A Molecular Dynamics Approach towards the Interfacial Properties of Sulfide- and Clay-Minerals

  • Author / Creator
    Hosseini Anvari, Monir
  • Interfacial properties have a regulatory role in processes which involve the coexistence of different phases. Although such macro-scale properties are determined by the intrinsic nature of the interface-sharing phases, it is possible to alter them in favor of the process objectives. This, however, is subject to understanding the underlying factors that control these properties at the micro-scale, which is achievable through atomistic simulations. In general, this study is oriented around investigation of the interfaces that occur in the context of froth flotation of sulfide minerals and the solvent-based extraction of bitumen from oilsands, through a molecular dynamics (MD) approach. In the first step, the size-dependence of contact angle, as a measure of surface wettability, was elucidated for the hydrophilic zinc sulfide (sphalerite) and the rather hydrophobic lead sulfide (galena). Determining the contact angles of a series of nano-sized water clusters provided an estimation of the line tension and the macroscopic contact angles of the two mineral surfaces, based on the modified Young’s equation. It was made evident that unlike the case of galena with a positive line tension, the favorable interactions between water and sphalerite would cause the microscopic contact angles to be smaller than the macroscopic value, yielding a negative line tension. The simulations were then extended to the collector-modified surface of sphalerite. Butylthiol molecules, made up of normal and branched alkyl tails, were grafted onto the adsorption sites of the surface at different coverages and in two distinct distributions – ordered and random. For a given butylthiol at a given site coverage, random surface distribution yielded a slightly larger contact angle, due to smaller patches of the bare surface being exposed to water molecules. The Test Area Method and the Kirkwood and Buff approach were adopted to estimate surface energies (γ_SV) of the bare sphalerite (110) surface and the collector monolayer, respectively. This led to estimation of the apparent surface energies and solid-water interfacial tensions (γ_SL), which both exhibited a linear inverse dependence on the surface coverage with a crossover point at 25% coverage. The results also revealed that at coverages above 85%, contact angles of the branched thiols are significantly lower than their normal counterparts. We then proceeded with studying the interface of water-cyclohexane mixtures with kaolinite, as a common host material for bitumen. On a dry clay basis, concentrations of w_(H_2 O)≈6 to 30% and w_(C_6 H_12 )≈14% were studied. Using the Gouy-Chapman theory, formation of water bridges between the two surfaces was attributed to the overlapping of the surface potentials in the interior region of the pore. Larger areas of the hydrophobic tetrahedral surface became water-wet with the increase in water concentration, to minimize the contact area between the oil and the water phase. Addition of sodium chloride to the aqueous phase at concentrations of 0.1, 0.5 and 1 M substantially improved the wetting of basal surfaces. At the highest salt concentration, breakage of the water bridge was observed, and a phase-separated, three-layer structure (water-cyclohexane-water) was formed within the nanopore, caused by the screening effect of the adsorbed counter-ions. The focus of the next step was put on the effect of different inorganic solutes on the interfacial properties of water and cyclohexane, out of and within a confined environment. Sodium decanoate was also included, as a representative organic ion which is prone to strong adsorption to the clay surfaces. Four aqueous phases were used, each containing one of the four solutes (NaCl, NaOH, CaCl2 and Ca(OH)2) at the concentration of 1.0 M. At the interface of water and neat cyclohexane, the more strongly hydrated ions, such as calcium and hydroxide, were more intensely depleted as compared to sodium and chloride. The interfacial tension increments were proportional to the ions’ surface exclusion. Upon addition of sodium decanoate to the cyclohexane phase, a small fraction of the solvated cations in the water phase drifted to the depletion zone. When such systems were confined in a kaolinite nanopore, the adsorption behavior of decanoate anions was determined by the nature of the solvated ions in water. With CaCl2, almost all of these organic ions were released from the octahedral surface, owing to the strong propensity of calcium towards the organic interface, and the inner-sphere adsorption of majority of chlorides to the octahedral surface.

  • Subjects / Keywords
  • Graduation date
    Spring 2019
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
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