pH dependent reactivity of boehmite surfaces from first principles molecular dynamics

Abstract

pH dependent interfacial chemistry depends upon the distribution and respective pKa values of different surface active sites. This is highly relevant to chemistry of nanoparticle morphologies that expose faces with...

Predicting Outcomes of Nanoparticle Attachment by Connecting Atomistic, Interfacial, Particle, and Aggregate Scales

“…Recently, Smith et al used ab initio molecular dynamics (AIMD) simulations to calculate the reactivity of oxygens on the boehmite (010), (101), (100), and (001) faces, finding that the pK a values are primarily determined by the coordination of oxygen with neighboring aluminum atoms. 55 The (010) plane possesses μ 2 and μ 4 oxygens, which are bound to two and four aluminum atoms, respectively, but only the μ 2 oxygen is expected to be reactive. The (101) face possesses two types of oxygens, denoted μ 1 and μ 2 , that are bound to one and two aluminum atoms.…”

“…The (100) and (001) additionally show μ 3 oxygens, but these faces are not strongly expressed on the boehmite platelets studied here. Using the p K a values from Smith et al, we have developed a surface complexation model to predict facet-dependent charging behavior. The model treats proton adsorption as a Stern layer phenomenon, with formal charges assigned to each binding site by the bond-valence approach .…”

“…Its crystal structure is highly anisotropic, and therefore, depending on the synthesis method, boehmite nanoparticles can exhibit a variety of morphologies and surface chemistries, thus providing an excellent opportunity for investigating the connection between particle shape and aggregation outcomes . Boehmite surfaces possess various hydroxyl groups whose protonation state is influenced by the solution conditions, , and it exhibits complex interfacial solution structure associated with a surface lattice exposing metal, oxygen and hydroxyl sites. − Such a nanomaterial presents an ideal model system for connecting nanoparticle surface reactivity to emergent aggregation behavior, as its surface chemistry, which includes metal ion, oxygen, and hydroxyl sites, is broadly representative of particles in geochemical and industrial settings. Moreover, boehmite aggregation has important implications for environmental management through its potential impacts on the rheological properties of legacy nuclear waste slurries destined for vitrification at the Unites States Department of Energy’s Hanford Site in Washington State. , …”

Macroscopic volume phase transitions in supramolecular gels directed by covalent crosslinking

“…24,25 Under these conditions, the pH of the gel was higher than the apparent pK a of 2 suggesting that the gel at pH 9.3 is dominated by the amine form (>50%) than the ammonium form of 2. Analysis of the equilibrium notions using Henderson-Hasselbalch equation 35,36 reveals ~80% deprotonation of 2 at pH 9.3. On further increasing the pH to 10.8 (with 2 equivalents of NaOH), the stiffness (G′) of the material drops dramatically due to the complete loss of charge on the ammonium group (~100% deprotonation of 2) and thereby destruction of the salt-bridge between 1 and 2 (Figure 2b, Figure S6).…”

Effect of Adsorbed Carboxylates on the Dissolution of Boehmite Nanoplates in Highly Alkaline Solutions

“…To mimic the 3 M NaOH/boehmite interface condition, 6% of the boehmite (010) surface −OH sites are deprotonated considering the (010) surface pk a value of 15.69. 43 Three potential surface site types are considered for the oxalate dianion adsorption, namely, a normal hydroxyl site (OH), deprotonated site (O − ), and surface Na + site where Na + is bonded to the deprotonated O (Figure S1). For comparison purposes, in all the systems, a sodium cation is tied to an O − site with harmonic spring (k = 5 kcal/mol).…”