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Langmuir 1996, 12, 6006-6010
Aqueous Properties of Resorcinol-type Calix[4]arenes Bearing Four Alkyl Side Chains Kayo Sugiyama,† Kunio Esumi,*,† and Yoshifumi Koide‡ Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan, and Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, Kurokami, Kumamoto 860, Japan Received May 6, 1996. In Final Form: August 19, 1996X The resorcinol-type calix[4]arenes bearing alkyl side chains ([4]Ar-Rn, n ) 4, 6, 8) have been prepared, and their aqueous properties at pH 13 have been studied by measuring the surface tension, NMR, and solubilization behavior of benzene. It is found from the surface tension measurements that the formation of micelles is promoted and the surface tension value at the critical micelle concentration cmc is decreased with increasing alkyl side chain length. The results of 2D-NMR and relaxation measurements suggest that the interaction of alkyl side chains in the micelles becomes weak, but the interaction between the resorcinol ring and the inner alkyl side group is increased with increasing alkyl side chain length. In the solubilization of benzene, benzene molecules are mainly incorporated in the resorcinol ring part of [4]Ar-R4, whereas they are solubilized in the alkyl side chains of [4]Ar-R6.
Introduction Calixarenes are cyclic oligomers combined with phenol units by methylene substituents and have the structure of metacyclophane. These compounds have been famous as inclusion compounds and synthesized from phenols and formaldehyde by Kammerer and Caesar.1 In addition, calixarenes have been investigated for host-guest chemistry research, and many reports have been published.2-8 To enhance the molecular recognition of host compounds with a hydrophobic pore, it is useful to use water rather than organic solvents. These calixarenes have some hydroxides on the benzene rings, and these groups cause strong intermolecular hydrogen bonding, so that in highpH solution these compounds can be dissolved in aqueous solution. For the case of many functionalized calixarenes, resorcinol-type [4]calixarenes with two hydroxides were synthesized by Hogberg.9 Since resorcinol-type [4]calixarenes have twice as many hydrophilic groups, they are more soluble in aqueous solution than normal calixarenes. Recently, resorcinol-type calix[4]arenes with alkyl side chains have been synthesized by Koide et al.10 They have found11 that a selective flotation of Cs+ ions is particularly enhanced by the calix[4]arenes. Although it is important to understand functions of calix[4]arenes in aqueous solution, only a few studies about the aqueous properties †
Science University of Tokyo. Kumamoto University. X Abstract published in Advance ACS Abstracts, November 1, 1996. ‡
(1) Kammerer, G. H.; Caesar, F. Macromol. Chem. 1972, 162, 1179. (2) Scheider, H.; Kramer, R.; Simova, S.; Scheider, U. J. Am. Chem. Soc. 1988, 110, 6442. (3) Ishikawa, Y.; Kunitake, T.; Masuda, T.; Otsuka, T.; Shinkai, S. J. Chem. Soc., Chem. Commun. 1989, 736. (4) Shinkai, S. Pure Appl. Chem. 1986, 58, 1523. (5) Gutsche, C. D.; Iqbal, M.; Nam, K. S.; Alam, I. Pure Appl. Chem. 1988, 60, 483. (6) Dei, L.; Casnati, A.; Nostro, P. L.; Baglioni, P. Langmuir 1995, 11, 1268. (7) Tanaka, Y.; Aoyama, Y. Bull. Chem. Soc. Jpn. 1990, 63, 3343. (8) Markowitz, M. A.; Janout, V.; Castner, D. G.; Regen, S. L. J. Am. Chem. Soc. 1989, 111, 8192. (9) Hogberg, A. G. S. J. Am. Chem. Soc. 1980, 102, 6046. (10) Koide, Y.; Oka, T.; Imamura, A.; Shosenji, H.; Yamada, K. Bull. Chem. Soc. Jpn. 1993, 66, 2137. (11) Koide, Y.; Sato, H.; Shosenji, H.; Yamada, K. Bull. Chem. Soc. Jpn. 1996, 69, 315.
S0743-7463(96)00443-X CCC: $12.00
of calixarenes have been reported.12,13 It is also worth elucidating the aggregation behavior of resorcinol-type calix[4]arenes in aqueous solution from the standpoint of conformational structure. In this study, the aqueous properties of resorcinol-type calix[4]arenes bearing alkyl side chains (chain lengths 4, 6, and 8) have been investigated by static surface tension, NMR, and solubilization measurements. Experimental Section Materials. 1,8,15,22-Tetraalkyl[14]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 and were recrystallized from methanol. Yield: [4]Ar-R4, 73%; [4]Ar-R6, 89%; [4]Ar-R8, 76%. Mp’s of all the [4]Ar-Rn: above 300 °C. IR s(KBr) of all the [4]Ar-Rn: νOH 3500-3000 and νCH 29002800 cm-1. 1H-NMR: [4]Ar-R4 (DMSO-d6, TMS) δ ) 0.9 (12H, CH3), 1.2 (8H, CH2), 4.3 (4H, CH), 6.2 (4H, Ar-H), 7.3 (4H, ArH), 8.9 (8H, OH); [4]Ar-R6 (DMSO-d6, TMS) δ ) 0.8 (12H, CH3), 1.2 (24H, CH2), 2.0 (8H, CH2), 4.2 (4H, CH), 6.1 (4H, Ar-H), 7.1 (4H, Ar-H), 8.8 (8H, OH); [4]Ar-R8 (CD3Cl, TMS) δ ) 0.9 (12H, CH3), 1.4 (4OH, CH2), 2.2 (8H, CH2), 4.3 (4H, CH), 6.1 (4H, ArH), 7.2 (4H, Ar-H), 9.4, 9.6 (8H, OH). Anal. (Corona 114 vaporpressure osmometer, DMF.) [4]Ar-R4 Found: C, 70.37; H, 7.53. Calcd for C40H48O8 + H2O: C, 71.19; H, 7.47. [4]Ar-R6 Found: C, 71.98; H, 8.40. Calcd for C48H64O8 + 2H2O: C, 71.61; H, 8.51. [4]Ar-R8 Found: C, 73.31; H, 9.14. Calcd for C56H80O8 + 2H2O: C, 73.33; H, 9.23. The chemical structure of [4]Ar-Rn is shown in Figure 1. Water was purified through a Milli-Q water system (Japan Millipore Ltd.) and was used throughout all experiments. The solution was adjusted at pH 13 with NaOH (Kanto Chemical Industry; purity 99.5%) of NaOD (Aldrich 30 wt % D2O solution). Benzene was used as a solubilizate (Wako Pure Chemical Industry). Methods. Resorcinol-type calix[4]arenes, [4]Ar-Rn, could be easily soluble in alkaline aqueous solution. The aqueous solutions were adjusted to pH 13 by addition of NaOH. The static surface tensions of [4]Ar-Rn aqueous solutions were measured by the Wilhelmy plate method (Kruss K12 tensiometer) at 25 °C. The change in the surface tension was very dependent on time; due to bulky structure it took more than 4 h to reach adsorption equilibrium. For NMR measurements, heavy water (D2O) (Aldrich 99.9%) was used as a solvent, and the pH was adjusted to 13 with NaOD (12) Shinkai, S.; Mori, S.; Koreishi, H.; Tsubaki, T.; Manabe, O. J. Am. Chem. Soc. 1986, 108, 2409. (13) Arimori, S.; Nagasaki, T.; Shinkai, S. J. Chem. Soc., Perkin Trans. 2 1995, 67.
© 1996 American Chemical Society
Resorcinol-type Calix[4]arenes
Figure 1. Chemical structure of [4]Ar-Rn (n ) 4, 6, 8). (Aldrich 30 wt % in D2O). The samples were prepared in D2O and were put directly into 5 mm NMR tubes. NMR spectra were obtained with a JEOL JMN EX400 spectrometer by using freshly prepared surfactant solutions in D2O at 25 °C. All chemical shifts were referred to HDO, 4.7 ppm. The errors did not exceed (0.2 Hz. The relaxation time measurements were made by using the inversion-recovery sequence for T1 and the Carr-PurcellMeiboom-Gill (CPMG) sequence for T2. The errors were within 10%. To investigate the proton atom conformation in a micelle, simultaneous measurements of NOESY, COSY, and COCONOSY were carried out for all the surfactants. If micelles are formed, a difference of conformation between monomer and micelle will be expected. Due to the very low critical micelle concentrations (cmc’s) of [4]Ar-R6,8 the measurements were performed only above the cmc’s. The solubility of benzene in aqueous solutions of [4]Ar-Rn was determined using NMR spectroscopy. NMR studies of solute/ micellar systems have primarily focused on elucidating the nature of the interactions between solute and micelle or on the location of solubilized species,14 mainly from the dependence of chemical shifts or peak area on surfactant or solute concentrations.15 NMR peak intensities or integrals were used to determine the solubility of benzene in micellar solutions.16 The surfactant solutions (1 mL) and benzene (1 mL) were mixed and shaken during 30 s. Then they were separated into two phases. The lower 1 mL was directly injected into a 5 mm NMR tube as sufficient height of solution, keeping the equilibrium state for 24 h. In the case of [4]Ar-R6 the measurements were performed at 5 mmol dm-3, which was above the cmc. However, for [4]Ar-R8 no results were obtained because of film formation with [4]Ar-R8 at the benzene/ solution interface. Saturated solutions of benzene in NaOD/D2O solution were also prepared by shaking excess benzene with NaOD/D2O in a tube to determine the solubility of benzene in NaOD/D2O solution. Then, using this solution, a 5 mmol dm-3 [4]Ar-R4 solution was prepared and measured in the same manner. Furthermore, the static surface tension of [4]Ar-Rn was measured to study the influence of benzene on the surface activity. All measurements were carried out at 25 °C.
Results and Discussion Static Surface Tension. The results for the static surface tension of [4]Ar-Rn as a function of the logarithm of the surfactant concentration are shown in Figure 2. The cmc values of [4]Ar-Rn (n ) 4, 6, and 8) taken from the breakpoint in the surface tension vs the concentration curve were 2.8, 0.04, and 0.008 mmol dm-3, respectively. The molecular occupied areas of [4]Ar-Rn (n ) 4, 6, and 8) calculated with the Gibbs adsorption equation were 1.54, 0.95, and 0.80 nm2 molecule-1, respectively. With (14) Guo, L. N.; Arnaud, I. J. Colloid Interface Sci. 1994, 163, 334. (15) Nakagawa, T.; Tori, K. Kolloid Z. Z. Polym. 1964, 194, 143. (16) Gadelle, F.; Koros, W. J.; Schechter, R. S. J. Colloid Interface Sci. 1995, 170, 57.
Langmuir, Vol. 12, No. 25, 1996 6007
Figure 2. Surface tension vs surfactant concentration of [4]Ar-Rn.
Figure 3. Assignments of 1H-NMR spectra for [4]Ar-Rn.
increasing side chain length, the surfactant molecules are adsorbed at the air/water interface more closely packed, although the shape of the molecules is less flat at the air/water interface. A considerable change in the surface properties of [4]Ar-Rn was observed between R4 and R6,8. In addition, the values of the surface tension at the cmc (γcmc) were 42 mN m-1 for [4]Ar-R4, 30 mN m-1 for [4]ArR6, and 28 mN m-1 for [4]Ar-R8, suggesting that the surface activities of [4]Ar-R6 and [4]Ar-R8 are higher than that of [4]Ar-R4. Thus, it is found that the surface properties of [4]Ar-Rn are appreciably altered with the alkyl side chain length. 1H-NMR. The assignments of 1D 1H-NMR spectra for [4]Ar-Rn are shown in Figure 3. The chemical shifts for [4]Ar-R4 are plotted vs the inverse concentration in Figure 4. Generally, from the chemical shifts vs the inverse concentration, the cmc’s of conventional surfactants can often be determined. In the case of [4]Ar-R4, the proton chemical shifts of the alkyl chain were not linearly changed against the inverse concentration. However, since the chemical shifts for [4]Ar-R4 against the concentration showed a similar behavior to that of conventional hydrocarbon surfactants that the magnetic field is shifted lower with increasing surfactant concentration, it can be said
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Sugiyama et al.
Figure 5. Spin-lattice relaxation rate vs inverse surfactant concentration of [4]Ar-R4.
Figure 4. Chemical shift vs inverse surfactant concentration of [4]Ar-R4.
that the interaction between the alkyl side chain protons occurs by intermolecular and intramolecular interaction. Further, the change of chemical shift of the alkyl side protons (peaks 3 and 4) was smaller than that of the alkyl side protons (peaks 5 and 6), probably due to a fixation of the alkyl side protons (peaks 3 and 4) in the resorcinol ring. In addition, the chemical shift of the resorcinol protons (peaks 1 and 2) was smaller than that of the alkyl side chain protons, suggesting a change of conformation for the resorcinol ring in micelles. Thus, it is seen from the experimental results that when a [4]Ar-R4 micelle is formed, the alkyl side protons near the resorcinol ring are fixed in the aggregates, but the alkyl side chains are more loose than the conventional hydrocarbon surfactant chains in the micelles. The results for the spin-lattice relaxation rate of [4]Ar-R4 are shown in Figure 5. In general, when micelles are formed, 1/T1 and 1/T2 rates are increased. However, in this study the alkyl side chain proton’s 1/T1 values were hardly decreased as well as those of the resorcinol protons. Moreover, a marked change of 1/T1 with the formation of a [4]Ar-R4 micelle was not recognized. Consequently, it is inferred that the aggregation of the alkyl side chains is relatively loose compared to that of the conventional hydrocarbon surfactant micelles. The aggregation number would be expected to be quite small. In the same manner, the relaxation of 1/T2 vs the inverse surfactant concentration is shown in Figure 6. The change of 1/T2 with the aggregation of the alkyl side chains was not observed, similar to the case for 1/T1. This behavior was also different from that of the conventional hydrocarbon surfactant micelles.17 2D NMR COCONOSY. In order to investigate the conformation of the alkyl side chains, 2D NMR measurements were performed. Simultaneously COSY and NOE(17) Chachaty, C. Prog. Nucl. Magn. Reson. Spectrosc. 1987, 19, 183.
Figure 6. Spin-spin relaxation rate vs inverse surfactant concentration of [4]Ar-R4.
SY (COCONOSY) were carried out under NOESY measurement parameters; PI3 500 ms TOD, CLP 1024. Here, the contour of the spectrum was cut at equal height for each sample. The COSY measurements were performed at below and above the cmc for [4]Ar-R4 (0.5 and 10 mmol dm-3) and at only above the cmc for [4]Ar-R6,8. There were some differences of the spectra among the three surfactants. Compared to the 0.5 and 10 mmol dm-3 NOESY for [4]Ar-R4 shown in parts a and b of Figure 7, there was no correlation at 0.5 mmol dm-3. On the other hand, we obtained some correlations at 10 mmol dm-3: the alkyl chain proton (peak 4) and the resorcinol proton (peak 1), and peak 5 and peak 6 had correlations. Since NOESY spectra indicate a correlation of hydrogen atoms located at short distances (e4.5 Å) in the spacial structure, it is suggested that, above the cmc of [4]Ar-R4, the resorcinol proton (peak 1) is near the base proton of the alkyl side chain and that both the end proton of the alkyl side and the neighbor proton exist in near space. As one can see from Figure 7c for [4]Ar-R6, peaks 1 and 3, peaks 1 and 4, and peaks 5 and 6 had correlations, respectively. Thus, some different conformations between [4]Ar-R4 and [4]Ar-R6 can be seen from each figure. From [4]Ar-R6 and [4]Ar-R8 NOESY (Figure 7d) spectra, it is confirmed that
Resorcinol-type Calix[4]arenes
Langmuir, Vol. 12, No. 25, 1996 6009
Figure 7. NOESY spectra of (a) [4]Ar-R4, 0.5 mmol dm-3, (b) [4]Ar-R4, 10 mmol dm-3, (c) [4]Ar-R6, 5 mmol dm-3, and (d) [4]Ar-R8, 5 mmol dm-3.
the correlation between peaks 1 and 2 is much stronger for [4]Ar-R8 than for [4]Ar-R6. Solubilization. Figure 8 shows the change in the chemical shifts of solubilized benzene in [4]Ar-R4 solution. Clearly, the benzene signal was shifted to the lower magnetic side, and in spite of the existence of benzene, a similar change in the chemical shifts against the concentration was observed. Below the cmc all the protons were influenced by benzene ring current, whereas above the cmc the alkyl side chain protons (peaks 4, 5, and 6) were hardly influenced by benzene ring current. Therefore it is suggested that benzene molecules below the cmc exist mainly near the alkyl side chain, but above the cmc they locate at the resorcinol ring of the calixarene. Below the cmc the chemical shift of benzene agreed with that of benzene in NaOD/D2O solution. With increasing surfac-
tant concentration, the signal was shifted to the higher magnetic side, suggesting that the environment around benzene molecules is changed due to the formation of micelles. Then we determined the amount of benzene incorporated. The result is shown in Figure 9. It can be seen that a considerable amount of benzene is incorporated in the monomer state of [4]Ar-R4. Above the cmc one benzene molecule was incorporated per surfactant (decreasing the amount of benzene solubilized in NaOD/D2O). From the results of the location of the solubilized benzene and of the amount of benzene incorporated, it is suggested that below the cmc benzene molecules exist in the alkyl side chain of [4]Ar-R4, but the penetration of benzene molecules into the alkyl chain of [4]Ar-R4 is made more difficult by forming aggregates of the [4]Ar-R4 molecule. The amount
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Sugiyama et al. Table 1.
1H-NMR
of [4]Ar-R6 Solution Containing Benzene Aa
peak number
δ/ppm
peak 1 peak 2 peak 3 peak 4 peak 5 peak 6
6.950 5.886 4.202 2.127 1.386 0.965
benzene peak
Bb δ/ppm
diff
[4]Ar-R6 Solution 6.951 -0.001 5.868 +0.018 4.204 -0.002 2.112 +0.015 1.368 +0.018 0.952 +0.013
6.951 5.868 4.204 2.118 1.352 0.939
-0.001 +0.018 -0.002 +0.015 +0.034 +0.026
Benzene in NaOD/D2O 7.365 7.339 +0.026
7.249
+0.016
δ/ppm
diff
a
[4]Ar-R6 solution prepared with benzene-saturated NaOD/D2O (contains 15.8 mmol dm-3 benzene) as solvent. b [4]Ar-R6 solution saturated with benzene (contains 40.6 mmol dm-3 benzene): The concentration of [4]Ar-R6 for both A and B is 5 mmol dm-3.
Figure 8. 1H-NMR chemical shifts of [4]Ar-R4 (closed symbols) and [4]Ar-R4/benzene (open symbols).
incorporated. This suggests that benzene molecules exist in the alkyl side chains above the cmc. This differs from the case for [4]Ar-R4. It is also found from the solubilized amount of benzene that three benzene molecules are incorporated per [4]Ar-R6 molecule. To investigate the effect of benzene on the surface activity and micelle formation of [4]Ar-Rn, the static surface tensions were measured. The cmc values for [4]Ar-R6 and [4]Ar-R8 were higher in the presence of benzene than those for [4]Ar-R6 and [4]Ar-R8 in the absence of benzene, but their γcmc values in the former were lower than those in the latter. On the other hand, there was no change in the surface tension curves for [4]Ar-R4 with and without benzene. These results might be correlated with the location of benzene molecules in [4]Ar-Rn aggregates. Conclusions
Figure 9. Solubilized amount of benzene in [4]Ar-R4 and [4]Ar-R6 solutions.
of benzene solubilized by [4]Ar-R6 (5 mmol dm-3) is also given in Figure 9. From the comparison of the chemical shifts for [4]Ar-R6 with different amounts of benzene solubilized shown in Table 1, one can see that the alkyl side chain peaks of [4]Ar-R6 are only shifted to the higher magnetic side in spite of the different amounts of benzene
The aqueous properties of [4]Ar-Rn (n ) 4, 6, and 8) at pH 13 are investigated using NMR, static surface tension, and solubilization techniques. At the air/aqueous solution interface, the occupied area of [4]Ar-Rn decreases with increasing alkyl chain length but is considerably greater than that of the conventional linear surfactants. From NMR measurements, some differences in the conformation of micelles are observed: In the micelles, the interaction of the alkyl side chains of [4]Ar-Rn is decreased, but the interaction between the resorcinol group and the inner alkyl chain is increased with increasing alkyl side chain length. Although the aggregation number cannot be determined, it seems that stereostructural control is strong in micelle formation, resulting in a quite small number. In the solubilization results, [4]Ar-R4 incorporates much benzene in the monomer state which is located near the alkyl side chains. Above the cmc benzene might exist not near the alkyl side chains but mainly in the resorcinol ring. Thus, the solubilization of benzene in [4]Ar-R4 solution is different from that of the conventional linear hydrocarbon surfactants. On the other hand, the results of 1H-NMR and static surface tension indicate that benzene is solubilized in the alkyl side chains of [4]Ar-R6 like the conventional linear hydrocarbon surfactants. LA960443+
