Ab Initio Calculations of Partial Charges at Kaolinite Edge Sites and

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Ab Initio Calculations of Partial Charges at Kaolinite Edge Sites and Molecular Dynamics Simulations of Cation Adsorption in Saline Solutions at and Above the pH of Zero Charge Gonzalo R. Quezada, Roberto E. Rozas, and Pedro G. Toledo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b05339 • Publication Date (Web): 30 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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The Journal of Physical Chemistry

Prepared for JPC C

Ab initio Calculations of Partial Charges at Kaolinite Edge Sites and Molecular Dynamics Simulations of Cation Adsorption in Saline Solutions at and Above the pH of Zero Charge Gonzalo R. Quezada,a Roberto E. Rozas,b Pedro G. Toledoc* aWater

Research Center for Agriculture and Mining (CRHIAM), Universidad de Concepción, Victoria 1295, Concepción, Chile bDepartment of Physics, Universidad del Bío-Bío, PO Box 5-C, Concepción, Chile cDepartment of Chemical Engineering and Laboratory of Surface Analysis (ASIF), Universidad de Concepción, PO Box 160-C, Correo 3, Concepción, Chile ABSTRACT: Molecular dynamics simulations are used to study adsorption of cations on the (010) kaolinite edge surface at and above the pH of zero charge. The cation solutions are highly concentrated and include alkali and alkaline-earth metals. It is known that the pHdependent edge surface of kaolinite is more reactive than the basal surfaces and more eager to adsorb metal ions, however knowledge at atomic scale is scarce regarding the structure of the surface edge, charge distribution, solvation, and structure of layers of adsorbed cations. First, ab initio calculations are used to determine the energetically most favorable surface terminations and the distribution of partial atomic charges on both neutral (protonated) and negatively charged (deprotonated) edge surfaces of kaolinite. Then, molecular dynamics simulations are used to study the solvation of kaolinite and the adsorption of cations. Results include density profiles of adsorbed cations, orientation profiles of water molecules close to the mineral surface for different cations, and the distance at which such surfaces become neutral or reverse their charges. Results compare well with available experimental and simulation data. Findings are expected to contribute to the selection or design of organic compounds that effectively adhere to kaolinite in aqueous electrolyte solutions in water recovery processes.

Corresponding author *E-mail: [email protected] (P.G.T.) Phone 56-41 2203658 ORCID Pedro G. Toledo: 0000-0003-2863-7997  Gonzalo R. Quezada: 0000-0002-8670-6260 Roberto E. Rozas: 0000-0002-6466-5162

ACS Paragon Plus Environment

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Prepared for JPC C 1

Introduction

The mining industry continues to make great efforts to close the water cycle, however, the loss of water to tailings remains prohibited. The problem is further complicated by the presence of fine colloidal material that does not respond to traditional flocculants, this material is mainly made up of clays and its concentration increases in overexploited and lowgrade deposits. In arid regions or in places with legislation against the use of fresh water in industrial processes, the use of water with varying degrees of salinity requires a better understanding of the impact of salts in the separation of water from clay-rich solids of which kaolinite is one of the most frequent. Kaolinite, the most abundant type of authigenic clay in mineral reservoirs and rock deposits, is a layered hydrated silicate mineral of composition Al2O3.2SiO2.2H2O, with one tetrahedral sheet of silica linked through oxygen atoms to one octahedral sheet of alumina. The morphology is platy and pseudo-hexagonal with a length-to-thickness ratio of about 10. There is an extensive literature on showing that kaolinite favorably adsorbs water, alkali and alkaline-earth ions, heavy metal ions, simple monomers, sugars, saturated hydrocarbons, and polar organic macromolecules,1-5 and that the edge face is more reactive than the negative cleavage face (see early works6,7 and also more recent8,9). The basal surface of kaolinite is believed to carry a nearly constant structural negative charge, but the charge on the edges depends on the solution pH10-15. Thus, pH control can play a key role in the flocculation of fine particles of kaolinite in salt water16-21 and in many other applications. The behavior of water at the edges is also of relevance for the initial stages of interlayer hydration, water needs to enter the interlayer via the edge regions and some atomic arrangements might indeed be more or less favorable for water entry. In the last decade, classical molecular dynamics simulations have been used to study the surface properties and adsorption mechanisms of kaolinite surfaces,22-31 however, the use of empirical forces without distinguishing the different chemical characters of the participating atoms can lead to wrong conclusions that are opposite to those from first-principles simulations (see related discussion by Tunega et al.32). For more fundamental studies of kaolinite adsorption and geometry optimization, ab initio quantum mechanical calculations26,27,34-40 and ab initio molecular dynamics simulations have been used,32,34,41-43 although still with limitations of time scale and range of phase space that may prevent the capture of important atomic events. Some atomic-molecular events may require time intervals that are currently forbidden for quantum calculations (see related discussion by Underwood et al.25). Most of the simulation studies have been conducted on the regular basal surfaces of kaolinite, both hydroxyl-aluminol and siloxane,22-27,29,32,33-35 and few on the edge surfaces28,36,42,43 and defective surfaces4,44 although edge sites are generally amphoteric and therefore control the pH-dependent properties of kaolinite, such as the adsorption of metal ions, organics and, in particular, flocculant polymers. Recently, a combination of quantum DFT and molecular simulation methods that can sample a greater range of phase space, such as molecular dynamics and Monte Carlo simulations, have been used to obtain a more fundamental atomic understanding of the adsorption processes on relevant surfaces of several minerals. Finite cluster and slab model representations of the minerals with vacuum-terminated surfaces have been used to determine surface terminations and charge distributions, this information is then used in


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Prepared for JPC C classical molecular dynamics simulations on more pertinent large-scale systems (Kroutil et al.45 on silica and Quezada et al. on silica and corundum46). Here, molecular dynamics simulations are used to study the adsorption of concentrated alkali and alkaline-earth metal cations on the (010) kaolinite edge surface at and above the pH of zero charge. First, ab initio calculations in finite clusters and slabs are used to determine the energetically most favorable kaolinite surface termination and the distribution of partial atomic charges on both neutral (protonated) and negatively charged (deprotonated) edge surfaces. Then, molecular dynamics simulations in large supercells are used to study the solvation of kaolinite and the adsorption of cations. Results include density profiles of adsorbed cations, orientation profiles of water molecules close to the mineral surface for different cations, and the distance at which the mineral surface becomes neutral or reverse the charge. 2

Methodology

The ClayFF-MOH force field, the classical ClayFF force field47 with recent metal−O−H angle bending terms,48.49 was used to model the interactions of a deprotonated (010) edge surface of kaolinite with different concentrated salt solutions. Deprotonation of reactive groups on the mineral surface causes the surface to be negatively charged. With deprotonation, the partial charges of the atoms are modified, but not the other interaction parameters of the ClayFF-MOH force field. The kaolinite surface was prepared by first cutting a kaolinite crystal parallel to the (010) plane, thus breaking the weakest bonds, and then hydroxylating all dangling bonds. For each unit cell, two Al-O bonds with a strength of ½ each and one Si-O bond with a strength of 1 are cut (Figure 1). The resulting surface atom layer, delimited by the zero-reference plane, is flexible while the inner layers are rigid (Figure 1). The arrangement of surface groups for the energetically most favorable termination of a neutral (010) edge surface of kaolinite include one AlOH2+1/2, one AlOH ―1/2 and one SiOH group per unit cell.36 The (010) edge model of kaolinite in Figure 1 corresponds to the AC1 termination used by Pouvreau et al.48 To determine the most favorable deprotonation reaction on this surface in the presence of an OH- group we used quantum mechanical calculations based on DFT with the Gaussian 09 package.50 For this purpose, a slab of kaolinite of 46 atoms was used with the B3LYP functional,51 the 3-21G basis functions were used for the Si, Al and H atoms and the 6-31+G (d,p) basis functions for the O atoms. Then, the partial atomic charges on the neutral and negatively charged (deprotonated) surfaces were derived for a kaolinite slab of 108 atoms using the natural bond orbital (NBO) method as implemented in the Gaussian 09 package. For the charge parametrization only atoms at least three bonds from the edges of the slab were considered. To distribute the charge generated by deprotonation of the surface, i.e., removal of a hydrogen, in the simulation supercell we proceeded as Kroutil et al.45 using Equation 1, where 𝑞𝑖, 𝑞𝑖0, and Δ𝑞𝑖 are calculated final atomic charges, atomic charges on the neutral surface and NBO charge correction for the deprotonated edge surface respectively. 𝑁𝑖 is the number of atoms of the type 𝑖 and 𝑛 is the number of deprotonated groups. Q´ is the negative charge per deprotonated site to be distributed over the remaining surface atoms. Kroutil et al.45 showed that this latter scheme of charge assignment for negatively charged surfaces was the best for


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Prepared for JPC C predicting interaction curves of a water molecule approaching the ionized oxygen of a deprotonated group.

(

𝑞𝑖 = 𝑞𝑖0 + Δ𝑞𝑖 +

𝑛[𝑄´ ― ∑𝑖𝑁𝑖Δ𝑞𝑖] ∑𝑖𝑁𝑖

)

(1)

Once the lateral surface groups were defined per unit cell of kaolinite that are ionized in the presence of alkaline water and once the partial atomic charges on the deprotonated surfaces were calculated, we defined the size of the simulation supercell relative to the unit cell and the number of deprotonated sites to obtain a given surface charge density. For the simulation of interactions of the kaolinite edge surface with different concentrated salt solutions, a rhombohedral supercell of 8 × 8 × 2 was created by replicating the unit cell with dimensions 𝐿𝑥 = 4.1184 nm, 𝐿𝑦 = 5.7226 and thickness of 1.7867 nm, with 𝜃 = 72.2° between 𝐿𝑥 and 𝐿𝑦. The cross section of the simulation box corresponded to 𝐿𝑥 × 𝐿𝑦sin (𝜃) of the slab. The height of the box was chosen 𝐿𝑧 = 10.000 nm to include the slab of kaolinite and the aqueous medium. A number of 16 ionized sites were regularly distributed on the surface to obtain a charge density of ca. -0.110 C/m2 corresponding to a pH value of 7 (Figure 1). The kaolinite slab was positioned at half height in the corresponding simulation box. Temperature was 300 K always. Salt cations and anions were added at uniformly distributed random positions in the simulation box maintaining a separation distance of at least 0.8 nm between atoms and 1 nm from the slab surface. Salt concentration was always high and equal to 0.66 M, equivalent to the ionic strength of seawater. This concentration includes the cations needed to compensate surface charge. When the interaction is in pure water, sufficient cations are added to the box to electro neutralize the charge of the slab. The same group of salts from our previous work was considered,46 the monovalent alkali series (LiCl, NaCl, KCl, RbCl and CsCl) and the divalent alkaline-earth series (MgCl2, CaCl2 and SrCl2). Then, water was added from an independent simulation in a box of equivalent size at 300 K, taking care to eliminate water molecules that overlap with the substrate and/or ions. To relax the system, first a force minimization step was performed by the steepest descent method, and then a 100 ps NVT simulation was run, both steps with cations and anions at fixed positions, to generate the corresponding hydration layers. Then, a 1 ns NpT simulation run was performed, with ions at fixed positions, in order to relax the pressure of the system to 1 bar; the Berendsen barostat was used with scaling only in the z-direction of the simulation box. Finally, an NVT production stage of 40 ns at 300 K was performed where all particles were allowed to move. The Gromacs 5.1.2 molecular dynamics simulation package52 was used with the SIMD instructions AVX_256 and GPU enabling acceleration of the calculations. The particle mesh Ewald method (PME) was used for long range corrections.53 The ClayFF-MOH force field was used to model the interactions of the kaolinite slab with the salt solutions.47,48,49 The SPC/E water model54 constrained with SETTLE55 was used to describe water. For the ions, Lennard-Jones 12-6 parameters derived from Li and Merz and Li et al. were used,56,57 adjusted to the SPC/E water model. The advantage of using these potentials is that they all share the same mixing rule. In all cases integration step was 2 fs and information was saved every 1 ps. The highest frequencies of our system make possible the use 2 fs without compromising the stability of the velocity-Verlet method. The Berendsen


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Prepared for JPC C modified thermostat and Berendsen barostat were used,58,59 with relaxation times of 0.1 and 2 ps, respectively. The van der Waals and Coulombic cutoff radii were 1.2 nm. 3

Results

3.1

Termination of the (010) surface and solvation

The arrangement of surface groups corresponding to the energetically most favorable termination of a neutral (010) edge surface of kaolinite is shown in Figure 1. Surface groups include one AlOH2+1/2, one AlOH ―1/2 and one SiOH per unit cell.36 In accordance with calculations by Liu et al.60 that predict that the free energy of desorption of one water molecule from a hydrated surface of gibbsite at 300 K is at least +10 kcal/mol. Several other terminations of the edge surface of kaolinite have been determined to be higher in energy or simply unstable. According to Kremleva et al.36 the main effect of solvation on the termination in Figure 1 is a proton rearrangement on the surface. The proton of the SiOH group move to the neighboring AlOH ―1/2 group forming a second AlOH2+1/2 group and a negatively charged SiO- group in the primitive unit cell. However, the later rearrangement was determined here to be 9 kcal/mol less favorable than the arrangement of groups in the neutral surface in Figure 1 and is thus not considered any further here. Thus, Figure 1 suggests that solvation of the (010) kaolinite surface begins with a first layer of water molecules adsorbed at the edge of the clay mineral to complete the octahedral Al(O,OH)6 sheet with the reactive group AlOH2+1/2 and also to cover surface irregularities generated by the “neutral” AlOH2+1/2-AlOH-1/2-SiOH moiety at the kaolinite edge termination. Additional layers of structured water by hydrogen bonds complete the solvation layer at the edge. Protonated site

Deprotonated site HI AlI

OB

OI HS OS

OA AlA

HA 𝑧

SiS

SiB

(a)

(b)

Figure 1 Kaolinite (010) edge surface model. (a) Side view. Close up to the surface indicating neutral and deprotonated Al sites, with Al, Si, O, H atoms represented by pink, yellow, red, and white spheres, blue is used to highlight surface oxygen atoms at the deprotonated Al site Al(OH)2― . The dashed black line defines the zeroreference plane located at 𝑧 = 0, at the center of surface aluminums. Surface atoms include aluminol (A) groups (AlA, OA, HA), silanol (S) groups (SiS, OS, HS), bulk (B) atoms (SiB, OB), and surface ionized (I) groups (AlI, OI, HI). (b) Top view. Kaolinite slab represented by a simulation supercell of 8 × 8 × 2 unit cells. 𝑥 × 𝑦 crystalline plane and uniform distribution of ionized surface aluminol groups, corresponding to a surface charge density of -0.110 C/m2 and pH 7.


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Prepared for JPC C 3.2

(010) surface deprotonation and charge derivation

Each of the surface groups from an AlOH2+1/2 ― AlOH ―1/2 ― SiOH moiety per unit cell at the kaolinite edge termination is reactive to alkaline water, however which group is the most reactive is unknown. Here, quantum mechanical calculations based on DFT were used to determine the most energy-favorable deprotonation reaction, that of the aluminol group or that of the silanol group. Figure 2 summarizes the calculations performed, clearly deprotonation of the aluminol group is energetically most favorable compared to deprotonation of the silanol group. Thus, at alkaline pH and above the pH of zero charge kaolinite becomes negatively charged with surface groups including two AlOH-1/2 and one SiOH per unit cell (Figure 1).

AlOH2OH + OH ― →Al(OH)2― + H2O Δ𝐸 = ―25.2 kcal/mol (a)

SiOH + OH ― →SiO ― + H2O Δ𝐸 = ―12.6 kcal/mol (b) Figure 2 Deprotonation reactions of surface groups at the (010) edge surface of kaolinite and corresponding energy changes, (a) aluminol group AlOH2OH, (b) silanol group SiOH. Al, Si, O, and H represented respectively by pink, grey, red and white spheres.

Partial atomic charges of the atoms on the neutral (010) edge surface from the ClayFF-MOH force field are summarized in Table 1. Scaled NBO charges, also calculated here, were so similar to the ClayFF-MOH charges that we decided to continue with the latter without additional changes. NBO charges were also calculated for a deprotonated edge surface and the differences with respect to the unscaled NBO charges of the neutral surface are also summarized in Table 1. The excess charge generated by the removal of a hydrogen in the deprotonation reaction in an aluminol site is 𝑄´ = ―0.475 𝑒. This charge is distributed only over the atoms of the kaolinite on the surface and according to Equation 1. Calculated partial charges on the various atom types at the deprotonated edge surface of kaolinite for the surface charge of -0.110 C/m2 are summarized in Table 1 in the fifth column. This latter value of surface charge was chosen to compare directly with the results of Zeitler et al.28 and also because it can be related to a specific pH value. The pH vs. charge curve for kaolinite is


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Prepared for JPC C obtained experimentally by titration, from which the pH corresponding to -0.110 C/m2 is obtained, pH ~ 7 according to data from Gupta et al.14 Table 1 shows that ionized oxygens (OI ) carry the same partial charge as surfaces oxygens in aluminol and silanol groups (OA,Os) and similar to that of bulk oxygens (OB). Therefore, the calculations of quantum mechanics reveal that the surface charge of kaolinite due to deprotonation is rearranged uniformly over the atoms rather than exclusively allocated to the ionized oxygens. For example, in the work of Zeitler et al. the excess charge resides only on the ionized oxygen resulting in a local strong charge of -1.1875 e.28 Table 1. Partial charges (𝑞𝑖) on the various atom types at the (010) edge surface of kaolinite for a surface charge density of −0.110 C/m2. Atom types are defined in Figure 1. Calculations based on a slab of kaolinite represented by a simulation supercell of 8 × 8 × 2 unit cells (Figure 3). Number of cells 𝑁𝑐 = 64 and number of ionized Al sites 𝑁𝐼 = 16. ClayFF-MOH charges for atoms of a neutral kaolinite surface (in this case AlI = AlA , OI = OA, HI = HA). QM corrections are the differences between NBO charges of the atoms on neutral and deprotonated surfaces (𝛥𝑞𝑖).

Atom type SiB SiS AlA AlI OB OA OI OS HA HI HS

3.3

Number of atoms 𝑵𝒊 𝑁𝑐 𝑁𝑐 2𝑁𝑐 ― 𝑁𝐼 𝑁𝐼 3𝑁𝑐 4𝑁𝑐 ― 𝑁𝐼 2𝑁𝐼 𝑁𝑐 4𝑁𝑐 ― 𝑁𝐼 2𝑁𝐼 𝑁𝑐

ClayFF-MOH 𝒒𝒊𝟎(𝒆)

QM corrections 𝜟𝒒𝒊(𝒆)

+2.100 +2.100 +1.575 +1.575 ―1.050 ―0.950 ―0.950 ―0.950 +0.425 +0.425 +0.425

―0.001 ―0.002 ―0.005 ―0.010 +0.010 +0.020 ―0.100 +0.010 ―0.010 ―0.015 +0.010

−0.110 C/m2 𝒒𝒊(𝒆) +2.0913 +2.0903 +1.5623 +1.5573 ―1.0477 ―0.9377 ―1.0577 ―0.9477 +0.4073 +0.4023 +0.4273

Cation adsorption

Axial density profiles are determined for alkali and alkaline-earth cations adsorbed on the (010) edge surface of two kaolinite surfaces, one neutral and the other charged (-0.110 C/m2). The zero plane is defined by the position of the aluminum atoms in the top-most layer of the edge surface (Figure 1). The cation profiles are shown in Figure 3, and as a reference also the mean positions of the peaks corresponding to the water density profiles. These mean positions of the water peaks are the same whether the water is pure or salty. Water density profiles are provided in Supporting Information. The positions of cations and solvation water molecules remain practically unchanged even if the surface is charged. Solvation of the (010) kaolinite surface is characterized by three water layers, the first correspond to water molecules which are adsorbed at the edge of the clay mineral to cover surface irregularities around deprotonated edge sites. Second and third water layers complete the solvation of the edge. The distance between water peaks is slightly less than 0.14 nm, the radius of a water molecule, which suggests that water layers are not arranged one above the other but rather overlap slightly. Molecules of water located beyond 0.6 nm from the zero plane correspond to bulk water. In Figure 3, cations forming inner-sphere complexes are identified by counting


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Prepared for JPC C the number of irreversible adsorption points on the kaolinite without water intermediation. Such positions on the cations would otherwise be occupied by coordination water molecules in the bulk. The inner-sphere complexes always occur with four adsorption points (IS4) or with two (IS2). The cations that form outer-sphere complexes (OS) are irreversibly adsorbed on the kaolinite with their hydration layers intact. For the identification of the different complexes z-density distribution, areal density distribution and RDF are used. Figure 3 clearly shows that the first solvation layer of the edge surface of kaolinite is permanent and that its location is inaccessible to the cations studied in this work. However, the electrostatic attraction between the ionized oxygens on the kaolinite surface and each of the alkali metal cations is strong enough for the hydration layers of the cations other than the first layer to be partially destroyed at the point of contact leading to IS complexes. The complexes according to Figure 3 are located at distances of ca. 0.2 nm for Li+ to ca. 0.3 nm for Cs+. These distances are in close agreement with cation radii with a first hydration shell compiled by Marcus from experiments and/or simulations.61 For negatively charged kaolinite surfaces these distances decrease a little as a consequence of the greater electrostatic attraction. Figure 3 shows that alkaline cations also form a second layer of complexes at distances of ca. 0.3 nm for Li+ to ca. 0.45 nm for Cs+. The cations of this layer form IS2 complexes (Li+, Na+, K+), located closer to the surface than the second solvation layer of the kaolinite, and OS complexes (Rb+, Cs+). This distribution is accentuated in the presence of negatively charged kaolinite. The location of the OS complexes reproduces cation radii with a second hydration shell published by Marcus.61 A third peak in the density profile of every alkali cation is weak enough to deserve further consideration. Density profiles for alkaline-earth cations are shown in Figure 3. Mg cations are adsorbed at the kaolinite surface forming OS complexes only, a single wide peak located at ca. 0.5 nm from the zero-reference plane, between the second and third solvation layers of kaolinite, suggests adsorption of fully solvated Mg cations in a thick interval from 0.4 to 0.6 nm from the zero plane. For negatively charged kaolinite surfaces, Ca and Sr cations present peaks corresponding to IS4 complexes at ca. 0.3 nm from the zero plane, between the first and second solvation layer of kaolinite, suggesting adsorption of cations with a compressed solvation layer or simply deprived of the outermost hydration layer in agreement with data of Marcus.61 Ca and Sr cations present a second adsorption peak corresponding to IS2 complexes at ca 0.4 nm from the zero plane. The detailed distribution of cations on the kaolinite presented here, quantity and location, is possible when the charge due to deprotonation is assigned in a uniform way to the surface oxygens, as it is done here, and it is not possible when the charge is assigned completely to the ionized oxygens.28 Adsorption peaks for divalent cations at distances > 0.4 nm from the zero plane are not considered any further.


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Prepared for JPC C

Li

IS4

+0.000 C/m2 -0.110 C/m2

12 10 8

IS2

6 4 2 0 0.0

OS 0.2

0.4

0.6

16

+

0.8

-3

14

Cation density [nm ]

Cation density [nm-3]

16

10 8

OS

6 4 2 0.2

0.4

14

Na

IS4

+0.000 C/m -0.110 C/m2

10 8 6 4

IS2

2 0 0.0

16

+

2

12

OS 0.2

0.4

0.6

0.8

+0.000 C/m2 -0.110 C/m2

12 10 8

IS2

6 4

IS4

OS

2 0 0.0

1.0

K

0.2

0.4

8

+0.000 C/m -0.110 C/m2 IS4

6 4 IS2

2 0 0.0

0.2

0.4

OS 0.6

0.8

1.0

+0.000 C/m2 2 -0.110 C/m

12 10

IS4

8


10 6

2+

IS2

6 4 2 0 0.0

OS 0.2

0.4

0.6

0.8

1.0

+

+0.000 C/m2 -0.110 C/m2

12 8

1.0

z [nm] Rb

14

0.8

Sr

14

z [nm] 16

0.6

16

+

2



10

1.0

z [nm]

16 12

0.8

Ca2+

14

z [nm] 14

0.6

z [nm]



16

2+

+0.000 C/m2 -0.110 C/m2

12

0 0.0

1.0

Mg

14

z [nm]

IS2

4 OS

2 0 0.0

0.2

0.4

0.6

0.8

1.0

z [nm] 16 Cation density [nm-3]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Cs

14

+

2

+0.000 C/m -0.110 C/m2

12 10 8 6 4

IS2 OS

2 0 0.0

0.2

0.4

0.6

0.8

1.0

z [nm]

Figure 3 Density profiles of alkali (left column) and alkaline-earth (right column) metals close to neutral and negatively charged (010) edge surfaces of kaolinite. Legends indicate surface charge density in C/m2. Mean position of peaks corresponding to the water density profile are represented by segmented vertical lines. IS4 and


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Prepared for JPC C IS2 are inner-sphere complexes respectively with four and two adsorption points. OS stands for outer-sphere complexes.

In general, for alkali and alkaline-earth metal cations, an increase in the charge of the kaolinite surface increases the adsorption of cations without altering the location of the adsorbed layers. We compare our results with those of Zeitler et al.28 for adsorption of Na and Ca on neutral and deprotonated surfaces of kaolinite with the same charge density.28 Our results are quite different and show much higher adsorption. In the neutral kaolinite, Na shows three adsorption peaks with maximum densities of ca. 0.5, 1 and 0.5 cation/nm3 (Figure 3) versus three peaks all with maximum densities of less than ca. 0.5 cations/nm3 (Zeitler et al.), on the other hand Ca shows two adsorption peaks with maximum densities of ca. 2 and 1 cations/nm3 (Figure 3) versus one peak with a maximum density of ca. 0.5 cations/nm3 (Zeitler et al.). In the negatively charged kaolinite, Na shows three adsorption peaks with maximum densities of ca. 16, 2 and 0.5 cation/nm3 (Figure 3) versus three peaks with maximum densities of ca. 6, 2 and 0.5 cations/nm3 (Zeitler et al.), on the other hand Ca shows three adsorption peaks with maximum densities of ca. 3, 6 and 1.5 cations/nm3 versus two peaks with maximum densities of ca. 7 and 0.5 cations/nm3 (Zeitler et al.). However, these differences are expected because we use the new metal-O-H angle bending parameters recently developed by Pouvreau et al.48,49 precisely to improve the modeling of several crystals, kaolinite in particular. An explanation, perhaps more fundamental, is that Zeitler et al. do not distribute the charge that originates from deprotonation, but rather keeps it concentrated, thus limiting the adsorption of cations. Cations are adsorbed in layers from the edge surface of the kaolinite. Each layer appears in the density profile as an adsorption peak between two minimum densities. The number of cations in each layer is obtained by integrating the respective adsorption peak. The total number of cations adsorbed is given by the simple sum of cations in the different layers. The surface adsorption density of a cation is obtained by dividing by the substrate area. Results are shown in Figure 4 for each cation for the first two adsorption peaks and total surface density. The surface density of adsorbed cations on the edge of kaolinite increases significantly with charge density of the mineral. The surface density of cations shows a sustained decrease as the size of monovalent cations increases, as expected, and a slight increase as the size of divalent cations increases (Figure 4c). This behavior reminds the adsorption of alkali and alkaline-earth metal cations on the (001) corundum surface. Our previous results on neutral corundum show adsorption densities slightly higher than on neutral kaolinite edge surfaces,46 most likely because the corundum surface presents a full carpet of aluminol groups, whereas the aluminol carpet of kaolinite is interrupted by inner silanol groups chemically less active than the surface aluminol groups. The much higher cation adsorption of negatively charged corundum compared to negatively charged kaolinite is also due to a higher surface charge density used by Quezada et al. in the corundum (-0.20 C/m2 vs -0.110 C/m2).46 The cation adsorption sequences on kaolinite edge surfaces, as in (001) corundum surfaces, are wellknown and have been satisfactorily explained by a “like absorbs like” concept; that is, high isoelectric point materials preferentially adsorb well-hydrated cations.


62-67

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0.6 0.4 0.2 0.0

Li Na K Rb Cs Mg Ca Sr

0.8

layer

0.6 0.4 0.2 0.0

2

2

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Li Na K Rb Cs Mg Ca Sr

-2

nd

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1.5 1.0 0.5 0.0

Li Na K Rb Cs Mg Ca Sr Cations

Cations

Cations

0.000 C/m 2 -0.110 C/m

(c) (a) (b) Figure 4 Cation adsorption density on the (010) kaolinite surface for charge densities -0.00 and -0.110 C/m2, (a) first layer, (b) second layer, (c) total. Integration of axial density profiles of monovalent and divalent cations is over the first two peaks.

The position coordinates of alkali and alkaline-earth cations are used to prepare adsorption maps on the kaolinite edge surface either neutral or negatively charged. Figures 5 and 6 show 2 × 2 nm2 adsorption layers of monovalent and divalent cations respectively on the negatively charged kaolinite surface for the first two layers of cations. Cations approaching the kaolinite surface can react with the surface groups of either the AlOH2+1/2-AlOH-1/2-SiOH moiety at a “protonated” site or the AlOH-1/2-AlOH-1/2-SiOH moiety at a “deprotonated” site. The maps clearly reveal the adsorption sites preferred by the cations, whether the adsorption is extensive or sparse. Li+ at one end of the alkali metal series, is adsorbed abundantly and only at selected sites of the kaolinite forming a first layer of IS4 complexes, while Cs+, at the other end of the series, adsorbs very little and scattered forming a first layer of IS2 complexes, notable Li+, Cs+ and all the other members of the series, are adsorbed at the same sites (Figure 5), i.e., interspersed between groups AlOH-1/2-AlOH-1/2 or between groups AlOH-1/2-SiOH. The maps clearly show that the cations avoid the spaces neighboring groups AlOH2+1/2. In the second layer the cations are directly adsorbed on the oxygens of the groups AlOH-1/2 in smaller quantity and more dispersed for cations of greater size. In the alkaline-earth metal series (Figure 6), Mg2+ is not adsorbed close to the kaolinite surface, and although the adsorption of Ca2+ and Sr2+ increases slightly with the size of the cations, in the form of IS4 complexes, the adsorption of each cation is not extensive. The adsorption of Mg2+ in the second layer occurs via OS complexes very far from the reference plane, adsorption is scarce and diffuse, whereas the adsorption of Ca2+ and Sr2+ in the form of IS2 complexes is greater and more localized over the groups AlOH-1/2-AlOH-1/2 and AlOH1/2-SiOH. That the divalent cations avoid being adsorbed between (first layer) or over (second layer) groups AlOH2+1/2 is even clearer in the maps.

Li +

Na +

K+


Rb +

Cs + 11


2nd layer

1st layer

Prepared for JPC C

Figure 5 2×2 nm2 adsorption maps of monovalent alkali metal cations on the (010) kaolinite surface for the first two layers of cations, surface charge is -0.110 C/m2. First layer composed of IS4 complexes for Li, Na and K, and IS2 complexes for Rb and Cs. Second layer composed of IS2 complexes for Li, Na and K, and OS complexes for Rb and Cs. Deprotonated aluminol groups are represented by blue spheres at the center of each map. Density of adsorbed cations in logarithmic color scale. Color bar at the bottom of the figure.

Ca2 +

Sr2 +

1st layer

Mg2 +

2nd layer

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 22

Figure 6 2×2 nm2 adsorption maps of divalent alkaline-earth metal cations on the (010) kaolinite surface for the first two layers of cations, surface charge is -0.110 C/m2. First layer composed of IS4 complexes for Ca and Sr. Second layer composed of OS complexes for Mg and IS2 complexes for Ca and Sr. Deprotonated aluminol groups are represented by blue spheres at the center of the map. Density of adsorbed cations in logarithmic color scale. Color bar at the bottom of the figure.

3.4

Surface charge neutralization and charge inversion

To determine the distance at which the kaolinite edge surface reverses its charge, the accumulated net charge was calculated by integration of the charge density profile in the zdirection normal to the surface with reference to the plane of zero charge. The results are


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Prepared for JPC C shown in Figure 7 for neutral and negatively charged kaolinite surfaces in water and in monovalent and divalent cation solutions. On neutral surfaces of kaolinite, in the presence of alkali metal cations, the surface charge is zero at ca. 0.24 nm from the zero-reference plane, the exact position at which cation adsorption begins to occur. The accumulated charge increases due to adsorbed cations and it does so more strongly according to the order Li+ > Na+ > K+ > Rb+ > Cs+. The accumulated charge in the diffuse double layer, between ca. 0.24 and 2.5 nm, is positive, and in the bulk, beyond 2.5 nm, is again zero. On negatively charged surfaces, in the presence of Li and Na ions, the first layer of IS4 complexes counterbalances the negative charge on the kaolinite at 0.24 nm from the zero-reference plane, beyond this distance the additional adsorption of Li in the double layer, second layer of IS2 and OS complexes, reverses the negative charge of the kaolinite surface, finally at ca. 2.5 nm the accumulated charge is again zero indicating that the liquid bulk phase has been reached. The adsorption of larger K, Rb and Cs cations is not enough to reverse the negative charge of the kaolinite, although it is sufficient to counterbalance it at ca. 2.5 nm. The impact of alkaline-earth cations in the accumulated charge on the neutral kaolinite is not very interesting because the adsorption of these cations is poor under these conditions. More interesting is the impact of these divalent cations when they are adsorbed onto a negatively charged kaolinite surface. In the presence of Ca ions the first two layers corresponding to IS4 and IS2 complexes are needed to counterbalance the negative charge of the kaolinite at 0.39 nm from the reference, and in the presence of Sr ions the first layer of IS4 complexes counterbalances the negative kaolinite at 0.27 nm from the reference, beyond these distances the adsorption of Ca2+ in the OS complexes and Sr2+ in the IS2 and OS complexes reverses the negative charge on the surface, at ca. 1.5 nm the surface charge is again zero. The layers of OS Mg2+ complexes located a little farther from the surface, ca. 0.5 nm, are sufficient to reverse the charge of the kaolinite. The divalent cations although adsorbed in low quantity are very effective in neutralizing the charge of the kaolinite at a short distance from the reference plane, that is, at ca. 1.5 nm. Our results on surface charge and charge inversion closely resemble those observed experimentally for corundum and agree qualitatively for example with results from the work of Martin-Molina et al.68 based on canonical Monte Carlo simulation studies. Neutralization of kaolinite edge charges is so effective in the presence of Li+ or any of the divalent cations that the surface charge accumulated in the diffuse double layer of adsorbed cations reverses the charge on the mineral. Adsorption of these cations triggers the formation of salt bridges for example with flocculant macromolecules. Therefore, this result may be a key aid in the selection of flocculants to separate kaolinite particles from salt water.


13


Prepared for JPC C 10 5

0.000 C/m

10

2

5

-5 -10 -15

z [nm]

-15

5

-20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 z [nm]

(a)

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Salt free LiCl NaCl KCl RbCl CsCl

-5 -10

-20 0.0 0.5 1.0 1.5 2.0 2.5 3.0

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0 Salt free LiCl NaCl KCl RbCl CsCl

Q [e]

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0 Salt free MgCl2

-5

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CaCl2

-10

SrCl2

-15

-20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 z [nm]

CaCl2

-10

SrCl2

-15

Salt free MgCl2

-5

-20 0.0 0.5 1.0 1.5 2.0 2.5 3.0 (c)

z [nm]

(d)

Figure 7 Cumulative net charge (units of electron charge e-) on the (010) kaolinite surface in the direction normal to the substrate (z-direction) in the presence of water and saltwater (0.66 M) for different surface charge densities, black dots represent the charge on the kaolinite. (a) and (c) neutral surface, (b) and (d) negatively charged surface. Mean position of peaks in the water density profile are represented by segmented vertical lines.

3.5

Water orientation

Finally, the effect of cations on the orientation of water molecules in the proximity of the interface between an edge surface of kaolinite and several cation solutions is analyzed. Orientation is given by cos (𝜙), where 𝜙 is the angle between the vector opposite to the water dipole and the normal to the mineral interface. Values of -1 indicates the water dipole points away from the mineral surface, i.e., water hydrogens point towards the surface, and the opposite when the value is 1. Values between -1 and +1 indicate partial orientation of the water molecules. For neutral and negatively charged kaolinite surfaces, Figure 8 summarizes the average of cos (𝜙) as a function of the distance from the zero-reference plane. In the presence of monovalent and divalent cations the solvation layer of an edge surface of kaolinite, either neutral or negatively charged, is structured by at least three layers of water. Figure 8 shows that water molecules in the three solvation layers of neutral kaolinite are oriented with the hydrogens pointing away from the mineral surface (cos (𝜙) > 0), and that very few water molecules, free water or coordination water of adsorbed cations, are


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Prepared for JPC C accommodated between the first and second solvation layers with the hydrogens pointing towards the mineral surface (cos (𝜙) < 0), it can also be observed that beyond the three water layers cos (𝜙)→0 typical of bulk water, Figure 8 also shows that this whole behavior is not different in the absence of cations. It can then be concluded that the cations, monovalent and divalent, have no significant effect on the orientation of the solvation water molecules on a neutral kaolinite edge surface. The effect of some cations on the solvation layer of a negatively charged edge surface of kaolinite is quite different. Since adsorption of cations does not alter the first solvation layer, the orientation of the water molecules remains that of the moiety AlOH2+1/2, that is, with the hydrogens pointing away from the mineral surface (cos (𝜙) > 0). The effect of the negative charge on the surface in the absence of cations causes the orientation of the water molecules between the second and third layers to change with respect to the neutral surface, in this case the hydrogens point towards the surface (cos (𝜙) < 0), in the direction of the ionized groups AlOH-1/2. The presence of cations in this case affects the orientation of the water molecules in the solvation layer. Li+ and Na+ maintain the orientation of the water molecules with the hydrogens pointing away from the mineral surface, however K+, Rb+ and Cs+ change the orientation of the water molecules of the second and third layers to such an extent that the hydrogens point towards the mineral surface. In the presence of divalent cations water molecules such as free water or coordination water of adsorbed cations, are accommodated between the first and second solvation layers with the hydrogens pointing towards the mineral surface (cos (𝜙) < 0). Ca2+ and Sr2+ maintain the orientation of the water molecules with the hydrogens pointing away from the mineral surface from the second solvation layer. Mg2+ changes the orientation of the water molecules located beyond the first hydration layer in such a way that hydrogens point towards the mineral surface; the tendency is to turn the dipoles of the water from the condition cos (𝜙) < 0 to the condition cos (𝜙) = 0, typical of bulk water without any structure. Thus, divalent cations do change the structure of the solvation layer of kaolinite. According to these results, it is clear that cations influence the ordering of water structure and the orientation of water dipoles with respect to the mineral surfaces, which is very important for the mobility of particles and also for the effective anchoring of macro(bio)molecules. The results resemble those obtained for the water-corundum interface first by Argyris et al.69 and more recently by ourselves.46


15


Prepared for JPC C 0.8 0.000 C/m

0.6

0.8

2


0.0 -0.2 -0.4 -0.6 0.2

0.4

0.6

0.8

cos()

cos()

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2

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0.0 Salt free MgCl2 CaCl2

-0.6 -0.8 0.0

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Salt free MgCl2

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CaCl2

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SrCl2 0.2

cos()

0.4

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z [nm]

z [nm]

cos()

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Page 16 of 22

1.0

-0.8 0.0

SrCl2 0.2

0.4

0.6

0.8

1.0

z [nm]

z [nm] C/m2)

Figure 8 Water orientation on neutral and negatively charged (-0.110 edge surface of kaolinite in the presence of water and saltwater (0.66 M). Mean position of peaks in the water density profile are represented by segmented vertical lines.

4 Conclusions Ab initio calculations indicate that the energetically most favorable termination of a neutral (010) edge surface of kaolinite is with surface groups including one AlOH2+1/2, one AlOH ―1/2 and one SiOH per unit cell in agreement with previous results. Solvation of this surface begins with a first layer of water molecules adsorbed at the edge to complete the octahedral Al(O,OH)6 sheet with the reactive group AlOH2+1/2 and also to cover surface irregularities generated by the “neutral” AlOH2+1/2-AlOH-1/2-SiOH surface moiety. Additional layers of structured water by hydrogen bonds complete the solvation layer. At alkaline pH and above the pH of zero charge, ab initio calculations indicate that deprotonation of the aluminol group is energetically most favorable than that of the silanol group and thus kaolinite becomes negatively charged with surface groups including two AlOH-1/2 and one SiOH per unit cell. Molecular dynamics simulations reveal that an increase in the charge of the kaolinite edge surface increases the adsorption of cations, alkali and


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Prepared for JPC C alkaline-earth metals, without altering the location of the adsorbed layers, in agreement with previous work on a few selected cations. Li+ and Na+ are adsorbed preferentially forming inner-sphere complexes. The other cations of the alkaline series form inner-sphere complexes and also outer-sphere complexes. Mg2+ is adsorbed forming outer-sphere complexes exclusively, and Ca2+ and Sr2+ are adsorbed forming inner and outer-sphere complexes, the latter with their solvation layers somewhat compressed. The surface density of cations shows a clear sustained decrease as the size of monovalent cations increases and a less clear increase as the size of divalent cations increases. Neutralization of kaolinite edge charges is so effective in the presence of Li+ or any of the divalent cations that the surface charge accumulated in the diffuse double layer of adsorbed cations reverses the charge on the mineral. This result may be a key aid in the selection of polymer flocculants to separate kaolinite particles from water with one class or another of salt. Also, monovalent and divalent cations influence the ordering of water structure and the orientation of water dipoles with respect to the mineral surfaces, particularly in charged surfaces, which is very important for the mobility of particles and also for the effective anchoring of macromolecules of specialty chemicals. Also, the results here provide a novel contribution to practical decisions about the use of salt water in the processing of clay-rich minerals and the choice of polymer flocculants in water recovery processes. Acknowledgements We thank Centro CRHIAM through Project Conicyt/Fondap/15130015 for financial support and The Southern GPU-cluster (SGPUC) UDEC funded by FONDEQUIP EQM150134 for computational support. The authors would like to thank Dr. J. A. Greathouse from Sandia National Laboratories, Albuquerque, New Mexico, United States, for sharing kaolinite coordinates with us. We thank three anonymous reviewers for several helpful suggestions. Supporting Information Axial density profiles of water in the presence of solutions of alkali and alkaline-earth metals close to neutral and negatively charged (010) edge surfaces of kaolinite at 300 K. Mean position of peaks are represented by segmented vertical lines. References 1. Wahlberg, J. S.; Fishman, M. J. Adsorption of Cesium on Clay Minerals (No. 1140). US Government Printing Office. 1962. 2. Yavuz, Ö.; Altunkaynak, Y.; Güzel, F. Removal of Copper, Nickel, Cobalt and Manganese from Aqueous Solution by Kaolinite. Water Research 2003, 37(4), 948952. 3. Srivastava, P.; Singh, B.; Angove, M. Competitive Adsorption Behavior of Heavy Metals on Kaolinite. J. Colloid Interf. Sci. 2005, 290, 28-38. 4. Bhattacharyya, K. G.; Gupta, S. S. Adsorption of a Few Heavy Metals on Natural and Modified Kaolinite and Montmorillonite: A Review. Advances J. Colloid Interf. Sci. 2008, 140(2), 114-131. 5. Jiang, M. Q.; Jin, X. Y.; Lu, X. Q.; Chen, Z. L. Adsorption of Pb (II), Cd (II), Ni (II) and Cu (II) onto Natural Kaolinite Clay. Desalination 2010, 252(1-3), 33-39.


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Prepared for JPC C 6. Schofield, R. K.; Samson, H. R. Flocculation of Kaolinite due to the Attraction of Oppositely Charged Crystal Faces. Discuss. Faraday Soc. 1954, 18, 135-145 7. Quirk, J. P. Negative and Positive Adsorption of Chloride by Kaolinite. Nature 1960, 188, 253. 8. Brady P. V.; Cygan R. T.; Nagy K. L. Molecular Controls on Kaolinite Surface Charge. J. Colloid Interf. Sci. 1996, 183, 356–364. 9. Bourg I. C.; Sposito G.; Bourg A. C. M. Modeling the Acid-Base Surface Chemistry of Montmorillonite. J. Colloid Interf. Sci. 2007, 312, 297–310. 10. Williams, D. J.; Williams, K. Electrophoresis and Zeta Potential of Kaolinite. J. Colloid. Interface Sci. 1978, 65, 79–87.   11. Zhou, Z., & Gunter, W. D. The Nature of the Surface Charge of Kaolinite. Clays Clay Min. 1992, 40(3), 365-368. 12. Schroth, B. K.; Sposito, G. Surface Charge Properties of Kaolinite. Clays Clay Min. 1997, 45(1), 85-91. 13. Tombácz, E.; Szekeres, M. Surface Charge Heterogeneity of Kaolinite in Aqueous Suspension in Comparison with Montmorillonite. Applied Clay Science 2006, 34(14), 105-124. 14. Gupta, V.; Hampton, M. A.; Stokes, J. R.; Nguyen, A. V.; Miller, J. D. Particle Interactions in Kaolinite Suspensions and Corresponding Aggregate Structures. J. Colloid Interf. Sci. 2011, 359(1), 95-103. 15. Zhu, X.; Zhu, Z.; Lei, X.; Yan, C. Defects in Structure as the Sources of the Surface Charges of Kaolinite. Applied Clay Science 2016, 124, 127-136. 16. Atesok, G.; Somasundaran, P.; Morgan, L. J. Adsorption Properties of Ca2+ on NaKaolinite and its Effect on Flocculation Using Polyacrylamides. Colloids and Surfaces 1988, 32, 127-138. 17. Lee, L. T.; Rahbari, R.; Lecourtier, J.; Chauveteau, G. Adsorption of Polyacrylamides on the Different Faces of Kaolinites. J. Colloid Interf. Sci. 1991, 147(2), 351-357. 18. Peng, F. F.; Di, P. Effect of Multivalent Salts-Calcium and Aluminum on the Flocculation of Kaolin Suspension with Anionic Polyacrylamide. J. Colloid Interf. Sci. 1994, 164(1), 229-237. 19. Laird, D. A. Bonding Between Polyacrylamide and Clay Mineral Surfaces. Soil Science 1997, 162(11), 826-832. 20. Wang, S.; Zhang, L.; Yan, B.; Xu, H.; Liu, Q.; Zeng, H. Molecular and Surface Interactions Between Polymer Flocculant Chitosan-G-Polyacrylamide and Kaolinite Particles: Impact of Salinity. J. Phys. Chem. C 2015, 119(13), 7327-7339. 21. Jeldres, R. I.; Piceros, E. C.; Leiva, W. H.; Toledo, P. G.; Herrera, N. Viscoelasticity and Yielding Properties of Flocculated Kaolinite Sediments in Saline Water. Colloids and Surfaces A 2017, 529, 1009-1015. 22. Vasconcelos, I. F.; Bunker, B. A.; Cygan, R. T. Molecular Dynamics Modeling of Ion Adsorption to the Basal Surfaces of Kaolinite. J. Phys. Chem. C 2007, 111(18), 6753-6762. 23. Yang, W.; Zaoui, A. Uranyl Adsorption on (001) Surfaces of Kaolinite: A Molecular Dynamics Study. Applied Clay Science 2013, 80, 98-106. 24. Li, X.; Li, H.; Yang, G. Promoting the Adsorption of Metal Ions on Kaolinite by Defect Sites: A Molecular Dynamics Study. Scientific Reports 2015, 5, 14377.


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Prepared for JPC C 25. Underwood, T.; Erastova, V.; Greenwell, H. C. Wetting Effects and Molecular Adsorption at Hydrated Kaolinite Clay Mineral Surfaces. J. Phys. Chem. C 2016, 120(21), 11433-11449. 26. Greathouse, J. A.; Geatches, D. L.; Pike, D. Q.; Greenwell, H. C.; Johnston, C. T.; Wilcox, J.; Cygan, R. T. Methylene Blue Adsorption on the Basal Surfaces of Kaolinite: Structure and Thermodynamics from Quantum and Classical Molecular Simulation. Clays Clay Min. 2015, 63, 185−198. 27. Greathouse, J. A.; Cygan, R. T.; Fredrich, J. T.; Jerauld, G. R. Adsorption of Aqueous Crude Oil Components on the Basal Surfaces of Clay Minerals: Molecular Simulations Including Salinity and Temperature Effects. J. Phys. Chem. C 2017, 121(41), 22773-22786. 28. Zeitler, T. R.; Greathouse, J. A.; Cygan, R. T.; Fredrich, J. T.; Jerauld, G. R. Molecular Dynamics Simulation of Resin Adsorption at Kaolinite Edge Sites: Effect of Surface Deprotonation on Interfacial Structure. J. Phys. Chem. C 2017, 121(41), 22787-22796. 29. Fazelabdolabadi. B.; Alizadeh-Mojarad, A. Molecular Dynamics Investigation into the Adsorption Behavior Inside {001} Kaolinite and {1014} Calcite Nano-Scale Channels: The Case with Confined Hydrocarbon Liquid, Acid Gases, and Water. Applied Nanoscience 2017, 7, 155-165. 30. Papavasileiou, K. D.; Michalis, V. K.; Peristeras, L. D.; Vasileiadis, M.; Striolo, A.; Economou, I. G. Molecular Dynamics Simulation of Water-Based Fracturing Fluids in Kaolinite Slit Pores. J. Phys. Chem. C 2018, 122(30), 17170-17183. 31. Ma, X.; Fan, Y.; Dong, X.; Chen, R.; Li, H.; Sun, D.; Yao, S. Impact of Clay Minerals on the Dewatering of Coal Slurry: An Experimental and Molecular-Simulation Study. Minerals 2018, 8(9), 400. 32. Tunega, D.; Gerzabek, M. H.; Lischka, H. Ab Initio Molecular Dynamics Study of a Monomolecular Water Layer on Octahedral and Tetrahedral Kaolinite Surfaces. J. Phys. Chem. B 2004, 108(19), 5930-5936. 33. Hobbs, J. D.; Cygan, R. T.; Kathryn L. Nagy, K. L.; Peter A. Schultz, P. A.; Mark P. Sears, M. P.  All-Atom Ab Initio Energy Minimization of the Kaolinite Crystal Structure. American Mineralogist 1997, 82, 657–662. 34. Tunega, D.; Benco, L.; Haberhauer, G.; Gerzabek, M. H.; Lischka, H. Ab Initio Molecular Dynamics Study of Adsorption Sites on the (001) Surfaces of 1:1 Dioctahedral Clay Minerals. J. Phys. Chem. B 2002, 106, 11515-11525. 35. Michalkova, A.; Robinson, T. L.; Leszczynski, J. Adsorption of Thymine and Uracil on 1:1 Clay Mineral Surfaces: Comprehensive Ab Initio Study on Influence of Sodium Cation and Water. Phys. Chem. Chem. Phys. 2011, 13(17), 7862-7881. 36. Kremleva, A.; Krüger, S.; Rösch, N. (2011). Uranyl Adsorption At (0 1 0) Edge Surfaces of Kaolinite: A Density Functional Study. Geochim. Cosmochim. Acta 2011, 75(3), 706-718. 37. Geatches, D. L.; Jacquet, A.; Clark, S. J.; Greenwell, H. C. Monomer Adsorption on Kaolinite: Modeling the Essential Ingredients. J. Phys. Chem. C 2012, 116, 22365−22374. 38. Johnson, E. R.; Otero-de-la Roza, A. Adsorption of Organic Molecules on Kaolinite from the Exchange-Hole Dipole Moment Dispersion Model. J. Chem. Theory Comput. 2012, 8, 5124−5131.


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Page 22 of 22

Prepared for JPC C

For Table of Contents Only

Protonated site

Deprotonated site H Al

O

O

O H

H O

Al

�

Si

Si

Kaolinite (010) edge surface sites


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Ab Initio Calculations of Partial Charges at Kaolinite Edge Sites and

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