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One More Extreme near the Critical Micelle Concentration: Optical Activity Anatoly I. Rusanov* and Aleksandr G. Nekrasov Mendeleev Center, St. Petersburg State University, 199034 St. Petersburg, Russian Federation Received June 21, 2010. Revised Manuscript Received July 20, 2010 It is reported the discovery of optical activity of micellar solutions of sodium dodecyl sulfate, cetyltrimethylammonium bromide, and poly(oxyethilene)(4)dodecyl ether as typical representatives of anionic, cationic, and nonionic surfactants. The surfactants exhibit dextrorotation. Since the optical activity of the ionic surfactants appears only above the Krafft point, it should be ascribed to micellar aggregates. Optical rotation as a function of the surfactant concentration displays a pronounced maximum near the critical micelle concentration (the subsequent decrease of optical activity corresponds to formation of spherical micelles that cannot possess optical activity by symmetry). For sodium dodecyl sulfate, the maximum correlates well with extremes of other properties of surfactant solutions (foaminess, foam stability, Gibbs’ and transversal elasticity of thin foam films), which was described earlier. Naturally, films and foams of sodium dodecyl sulfate solutions also demonstrate optical activity.
In 2004, we (together with the late Professor Krotov) reported a maximum of foaminess of the sodium dodecyl sulfate (SDS) aqueous solution near the critical micelle concentration (CMC) and accompanying maxima of some properties of films and foams prepared from the SDS solution: the film elasticity, the foam stability, the transversal elasticity modulus, and the thin film thickness as functions of concentration at a given disjoining pressure.1 All the maxima were located at the same concentration (6.9 mol/m3), which was close to but a bit smaller than the CMC at 20 °C (ranging within 7.6-8.25 mol/m3 according to the literature data2). This time, we present one more case of an extreme at the same concentration, a maximum of optical activity (i.e., optical rotation). The experiments were carried out with the same SDS (Fluka, >99% grade) as used earlier.1 As is well-known, SDS does not possess optical activity, but it appears in the course of micellar aggregation. Figure 1a shows the dependence of the specific rotation of SDS [R] on the SDS concentration in aqueous solution at 20 °C. The maximum effect is observed very close to concentration 6.9 mol/m3 that was reported earlier for other properties. Those properties have only a little in common with optical activity, but one can say that all kinds of activities fixed occur within the range of a maximum disorder in a micellar system. Near the CMC, the process of aggregation of surfactant ions proceeds intensively, with the formation of multiply sized and shaped aggregates. Some of them seem to be chiral and exhibit optical activity. As the aggregates become complete spherical micelles, the optical activity decreases and almost disappears since spherical bodies cannot possess optical activity by the symmetry condition. To confirm that the optical activity observed was related to the aggregation process, we repeated the experiment below the Krafft point (ranging within 8-16 °C2) where micellization cannot occur. No optical activity was observed at 7 °C. The above experiment was carried out with the natural light. To verify the influence of the light wavelength, the experiment was performed with monochromatic beams of the wavelengths of *To whom correspondence should be addressed. E-mail: rusanov@ar1047. spb.edu. (1) Rusanov, A. I.; Krotov, V. V.; Nekrasov, A. G. Langmuir 2004, 20, 1511. (2) Van Os, N. M.; Haak, J. R.; Rupert, L. A. M. Physico-Chemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: Amsterdam, 1993.
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0.45, 0.48, 0.57, and 0.61 μm. The results (presented this time, by necessity, in terms of the absolute optical rotation R) are shown in Figure 2. For all the wavelengths, the maxima of optical activity were observed at the same concentration (close to 6.9 mol/m3), and the maxima heights obey Biot’s law predicting the optical rotation to be roughly inversely proportional to the square of the wavelength. This also confirms that we deal with the ordinary optical activity. However, the concentration dependence of the optical rotation is quite unusual and contradicts Biot’s law that requires optical activity to be proportional to concentration. The explanation is simple: Biot’s law deals with ordinary not associated solutions containing only a single kind of solute particle, whereas a micellar system contains particles of various size, shape, and composition. In addition, Biot’s law implies c to be the concentration of an optically active substance, whereas we operate with the total concentration of the solute (the amount of an optically active fraction of matter remains unknown). As a result, Biot’s law (concerning its part related to concentration) turns out to be inapplicable to an aggregative system. The optical effect can be also observed by measuring the output light intensity I at the fixed crossed position of the Nicols. Naturally, the intensity I is very small in this case as compared with the input light intensity I0. Nevertheless, the intensity I is reliably measurable and variable due to rotation of the polarization plane at changing concentration. Figure 3 shows the dependence of I/I0 on the SDS concentration at 20 °C. We again see a maximum at the same place as it was found for other properties. Naturally, films and foams formed from SDS aqueous solutions also exhibit optical activity. Figure 4 illustrates the optical activity of a single flat film of SDS solution of a thickness of about 200 nm at concentration 6.9 mol/m3 corresponding to the maximum optical activity of the solution. Diminishing the angle j between the light beam and the film plane leads to increasing the light path length inside the film, which causes an increase in the optical rotation unless the light spot on the film becomes larger in area than the film itself (Figure 4). We also tested foam formed from the SDS solution of the same concentration with the following structural characteristics: average cell radius, 1 mm; phase volume ratio, 2280; foam film thickness, 35 nm. Placing the foam layer of a thickness of 15 mm between the polarizer and the analyzer crossed (with zero output light intensity without the
Published on Web 07/27/2010
DOI: 10.1021/la102514a
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Figure 1. Specific rotation [R] versus concentration for the aqueous solutions of SDS at 20 °C (a) and CTAB at 30 °C (b).
Figure 2. Optical activity of the SDS solution using monochromatic light of wavelengths 0.45 (1), 0.48 (2), 0.57 (3), and 0.61 μm (4).
Figure 3. Relative light intensity versus concentration for the SDS solution at 20 °C.
foam layer) produces the light intensity I/I0=1/58. This shows the foam to be capable of optical activity. As SDS is a typical example of anionic surfactants, we also tested cetyltrimethylammonium bromide (CTAB, Acros, >99% grade), representing the class of cationic surfactants, for optical activity. The experiment carried out at 20 °C found no optical activity, and this was natural since the Krafft point of CTAB is 26 °C.2 The result obtained for the CTAB aqueous solution at 30 °C (using natural light) is shown in Figure 1b. Comparing Figure 1a and b, we see a great similarity. The only difference is that the maximum in Figure 1b lies to the right from the CMC. Since the CTAB CMC database for 30 °C is poor (only one source2), we verified the CMC value using the conductivity method, 13768 DOI: 10.1021/la102514a
Figure 4. Relative light intensity versus the angle j between the light beam and the film surface for the film of a thickness of 200 nm formed from the SDS solution of concentration 6.9 mol/m3 at 20 °C.
but obtained practically the same result (0.94 versus 0.87 mol/m3; see Figure 1b). As for the mutual positions of the maximum and the CMC value, it may be related to the definition of the CMC itself. It follows from the theory of micellization3 that the CMC corresponds to the beginning of micellization and refers to the earlier stage of micellization, the larger the micelle size is. Since the hydrocarbon tail of the CTAB molecule is longer than that of SDS, CTAB micelles are larger and their CMC corresponds to the earlier stage of micellization than in the case of SDS. However, this all is of no principal significance, and another matter is more important. As is seen from Figure 1a and b, the formation of spherical micelles is completed well above the CMC (at about 15 mol/m3 for SDS and at about 8 mol/m3 for CTAB), so that not only concentrations below the CMC but also a certain concentration region above the CMC can be regarded as corresponding to the premicelle state. Passing to nonionic surfactants, we have to expect a considerably smaller optical activity simply from the fact that micellization occurs at concentrations by 2-3 orders of magnitude smaller than for the case of ionic surfactants. Thus, optical activity can be observed if it is sufficiently high. We illustrate this with the solution of polyoxyethylene(4)mono-n-dodecyl ether (Acros, 99% grade), which is a popular nonionic also known as Brij-30. To show what measurable quantities we deal with, Figure 5 exhibits the variation of absolute (not specific) optical rotation with concentration (R should be multiplied by 1.25 104 to pass to the specific optical rotation [R]). A sharp maximum of R corresponds to the (3) Rusanov, A. I. Micellization in Surfactant Solutions; Russian Scientific Reviews Series, Chemistry Reviews, Vol. 22; Gordon and Breach: Reading, 1996.
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Figure 5. Optical rotation R versus concentration for the aqueous solution of Brij-30 at 20 °C.
concentration 13 g/m3 and lies within the CMC region ranging2,4 from 12.73 to 23.2 g/m3 (our value is 13.85 g/m3; a large scatter of experimental data for nonionics is known to originate from the difficulty of their purification). Figure 5 is quite analogous to Figure 1 and confirms that the optical activity of micellar aggregates can happen for any type of surfactant. However, we earlier tested solutions of dodecanoyl-N-methylglucamide (which is also a colloidal nonionic surfactant) and found no optical activity. Therefore, we can say that micellar optical activity is not a universal phenomenon and depends on the nature of a system, not speaking about the possible role of admixtures. Since we ascribed optical activity to micellar aggregates, the question can arise concerning the possible role of surfaces (a surfactant monolayer at the free solution surface or a liquidcrystalline layer at the solution/solid boundary). To elucidate the situation, we checked the proportionality between the optical rotation and the light path length inside the surfactant solution. The proportionality was found to be fulfilled perfectly, and this shows that all what we observed referred to the solution bulk but not to the surfaces. In this way, the dominant role of micellar aggregates was confirmed. There are two aspects in this work. First, it contributes to the problem of chirality that has been known in chemistry for a long (4) Dar, A. A.; Rather, G. M.; Das, A. R. J. Phys. Chem. B 2007, 111, 3122.
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time. The phenomenon we observed refers to supramolecular chirality, that is, chirality at the supramolecular level, which can originate from molecular chirality as its continuation or can arise from achiral components as a result of their interaction.5 The expression of supramolecular chirality in surfactants,6 micelles7 (composed of chiral monomers), and self-assemblies in solutions was described and reviewed8 in the literature. In all cases, however, they considered so-called “noncovalent synthesis”, that is, a binding process leading to the formation of chemical structures, for instance, hydrogen-bonded assemblies. We dealt with the process of another nature, the simple micellar aggregation of an achiral surfactant caused by the hydrophobic effect, and we met the fact that the hydrophobic effect is also capable of creating chiral structures. This kind of supramolecular chirality can be called aggregative chirality. A similar phenomenon (but with another nature of forces) was investigated at crystallization9 (crystalline nuclei are similar to micelles in some respects) where the role of cryptochiral (undetectable) admixtures in the origin of chirality was discussed. In our case, there were no detectable optically active impurities, but, naturally, the presence of cryptochiral admixtures cannot be excluded. The second aspect concerns colloid science. We suggest the optical rotation as a new tool for investigating micellization processes, which allows us to determine experimentally the concentration of complete formation of spherical micelles. This concentration is significantly different from the CMC and is a new characteristic of micellization. Unfortunately, the method is not universal since it implies the optical activity of aggregates, which does not exist in every micellar solution. Acknowledgment. This work was supported by the Presidential program of support of leading Russian scientific schools (Grant 6291.2010.3). (5) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, 1995. (6) Sommerdijk, N. A. J. M.; Lambermon, M. H. L.; Feiters, M. C.; Nolte, R. J. M.; Zwanenburg, B. Chem. Commun. 1997, 1423. (7) Roy, S.; Das, D.; Dasgupta, A.; Mitra, R. N.; Das, P. K. Langmuir 2005, 21, 10398. (8) Mateos-Timoneda, M. A.; Crego-Calama, M.; Reinhoudt, D. N. Chem. Soc. Rev. 2004, 33, 363. (9) Viedma, C. Cryst. Growth Des. 2007, 7, 553.
DOI: 10.1021/la102514a
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