Langmuir 2005, 21, 2713-2720
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Thermodynamic Properties of the Unique Self-Assembly of {Mo72Fe30} Inorganic Macro-Ions in Salt-Free and Salt-Containing Aqueous Solutions Guang Liu†,‡ and Tianbo Liu*,†,‡ Department of Chemistry, Lehigh University, Bethlehem, Pennsylvania 18015, and Department of Physics, Brookhaven National Laboratory, Upton, New York 11973 Received August 23, 2004. In Final Form: January 5, 2005 Static and dynamic laser light scattering techniques are used to monitor the slow self-assembly of 2.5-nm-diameter, hollow spherical, fully hydrophilic heteropolyoxometalate {Mo72Fe30} macro-ions into single-layer vesicle-like “blackberries” (averaging ∼50-60 nm in diameter) in dilute salt-free and saltcontaining aqueous solutions, to obtain the thermodynamic properties of the unique self-assembly. A very high activation energy is observed during the transition from the single ion (general solute state) to blackberries (so-called “second solute state”), which might be responsible for the interestingly slow selfassembly process in dilute solutions. The thermodynamic parameters of the blackberry formation can be affected by adding simple electrolytes into the solution, because the electrostatic interactions are responsible for the unique self-assembly, and the effects of various anions and cations (in the low salt concentration regimes) are discussed. Multivalent anions make the single {Mo72Fe30} macro-ions more stable and make the blackberry formation more difficult. Small cations carrying more charges tend to accelerate the selfassembly process. This is the first study on the thermodynamic properties of the novel self-assembly in dilute solutions and the equilibrium and transition between the two solute states of macro-ions in solution.
Introduction It is our expectation that soluble inorganic ions distribute homogeneously in dilute solutions. However, this widely accepted concept might not be valid anymore when the inorganic ions reach the size of nanometer scale (i.e., macro-ions). Recently we reported that highly soluble, fully hydrophilic, wheel-shaped, 3.6-nm-sized giant polyoxomolybdate (POM) compounds, with each carrying a moderate amount (∼15) of delocalized charges, tend to self-associate into large, single-layer vesicle-like “blackberry” structures.1 A delicate balance between the repulsive electrostatic interactions and the attractive counterion effect (so-called “like-charge attraction”2), along with van der Waals force, could be responsible for the blackberry formation. Additionally, each POM anion is fully covered by a layer of water via chemical bonding, and the unique water assemblies between the POMs in the blackberries may play an important role via hydrogen bonds.1 The major difference between the POM supramolecular blackberries and the regular inorganic aggregates is that the former one is formed by highly soluble ions and itself is still a thermodynamically stable state, while the latter one is not thermodynamically stable and tends to further grow into larger aggregates until precipitation occurs. Such unique POM blackberries are also different from the vesicles formed by surfactants and biolipids via hydrophobic interactions, because the POM anions are fully hydrophilic due to the large number of charges and the chemically attached external water layer. We name the novel vesicle-like blackberry structures the “second solute state”, by referring to the homogeneous distribution of * To whom correspondence should be addressed. E-mail: TIL204@ lehigh.edu. † Lehigh University. ‡ Brookhaven National Laboratory. (1) Liu, T.; Diemann, E.; Li, H.; Dress, A. M. W.; Mu¨ller, A. Nature 2003, 426, 59. (2) Sogami, I.; Ise, N. J. Chem. Phys. 1984, 81, 6320.
the inorganic ions as the “general solute state” or the “first solute state”. The various “giant wheels” (such as {Mo154}3 and {Mo176}4) are some examples of giant strong electrolytes. More accurately, they can be treated as NaHSO4-type electrolytes that easily ionize in polar solvents. On the other hand, there also exist some giant polyoxometalates that show the behaviors of weak electrolytes, e.g., the less soluble, 2.5-nm-sized polyoxometalate containing the hollow, spherical “Keplerate” structure, [MoVI72FeIII30O252(CH 3 COO) 12 {Mo 2 O 7 (H 2 O)} 2 {H 2 Mo 2 O 8 (H 2 O)}(H 2 O) 91 ]‚ ∼150H2O ({Mo72Fe30}).5,6 The {Mo72Fe30} aqueous solutions are acidic as a result of partial deprotonation of the H2O ligands attached to the Fe centers.6 Consequently, the originally neutral {Mo72Fe30} clusters become negatively charged in solution, with each anion carrying only several localized charges.7 Overall, the {Mo72Fe30} clusters can be treated as a H3PO4-type weak acid. This compound can also self-assemble in solution to form blackberry structures.8,9 The differences in the chemical properties lead to different self-assembly behaviors between the giant wheels and the {Mo72Fe30}. The mechanisms of their blackberry formation are different: for {Mo72Fe30}, the water nanoassemblies change since the water layer is broken due to the deprotonation. The temporary Fe-O-H‚‚‚O-Fe link(3) Mu¨ller, A.; Krickemeyer, E.; Meyer, J.; Bo¨gge, H.; Peters, F.; Plass, W.; Diemann, E.; Dillinger, S.; Nonnenbruch, F.; Randerath, M.; Menke, C. Angew. Chem., Int. Ed. Engl. 1995, 34, 2122. (4) (a) Mu¨ller, A.; Koop, M.; Bo¨gge, H.; Schmidtmann, M.; Beugholt, C. Chem. Commun. 1998, 1501. (b) Jiang, C. C.; Wei, Y. G.; Liu, Q.; Zhang, S. W.; Shao, M. C.; Tang, Y. Q. Chem. Commun. 1998, 1937. (5) Mu¨ller, A.; Ko¨gerler, P.; Dress, A. W. M. Coord. Chem. Rev. 2001, 222, 193 and references therein. (6) Mu¨ller, A.; Sarkar, S.; Shah, S. Q. N.; Bo¨gge, H.; Schmidtmann, M.; Sarkar, S.; Ko¨gerler, P.; Hauptfleisch, B.; Trautwein, A. X.; Schu¨nemann, V. Angew. Chem., Int. Ed. 1999, 38, 3238. (7) Liu, T. J. Clust. Sci. 2003, 14, 215. (8) Liu, T. J. Am. Chem. Soc. 2002, 124, 10942; 2004, 126, 406 (Add./ Cor.). (9) Liu, T. J. Am. Chem. Soc. 2003, 125, 312.
10.1021/la047897o CCC: $30.25 © 2005 American Chemical Society Published on Web 02/18/2005
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ing between adjacent {Mo72Fe30} anions might contribute as an additional driving force to help the self-assembly of the blackberries. Such a unique process might be responsible for the fact that many {Mo72Fe30} blackberries are not in perfect spherical shape. Moreover, the kinetics of the {Mo72Fe30} blackberry formation also shows many unique but interesting behaviors. For example, recently we found that it took several months or even a year for the completion of blackberry formation in dilute (
