Advanced Imaging of Organic Ligand-Mineral Interactions
Drs. Paul Fenter and Neil Sturchio of Argonne National Laboratory utilized the Advanced Photon Source to demonstrate that the reactive organic ligand, stearate, has a systematic, ordered structure when attached to a calcium carbonate mineral substrate. X-ray reflectivity measurements of the model carbonate-stearate-methanol system show that the surface structure of the mineral is altered by attachment of the stearate. The stearate molecules in the thin surface film are aligned at a high angle to the mineral surface and separate the methanol solvent from the mineral surface with a layer of organic matter. Stearate molecules are constantly being added to or removed from this layer in a dynamic process. Within measurement error, the stearate molecular coverage is equal to the to the density of calcium sites on the mineral surface. This strongly suggests that that the adsorption mechanism can be uniquely identified as chemisorption, replacing a Ca-O bond. It takes only a short time for the surface to reestablish equilibrium after a change the solution concentration, suggesting that the adsorbed coverage is a result of dynamic equilibrium between desorption and adsorption processes. The ability to make these types of quantitative measurements is a significant step forward in our ability to understand the interactions among a wide range of natural and manmade organic compounds and mineral surfaces in the shallow earth.
