorimeter was used for the determination of micellization and heat capacity of the resulting solution. For the details of thermometric measurements we refer to our earlier publications.26-28 All measurements were at least duplicated and the average values are reported and used for calculation. The measurements were taken at a temperature of 303 K with fluctuations of (0.0002 °C. Fluorometric measurements were taken in a Shimadzu RF 540 spectrofluorophotometer, Japan, at 30 ( 0.01 K. Measurement details can be found in earlier publications.29,30 Conductance measurements were taken with a Jenway (U.K.) conductometer in a cell of cell constant 0.3 cm-1. The measurements procedure was the same as earlier.27
Results and Discussion cmc of AOT. The critical miceller concentration (cmc) of AOT was essentially determined by the method of calorimetry. The fluorometric and conductometric methods were used for verification. Good agreement among the methods has been obtained. Several thermograms depicting the cmc points on them are exemplified in Figure 1A. The evaluation of cmc by fluorometry and conductometry is illustrated in Figure 1B. The measured cmc values in EO + Bu and EO + CA media are presented in Tables 1 and 2, respectively. In the case of Bu, there has been an initial decrease in cmc followed by an increase. Thus, a minimum (at 10% EO, w/w) and a maximum (at 50% EO, w/w) have been observed. For the EO + CA medium only a maximum of 50% EO (w/w) has been observed. A three-dimensional representation is shown in parts A and B of Figure 2, where the base axes are in moles of EO and Bu (or CA). The percentages of EO at the trough and crest points are indicated on the cmc profiles. A similar phenomenon of minimization of cmc of AOT at a typical composition of binary solvent media from combinations with formamide, ethylene glycol, dioxane, and water has been recently reported.31 A compositiondependent, but opposing, effect on the self-organization of AOT molecules in the mixed solvent media has been envisaged. A changeover phenomenon from normal micelle to reverse micelle is not ruled out. The polarity of a medium becomes a determining factor for the type of self-assembly in a surfactant solution. Although high polarity favors the formation of normal micelles, reverse micelles result in a low polar medium. Bu is a low polar medium (dielectric constant D ) 20 at 25 °C); both EO (D ) 4.75 at 29 °C) and CA (D ) 7.2 at 29 °C) are weaker polar than Bu. The D of Bu is taken from the literature,32 whereas those of EO and CA have been determined with a Delica impedance bridge (Model 12K), Japan, in our laboratory. Formation of normal (24) Haque, Md. E.; Das, A. R.; Moulik, S. P. J. Phys. Chem. 1995, 99, 14032. (25) Moulik, S. P.; Haque, Md. E.; Jana, P. K.; Das, A. R. J. Phys. Chem. 1996, 100, 701. (26) Das, M. L.; Bhattacharya, P. K.; Moulik, S. P.; Das, A. R. Langmuir 1992, 8, 2135. (27) Jana, P. K.; Moulik, S. P. J. Phys. Chem. 1991, 95, 9525. (28) Ray, S.; Bisal, S. R.; Moulik, S. P. J. Chem. Soc., Faraday Trans. 1 1993, 89, 3277. (29) Bhattacharya, S. C.; Das, H. T.; Moulik, S. P. J. Photochem. Photobiol. A: Chem. 1993, 71, 257. (30) Bhattacharya, S. C.; Das, H. T.; Moulik, S. P. J. Photochem. Photobiol., A: Chem. 1993, 74, 239. (31) Mukherjee, K.; Mukherjee, D. C.; Moulik, S. P. J. Phys. Chem. 1994, 98, 4713.
Figure 1. Representative thermograms of AOT in pure and mixed solvents at 303 K. Part A shows calorimetric thermograms: (a) 1:1 (w/w) Bu/EO; (b) EO only; (c) CA only; (d) 1:4 (v/v) CA/EO. S and E indicate the start and end of a run. Scale divisions are in centimeters. For the ordinate, 25.5 cm ≡ 1 mV (for curve c, 2.55 cm ≡ 1 mV). For the abcissa, 1 min ≡ 0.037 mmol AOT addition for curves a, b, and d, and 0.02 mmol AOT addition for curve c. Part B, curve e shows a fluorometric verification with 1:9 (v/v) EO/Bu. Part B curve f shows a conductometric verification with Bu only. Table 1. cmc of AOT in EO/Bu Mixture and specific heatsa of the Resulting Solutions cmc/mM EO/wt % 0.00 4.98 10.43 14.26 20.76 34.38 51.17 67.70 80.74 90.42 100.00
cal
Cp/J g-1 K-1
flu
cond
1.90 ( 0.36 2.10 ( 0.26 1.05 ( 0.21 0.70 ( 0.15 0.64 ( 0.10 0.91 ( 0.18 2.68 ( 0.43 2.65 ( 0.35 2.68 ( 0.41 2.71 ( 0.40 2.28 ( 0.34 2.20 ( 0.38 1.73 ( 0.28 1.70 ( 0.30 1.20 ( 0.22 1.30 ( 0.21
exptl
calcd
2.185 ( 0.11 2.438 ( 0.07 2.337 ( 0.05 2.352 ( 0.05 2.131 ( 0.09 2.318 ( 0.07 2.267 ( 0.05 1.966 ( 0.04 2.046 ( 0.04 2.268 ( 0.05 2.379 ( 0.06
2.346 2.348 2.349 2.350 2.352 2.357 2.363 2.368 2.372 2.375
a Specific heat of EO is not available in literature. That of Bu is obtained from the specific heats of lower alkanols32 by the interpolation method. The experimental specific heat of EO was used for the calculation of the specific heat of the mixture.
Table 2. cmc of AOT in EO/CA Mixture and Specific Heatsa of the Resulting Solutions cmc/mM EO/wt %
cal
0.00 32.17 48.68 65.48 79.14 100.00
0.94 ( 0.21 1.85 ( 0.30 2.80 ( 0.40 2.10 ( 0.31 1.50 ( 0.25 1.20 ( 0.22
Cp/J g-1 K-1 fluo
exptl
calcd 2.922 2.789 2.655 2.546
1.30 ( 0.18
3.180 ( 0.13 2.031 ( 0.08 2.550 ( 0.07 1.743 ( 0.08 2.066 ( 0.06 2.379 ( 0.07
a
Specific heats of EO and CA are not available in literature. The experimental specific heats of EO and CA were used for the calculation of the specific heats of the mixture.
micelle in Bu and reverse micelle of AOT in EO and CA is anticipated. The conductance behavior of AOT in pure Bu and Bu with 20% EO has cmc breaks such as normal
3286 Langmuir, Vol. 13, No. 13, 1997
Majhi et al. Table 3. Energetics of Micellization of AOT in EO/Bu Mixture at 303 K EO/wt %
-∆Gm°/kJ mol-1
∆Hm°/kJ mol-1
∆Sm°/J mol-1 K-1
0.00 4.98 10.43 14.26 20.76 34.38 51.17 67.70 80.74 90.42 100.0
31.6 ( 0.97 34.6 ( 1.02 36.6 ( 1.10 35.3 ( 1.00 29.8 ( 0.80 29.8 ( 0.67 29.8 ( 0.75 30.7 ( 0.75 30.8 ( 0.87 32.0 ( 0.82 33.9 ( 0.93
2.23 ( 0.11 2.43 ( 0.12 2.84 ( 0.11 2.23 ( 0.07 -1.18 ( 0.06 -1.64 ( 0.08 -0.23 ( 0.01 -0.17 ( 0.01 -1.11 ( 0.06 -2.26 ( 0.04 -3.61 ( 0.11
112.5 ( 3.55 122.0 ( 3.76 130.0 ( 3.99 124.0 ( 3.53 95.0 ( 2.80 93.0 ( 2.47 98.0 ( 2.50 102.0 ( 2.50 98.0 ( 3.07 98.0 ( 3.00 100.0 ( 3.43
Table 4. Energetics of Micellization of AOT in EO/CA Mixture at 303 K EO/wt % -∆Gm°/kJ mol-1 -∆Hm°/kJ mol-1 ∆Sm°/J mol-1 K-1 0.00 32.17 48.68 65.48 79.14 100.00
Figure 2. CA/EO (A) and Bu/EO (B) dependent cmc profiles of AOT at 303 K.
35.1 ( 1.15 32.5 ( 0.97 29.6 ( 0.74 31.1 ( 0.75 32.8 ( 0.85 33.9 ( 0.95
12.31 ( 0.25 8.30 ( 0.33 0.10 ( 0.005 0.22 ( 0.01 1.60 ( 0.09 3.61 ( 0.14
formation in most of the combinations, the methods of calorimetry and fluorometry clearly indicate such a formation. The normal micelle formation in Bu is opposed by the increasing proportion of EO. The cmc thus increases. The initial decrease in cmc and its minimum at 10% EO suggest formation of an AOT/EO mixed micelle, which afterward is not favored for the increasing nonpolarity of the medium by the increased EO presence. Why the cmc shows ups and downs in the solvent composition profiles rests on the solvent properties, viz., polarity, weak intra- and intermolecular interactions, hydrophobicity, solvation, orientation, etc. Ups and downs in the enthalpy of micellization have been also observed, which will be discussed in the next section. A clear-cut explanation of the observed facts ought to be difficult but needs exploration through further physical measurements. Energetics of Micellization. The energetic parameters of AOT micellization in EO + Bu and EO + CA mixed media are presented in Tables 2 and 3, respectively. The spontaneity of micellization is revealed from negative values of the free energy for the process expressed per unit mole of monomer of AOT (∆Gm°) calculated according to the relation
∆Gm° ) 2RT ln cmc
Figure 3. EO-dependent conductance characteristics of 1 mmol dm-3 AOT in Bu.
micelle formation (Figure 1). At higher wt % of EO the conductance is low and hardly changes with [AOT]; reverse micelle formation is thus anticipated. In Figure 3 the dependence of conductance of 1 mmol dm-3 AOT with increasing EO in Bu is presented. A sharp fall after 20% with a steady value afterward is observed. Reverse orientation of AOT molecules in the micelles at EO > 20% is considered. The 3% (w/w) water content as impurity in AOT causes it to readily form reverse micelles also in pure CA and EO and their mixtures. Although the method of conductance is not responsive to showing micelle (32) Handbook of Chemistry and Physics, 55th ed.; CRC Press: Cleveland, OH, 1974.
75.0 ( 4.62 80.0 ( 4.29 97.0 ( 2.45 102.0 ( 2.50 103.0 ( 3.10 100.0 ( 3.58
(1)
assuming total counterion association in low polar media herein studied. The values obviously have the modulation equivalent to that of cmc depending on the solvent composition. The enthalpies of micellization (∆Hm°) directly determined by the method of calorimetry have been found to be both positive and negative. In the EO + Bu medium, on the lower side of EO addition, the self-aggregation property of AOT is endothermic but is exothermic with a larger presence of EO in the medium. In the case of EO + CA, the process is exothermic. The graphical representation (Figure 4) of the enthalpy values in the EO + Bu medium shows an endothermic maximum at 10% EO, an exothermic minimum at 35% EO, and a second exothermic maximum at 55% EO. The minimum and maximum in ∆Gm° (a direct reflection of cmc), on the other hand, correspond to 10% and 50% EO, respectively. In the EO + CA medium, there occurs only one exothermic maximum at 55% EO, nearly comparable with ∆Gm° maximum at 50% EO.
Micellization of Aerosol-OT
Figure 4. ∆Hm° vs wt % EO profiles in Bu + EO and CA + EO media at 303 K.
The entropies of micellization (∆Sm°) have shown variations more or less comparable with that of ∆Hm° with respect to the medium composition. The values are all large and positive, supporting the general observations that micellization is essentially associated with a large positive entropy change. The three energetic parameters are profiled together in Figures 5 and 6 for EO + Bu and EO + CA media, respectively. The varying interdependence among the parameters is obvious. The base curves in the plots representing ∆Hm° - ∆Sm° correlation are nonlinear, suggesting a nonexact compensation feature between the two. Good enthalpy/entropy compensations have been, on the other hand, reported earlier.22,31 In the present system, the ∆Gm° variation with composition is not in a narrow range. Consequently, ∆Hm° and ∆Sm° are not linearly proportional. The polarity of Bu is greater than that of CA and EO. It is expected that AOT forms a normal micelle in Bu and reverse micelles in CA and EO as well as in the mixed media of EO + Bu and EO + CA. The composition at which a normal micelle changes to a reverse micelle in an EO + Bu medium is not easy to ascertain. The CMC
Langmuir, Vol. 13, No. 13, 1997 3287
maximum at 50% EO is not the reversal point, since the percentage of EO is considerable (compare results of Figure 2). Moreover, a similar maximum has also been observed in an EO + CA mixed medium where both solvents favor reverse micelle formation. The minimum and maximum in CMC are sovlent effects that stem from various factors mentioned in the previous section. The overall positive ∆Sm° suggests release of order during the self-assembling process of AOT; a structured microenvironment surrounding the AOT molecules is envisaged. With the increase in the percentage of EO in a Bu + EO mixture, the ∆Sm° variation fluctuates comparably with ∆Hm°; maximum disordering at 10% EO followed by its minimum at 33% and a further increase reaching to a maximum beyond 50% of EO are observed (Figure 7). Thus, EO + Bu has more than one level of structural transition during micellization of AOT depending on solvent composition. In the case of EO + CA, the ∆Sm° reaches a maximum at 50% EO. Thus, a continuous disordered condition prevails in this mixed medium. Although desolvation of AOT prior to micellization has an endothermic contribution in the overall heat change, its self-association and intermolecular EO/CA association (via nonpolar and polar interaction) have exothermic contributions. The overall balance is manifested in the calorimetric measurements, which shows composition-dependent enthalpic signs and magnitudes. The overall energetics suggest EO + CA to be less complex than EO + Bu. The thermodynamics of AOT micellization in EO/Bu and EO/CA media may be compared with AOT micellization in water/dioxane (Wa/Dx) and ethylene glycol/ dioxane (Eg/Dx) media.31 The cmc values herein reported are on the whole lower than those in Wa/Dx and Eg/Dx media. The ∆Gm° values produced both maximum and minimum with composition (Tables 3 and 4). A minimum and maximumin ∆Gm° with solvent composition were also observed in Wa/Dx and Eg/Dx media, respectively. The enthalpy values in the former were both negative and positive, as in EO/Bu, but the values were all positive in the latter (in the EO/CA medium negative ∆Hm° were obtained). The ∆Sm° values were all positive and about 50% lower than those obtained in EO/Bu and EO/CA media. In contrast to the previous observation, the ∆Hm° and ∆Sm° did not maintain a fair correlation between them. More spontaneity with higher positive entropy change occurred in EO/Bu and EO/CA media compared to that in Wa/Dx and Eg/Dx media for the self-organization of AOT.
Figure 5. Three-coordinate representation of the energetic parameters for AOT micellization in Bu medium at 303 K.
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Majhi et al.
Figure 8. Dependence of I1/I3 of pyrene on AOT in 1:9 (v/v) Bu/EO medium at 303 K. Figure 6. Three-coordinate representation of the energetic parameters for AOT micellization in CA + EO medium at 303 K.
35% EO. Thereafter, the solvent structure becomes loose. For the EO/CA medium Cpexp < Cpcalc, the mixed medium is more fluid, and the Cpcalc ignoring interaction is thus greater than Cpexp. Micellar Micropolarity. The ratio of the first and the third vibronic peaks of the fluorescence emission spectra of pyrene can be a measure of the microenvironment of the probe molecule.24 This ratio decreases with decreasing local polarity of the probe. For the mixed medium of 9:1 for EO:Bu (v/v), the I1/I3 values were determined with varying [AOT]. The results are presented in Figure 8. An initial rapid decline in I1/I3 followed by the formation of a plateau at 2.0 mmol dm-3 is observed. The value is close to the calorimetrically determined cmc (1.7 mmol dm-3) given in Table 1. The very weakly polar pyrene preferentially shelters near the nonpolar tail region of AOT, which is detected by the I1/I3 ratio up to the stage of reverse micelle formation. Beyond that pyrene finds no change in the environment so that the I1/I3 values virtually come to a halt. The results reveal that the studied EO:Bu medium (9:1) is more polar than the peripheral region environment of the compartments of the AOT reverse micelles. Conclusions
Figure 7. ∆Sm° variation with EO in Bu + EO and CA + EO media at 303 K.
The process was thus more favored in the EO/Bu and EO/CA mixed environment. The Cpexp and Cpcalc (on the basis18,26 of ∑giCpi, where gi and Cpi stand for mass fraction and Cp of the ith component, respectively) reveal initial greater structure of the medium than expected without interaction (Cpexp > Cpcalc) up to
(1) The micellization of AOT in EO + C Bu A media is composition dependent; both minimum and maximum in cmc have been observed. (2) The micellization can be both exothermic and endothermic in EO + Bu and only exothermic in EO + CA. The ∆Hm° also manifests a composition-dependent maximum and minimum. (3) The ∆Sm° values are all positive, which is again composition dependent, and ∆Hm° and ∆Sm° do not linearly correlate with each other. Acknowledgment. The work was financially supported by the University Grants Commission with a fellowship to Pinaki R. Majhi. LA960940Q
