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Accurately Tuning the Charge on Giant Polyoxometalate Type Keplerates through Stoichiometric Interaction with Cationic Surfactants Melissa L. Kistler, Komal G. Patel, and Tianbo Liu* Department of Chemistry, Lehigh University, 6 East Packer Avenue, Bethlehem, Pennsylvania 18015 Received January 31, 2009. Revised Manuscript Received March 3, 2009 We report an approach of exploring the interaction between cationic surfactants and a type of structurally well-defined, spherical “Keplerate” polyoxometalate (POM) macroanionic molecular clusters, {Mo72V30}, in aqueous solution. The effectiveness of the interaction can be determined by monitoring the size change of the “blackberry” supromolecular structures formed by the self-assembly of {Mo72V30} macroions, which is determined by the effective charge density on the macroions. Long-chain surfactants (CTAB and CTAT) can interact with {Mo72V30} macroions stoichiometrically and lower their charge density. Consequently, the blackberry size decreases continuously with increasing surfactant concentration in solution. On the other hand, for short-chain surfactants (e.g., OTAB), a larger fraction of surfactants exist as discrete chains in solution and do not strongly interact with the macroions. This approach shows that a controllable amount of suitable surfactants can accurately tune the charge on large molecular clusters.

Introduction Polyoxometalates (POM) represent a large series of metaloxide inorganic molecular clusters (some are nanometers in size) that are structurally well-defined and show a wide variety of molecular and electronic structures,1,2 which exhibit unparalleled structural, magnetic, and electronic properties.3-5 POMs offer unique opportunities in both fundamental studies and practical applications in various fields.6 A few types of large POMs exist as macroanions in solution, with most of them carrying inherent charges. For a given type of POM, its charge number can be altered by applying different types of ligands during synthesis. For example, each “Keplerate” {Mo132} cluster7a (2.9 nm hollow, porous spherical structure) theoretically carries ∼42 charges when there are acetate ligands inside its molecular skeleton (full formula (NH4)42[Mo132O372(CH3COO)30(H2O)72] 3 ca.300H2O 3 ca.10 CH3COONH4). If the acetate ligands are replaced by sulfates, the cluster can carry up to ∼72 negative charges.7b The name of “Keplerate” is specifically given *E-mail: [email protected]. (1) (a) Polyoxometalates special issue: Hill, C. L., Ed. Chem. Rev. 1998, 98, 1:: 387. (b) Muller, A.; Roy, S. Coord. Chem. Rev. 2003, 245, 153. :: :: (2) Muller, A.; Kogerler, P.; Dress, A. W. M. Coord. Chem. Rev. 2001, 222, 139. :: :: :: (3) Muller, A.; Krickemeyer, E.; Das, S. K.; Kogerler, P.; Sarkar, S.; Bogge, H.; :: Schmidtmann, M.; Sarkar, S. Angew. Chem., Int. Ed. 2000, 39, 1612. (b) Muller, A.; :: :: Das, S. K.; Kogerler, P.; Bogge, H.; Schmidtmann, M.; Trautwein, A. X.; :: Schunemann, V.; Krickemeyer, E.; Preetz, W. Angew. Chem., Int. Ed. 2000, 39, 3413. (4) (a) Grigoriev, V. A.; Cheng, D.; Hill, C. L.; Weinstock, I. A. J. Am. Chem. Soc. 2001, 123, 5292. (b) Grigoriev, V. A.; Hill, C. L.; Weinstock, I. A. J. Am. Chem. Soc. 2000, 122, 3544. :: :: (5) Cronin, L.; Kogerler, P.; Muller, A. J. Solid State Chem. 2000, 152, 57. :: (6) (a) Kurth, D. G.; Volkmer, D.; Ruttorf, M.; Ritcher, B.; Muller, A. Chem. :: :: Mater. 2000, 12, 2829. (b) Muller, A.; Krickemeyer, E.; Meyer, J.; Bogge, H.; Peters, F.; Plass, W.; Diemann, E.; Dillinger, F.; Nonnenbruch, F.; Randerath, M.; :: Menke, C. Angew. Chem., Int. Ed. 1995, 34, 2122. (c) Muller, A.; Plass, W.; :: Krickemeyer, E.; Dillinger, S.; Bogge, H.; Armatage, A.; Proust, A.; Beugholt, C.; Bergmann, U. Angew. Chem., Int. Ed. 1994, 33, 849. :: :: (7) (a) Muller, A.; Krickemeyer, E.; Bogge, H.; Schmidtmann, M.; Peters, F. :: :: Angew. Chem., Int. Ed. 1998, 37, 3360. (b) Muller, A.; Krickemeyer, E.; Bogge, H.; Schmidtmann, M.; Roy, S.; Berkle, A. Angew. Chem., Int. Ed. 2002, 41, 3604. (c) :: Muller, A. Nature (London) 2007, 447, 1035. (d) Kong, X.; Ren, Y.; Long, L.; Zheng, Z.; Huang, R.; Zheng, L. J. Am. Chem. Soc. 2007, 129, 7016.

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to the structures (including polyoxometalate clusters and other compounds) that contain platonic and archimedean solids one inside another, which resemble an early model of the solar system.7c,7d In single crystals, the macroanions are balanced by small cations (here NH+ 4 ). Obviously, there is a limit to which the charge on a specific type of POM can be adjusted, and it is difficult to accurately and continuously tune the charge density on POMs, especially in solutions. Several types of Keplerate POMs, such as {Mo72Fe30}8a and the structurally analogous compound {Mo72Cr30}8b (full formulae: [Mo72Fe30O252(CH3COO)12{Mo2O7 (H2O)}2{H2Mo2O8(H2O)}(H2O)91] 3 ca. 150 H2O and [{Na (H2O)12}⊂{Mo72Cr30O252(CH3COO)19(H2O)94}] 3 ca. 120 H2O, respectively), can be treated as weak acids. They exist as almost neutral molecules in crystals but can partially deprotonate their water ligands coordinated to the non-Mo metal centers when dissolved in aqueous solution, resulting in the formation of POM macroanions. The charge density of the macroanions can then be adjusted with solution pH.9,10 However, most POMs can only survive in a limited pH range (usually in weak acidic solutions), and it is sometimes difficult to accurately measure the charge density of macroions in solution. Such problems, together with the very limited number of available “weak acid”type POM clusters, make the approach involving accurately tuning charge density by pH adjustment not practical in many cases. On the other hand, accurately tuning the charge density of POM macroions is important, especially if considering the fact that the charge density of POM macroions often cannot be easily tuned to desired values from synthesis. Here, we demonstrate a simple and reliable way of tuning charge density on macroions :: :: (8) (a) Muller, A.; Sarkar, S.; Shah, S. Q. N.; Bogge, H.; Schmidtmann, M.; :: :: Sarkar, S.; Kogerler, P.; Hauptfleisch, B.; Trautwein, A. X.; Schunemann, V. :: Angew. Chem., Int. Ed. 1999, 38, 3238. (b) Todea, A. M.; Merca, A.; Bogge, H.; van Slageren, J.; Dressel, M.; Engelhardt, L.; Luban, M.; Glaser, T.; Henry, M.; :: Muller, A. Angew. Chem., Int. Ed. 2007, 46, 6106. :: (9) Kistler, M. L.; Liu, T.; Gouzerh, P.; Todea, A. M.; Muller, A. Dalton Trans., accepted. (10) Liu, T.; Imber, B.; Diemann, E.; Liu, G.; Cokleski, K.; Li, H.; Chen, Z.; :: Muller, A. J. Am. Chem. Soc. 2006, 128, 15914.

Published on Web 04/09/2009

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Figure 1. (a) Structure of the 2.5-nm-diameter “Keplerate” {Mo72V30} molecular cluster reprinted from ref 15b, page 5262, copyright 2005,

permission from Royal Chemical Society.15a (b) Structure of alkylammonium cationic surfactants, where n is the number of CH2 groups in the hydrocarbon chain. (c) Blackberry structure formed by {Mo72V30} macroanions in aqueous solution (with average hydrodynamic radii 25-120 nm depending on the type and amount of cationic surfactant added). The packing of POMs on blackberry surface is not as ordered as it suggests in the cartoon.

Figure 2. Gradually introducing alkyltrimethylammonium halide cationic surfactants into the dilute aqueous solution of {Mo72V30} clusters is expected to gradually decrease the charge density on the {Mo72V30} macroions. Transition from discrete {Mo72V30} macroanions to blackberry structures to finally insoluble, surfactant-encapsulated (almost neutrally charged) clusters can be achieved during the process. The average blackberry size increases with increasing surfactant amount (i.e., decreasing charge density on {Mo72V30}).

by using cationic surfactants. The idea of interacting POM macroanions with cationic surfactants to form complex structures has been extensively reported before.11-13 Different groups have applied various types of cationic surfactants to strongly interact with the POMs and form a full hydrophobic layer. For example, Kurth et al. showed that the addition of cationic surfactants DODA-Br (dioctyl-dimethyl ammonium bromide, full formula (C18H37)2(CH3)2NBr) to POM clusters to form surfactant-encapsulated clusters (SECs), where DODA cations were used to replace all the counterions (∼40 NH+ 4 counterions in the case of {Mo132}12a and ∼20 counterions in the case of {Mo57V6},11bwith a full formula [H3Mo57V6(NO)6O183(H2O)18], respectively). Such surfactant-POM complexes are hydrophobic in nature and almost all the negative charges on the POMs have been neutralized by the cationic surfactants. These complexes have been used to create different SECs such as self-assembled multilayers for thin films, lamellar liquid crystal phases, multilamellar structures, nanorods on graphite, and materials combining desired properties of POMs and surfactants.12,13 On the other hand, some other :: (11) (a) Kurth, D. G.; Lehmann, P.; Volkmer, D.; Muller, A.; Schwahn, D. J. Chem. Soc., Dalton Trans. 2000, 3989–3998. (b) Kurth, D. G.; Lehmann, P.; :: :: Volkmer, D.; Colfen, H.; Koop, M. J.; Muller, A.; Du Chesne, A. Chem. Eur. J. 2000, 385. (12) (a) Liu, S; Volkmer, D; Kurth, D. G. J. Clust. Sci. 2003, 14, 405. (b) Li, W.; Bu, W.; Li, H; Wu, L.; Li, M. Chem Commun. 2005, 3785. (c) Volkmer, D.; :: Bredenkotter, B.; Tellenboker, J.; Kogerler, P.; Kurth, D. G.; Lehmann, P.; Schnablegger, H.; Schwahn, D.; Piepenbrink, M.; Krebs, B. J. Am. Chem. Soc. 2002, 124, 10489. (d) Wu, P.; Volkmer, D.; Bredenkotter, B.; Kurth, D. G.; Rabe, J. P. Langmuir 2008, 24, 2767. (13) (a) Bu, W.; Li, H.; Li, W.; Wu, L.; Zhai, C.; Wu, Y. J Phys. Chem. B 2004, 108, 12776.(b) Zhang, H.; Lin, X.; Yan, Y.; Wu, L. Chem. Commun. 2006, 4575. (c) Yin, S.; Li, W.; Wang, J.; Wu, L. J. Phys. Chem. B 2008, 112, 3983.

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groups have performed fundamental studies on the interactions between POM anions and cations in solution and the effects of ion-pairing.14 The novelty of the current work is to use limited amounts of surfactants to interact with the POMs and try to gradually change their charge density. If successful, this work will offer an approach to accurately adjust the charges on the POMs while the POMs are still soluble in water or other polar solvents. Obviously, this approach requires the assumptions that (1) the surfactants interact with POMs stoichiometrically; (2) we have a reliable way to monitor the change of charge density on the POMs during the process. However, it is not easy to ensure either assumption. For surfactants with different tail lengths, their interactions with POM anions should be different even in the low concentration regime (below the critical micelle concentration, or CMC, of the surfactant). In this paper, we demonstrate a study of the interaction of different cationic surfactants with {Mo72V30} macroanions (crystalline compound Na8K14(VO)2[{Mo(Mo)5O21(H2O)3}10{Mo (Mo)5O21(H2O)3(SO4)}2{VO(H2O)}20{VO}10({KSO4}5)2] 3 ca. 150 H2O).15 {Mo72V30} is a type of 2.5-nm-sized, spherical “Keplerate” cluster and has an overall structure similar to that of {Mo72Fe30},8a but carries 26 inherent charges due to the ligands (14) (a) Kim, K.; Pope, M. T. J. Am. Chem. Soc. 1999, 121, 8512. (b) Sadakane, M.; Dickman, M. H.; Pope, M. T. Angew. Chem., Int. Ed. 2000, 39, 2914. (c) Leroy,  F.; Miro, P.; Poblet, J. M.; Bo, C.; Avalos, J. B. J. Phys. Chem. B 2008, 112, 8591. :: :: (15) (a) Muller, A.; Todea, A. M.; van Slageren, J.; Dressel, M.; Bogge, H.; Schmidtmann, M.; Luban, M.; Engelhardt, L.; Rusu, M. Angew. Chem., Int. Ed. :: :: 2005, 44, 3857. (b) Botar, B.; Kogerler, P.; Muller, A.; Garcia-Serres, R.; Hill, C. L. :: Chem Commun. 2005, 5621. (c) Botar, B.; Kogerler, P.; Hill, C. L. Chem Commun. :: 2005, 3138. (d) Botar, B.; Kogerler, P.; Hill, C. L. J. Am. Chem. Soc. 2006, 5336.

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inside its skeleton. Each {Mo72V30} cluster is composed of 20 octahedrally coordinated vanadium centers and water ligands, while the other 10 vanadium centers are square pyramidally coordinated by VdO groups but without attached water ligands. Each {Mo72V30} can partially deprotonate the water ligands (∼5 from each cluster), which are attached to the equatorial vanadium centers and point outside when dissolved in aqueous solution.9 Thus, in pure water, each {Mo72V30} cluster theoretically carries ∼31 charges on average, with the counterions including K+, Na+, VO2+, and H+. To monitor the effective charge density on {Mo72V30} macroions in the presence of surfactants, we use the unique but reliable relationship between the charge density of macroions and the size of their self-assembled structures. Unlike soluble small inorganic ions, which distribute homogeneously in dilute solutions, we found that various types of hydrophilic macroions could selfassemble in solution to form large, relatively uniform, single layer, “blackberry”-type structures, as characterized by static and dynamic laser light scattering (SLS and DLS), TEM, SEM, and AFM studies.16,17 Blackberry formation has been observed by various POM clusters such as the wheel-shaped {Mo154},16a the “Keplerate” {Mo132}17 and {Mo72Fe30},16b,16c and some giant polyoxotungstate clusters.16d In comparison, the strong electrolyte-type of POMs and the weak electrolyte-type of POMs show different self-assembly mechanisms.16b,16c,17 The blackberry structures display intriguing properties, such as the obvious soft nature, which is considerably unusual for inorganic species.16 The formation of blackberries may require months to reach equilibrium state at room temperature, but it can be accelerated at higher temperatures.16 Previous studies by our group revealed that the driving forces behind blackberry formation are not primarily due to van der Waals forces, hydrophobic interactions, or chemical interactions.16 Instead, we believe that the counterion-mediated attraction might be important, along with hydrogen bonding.10,16b,17a,17c This separates POM solutions from wellknown colloid suspensions and surfactant solutions. {Mo72V30} behaves similarly to {Mo132} in acetone-water mixtures.9 For {Mo132} and {Mo72V30} in solutions of pure water, blackberry formation does not occur due to the high charge density of the individual macroanions.9,17 However, when acetone is added into the solution the charge density of the macroanions is lowered in response to more significant association of the counterions to the discrete POM clusters, which induces the blackberry formation (with only ∼3 vol % acetone added).9,17 The blackberry size increases with increasing acetone content, and a linear relationship was found between the average blackberry size and the reversed dielectric constant.17b In the case of {Mo72Fe30}, a weak acid in solution, its blackberry size decreases with increasing solution pH due to the gradual increase of charge density at higher pH conditions.10 In general, the blackberry size is very sensitive to the charge density of macroions: it decreases monotonically with increasing macroionic charge density in a given experimental condition. This unique feature of macroions can be applied to explore the interaction between cationic surfactants and POM macroanions. :: (16) (a) Liu, T.; Diemann, E.; Li, H.; Dress, A. W. M.; Muller, A. Nature (London) 2003, 426, 59. (b) Liu, T. J. Am. Chem. Soc. 2002, 124, 10942. (c) Liu, G.; :: :: Liu, T. Langmuir 2005, 21, 2713. (d) Schaffer, C.; Merca, A.; Bogge, H.; Todea, A. :: M.; Kistler, M. L.; Liu, T.; Thouvenot, R.; Gouzerh, P.; Muller, A. Angew. Chem., Int. Ed. 2009, 48, 149. (17) (a) Kistler, M. L.; Bhatt, A.; Liu, G.; Casa, D.; Liu, T. J. Am. Chem. Soc. 2007, 129, 6453. (b) Verhoeff, L.; Kistler, M. L.; Bhatt, A.; Pigga, J.; Groenewold, J.; Klokkenburg, M.; Veen, S.; Roy, S.; Liu, T.; Kegel, W. K. Phys. Rev. Lett. 2007, 99, 066104. (c) Schmitz, K. J. Phys. Chem. B 2009, 113, 2624.

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We expect that the size of the {Mo72V30} blackberries formed in the presence of different amounts of cationic surfactants can be used as an indication to determine whether the strong interaction between surfactants and {Mo72V30} is stoichiometric, i.e., whether all the surfactants are strongly associated with {Mo72V30} macroions via electrostatic interaction. Specifically, we are aiming to understand the effect of surfactants’ alkyl tail length on the interaction. This type of interaction represents a general model of interactions between relatively small, monovalent (but with hydrophobic portion) cations and large (porous) macroanions in solution.

Experimental Section Sample Preparation. {Mo72V30} single crystals were synthe-

sized according to the literature15a and then dissolved in water at a concentration of 1.0 mg/mL. Cationic surfactants used in this study were cetyltrimethylammonium bromide (CTAB), cetyltrimethyltetradecylammonium chloride (CTAT), dodecyltrimethylammonium bromide (DTAB), and octyltrimethylammonium bromide (OTAB). Surfactant solutions were prepared by directly dissolving surfactant powders into deionized water. Equal volumes of (freshly prepared) {Mo72V30} and surfactant solutions were then mixed together (i.e., the final concentration for {Mo72V30} was 0.5 mg/mL), then tightly sealed in glass vials and heated at 30 °C for several days. Static and Dynamic Laser Light Scattering. A commercial Brookhaven Instrument light-scattering spectrometer was used for both the DLS (with a BI-9000AT digital correlator) and the absolute integrated scattered intensity (SLS) measurements. The CONTIN method18 was used to analyze the DLS data and to calculate the hydrodynamic radius (Rh) of the particles in solution. DLS measurements further provide information on the particle-size distribution in solution from a plot of ΓG(Γ) versus Rh. The peak areas in the CONTIN analysis18 denote the relative contributions to the total scattered intensity from the corresponding types of particles. SLS experiments were performed at scattering angles between 30° and 100°, at 5° intervals. The basis of the SLS data analysis is the Rayleigh-Gans-Debye equation,16 which is used to obtain the radius of gyration (Rg) and the weight-average molecular weight (Mw) of the particles in solution.

Results and Discussion {Mo72V30} Clusters in Aqueous Solution and Water/ Acetone Mixed Solvents. Each {Mo72V30} cluster theoretically carry up to 31 negative charges in aqueous solution as a result of deprotonation (∼5) and its inherent charges (26) from its synthesis as a crystalline salt.9,15 The {Mo72V30} clusters can be quickly dissolved in water; at 1.0 mg/mL the solution pH = 3.6. No self-assembly behavior is observed in such aqueous solutions due to the strong electrostatic repulsion between the highly charged macroions. In water/acetone mixed solvents (3-70 vol % acetone), blackberry formation can be observed.9 This behavior is very similar to {Mo132}, as we reported previously.17 Considering that the role of acetone is to decrease the solvent’s dielectric constant (therefore decrease the effective charge density on macroions due to stronger counterion association), and only a small amount of acetone (3 vol %) is needed to move the {Mo72V30} macroions into the blackberry region, we speculate that, when the charge density of the {Mo72V30} macroanions in aqueous solution is slightly decreased, (18) Provencher, S. W. Biophys. J. 1976, 16, 29. (19) (a) Beyer, K; Leine, D.; Blume, A. Colloids Surf. B: Biointerfaces 2006, 49, 31. (b) Mukerjee, P; Mysels, K. J. Nat. Bur. Std. 1970, 36, 104. (c) Jang, J.; Lee, K. Chem Commun. 2002, 1098.

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blackberry formation should also occur based on our experience on other types of POM macroions. Interaction Between {Mo72V30} Macroions and LongTailed Surfactants CTAB and CTAT. For a typical experiment, aqueous solutions containing water-soluble alkyltrimethylammonium halide surfactants, such as CTAB or CTAT, were introduced to freshly prepared aqueous solutions of {Mo72V30}. It should be mentioned that almost all the surfactant concentrations in the final solutions we have studied are below their respective critical micelle concentration (CMC) values (values are shown in Table 1). At the high end of the CTAT surfactant concentration range, the surfactant concentration used (0.229 mg/mL) barely meets its CMC value. Therefore, we expect that no surfactant micelles form in any of the solutions we have studied, i.e., those surfactant chains should exist as discrete chains in aqueous solution if ignoring the presence of {Mo72V30} macroions. In solutions containing 1:1 molar ratio of {Mo72V30} to CTAB or CTAT surfactant ({Mo72V30} concentration is ∼0.5 mg/mL, or 0.025 mM), no blackberry formation occurs in all solutions, as the total scattered intensities for such solutions measured by SLS technique are all considerably low (not much higher than that of pure water), indicating that there is no selfassembled large structures present in solution. It suggests that the charge density of the {Mo72V30} macroanions is still too high to self-assemble. This is not surprising considering the limited amount of surfactant added; even if they fully interact with the POM macroions, they can only decrease the charge number on each cluster by one on average. At a {Mo72V30} to surfactant molar ratio of 1:1.2, the charge density of the {Mo72V30} should have been further lowered. Supramolecular structure formation can be observed in the solutions of two long-tail surfactants: CTAB and CTAT, as indicated by significantly increased scattered intensity from SLS studies. CONTIN analysis18 on the DLS measurements on the POM solutions containing CTAB or CTAT also shows the supramolecular structure formation, with the average hydrodynamic radii (Rh) of the assemblies as 29 and 25 nm, respectively, as shown in Figure 3. The sizes of the supramolecular structures are not very different from those of the corresponding CTAB and CTAT surfactant micelles (23 and 35 nm, respectively). However, the surfactant concentration is far less (15-20 times lower) than their CMC values and no supramolecular structure formation is observed in the surfactant solutions at the same concentrations but without {Mo72V30} clusters. Therefore, we can conclude that the self-assembly is not due to the surfactant micelle formation, but due to the {Mo72V30} macroions. The almost identical values of the average Rg (obtained from SLS studies) and Rh suggest that such supramolecular structures are hollow in nature and very likely blackberry-type structures.16 Blackberry formation indicates that the net charge density on the POM macroions has been effectively decreased so that the attractive forces among those {Mo72V30} macroanions become dominant. The blackberry sizes are relatively small from our previous experience, suggesting that the charge density on {Mo72V30} is still near the high end of the charge density range favoring blackberry formation.17 This is reasonable because, at this moment, only 1-2 charges on each {Mo72V30} are neutralized by surfactants. Considering that each {Mo72V30} can only interact with one or two surfactants, on average, the mechanism of the self-assembly should still be the same as those in surfactant-free systems, such as {Mo72V30} in water/acetone mixed solvents and {Mo72Fe30} in aqueous solution, i.e., due to counterion-mediated attractions and hydrogen bonds. MeanLangmuir 2009, 25(13), 7328–7334

Article Table 1. CMC Values and Micellar Sizes at Room Temperature for the Four Types of Cationic Surfactants Used for the Current Study, Where n is the Number of -CH2 Groups in the Hydrophobic Alkyl Tail surfactant

n

CTAB CTAT DTAB OTAB

15 13 9 7

CMC at 25 °C (mg/mL)/(mM) Rh of micelle (nm) 19c 0.33/0.9119a 0.23/0.7719a 2.54/8.219a 32.8/13019b

23 35 12 9

while, it suggests that there exists a critical surfactant concentration (CSC) for the {Mo72V30}/surfactants solutions as blackberry formation is not observed in solutions with less than ∼0.9  10-4 M DTAB. The solutions are still homogeneous and very stable in the presence of the large blackberry structures, as we have reported before.17 By further increasing the CTAB or CTAT surfactant concentration in {Mo72V30} solution continuously, the average Rh of the blackberries continues to increase in response, as shown in Figures 3 and 4. The close correlation between the average Rh value of the blackberries and the surfactant concentration is obvious for the whole surfactant concentration range up to a {Mo72V30}/surfactant molar ratio of 1:15, at which the average Rh values are both around 114 nm for {Mo72V30} blackberries in the presence of CTAB and CTAT, as shown in Figure 4. The continuous increase in the blackberry size during the whole process suggests that the effective charge density on the {Mo72V30} macroanions decreases continuously with increasing surfactant concentration. A key question is whether the two surfactants interact with {Mo72V30} macroions stoichiometrically. From the curves in Figure 4, we tend to believe a positive answer. The changes of Rh value in response to the amount of added surfactant are almost identical in the whole surfactant concentration range for CTAB and CTAT. This is an indication that these two types of longtailed surfactants, although they have different alkyl chain lengths, behave almost the same when interacting with {Mo72V30} macroions. On the other hand, if the interaction is not stoichiometric, more CTAB surfactants should interact with each {Mo72V30} than CTAT surfactants do for a given surfactant concentration. As the result, we can expect to see different Rh values in a given surfactant concentration for CTAB and CTAT, which is not the case in Figure 4. Additionally, the similar CSC values for the two types of surfactants also suggest that they behave very similarly when interacting with {Mo72V30} macroions (on the other hand, their CMC values in aqueous solution are different due to their different alkyl chain lengths). Therefore, logically we can conclude that long-tail surfactants such as CTAB and CTAT can be used to accurately adjust the charge density on {Mo72V30} macroions. Furthermore, it seems that the hydrophobic interaction is still not responsible for the blackberry formation of {Mo72V30} macroions when some hydrophobic alkyl chains are present on the surface of macroions; otherwise, we should see some difference in blackberry size as the CTAB surfactants possess longer chains. However, hydrophobic interaction might also play a minor role for the self-assembly of {Mo72V30} macroions by offering additional attractive forces. For the whole study shown in Figure 4, we observe only one mode (corresponding to a single size distribution) in the CONTIN analysis18 of DLS studies which should be attributed to {Mo72V30} blackberries. Even for the data corresponding the highest surfactant concentration (POM/surfactant molar ratio of 1:15, or surfactant concentration of ∼0.79 mM), there is no bimodal distribution in the CONTIN18 result which might DOI: 10.1021/la900394z

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Figure 3. CONTIN analysis18 of {Mo72V30}/CTAB and {Mo72V30}/CTAT solutions with different {Mo72V30}/surfactant molar ratios.

Figure 4. Average hydrodynamic radius (Rh) of the {Mo72V30} blackberries in aqueous solutions containing CTAB or CTAT, measured by DLS. The concentration of {Mo72V30} is 0.5 mg/mL, equivalent to a molar concentration of 2.6  10-5 M. Experiments were performed at 90° scattering angle.

Figure 5. Average hydrodynamic radius (Rh) of the {Mo72V30} blackberries formed in aqueous solution containing different amount of DTAB surfactants, as measured by DLS. (Inset) CONTIN analysis18 of the same DLS studies. Experiments were performed at 90° scattering angle.

indicate the presence of both the blackberry structures and the surfactant micelles in this solution. Additional LLS studies on the aqueous solutions containing only surfactants at corresponding concentrations confirm that no micelles exist at these surfactant concentrations. With more and more surfactants interacting with {Mo72V30} clusters, the solubility of {Mo72V30} clusters continues to drop due to the decrease of charge density and the increase 7332 DOI: 10.1021/la900394z

of their surface hydrophobicity (from the long alkyl chains of the surfactants). When the molar ratio of surfactant/{Mo72V30} is over 15, the clusters cannot be soluble in solution anymore and precipitation can be observed at the bottom of sample vials. Interaction Between {Mo72V30} Macroions and DTAB. The above observations and conclusions do not completely apply when interacting {Mo72V30} with shorter surfactants such as DTAB (with alkyl chains n = 9), which are less hydrophobic in solution, and therefore, weaker interactions with macroanions can be expected (that is, more DTAB surfactants will remain as discrete chains in solution and not interact strongly with {Mo72V30}). For 0.5 mg/mL {Mo72V30} aqueous solution, blackberry formation starts when ∼1.0  10-4 M DTAB surfactant (i.e., at ∼1:4 molar ratio of {Mo72V30} to surfactant) is introduced, as shown in Figure 5. Below this surfactant concentration, the {Mo72V30} macroions still exist as discrete ions. That is, the CSC value for DTAB in 1.0 mg/mL {Mo72V30} solution is considerably higher than those for CTAB and CTAT (about doubled). The CSC values in CTAB and CTAT solutions (which are almost identical) can be treated as the minimum amount of surfactant needed for decreasing the charge density on {Mo72V30} macroions to the blackberry formation range, if we assume that both surfactants fully interact with {Mo72V30} macroions. Thus, the higher CSC value in DTAB solution suggests that, for DTAB, a greater amount of surfactant than for CTAB and CTAT is needed to decrease the charge density on {Mo72V30} clusters to the same level. Therefore, it is an indication that DTAB surfactants do not interact stoichiometrically with {Mo72V30} macroions in dilute solution due to their weaker hydrophobicity, i.e., there are still some discrete DTAB surfactant chains in {Mo72V30} solution. Beyond the CSC, in DTAB solution the size of {Mo72V30} assemblies increases with increasing surfactant concentration, similar to {Mo72V30}/CTAB and {Mo72V30}/CTAT solutions. However, the {Mo72V30}/DTAB blackberries are smaller in size than the other two at a given {Mo72V30}/surfactant ratio. For example, at a {Mo72V30}/surfactant molar ratio of 1:7, blackberries in the {Mo72V30}/DTAB solution have an average Rh of 58 nm while those in {Mo72V30}/CTAB and {Mo72V30}/CTAT solutions have average Rh values of 76 and 72 nm, respectively (Table 2) (the experimental error bar is around (4 nm). The smaller blackberry size suggests higher effective charge density on the {Mo72V30} macroions. It indicates that, beyond the CSC value, the interaction between DTAB and {Mo72V30} is still not stoichiometric. The obviously smaller size of the {Mo72V30}/ DTAB blackberries shows that the effective charge density on {Mo72V30} clusters is higher with DTAB than with the same amount of CTAT or CTAT. This clearly suggests that the Langmuir 2009, 25(13), 7328–7334

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Table 2. Critical Surfactant Concentrations of Four Different Cationic Surfactants in the Aqueous Solution of 0.5 mg/mL {Mo72V30}, and the Average Rh Values of the {Mo72V30} Blackberry Structures in the Presence of Various Surfactants with Surfactant/POM Molar Ratios of 1:5, 1:7, and 1:10, Respectivelya

n (chain length) CSC (surf./POM molar ratio) Rh of blackberries (nm) at surf./POM molar ratio of 1:5 Rh of blackberries (nm) at surf./POM molar ratio of 1:7 Rh of blackberries (nm) at surf./POM molar ratio of 1:10 a The Rh values have error bars of (4%.

Figure 6. Average hydrodynamic radius (Rh) of the 0.5 mg/mL {Mo72V30} blackberries formed in aqueous solutions containing OTAB surfactant, as measured by DLS. (Inset) CONTIN analysis18 of the corresponding DLS studies. Experiments were performed at 90° scattering angle.

interaction between {Mo72V30} and DTAB is weaker than those between {Mo72V30} and CTAB or CTAT. Additionally, we are certain that the supramolecular structures formed in {Mo72V30}/ DTAB solutions are {Mo72V30}-based blackberries but not surfactant micelles, as the DTAB micelles have an average Rh of only 12 nm (Table 1). Interaction Between {Mo72V30} Macroions and ShortTailed Surfactant OTAB. OTAB has an alkyl chain that contains eight carbons and is shorter than the above-studied surfactants. Therefore, we can expect that the interaction between OTAB and {Mo72V30} macroions is much less effective than the previous ones. Blackberry formation is first observed at the molar ratio of 1:5 {Mo72V30}/OTAB. This CSC value is slightly higher than that for {Mo72V30}/DTAB solutions, which is reasonable if considering the weaker hydrophobicity of OTAB. Consequently, the self-assembled blackberry structures in OTAB solution are also smaller in size; their average Rh values in the solution with a {Mo72V30}/OTAB molar ratio of 1:5 is ∼45 nm, which further confirms that fewer OTAB surfactants have closely associated with {Mo72V30} macroions at this surfactant concentration than the previous ones. The blackberries are much larger than OTAB micelles (Rh = 9 nm, Table 1), and therefore, the supramolecular structures are not OTAB micelles. It is not a surprise to see that the surfactant with the shortest alkyl chain has the highest the CSC value and the smallest blackberry size at a given {Mo72V30}:surfactant molar ratio. However, we notice an interesting but unexpected feature of the {Mo72V30}/OTAB solutions: the average size of the {Mo72V30}/OTAB blackberries does not increase significantly with increasing OTAB concentration. Instead, it remains roughly constant at Rh ∼45 nm when the {Mo72V30}/OTAB molar ratio increases to up to 1:20 (the average Rh values fluctuate a little with Langmuir 2009, 25(13), 7328–7334

CTAB

CTAT

DTAB

OTAB

15 1:1.2 67 76 92

13 1:1.2 65 72 92

10 1:4 56 58 83

7 1:5 45 45 45

surfactant concentrations but are all in the 40-50 nm range), as shown in Figure 6, contradicting our observations in other types of surfactant solutions. A reasonable explanation is that, when more OTAB surfactants are introduced to the solution, most of them exist as discrete molecules and do not interact with {Mo72V30} macroions closely. The solutions with {Mo72V30}/OTAB molar ratio over 1:20 are still stable. In the presence of a larger amount of OTAB surfactants, the assembly size increases (Figure 6), suggesting that more surfactants interact with {Mo72V30} macroions which further decreases the charge density on {Mo72V30}. The continuous increase of Rh values with increasing surfactant concentration is similar to the previous cases of DTAB, DTAB, and CTAB. It seems that the interaction between OTAB and {Mo72V30} macroions exists at two critical surfactant concentrations, located at {Mo72V30}/OTAB molar ratios of 1:5 and 1:20, respectively. The first one denotes the starting point for the self-assembly; the second one denotes the transition point, after which the interaction between OTAB and {Mo72V30} start to show dependence on surfactant concentration. Between the two critical points, the amount of OTAB surfactants associated with each {Mo72V30} macroion is almost constant.

Conclusions In summary, we report using the formation of self-assembled blackberry structures by the large “Keplerate” POM {Mo72V30} macroions and their corresponding sizes in the presence of cationic surfactants to understand the interaction between cationic surfactants and macroanions. With the large amount of inherent charges, {Mo72V30} clusters stay as discrete anions and do not self-assemble in dilute aqueous solutions. Cationic surfactant molecules can strongly interact with {Mo72V30} macroions and lower their effective charge density. This induces the blackberry formation, and blackberry size increases with decreasing effective charge density. For the first time, we are able to quantitatively estimate the amount of surfactants interacting with macroions. For surfactants with long alkyl tails (CTAB and CTAT), almost all of them strongly interact with {Mo72V30} macroions in dilute aqueous solutions. The blackberry size increases monotonically with increasing surfactant amount in solution. In this regime, both surfactants behave very similarly. When the surfactant chains become shorter (e.g., DTAB), more surfactants will stay as discrete chains in solution, resulting in a weaker surfactant{Mo72V30} interaction and a higher critical surfactant concentration required for the self-assembly of {Mo72V30} macroions. Consequently, smaller blackberries are assembled at a given molar ratio of {Mo72V30}/surfactant. Further decrease of the surfactant chain length (e.g., OTAB) results in more surfactants existing as discrete chains in solution and not interacting with the macroions, and the relation between the surfactant concentration and macroionic charge density becomes DOI: 10.1021/la900394z

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less relevant. By understanding the interactions of the POMs macroanions and cationic molecules, we can more accurately control and modify the surfactant-encapsulated clusters and the size of the self-assembled structures.

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Acknowledgment. T.L. acknowledges support of this work by the National Science Foundation (CHE0723312), the American Chemical Society (PRF 46294-G3), the Alfred P. Sloan Foundation and Lehigh University.

Langmuir 2009, 25(13), 7328–7334