Langmuir 1997, 13, 4767-4769
4767
Notes Conformational Behavior of Aqueous Micelles of Sodium N-Dodecanoyl-N-methylglycinate Giorgio Cerichelli Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita` degli Studi de L’Aquila, Via Vetoio, 67010 Coppito Due (AQ), Italy Luciana Luchetti* Dipartimento di Scienze e Tecnologie Chimiche, Universita` degli Studi di Roma “Tor Vergata”, Via della Ricerca Scientifica, 00133 Roma, Italy Giovanna Mancini* Centro CNR di Studio sui Meccanismi di Reazione c/o Dipartimento di Chimica, Universita` degli Studi di Roma “La Sapienza”, P.le Aldo Moro 5, 00185 Roma, Italy
Figure 1. 1H (a) and 13C NMR (b) spectra of the 12-CH3 group of 1 in D2O at 25 °C. [1] ) 0.0270.
Received December 30, 1996. In Final Form: May 5, 1997
Chart 1
Introduction It is well-known that the 1H and 13C NMR spectra of amides may show two different signals for each of the nuclei. This can be observed if the rotation about the amide bond is slow compared to the NMR time scale, and it is due to the fact that a Z-isomer nucleus is magnetically nonequivalent to the E-isomer nucleus. Several papers have been published on NMR studies of surfactants with amide bonds, and N-acyl-N-alkylglycines and their salts have been studied.1-3 In particular, for sodium N-dodecanoyl-N-methylglycinate (1) literature data report that the population of the E-isomer of 1 increases with increasing concentration and that two different signals for each of the head group nuclei are observed in the 1H and 13C NMR spectra.1a,3 In spite of these observations, when we examined NMR spectra, we found some interesting features not mentioned before. Experimental Section Materials. Sodium N-dodecanoyl-N-methylglycinate (1) is a commercial material (Fluka). It was purified by repeated crystallizations with ethanol and diethyl ether. NMR. 1H and 13C NMR measurements have been carried out on a Brucker AC300P instrument operating at 300.13 and 75.468 MHz for 1H and 13C, respectively. 1,4-Dioxane was used as internal standard.
Results and Discussion 1H
NMR spectrum of 0.02 M 1 in a nonmicellizing The solvent such as CD3OD shows two well-resolved signals each for NCH3, NCH2, and 2-CH2. This is due to the two conformations, as shown in Chart 1, indicated as 1-Z and 1-E, respectively. According to the literature,4 it is possible to assign the resonance lines and determine the ratio [1-E]/ * To whom correspondence should be addressed. (1) (a) Takahashi, H.; Nakayama, Y.; Hori, H.; Kihara, K.; Okabayashi, H.; Okuyama, M. J. Colloid Interface Sci. 1976, 54, 102. (b) Okabayashi, H.; Kihara, K.; Okuyama, M. Chem. Abstr. 1977, 87, 54871z. (2) Okabayashi, H.; Yoshida, T.; Terada, Y.; Matsushita, K. J. Colloid Interface Sci. 1982, 87, 527. (3) Yahagi, K.; Tsujii, K. J. Colloid Interface Sci. 1987, 117, 415. (4) Ambu¨hl, M.; Bangerter, F.; Luisi, P. L.; Skrabal, P.; Watzke, H. J. Langmuir 1993, 9, 36.
S0743-7463(96)02136-1 CCC: $14.00
[1-Z] by integration. The ratio is 0.85 and indicates that 1-Z is only slightly more stable than 1-E, when they are monomers. When 1H and 13C NMR spectra of 0.0270 M 1 are run in D2O, the 12-CH3 signal is formed by two overlapped triplets and two peaks, respectively, shown in Figure 1. In particular, in the 1H NMR spectrum the triplet at δ ) 0.871 ppm (J ) 7 Hz) can be assigned to 1-E and that at δ ) 0.864 ppm (J ) 7 Hz) to 1-Z. Because this peculiarity was not reported before, we have examined 1H and 13C NMR spectra in a wide range of concentrations. In Table 1 we report the effect of concentration of 1 on 1H NMR chemical shift and on the ratio [1-E]/[1-Z] in D O 2 at 25 °C. More extensive data, including 13C NMR spectra at various concentrations, are reported as Supporting Information (Tables S1 and S2). In agreement with literature data,1a the ratio [1-E]/[1-Z] increases asymptotically and reaches a constant value at high [1]. The ratio in micellized surfactant ([1-E]/[1-Z] ) 3.1) is different from that in CD3OD, which is very close to the value in D2O at concentrations below the cmc, i.e., to the value for monomeric surfactant. This observation indicates that the isomer population depends on the aggregation and not on the solvation of the polar head groups. In fact, it is commonly accepted that the micellar Stern layer polarity is very similar to those of CH3OH and C2H5OH.5 In the 1H NMR spectra the 12-CH3 signal appears as two overlapped triplets in the range of concentration 0.0147-0.0486 M. In particular the splitting is observed for concentrations just above the cmc, that is 0.013 M, (5) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1015.
© 1997 American Chemical Society
4768 Langmuir, Vol. 13, No. 17, 1997
Notes
Table 1. Effect of Concentration of 1 on 1H NMR Chemical Shift and on the Ratio [1-E]/[1-Z]a NCH2
NCH3
2-CH2
12-CH3
[1], M
1-E
1-Z
1-E
1-Z
1-E
1-Z
0.0101 0.0108 0.0135 0.0147 0.0243 0.0486 0.0567 0.1130 0.1800 0.4500 0.8100
3.902 3.902 3.902 3.900 3.891 3.886 3.888 3.887
3.967 3.967 3.965 3.963 3.937 3.917 3.917 3.904
3.072 3.072 3.071 3.068 3.056 3.052 3.052 3.051 3.053 3.062 3.071
2.910 2.910 2.910 2.909 2.899 2.890 2.889 2.886 2.886 2.894 2.903
2.439 2.439 2.438 2.434 2.418 2.410 2.413 2.409 2.411 2.418 2.427
2.292 2.293 2.293 2.291 2.284 2.279 2.279 2.276 2.276 2.287 2.296
a
3.889 3.898 3.908
1-E
1-Z
[1-E]/[1-Z]
0.851 0.851 0.854 0.856 0.870 0.875
0.88 0.85 0.93 0.97 1.6 2.1 2.2 2.6 2.8 3.0 3.1
0.853 0.862 0.871 0.877 0.878 0.880 0.887 0.896
Values of δ, relative to 1,4-dioxane (δ ) 3.750) as internal standard, at 25 °C. Table 2. Changes in
13C
NMR Chemical Shifta
1-E CO2 NCH2 NCH3 CO 2-CH2 3-CH2 10-CH2 11-CH2 12-CH3
δb 176.47 51.77 36.76 176.81 32.77 24.54 31.11 21.97 13.34
1-Z ∆δE
δb
∆δZc
-0.44 -0.04 -0.20 -1.30 0.30 0.42 0.76 0.54 0.32
176.18 54.23 34.98 177.27 32.69 24.83 31.11 21.97 13.34
-0.75 -0.10 -0.22 -1.72 0.01 0.47 0.76 0.54 0.35
c
a Values of δ, relative to 1,4-dioxane (δ ) 66.50) as internal standard, at 25 °C. b [1] ) 0.0101. c [1] ) 0.8100.
Figure 2. 13C NMR spectra of 1 in D2O at 25 °C. [1] ) 0.0101 (a), 0.0270 (b), and 0.180 (c). CO2 and CO signals are not reported.
determined by conductivity measurements and in agreement with literature data.1a,3 We attempted to calculate the cmc of each isomer by 1H NMR, in order to show any differences. According to the literature,6 we plotted the variation of chemical shift versus concentration and considered as the cmc the concentration which corresponded to the inflection point of the plot. We examined the NCH3, NCH2, and 2-CH2 chemical shifts for both isomers and found that the 1-E and 1-Z cmc values were slightly different, 0.007 and 0.008 M, respectively, although the difference was very close to the experimental error. These values are rather different from the value reported before and from literature data because in this case we do not consider the analytical concentration of the surfactant but the concentration of each isomer, i.e., the analytical concentration multiplied by the mole fraction, obtained by integration. In our opinion, the presence of two 12-CH3 signals is not due to the transmission of the head conformation along the hydrophobic chain. In order to explain this observation on the 1H NMR spectra, we observed 13C NMR chemical shifts at various concentrations. In Figure 2 we report a selection of 13C NMR spectra. They were chosen in order to show the spectra of the monomer (0.0101 M, a), of the micelle (0.180 M, c), and of an intermediate concentration (0.0270 M, b). 13C NMR chemical shifts depend on [1]. In particular, the peaks relative to CO2, NCH2, NCH3, and CO are shifted upfield by the increase of [1], whereas those of 3-CH2, 10-CH2, 11-CH2, and 12-CH3 are shifted downfield. The (6) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975.
micellization is accompanied by a change from gauche to trans conformation in the chain that causes a downfield shift.7 For the head group, changes in chemical shifts at the micellar surface should also depend on micellar head group interactions and hydration.8 For the head group signals, changes of 1-E chemical shift with respect to [1] (∆δE), reported in Table 2, are smaller than those of 1-Z (∆δZ). For the tail signals changes are similar and, when they differ, they are slightly larger for 1-Z. ∆δE and ∆δZ are smaller for 12-CH3 and 3-CH2 and larger for carbon nuclei in the middle of the hydrophobic chain. Even if assignment was not made, the signals relative to overlapped 4-CH2-9-CH2 peaks show a downfield shift, of about 1 ppm. This effect is in agreement with the chain defolding due to aggregation, which is more relevant for central carbon atoms.9 In the 13C NMR spectra every resolved carbon nucleus causes two signals, at least in a range of concentration. The signals relative to overlapped 4-CH2-9-CH2 peaks, reported for [1] ) 0.0630 in Figure 3, are very interesting. Even if they are not well resolved, it is clear that they are produced by more than six nuclei; by assuming that the higher peaks are caused by more than one nucleus, it is possible to reckon twelve signals. This confirms that each carbon atom produces two peaks. For [1] just above the cmc, the 13C NMR signal of 12CH3 is formed by two peaks and 1-Z is upfield with respect to 1-E; at high [1] the situation is reversed and 1-Z is downfield with respect to 1-E. That means that in a range of concentration the two peaks have the same 13C NMR chemical shift, under our experimental conditions; this equivalence is also present in 1H NMR spectra, when the two signals coincide and only a triplet is observed. Another interesting observation is the lack of NCH2 and CO2 signals in a range of concentration. In particular, (7) Grant, D. M.; Cheney, B. V. J. Am. Chem. Soc. 1967, 89, 5315. (8) Ahlnas, T.; Soederman, O. Colloids Surf. 1984, 12, 125. (9) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G.; Bunton, C. A. J. Colloid Interface Sci. 1993, 160, 85.
Notes
Langmuir, Vol. 13, No. 17, 1997 4769
Figure 3. 13C NMR spectrum of 4-CH2-9-CH2 groups of 1 in D2O at 25 °C. [1] ) 0.0630. Scheme 1
they broaden at [1] > 0.0243, disappear into the noise, and sharpen again at increasing [1]. In our opinion this experimental feature is in agreement with Scheme 1, in which the equilibria involved in this system are reported. In general, the equilibria between 1-E and 1-Z (1 and 4 in Scheme 1) are slow, and those between monomer and micelle (2 and 3 in Scheme 1) are fast with respect to the NMR time scale. Only equilibrium 1 is present at [1] < cmc, while equilibrium 4 becomes more important with increasing [1]. At the concentration in which 1-E starts to become consistently predominant with respect to 1-Z, there is the occurrence of equilibrium 3. The formation of 1-Emic decreases the concentration of 1-Emon from equilibrium 1 that is accelerated. As a consequence, we
observe the broadening of the signals. On the other hand, with increasing [1], 1-Z also aggregates and, with the occurrence of equilibrium 4, the signals sharpen. The fact that each carbon nuclei produces two signals does not depend on the amide bond conformation because this effect should not be transmitted along the tail, but we rather attribute it to the presence of two separated domains; i.e., the two isomers micellize on the basis of a different stereochemical code in conformations that give two different NMR spectra. In fact, in a nonmicellizing solvent, such as CD3OD, and in D2O at concentrations below the cmc, 13C NMR spectra do not reveal the splitting in the chain carbon atoms from 4-CH2 to 12-CH3, showing that in the monomer the head group conformation does not control the conformation of the hydrophobic tail (Figure 2a). On the other hand, under aggregating conditions, the two isomers organize differently because each of them recognizes different stereochemical information. Because they are in conformational equilibrium, the recognition is able not only to select but also to induce the isomer that yields the more stable domain. It is impossible to discriminate if we are in the presence of distinct regions of 1-E and 1-Z within a micelle or of 1-E and 1-Z micelles. In the case of distinct regions within the same aggregate, we could make the hypothesis that the two domains are located according to the steric hindrance of the head group in a nonspherical aggregate; i.e., the two isomers could be located in regions which differ for the curvature radius. The presence of domains and the fact that the percentage of 1-E increases with increasing [1] are a phenomenon of molecular recognition that, according to Lehn, “can be defined as a process involving both binding and selection of substrate”.10 Acknowledgment. Support of this work by CNR (Progetto Strategico Beni Culturali, Roma) and the MURST is gratefully acknowledged. Supporting Information Available: Tables showing the effect of the concentration of 1 on 1H and 13C NMR chemical shift and on the ratio [1-E]/[1-Z] (5 pages). Ordering information is given on any current masthead page. LA962136+ (10) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1988, 27, 89.
