Effects of layer-charge distribution on the thermodynamic and microscopic properties of Cs-smectite

Liu, Xiandong; Lu, Xiancai; Wang, Rucheng; Zhou, Huiqun

doi:10.1016/j.gca.2008.01.028

Cited by 77 publications

(141 citation statements)

References 58 publications

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“…The amount of negative layer charge is the most important factor determining the surface confining effects on water of smectite, and the distribution of layer charge is another important factor [38]. As shown in Table 1, NAu and SAz both contain 1 negative layer charge per unit cell while SWy only contains 0.75 charges.…”

Section: Influence Of Layer-charge Amount and Distributionmentioning

confidence: 99%

Hydration of methane intercalated in Na-smectites with distinct layer charge: Insights from molecular simulations

Zhou

Liu

et al. 2011

Journal of Colloid and Interface Science

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Section: Influence Of Layer-charge Amount and Distributionmentioning

confidence: 99%

Hydration of methane intercalated in Na-smectites with distinct layer charge: Insights from molecular simulations

Zhou

Liu

et al. 2011

Journal of Colloid and Interface Science

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“…Therefore, the models containing two water layers are focused on in this study. Based on our previous studies (Liu and Lu, 2006;Liu et al, 2008a), the 2-layer hydrate has about 10 water molecules per unit cell. With (4a Â 2b Â 2c) models, we carry out NPT MD simulations under ambient conditions to determine the basal spacing values.…”

Section: Systemsmentioning

confidence: 99%

“…For example, according to numerous experimental and theoretical studies, it has been well recognized that in interlayer spaces, water can form integer-number molecular layers and organic ions (e.g. alkylammoniums) can form layering or paraffin configurations depending on the layer-charge characteristics (Mooney et al, 1952a,b;Bérend et al, 1995;Boek et al, 1995;Skipper et al, 1995a,b;Karaborni et al, 1996;Cases et al, 1997;Chang et al, 1998;Skipper, 1998;Smith, 1998;Sutton and Sposito, 2001;Tambach et al, 2004a,b;Lagaly et al, 2006;Liu and Lu, 2006;Skipper et al, 2006;Liu et al, 2007Liu et al, , 2008aLiu et al, , 2009Cygan et al, 2009;Suter et al, 2009;Anderson et al, 2010).…”

Section: Introductionmentioning

confidence: 99%

Acidities of confined water in interlayer space of clay minerals

Liu

Wang

et al. 2011

Geochimica et Cosmochimica Acta

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The acid chemistry of confined waters in smectite interlayers have been investigated with first principles molecular dynamics (FPMD) simulations. Aiming at a systematic picture, we establish the model systems to take account of the three possible controlling factors: layer charge densities (0 e, 0.5 e and 1.0 e per cell), layer charge locations (tetrahedral and octahedral) and interlayer counterions (Na + and Mg 2+). For all models, the interlayer structures are characterized in detail. Na + and Mg 2+show significantly different hydration characteristics: Mg 2+ forms a rigid octahedral hydration shell and resides around the midplane, whereas Na + binds to a basal oxygen atom and forms a very flexible hydration shell, which consists of five waters on average and shows very fast water exchanges. The method of constraint is employed to enforce the water dissociation reactions and the thermodynamic integration approach is used to derive the free-energy values and the acidity constants. Based on the simulations, the following points have been gained. (1) The layer charge is found to be the direct origin of water acidity enhancement in smectites because the neutral pore almost does not have influences on water dissociations but all charged pores do. (2) With a moderate charge density of 0.5 e per cell, the interlayer water shows a pKa value around 11.5. While increasing layer charge density to 1.0 e, no obvious difference is found for the free water molecules. Since 1.0 e is at the upper limit of smectites' layer charge, it is proposed that the calculated acidity of free water in octahedrally substituted Mg 2+ -smectite, 11.3, can be taken as the lower limit of acidities of free waters. (3) In octahedrally and tetrahedrally substituted models, the bound waters of Mg 2+ show very low pKa values: 10.1 vs 10.4. This evidences that smectites can also promote the dissociations of the coordinated waters of metal cations. The comparison between the two Mg 2+ -smectites reveals that different layer charge locations do not lead to obvious differences for bound and free water acidities.

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“…Extensive studies have detailed the structures and properties of basal surfaces. The surfaces have been shown to be terminated with siloxanes, the Si-O rings of which are important adsorbing sites for many cations (Cygan et al, 2004(Cygan et al, , 2009Bergaya et al, 2006;Liu and Lu, 2006;Liu et al, 2007Liu et al, , 2008aAnderson et al, 2010). In contrast, the edge surfaces have more complex structures and thus more subtle properties.…”

Section: Introductionmentioning

confidence: 99%

Atomic-scale structures of interfaces between phyllosilicate edges and water

Liu

Meijer

et al. 2012

Geochimica et Cosmochimica Acta

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We report first-principles molecular dynamics (FPMD) studies on the structures of interfaces between phyllosilicate edges and water. Using FPMD, the substrates and solvents are simulated at the same first-principles level, and the thermal motions are sampled via molecular dynamics. Both the neutral and charged silicate frameworks are considered, and for charged cases, the octahedral (Mg for Al) and tetrahedral (Al for Si) substitutions are taken into account. For all frameworks, we focus on the commonly occurring (0 1 0)-and (1 1 0)-type edge surfaces.With constrained FPMD, we calculated the free energy of the leaving processes of coordinated water of octahedral cations; therefore, the coordination states of those edge cations are determined. For (0 1 0)-type edges, both the 5-and 6-fold coordination states of Al are stable and occur with a similar probability, whereas only the 5-fold coordination is stable for Mg cations. For (1 1 0)-type edges, only the 6-fold states of Al cations are stable. However, for Mg cations, both coordination states are stable. In the 5-fold case, the solvent water molecules form H-bonds with the bridging oxygen atoms (i.e., "MgAOASi"). The free energy results indicate that there should be a considerable number of 5-fold coordinated octahedral sites (i.e., "MgAOH/ "AlAOH) at the interfaces. The interfacial structures and acid/base groups were determined by detailed H-bonding analyses.(1) Bridging oxygen sites. For (0 1 0) edges, the bridging oxygen atoms are not effective proton-accepting sites because they are inaccessible from the solvent. For (1 1 0) edges, the oxygen atoms of neutral and octahedrally substituted frameworks (i.e., "MAOASi" for M@Al/Mg) are proton-accepting sites. For T-sheet substituted cases, the bridging oxygen site becomes a proton-donating group (i.e., "AlAOHASi") through the capture of a proton. (2) T-sheet groups. All T-sheet edge groups are "MAOH (M@Si/Al) and act as both proton donors and acceptors. (3)O-sheet groups. For (0 1 0) edges with 6-fold Al, the active surface groups include "AlA(OH)(H 2 O) and "AlA(OH) 2 , whereas for Mg cations, the edge group is "MgA(OH 2 ) 2 . For 5-fold coordination, the active groups are "AlAOH. At (1 1 0) edges, proton-donating sites include "AlAOH, "AlAOH 2 and "MgAOH 2 groups. Overall, our results reveal the significant effects of isomorphic substitutions on the interfacial structures, and the models provide a molecular-level basis for understanding relevant interfacial processes.

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Effects of layer-charge distribution on the thermodynamic and microscopic properties of Cs-smectite

Cited by 77 publications

References 58 publications

Hydration of methane intercalated in Na-smectites with distinct layer charge: Insights from molecular simulations

Hydration of methane intercalated in Na-smectites with distinct layer charge: Insights from molecular simulations

Acidities of confined water in interlayer space of clay minerals

Atomic-scale structures of interfaces between phyllosilicate edges and water

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