Unique structures in molybdenum blue solutions reveal possible new solute state

For nearly 200 years, scientists have known that the elements molybdenum and oxygen can form various large molecules, which usually impart a unique blue color to aqueous solutions. Only recently have scientists been able to isolate these molecules, but no one was able to explain their supramolecular structure in solution, until now. In a paper scheduled to appear in an upcoming issue of the Journal of the American Chemical Society (available online August 20), Tianbo Liu, a physicist at the U.S. Department of Energy’s Brookhaven National Laboratory, describes the unique “blackberry” structure, which may represent a new, stable solute state never seen before.

“The nature of ’molybdenum blue solutions’ has remained a fascinating enigma for inorganic chemists since the late 1700s and early 1800s,” said Liu. In 1826, scientists discovered the first so-called polyoxomolybdate (POM) molecules with a chemical formula of Mo5O14, and realized that the electronic state of the molybdenum atoms was responsible for the blue color in solution. However, the molybdenum blue solutions contained many more complicated molecules. For a long time, scientists were unable to isolate these molecules.

Recently, however, scientists have isolated several different polyoxomolybdate molecules from various molybdenum blue solutions — all “giant” compared to other inorganic molecules (see http://www.bnl.gov/bnlweb/pubaf/pr/2002/bnlpr_spotlights_2002.htm). Unlike other water-soluble inorganic compounds, such as common table salt (NaCl), giant POMs do not exist as single ions in water. Instead, they cluster together. But scientists were still unable to understand the structures of these aggregates, even with the help of electronic microscopes.

Now, using static and dynamic laser light scattering — techniques formerly reserved for larger particles and polymers — Liu has deciphered the structure of these inorganic POM clusters. “Once we found how big these molecules were [2.5-5.1 nanometers, or billionths of a meter, aggregating in clusters as large as 70-300 nanometers], we realized we could use laser light scattering to decipher the structure,” said Liu.

The laser light scattering technique works similar to the way we see objects by looking at the light that bounces off of them, except that the scientists use highly focused laser light and detectors that can “see” details on a much smaller scale than the human eye.

Using these techniques Liu was able to determine the radius of the individual particles and the particle clusters, the size distribution of the clusters, how far from the center the mass of the clusters is distributed, and the mass of the clusters. Putting all these pieces together, Liu has concluded that hundreds of individual POM molecules form hollow, spherical clusters, where all of them are clustered around the surface of the sphere.

Yet this solution to the structural enigma has now opened another mystery, says Liu. “What is the new physics behind this structure?” he asks. Unlike sodium and chloride ions, which distribute evenly in solution, or larger, charged particles like DNA or proteins, which form large clusters and precipitate out, POMs form stable clusters and remain in solution.

“We believe we are seeing a new, thermodynamically stable state for solutes, where large-size, single molecules with a limited amount of charge on the surface will all form hollow spherical clusters,” says Liu. The hollow vesicle structure allows the particles to remain suspended. Liu likens the new structure to a blackberry.

“We are still looking for theoretical explanations for the new solute state,” says Liu. He has found that some other giant molecules with different shapes also adopt this new structure in solution, suggesting that the hollow spherical structure may be a universal state for certain solutes.