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Atomistic Simulations of the Dioctahedral 2:1 Phyllosilicate Edge-Water Interface: Structure, Dynamic Properties, and Cation Complexes

  • Author(s): Newton, Aric George
  • Advisor(s): Sposito, Garrison
  • et al.
Abstract

The abundance and reactivity of 2:1 phyllosilicate minerals (e.g. mica, vermiculite, smectite, illite, and pyrophyllite) have made them a subject of much research. The permanent structural and variable edge charge that these minerals possess provides a diverse set of modes through which surface reactions occur. Surface reactions due to permanent charge occur at the basal surface or in the interlayer of the phyllosilicate sheet. These reactions are well characterized and understood at multiple length scales in geochemistry (i.e. from the angstrom to the micron scale). This understanding arises from the detailed atomic-scale description of the bulk mineral structure and the origin of the permanent charge in isomorphic substitutions within the mineral structure. Variable charge arises from acid-base reactions at the edges of the 2:1 phyllosilicates sheets. These edge surface reactions are important in the retention of ions, stabilization of soil organic matter, rheological properties and colloidal behavior of clay minerals, and the dissolution kinetics of the 2:1 phyllosilicates. Despite the importance of the edge to the reactivity and other surface properties of these minerals, an atomistic description of the edge structure is lacking.

Periodic bond chain (PBC) theory identifies the dominant edges and nanoparticulate morphologies of 2:1 phyllosilicate minerals. Initial atomic structures of the 2:1 phyllosilicate edges and pyrophyllite nanoparticle defined by PBC theory were simulated using molecular dynamics (MD). The equilibrium edge structures reveal that the chemistry of the aluminum (Al) at the edge-water interface plays an important role in the inherent disorder of the 2:1 phyllosilicate edges. The edge Al atoms respond to increases in local surface charge by decreasing the number of coordinating oxygen atoms. The edge Al atoms assume four different polyhedral morphologies (i.e. octahedral, square pyramidal, trigonal bipyramidal, and tetrahedral Al polyhedra). The development of a trigonal bipyramidal Al structure indirectly creates an inverted Si tetrahedron that is consistent with a near-forgotten model of smectite structure. This non-planar and irregular structure of the mineral edge creates an interfacial water structure that is distinct from that of the 2:1 phyllosilicate basal surface. The water self-diffusion coefficient at the surface is two orders of magnitude less at the interface than in the bulk water. Monovalent cation complexes at the edge interface almost exclusively form inner-sphere surface complexes. These cationic edge surface complexes can form at any one of three non-equivalent vacancies at the 2:1 phyllosilicate edge. Dimeric surface complexes begin to form with increasing cation surface excess as the pH increases.

These MD simulations of the 2:1 phyllosilicate edges, nanoparticles, and cationic edge surface complexes demonstrate the complexity of the edge surface structure and reactivity. These insights from MD simulations provide support for interpretations of experimental results that heretofore could not be tested. In addition, these results demonstrate that a high-quality set of atomic interaction parameters derived from ab initio simulations of atomic clusters can be used to predict complex mineral structures.

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