Acidity of edge surface sites of montmorillonite and kaolinite

Abstract

Acid-base chemistry of clay minerals is central to their interfacial properties, but up to 10 now a quantitative understanding on the surface acidity is still lacking. In this study, with first principles 11 molecular dynamics (FPMD) based vertical energy gap technique, we calculate the acidity constants of 12 surface groups on (010)-type edges of montmorillonite and kaolinite, which are representatives of 2:1 13 and 1:1-type clay minerals, respectively. It shows that ≡Si-OH and ≡Al-OH 2 OH groups of kaolinite 14 have pKas of 6.9 and 5.7 and those of montmorillonite have pKas of 7.0 and 8.3, respectively. For each 15 mineral, the calculated pKas are consistent with the experimental ranges derived from fittings of 16 titration curves, indicating that ≡Si-OH and ≡Al-OH 2 OH groups are the major acidic sites responsible to 17 pH-dependent experimental observations. The effect of Mg substitution in montmorillonite is 18 investigated and it is found that Mg substitution increases the pKas of the neighboring ≡Si-OH and ≡Si-19 OH 2 groups by 2~3 pKa units. Furthermore, our calculation shows that the pKa of edge ≡Mg-(OH 2 ) 2 is 20 as high as 13.2, indicating the protonated state dominates under common pH. Together with previous 21 adsorption experiments, our derived acidity constants suggest that ≡Si-O-and ≡Al-(OH) 2 groups are the 22 most probable edge sites for complexing heavy metal cations.

Interlayer Structures and Dynamics of Arsenate and Arsenite Intercalated Layered Double Hydroxides: A First Principles Study

“…Recently, precise information of the speciation, structures, and acidities of As in aqueous solutions has been obtained by using FPMD [54], indicating that FPMD can provide precise information in As-bearing systems. The efforts to explore the structural and thermodynamic properties of layered minerals such as gibbsite [55] and clay minerals [56][57][58][59] also obtained results consistent with experiments. In this study, we employ first principles simulation techniques to investigate basal spacings and interlayer properties of arsenate and arsenite intercalated LDHs.…”

Molecular Dynamic Simulations of Montmorillonite–Organic Interactions under Varying Salinity: An Insight into Enhanced Oil Recovery

“…Basal {001} surfaces of montmorillonite (the periodic models have no edge sites) are relatively pH stable and unlikely to vary within the pH range encountered within a typical oil reservoir. 24,25 All organic molecules used in this study were created using the Avogadro molecular editing suite. 26 Periodically replicated supercells contained one layer of montmorillonite composed of 84 unit cells (12 x 7 x 1), dimensions of approximately 6 x 6 x 6 nm, and a d -spacing of approximately 5 nm.…”

Na-Montmorillonite Edge Structure and Surface Complexes: An Atomistic Perspective

“…This pH condition imposes some limitations on our findings; however, the results should be considered in light of the calculated pK a s of the B edge surface groups. The calculated pK a s for the B edge are~7 for silanol groups and 8 for aluminol but are subject to relatively large uncertainties that result in the overlap of the two values [25][26][27]. On the AC edge, the pK a for the amphoteric Al site (i.e., ≡Al-OH 2 ) is 5.5 [26].…”

“…When the surface octahedra include isomorphic substitutions, however, this homogeneous assignment is perhaps insufficient. The observed spontaneous transfer of protons on the pyrophyllite B edge surface during DFT-MD simulations [20,24,34]; the effects of isomorphic substitution on the pK a of edge surfaces groups [25]; and the considerable difference between the first hydrolysis constants of Mg and Al in solution (pK a1 = 11.4 v. 5.0, respectively [55]) all suggest that the investigation of alternative initial surface proton distributions is prudent. Liu, et al [24] used constrained DFT-MD to calculate the free-energy profile for H 2 O dissociation from the phyllosilicate edges and identified variations in the surface functional groups when Mg is substituted for solvent accessible Al sites.…”

“…These edge structures have been the initial configuration for atomistic simulations of the 2:1 clay edge at both the quantum mechanical (QM) and molecular mechanical (MM) levels of theory. Many QM simulations have been used for a detailed characterization of the 2:1 edge structures, surface energies, acid-base reactivity, and cationic surface complexes [18][19][20][21][22][23][24][25][26][27]. These simulations are the most theoretically rigorous available, but the theoretical rigor comes with high computational costs that limit the model size and the duration of any QM-based molecular dynamics (MD) simulations.…”