DFT Study of Polymorphism in Al(OH)3: A Structural Synthon Approach

Concentration dependent interfacial chemistry of the NaOH(aq): gibbsite interface

Caustic conditions are often employed for dissolution of a wide variety of minerals, where ion sorption, surface diffusion, and interfacial organization impact surface reactivity. In the case of gibbsite, γ-Al(OH)₃, the chemistry at the NaOH_(aq) interface is deeply intertwined with industrial processing of aluminum, including metal production and the disposition of Al-containing wastes. To date, little is known about the structure, speciation, and dynamic behavior of gibbsite interfaces (and that of many other minerals) with NaOH_(aq)-particularly as a function of ionic strength. Yet concentration-dependent interfacial organization and dynamics are a critical starting point to develop a fundamental understanding of the factors that influence dissolution. This work reports equilibrium molecular dynamics simulations of the γ-Al(OH)₃:NaOH_(aq) interface, revealing the sorption behavior and speciation of ions from 0.5-10 M [NaOH]. As inner-sphere complexes, Na⁺ primarily coordinates to the side of the gibbsite hexagonal cavities, while OH^- accepts hydrogen-bonding from the surface-OH groups. The mobility of inner-sphere Na⁺ and OH^- ions is significantly reduced due to a strong surface affinity in comparison to previous reports of NaCl, CaCl₂, or BaCl₂ electrolytes. At high [NaOH], contact ion pairing that is observed in the bulk solution is partially disrupted upon sorption to the gibbsite surface by the individual ion-surface interactions. The molecular-scale changes to surface speciation and competition between ion-surface vs. ion-ion interactions influence surface characterization of gibbsite and potential dissolution processes, providing a valuable baseline for starting conditions needed within future reactive molecular simulations.

Dehydration-Rehydration Studies on Polytypes of Chloride and Nitrate Layered Double Hydroxides of Nordstrandite and Bayerite: a Comparative Study

In this work, we report for the first time, the dehydration-rehydration studies of nordstrandite-derived layered double hydroxides (LDHs) of Li and Al, n-[Li-Al-X] (X = Cl^- and NO₃ ^-) (n-nordstrandite derived). n-[Li-Al-NO₃], an orthorhombic phase, dehydrated at 180 °C to a monoclinic phase. Refinement placed the NO₃ ^- ions parallel to the hydroxide layers. The dehydration showed no change in basal spacing. The monoclinic n-[Li-Al-Cl] dehydrated at 160 °C with a 0.49 Å compression in basal spacing to an orthorhombic polytype. We compared our results with the published results of their bayerite counterparts b-[Li-Al-X] (b-bayerite derived) and observed that though n-[Li-Al-X] and b-[Li-Al-X] LDHs have similar structures, their dehydrated phases are structurally different. We also report the refinement of b-[Li-Al-Cl] (DH). Previous studies attribute the basal spacing values to the (i) degree of hydration and (ii) orientation of anions in the interlayer. We observe that basal spacing is a manifestation of the symmetry of the crystal. Dehydration of nitrate intercalated LDH, which proceeds from an orthorhombic symmetry to a monoclinic symmetry with no decrease in the interlayer spacing, is attributed to sliding of the hydroxyl layers in the ab-plane due to the increase in the β value. This sliding stabilizes the interlayer through weak long-range electrostatic forces that mainly contribute to the stabilization of the layered structure at separations much larger than the effective radius of hydrogen bonds. Such stabilization would negate the need for the layers to compress, thus conserving the basal spacing in n-[Li-Al-NO₃] (DH).

Synthon Approach to Structure Models for the Bayerite-Derived Layered Double Hydroxides of Li and Al

The Br^- ion intercalated layered double hydroxide (LDH) of Li and Al obtained from the bayerite-Al(OH)₃ precursor crystallizes in a structure different from that of the gibbsite-Al(OH)₃ derived counterpart. Additionally, it undergoes temperature- and humidity-induced reversible interpolytype transformations. The dehydrated LDH (T ≈ 120 °C) adopts a structure of hexagonal symmetry (space group P3̅1m) and comprises a stacking of the metal hydroxide layers arranged one above another. On cooling and rehydration, the LDH adopts a structure of monoclinic symmetry with a stepwise increase in the stacking angle, β. Using the structural synthon approach, based on the systematic elimination of the principal symmetry elements of the hexagonal crystal, structure models were generated for each of the two hydration steps (relative humidity ∼50%, >70%) and the structures refined (space group C2/m). The refined structures show that the interpolytype transitions are a result of rigid translations of successive metal hydroxide layers relative to one another by translation vectors (1/6, 0, 1) and (1/3, 0, 1), respectively.