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Self-Assembly of Yttrium-Containing Lacunary Polyoxotungstate Macroanions in Solution with Controllable Supramolecular Structure Size by pH or Solvent Content Padmaja P. Mishra,† Jing Jing,‡ Lynn C. Francesconi,‡ and Tianbo Liu*,† Department of Chemistry, Lehigh UniVersity, Bethlehem, PennsylVania, 18015, and Department of Chemistry, Hunter College, City UniVersity of New York, New York, New York, 10021 ReceiVed May 1, 2008. ReVised Manuscript ReceiVed June 12, 2008 The self-assembly and the formation of “Blackberry” type supramolecular structures for a type of Yttrium-containing polyoxometalate (K15Na6(H3O)9[(PY2W10O38)4(W3O14)] · 9H2O, or {P4Y8W43}) macroanions is characterized by using static and dynamic light scattering techniques. {P4Y8W43} macroions are found to form hollow, spherical, single-layer “blackberry” structures in water and water-acetone mixed solvents. Very interestingly, the blackberry size can be accurately controlled by either changing acetone content in water-acetone mixed solvents, or by changing solution pH in aqueous solution. The blackberry size increases with decreasing pH (lower charge density) or higher acetone content in the mixed solvent (lower dielectric constant) and the blackberry size can change in responding to the change of external conditions. This indicates that the {P4Y8W43} macroanions possess the properties of both “strong electrolyte type” and “weak electrolyte type” macroions, as we outlined previously. This is due to the special chemical feature of such clusters, which can be treated as Na2HPO4-type electrolytes in solution. The kinetics of the blackberry formation can be controlled by temperature.
Introduction Polyoxometalates (POM) are a distinct class of oligomeric, structurally well-defined multinuclear complexes with interesting optical, electric and magnetic properties leading to relevance in many fields.1 Moreover, POMs, especially those containing Keggin and Wells-Dawson moieties, are chemically robust and can be easily modified with respect to incorporation of transition metal ions, charge, size potential and can be rendered soluble in water or in organic solutions. Rare earth-substituted, especially Lanthanide (Ln)-substituted POMs can offer unique functionality to POMs such as excellent luminescence properties. These POMs have found applications in the fields of catalysis, medicine, material sciences, etc.2–4 (Scheme 1). Though the single crystal structures of some types of Lnsubstituted POMs have been determined, their solution behavior remains unknown. This is an interesting topic because such clusters exist as macroions in solution. Recently, we found that the hydrophilic, soluble ions possess very unique solution properties when they are large (i.e., reaching the nanometer scale). Contrary to the widely accepted theory that soluble inorganic ions should distribute homogeneously in dilute solutions, such macroions, such as POM macroanions and cationic metal-organic * To whom correspondence should be addressed. E-mail:
[email protected]. † Lehigh University. ‡ City University of New York.
(1) (a) Polyoxometalate Chemistry: From Topology Via Self-Assembly to Application (Eds.: Pope, M. T.; Mu¨ller, A.), Kluwer, Dordrecht, Netherlands 2001. (b) Liu, S.; Kurth, D. G.; Bredenkotter, B.; Volkmer, D. J. Am. Chem. Soc. 2002, 124, 12279. (c) Mu¨ller, A.; Kogerler, P.; Dress, A. W. M. Coord. Chem. ReV. 2001, 222, 139. (d) Hill, C. L. Chem. ReV. 1998, 98, 1–387. (2) (a) Pope, M. T.; Mu¨ller, A. Chem. ReV. 1998, 98, 1. (b) Pope, M. T.; Mu¨ller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. (c) Grigoriev, V. A.; Cheng, D.; Hill, C. L.; Weinstock, I. A. J. Am. Chem. Soc. 2001, 123, 5292. (3) (a) Aspinall, H. C. Chem. ReV. 2002, 102, 1807. (b) Molander, G. Chem. ReV. 1992, 92, 29. (c) Shibasaki, M.; Yoshikawa, N. Chem. ReV. 2002, 102, 2187. (d) Kobayashi, S.; Kawamura, M. J. Am. Chem. Soc. 1998, 120, 5840. (e) Aspinall, H. C.; Dwyer, J. L. M.; Greeves, N.; McIver, E. G.; Woolley, J. C. Organometallics 1998, 17, 1884. (4) (a) Wang, J.; Wang, H.; Liu, F.; Fu, L.; Zhang, H. Mater. Lett. 2003, 416, 1210. (b) Wang, X. L.; Wang, Y. H.; Hu, C. W.; Wang, E. B. Mater. Lett. 2002, 56, 305.
Scheme 1. Hollow, Single-Layer, Spherical Blackberry Structures Formed by the Self-Assembly of {P4Y8W43} Macroions in Water and in Acetone/Water Mixturesa
a The macroions on blackberry surface do not form long-range ordered packing (as hinted by this cartoon picture).
cages, behave differently.5 The POMs are highly soluble in polar solvents due to a large number of surface water ligands and their negative charges. These macroanions are balanced by small counter-ions, such as Na+ and NH4+, in solution. Such POM macroions, though hydrophilic in nature and soluble in polar solvents, tend to self-assemble into large, single layer, “blackberry”-type structures, which is in equilibrium with discrete macroions in solution.6 The phenomenon itself is very intriguing as soluble ions are expected to stay discretely in dilute solutions. The blackberry size, ranges from tens to hundreds of nanometers and with a narrow size distribution, can be accurately controlled by adjusting the change density of macroions and the solvent quality. The formation of spherical blackberry structures has been observed in the solutions of various POM macroanions, such as wheel-shaped {Mo154} and {Mo176}, “Keplerate” {Mo132} and {Mo72Fe30}, as well as {P8W48Cu20},5–8 suggesting that the supramolecular structure is quite universal for the macroions with different molecular shapes. Previous studies from our group, based on weak electrolytetype, spherical 2.5-nm-diameter heteropolyoxometalate “Ke(5) (a) Liu, T. J. Am. Chem. Soc. 2002, 124, 10942. (b) Liu, T. J. Am. Chem. Soc. 2003, 125, 312. (c) Liu, T. Nature 2003, 426, 59. (d) Kistler, M. L.; Bhatt, A.; Liu, G.; Casa, D.; Liu, T. J. Am. Chem. Soc. 2007, 129, 6453. (e) Li, D.; Zhang, J.; Landskron, K.; Liu, T. J. Am. Chem. Soc. 2008, 130, 4226. (6) Liu, G.; Liu, T. J. Am. Chem. Soc. 2005, 127, 6942. (7) Liu, G.; Liu, T.; Mal, S. S.; Kortz, U. J. Am. Chem. Soc. 2006, 128, 10103. (8) Liu, G.; Liu, T. Langmuir 2005, 21, 2713.
10.1021/la801366r CCC: $40.75 2008 American Chemical Society Published on Web 08/05/2008
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the fact that each cluster has nine H3O+ counter-ions, its charge density can be affected by solution pH. This unique macroion offers us a valuable opportunity to explore the effects of solvent quality and the pH at the same time.
Experimental Section
Figure 1. Ball and stick representation of the Y-tetramer (K15Na6(H3O)9[(PY2W10O38)4(W3O14)] · 9H2O). Purple represents W, red represents O, pink represents P, light orange represents Y(III).
plerate” {Mo72Fe30} macroanions and strong electrolyte-type, spherical, 2.9-nm-diameter “Keplerate” POM {Mo132}, has exhibited that the major driving forces responsible for the blackberry formation are not due to either the hydrophobic interactions (because the POMs do not have any hydrophobic parts) or van der Waals forces. This distinguishes the macroionic solution from well-known systems such as surfactants and colloids. We believe that the counter-ion-mediated attraction is critical for the blackberry formation, helped by the hydrogen bonds formed between the adjacent macroanions.6,7 The “strong electrolyte” type and “weak electrolyte” type macroions show different self-assembly mechanisms.8 Strong electrolytes, such as {Mo132}, {Mo154} and {Mo368} clusters, all contain inherent electrons and therefore exist as ionic-type compound in crystals and macroanions in solution.8 On the other hand, weak electrolyte {Mo72Fe30}, being neutral clusters in crystals, can be treated as a weak acid due to the presence of 30 FeIII(H2O) groups on the surface which allow partial, pH-dependent deprotonations. Consequently, the self-assembly behavior of {Mo72Fe30} macroions (which carry a small number of localized charges) depends on solution pH and show very unique kinetic properties. Some ”strong electrolyte”-type POMs are found to form blackberries in water, but some do not exhibit such behavior due to their very high charge density (therefore strong repulsion) in water, such as {Mo132} macroanions which carry 42 inherent charges. However, {Mo132} macroanions can form blackberries in weaker polar solvents, such as water/acetone mixed solvents, in which the charge density of macroions will decrease.5c We found that the blackberry size increases monotonically with increasing acetone content, and there is a linear relationship between the blackberry size and the inversed dielectric constant of the solvent.5c The transition from discrete macroions to blackberries and vice versa can be achieved by only adjusting solvent content. On the contrary, for such strong electrolyte type POMs, changing solution pH will not affect their charge density significantly since their charges are mostly inherent charges. Therefore, so far no POM system can show the blackberry formation and variation in the blackberry size dependent on both pH and solvent content. The nature of charges from the two types of POMs are different (delocalized vs localized charges), which further prevents us from establishing a quantitative connection between the effect of charge density and the effect of solvent content from there. Here, for the first time we have identified a model system which can solve the problems mentioned above: a lanthanide-substituted POM {P4Y8W43} (K15Na6(H3O)9[(PY2W10O38)4(W3O14)] · 9H2O, Figure 1). {P4Y8W43} has a tetramer structure and carries ∼30 charges. Furthermore, due to
Sample Preparation. (K15Na6(H3O)9[(PY2W10O38)4(W3O14)] · 9H2O, Figure 1) was synthesized by a modification of the procedure published earlier.9 In brief, solid Na9PW9O34 0.15H2O10 (2.2 g, 0.81 mmol) was added to a solution of YCl3 · 6H2O (0.28 g, 0.95. mmol) in H2O (7.5 mL). The resulting suspension was heated to about 80 °C, and a clear solution was formed. Solid KCl (1.4 g, 19 mmol) was added to the hot solution, and the solution was heated for an additional 5 min. The resulting slurry was cooled to room temperature and then in an icebath. The solid was collected by filtration, washed with cold H2O, and recrystallized from hot H2O (yield: 2.1 g, 72% based on YCl3). X-ray quality crystals of K15Na6(H3O)9[(PY2W10O38)4(W3O14)] were grown at -10 °C from a clear solution of K15Na6(H3O)9[(PY2W10O38)4(W3O14)] in H2O saturated with KCl. A series of 0.3 mg/mL {P4Y8W43} solutions were prepared by dissolving {P4Y8W43} single crystals in deionized water and in water/ acetone mixed solvents with acetone content ranging from 0-70% by volume. The solutions were tightly sealed in glass light scattering vials. Additional amounts of acetone or water were added to the solutions, if changing the solvent quality was required for the experiment. The pH of the aqueous solution of {P4Y8W43} can be adjusted by introducing small amounts of 0.1N HCl solution, and be monitored by an ORION pH-meter. 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 method was used to analyze the DLS data and to calculate the hydrodynamic radius (Rh) of the particles.11 DLS measurements further provide information on the particle-size distribution in solution from a plot of ΓG(Γ) versus Rh. 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,11 which is used to obtain the radius of gyration (Rg) and the weightaverage molecular weight (Mw) of the particles in solution. The details of SLS and DLS principles can be found in our previous publications.1b,5–8
Results and Discussions Self-Assembly of {P4Y8W43} Macroions in Aqueous Solution. In single crystals, the dimension of {K15Na6(H3O)9[(PY2W10O38)4(W3O14)} cluster is around 4.0 × 2.0 × 2.0 nm3. The {P4Y8W43} clusters are moderately soluble in water (solubility ∼1.2 mg/mL at room temperature) and mostly stable in basic conditions. The clusters carry negative charges, after releasing their counter-ions including K+, Na+ and H3O+ ions in solution. The measured pH of the solution after dissolving 0.3 mg of {P4Y8W43} in water is 6.5, indicating that only tiny amount of H3O+ groups are released into solution. That is, at this stage the counter-ions are mostly K+ and Na+. It is very likely that the {P4Y8W43} tetramer remains as a tetrameric structure in solution, because the W-183 and P-31 NMR studies in {P4Y8W43} aqueous solution at pH > 5 show the appropriate symmetry (C2) for the tetramer structure.9 Also, in solution, the excitation spectrum for the Eu(III) analog that is isostructural with the Y(III) analog, shows four distinct Eu(III) environments consistent with C2 symmetry.9 (9) Howell, R. C.; Perez, F. G.; Jain, S.; Dew, W. H.; Rheingold, A. L.; Francesconi, L. C. Angew. Chem., Int. Ed. 2001, 113, 4155. (10) Klemperer, W. G. Inorg. Syn. 1992, 27, 71. (11) Provencher, S. W. Biophys. J. 1976, 16, 29.
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Figure 2. (a) CONTIN analysis of the DLS measurement monitors the slow self-assembly of {P4Y8W43} macroions in aqueous solution with time. (b) The corresponding increment in the scattered intensity with time monitored by SLS.
In the freshly prepared dilute solutions (0.1-0.5 mg/mL), the clusters essentially exist as discrete macroanions, showing no signs of self-association. This is confirmed by the SLS and DLS studies: very weak scattered intensities are recorded in the SLS studies, showing the absence of any kind of large supramolecular structure formation; furthermore, DLS measurements with CONTIN analysis also shows no peak corresponding to large structures. This type of solution behavior is very similar to the soluble simple electrolytes (e.g., NaCl). SLS and DLS techniques are powerful tools for monitoring the formation of supramolecular structure in solution.12 Soon after the dissolution of the {P4Y8W43} tetramers, a continuous increase in the scattered intensity by SLS measurements can be observed, suggesting the slow but continuous formation of large structures. At the same time, a new mode (usually with narrow size distribution) corresponding to larger structures can be identified by DLS studies. The scattered intensity from {P4Y8W43} macroionic solution increased dramatically when measured after 48 h and it kept increasing for the next 10-12 days until finally became stabilized, suggesting that the equilibrium between the discrete {P4Y8W43} tetramer macroions and the large supramolecular structures had been reached (Figure 2b). During this period of time, the hydrodynamic radius (Rh) of the large structures measured by DLS first increased from 39 to 56 nm, then became stabilized after 12 days. A typical CONTIN analysis from a DLS study on a 0.3 mg/mL {P4Y8W43} macroion solution on the 12th day of the experiment is shown in Figure 2a. The peak due to discrete {P4Y8W43} macroions cannot be identified because the maccroions contribute only a tiny portion to the total scattered intensity. After two weeks, no further change in the average Rh of the supramolecular structures is observed, although the total scattering intensity from the solution still continued to increase for a while. As discussed before, this behavior indicates that the (12) (a) Moradian-Oldak, J.; Leung, W.; Fincham, A. G. J. Struct. Biol. 1998, 122, 320. (b) Sedlak, M. J. Phys. Chem. B 2006, 110, 4329. (c) Ermi, B. D.; Amis, E. J. Macromolecules 1998, 31, 7378.
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supramolecular structures will not grow larger in solution; instead, more and more large assemblies are formed with time.5b,7 A weak angular dependence of the Rh value indicates that the assemblies formed in the solution are mostly spherical, consistent with our early observations obtained for other POM solutions.5 The measured Rh value for the large assemblies in {P4Y8W43} macroionic solution does not show any concentration dependence for a concentration range from 0.1 mg/mL to 0.5 mg/mL {P4Y8W43}. As we previously reported, there exists a critical association concentration (CAC) for the blackberry formation process, and the blackberry size does not change with increasing cluster concentration above CAC.6 For {P4Y8W43}, the CAC value must be very small. The structure of the large assemblies can be obtained by studying the relation between their Rh and radius of gyration (Rg) values, the latter was determined by SLS studies. It is noteworthy that the Rg and Rh values of the spherical assemblies are roughly the same. In contrast, for a solid spherical particle, Rg ) 0.77 Rh. The ratio of Rg/Rh will rise if more mass in a sphere distributes closer to the surface. If the spherical aggregates have all their mass on their surfaces, Rg/Rh ) 1. Therefore, the aggregates obtained for the {P4Y8W43} after several days are very likely to be hollow spheres, i.e., same as the “blackberry” structure we have identified before in other macroanionic and macrocationic solutions.5,13 Another obvious similarity between the current system and the previous POM solutions we have studied (especially those “weak electrolyte” type POMs) is the very slow blackberry formation.5b As we stated earlier, the very slow process suggests a very high energy barrier between the discrete macroions and the blackberries.8 This barrier is much more significant for “weak electrolyte” type POMs such as {Mo72Fe30} because the charges on their surface are localized, which will decrease the effective collisions between POM macroanions involving counter-ions. The blackberry structures formed by {P4Y8W43} macroions are very stable. Once the equilibrium is reached in the solution, keeping the solutions at room temperature for months does not result in any obvious change in either scattered intensity as determined from SLS measurements, or in the average Rh value of the assembly structures as determined by DLS measurements. It is also observed that diluting the original solution with water does not change the blackberry size. As we have described earlier, the blackberry state is a free-energy-favored state that is more stable than the homogeneous distribution of macroions in solution.14 Effect of pH on the Self-Assembly Process of {P4Y8W43} in Aqueous Solution. We have realized that in determining the fascinating behavior of POM macroionic solutions, the small counter-ions play an important role. The disparity in size and charge density between cations and anions in such solutions leads to the partial association of counter-ions around macroanions, similar to the case of polyelectrolyte solutions, which is called counter-ion condensation or counter-ion association.15 In other words, the small cations are not totally free in solution. For {P4Y8W43} macroions, their small counter-ions include K+, Na+ and H3O+, from their chemical formula. The existence of H3O+ groups indicates that the amount of counter-ions, i.e., the charge density on {P4Y8W43} macroanions, could be affected by solution (13) Zhang, J.; Keita, B.; Nadjo, L.; Mbomekalle, I. M.; Liu, T. Langmuir 2008, 24. (14) Liu, G.; Liu, T. Langmuir 2005, 21, 2713. (15) (a) Sedlak, M. J. Chem. Phys. 1994, 101, 10140. (b) Sedlak, M. J. Chem. Phys. 2002, 116, 5256. (c) Ermi, B. D.; Amis, E. J. Macromolecules 1996, 29, 2701. (d) Zhang, Y.; Douglas, J. F.; Ermi, B. D.; Amis, E. J. Macromolecules 2001, 29, 3299. (e) Schmitz, K. S.; Lu, M.; Singh, N.; Ramsay, D. J. Biopolymers 1984, 23, 1637.
Self-Assembly of Lacunary Polyoxotungstate Macroanions
Figure 3. Comparison of change of blackberry size (in the hydrodynamic radius) with (a) solution pH with respect to {Mo72Fe30} and (b) acetone content in the solvent with respect to {Mo132}.
pH. On the other hand, the nine H3O+ groups on each cluster may not completely leave the cluster when dissolved in water. We can further speculate that the degree of deprotonation for the {P4Y8W43} clusters might be pH-dependent, which indicates a weak acid nature for the clusters. The {P4Y8W43} aqueous solution is almost neutral, indicating that only a small amount of H3O+ groups are released into solution, i.e., the counter-ions upon dissolution are mostly K+ and Na+. As described in the above section, we have obtained spherical “blackberry”-type supramolecular structure in this neutral solution, with an average Rh value of about 56 nm. A series of solutions at different pH values were prepared by introducing a small amount of HCl or NaOH solutions and studied in order to verify the effect of pH on the formation of these assemblies. Large blackberry assemblies can still be observed in such solutions, but their sizes show obvious pH dependence, as shown in Figure 3. When the solution becomes more acidic, the Rh value of the blackberry assemblies increases gradually. At pH 4.0, the Rh value is ∼67 nm. On the other hand, the average Rh value decreases when the solution becomes more basic, reaching ∼47 nm when the pH is adjusted to 10. This relation between pH and Rh values is similar to the case of {Mo72Fe30} aqueous solutions.16 {Mo72Fe30} is a weak acid and its degree of deprotonation (i.e., charge density on macroions) increases with increasing pH. This leads to smaller blackberry size at higher pH. In our current {P4Y8W43} solutions, the change of H+ concentration also affects the self-assembly process and especially the blackberry size. This should be due to the variation of the degree of the deprotonation/protonation of the clusters and hence the strength of the electrostatic interaction between them with the change in pH. Overall, {P4Y8W43} clusters can be treated as a salt of a weak acidssimilar to the case of Na2HPO4. Therefore, at lower pH conditions, the protonation process (more H3O+ ions from the solution will reside with the clusters) will further decrease (16) Liu, T.; Imber, B.; Diemann, E.; Liu, G.; Cokleski, K.; Li, H.; Chen, Z.; Mu¨ller, A. J. Am. Chem. Soc. 2006, 128, 15914.
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the number of charges on them. This explains why the blackberry size further increases when pH goes below 6.5, where deprotonation process is almost negligible. Comparison of the pH-Rh curve in Figure 3 with the corresponding curve we obtained previously in {Mo72Fe30} solutions, we notice that at a given pH, the {P4Y8W43} blackberry size is much larger (for example, at pH 6, Rh ) 60 nm for {P4Y8W43} blackberries and Rh ) 16 nm for {Mo72Fe30} blackberries). A major reason for the size difference is that at a given pH, {Mo72Fe30} clusters carry more charges (due to deprotonation) than {P4Y8W43}, based on pH-titration studies. This factor is particularly important for high pH conditions where {Mo72Fe30} carries quite a few charges. Another factor for the size difference is the charge density of macroions. {Mo72Fe30}, a 2.5-nm-diameter sphere, has a total surface area much smaller than that of {P4Y8W43}. Consequently, the charge density on {Mo72Fe30} would be higher even though both types of clusters carry the same amount of charges. Higher charge density leads to stronger electrostatic repulsion between macroanions and therefore results in smaller blackberry structures. {Mo72Fe30} clusters remain as discrete molecules at pH < 2.9, as the clusters (a weak acid) are almost charge neutral in nature. We expect that we might also obtain discrete {P4Y8W43} neutral molecules if we continue to decrease the solution pH. Obviously, fully protonated {P4Y8W43} is a stronger acid than {Mo72Fe30}. Therefore we would need to investigate lower pH conditions. Unfortunately, we are unable to observe the same phenomenon in {P4Y8W43} solutions due to decomposition of the {P4Y8W43} macroions at pH e 3.5. Effect of Solvent Content. For typical “strong electrolyte” type macroions, such as {Mo132} macroanions (balanced by 42 NH4+ cations), changing solution pH will not affect their charge density or the self-assembled structure. Instead, introducing another miscible, less polar solvent like acetone can enhance the interaction between the macroanions and the small counter-ions, i.e., more counter-ions will be closely associated with the macroions and decrease their effective charge density. For {Mo132} macroions, introducing acetone into their aqueous solution gradually induces the blackberry formation and the blackberry size increases with increasing acetone content. We noticed that there was a linear relationship between the blackberry size and the solvent content (in 1/ε, with ε being the solvent’s dielectric constant). Furthermore, a general equation was presented to describe the average blackberry radius R:17
R≈
-48λBu Ψ2
(1)
and the Bjerrum length
λB )
e2 4πε0εRkT
(2)
with u, Ψ, ε0 and εR being the attractive interaction between macroions, the Zeta potential of the blackberries, as well as water and the solvent dielectric constants, respectively. The {P4Y8W43} clusters also carry quite a few inherent charges when dissolved in water. Consequently, we can examine the effect of solvent content on their self-assembly behavior. Different from the effect of changing pH in aqueous solution which only changes the degree of deprotonation on the clusters, changing solvent quality will affect the state of all counter-ions. A series (17) Verhoeff, A. A.; 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.
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Figure 4. Monitoring the change of {P4Y8W43} blackberry size (in Rh) with time in different water/acetone mixed solvents.
Figure 5. Linear relation is found between the {P4Y8W43} blackberry Rh and 1/ε, which is predicted by eq 1.
of 0.3 mg/mL {P4Y8W43} solutions in water/acetone mixed solvents with water content ranging from 10 to 70% v/v were prepared. Blackberry formation is observed in all the solutions. As shown in Figure 3b, the average Rh of the blackberries increases with increasing acetone content in solution, but the slope in Figure 3b is not as large as that in {Mo132} system. The size of the blackberries ranges from ∼56 nm in 10% v/v acetone solution to ∼72 nm in 70% v/v acetone solution. The size distributions of the blackberry structures formed in different solvents are very similar. The growth of the blackberries is again a very slow process, similar to our previous examples. The blackberry size slightly increases during the first 10 days, and then gradually become stabilized afterward, as shown in Figure 4. Combined with SLS studies, we again notice that the values obtained for Rg and Rh are almost the same for the supramolecular assemblies formed in different solvents, starting from 0% v/v acetone to 70% v/v acetone, indicating the formation of hollow, vesicle-like blackberry-type structures. This result is interesting because it hints that the increase of charge density on macroanions leads to more significant attraction among them, that is, likecharge attraction.18 When the acetone content is increased to more than 80% v/v, the clusters become less soluble in solution. We then chose lower macroionic concentrations for our study to ensure that the clusters still form “real solutions”. No sign of supramolecular structure formation is obtained for quite a long period of time in such solutions, since the SLS studies show very weak scattered intensities, and the DLS measurements do not reveal any mode corresponding to large supramolecular structures. This clearly indicates that the less soluble {P4Y8W43} clusters now remain as discrete clusters in solution. In a poorer solvent, the counterion condensation around individual clusters becomes more significant, and as a result, the clusters become almost charge neutral in solution. It shows that the clusters do not tend to attract with each other strongly in the absence of their negative charges and proves that the van der Waals forces are not the major attractive forces responsible for the self-assembly process. If that would be the case, more serious aggregation should occur in solution with higher acetone content, as the loss of charge on the macroions decreases the electrostatic repulsion between macroanions and does not greatly affect the van der Waals attractive forces. We reached similar conclusions in studies of the “weak electrolyte” {Mo72Fe30} macroions in water/methanol mixed solvents, and “strong electrolyte” {Mo132} in water/acetone mixed solvents,.5a,6 The change in Rh with the increase in acetone percentage qualitatively follows the same trend as has been observed before for {Mo132}. In case of {Mo132}, Rh for 10%
acetone content was found to be almost ∼51 nm, which is comparable to the Rh ) 56 nm for {P4Y8W43}. Quantitatively, when the acetone content increased from 10% to 70% v/v, the average Rh value for {P4Y8W43} blackberries moderately increases from 56 to 68 nm, whereas a 2-fold increase in Rh was observed for the {Mo132} blackberries. The increment of Rh value with increasing solvent′ dielectric constant in {P4Y8W43}/water/acetone system strictly follows eq 1. A linear relationship is found between the Rh of the blackberries and the 1/εR, as shown in Figure 5. This behavior is consistent with a stabilization mechanism based on Coulomb repulsion combined with charge regulation. Comparing this curve with the corresponding curve for {Mo132}/water/acetone system, the major difference is that the current one has a much smaller slope, i.e., the Rh value increases much slower with increasing acetone content. According to eq 1, the smaller slope suggests a weaker attractive interaction between the macroanions on blackberry surface. We might attribute this to the types of counter-ions. In {Mo132} solutions, the counter-ions are NH4+, which are significantly larger than the counter-ions in {P4Y8W43} solutions (mainly K+ and Na+). Smaller counter-ions have less tendency to closely associate with macroions. Therefore, when the acetone content in solution increases, the counter-ion association will increase more significantly in {Mo132} solutions, resulting in much larger increase in the blackberry size. In pure aqueous solution, {P4Y8W43} macroions form fairly large blackberries and {Mo132} macroions exist as discrete ions, this is due to the fact that {Mo132} macroions have much higher charge density than {P4Y8W43} (more charges and less surface area). Changing pH vs Changing Solvent ContentsAre They Identical? {P4Y8W43} macroions is the first system we have explored so far which shows size-dependent blackberry formation at both different pH and different solvent contents. This is due to the unique feature of this cluster, which can be treated as a Na2HPO4-type compound. It contains both “free” counterions (K+ and Na+) which can easily be released into solution and hydronium groups which can partially deprotonate with the degree of deprotonation depending on solution pH. This unique feature offers us an opportunity to compare the similarities and differences between the two processes: the change of blackberry size by changing pH or solvent content. From Figure 3, it is obvious that larger {P4Y8W43} blackberries can form either at a lower pH or in a solvent with higher acetone content. The two processes are different. Changing pH only changes the number of protons released from each cluster, but does not change the number of inherent charges on the clusters (e.g., those charges balanced by the counter-ions in crystals), and the number of nonproton counter-ions in solution (e.g., K+ and Na+). Furthermore, the charges generated from deprotonation are localized charges, which are different from the inherent
(18) (a) Sogami, I.; Ise, N. J. Chem. Phys. 1984, 81, 6320. (b) Ise, N. Proc. Jpn. Acad., Ser. B 2002, 78, 129. (c) Yamanaka, J.; Yoshida, H.; Koga, T.; Ise, N.; Hashimoto, T. Phys. ReV. Lett. 1998, 80, 5806.
Self-Assembly of Lacunary Polyoxotungstate Macroanions
Langmuir, Vol. 24, No. 17, 2008 9313
simplified equation for SLS, the scattered intensity is proportional to the solute mass M and concentration C:
I∝C·M
Figure 6. Determination of the activation energy (Ea) for the {P4Y8W43} blackberry formation by measuring the formation speed at 25, 40 and 50 °C. The insert shows the change of scattered intensity at those temperatures, as monitored by SLS. CONTIN analysis of the DLS measurements on the {P4Y8W43} blackberry formation in the aqueous solution at 40 °C.
chargessin many cases they are delocalized. On the other hand, changing solvent quality will increase the cation absorption on the cluster surface. This process will affect the concentrations of all types of counter-ions in solution. Overall, we expect that changing solvent content should have a stronger impact on the charge density of the clusters. Effect of Temperature on the Blackberry Formation. Similar to other POM solutions, the blackberry formation in {P4Y8W43} solutions is also very slow. This can be confirmed by studying the effect of temperature on the formation of these novel supramolecular structures. For example, a 0.3 mg/mL {P4Y8W43} aqueous solution was kept at 40 °C and the Rh and Rg were measured periodically. The process of the blackberry formation becomes faster at a higher temperature. The equilibrium is reached in about 12-14 days at 25 °C, but can be achieved within 7-8 days at 40 °C. This indicates that the energy barrier between single macroions and the blackberry structures becomes lower with raising temperature. Meanwhile, the blackberry size does not show any temperature dependence. Also, the average size of the blackberries does not increase significantly with time, as shown in Figure 6. To study the temperature dependence of the blackberry formation, we use SLS to monitor the change of the scattered intensity (I) at a 90° scattering angle (I90) with time. From the
Figure 7. Determination of the activation energy (Ea) for the {P4Y8W43} blackberry formation by measuring the formation speed at 25, 40 and 50 °C. The insert shows the change of scattered intensity at those temperatures, as monitored by SLS.
(3)
Therefore, with the continuous blackberry formation, the I90 will increase accordingly. The contribution from discrete macroions to I90 is negligible. The intensity-time curves obtained form {P4Y8W43} solutions are very similar to those of {Mo72Fe} solutions, as we reported before.5b,8 The blackberry formation is slow, and roughly follows the rule of a first-order reaction. The process can be significantly accelerated by increasing solution temperature, as shown in Figure 7(insert). The activation energy of the transition from discrete macroions to blackberry structures can be estimated by studying the temperature dependence of k values (Arrhenius equation), which can be obtained by measuring the initial speed of blackberry formation at different temperatures:
k ) Ae-Ea⁄RT
(4)
with A, Ea, R, and T being a frequency factor, the activation energy, the universal gas constant, and temperature, respectively.5b As shown in Figure 7, the Ea value is calculated as 99.7 kJ/mol which is considered quite high even for regular chemical reactions at room temperature. Therefore, we can conclude that the slow blackberry formation in {P4Y8W43} solution is due to the high energy barrier between the two states. Raising temperature can significantly decrease the energy barrier. This value is also comparable to that obtained in the {Mo72Fe30}/water system (∼115 kJ/mol), suggesting that the two processes are similar.
Conclusion In summary, we demonstrate here that a type of Yttriumcontaining polyoxotungstate, {P4Y8W43} clusters, can selfassemble into spherical, single-layer vesicle-like supramolecular blackberry structures in dilute aqueous solutions. {P4Y8W43} clusters are unique because they possess two different types of counter-ions - the inherent counter-ions such as K+ and Na+, as well as the H3O+ ions due to the deprotonation/protonation equilibrium. As the result, {P4Y8W43} clusters behave like NaHPO4 in solution. Its charge density can be adjusted by changing solution pH (changing the deprotonation/protonation equilibrium) or changing solvent content (changing the degree of counter-ion association). Consequently, {P4Y8W43} becomes the first system which shows that their blackberry size can be controlled by either varying pH or varying solvent content. This offers us an opportunity to compare the two processes. The blackberry formation process becomes faster at elevated temperatures, indicating that a high energy barrier exists between the discrete macroion state and the blackberry state. Acknowledgement. T.L. thanks supports from the NSF (CHE0545983), ACS-PRF (46294-G3) and Lehigh University. L.C.F. thanks supports from NSF (CHE 0414218) and NIH (S06 GM60654 (SCORE)). Research Infrastructure at Hunter College is partially supported by NIH-Research Centers in Minority Institutions Grant No. RR03037-08. LA801366R
