Molecular Dynamics Simulations of Quartz (101)âWater and

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Molecular Dynamics Simulations of Quartz (101)-Water and Corundum (001)-Water Interfaces: Effect of Surface Charge and Ions on Cation Adsorption, Water Orientation, and Surface Charge Reversal Gonzalo Quezada, Roberto E. Rozas, and Pedro G. Toledo J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08836 • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 24, 2017

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The Journal of Physical Chemistry

Prepared for JPC C Molecular Dynamics Simulations of Quartz (101)-Water and Corundum (001)-Water Interfaces: Effect of Surface Charge and Ions on Cation Adsorption, Water Orientation, and Surface Charge Reversal Gonzalo Quezada,a Roberto E. Rozas,b Pedro G. Toledoa* a

Chemical Engineering Department and Surface Analysis Laboratory (ASIF), University of Concepción, PO Box 160-C, Correo 3, Concepción, Chile b Department of Physics, University of Bío-Bío, PO Box 5-C, Concepción, Chile *Corresponding author's email address: [email protected] ABSTRACT Quartz and corundum surfaces in water are capable of adsorbing and releasing protons a behaviour attributed to the amphoteric character of their silanol and aluminol surface groups respectively. Here, ab initio calculations are used to obtain different charge densities on crystalline (101) quartz and (001) corundum surfaces and the corresponding charge delocalization after deprotonation of the silanol and aluminol groups respectively. Then, classical molecular dynamics simulations are used to study the interaction of water with the charged quartz and corundum surfaces in the presence of aqueous solutions of monovalent alkali and alkaline-earth metal chlorides. Results include density profiles of adsorbed cations, and the effect of cations on the orientation profiles of water molecules close to the mineral surfaces and the distance at which such surfaces become neutral or reverse their charges. In all cases where there are experimental or simulation data, the results here compare very well. The adsorption density of cations on quartz increases with the size of the cations, either monovalent or divalent. The density of adsorbed monovalent cations on corundum decreases for larger cation sizes, while this behaviour on quartz is the opposite. In both cases the adsorption of cations is enhanced by the increase of the surface charge. Adsorption on corundum is much extensive compared to quartz for all surface charges and cations. The sequence of simulations of cation adsorption on silica and alumina provide support to the idea that high isoelectric point materials preferentially adsorb well-hydrated cations and low isoelectric point materials preferentially adsorb poorly-hydrated cations. The results of this work are expected to contribute to improving current knowledge on the interaction of mineral oxides with macromolecules, such as polyelectrolytes in solid-liquid separation processes and biomolecules in lung inflammatory processes. 1

Introduction

Silicates and aluminas are the most abundant minerals of the earth crust. Quartz, the second most abundant, is a silicate made of pure silica and corundum is the most common form of pure alumina. The physical chemistry of the interface between these inorganic oxides and water is of great interest for a number of applications including geological,1,2,3,4 biological,5,6 and technological.7,8 Both minerals in water are capable of adsorbing and releasing protons, a behaviour attributed to the amphoteric character of their silanol and aluminol surface groups, respectively. Thus, silanol groups can exist in deprotonated

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Prepared for JPC C (Si − O ), unprotonated (Si − OH) and protonated (Si − OH ) states. On the other hand, aluminol groups can exist in deprotonated (Al − O ), unprotonated (Al − OH) and protonated (Al − OH ) states. The silanol groups become partially protonated below the pH corresponding to the point of zero charge (pzc) and deprotonated above such a point. As the pH increases, the number of deprotonated silanol and aluminol groups and the negative charge of the surface increase. As the pH decreases, the number of protonated silanol and aluminol groups and the positive charge of the surface increase. The point of zero charge occurs at pH ~ 2 for silica9,10 and pH ~ 5 to 6 for alumina.11,12 The physical chemistry of the quartz-water and corundum-water interfaces is further complicated by the presence of electrolytes, currently of central interest in mineral processing because freshwater shortages have pushed the mining industry to use seawater as is or partially desalinated.13,14,15,16,17 Most studies reveal that adsorption of monovalent cations on silica follows the inverse Hofmeister series, which orders ions from the least hydrated (so-called breaker ions) to the most hydrated (so-called maker ions), with Cs adsorbing in greater quantities than Li.18,19,10 Investigations revealed that as expected the magnitude of the negative zeta potential in concentrated monovalent electrolytes at pH above the isoelectric point (iep) increases as the Hofmeister series.20,21,22,10 On the other hand, the specific adsorption of counterions to the surface of the silica changes the iep to higher values with unexpected consequences. For example, at salt concentrations as high as 1 M and pH below the shifted iep, the yield stress and viscosity of silica suspensions follows the Hofmeister series, however above the shifted iep the yield stress and viscosity follows the inverse series.23,24,10 Conversely, high iep materials such as alumina show that both ion adsorption25,26 and magnitude of the negative zeta potential27,28 follows the inverse Hofmeister series. To explain these behaviors (and others), supramolecular hydration models have been proposed for both ions and surfaces, which show excellent agreement with experimental results24,29 including charge reversal at high electrolyte concentration.29 However, a molecular approach is needed to obtain a detailed description of the interfaces between charged oxides and water in the presence of electrolytes at high concentration, and in particular a detailed description of the structure and orientation of water in the vicinity of the oxide surfaces. Molecular dynamics (MD) simulations are particularly suitable for these purposes. Numerous MD simulations of the quartz-water interface have been carried out over a range of pH values. Argyris et al.30 employed MD simulations to study the dynamic properties of water at the silica-water interface in the absence of electrolytes. Notman and Walsh31 reported molecular dynamics simulations of fully hydroxylated quartz surfaces in explicit water in the presence of amino acid side chains but not electrolytes. They found that at least two layers of structured water form on the quartz surface driven by the formation of a strong hydrogen bond network. Skelton et al. 32 used classical (MD) simulations of quartz interacting with water according to different classical force fields and compared to ab initio molecular dynamics (AIMD). These authors concluded that ClayFF, the original force field for neutral clay materials developed by Cygan et al.,33 treats the hydrogen bonding more accurately. Butenuth et al.34 developed and validated another force field for modelling the non-bonded interactions between natively charged amorphous silica surfaces and water, their force field provided a correct description of the aqueous interface for both charge-neutral and negatively-charged surfaces, but no electrolytes were considered. Emami et al.35 introduced a silica force field resolving numerous shortcomings


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Prepared for JPC C of prior silica force fields over the last decades and reduced uncertainties in computed interfacial properties from at least one order of magnitude. The force field of Emami et al. enabled accurate computational predictions of aqueous interfacial properties of all types of silica. Dewan et al.36 performed MD simulations of the aqueous phase of a model of a negatively charged amorphous silica and found that water orientation and distribution of ions strongly depends on the identity of cations, the study was limited to Na+ and Cs+. Kroutil et al.37 modified the ClayFF force field to describe negative charging of the (101) quartz surface above its point of zero charge. The same authors then used MD simulations to evaluate the influence of different surface charge densities on the interfacial water and adsorption of Na+, Rb+ and Sr2+. DelloStritto et al.38 used ab initio MD simulations to study the effect of Na+, Rb+, Mg2+ and Sr2+ on the structure and dynamics of the quartz (101)water interface. MD simulations of the alumina-water interface have also been performed although in a considerably smaller number than the silica-water interface. Adiga et al.39 used classical MD simulations to determine the effect of hydroxilation on the surface structure of amorphous alumina (see also Blonski and Garofalini40). Argyris41 performed MD simulations to investigate interfacial water properties at the alumina surface, they used the CLAYFF force field to simulate the solid substrate. More recently, Yeh et al.42 performed MD simulations to study adsorption of catechol to alumina surfaces in both anhydrous and aqueous conditions, they also used the CLAYFF force field. None of these studies considered charged alumina surfaces nor the effect of electrolytes in the aqueous solution. MD simulations of solid-aqueous interfaces where the solid is neither quartz nor alumina also have clarified the structure and orientation of water close to the solid surface.43,44,45 The most recent silica-water and alumina-water force fields incorporate deprotonated silanol and aluminol groups respectively and are easily transferable into various MD software packages. Here, we use ab initio calculations to obtain charge densities on crystalline (101) quartz and (001) corundum surfaces and the corresponding charge delocalization after varying degrees of deprotonation of the surface silanol and aluminol groups respectively. Then, we use classical MD simulations to study the interaction of water with the charged quartz and corundum surfaces in the presence of aqueous solutions of the complete series of monovalent alkali and alkaline-earth metal chlorides. We used the CLAYFF force field modified to incorporate deprotonation of both silanol and aluminol surface groups. Results include density profiles, total number of cations adsorbed, sequentially adsorbed layers of monovalent and divalent cations and adsorption patterns, effect of cations on the orientation of water molecules close to the surfaces, and the distance at which the surface becomes neutral or reverses its charge. The results of this work are expected to contribute new knowledge on the interaction of mineral oxides with macromolecules in saltwater, for example, polyelectrolytes in solid-liquid separation processes and biomolecules in lung inflammatory processes. 2

Methodology

Information on the crystalline structure of quartz (silica, SiO2) and corundum (alumina, Al2O3) was obtained from the Mindat database.46 In the case of quartz crystals, application of an external stress reveals a large number of planes of cleavage, however experimental studies have shown that the crystalline forms with the highest occurrence are the correlated



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Prepared for JPC C forms (101) and (011). On the other hand, calculations with functional theory of periodic density show that form (101) possesses the lowest energy in both relaxed and non-relaxed surfaces correlating very well with the highest probability of experimentally observed cleavage. Corundum on its part is the most common form of alumina and its most important surface is the basal plane (001). Quartz is less dense than corundum and thus its surface is more open and ondulated. This background was enough for our interest to focus on the surfaces (101) for quartz and the (001) for corundum. These surfaces being the most recurrent in nature are therefore the most studied by molecular simulations.47,37,42,41 The quartz surfaces were prepared as follows, a quartz slab of six Si layers thick was cut, as a result one of the surfaces remained neutral but the other was deficient in oxygen, then to complete the crystalline quartz network the missing oxygens were added so that the two surfaces were neutral, finally the surface oxygens on one side of the slab were converted into silanol groups (Si − OH). The same procedure was used to prepare the corundum surfaces, except that in this case the slab is nine Al layers thick, and the surface oxygen on one side of the slab were converted into aluminol groups (Al − OH). The surface layers of the slabs are flexible, while the inner layers are rigid. To relate pH to the number of surface ionized groups, alternatively called active sites, experimental pH-charge data was used for quartz and alumina. For a given charge density, we calculate the number of ionized groups that are then tagged on the surfaces according to a uniform distribution. For example, Figure 1 shows quartz and corundum surfaces for a charge density of −0.12 C/m2,37 and −0.20 C/m2,48 respectively, corresponding to pH 11. The silica surface immersed in water has silanol groups pointing out-of-the plane of the surface toward the liquid water and silanol groups in the plane of the surface. In plane and out of plane silanols alternate on the crystal surface. Out of plane silanol groups donate strong H bonds while in plane silanol groups accept weak H bonds. Here, only out of plane silanols are considered to be ionized. In the case of corundum, the aluminol groups are at the same level so that all them are equally likely to be ionized. Figure 1 shows the complete plane of the silica surface used here with dimensions × = 3.9328 × 5.5028 nm2 and thickness 2.2815 nm for silica and the complete plane of the corundum surface used here with dimensions × = 4.9704 × 3.8256 nm2 and thickness 2.1579 nm. In this work, three values of charge density for silica (-0.03, -0.06 and -0.12 C/m2) and two values for corundum (0.00 and -0.20 C/m2) were used, corresponding to pH 7, 9 and 11 for silica (these values which were taken from Kroutil et al.37 are supported by Goloub et al.49) and pH 9 and 11 for corundum,48 all above the pH corresponding to the point of zero charge.


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Prepared for JPC C

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Figure 1. Crystalline (101) quartz and (001) corundum surfaces, segments yellow (Si), red (O), and white(H) compose silanol groups in quartz and segments pink (Al), red (O), and white (H) compose aluminol groups in corundum. (Top) × ! crystalline planes and uniform distribution of ionized surface groups (blue spheres represents ionized O), pH is 11, surface charge −0.12 C/m2 for quartz (a) and −0.20 C/m2 for corundum (b). (Middle) Side views at the bottom of each plane. (Bottom) IP and OP in (a) stand respectively for in-plane and out-of-plane surface silanol groups, the group ending with the blue sphere corresponds to an ionized OP silanol group in (a) and an ionized aluminol group in (b), the segmented lines define zero reference planes for quartz and corundum.



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Prepared for JPC C The cross section of the simulation boxes corresponded to the dimensions × for each substrate and the height was " = 10 nm. Each substrate was positioned at half height in the corresponding simulation box. Salt, cations and corresponding anions, was added at uniformly distributed random positions but maintaining a separation distance of at least 0.8 nm between atoms and 1 nm from the slab surface, the salt concentration was always high and equal to 0.66 M, equivalent to the ionic strength of seawater. When the interaction is in pure water, sufficient sodium cations are added to the box to electroneutralize the charge of the silica surfaces. The ions considered in this study were 8 positive and 1 negative all constituents of the seawater. Positive ions corresponded to the alkali metal series, that is, Li+, Na+, K+, Rb+, Cs+, and the alkaline-earth metal series, that is, Mg2+, Ca2+, Sr2+, the negative ion was the chloride anion Cl-. Then, water was added from an independent simulation in a box of equivalent size at a temperature of 300 K, taking care to eliminate water molecules that overlap with the corresponding substrate and 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 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 5.0.2 molecular dynamics simulation package50 was used with the SIMD instructions AVX_256 and GPU enablement to accelerate the calculations. In all cases the integration step was 2 fs and information was saved every 1 ps. The Berendsen modified thermostat and Berendsen barostat were used, 51,52 with relaxation times of 0.1 ps and 2 ps, respectively. The vdW and coulombic cut-off radii were 1.2 nm. The particle mesh Ewald method (PME) was used53 to compute long range interactions. The CLAYFF force field33 was used to model interactions in both substrates. The methodology of Kroutil et al.37 was used to determine the local charge in the surface atoms, procedure that introduces corrections to the surface charge due to protonation and deprotonation processes. Thus, we used the Gaussian software package for Hartree-Fock and DFT calculations. A slab of 170 atoms was used for silica and 80 for alumina, initially a calculation was made with Hartree-Fock which was used as the initial value in DFT calculations with B3LYP functional from the Gaussian 9 Program. In both calculations 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. Partial charges were derived by the natural orbital bond method (NBO) as implemented in Gaussian. The atoms located at three bonds from the edges of the slab were used for the parametrization of the charges. The calculations were made for a neutral slab and a slab where a central silanol or aluminol is left without a hydrogen atom, whereby the differences in charges resulting from the ionization of the silanol or aluminol charged groups are obtained. The SPC/E water model was used to describe water. For the ions, Lennard-Jones 12-6 parameters derived from Li and Merz54 and Li et al.55 were used adjusted to the SPC/E water model.56 The advantage of using these potentials is that they all share the same mixing rule.


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Prepared for JPC C 3

Results

3.1

Cation density profiles

Density profiles on the direction perpendicular to the substrate (z-direction) are determined for alkali and alkaline-earth cations adsorbed on the (101) quartz surface and on the (001) corundum surface for different charge densities. To compare the various profiles, reference planes are defined. For quartz, the zero plane is defined by the average position of the silicon atoms in the topmost layer, which includes silicon atoms in silanol groups (Si − OH), both in plane and out of plane, and superficial silicon atoms (O − Si − O). For corundum, the zero plane is defined by the average position of the aluminum atoms in the topmost layer, which includes aluminum atoms in aluminol groups (Al − OH) both in plane and out-of-plane. The profiles are shown in Figure 2; for quartz is characteristic the presence of two peaks of high intensity and for corundum is characteristic the presence of three peaks. As can be seen, the peaks of each cation have a well defined position, relative to the corresponding zero plane, which practically does not change with surface charge density. Clearly, for a given surface, the height of the peaks depends heavily on charge density and cation. Figure 3 summarizes the position of the various density peaks relative to the corresponding zero planes for quartz (Figures 2a and 2b) and corundum (Figures 2c and 2d). Clearly the ability of corundum to adsorb cations is several times greater than that of quartz. In quartz, the position of the first density peak is practically independent of the cation type, which accommodate their hydration shells to position ca. 4 Å from the zero plane, with the exception of Mg2+ which remains heavily hydrated and thus can not approximate further to the surface as the other cations. The position of the second density peak varies with the cation type and for both monovalent and divalent cations, the position moves away from the surface as the cation size increases. For monovalent cations the difference between the first two density peaks varies between 0.14 nm for Li+ and 0.25 nm for Cs+, and for divalent cations is ca. 0.14 nm. We recently found similar differences in adsorption of cations on partially hydrolyzed polyacrylamide molecules,17 that is, cations with high charge density (such as Li+) conform a second layer that partially penetrates the first to get closer to the charged surface and therefore the second density peak appears close to the first, and large cations with low charge density (as Cs+) conform a second layer that can not penetrate the first and so the second density peak appears far from the first, we assume the same happens on the quartz surface. The inclusion of the density of the water molecules in Figure 2 helps in the assessment of the degree of hydration of the adsorbed cations. The monovalent cations compete for the position with the molecules of water (1st water peak), some of them rearrange their water of coordination in the zone of "contact" with the silica to try a more intimate adsorption, consequently the cation peak appears closer to the surface than the water peak. This effect is more pronounced in highly hydrated cations (maker ions in the Hofmeister nomenclature). For example, Li+ is devoid of water on the side of the silica contact, but on the opposite side its coordination water coincides with the first hydration layer of the silica. In contrast, the position of Cs+ coincides with the first layer of water, thus it can be deduced that Cs+ is adsorbed non-hydrated. The position of the second peak of water is clearly gained by water, with some interruptions due to a second low-density cation layer. Among the divalent cations, Mg2+ does not release its



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Prepared for JPC C coordination water, so it adsorbs away from the silica surface, the peak appears between two peaks of water, remarkable is that the distance between the peaks of water is approximately 0.25 nm suggesting it is water of coordination of the Mg. The position of these peaks is also shared by a hydration layer of the silica. In corundum, the position of the first density peak moves away from the surface as the size of the cation increases, for both monovalent and divalent cations. For example, the first layer of Li+ cations is located at 0.2 nm from the zero plane (Figure 3), substracting from this the bare Li+ radius (0.071 nm) leads to 0.129 nm which should approximate the size of the O- in a typical Al − O group at the corundum surface. Adding 0.129 nm to the bare Na+ radius recovers the positions of the first density peak for the Na+ cation in Figure 2c. Due to the larger size of the cations K+, Rb+ and Cs+, the corresponding first density peak is located at more than 0.3 nm and due to their low charge density the number of cations adsorbed is almost imperceptible. Thus, the high charge density of the Li+ and Na+ cations maximizes the attraction towards the charged corundum sites, whereby they are adsorbed in a first layer in a non-hydrated state. The second density peak for Li and Na is located at nearly 0.34 nm and is far more intense than the first. For these cations, the difference between the first two density peaks is ca. 0.14 nm, and thus the high cationcorundum attraction leads to a second cation layer that partially penetrates the first to get closer to the disociated aluminol groups. The cations K+, Rb+ and Cs+ present a second density peak whose location increases with the size of the cation, becoming more than 0.4 nm in the case of Cs+. Thus Li+ and Na+, their second density peak, and K+, Rb+ and Cs+, also their second density peak which decreases in intensity from K+ to Cs+, conform a layer of hydrated cations. For divalent cations, Figure 2d shows that the difference between the second and third density peaks varies between 0.2 and 0.3 nm, the size of one molecule of water in bulk, suggesting that the third peak corresponds to a layer of hydrated cations with little deformation in their water shells. The Mg2+ cation, as in quartz, is only adsorbed in a strongly hydrated state and away from the surface of the corundum. The position of the adsorbed cations coincides with the optimized position reported by Yong et al.57 Figure 2 clearly shows that the monovalent maker cations compete for the closest position to the corundum surface, especially Li+ and Na+. Breaker cations do not gain the closest position and only compete with water for the second closest position. At low pH, the divalent cations are adsorbed between the two first layers of hydration of the corundum, these two layers include the coordination waters of the divalent metals. The position of the third layer of water is shared with divalenes cations.


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Prepared for JPC C

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Cation (a)

Cations (b)

Figure 3. Positions of density peaks for alkali and alkaline-earth metal cations relative to the zero plane of the (101) quartz surface for charge densities −0.03, −0.06, −0.12 C/m2 (a) and the (001) corundum surface (b) for charge densities 0.00, −0.20 C/m2. For each surface the positions overlap showing independence of charge density (charges are not shown in the figures).

3.2

Cation adsorption

The total number of cations adsorbed on quartz and corundum is obtained by integration of the axial density profiles of cations in Figure 2. The corresponding surface densities are obtained by dividing the number of cations by the substrate area. For quartz the integration is done from zero to the second minimum, which corresponds to 2 layers, and for corundum up to the third minimum, which corresponds to 3 layers. The decision to integrate up to these minima is because the density profiles do not show adsorption beyond these limits, which was verified in adsorption maps. The results are shown in Figure 4. In general, the surface density of adsorbed cations on the (101) quartz surface shows a sustained increase as the size of the cation increases or alternatively as the cation is less hydrated for both monovalent and divalent cations. At high pH at which quartz has a high surface charge, Li+ breaks the trend but does not get adsorbed more than the larger cations. The adsorption density of divalent cations is lower than that of monovalent cations due to their high electric charge density that makes them more effective to screen the quartz charge. The adsorption density of monovalent cations on the the (001) corundum surface follows the opposite behaviour to quartz, that is, the surface density of adsorbed cations decreases as the size of the cation increases and this effect is pronounced as the pH increases. Moreover, adsorption on corundum is much extensive compared to quartz for all surface charges and cations. Figure 4 shows that Mg2+ is not adsorbed as such in either quartz or corundum at high pH, because this is the condition that favors precipitation of its hydroxides. The cation adsorption sequences on silica and alumina are well known and have been satisfactorily explained by a “like absorbs like” concept, that is, high isoelectric point materials preferentially adsorb well-hydrated cations and low isoelectric point materials preferentially adsorb poorly hydrated cations.58,23,10,15,16,17 However, the results in Figure 4 are important because for the first time they provide molecular support to such rule of thumb. Our results compare well with experimental data and simulation results available


Page 11 of 27

Prepared for JPC C for quartz and corundum in the presence of some of the cations considered here. For example, for Rb+ on quartz, for a surface charge of -0.11 C/m2, the experimental value of adsorption in the first two layers is 0.23 ± 0.03 Rb/AUC where A&' = 0.338 nm2 is the area of a unit cell along the (101) quartz surface,59 while the value obtained by simulation using molecular dynamics for the first two layers is 0.20 Rb/AUC,37 very close to the experimental value. Interpolating our adsorption data of Rb for a surface charge of -0.11 C/m2 we find that the adsorption value for the first two layers is ca. 0.63 Rb/nm2 or 0.21 Rb/AUC or 1.05 µmol/m2 which is in excellent agreement with data from Bellucci et al.59 and Kroutil et al.37 Our results for the adsorption of Na+, Rb+ and Sr2+ on quartz at different densities of surface charge are in close agreement with the Kroutil et al. simulation results; both results for the first two layers of adsorbed cations, as shown in Table 1. This agreement suggests that our results for cations as yet untested, experimentally or computationally, are correct and can serve as a basis for comparison in future studies. Table 1. Amount of adsorbed cations in (mol/m2 from the simulations here and from those of Kroutil et al.37

This work 0.313 0.393 0.220

Surface charge density, C/m2 -0.03 -0.06 -0.12 Kroutil et al. This work Kroutil et al. This work Kroutil et al. 0.305 0.484 0.470 1.004 1.060 0.335 0.655 0.535 1.122 1.040 0.156 0.324 0.310 0.712 0.610

2,5

1,4

-0.03 C/m 2 -0.06 C/m 2 -0.12 C/m

1,2 1,0 0,8 0,6 0,4 0,2 0,0

2

2

2

2

Adsorption [1/nm ]

Na+ Rb+ Sr2+

Adsorption [1/nm ]

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


0.00 C/m 2 -0.20 C/m

2,0 1,5 1,0 0,5 0,0



Cations

Cations

(a)

(b)

Figure 4. Cation adsorption density on the (101) quartz surface for charge densities −0.03, −0.06, −0.12 C/m2 (a) and the (001) corundum surface (b) for charge densities 0.00, −0.20 C/m2. Integration of axial density profiles of monovalent and divalent cations is over the first two peaks.



Prepared for JPC C 3.3

Adsorption maps

Sequentially adsorbed layers of monovalent and divalent cations on quartz and corundum are calculated for all the charge densities considered in this study. The density of adsorbed cations changes in the different simulation cases but not the adsorption sites. Thus only results for quartz with charge density −0.06 C/m2 and for corundum with charge density −0.20 C/m2 are shown and limited to high-intensity density peaks. Figures 5 and 6 show 2 × 2 nm2 adsorption layers of monovalent and divalent cations respectively on the (101) quartz surface for the first two layers of cations. In the first layer the adsorption pattern is very clear, Li+ is preferentially adsorbed onto the deprotonated out-of-plane silanol, and secondarily onto protonated in-plane silanols. The pattern changes in the second layer, Li+ is adsorbed in its hydrated state and preferably around the deprotonated out-of-plane silanol, which is already covered by a first layer of Li+, in this second layer hydrated Li+ cations are accommodated in the empty spaces between neighboring unprotonated in-plane silanols, the resulting adsorption map is not homogeneous. The adsorption pattern of Cs+ is the opposite to Li+, Cs+ because its large size is preferentially adsorbed around the deprotonated out-of-plane silanol and in the valleys offered by the unprotonated in-plane silanols, but in the second layer the adsorption is complete and uniform. Na+, K+ and Rb+ follow intermediate behaviors between Li+ and Cs+. In the case of divalent cations results show that Mg2+ is not adsorbed in a first layer due to its water shells, but in a second layer around the deprotonated out-of-plane silanol and in the interstices between neighboring unprotonated in-plane silanols. Unlike Mg2+, Ca2+ and Sr2+ cation accommodate their water shells to be adsorbed on the first layer directly over the deprotonated out-of-plane silanol and in a second layer around this silanol and in the interstices between neighboring unprotonated in-plane silanols. The resulting adsorption layers of divalent cations are fairly incomplete although charge screening in all cases is very efficient. Na

K

Rb

Cs

1st layer

Li

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 27

Figure 5. 6 × 6 nm2 adsorption maps of monovalent alkali metal cations on the (101) quartz surface for the first two layers of cations, surface charge is -0.06 C/m2. Out-of-plane deprotonated silanol are represented by a red sphere at the center of the map. This silanol group is shown in the 2nd layer only as a reference. Density of adsorbed cations in logarithmic color scale. Colorbar at the bottom of the figure.


Page 13 of 27

Prepared for JPC C

Ca

Sr

1st layer

Mg

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


Figure 6. 6 × 6 nm2 adsorption maps of divalent alkaline-earth metal cations on the (101) quartz surface for the first two layers of cations, surface charge is -0.06 C/m2. Out-of-plane deprotonated silanol are represented by a red sphere at the center of the map. This silanol group is shown in the 2nd layer only as a reference. Density of adsorbed cations in logarithmic color scale. Colorbar at the bottom of the figure.

Figures 7 and 8 show 2 × 2 nm2 adsorption layers of monovalent and divalent cations respectively on the (001) corundum surface for the first three layers of cations. In the first layer Li+ is preferentially adsorbed around deprotonated aluminols occupying the interstices in the slightly distorted hexagonal close packing formed by surrounding oxygens. The pattern changes in the second layer, Li+ is adsorbed in its hydrated state and preferentially on the deprotonated aluminols. In the third layer Li+ repeats the adsorption patern of the first layer. The resulting adsorption map is discontinuous, with many space free of cations, although charge screening in all cases is very efficient. The adsorption pattern of Cs+ is similar to that of Li+ although less abundant due to the large size of Cs+. In the second layer Cs+ is adsorbed around the deprotonated aluminols forming a pattern of six-point stars, clearly the Cs+ cations avoid being adsorbed just over the deprotonated aluminols. Finally, in the third layer the adsorption of Cs+ is scarce with cations being adsorbed in the interstices left in the previous layers forming a thin and discontinuos cation covering of the corundum surface. As can be seen in Figure 7, Na+, K+ and Rb+ follow intermediate behaviors between Li+ and Cs+. Figure 8 shows that Mg2+ is not adsorbed at all, because at the high pH of the system it is known that Mg2+ is not available as free cation but as hydroxide. Unlike Mg2+, Ca2+ and Sr2+ cation accommodate their water shells to be adsorbed on the first layer about a deprotonated aluminol group and then almost on top in the second and third layers. The resulting adsorption layers of divalent cations are fairly incomplete although charge screening in all cases is very efficient.



Prepared for JPC C Na

K

Rb

Cs

3rd layer

2nd layer

1st layer

Li

Figure 7. 6 × 6 nm2 adsorption maps of monovalent alkali metal cations on the (001) corundum surface for the first three layers of cations, surface charge is -0.20 C/m2. Out-of-plane deprotonated aluminol groups are represented by red spheres. This aluminol group is shown in the 2nd and 3rd layers only as a reference. Density of adsorbed cations in logarithmic color scale. Colorbar at the bottom of the figure.

Ca

Sr

2nd layer

1st layer

Mg

3rd 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 14 of 27

Figure 8. 6 × 6 nm2 adsorption maps of monovalent alkaline-earth metal cations on the (001) corundum surface for the first three layers of cations, surface charge is -0.20 C/m2. Out-of-plane deprotonated aluminol groups are represented by red spheres. This aluminol group is shown in the 2nd and 3rd layers only as a reference. Density of adsorbed cations in logarithmic color scale. Colorbar at the bottom of the figure.

There is a striking difference as regards the lateral arrangement of ions on the quartz and corundum substrates. Particularly, for Rb+ on corundum, and for other ions too, the


Page 15 of 27

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


Prepared for JPC C adsorption of the first two layers is essentially crystal-like. This interesting feature has attracted attention recently60 as well as ours. 3.4

Water orientation

The effect of cations on the orientation of water molecules close to quartz-water and corundum-water interfaces is evaluated. Orientation is given by cos (;), where ; is the angle between the vector opposite to the water dipole and the normal to the substrate either quartz or corundum. Values of -1 indicates the water dipole points away from the oxide 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. Figures 9 and 10 correspond to the average of cos(;) over the entire mineral surface respectively for quartz and corundum, for different surface charges, in water and in the presence of monovalent and divalent cation solutions. The results in Figures 9 and 10 show that in the absence of electrolytes the quartz and corundum surfaces induce orientational order on water molecules located near the oxide-water interface which leads to two layers of water in quartz and six layers, possibly seven, in corundum. The orientation of the water molecules in the first layer, near the surface, is mostly with the hydrogens pointing away from the surface (cos(;) → 0.9, i. e. , ; → 25°), in the second layer the orientation is mostly with the hydrogens pointing towards the surface (cos(;) → −0.5, i. e. , ; → 120°), and so on. These water layers define a zone of compact water whose thickness in quartz is ca. 0.5 nm, much smaller than in corundum which is ca. 1.25 nm. Following the compact zone, the orientation of the water molecules on quartz continues to show oscillations suggesting a layered arrangement, however the water molecules although somewhat adjust their orientation mostly keep it with the hydrogens pointing away from the surface (cos(;) < 0). This layer which we defined as semicompact has a thickness of ca. 0.5 nm. This layer does not exist in the case of corundum. Then follows a diffuse layer with cos(;) < 0 that vanishes at the bulk water. It may be surprising that water molecules in the first layer of the compact zone of both quartz and corundum point with the hydrogens away from the surface, anionic in most cases, however it should be remembered that the surfaces not only count with deprotonated silanol/aluminol groups but also with unprotonated silanol/aluminol groups in and out of the zero plane which are ready for H bonding with water with orientation cos(;) > 0. In any case, the most frequent water orientations are those that align with the water density profile as shown in Figure 10, the other orientations with the oxygen of the water pointing towards the surface correspond to an insignificant group of water molecules (no significant water density peak in Figure 10), close to the surface in the case of silica. This also occurs on the corundum surface. However, in the case of corundum there is actually a population of water molecules with their dipole facing the surface, followed by another population of molecules with their dipole oriented away from the surface and then a third population with their dipole facing the surface again. It does not mean that the water molecules alternate orientation, it is a specific aspect imposed by the crystalline structure of corundum. The first population of water molecules is topologically distributed in one zone (OH-terminated), the second population in another zone (O--terminated), and so on. This finding was first observed by Argyris et al.41



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

Prepared for JPC C We next analyze the effect of electrolytes on the orientation of the water located near the quartz-water interface. In Figure 4 it is clear that among all the monovalent cations, Li+ is the less adsorbed on quartz, and that although its adsorption increases with the surface charge, or pH, never exceeds the adsorbed amount of large cations such as Cs+. However, the high charge density of Li+ is enough to attract and orient a large number of water dipoles, in addition to those oriented by the unprotonated silanol surface groups, in a first water shell at the quartz surface, with cos(;) > 0, i.e., with water hydrogens pointing away from the adsorbed cations and/or the quartz surface, which conditions the arrangement of a smaller but still important number of water molecules in a series of layers alternating the orientation of the water molecules. Figure 9 shows that Li+ cations, compared to pure water, gives a stronger orientation to a larger number of water molecules in a compact zone with six, possibly seven, water layers and thickness of ca. 0.75 nm. In the presence of Li+ there is no semicompact zone nor diffuse layer. In the presence of other alkali metals the orientation of the water is not as strong as in the presence of Li+, however, it is somewhat greater than in pure water. The capacity of monovalent cations to orient water molecules in the vicinity of the quartz surface is most clearly seen at high surface charge or at high pH, such capacity follows the order Li+ > Na+ > K+ > Rb+ > Cs+ > water. The divalent cations are adsorbed in less quantity than the monovalent cations due to their higher electrical charge density, so they are more effective in screening the quartz surface charge, such adsorption increases slightly with surface charge or pH. The adsorption of divalent cations in general increases with the cation size, so that the small cation Mg2+ is less adsorbed than Rb+ (an even less than the large monovalent cation Cs+). However, as with Li+, the high Mg2+ charge density is sufficient to attract and orient a large number of water dipoles, in addition to those oriented by the unprotonated silanol surface groups, in a first shell of water on the quartz surface with cos(;) > 0, i.e., with water hydrogens pointing away from the adsorbed cations and/or the quartz surface, which conditions the arrangement of a smaller but still important number of water molecules in a second water shell with cos(;) < 0, and so on. Figure 9 shows that Mg cations, compare to pure water, are able to orient more strongly a greater number of water molecules close to the quartz surface conforming a compact zone with thickness of ca. 0.75 nm. The divalent cations have similar capacity to orient water molecules in the vicinity of the quartz surface, however the Sr2+ cation with its low charge density shows capacity similar to that of Mg2+, the explanation here has to do with the greater amount of adsorbed Sr cations, apparently the water orientation capacity is the result of a balance between electric charge density and adsorption density. Summarizing, the capacity of divalent cations to orient water molecules in the vicinity of the quartz surface follows the order Mg2+ ~ Sr2+ and Mg2+ > Ca2+. The number of monovalent cations adsorbed on the (001) corundum surface decreases as the size of the cation increases, reverse trend to quartz (see Figure 4). The adsorption density of divalent cations on corundum is lower than that of monovalent cations and slightly increases as the size of the cation increases, same trend as quartz (see Figure 4). Figure 10 shows for the monovalent series that Li+, with its high electric charge density, is the one that most affects the orientation of the water molecules in the vicinity of the corundum surface. Figure 10 shows that this orientation follows the order Li+ > Na+ > K+ > Rb+ > Cs+ > water. The divalent cations have similar capacity to orientate water molecules in the vicinity of the corundum surface, ca. 1.25 nm, such capacity follows the order Mg2+ > Ca2+ > Sr2+ > water. Mg2+ is not shown in Figures 9 and 10 at the highest surface charges because under this condition this cation exists only as hydroxide.


Page 16 of 27

Page 17 of 27

Prepared for JPC C 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. 1.0 -0.03 C/m

1.0

2

Quartz Surface

0.5

-0.03 C/m

0.5

;

2

Interface Normal

w Li Na K Rb Cs

0.0 -0.5 -1.0 0.0

cos(φ)

cos(φ)

(

0.5

1.0

1.5

0.0 w Mg Ca Sr

-0.5 -1.0 0.0

2.0

0.5

z [nm] -0.06 C/m

-0.06 C/m

cos(φ)

cos(φ)

2.0

2

0.5 w Li Na K Rb Cs

0.0 -0.5

0.5

1.0

1.5

0.0 w Mg Ca Sr

-0.5 -1.0 0.0

2.0

0.5

z [nm]

1.0

1.5

2.0

z [nm]

1.0 -0.12 C/m

1.0

2

-0.12 C/m

2

0.5 cos(φ)

0.5 w Li Na K Rb Cs

0.0 -0.5 -1.0 0.0

1.5

1.0

2

0.5

-1.0 0.0

1.0 z [nm]

1.0

cos(φ)

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


0.5

1.0

1.5

0.0 w Mg Ca Sr

-0.5

2.0

-1.0 0.0

z [nm]

0.5

1.0

1.5

2.0

z [nm]

Figure 9. Water orientation on the (101) quartz surface in the presence of water and saltwater (0.66 M) for charge densities −0.03, −0.06, −0.12 C/m2. The inset in the upper right frame shows the definition of ;, the



Prepared for JPC C angle between the vector opposite to the water dipole and the normal to the quartz-water interface. Mean position of peaks in the water density profile are represented by segmented vertical lines.

Our results are consistent with those of Argyris et al.30 for the silica-water interface, the net orientation of the first layer of water on partially hydroxylated silica is the same. Our results are also consistent with those of Argyris et al.41 for the corundum-water interface. The orientation of the first layer of water on the corundum and the successive layers is the same according to both models, the bulk condition in both models is reached at a distance of ca. 1 nm from the first layer of water. 1.0

1.0 0.00 C/m

2

Corundum Surface

0.5

0.00 C/m

0.5

;

w Li Na K Rb Cs

0.0 -0.5 -1.0 0.0

cos(φ)

cos(φ)

(

0.5

1.0

1.5

-1.0 0.0

w Mg Ca Sr 0.5

1.0

1.5

2.0

z [nm]

1.0

1.0 -0.20 C/m

2

-0.20 C/m

2

0.5 cos(φ)

0.5 w Li Na K Rb Cs

0.0 -0.5 -1.0 0.0

Interface Normal

-0.5

2.0

2

0.0

z [nm]

cos(φ)

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 18 of 27

0.5

1.0

1.5

0.0 w Mg Ca Sr

-0.5

2.0

-1.0 0.0

z [nm]

0.5

1.0

1.5

2.0

z [nm]

Figure 10. Water orientation on the (001) corundum surface in the presence of water and saltwater (0.66 M) for charge densities A. AA, −A. 6A C/m2. The inset in the upper right frame shows the definition of B, the angle between the vector opposite to the water dipole and the normal to the corundum-water interface. Mean position of peaks in the water density profile are represented by segmented vertical lines.

3.5

Surface charge neutralization and charge inversion

The net charge on each surface in the direction normal to the surface (z-direction) is calculated to determine the distance at which the surface becomes neutral or reverses its charge. The results for the (101) quartz surface are shown in Figure 11 for different surface charges in water and in monovalent and divalent cation solutions. In the presence of cations from the monovalent series the results show that the net charge is neutralized at approximately 2 nm from the zero plane when the surface charge is -0.06 C/m2, slightly


Page 19 of 27

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


Prepared for JPC C closer when the charge is -0.03 C/m2 and slightly further when the charge is -0.12 C/m2. The neutralizing capacity of the monovalent cations follows the Li+ >> Na+ > K+ > Rb+ > Cs+ >> water. On the other hand, the divalent cations, which are known to be adsorbed by quartz in a much smaller number than the monovalent cations (see Figure 4), due to their high charge density are very effective in neutralizing the quartz surface. Figure 11 shows that a single layer of divalent counterions is sufficient to neutralize the quartz surface completely at low surface charge, -0.03 C/m2, and to overcharge the surface with consequent charge inversion at medium and high surface charge (-0.06 and -0.12 C/m2). Neutralization and charge reversal occurs at ca. 0.5 nm from the zero plane and ca. 0.2 nm from the O- of the ionized silanol groups, suggesting that the first layer of counterions is adsorbed in the hydrated state with at least a hydration shell (the average hydrated radius of monovalent and divalent cations with a single water shell is 0.255 nm; the smallest radius is 0.208 nm which corresponds to Li+ and the largest is 0.314 which corresponds to Cs+). The neutralizing capacity of the divalent cations follows the series Mg2+ >> Ca2+ > Sr2+ >> water. It is important to note that when surfaces are exposed to pure water, or in the presence of a small number of Na cations necessary to adjust the pH, in general the neutralization in water at any surface charge is slow and is reached at distances of at least 4 nm. In the case of neutral corundum, there are no dissociated aluminol groups, Figure 12 shows that in any case adsorption of both monovalent and divalent cations occurs. Neutralization of corundum is so effective that the surface charge is reversed in the presence of both monovalent and divalent cations. When the surface charge of corundum is 0.00 C/m2 and -0.20 C/m2 the overcharge leads to an inversion of the surface charge in the presence of both monovalent and divalent cations. The neutralizing capacity of the monovalent cations follows the series Li+ > Na+ > K+ > Rb+ > Cs+ >> water. Figure 12 shows that the larger the size of the cation the greater the number of layers required for neutralization. It is interesting to note that the first layers of Li+ and Na+ are located very close to 0.2 nm from the zero plane, corroborating the non-hydrated state of these cations, as expected from the axial density profiles (Figure 2), the following Li layers are located at greater distances and in hydrated state. The neutralizing capacity of the divalent cations follows the series Mg2+ >> Ca2+ > Sr2+ >> water. Again, Figure 12 shows that the larger the size of the cation the greater the number of layers required for neutralization. The first layers of Mg2+, Ca2+ and Sr2+ are located at distances > 0.6 nm either because the cation is heavily hydrated, case of Mg2+, or the size is too big, case of Ca2+ and Sr2+. Finally, it is important to note that when the surfaces are exposed to water, or in the presence Na+ cations which are necessary to adjust the pH, in general the neutralization in water for any surface charge is reached at distances of barely 0.3 nm from the zero plane. Again Mg2+ is not shown in Figures 11 and 12 for the largest surface charges because at the corresponding high pH of 11 it is known that magnesium is not available as free cation but as hydroxide. Our results from molecular dynamics reproduce surface charge and charge inversion observed experimentally and agree at least qualitatively for example with results from the pioneering work of Martin-Molina et al.61 based on canonical Monte Carlo simulation studies and those of Parson and Ninham29 based on a modified Poisson-Boltzmann approach that includes nonelectrostatic ion-surface dispersion interactions.



Prepared for JPC C 5

5 -0.03 C/m

-0.03 C/m2

2

0

-5

w Li Na K Rb Cs

-10 -15

Q/e

Q/e

0

-5 -10

w Mg Ca Sr

-15

-20

-20 0

1

2

3

4

0

1

z [nm]

3

4

5 -0.06 C/m

2

0

-0.06 C/m

0

-5

w Li Na K Rb Cs

-10 -15

Q/e

Q/e

2 z [nm]

5

2

-5 -10

w Mg Ca Sr

-15

-20

-20 0

1

2

3

4

0

1

z [nm]

2

3

4

z [nm]

5

5 -0.12 C/m

2

0

-0.12 C/m

0

-5

w Li Na K Rb Cs

-10 -15

Q/e

Q/e

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 20 of 27

-5 -10

w Mg Ca Sr

-15

-20

2

-20 0

1

2

3

4

0

z [nm]

1

2

3

4

z [nm]

Figure 11. Dimesionless net charge (charge Q normalized by the electron charge C ) above the (101) quartz surface on the direction normal to the substrate (z-direction) in the presence of water and saltwater for different surface charges, black dots represent the net charge number of the surface devoid of cations.


Page 21 of 27

Prepared for JPC C

20 0.00 C/m

20

2

0.00 C/m

2

10

0

w Li Na K Rb Cs

-10 -20

Q/e

Q/e

10

0 -10

w Mg Ca Sr

-20

-30

-30 0

1

2

3

4

0

1

z [nm]

2

3

4

z [nm]

20 -0.20 C/m

20

2

10

-0.20 C/m2

10

0

w Li Na K Rb Cs

-10 -20 -30 0

1

2

3

Q/e

Q/e

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


0 -10

w Mg Ca Sr

-20 -30 4

0

z [nm]

1

2

3

4

z [nm]

Figure 12. Dimesionless net charge (charge Q normalized by the electron charge C ) above the (001) corundum on the direction normal to the substrate (z-direction) in the presence of water and saltwater for different surface charge densities, black dots represent the net charge number of the surface devoid of cations.

4

Conclusions

Computer simulations results of quartz (101)-water and corundum (001)-water interfaces in the presence of aqueous solutions of monovalent alkali and alkaline-earth metal chlorides lead to the following conclusions. For each surface, sequentially adsorbed layers of cations are different in the adsorption density but not in the adsorption sites preferred by the cations. The adsorption density of cations on quartz shows a sustained increase as the size of the cation increases for both monovalent and divalent cations. The adsorption density of divalent cations is lower than that of monovalent cations due to their high electric charge density that makes them more effective to screen the quartz charge. The adsorption density of monovalent cations on corundum follows the opposite tendency to quartz, the number of cations adsorbed decreases as the size of the cation increases and this effect becomes more pronounced as the surface charge, or equivalently the pH, increases. Adsorption on corundum is much more extensive compared to quartz for all surface charges and cations.



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Prepared for JPC C At high pH, Mg2+ is not adsorbed as such in either quartz or corundum because this condition favors precipitation of its hydroxides. The sequences of cation adsorption on silica and alumina are known and have been satisfactorily explained by a “like adsorbs like” concept, that is, high isoelectric point materials preferentially adsorb well-hydrated cations and low isoelectric point materials preferentially adsorb poorly-hydrated cations. The results here are important because they provide support at molecular level to this rule of thumb. The results reveal that quartz is actually less hydrated (less structure maker) than corundum (more structure maker); in the absence of electrolytes the layer of hydration in quartz has a thickness of ca. 0.5 nm whereas in corundum has a thickness of ca. 1.25 nm. The capacity of monovalent cations to orient water molecules in the vicinity of quartz and corundum surfaces, in addition to that of unprotonated surface groups, follows the order Li+ > Na+ > K+ > Rb+ > Cs+ and that of divalent cations is equivalent between them, however the range of influence in quartz is much less, ca. 1 nm, than in corundum, ca. 1.5 nm. Neutralization of quartz by the monovalent cations follows the order Li+ >> Na+ > K+ > Rb+ > Cs+ >> water, neutralization occurs at ca. 2 nm from the zero plane when the surface charge is moderate. Regarding divalent cations, a single layer is sufficient to neutralize the quartz surface completely at low surface charge and to overcharge the surface with consequent charge inversion at medium and high surface charge. Neutralization and charge reversal by divalent cations follow the order Mg2+ >> Ca2+ > Sr2+ >> water and occurs at ca. 0.5 nm from the zero plane. Neutralization of corundum is so effective that the surface charge is reversed in the presence of both monovalent and divalent cations. The neutralizing capacity of the monovalent cations follows the series Li+ > Na+ > K+ > Rb+ > Cs+ >> water, the larger the size of the cation the greater the number of layers required for neutralization of corundum. The neutralizing capacity of the divalent cations follows the series Mg2+ >> Ca2+ > Sr2+ >> water. Again, the larger the size of the cation the greater the number of layers required for neutralization. The first layers of Mg2+, Ca2+ and Rb2+ are located at distances > 0.6 nm either because the cation is heavily hydrated, case of Mg2+, or the size is too large, case of Ca2+ and Rb2+. Our results compare well with experimental data and simulation results available for quartz and corundum in the presence of some of the cations considered here. The results of this work are expected to contribute to a better understanding of the interaction of mineral oxides with macromolecules. Acknowledgements We thank Centro CRHIAM Project Conicyt/Fondap-15130015 for financial support. G. Quezada thanks University of Concepción and CONICYT-Chile for graduate student fellowships. The authors would like to thank Dr. Fernando Vallejos-Burgos from Center for Energy and Environmental Science at Shinshu University, Matsumoto, and Dr. Adam Skelton from School of Health Sciences at University of KwaZulu-Natal, Durban, for invaluable help with Gaussian calculations for corundum. We thank two anonymous reviewers for helpful suggestions.


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Prepared for JPC C TOC Graphic Na

K

2nd layer

1st layer

Li

3rd layer

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Rb

Cs

Molecular Dynamics Simulations of Quartz (101)âWater and

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