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Wormlike Micelles from a Cage Amine Metallosurfactant George A. Koutsantonis* and Gareth L. Nealon Chemistry, School of Biomedical, Biomolecular and Chemical Sciences, UniVersity of Western Australia, 35 Stirling Highway, Crawley, WA, Australia 6009
Craig E. Buckley and Mark Paskevicius Department of Applied Physics, Curtin UniVersity of Technology, GPO Box U 1987, Perth, WA, Australia 6845
Laurent Douce IPCMS-GMO, 23 rue du Loess, BP 43 Strasbourg, Cedex 2 67034, France
Jack M. Harrowfield Institut de Science et d’Inge´ nierie Supramole´ culaires, UniVersite´ Louis Pasteur, 8 alle´ e Gaspard Monge, Strasbourg 67083, France
Alasdair W. McDowall Institute for Molecular Bioscience and the Centre for Microscopy and Microanalysis, The UniVersity of Queensland, St. Lucia, QLD, Australia 4072 ReceiVed May 2, 2007. In Final Form: September 3, 2007 We have shown that copper and cobalt metallosurfactants derived from Cu(II) and Co(III) complexes of a macrobicyclic hexamine (“cage”) can form wormlike micelles in aqueous solution that may coexist with or easily interconvert with vesicle structures. The cylindrical micelle structures are unusual for triple-chain surfactants with a single headgroup and are not easily accounted for using geometrical packing arguments. The solution behavior has been characterized by cryo-TEM and SAXS measurements. Both the Cu and Co compounds display viscoelastic solutions at 1 wt %, indicating that such behavior may be anticipated for the full variety of stable metal complexes formed by the cage headgroup, auguring applications based on the incorporation of metallo aggregates into mesoporous silica structures.
Metallosurfactants are an emerging class of materials that offer interesting alternatives to traditional “organic” surfactants because of the range of properties inherent to complexed metal ions.1,2 The introduction of a metal ion center can impart magnetic and electronic properties as well as the redox and catalytic activity of the complex to the surfactant system, which of course can be concentrated at an interface, be it polar/apolar (e.g., micelles, vesicles), solid/liquid (e.g., monolayers), or liquid/gas (e.g., Langmuir-Blodgett films). Cationic surfactants have general applications such as biocidal agents,3 and there has been recent interest in their use as DNA delivery agents for gene therapy.4,5 Given the remarkable stability and variety of electronic, magnetic, and redox properties of metal complexes of the sarcophagines,6 these entities are of appeal for use as the cationic headgroup of a surfactant.7 Initial investigations of this prospect were based on a series of single- and double-chain derivatives * Corresponding author. E-mail:
[email protected]. Fax: (+61) 8 6488 7247. (1) Donnio, B. Curr. Opin. Colloid Interface Sci. 2002, 7, 371-394. (2) Griffiths, P. C.; Fallis, I. A.; Chuenpratoom, T.; Watanesk, R. AdV. Colloid Interface Sci. 2006, 122, 107-117. (3) Steichen, D. S. Cationic Surfactants. In Handbook of Applied Surface and Colloid Chemistry; Holmberg, K., Ed.; John Wiley & Sons: New York, 2002; Vol. 1, pp 309-348. (4) Mahato, R. I.; Rolland, A.; Tomlinson, E. Pharm. Res. 1997, 14, 853-859. (5) Zhang, S.; Xu, Y.; Wang, B.; Qiao, W.; Liu, D.; Li, Z. J. Controlled Release 2004, 100, 165-180. (6) Sargeson, A. M. Pure Appl. Chem. 1986, 58, 1511-22. (7) Sargeson, A. M. Coord. Chem. ReV. 1996, 151, 89-114.
of Co(III) sarcophagines, with activity against rat tapeworms and other parasites demonstrated for some of the complexes.8,9 Although these derivatives involved relatively long-chained substituents, there is also evidence that some aspects of surfactant behavior arise even with chains as short as C4.10 Compound 1 was synthesized via reaction of 3,4,5tris-dodecyloxy benzyl chloride with [Co(NH2)(CH3)sar]3+, giving the product as a waxy orange solid. This compound displays good solubility in common organic solvents and remarkable solubility in water as the acetate salt. (The use of other anions such as chloride, perchlorate, and triflate gave products that were completely insoluble in water.) The surfactant behavior of the complex was confirmed by conductivity measurements, which showed a clear break point between two linear curves, indicating a critical micelle concentration of (1.5 ( 0.3) × 10-5 M. This value was further supported by dye-micellization experiments and is comparable to those for other triple-chain ammonium amphiphiles.11 This might appear to be counterintuitive because the large headgroup charge (8) Behm, C. A.; Boreham, P. F. L.; Creaser, I. I.; Korybut-Daszkiewicz, B.; Maddalena, D. J.; Sargeson, A. M.; Snowdon, G. M. Aust. J. Chem. 1995, 48, 1009-1030. (9) Walker, G. W.; Geue, R. J.; Sargeson, A. M.; Behm, C. A. J. Chem. Soc., Dalton Trans. 2003, 2992-3001. (10) Harrowfield, J. M.; Koutsantonis, G. A.; Nealon, G. L.; Skelton, B. W.; White, A. H. Eur. J. Inorg. Chem. 2005, 2384-2392. (11) Kunitake, T.; Kimizuka, N.; Higashi, N.; Nakashima, N. J. Am. Chem. Soc. 1984, 106, 1978-1983.
10.1021/la701283b CCC: $37.00 © 2007 American Chemical Society Published on Web 10/19/2007
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Figure 1. (A) 1 wt % solution, negatively stained. (B) 10 wt % solution, negatively stained. (C) 1 wt % solution, cryo-TEM. (D) 10 wt % solution, cryo-TEM. Scheme 1. Synthesis of Surfactant Complexes 1 and 2
(3+) of the cobalt complex should inhibit micelle formation because of electrostatic repulsions, but this behavior is not uncommon to metallosurfactants and emphasizes the level of counterion association that occurs during micellization2 and may help to explain the aggregate structures formed. (See below.) The Cu(II) analogue of 1, complex 2 (Scheme 1), can be synthesized in a similar manner, and solutions of both 1 and 2 in water display viscoelastic behavior at concentrations e1 wt %, suggesting the presence of long cylindrical or “wormlike”
micelles.12 At 10 wt %, the solutions became extremely viscous, so much so that a sample of a solution could be inverted without any appreciable flow of the material for tens of seconds. To confirm the presence of wormlike micelles, solutions of 1 at 1 and 10 wt % were imaged by TEM using a negative staining technique employing uranyl acetate to enhance contrast. Representative images for the 1 wt % solution are shown in Figure 1, which clearly indicates the presence of long flexible (12) Hoffman, H.; Ebert, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 902-912.
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Figure 2. Cryo-TEM images. (A) 1 wt % solution showing vesicles. The arrow indicates cubic crystalline ice. (B) 1 wt % solution showing broken vesicles. Black arrows indicate bilayer fragments, and white arrows indicate wormlike micelles. (C) Wormlike micelles and vesicles coexisting. (D) 10 wt % solution, concentrated wormlike micelles or possibly a sponge phase.
filaments, consistent with the expected wormlike micelles. The start and endpoint of any one micelle is difficult to determine, but it is clear that the majority of the structures are hundreds of nanometers in length and most probably of micrometer dimensions. Images for the 10 wt % solution (Figure 1) indicate that worms are also present at this concentration and are more abundant. However, it remains difficult to determine the length of the micelles from these images, and it is not possible to determine if they are branched. Given that a staining agent and drying steps may modify the delicate solution structures present,13 we turned to cryogenic TEM14 (cryo-TEM) to confirm the presence of wormlike micelles. Imaging of the resultant vitrified solution allows the direct visualization of the trapped aggregates, without the use of any additives or drying steps.15,16 Cryo-TEM images at 1 wt % (Figure 1) showed that the dominant structures present were wormlike micelles, consistent with the negatively stained images. Again, the structures are highly tortuous, with lengths exceeding hundreds of nanometers, and measurements of the images indicated a (13) Kilpatrick, P. K.; Miller, W. G.; Talmon, Y. J. Colloid Interface Sci. 1985, 107, 146-158. (14) Dubochet, J.; McDowall, A. W. J. Microsc. 1981, 124, RP3-4. (15) Dubochet, J.; Adrian, M.; Chang, J.-J.; Homo, J.-C.; Lepault, J.; McDowall, A. W.; Schultz, P. Q. ReV. Biophys. 1988, 21, 129-228. (16) Almgren, M.; Edwards, K.; Karlsson, G. Colloids Surf., A 2000, 174, 3-21.
micellar diameter of ca. (6 ( 1) nm. Molecular models of the cation (Spartan, Wavefunction, California) indicated a fully extended molecular length of ∼30 Å, which is in good agreement with the expected value from the TEM images (given that two fully extended molecules placed end-to-end define the maximum diameter of a micelle). Vesicles were also observed at this concentration, often coexisting with the wormlike structures,17,18 and their general morphology was spherical with a range of diameters from ca. 30-300 nm. Exceptions to the spherical shape were also observed, with long elongated vesicles (up to ca. 1 µm in one dimension in some cases) occasionally observed (Figure 2). An interesting series of structures was also observed (in limited abundance) in which it appeared that some vesicles were in the process of being formed or were breaking apart, displaying open edges that were often accompanied by structures resembling wormlike micelles (Figure 2). At this point, it is worth pointing out that microstructural changes in solution aggregates of surfactants are known to occur occasionally during the sample preparation step in cryo-TEM, commonly believed to be a result of the shear applied to the sample during the blotting stage of the experiment.19,20 Images at 10 wt % (Figure 1) were difficult (17) Danino, D.; Talmon, Y.; Zana, R. Langmuir 1995, 11, 1448-1456. (18) Karukstis, K. K.; McDonough, J. R. Langmuir 2005, 21, 5716-5721. (19) Gonzalez, Y. I.; Kaler, E. W. Curr. Opin. Colloid Interface Sci. 2005, 10, 256-260.
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Figure 3. SAXS data from the 10 wt % solution. The unified fit to the data and its component levels are shown. Level 1 represents the scattering contribution from the headgroups, level 2 represents the scattering contribution from the surfactants within the micelles, and level 3 represents the scattering from whole micelles.
to interpret because of the high density of features present, but they appear to indicate a dense collection of criss-crossed wormlike micelles. The random pattern that results from this arrangement may indicate a local bilayer structure reminiscent of a “sponge” phase21 (Figure 2), and we cannot rule out this possibility at this stage. A common method for rationalizing the aggregate structures of surfactants is the use of a model based on geometrical packing constraints. This model uses a “critical packing parameter” of S ) V/al, where V is the volume of the tails, a is the effective headgroup area, and l is the extended length of the alkyl chains.22,23 Thus, a value of 1/3 < S < 1/2 indicates cylindrical micelles, and 1/2 < S < 1 indicates the existence of vesicles and flexible bilayers. These general principles can be applied to help explain the aggregation behavior of a wide range of surfactants. Examples can be found by considering the behavior of di-, tri-, and tetrameric surfactants that generally display markedly different and more varied behavior than do their monomeric counterparts and can be rationalized by considering the effect that the length of the spacer group has on the headgroup separations.24 The application of these principles is not limited to “organic” surfactants, as shown by the fact that the introduction of four alkyl groups into a Ru(II) metallosurfactant leads to the generation of reverse
vesicle and micelle structures in toluene and hexane, respectively.25 In the case of compounds 1 and 2, values of V and l can be calculated for three C12 tails,26 allowing the determination of the effective headgroup area a predicted for the aggregates, which gives values of 125 < a < 190 Å2 for cylindrical micelles and 125 < a < 65 Å2 for vesicles. Unfortunately, it is difficult to categorize the aromatic portion of the molecule as forming part of the “tail” or, as a result of the ether and amide functionalities, part of the headgroup. If the tail portion is defined as that volume that excludes water,23 then it is perhaps reasonable to exclude the aromatic ring from the calculation of the tail volume and length because the polar functional groups on the aromatic ring lend themselves to hydration. The estimated cross-sectional area of the cobalt cage headgroup is ca. 70-80 Å2,28 and thus using these geometric arguments, it might be expected that the vesicles are the preferred structures. The presence of significant electrostatic repulsions between Co(III)‚‚‚Co(III) headgroups, which might be heavily hydrated, would go some way toward increasing the effective headgroup area of the complex and thus would help explain the presence of the cylindrical micelles. However, drawing parallels with the extensive solid state investigations of the complexes of sarcophagines,27 it might be expected that the acetate anions would be readily accommodated within the headgroup
(20) Lu, B.; Li, X.; Scriven, L. E.; Davis, H. T.; Talmon, Y.; Zakin, J. L. Langmuir 1998, 14, 8-16. (21) Ponsinet, V.; Talmon, Y. Langmuir 1997, 13, 7287-7292. (22) Israelachvili, J. N.; Mitchell, D. J.; Ninham, B. W. J. Chem. Soc., Faraday Trans. 2 1976, 72, 1525-1568. (23) Tanford, C. J. Phys. Chem. 1972, 76, 3020-3024. (24) Zana, R. AdV. Colloid Interface Sci. 2002, 97, 205-253.
(25) Domı´nguez-Gutie´rrez, D.; Surtchev, M.; Eiser, E.; Elsevier, C. J. Nano Lett. 2006, 6, 145-147. (26) Hamley, I. W. Introduction to Soft Matter: Polymers, Colloids, Amphiphiles and Liquid Crystals; John Wiley and Sons: Chichester, England, 2000. (27) Harrowfield, J. M. Supramol. Chem. 2006, 18, 125-136. (28) Geue, R. J.; Hambley, T. W.; Harrowfield, J. M.; Sargeson, A. M.; Snow, M. R. J. Am. Chem. Soc. 1984, 106, 5478-5488.
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region, aided by extensive “H-bond chelation” by the coordinated NH groups, and in fact the concentration dependence of the NMR spectra of simple cages such as [Co(NH3)2sar]5+ in aqueous solution is thought to be caused by ion pairing with the anion.28 Unfortunately, drawing conclusions about the micelle structure at the molecular level based on these geometrical arguments is fraught with danger,29 particularly in light of the assumptions made above. The model assumes a fluidlike packing of the chains in the interior of the micelle, which might not be valid for these triple-chain amphiphiles,11 and directional attractive forces such as hydrogen-bonding networks that would act to stabilize the micelle are not accounted for. What is clear is that the conversion between the wormlike micelles and vesicles appears to be facile, which might indicate a relatively low energy barrier for their interconversion. Small-angle X-ray scattering (SAXS) was performed on 1 and 10 wt % solutions of 1 in order to obtain structural information from the micelles. The SAXS data was collected at relatively high q (0.039-0.85 Å-1) in order to analyze the very small structural features that are present within the micelles. Structural information exists over the entire q range studied by SAXS, as shown in Figure 3 where the pattern is dominated by two peaks that are indicative of structural correlations that exist within the micelles. Each SAXS pattern was collected for 3 h, but a moderate amount of noise exists in the data that is due to the subtraction of high background scattering from the capillary. However, the noise does not impact the data significantly because the structural features are easily resolvable. The data was modeled using the unified equation30 with the Irena package31 for Igor Pro (Wavemetrics, Oregon), as shown in Figure 3 for the 10 wt % solution. The unified model incorporates Guinier and power-law contributions to scattering as well as contributions from a structure factor for weak correlations. For the structure factor to be applied, the system must be consistent with weak correlations that can be verified via a fit to the data using the unified model. The system is considered to be weakly correlated when the structural packing factor is below a value of 4,32 which is true for the current system where packing factors of 3.8 and 1.2 were obtained for peaks at q ) 0.15 and 0.63 Å-1, respectively. There are three distinct regions present in the scattering data that have been assigned number levels in the unified model. These levels have been interpreted as observed scattering from different parts of the micelle structure. In this model, level 1 results from scattering from the small headgroup structures in the surfactants that are found to have a radius of gyration (Rg) of 3.7 ( 0.5 Å and an average headgroup-headgroup correlation distance of 9.7 ( 0.5 Å. Level 2 results from scattering from the whole surfactant units within the micelles that are found to have an Rg of 10.7 ( 0.5 Å and an average surfactant-surfactant correlation distance of 34.9 ( 0.5 Å. Level 3 results from the scattering from the whole micelles, but this region was only partially fit (assuming a smooth surface) because of the lack of data at low q. The surfactant-surfactant correlation distance is the average distance (29) Svenson, S. J. Dispersion Sci. Technol. 2004, 25, 101-118. (30) Beaucage, G.; Schaefer, D. W. J. Non-Cryst. Solids 1994, 172-174, 797-805. (31) Ilavsky, J. Irena, 2.10; Argonne, Illinois, 2005. (32) Beaucage, G.; Ulibarri, T. A.; Black, E. P.; Schaefer, D. W. Hybrid OrganicInorganic Composites. In ACS Symposium Series; Mark, J. E., Lee, C. Y.-C., Bianconi, P. A., Eds.; American Chemical Society: Washington, DC, 1994; Vol. 585, pp 97-111.
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between the centers of electron density for each of the surfactant units in a micelle. The diameter of the micelle can be approximated by using the surfactant-surfactant correlation distance (34.9 Å) as the average distance between two points on a circle (which is equal to one-quarter of the circumference),33 which gives a diameter of 45 ( 5 Å between centers of electron density. This value agrees only within experimental uncertainty with the cylindrical micelle diameter as observed by TEM because of the discrepancy between the surfactant correlation distance (between centers of surfactant electron density) given by SAXS and the tip-tip surfactant distance (micelle diameter) given by TEM. However, it is fair to say that the SAXS measurements, which were made with no sample preparation, provide the same results for micelle size as TEM, which was performed on samples that underwent some degree of sample preparation. It should also be noted that the SAXS pattern for the 1 wt % solution also showed the presence of a peak at q ) 0.15 Å-1 but less scattering and more noise were observed because of the lower solution concentration. The presence of a peak at the same q for both the 1 and 10 wt % solutions indicates the presence of the same structural correlations in both systems. The correlations are ∼35 Å, which have been attributed to the surfactant-surfactant correlations within the micelle structure. The presence of the same correlation in both the 1 and 10 wt % solutions indicates that the micelle structure is present in both concentrations and that the micelles do not undergo a structural change between these concentrations. We have shown that metallosurfactant 1 can form wormlike micelles in aqueous solution that may coexist with or easily interconvert with vesicle structures. The cylindrical micelle structures are unusual for triple-chain surfactants with a single headgroup and are not easily accounted for using geometrical packing arguments. They possibly reflect the presence of directional attractive forces in the headgroup region as well as a lack of complete fluidity of the alkyl chains. The wide variety of stable metal complexes formed by the cage headgroup makes these exciting materials for possible application in the production of mesoporous silica structures loaded with metal aggregates for a variety of catalytic applications.34 We are currently investigating their use in this area as well as determining the phase behavior of the material at higher concentrations, which might lead to some interesting lyotropic liquid-crystal phases.1 Acknowledgment. We thank the University of Western Australia for funding. G.L.N. and M.P. were the holders of Australian Postgraduate Awards, and M.P. was the holder of an AINSE Postgraduate Research Award. C.E.B. acknowledges the financial support of the Australian Research Council through REIF grant R00107962 2001, which enabled the SAXS studies to be undertaken. We also acknowledge technical, scientific, and financial assistance from the NANO-MNRF. Supporting Information Available: Experimental details for the synthesis of 1 and 2, SAXS and TEM work, and the cmc determination for 1. This material is available free of charge via the Internet at http://pubs.acs.org. LA701283B (33) Nishikawa, T.; Motter, A. E.; Lai, Y.-C.; Hoppensteadt, F. C. Phys. ReV. E 2002, 66, 046139/1-046139/5. (34) Kim, W.-J.; Yang, S.-M. Langmuir 2000, 16, 4761-4765.
