Computer Model Simulates Complex Chemical Dissolution
Scientists from Rice University in Houston, Texas, US, and the University of Bremen's Center for Marine Environmental Sciences (MARUM) in Germany, have combined cutting-edge experimental techniques with computer simulations to find a new way of predicting how water dissolves crystalline structures.
The study, featured on the cover of the Journal of Physical Chemistry C, revealed the team's method was more efficient at predicting the dissolution rates of crystalline structures - like those in natural stone and cement - in water than previous methods. The research could have wide-ranging impacts in such diverse areas as water quality and planning, environmental sustainability, corrosion resistance, and cement construction.
"We need to gain a better understanding of dissolution mechanisms to better predict the fate of certain materials, both in nature and in man-made systems," says lead investigator Andreas Lüttge, a professor of mineralogy at MARUM and professor emeritus and research professor in Earth science at Rice. His team specializes in studying the thin boundary layer that forms between minerals and fluids.
Boundary layers are ubiquitous in nature; they occur when raindrops fall on stone, water seeps through soil, and the ocean meets the sea floor. Scientists and engineers have long been interested in accurately explaining how crystalline materials, including many minerals and stones, interact with and are dissolved by water. Rate calculations for these dissolution processes are critical in many fields of science and engineering.
The animation, the result of a Monte Carlo simulation, shows a simplified, idealized crystal (Kossel-Stranski crystal). Each atom in the crystal is color coded according to the number of bonds that attach it to the surface. The fewer the bonds, the greater the probability that the atom detaches from its current position and ultimately dissolves in the surrounding fluid. As the simulation breaks bonds with nearest neighbors, the color (bonding status) of each atom changes. Atoms with five bonds are shown in red-brown, with four in tan, with three in yellow, and with two in red. Single-bonded atoms (so-called adatoms) are shown in pink, but are very rarely observed in this particular simulation.
"The new model simulates the dissolution kinetics at the boundary layer with greater precision than earlier stochastic models operating at the same scale," Kurganskaya notes. "Existing simulations rely on rate constants assigned to a wide range of possible reactions, and as a result, the total material flux from the surface has an inherent variance range - a plus or minus factor that is always there."
The team's simulations more accurately represent real processes because models incorporate actual measurements from cutting-edge instruments and high-tech materials, including glass ceramics and nanomaterials. With a special imaging technique called 'vertical scanning interferometry,' which MARUM and Rice helped develop, the team scanned the crystal surfaces of both minerals and manufactured materials to generate topographic maps with a resolution of a just a few nanometers, or billionths of a meter.
"We found that dissolution rates that were predicted using rate constants were sometimes off by as much as two orders of magnitude," says Lüttge. The new prediction method could revolutionize the way engineers and scientists make calculations related to a myriad of things, including the stability of building materials and the longevity of materials used for radioactive waste storage.
"Further work is needed to prove the broad utility of the method," he said. "In the next phase of research, we plan to test our simulations on larger systems and over longer periods." -- by Jade Boyd, © i SGTW
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