Article pubs.acs.org/Langmuir
Generation of a Chiral Giant Micelle Thiago H. Ito,† Airton G. Salles,‡ Jacks P. Priebe,† Paulo C. M. L. Miranda,‡ Nelson H. Morgon,† Dganit Danino,§ Giovanna Mancini,∥ and Edvaldo Sabadini*,† †
Department of Physical-Chemistry, Institute of Chemistry, and ‡Department of Organic Chemistry, Institute of Chemistry, University of Campinas, P.O. Box 6154, Campinas, SP 13084-862, Brazil § Departament of Biotechnology and Food Engineering, Technion−Israel Institute of Technology, 32000, Haifa, Israel ∥ CNR−Istituto di Metodologie Chimiche, Via Salaria km 29.300, Monterotondo Scalo, 00016 Rome, Italy ABSTRACT: Over the past few years, chiral supramolecular assemblies have been successfully used for recognition, sensing and enantioselective transformations. Several approaches are available to control chirality of discrete assemblies (e.g., cages and capsules), but few are efficient in assuring chirality for micellar aggregates. Optically active amino acid-derived surfactants are commonly used to generate chiral spherical micelles. To circumvent this limitation, we benefited from the uniaxial growth of spherical micelles into long cylindrical micelles usually called wormlike or giant micelles, upon the addition of cosolutes. This paper describes the unprecedented formation of chiral giant micelles in aqueous solutions of cetyltrimethylammonium bromide (CTAB) upon increasing addition of enantiopure sodium salt of 1,1′-bi-2-naphthol (Nabinaphtholate) as a cosolute. Depending on the concentrations of CTAB and Na-binaphtholate, chiral gel-like systems are obtained. The transition from spherical to giant micellar structures was probed using rheology, cryo-transmission electron microscopy, polarimetry, and electronic circular dichroism (CD). CD can be effectively used to monitor the incorporation of Nabinaphtholate into the micelle palisade as well as to determine its transition to giant micellar structures. Our approach expands the scope for chirality induction in micellar aggregates bringing the possibility to generate “smart” chiral systems and an alternative asymmetric chiral environment to perform enantioselective transformations.
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INTRODUCTION The confined environment within chiral supramolecular assemblies has been shown to impart notable selectivity and has created new avenues for chemists to explore it for enantioselective guest recognition and sensing1−7 as well as regio- and stereoselective transformations.8−14 Nevertheless, the highly dynamical character of noncovalent interactions makes the synthesis of such assemblies a tricky task.15 Diverse approaches to their construction are available to the supramolecular chemists, and these strategies rely upon properties such as reversibility, self-correction, and self-recognition in order to guarantee the formation of chiral assemblies.16 Whereas the use of such approaches to control chirality of discrete supramolecular assemblies (e.g., cages and capsules) is well developed,17−22 controlling chirality of micellar aggregates remains elusive. To the best of our knowledge, chiral micelles are obtained solely from optically active amino acid-derived surfactants23−25 or diffusion of amino acid molecules onto the Stern layer of preformed micelles.26 Here we induce and control the uniaxial growth of spherelike micelles by the addition of cosolutes forming long cylindrical micelles, usually called wormlike micelles (WLMs) or giant micelles.27,28 Such an approach brings about broader opportunities to generate © XXXX American Chemical Society
chiral micellar aggregates once cosolutes possessing other functionalities than amino acids are used. Additionally, the entanglement of the micellar chains results in supramolecular and chiral gel-like systems. Herein, we describe the unprecedented formation of chiral giant micelles in aqueous solutions of cetyltrimethylammonium bromide (CTAB) upon increasing addition of enantiopure sodium salt of 1,1′-bi-2-naphthol (Na-binaphtholate) as a cosolute (Figure 1). The key feature of this system relies on atropoisomerism of the C2-symmetric 1,1′-bi-2-naphthol which assures the induction of chirality in the micellar aggregates. The giant micelles were characterized by rheology and cryotransmission electron microscopy (Cryo-TEM). Moreover, changes observed by polarimetry and electronic circular dichroism may be used to probe the threading of Nabinaphtholate into the CTAB micelles and their further transition to giant micelles. Received: June 27, 2016 Revised: August 4, 2016
A
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Figure 1. Formation of chiral giant micelles upon the addition of enantiopure Na-binaphtholate to spherical micelles of CTAB. Na-binaphtholate micelles elongate, going into the giant micellar regime.
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EXPERIMENTAL SECTION
hydroxyl groups permits the electrostatic balance between negatively charged oxygen and positively charged headgroup of CTA. Additionally, with the ionization, the naphthyl rings could be maintained at the micellar interface, assuring their appropriated positioning into the micelle palisade to promote the cation−π interaction. According to Mahadevi et al.,30 this interaction is quite favorable to stabilize many chemical structures and even giant micelles.31 Given that cationic surfactants have been used previously to produce giant micelles, CTAB was an attractive choice as a surfactant and a proper starting point for these studies. Evidence of giant micelles formation in the dilute regime, produced by aqueous solutions of CTAB and Na-binaphtholate, is obtained when the solutions are swirling and the movement of the bubbles trapped is followed. In some [CTAB]/[Na-binaphtholate] proportions, due to the viscoelastic character of the WLMs solution, the bubbles move in an opposite trajectory (backward) when the swirling of the vial is interrupted.32 Highly viscoelastic solutions can be obtained in the semidilute regime, as shown in the photos of Figure 2, taken as soon as the vials were inverted. While the solution with 100 mmol L−1 CTAB and 10 mmol L−1 Na-binaphtholate flow readily, the flow is quite slow for 15 or 25 mmol L−1 of the aromatic cosolute. The highly viscoelastic character in these two cases is due to the entanglement of the chains of the giant micelles.
Chiral WLM Preparation. The surfactant cetyltrimethylammonium bromide (CTAB) and 1,1′-bi-2-naphthol (R and S) were obtained from Sigma-Aldrich. They were used as received. Solutions of CTAB and 1,1′-bi-2-naphthol (R and S) were prepared using ultrapure water (18 MΩ cm) and conditioned at pH 10 by using NaOH 0.1 mmol L−1, in order to obtain the sodium salt of 1,1′-bi-2-naphthol (Na-binaphtholate). WLMs (wormlike micelles) solutions were then prepared by mixing solutions of CTAB and Na-binaphtholate. Rheological Measurements. Flow curves were obtained on a Haake RS1 rheometer equipped with a water bath and plate−plate sensor (35 mm diameter and 1 mm gap). The temperature was maintained at 25 °C. Cryo-Transmission Electron Microscopy (Cryo-TEM). Samples for Cryo-TEM were prepared in the Vitrobot (FEI, Netherlands) at 25 °C and at saturation. Drops of ∼6 μL of the solutions were placed on a 400-mesh TEM copper grid covered with a perforated carbon film. The excess of the solutions was removed, in order to produce a thin liquid film and were held on the grid for 10 s to allow structures relaxation from shearing effects caused by the blotting. The samples were then plunged into liquid ethane (−183 °C) to form vitrified specimens and transferred to liquid nitrogen (−196 °C) for storage. Vitrified specimens were examined at temperatures below −175 °C using a Gatan 626 cryo holder in a Tecnai T12 G2 TEM (FEI, Netherlands) operating at 120 kV. Images were recorded on a Gatan UltraScan 1000 using the DigitalMicrograph software (Gatan, U.K.). Polarimetry. The optical rotation angle of Na-binaphtholate solutions in different concentrations, with and without CTAB (100 mmol L−1), were obtained with a PerkinElmer polarimeter using a lamp with a wavelength of 365 nm and a cell with 100 mm optical path. The measurements were obtained at 25 °C. Electronic Circular Dichroism. The ellipticity, θ, of the Nabinaphtholate solutions in different concentrations with and without CTAB (100 mmol L−1) were measured in a spectrometer JASCO model J-720 with a xenon lamp of 450 W in the range between 300 and 420 nm. A cuvette with 1 mm of path length was used and the sample was maintained at 25 °C.
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RESULTS AND DISCUSSION Initially, solutions of giant micelles were prepared following previous reports of our group29 via subcomponents assembly of CTAB and enantiopure Na-binaphtholate. We have chosen (S)- and (R)-Na-binaphtholate as chiral cosolutes for this study. We envisaged that C2-symmetric structures would reduce the number of different arrangements in the micelle palisade when compared to nonsymmetrical structures possibly having a beneficial effect on the induction of chirality. Binaphtholate has the required prerequisites to induce micellar growth in cationic micelles. The ionization of the
Figure 2. Gravitational effect on solutions containing 100 mmol L−1 CTAB and 10, 15, and 25 mmol L−1 (R)-binaphtholate. The photo was taken as soon as the vials were inverted. B
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= 5 mmol L−1 with [(R)-binaphtholate] = 1.0 mmol L−1 only globular micelles are observed. However, for the same CTAB concentration, but combined with 1.7 mmol L−1 (R)binaphtholate, multiconnected WLMs are obtained. The ratio [CTAB]/[(R)-binaphtholate] in this case is ≈3. To investigate the behavior of Na-binaphtholate enantiomers in aqueous solutions and in CTAB (100 mM) micellar solutions, the specific rotation at 589 nm was measured (Figure 5). Measurements showed distinct behavior of Na-
Variations of the zero-shear viscosity (η0) of solutions containing 100 mmol L−1 CTAB and increasing concentrations of (R)- and (S)-binaphtholate, as a function of binaphtholates concentrations, are shown in Figure 3. The results indicate that
Figure 3. Plot of the variation of the zero shear viscosity for 100 mmol L−1 CTAB and increasing concentrations of (R)- or (S)-binaphtholate. The measurements were carried out at 25 °C and at pH 11. The lines are only guides for the eyes. Figure 5. Observed optical rotation of (R)- and (S)-Na-binaphtholate as a function of binaphtholate concentration (from 0 to 25 mmol L−1) for only (R)- or (S)-Na-binaphtholate and (R)- or (S)-Nabinaphtholate with 100 mmol L−1 of CTAB. The temperature was maintained at 25 °C and pH 11. The solid lines were included to indicate the deviation from the linearity for (R)- or (S)-Nabinaphtholate with CTAB above 10 mol L−1.
η0 rapidly increases when Na-binaphtholate concentration is higher than 10 mmol L−1, which is associated with the onset point of WLMs formation. Beyond 20 mmol L−1, η0 decreases as predicted by Rehage−Hoffmann33 for WLMs formed by a combination of a cationic surfactant and an anionic cosolute. Interestingly, one maximum only is found for η0, as typically is found with catanionic mixtures34,35 or cationic mixtures with simple salts,36 and not two peaks as is characteristics for cationic surfactants with hydrotropes.33 For concentrations higher than 25 mmol L−1, two aqueous biphasic systems are obtained. Additionally, the curves of Figure 3 coincide, as expected, independent if the giant micelle is formed with (R) or (S) isomers, or even with a racemic binaphtholate mixture (not shown). Cryo-TEM images recorded under low dose conditions (Figure 4) of solutions in the viscoelastic region provide another evidence of WLMs.37 In the image on the top, [CTAB]
binaphtholate enantiomers in the presence of CTAB. Nabinaphtholate enantiomers in CTAB micellar solutions were found to produce an inversion in the sign of specific rotation when compared to aqueous solutions. It has been long established that significant variations in specific rotation of chiral biaryl compounds would take place under a conformational change from s-trans to s-cis and vice versa.38−40 In aqueous solution, Na-binaphtholate enantiomers would adopt s-trans conformation in order to decrease electron repulsion between negatively charged oxygen atoms39,40
Figure 4. Cryo-TEM images of vitrified solutions with [CTAB] = 5 mmol L−1 and [(R)-Na-binaphtholate] = 1.0 mmol L−1 (left) and 1.7 mmol L−1 (right). C
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Figure 6. CD spectra as a function of (R)- and (S)-Na-binaphtholate concentration, in absence and in the presence of CTAB (100 mM). The measurements were carried out at 25 °C and pH 11. Band change around 325 nm from monosignate to bisignate in CTAB micellar solution (at 10 mmol L−1) provides an insight into the transition into the giant micelles regime.
some relevant points concerning the formation of giant micelles can be considered. In a simple consideration, two opposite forces drive the formation of a micellar aggregate: the hydrophobic force, in which the surfactant molecules are pulled out from the aqueous phase into the micellar pseudophase, and working in an opposite way there is the headgroup Coulombic repulsion.41 The aggregates architecture depends on the packing of the surfactant molecules at the micellar surface and a model was proposed by Mitchel and Ninham42 and Israelachvili43 to describe the aggregates. According to this model, a critical packing parameter (cpp) related to the headgroup area (a0), the extended length (lc), and the volume (v) of the hydrophobic part of the surfactant molecule (see eq 1 in Figure 7) indicates the (predominant) architecture of the aggregates. Spherical ionic micelles of CTAB can be transformed to WLMs by changing cpp (which assumes values between 0.33 and 0.5) through the insertion of the ring of the aromatic anions into the micellar palisade layer, leading to screening of the surfactant headgroup charge and decreasing a0. A giant micelle has two characteristic regions: one, with low curvature (the cylindrical body), and another with high curvature (the end-caps).44 The electrostatic interactions between the ionic surfactants reduce the scission energy and thus contribute to the formation of end-caps, favoring the formation of spherical instead of cylindrical aggregates. However, the electrostatic neutralization by the counterions (for example binaphtholate) favors micellar growth.45 As represented in Figure 1, binaphtholate penetrates into the spherical micelles of CTAB when its concentration is lower than 10 mmol L−1. Possibly some micellar asymmetry is
whereas in CTAB micellar solutions Na-binaphtholate enantiomers would adopt s-cis conformation leading to the observed inversion in the sign of specific rotation. We hypothesize such conformational change is due to threading of Na-binaphtholate into the spherelike micelle palisade wherein negatively charged oxygen atoms would be exposed to the aqueous phase and hydrophobic naphthyl rings would penetrate into the micelles. We also observed a deviation from linearity for the curve of specific rotations as a function of Nabinaphtholate concentration at 10 mmol L−1 (Figure 5). Interestingly, as described above (Figure 3), transition from spherelike micelles to giant micelles occurs at this concentration, suggesting such deviation might be related to the onset of giant micelles. These results prompted us to explore further the transition into the giant micellar regime by circular dichroism spectroscopy (CD). Changes in the CD signal also probe the transition of CTAB micelles from spherical to giant (Figure 6). In the 10−15 mmol L−1 concentration range, the shape of the band at ca. 325 nm changes from a monosignate to a bisignate. Further in the 15− 25 mmol L−1 concentration range, the intensity of the first Cotton effect (at longer wavelength) decreases while that of the second Cotton effect (at shorter wavelengths) increases. This result indicates the occurrence of intermolecular chromophore coupling at concentrations of Na-binaphtholate >10 mmol L−1, thus suggesting that the transition from spherical to giant micelles involves a change in the arrangement of Nabinaphtholate within the aggregates. We did not observe any evidence for helical chirality in these aggregates. In order to understand the correlation between the morphological change and the variation in the CD signal, D
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L−1) follows the onset of the giant micellar regime. Our results also demonstrate the use of a cosolute other than amino acids to generate chiral micelles, expanding the scope to generate chiral micellar aggregates. The entanglements of the chains of the giant micelles produce chiral gel-like system in the semidilute regime. We envisage that the possibility to generate and control gelation-induced supramolecular chirality will allow the development of “smart” chiral materials such as chiroptical devices and chiral sensors.49 Additionally, we anticipate a chiral version of giant micelles may provide an asymmetric environment to perform enantioselective reactions, and their topological features possibly will have an impact on the kinetics of the reaction. Further investigations are ongoing to extend the nature of chiral cosolutes able to generate giant micelles, thus adding broader scope to our approach.
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Figure 7. Critical packing parameter (cpp) according to the model proposed by Mitchel and Ninham42 and Israelachvili.43
AUTHOR INFORMATION
Corresponding Author
*E-mail
[email protected] (E.S.). Author Contributions
induced due to the incorporation of this cosolute at this stage. However, the number of binaphtholate per micelle is low, and only small clusters can be formed inside this aggregate. Above 10 mmol L−1, a large amount of binaphtholate is incorporated into the aggregates, leading to lowering of the energy of the surfactant molecules at the cylindrical region relative to the end-caps. Therefore, the spherical micelles transition to cylindrical aggregates, minimizing the energy of the end-caps. The melting mechanism of spherical to giant micelles was recently described by Jensen et al., considering an analogy to catalyzed stepwise polymerization reactions.46 By using isothermal titration calorimetry, Ito et al. showed that at the spherical to giant micelles transition a large amount of aromatic anions is incorporated into the micellar palisade of the cationic micelle.47,48 The results revealed a characteristic sharp exothermic signal at the transition, characterizing a highly cooperative process. Possibly the energy released in this process has a large contribution from cation−π interaction. Therefore, the density of binaphtholate molecules in the giant micelles is relatively high. The positioning of the binaphtholates at the micellar interface and the relative long micellar persistent length allow the coupling between the vicinal aromatic molecules. Two hypotheses can explain the occurrence and evolution of bisignate CD bands. According to the first hypothesis Na-binaphtholate ions are randomly distributed at the interface, and coupling occurs between vicinal chromophores. By increasing the chromophore concentration, molecular packing changes, thus affecting the coupling itself. As a second more reliable hypothesis, Na-binaphtholate segregates, at high concentration, on the surface of the aggregates, also due to π−π interaction, and forms clusters whose shape and size evolve as a function of concentration.
T.H.I. and A.G.S. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Fundaçaõ de Amparo à Pesquisa do Estado de São Paulo (FAPESP, São Paulo, Brazil) and the Conselho Nacional ́ ́ de Desenvolvimento Cientifico e Tecnológico (CNPq, Brasilia, Brazil) for financial support and fellowships. D.D. acknowledges the support of the ISF (grant 741/11) and the Russell Berrie Nanotechnology Institute (RBNI). E.S. acknowledges the support of CNPq (grant 301016/2015-1).
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REFERENCES
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CONCLUSIONS We showed the use of enantiopure sodium salt of 1,1′-bi-2naphthol in the presence of CTAB to generate unprecedented chiral giant micelles. A transition from spherical to giant micellar structures was probed using rheology, Cryo-TEM, polarimetry, and circular dichroism. In the polarimetry, deviation from linearity for the curve of specific rotations of micellar solutions above 10 mmol L−1 indicates the transition. In circular dichroism, a band change at ca. 325 nm from monosignate to bisignate in micellar solutions (at 10 mmol E
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