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Montmorillonite layer edge surfaces have pH-dependent properties, which arises from the acid-base reactivity of their surface functional groups. Edge surface acidity (with intrinsic reaction equilibrium constant, pKa) is a chemical property that is affected by crystal structure. While a cis-vacant structure predominates in natural montmorillonites, prior molecular-level studies assume a centrosymmetric trans-vacant configuration, which potentially leads to an incorrect prediction of montmorillonite acid-base surface properties. We computed intrinsic acidity constants of the surface sites of a montmorillonite layer with a cis-vacant structure using the first-principles molecular dynamics-based vertical energy gap method. We evaluated pKa values for both non-substituted and Mg-substituted layers on common edge surfaces (i.e., surfaces perpendicular to [010], [01̅0], [110], and [1̅1̅0] crystallographic directions). The functional groups ≡Si(OH), ≡Al(OH2)2/≡Al(OH)(OH2), and ≡SiO(OH)Al sites on surfaces perpendicular to [010] and [01̅0] and ≡Si(OH)U, ≡Si(OH)L, ≡Al(OH2), and ≡Al(OH2)2 on surfaces perpendicular to [110] and [1̅1̅0] determine the proton reactivity of non-substituted cis-vacant edge surfaces. Moreover, the structural OH sites on edge surfaces had extremely high pKa values, which do not show reactivity at a common pH. Meanwhile, Mg2+ substitution results in an increase in pKa values at local or adjacent sites, in which the effect is limited by the distance between the sites. A surface complexation model was built with predicted pKa values, which enabled us to predict surface properties as a function of pH and ionic strength. Edge surface charge of both trans- and cis-vacant models has little dependence on Mg2+ substitutions, but the dependence on the crystal plane orientation is strong. In particular, at pH below 7, edge surfaces are positively or negatively charged depending on their orientation. Implications of these findings on contaminant adsorption by smectites are discussed.

Knowing the influence of isomorphic substitutions on the acid-base properties of smectite edge surfaces is an important aspect of the detailed understanding of clay minerals' interfacial properties with implications in the modeling of adsorption processes. We investigated the intrinsic acidity constants of Fe(II)/Fe(III) and Al(III) substituted edge surface sites of montmorillonite with a cis-vacant structure, which includes four crystallographic orientations perpendicular to [010], [0 1 0], [110], and [ 1 1 0], using the first-principles molecular dynamics (FPMD) based vertical energy gap method. Fe(II) and Fe(III) substitutions resulted, respectively, in a significant increase and decrease in pKa values of amphoteric groups directly associated with Fe octahedra. In addition, Fe(II) substitution increased the pKa values of the neighboring silanol sites, while Fe(III) substitution had a weak influence on these sites. The Al-substituted tetrahedra had amphoteric sites with higher pKa values than the non-substituted Si tetrahedra, and they increased significantly the pKa values of the sites bridging the tetrahedral and octahedral sheets on surfaces perpendicular to [010] and [110]. The acid-base properties of substituted and non-substituted surface sites of cis-vacant montmorillonite were used to build a state-of-the-art surface complexation model, which successfully reproduced the best available experimental acid-base titration data. This model was further used to predict acid-base properties of dioctahedral smectites (montmorillonite, beidellite, and nontronite) according to their cis- or trans-vacant structures and their layer chemistry. According to these predictions, these smectites exhibit very similar overall pH buffering properties despite significant differences in structure and chemistry. A detailed analysis of the acid-base properties as a function of crystallographic directions demonstrated, however, that these differences should have a large influence on the adsorption of ionic species. [ABSTRACT FROM AUTHOR]

Multinuclear U(VI) species may be dominant in aqueous solutions under environmental conditions, while the structures of the multinuclear U(VI) species on mineral surfaces remain unclear. This work reports the structural and bonding properties of the possible surface complexes of three aqueous multinuclear U(VI) species, i.e., (UO2)2(OH)3+, (UO2)2(OH)22+ and (UO2)3(O)(OH)3+, on the hydroxylated α-SiO2(001) surface based on density functional theory (DFT) calculations. The results show that (UO2)2(OH)22+ and (UO2)3(O)(OH)3+ tend to form end-on structures at ≡SiO(H)SiO(H) sites, whereas (UO2)2(OH)3+ prefers a side-on structure at ≡SiO(H)O(H)-SiO(H)O(H) sites. The main driving forces for the formation of the multinuclear U(VI) surface complexes are electrostatic interactions and partially covalent chemical bonds. The Os-2p orbital hybridizes strongly with U-5f and U-6d orbitals, with a decreasing binding strength in the sequence of (UO2)2(OH)3+ > (UO2)2(OH)22+ > (UO2)3(O)(OH)3+ for the adsorption at the same type of surface sites. For the adsorption of the same multinuclear U(VI) species, the binding energy increases with the deprotonation extent of the identical sites. In addition, hydrogen bonds between surface hydroxyls and coordination waters as well as the acyl oxygen of uranyl moieties contribute to the formation of the multinuclear U(VI) surface complexes. The U-5f electron delocalization of far-side U atoms in the end-on structures of (UO2)2(OH)22+ and (UO2)3(O)(OH)3+ surface complexes also contributes slightly to the overall binding energy. Overall, this study provides insights into the adsorption behavior of multinuclear U(VI) on silica. [ABSTRACT FROM AUTHOR]

To tune the complexation and solvent extraction performance of the ligands with a 1,10-phenanthroline core for trivalent actinides (An3+) and lanthanides (Ln3+), we synthesized two new asymmetric tetradentate ligands with pyrazole and amide groups, i.e., L1 (N,N-diethyl-9-(5-ethyl-1H-pyrazol-3-yl)-1,10-phenanthroline-2-carboxamide) and its analogue L2 with longer alkyl chains (N,N-dihexyl). The complexation of the ligands with Ln3+ was confirmed by 1H NMR titration and X-ray crystallography, and stability constants were measured in methanol by spectrophotometric titration. The asymmetric ligands exhibited an improved performance in terms of selective solvent extraction of Am3+ over Eu3+ in strongly acidic solutions compared to their symmetric analogues. The improved selectivity of the asymmetric ligands was interpreted theoretically by density functional theory simulations. This study implies that combining different functional groups to construct asymmetric ligands may be an efficient way to tune ligand performance with regard to An3+ separation from Ln3+. [ABSTRACT FROM AUTHOR]

Homogeneous confinement of metal oxide nanoparticles into the heteroatom‐doped carbon matrix is highly desirable but remains challenging to develop high‐performance anode materials. Herein, a facile simultaneous polymerization of 2‐thiophenemethanol (ThM) and titanium ethoxide (TTEO) is proposed to incorporate ultrafine anatase TiO2 nanocrystals into the continuous S‐doped carbon framework by carbonizing the as‐produced TiO2/poly(2‐thiophenemethanol) composites. This in situ crafting process spatially suppresses the growth of TiO2 by the polymerization of ThM, thus producing size‐adjustable TiO2 nanocrystals (5–50 nm) by tailoring the ratio of TTEO to ThM. It is found that smaller TiO2 nanocrystals are generated using the lower TTEO‐to‐ThM ratio. When used as lithium‐ion batteries (LIBs) anode, TiO2/S‐doped carbon (TSC) shows larger reversible capacity, superior rate capability, and better cycling stability compared to pure TiO2. A typical TSC with the smallest TiO2 nanocrystals (8 nm) shows the reversible capacity as high as 386 mAh g−1 at 50 mA g−1, and retains 55% of initial capacity at 500 mA g−1, and 96.8% capacity after 500 cycles at 1000 mA g−1. The excellent electrochemical performance is attributed to the shortened ion diffusion path of ultrafine TiO2 and the improved electronic conductivity, structural stability, and surface/interface nature by the S‐doped carbon matrix. [ABSTRACT FROM AUTHOR]

Multinuclear U(VI) species may be dominant in aqueous solutions under environmental conditions, while the structures of the multinuclear U(VI) species on mineral surfaces remain unclear. This work reports the structural and bonding properties of the possible surface complexes of three aqueous multinuclear U(VI) species, i.e. , (UO 2 ) 2 (OH) 3+ , (UO 2 ) 2 (OH) 2 2+ and (UO 2 ) 3 (O)(OH) 3 + , on the hydroxylated α-SiO 2 (001) surface based on density functional theory (DFT) calculations. The results show that (UO 2 ) 2 (OH) 2 2+ and (UO 2 ) 3 (O)(OH) 3 + tend to form end-on structures at SiO(H)SiO(H) sites, whereas (UO 2 ) 2 (OH) 3+ prefers a side-on structure at SiO(H)O(H)-SiO(H)O(H) sites. The main driving forces for the formation of the multinuclear U(VI) surface complexes are electrostatic interactions and partially covalent chemical bonds. The O s -2p orbital hybridizes strongly with U-5f and U-6d orbitals, with a decreasing binding strength in the sequence of (UO 2 ) 2 (OH) 3+ > (UO 2 ) 2 (OH) 2 2+ > (UO 2 ) 3 (O)(OH) 3 + for the adsorption at the same type of surface sites. For the adsorption of the same multinuclear U(VI) species, the binding energy increases with the deprotonation extent of the identical sites. In addition, hydrogen bonds between surface hydroxyls and coordination waters as well as the acyl oxygen of uranyl moieties contribute to the formation of the multinuclear U(VI) surface complexes. The U-5f electron delocalization of far-side U atoms in the end-on structures of (UO 2 ) 2 (OH) 2 2+ and (UO 2 ) 3 (O)(OH) 3 + surface complexes also contributes slightly to the overall binding energy. Overall, this study provides insights into the adsorption behavior of multinuclear U(VI) on silica.