Ab initio molecular dynamics free energy study of enhanced copper (II) dimerization on mineral surfaces

Abstract

Understanding the adsorption of isolated metal cations from water on to mineral surfaces is critical for toxic waste retention and cleanup in the environment. Heterogeneous nucleation of metal oxyhydroxides and other minerals on material surfaces is key to crystal growth and dissolution. The link connecting these two areas, namely cation dimerization and polymerization, is far less understood. In this work we apply ab initio molecular dynamics calculations to examine the coordination structure of hydroxide-bridged Cu(II) dimers, and the free energy changes associated with Cu(II) dimerization on silica surfaces. The dimer dissociation pathway involves sequential breaking of two Cu2+-OH− bonds, yielding three local minima in the free energy profiles associated with 0-2 OH− bridges between the metal cations, and requires the design of a (to our knowledge) novel reaction coordinate for the simulations. Cu(II) adsorbed on silica surfaces are found to exhibit stronger tendency towards dimerization than when residing in water. Cluster-plus-implicit-solvent methods yield incorrect trends if OH− hydration is not correctly depicted. The predicted free energy landscapes are consistent with fast equilibrium times (seconds) among adsorbed structures, and favor Cu2+ dimer formation on silica surfaces over monomer adsorption.

Structure, Stability, and Acidity of the Uranyl Arsenate Dimer in Aqueous Solution

“…Therefore, we calculate the PMFs using the U–U distance as a 1-D reaction coordinate, which can reflect the total energy changes in the U–As system. It is worth noting that the recent work for Cu dimerization used a designed reaction coordinate, which allows the dimer to dissociate into two monomers quickly by refining the distances between intermediates O and Cu . Alternatively, we employed the U–U distance as the reaction coordinate in order to find a possible reaction pathway.…”

“…To accurately calculate the dissociation free energies Δ G , we integrate the free-energy profiles Δ F ( Q ) as a function of the reaction coordinate Q and account for the entropic contribution from a standard state 1.0 M ideal concentration solution ,, normalΔ G = prefix− k normalB T 0.25em log ( ∫ normalΩ ( Q ) normald Q 0.25em exp [ Δ F false( Q false) k B T ] / V normalo ) Here, V o is the volume associated with 1.0 M aqueous solution (1662 Å 3 ) and Ω is the surface area of the sphere defined by one particle moving around the other.…”

“…It is worth noting that the recent work for Cu dimerization used a designed reaction coordinate, which allows the dimer to dissociate into two monomers quickly by refining the distances between intermediates O and Cu. 43 Alternatively, we employed the U−U distance as the reaction coordinate in order to find a possible reaction pathway. Nevertheless, additional reaction coordinates controlling U bonding (e.g., U−H 2 O or U−AsO 4 3−…”

“…To accurately calculate the dissociation free energies ΔG, we integrate the free-energy profiles ΔF(Q) as a function of the reaction coordinate Q and account for the entropic contribution from a standard state 1.0 M ideal concentration solution 43,69,74…”

“…Recently, quantum chemistry methods have been successfully applied to investigate the thermodynamic properties of metal ions and their complexes, including those of U. − For example, classical molecular dynamics (CMD) techniques have been used to calculate the structural information of uranyl ions and uranyl carbonate species. ,,− Static density functional theory (DFT) calculations together with experiments have revealed the adsorption mechanism of uranyl ions and the possible structures of the uranium arsenate complex . Combining electronic structure calculation with molecular dynamics sampling, first-principles molecular dynamics (FPMD) have served as a more effective approach to probe the thermodynamic properties of uranium-related species in aqueous solution. ,, For instance, the structural information and nuclear magnetic resonance (NMR) properties of UO 2 2+ , UO 2 (CO 3 ) 3 4– , and (UO 2 ) 3 (CO 3 ) 6 6– in water were explored by using the FPMD method .…”