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Interfacial properties of calcium montmorillonite in aqueous solutions: Density functional theory and classical molecular dynamics studies on the electric double layer
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Engineering.ORCID iD: 0000-0002-9629-2196
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The swelling properties of Bentonite are highly affected by clay content and the clay-water interactions that arise from the ion distribution in the diffuse double layer formed near the charged montmorillonite (or smectite) surfaces. Existing continuum models describing the electric double layers, such as classical Poisson-Boltzmann and DLVO theory, ignore the ion-ion correlations, which are especially important for multivalent ions at high surface charge and ionic strength. To better understand the clay-water interactions, atomistic models were developed using both density functional theory of fluids (DFT) as well as classical molecular dynamics (MD) methods. In order to increase our understanding of water-saturated, swelling smectite clays, a DFT, technique was initially developed that allowed more accurate predictions of important thermodynamic properties of the diffuse double layers. This DFT approach was then extended to handle systems with mixtures of different sizes and charges. The extended DFT model was verified against experiments and Monte-Carlo simulations. One practical application was to predict the ion exchange equilibria in Bentonite clays, which have wide practical usage in different areas. Nevertheless, in the DFT work it was realized that DFT demands that the particles, ions in this case, which are described as hard spheres, realistically cannot be described as such at low water loadings, when ion specific hydration forces govern the electric double layer properties. To study how the deformation of the hydration shells of Ca2+ influences the properties of compacted smectite clays, MD simulations using the CLAYFF forcefield were employed in order to account for the deformation of the hydration shells. Comparisons of DFT and MD modeling then allowed to demonstrate under which conditions DFT modeling becomes increasingly inaccurate and when it still can give accurate results.

Abstract [sv]

Under senare år har mycket forskning ägnats åt att förstå egenskaperna hos svällande leror som används för att skydda mot läckage av föroreningar från kontaminerade områden och från framtida slutförvar av radionuklider. Den fria svällningen förorsakas av de starka osmotiska krafter som uppstår när vatten tränger in mellan de tunna elektriskt negativt laddade lermineralskikten och löser de laddningskompenserande jonerna i det diffusa dubbelskiktet. I flera arbeten användandes av sk. kontinuum-teori har vattenmolekylens form, specifika orientering och bindning till katjonerna i de nanometerstora utrymmen mellan lerpartiklarna ej beaktats samt ej heller hur de hydratiserade jonerna orienteras på de atomärt ojämna ytorna. Detta möjliggörs dock genom modellering av de enskilda atomernas och jonernas interaktioner med molekyldynamik simuleringar, MD. I detta arbete har programmet Gromacs använts tillsammans med kraftfältet CLAYFF för att studera dessa fenomen i montmorillonitleror med natrium- och kalciumjoner. Simuleringarna visar att natrium bildar transienta innersfärkomplex vilka orienterar sig i bi-triangulära fördjupningar på ytan, ungefär 3.8 Å från mitt-planet mellan lerytorna. Denna orientering observeras ända upp till att avståndet mellan ytorna ökat till större än motsvarande fem lager vattenmolekyler mellan lerpartiklarnas ytor. Detta sker inte med kalcium, oberoende av avståndet mellan ytorna. Natriumjoner koordineras med fyra vattenmolekyler och en syreatom på leran vid ett lager vatten mellan ytorna och med fem till sex vattenmolekyler, ortogonalt orienterade med ökande mängd vatten mellan ytorna, och med en hydratiserad jon-radie av 3.1 Å. Kalcium koordinerar till sju vattenmolekyler vid ett vattenlager mellan ytorna, men ökar till åtta ortogonalt orienterade vattenmolekyler med en jonradie på 3.3 Å vid större avstånd. Generellt visas att när avståndet mellan lerytorna är mindre än ca 10 Å, deformeras de annars symmetriskt hydratiserade jonerna. En jämförelse mellan MD simuleringar och med klassisk täthetsfunktionalteori, DFT, visar att den senare inte kan beskriva hur yttersfärkomplexen samverkar med laddningarna bundna närmast ytan, dvs i Stern-lagret.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2017. , p. 52
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 17
Keywords [en]
Clay minerals, Bentonite, density functional theory, molecular dynamics, GROMACS, electric double layer, electrolytes, ionic liquids, primitive model, CLAYFF forcefield.
National Category
Chemical Engineering
Research subject
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-204648ISBN: 978-91-7729-318-7 (print)OAI: oai:DiVA.org:kth-204648DiVA, id: diva2:1085947
Public defence
2017-05-03, Kollegiesalen, Brinellvägen 8, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20170403

Available from: 2017-04-03 Created: 2017-03-30 Last updated: 2017-04-12Bibliographically approved
List of papers
1. A systematic comparison of different approaches of density functional theory for the study of electrical double layers
Open this publication in new window or tab >>A systematic comparison of different approaches of density functional theory for the study of electrical double layers
2015 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 142, no 19, article id 194110Article in journal (Refereed) Published
Abstract [en]

Based on the best available knowledge of density functional theory (DFT), the reference-fluid perturbation method is here extended to yield different approaches that well account for the cross correlations between the Columbic interaction and the hard-sphere exclusion in an inhomogeneous ionic hard-sphere fluid. In order to quantitatively evaluate the advantage and disadvantage of different approaches in describing the interfacial properties of electrical double layers, this study makes a systematic comparison against Monte Carlo simulations over a wide range of conditions. The results suggest that the accuracy of the DFT approaches is well correlated to a coupling parameter that describes the coupling strength of electrical double layers by accounting for the steric effect and that can be used to classify the systems into two regimes. In the weak-coupling regime, the approaches based on the bulk-fluid perturbation method are shown to be more accurate than the counterparts based on the reference-fluid perturbation method, whereas they exhibit the opposite behavior in the strong-coupling regime. More importantly, the analysis indicates that, with a suitable choice of the reference fluid, the weighted correlation approximation (WCA) to DFT gives the best account of the coupling effect of the electrostatic-excluded volume correlations. As a result, a piecewise WCA approach can be developed that is robust enough to describe the structural and thermodynamic properties of electrical double layers over both weak- and strong-coupling regimes.

Keywords
Chemical bonds, Intelligent systems, Monte Carlo methods, Perturbation techniques, Thermodynamic properties, Columbic interactions, Coupling parameters, Electrical double layers, Interfacial property, Perturbation method, Strong-coupling regime, Weak-coupling regime, Weighted correlation
National Category
Physical Sciences Chemical Sciences
Identifiers
urn:nbn:se:kth:diva-169261 (URN)10.1063/1.4921376 (DOI)000355006200013 ()26001450 (PubMedID)2-s2.0-84930000193 (Scopus ID)
Note

QC 20150615

Available from: 2015-06-15 Created: 2015-06-12 Last updated: 2017-12-04Bibliographically approved
2. Density functional theory of electrolyte solutions in slit-like nanopores II. Applications to forces and ion exchange
Open this publication in new window or tab >>Density functional theory of electrolyte solutions in slit-like nanopores II. Applications to forces and ion exchange
2016 (English)In: Applied Clay Science, ISSN 0169-1317, E-ISSN 1872-9053, Vol. 132, p. 561-570Article in journal (Refereed) Published
Abstract [en]

An extended reference fluid density approach/weighted correlation approximation (RFD/WCA) of density functional theory (DFT) for size-asymmetric electrolytes presented in part I is applied to calculate the forces and the ion exchange for Ca- and Na-montmorillonite systems in equilibrium with salt solutions. Our modeling shows that the DFT calculations are in excellent agreement with Monte Carlo simulations and experimental results. The results indicate that the ion size plays an important role in force-distance relation. Due to the excluded volume effect, the osmotic pressure curve predicted by DFT is shifted towards larger separation distances with increasing the diameter of counterions. Additionally, the interaction can be switched from attraction to repulsion with increasing diameter of counterions from standard to hydrated ionic size. Furthermore, the quantitative characterization of the exchange of calcium for sodium at room temperature on Wyoming bentonite is investigated with the DFT modeling in aqueous solutions at pH 7.0. It is found that a significant variation of the selectivity coefficient could be observed with the surface charge density, ionic diameter and interlayer separations. This implies that ion selectivity in compacted bentonite differs from that in dilute smectite dispersions.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Density functional theory, Montmorillonite, Ion exchange, Osmotic pressure, Swelling pressure
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:kth:diva-196377 (URN)10.1016/j.clay.2016.08.006 (DOI)000385600600065 ()2-s2.0-84991585055 (Scopus ID)
Note

QC 20161129

Available from: 2016-11-29 Created: 2016-11-14 Last updated: 2017-06-28Bibliographically approved
3. Density functional theory of electrolyte solutions in slit-like nanopores I. The RFD/WCA approach extended to non-restricted primitive model
Open this publication in new window or tab >>Density functional theory of electrolyte solutions in slit-like nanopores I. The RFD/WCA approach extended to non-restricted primitive model
2017 (English)In: APPLIED CLAY SCIENCE, ISSN 0169-1317, Vol. 135, p. 526-531Article in journal (Refereed) Published
Abstract [en]

An extended reference fluid density approach/weighted correlation approximation (RFD/WCA) of density functional theory (DFT) is tested for the description of the non-restricted primitive model of electrolyte solutions in slit-like nanopores. The RFD/WCA approach of modeling size-asymmetric ions is validated by reproducing the density and electrostatic potential profiles of planar electrical double layer systems in the presence of mixtures of mono- and multivalent ions. The results from the DFT agree quite well with Monte Carlo simulations and satisfy the contact density sum rules of ionic fluid under a wide range of conditions. These findings suggest that a generic RFD/WCA approach can be formulated within the framework of the restricted and non-restricted primitive model to further investigate the swelling and ion exchange of clay minerals.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Density functional theory, Electrical double layer, Non-restricted primitive model, Clay mineral
National Category
Materials Chemistry
Identifiers
urn:nbn:se:kth:diva-200190 (URN)10.1016/j.clay.2016.09.026 (DOI)000390621800058 ()2-s2.0-85003002054 (Scopus ID)
Note

QC 20170123

Available from: 2017-01-23 Created: 2017-01-23 Last updated: 2017-04-03Bibliographically approved
4. Atomistic simulations of cation hydration in sodium and calcium montmorillonite nanopores
Open this publication in new window or tab >>Atomistic simulations of cation hydration in sodium and calcium montmorillonite nanopores
2017 (English)In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 147, no 8, article id 084705Article in journal (Refereed) Published
Abstract [en]

facile synthesis of 3d-metal based electro-catalysts directly incorporated into a carbon support was carried out by.-radiation. Transition metals of period 4, i.e. Ni and Co, were precipitated and reduced from their respective salt solutions. The obtained materials were characterized by XRD, SEM, SQUID and the BET methods. Thereafter, the electrodes for fuel cells were fabricated out of synthesized material and their electrochemical performance for the oxygen reduction reaction in 6 M KOH was measured. Although the concentrations of Co and Ni in the electrode material were low (3.4% Co and 0.4% Ni) after reduction by irradiation, both the Ni and Co-based gas diffusion electrodes showed high catalytic activity for oxygen reduction both at room temperature and at 60 degrees C.

Place, publisher, year, edition, pages
American Institute of Physics (AIP), 2017
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-204898 (URN)10.1063/1.4992001 (DOI)000409143100035 ()2-s2.0-85028431057 (Scopus ID)
Note

QC 20170403

Available from: 2017-04-03 Created: 2017-04-03 Last updated: 2018-12-04Bibliographically approved

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