Langmuir 1999, 15, 6591-6593
Mixed Aqueous Properties of Resorcinol-type Calix[4]arenes Bearing Four Alkyl Side Chains and Cationic Surfactant Kunio Esumi,*,† Kei Syoji,† Munetaka Miyazaki,† Kanjiro Torigoe,† and Yoshifumi Koide‡ Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan, and Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, Kurokami, Kumamoto 860-0862, Japan Received October 14, 1998. In Final Form: March 16, 1999
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is given in Figure 1. Dodecyltrimethylammonium bromide (DTAB) was obtained commercially and was recrystallized from acetone several times. Water was purified through a Milli-Q system. The other chemicals were of analytical grade. Methods and Measurements. Since resorcinol-type calix[4]arenes, [4]Ar-Rn, are easily dissolved in alkaline aqueous solution, the mixed surfactant solutions were adjusted to pH 13 by addition of NaOH. The static surface tensions of these aqueous solutions were measured by the Wilhelmy-plate method (Kruss K12 tensiometer) at 25 °C. Fluorescence spectra of pyrene in the mixed aqueous surfactant solutions were obtained using a fluorescence spectrophotometer (Hitachi 650-10S). The concentration of pyrene was 1 × 10-6 mol dm-3, and the excited wavelength was 335 nm. All measurements were carried out at 25 °C.
Introduction
Results and Discussion
Calixarenes are cyclic oligomers combined with phenol units by methylene substituents and have the structure of metacyclophane. These compounds are known as inclusion compounds.1,2 Recently, resorcinol-type calix[4]arenes bearing four alkyl side chains have been synthesized and their aqueous properties have been characterized:3-5 The interaction between the resorcinol group and the inner alkyl side chain is increased with increasing alkyl side chain length, and the solubilization behavior by calix[4]arenes is considerably affected by the type of solubilizate. Thus, these calix[4]arenes bearing four alkyl side chains show interesting physicochemical properties as unique surfactants. Mixtures of different surfactants often exhibit synergism in their effects on the properties of the systems. Such synergism has been observed as lowering critical micelle concentrations (cmc’s) and surface tensions compared with those of the unmixed surfactants alone.6 In particular, opposite charged surfactants can interact strongly and sometimes form vesicles. Although many mixed surfactant systems have been extensively studied, there are few reports for the behavior of mixtures of calixarenes and other surfactants so that it is worthwhile to characterize the physicochemical properties of a mixed surfactant system involving calixarenes. The aim of this work is to investigate the physicochemical properties of mixtures of calix[4]arenes and a cationic surfactant, dodecyltrimethylammonium bromide, by surface tension and fluorescence measurements.
Before studying the physicochemical properties of mixed solutions of [4]Ar-Rn and DTAB, it is important to check whether their solutions precipitate or not. In the case of the [4]Ar-R4/DTAB system, some precipitation occurred in solutions with total concentrations more than 0.1 mmol dm-3 in the range of mole fraction (R) of [4]Ar-R4, 0.2-0.6, also at concentration exceeding 5 mmol dm-3 at R ) 0.8. In the case of the [4]Ar-R6/DTAB system, some precipitation occurred in solutions with total concentrations of a more than 0.3 mmol dm-3 at R ) 0.2, at 0.5 mmol dm-3 at R ) 0.5, at 1 mmol dm-3 at R ) 0.6, and at 5 mmol dm-3 at R ) 0.8. In addition, the [4]Ar-R8/DTAB system exhibited some precipitation at total concentrations above those for the [4]Ar-R6/DTAB system over the whole range of R. Thus, the concentration of the precipitation region for [4]Ar-Rn/DTAB system becomes higher as the alkyl side chain length of [4]Ar-Rn becomes longer. This result suggests that the interaction between [4]Ar-Rn and DTAB increases with decreasing alkyl side chain length of [4]Ar-Rn. This precipitation behavior for the [4]Ar-Rn/DTAB system is different from that for conventional surfactant mixtures of opposite charge. Figure 2 shows the surface tension curves as a function of total surfactant concentration for the [4]Ar-Rn/DTAB mixed systems. The surface tensions at various mole fractions of [4]Ar-Rn decreased with increasing total surfactant concentration, and each surface tension curve showed a break point which was taken as a mixed cmc. In the cases of [4]Ar-R4/DTAB and [4]Ar-R6/DTAB systems, the mixed cmc’s were smaller than that of the respective surfactants’ cmc’s, while they were intermediate between the respective surfactants’ cmc’s for the [4]Ar-R8/DTAB system. Further, it is interesting to compare the surface tension values at the cmc (γcmc) for the different mixed systems. The γcmc for the [4]Ar-R4/DTAB system was higher than that of DTAB, but the γcmc for the [4]Ar-R6/DTAB system was slightly lower. In the case of the [4]Ar-R8/ DTAB system, the γcmc was lower than that of DTAB except R ) 0.8. Thus, the interaction between [4]Ar-Rn and DTAB depends on the alkyl side chain length of [4]Ar-Rn. To obtain the relationship between the mixed cmc and the mole fraction of [4]Ar-Rn, the results of mixed cmc versus more fraction of [4]Ar-Rn are plotted in Figure 3. The mixed cmc’s decrease very sharply with increasing mole fraction of [4]Ar-Rn, and increase at high [4]Ar-Rn mole fraction for the three systems. These changes in the mixed cmc’s are typical for opposite charged mixed surfactant systems (anionic-cationic) and deviate appreciably from ideal mixing. The regular solution theory
Experimental Section Materials. 1,8,15,22-Tetraalkyl[4]metacyclophan-3,5,10,12,17,19,24,26-octols ([4]Ar-Rn where Rn indicates alkyl side chains) were prepared by condensation of resorcinol with long chain alkanals at 70-75 °C in the presence of 20% HCl catalyst. The products were recrystallized from methanol. The structures of these compounds were confirmed by IR (KBr), 1H NMR, and C, H, N elementary analyses. The chemical structure of [4]Ar-Rn † ‡
Science University of Tokyo. Kumamoto University.
(1) Scheider, H.; Kramer, R.; Simova, S.; Scheider, U. J. Am. Chem. Soc. 1988, 110, 6442. (2) Tanaka, Y.; Aoyama, Y. Bull. Chem. Soc. Jpn. 1990, 63, 3343. (3) Sugiyama, K.; Esumi, K.; Koide, Y. Langmuir 1996, 12, 6006. (4) Koide, Y.; Li, B.; Okubayashi, S.; Shosenji, H.; Esumi, K. J. Jpn. Oil Chem. Soc. 1997, 46, 767. (5) Koide, Y.; Li, B.; Kawaguchi, Y.; Shosenji, H.; Esumi, K. J. Jpn. Oil Chem. Soc. 1998, 47, 57. (6) Holland, P. M.; Rubingh, D. N. Mixed Surfactant Systems; Holland, P. M., Rubingh, D. N., Eds.; ACS Symposium Series 501; American Chemical Society: Washington, DC, 1992; Chapter 1.
10.1021/la9814384 CCC: $18.00 © 1999 American Chemical Society Published on Web 07/01/1999
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Notes
Figure 1. The chemical structure of calix[4]resorcinarenes bearing four alkyl side chains.
has been one of the most widely used developments in modeling mixed surfactant systems.6 However, since the interaction term with counterion effects is not included in the regular solution theory, the modified regular solution theory including counterion binding is required. In this study, the interaction parameter (β) was calculated by using the modified regular solution theory developed by Shinoda.7 The equation used is given below.
β ) ln(R1cmc(1+Kg)/x1cmc(1+Kg1))/(1 - x1)2 ) ln(R2cmc(1+Kg)/x2cmc(1+Kg2))/(1 - x2)2 where x and R are the mole fractions in the mixed micelles and in the total surfactants, respectively, Kg is the degree of counterion binding to the mixed micelles, which is assumed that Kg ) x1Kg1 + (1 - x1)Kg2. From the results of cmc change with pH, a Kg value of [4]Ar-Rn was obtained to be 0.35 and a Kg value of 0.73 was used for DTAB.8 The interaction parameter (β) was calculated to be -25.3 for the [4]Ar-R4/DTAB system, -21.4 for the [4]Ar-R6/DTAB system, and -10.4 for the [4]Ar-R8/DTAB system. A negative β suggests the interaction to be energetically favorable. Such large deviations from ideality are often seen in anionic/cationic mixed systems.9 Although [4]ArRn systems have some hydroxyl groups with benzene rings and these groups form a strong intermolecular hydrogen bonding, [4]Ar-Rn can also lose a proton at high pH in an aqueous solution and behave like an anionic surfactant. These interactions become smaller with increasing alkyl side chain length of [4]Ar-Rn. Accordingly these large negative β values, indicative of a strong interaction between [4]Ar-Rn and DTAB, are predominantly due to electrostatic interaction. From 1H NMR relaxation and 2D 1H NMR measurements,3 it has been suggested that the micelles of [4]Ar-Rn consist of a cone shape conformation with aggregation of the alkyl side chains. It seems that mixed micelles are formed from a complicated conformation consisting of interactions between the headgroups of DTAB and the ionized hydroxyl groups of [4](7) Shinoda, K. Colloidal Surfactants; Shinoda, K., Tamamushi, B., Nakagawa, T, Isemura, T., Eds.; Academic Press: New York, 1963; Chapter 1. (8) Malliaris, A.; Binana-Limbele, W.; Zana, R. J. Colloid Interface Sci. 1986, 110, 114. (9) Holland, P. M. Mixed Surfactant Systems; Holland, P. M., Rubingh, D. N., Eds.; ACS Symposium Series 501; American Chemical Society: Washigton, DC, 1992; Chapter 2.
Figure 2. Surface tension of aqueous solutions of [4]Ar-Rn/ DTAB mixtures vs log of the total surfactant concentration: (a) [4]Ar-R4/DTAB; (b) [4]Ar-R6/DTAB; (c) [4]Ar-R8/DTAB. R is the mole fraction of [4]Ar-Rn in total surfactant.
Ar-Rn with perhaps some association between the alkyl side chains of [4]Ar-Rn and the hydrophobic chain of DTAB. Microenvironmental polarities of the single and mixed micelles were investigated by using pyrene. Fluorescence probes have been used to estimate the polarity at the micelle-water interface for various surfactant micelles.10 In this study, the polarity of the micelles was evaluated (10) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1977, 81, 2176.
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Figure 4. Variations of I1/I3 of pyrene in [4]Ar-Rn/DTAB mixtures: (a) [4]Ar-R6/DTAB; (b) [4]Ar-R8/DTAB. R is the mole fraction of [4]Ar-Rn in total surfactant.
Figure 3. Mixed cmc of [4]Ar-Rn/DTAB mixtures: (a) [4]ArR4/DTAB; (b) [4]Ar-R6/DTAB; (c) [4]Ar-R8/DTAB. The plotted points are experimental results, the solid line is the prediction of the modified regular solution theory, and the dashed line is the prediction for ideal mixing. R is the mole fraction of [4]Ar-Rn in total surfactant.
by the intensity ratio I1/I3 of the first and third vibronic bands of monomeric pyrene. Figure 4 shows the variations of I1/I3 as a function of total surfactant concentration. Here,
only the data of [4]Ar-Rn (n ) 6, 8)/DTAB systems were given because the vibronic bands of monomeric pyrene for the [4]Ar-R4/DTAB system could not be easily assigned. As the total surfactant concentration increased, the I1/I3 ratio decreased abruptly at the cmc. The I1/I3 values above the cmc’s were larger than that of DTAB alone for the [4]Ar-R6/DTAB system, while their values were very close to that of DTAB alone except R ) 0.2 for the [4]Ar-R8/ DTAB system. These results suggest that the polarity of mixed micelles for the [4]Ar-R6/DTAB system is higher than that for the [4]Ar-R8/DTAB system. At the present time, an adequate explanation cannot be given in the result of I1/I3 with R ) 0.2 for both mixed systems. It can be concluded from the results of surface tension measurements that the mixtures of [4]Ar-Rn and DTAB show considerably lower mixed cmc’s, similar to other oppositely charged surfactant systems, and the interaction parameter for the mixed micelles, calculated using the modified regular solution theory, increases from -25.2 to -10.4 with increasing side chain length of [4]Ar-Rn. It is also found from the fluorescence measurements that the micropolarity of mixed micelles of [4]Ar-R6/DTAB is higher than that of [4]Ar-R8/DTAB. LA9814384
