Aqueous Electrolyte Solution Interface: Molecular Dynamic

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Quartz/aqueous Electrolyte Solution Interface: Molecular Dynamic Simulation and Interfacial Potential Measurements Zlatko Brklja#a, Danijel Namjesnik, Johannes Lutzenkirchen, Milan Predota, and Tajana Preocanin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b04035 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 3, 2018

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The Journal of Physical Chemistry

Quartz/aqueous Electrolyte Solution Interface: Molecular Dynamic Simulation and Interfacial Potential Measurements

Zlatko Brkljača1, Danijel Namjesnik1, Johannes Lützenkirchen2, Milan Předota3, Tajana Preočanin1,*

1

Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102A, HR10000 Zagreb, Croatia

2

Institut für Nukleare Entsorgung, Karlsruher Institut für Technologie, P.O. Box 3640, 76021 Karlsruhe, Germany 3

Institute of Physics and Biophysics, Faculty of Science, University of South Bohemia, Branišovská 1760, 37005 České Budějovice, Czech Republic

ABSTRACT In this complementary experimental and theoretical study, we employ surface and electrokinetic potential measurements and equilibrium molecular dynamics (EMD) techniques to study the electrical interfacial layer between aqueous solutions of electrolytes and an oxide solid surface. More specifically, we investigate the behavior of a prototypical model system consisting of the (0001) quartz surface in contact with aqueous solutions of alkali metal salts under different conditions. The inner surface potential and electrokinetic ζ-potential were measured by means of single crystal electrodes and via streaming current measurements, respectively. Calculated ζ-potentials allowed us to benchmark MD simulations against experiment, thereby, on the one hand, verifying the validity of our strategy, and on the other hand, enabling a detailed molecular picture of the investigated phenomena and elucidating the role of both water and ions in the formation of the multilayered quartz/aqueous electrolyte interface.

* Corresponding author: Tajana Preočanin, e-mail: [email protected]

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INTRODUCTION Quartz is one of the most common minerals that occurs in the environment. The quartz (0001) crystal face is the most stable plane with the lowest surface energy, and is often considered as a "model surface", convenient for modelling SiO2 materials and hydrophilic surfaces in general.1 In aqueous electrolyte solution surface silica atoms react with water and form amphoteric ≡SiOH silanol surface sites. The extent of the surface protonation and deprotonation of these silanol groups depends on pH and the composition of the aqueous electrolyte solution. Surface concentrations of positively and negatively charged surface groups determine the overall surface charge and ion distributions as well as the orientation and diffusion of water molecules within the interfacial layer. Surface charging and formation of the electrical interfacial layer (EIL) are complex and mutually related processes. The electrostatic surface potential is determined by the charge distribution at the quartz/electrolyte solution interface, resulting from an interplay of electrostatic and van der Waals interactions with a key role of surface charge and interfacial structure of the solvent. The inner surface potential, Ψ0, is the electrostatic potential at the solid plane exposed to the liquid medium. Since this potential markedly affects the state of charged species bound to the surface, it plays a dominant role in surface equilibration. The expressions for the inner surface potential depend on the assumed surface complexation model.2 However irrespective of the model, the inner surface potential depends on the bulk concentration of the potential determining ions (H+/OH– in the case of quartz), the thermodynamic equilibrium constants of surface complexation and the ratio of surface concentrations of the charged groups.3 The measurement of the inner surface potential, enabled by construction of single crystal electrodes,4 provides important information on the equilibrium at the interfacial layer and enables a critical examination of the theoretical models describing the interfacial equilibrium.5–7 A single crystal electrode (SCrE) consist of a single crystal mounted to a poly(methyl methacrylate) (PMMA) holder. Ideally, one specific crystal plane is exposed to the aqueous electrolyte solution and measurements of the electrode potential with respect to a reference electrode provides information about surface complexation and distribution of ions within the electrical interfacial layer. A few limitations of this method make its application somewhat difficult. This includes the required calculation of an absolute inner surface potential from the measured relative electrode potential,5 the high resistance of the single crystal and titration hysteresis.8 The electrokinetic potential, often called ζ-potential, is assumed to occur at the hypothetical slip (or shear) plane that divides the stagnant from the 2 ACS Paragon Plus Environment

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mobile part of the EIL. The position of the slip plane distance has often been estimated, by fitting experimental data,9 to be about 1 nm from the metal oxide surface. Molecular dynamic studies have attempted to explain the molecular origins of the electrokinetic potential and the location of slip plane.10,11,12 Experimental methods for determination of electrokinetic potential are based on electrokinetic properties of surfaces13 such as mobilities of colloid particles in an applied electrical field (electrophoresis). Streaming potential and streaming current are electrokinetic phenomena14 caused by an aqueous electrolyte driven by a pressure gradient through a microchannel with electrically charged solid walls. The charge separation at the solid-liquid interface results in the motion of the charge and therefore a net electrical current. The measured value of streaming potential difference or current is related to the ζ-potential of the charged surface.15,16 Molecular dynamics (MD) simulations represent a powerful theoretical tool enabling the study of atomistic details of complex systems, thereby complementing and helping to explain at the molecular scale the nature of the experimentally observed macroscopic phenomena. With respect to the interaction between surfaces and aqueous solutions, MD techniques have been widely applied in studies of both solid surface/aqueous17–20 and soft surface/aqueous interfaces.21–23 Quartz/water interfaces have also been extensively studied using MD techniques. In this respect, Du and de Leeuw24 investigated the interactions of bare and hydroxylated quartz (0001) surfaces with water, observing several ordered monolayers of water on the bare surface. More recently, Argyris et al.25 studied the properties of water at the silica/water interface in the absence of electrolytes. On the other hand, Adeagbo et al.26 studied the behavior of water confined between two (0001) α-quartz surfaces using CarParrinello MD simulations, and found that water molecules rapidly reacted with the Siterminated quartz surface, leading to hydroxylation of both surfaces. Yang and Wang showed, using classical MD techniques that a monolayer of water on the hydroxylated quartz (0001) surface adopts a flat two-dimensional (2D) structure, where the water molecules are oriented with water hydrogens towards the surface to satisfy hydrogen bonding between water and the surface hydroxyls.27 Skelton et al.28 performed a comparison of different parameterizations for quartz/water simulations. They tested different classical descriptions (force fields) against ab initio MD, finding that the original force field for neutral clay materials, namely ClayFF,29 outperforms other parameterizations.



The major problem in investigating quartz/water and even more so, quartz/aqueous solution interfaces by MD, was the inability to cover a wide pH range, as force fields incorporating deprotonated silanol groups, necessary to study the system at pH conditions above the point of zero charge (pHpzc), were quite rare. In this respect, Hassanali et al.30 expanded their force field for a neutral amorphous silica surface with new siloxide (Si−O−) parameters. However, the use of both Buckingham and Lennard-Jones potentials limited the compatibility of their force with common biomolecular force fields.31 Kroutil et al.,31 following previous work,28,32 modified the ClayFF force field to describe negatively charged (101) quartz surface above its pHpzc, allowing the evaluation of the influence of negative surface charge on interfacial water and to study the adsorption of Na+, Rb+ and Sr2+.31 Very recently, in a detailed MD study the adsorption of monovalent alkali metal cations (Li+, Na+, K+, Rb+ and Cs+) on the quartz (101) surface was investigated, employing the ClayFF force field.33 On the other hand, ab initio MD simulations attempted to explain the effect of dissolved cations in the vicinity of the (101)34 quartz/water surface and the dependence of the acidity of silanol sites on the dissolved electrolyte on (001)35 quartz/water interfaces, providing valuable insight in the reactivity of these surfaces, also aiding in parameterization of classical force fields. Significant progress with respect to the parameterization of quartz/aqueous solutions, and more generally, silica/aqueous solution interfaces, was achieved in the work of Emami et al.36 These authors introduced a silica force field, thus resolving a number of deficiencies in computed interfacial properties, which in turn enables accurate computational predictions of aqueous interfacial properties for all types of silica in a wide pH range.36 In this study, we aim to investigate the behavior of water and ions at (0001) quartz /aqueous electrolyte interface as well as their influence on the interfacial properties, for 2 ≤ pH ≤ 9. For this purpose, we chose to investigate different electrolytes in contact with quartz, namely aqueous solutions of NaCl, KCl, NaBr and KBr, by means of surface potential measurements via SCrE and streaming current experiments, on one hand, and using classical MD techniques on the other hand, utilizing the aforementioned Emami et al.36 force field. We validate our choice of the force field by first calculating the ζ-potential for the investigated systems and by comparing the obtained results with experiments. ζ-potential have been previously calculated using MD techniques for rutile nanoparticles,37 and were elucidated by means of electroosmotic flow simulations for a generic surface with five different charge densities.38 Only very recently, Předota et al.12 have connected this macroscopic measure to the microscopic realm through both non-equilibrium MD (NEMD) simulations of electroosmotic 4 ACS Paragon Plus Environment

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flow and equilibrium molecular dynamics (EMD) simulations, showing that equilibrium MD performs reasonably well in the case of monovalent cation species.12 Motivated by these results, we decided to use the latter technique, namely EMD, to calculate ζ-potential following the methodology presented in Předota et al.12 Having validated our simulations by comparing with experimental results, we explore in detail the behavior of both the investigated electrolytes and water at the (0001) quartz surface for 2 ≤ pH ≤ 9 and their influence on the interfacial properties of quartz/aqueous electrolyte solutions.

Molecular dynamics methodology Proper propagation of MD simulations necessitates the choice of a suitable force field (parameters), governing all interactions in the investigated systems. To describe the (0001) quartz surface we decided to employ the force field of Emami et al.36, which enables modeling the influence of variable pH conditions on the surface of the quartz crystal. This is accomplished indirectly by changing the fraction of siloxide groups (≡SiO–) on the quartz surface (0, 9 and 18 % of siloxide groups corresponding to pH ≈ 3 (pHpzc), pH ≈ 6 and pH ≈ 9, respectively).36 Ions in the system were described using the standard Amber force field,39 which is compatible with the Emami et al.36 parameter set, while water was described using the standard TIP3P model. We propagated fully atomistic simulations of aqueous electrolyte solution of the aforementioned salts of alkali metals confined between two identically charged (0001) quartz surface using GROMACS,40 setting the bulk concentration of dissolved salts to approximately 0.4 mol dm–3. Overall, we simulated 12 systems, covering four salts (NaCl, NaBr, KCl, KBr) and three different pH conditions. An example of the prepared simulation box is shown in Figure 1. The corresponding distribution of deprotonated siloxide sites is shown in Figure 2.



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Na+

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pH ≈ 3

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Figure 2. Models of the (0001) quartz surface (viewed in the z-direction, see Figure 1) containing 0, 9 and 18 % of siloxide groups, representing pH ≈ 3, pH ≈ 6 and pH ≈ 9, respectively. Oxygens of charged siloxide groups are depicted as red spheres.

To prepare the systems, we first employed semi-isotropic NPT simulations (p = 1 bar), in which the x and y directions (Lx = 34.5 Å, Ly = 35.5 Å), corresponding to the force-field optimized (0001) quartz crystal lattice, were kept constant, while the z direction (perpendicular to the surface) was allowed to change. The atoms were allowed to move freely in all directions at this stage. In this way we obtained systems in which bulk water possesses 6 ACS Paragon Plus Environment

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the correct bulk density, namely ≈ 0.98 g cm–3, irrespective of the dissolved salt, which is the expected density for TIP3P water model. A large vacuum gap in the z direction was added (end slab to slab distance is measured at approximately 12 nm, with a vacuum gap of approximately 88 nm imposed), making the simulation effectively periodic in twodimensions, although periodic boundary conditions in three dimensions are applied. This is because 3D Ewald summation is significantly faster compared to 2D Ewald. After obtaining equilibrated system dimensions, we replace the crystal geometries obtained during semiisotropic NPT with minimized (perfect) crystal geometry. In the subsequent canonical (NVT) simulations, the majority of quartz atoms were kept frozen to achieve proper interfacial properties leaving only the first two layers free to move. The temperature in the simulations was controlled by the Nosé-Hoover thermostat (T = 300 K). After the initial relaxation and equilibration of the prepared system, each MD simulation was propagated for 100 ns, constituting production runs.

Experimental methodology Quartz (0001) single crystals for surface potential measurements (10 mm × 10 mm × 0.5 mm; polished on one side) were obtained from SurfaceNet GmbH (Germany). To remove organic contamination prior to the measurements the crystals were soaked in acetone overnight and subsequently washed by ethanol and finally by MilliQ water. All solutions were prepared using deionized and decarbonized water (> 18 MΩ cm) and analytical grade chemicals. The aqueous electrolyte solutions were prepared by dilution of standard acid and base solutions (NaOH, KOH, HCl, HBr: Fluka, Fixanal, c = 0.1 mol dm–3); and dissolution of weighted amount of salt (NaCl, KCl, NaBr, KBr: Fluka, puriss p.a.). A quartz (0001) single crystal electrode was constructed,41 and the electrode potentials with respect to the reference electrode (Ag|AgCl|3M KCl) were measured with a Methrom pH meter: Cu(s) | conductive paint | single crystal | aqueous electrolyte solution | reference electrode The measuring system was thermostated (t = 25 ± 0.1 °C) and was kept under argon atmosphere during titration. The data were continuously collected and plotted against time so that the stability of the reading could be verified in real time. The pH was measured by a 7 ACS Paragon Plus Environment


combined glass electrode using a Metrohm 826 pH meter. The pH-electrode was calibrated using standard buffers. The surface potential values7,42,43 were calculated from the measured SCr electrode potentials following the standard procedure, namely assuming that the point of zero potential for quartz (0001)/aqueous electrolyte solution interface is equal to the isoelectric point. The streaming current measurements,44 on the quartz (0001) crystal plane were carried out using a commercial device from Anton Paar (Graz) with platinum electrodes, at room temperature. The conductivity electrode was calibrated using standard KCl solution as previously described. The quartz (0001) inner surface potential in aqueous sodium chloride solution (Ic = 0.01 mol dm–3, and Ic = 0.001 mol dm–3) and the electrokinetic potential of quartz (0001) in 0.001 mol

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pH Figure 3. Measured inner surface potentials Ψ0: (•; 0.01 mol dm–3) and (♦; 0.001 mol dm–3) and electrokinetic potentials ζ: (○; 0.001 mol dm–3) of quartz (0001)/sodium chloride solution at t = 25 °C.

For the surface potential measurements, contrary to a standard titration where the electrode is continuously immersed in a solution of variable composition, we decided to employ a batch 8 ACS Paragon Plus Environment

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method, in which multiple solutions with constant and well-defined compositions are used. This method has advantages over standard titration such as, better reproducibility, precision, and shorter overall experimental duration, since the time needed to equilibrate the solution at a certain pH is avoided. The main disadvantage of this method lies in the fact that rinsing of the single crystal electrode with deionized water between two measurements might change the crystal surface. This effect was tested by repeatedly switching solutions of different pH and different salt compositions. From those tests it was concluded that in the case of the quartz single crystal electrode, equilibrium is achieved within 30 min, and that the system was completely reversible with regard to switching between different solutions. The influence of different salts (NaCl, KCl, NaBr, KBr) on the inner surface potentials the solutions of different pH (2, 6 and 9) was studied by dissolving the required amount of each salt in deionized water, and adjusting the pH by addition of acid or base (HCl, HBr, NaOH or KOH). The respective ionic strengths were Ic = 0.01 mol dm–3 (Ic = 0.02 mol dm–3 in the case of pH = 2). The quartz single crystal electrode potential was measured after equilibration (approximately 30 min). Between two measurements, the quartz electrode was thoroughly rinsed with deionized water, but without wiping. The reversibility of the measured electrode potential was tested. The quartz (0001) inner surface potentials were evaluated from the measured electrode potentials, being shifted according to the reference value, which corresponds to the obtained Ψ0 of Q-NaCl at identical pH and Ic conditions 0.01 mol dm–3 (Figure 4).



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pH Figure 4. Measured inner surface potentials Ψ0 (full symbols) and electrokinetic potentials ζ (empty symbols) of quartz (0001) at 0.01 mol dm–3 and t = 25 °C. ζ at low pH was measured at slightly higher salt levels (0.03 mol dm–3) to suppress the action of the protons relative to the cation of the background electrolyte.

To perform electrokinetic measurements a certain amount of salt solution (NaCl, NaBr, KCl, KBr) was added to the aqueous solution at different pH whereupon the streaming current of the quartz (0001) plane was measured. The effect of salt concentrations on the electrokinetic potential is presented on Figure 5. As expected, by addition of electrolyte, due to electrostatic interactions, the magnitude of the electrokinetic potential decreases. The effect of pH on the electrokinetic potential is presented on Figure 4. The presented values of electrokinetic potentials are obtained by interpolating the experimental data from Figure 5 and extracting the values at Ic = 10 mmol dm–3. Overall, at pH 6 and 9, where the quartz surface is negatively charged, potassium ions more strongly shield the quartz surface compared to sodium ions. The effect of the anion follows the sequence Br– < Cl– for all examined pH values. The stronger binding of chlorides compared with binding of bromides is consistent with the single crystal electrode results at the same electrolyte concentration 0.01 mol dm–3 (Figure 4).


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Figure 5. Electrokinetic potential of quartz (0001) in different salt solutions at a) pH ≈ 6, and b) pH ≈ 9, obtained by means of streaming current measurements at t = 25 °C. The values presented in Figure 4 are obtained by interpolating the experimental data (corresponding curves), and extracting the values at Ic = 10 mmol dm–3. ŠTO je s crtama koje povezuju podatke – Popravio sam regresiju na Figure 5a u dogovoru sa Zlatkom

The effects of iodide salts (KI, NaI) on the quartz surface were also examined by SCrE and streaming current measurements. The quartz electrode potential in the contact with aqueous iodide salts changed by more than 100 mV during equilibration (electrode potential raw data 11 ACS Paragon Plus Environment


are given in SI). The presence of iodine (I2) on the quartz surface was qualitatively confirmed by UV-VIS spectroscopy (see SI). The adsorbed species cannot be removed from contaminated quartz surface by simply rinsing with deionized water, only wiping with ethanol, followed by rinsing with deionized water returns the electrode to the initial state. The specific adsorption of iodine on quartz is documented in the literature.45–53 Since the presence of I2 in the concentrated aqueous iodide solutions could not be avoided, the results obtained by SCrE cannot be compared with results obtained for chlorides and bromides (for the same reason MD simulations for iodides were not performed) and we excluded them from further discussion. The effect of iodide on the quartz surface during streaming current measurements was significantly less pronounced. This observation stems from the fact that streaming current measurements are performed on considerably shorter timescales compared to SCrE measurements.


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MD Simulations

Figure 6. Interface-normal number density profiles of water at quartz (0001) surface at pH = 3 (green line), pH = 6 (violet dashed line), and pH = 9 (blue dotted line) as obtained from the MD simulations of Q-NaCl system. Black dashed lines denote positions of three peaks.

Structure of Water Perpendicular to the Surface. We start by exploring the structure of water molecules in the vicinity of the quartz surface. The interface-normal number density profiles of water from the MD simulations are shown in Figure 6 for Q-NaCl, as the calculated number density profiles of water remain virtually identical when other electrolytes, i.e., NaBr, KCl and KBr, are used (see Figure S1). This suggests that the surface charge, stemming from varying the number of siloxide groups at the (0001) quartz surface (see Molecular Dynamics Methodology), and not the electrolyte type, plays the predominant role with respect to the behavior of water at quartz/aqueous electrolyte solution interfaces. The point of origin, z = 0, is taken to be at the average position of hydroxyl oxygen atoms on the quartz surface, which we consider as the end of the quartz slab. We find that water shows three distinct peaks, corresponding to three water layers, positioned at approximately 3, 6 and 9.5 Å above to quartz surface, respectively, with the positions of layers being independent of pH. While profiles at different pH values visibly differ, the overall number of water molecules in its first layer, obtained by integrating the profiles up to the first minimum, is very similar in all investigated cases. Water approaches the quartz surface closer at higher pH, while at pH = 13 ACS Paragon Plus Environment


3 the first peak of water density is highest. Overall we find that the layering profile of water in the interfacial region tends to decrease with the increase in pH (adsorption maps of water at the (0001) quartz surface are given in SI, Figure S3). Water Orientation at the Interface. We have also investigated the quartz/aqueous electrolyte solution interface by analyzing the orientational preference of water close to the surface. Similarly to the interface-normal number density profiles of water (Figure 6) the orientation of water molecules typically does not depend on the type of the electrolyte used, with pH again playing the key role. Here, we present orientational profiles of water for the QNaCl system (Figure 7a). Orientational profiles for other electrolytes are presented in Figure S2 (see SI). Interfacial water closest to the quartz surface, at approximately 1.5 Å from the surface, shows only weak preferential orientation at pH = 3, with water oxygens having a weak tendency to point towards the surface. On the other hand, at pH = 6 and pH = 9, interfacial water shows strong orientational ordering with hydrogen atoms pointing toward the negatively charged quartz interface (Figure 7a). It is important to notice that, while interfacial water shows either weaker (pH = 3) or stronger (pH = 9) orientational ordering, the molecules in the center of the first water layer, situated at ≈ 3 Å from the surface (Figure 6), show very weak ordering. We inspected this region in detail, as even a mixture of well-defined orientations of water towards and away from the surface can yield nearly zero average dipole. However, by inspecting the distribution of dipole angles of water molecules in the aforementioned region we find that it actually closely corresponds to the one found in the bulk, with water molecules showing no preferred orientation towards or away from the surface. Bulk-like properties can be observed already at ≈ 10 Å from the interface at pH = 3 and at approximately 15 Å from the quartz for pH = 6 or 9, implying (as might be expected) somewhat stronger influence of negatively charged quartz surfaces.


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Figure 7. Average cosine of the angle between water dipole and surface normal (denoted by θ) as a function of distance from quartz (0001) surface for a) Q-NaCl system at pH = 3, 6, and 9, and for b) all investigated quartz/aqueous electrolyte solution interfaces at pH = 9. Water dipole and surface normal are depicted as red and blue arrow, respectively. Again, z = 0 is taken to be the average position of hydroxyl oxygen atoms on quartz surface.

While we find that the behavior of water does not depend on the type of dissolved electrolyte, with virtually the same profiles for all quartz/aqueous electrolyte systems at pH =3 and pH = 6 (Figure S2), respectively, we observe noticeable differences in the orientational profiles of water at pH = 9. More precisely, water exhibits distinctly different second (≈ 3.5 Å) and third (≈ 6 Å) peaks in the presence of sodium salts as opposed to potassium salts (Figure 7b). This finding is the first indication of differences in the system properties, under basic conditions, when different alkali metal ions are present. Motivated by this we shift our focus to ions, and interface-normal number density profiles of cations and anions, for all investigated quartz/aqueous electrolyte systems.

Ions – Number Density Profiles. We have further investigated the quartz/aqueous electrolyte solution properties by analyzing ion distributions at the interface. Figure 8 presents interfacenormal number density profiles of both cations and anions for all investigated pH values. In general, we observe that, relatively small differences between potassium and sodium distributions are seen already at pH = 3. As the surface becomes more negatively charged, i.e., at pH = 6 and especially at pH = 9, the differences in the behavior of the two cations become more evident. More precisely, we find that at the neutral surface (pH = 3), rather small differences are found between cation and anion distributions, respectively, regardless of the salt. However, even at the neutral surface, sodium ions tend to behave differently from 15 ACS Paragon Plus Environment


potassium ions. In this respect, potassium salts exhibit more pronounced peaks in their interface-normal number density distributions in the first water layer (first peak in the upper leftmost panel, 3 Å from the surface). At the same time, we find that sodium ions are shifted by approximately 0.5 – 1 Å toward the quartz surface compared to potassium ions (Figure 8, upper left panel, compare the first three peaks for sodium and potassium ion distributions). Interestingly, both sodium and potassium ions tend to occupy minima in the number density distribution of water, namely at around 4.5 Å and 7.5 Å from the surface (compare the positions of the two most dominant peaks in potassium and sodium ion profiles, Figure 8, with Figure 6). This tendency is more pronounced in the case of sodium ions. Such behavior of cations, i.e., the tendency to avoid entering the three interfacial water layers (Figure 6), can be tentatively explained by the strong organization of water due to hydrogen bonding makes these positions energetically more favorable for water but less favorable for cations. The same phenomenon is observed for anions (Figure 8, lower leftmost panel), but significantly less pronounced. A first noticeable difference between the different cations occurs at pH = 6 (Figure 8, upper middle panel), where sodium ions, both in Q-NaCl and Q-NaBr, strongly occupy the region at approximately 2 Å from the surface, denoting the second inner-sphere complex between Na+ and exposed SiO– groups, i.e. complexes in which the majority of cations interacts with the SiO– groups directly. The smaller of the two investigated cations, i.e. sodium ions, tend to approach quartz surface more closely, with their first peak (first inner-sphere complex in which cations directly interact with buried SiO– groups) at ≈ 1 Å, following the trend observed at pH = 3 (Figure 8, left upper panel). Furthermore, we find that potassium ions also strongly populate the region between 1.5 Å – 2.5 Å from the surface. However, different from sodium ions, they exhibit two weakly separated peaks (positioned at 1.6 and 2.4 Å). It is worth noting that the third peak, found both in potassium and sodium ion distributions, lies between the first and second interfacial water layer, as was observed in their profiles at pH = 3 (Figure 8, upper leftmost panel, second peak, lying at ≈ 4.5 Å). Unlike cations, anions do not show pronounced differences at pH = 6, regardless of the dissolved salt.


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Figure 8. Interface-normal number density profiles of cations (upper panel) and anions (lower panel) in their respective systems at quartz (0001) surface at three investigated pH values. z = 0 to be the average position of hydroxyl oxygen atoms on quartz surface.

Under basic conditions (pH = 9), we observe the most pronounced differences in the behavior of the two alkali metal cations. Sodium cations exhibit a significantly stronger first peak in their distribution compared to pH = 6. Specifically, we observe predominantly the first innersphere ≡SiO– Na+ complex (at approximately 1 Å from the surface). The second strong peak belongs to the second inner-sphere ≡SiO– Na+ complex, with sodium ions residing in the region ≈ 2.3 Å above the quartz surface. Qualitatively, a completely different profile is obtained for potassium ions, with a single strong peak between the two sodium peaks at approximately 1.8 Å. For both pH = 6 and pH = 9, the behavior of the cations is virtually independent of the anion. This should be expected since the interfaces at negative surfaces are dominated by cations. Chlorides and bromides behave very similarly in Q-NaCl/Q-NaBr and Q-KCl/Q-KBr systems. While chlorides and bromides in sodium salts show only a rather small peak at approximately 3.7 Å above the quartz surface, the anion profiles in potassium salts are significantly better defined, particularly at pH = 9. There we observe two strong peaks of anions at around 4.3 Å and 6.8 Å from the quartz surface showing their preference 17 ACS Paragon Plus Environment


toward regions in which the interface-normal number density profiles of water reach minima (see Figure 6). Overall, we find that cations play the predominant role with respect to the differences observed in the behavior of the investigated sodium and potassium salts, with the distinctions becoming more pronounced at more basic conditions, i.e., at more negatively charged quartz surfaces. On the other hand, anions play a role at the neutral surface, with small differences in the behavior of chloride and bromide salts, respectively (Figure 8, bottom leftmost panel). At higher pH values, their behavior is virtually identical, irrespective of the dissolved salt (pH = 6). At pH = 9, chloride and bromide anions, originating from the salt of the same alkali metal, possess very similar profiles, with the profile of the same anion being both qualitatively and quantitatively completely different when that anion species originates from a different cation – anion combination (Figure 8, bottom rightmost panel). Adsorption maps showing the distribution of cations in their first and second interfacial layers are discussed in SI (Figures S4 and S5). We now turn our attention to two important system properties enabling us direct comparison of simulations with experiments, namely electrostatic potential and ζ-potential.

Electrostatic Potentials. The profiles of the electrostatic potentials, shown in Figure 9, were calculated by double integration of the charge distribution, taking into account contributions from the entire system. The electrostatic potential is generally dominated by damped oscillations around zero, which originate from the water contributions due to preferred orientation of water dipoles and inhomogeneous water density. As expected, at the neutral quartz surface, the profiles essentially do not depend on the combinatin of cation – anion pairs. In general, due to their excess at the interface compensating the negative surface charge, the effect of cations, rather than anions, increases with negative surface charge, but is limited mostly to the region of inner-sphere and first outer-sphere adsorption peaks, i.e., around 3–7 Å, being most pronounced at around 5 Å above the quartz surface.


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Figure 9. Electrostatic potential at the quartz/aqueous electrolyte solution interface calculated taking into account all atoms including water at a) pH = 3, b) pH = 6, and c) pH = 9. All curves were shifted to zero potential in the bulk.

While electrostatic potentials show very similar profiles for pH = 3 and pH = 6, under basic conditions (pH = 9) a difference is observed when Q-NaCl and Q-NaBr are compared to QKCl and Q-KBr systems. This difference stems from two effects, namely different sodium and potassium ion distributions at the negatively charged quartz surface at pH = 9 (Figure 8, upper rightmost panel), and the concomitant orientational behavior of water (Figure 7). From the electrostatic potentials, it should be in principle possible to infer ζ-potential, traditionally described as the value of the electrostatic potential at the slipping plane. However, it was recently found12 that it is not possible to locate a clearly defined slipping plane that can be linked to the macroscopically observable ζ-potential, thereby preventing direct determination of this property from the electrostatic potential. We thus decided to calculate a ζ-potential from two axial profiles accessible from the performed equilibrium MD simulations, namely using charge density and the viscosity profile in the direction perpendicular to the solid surface, following the procedure described in the Supporting Information of Ref. 12 and elaborated in more details in the following section.

ζ-Potentials. In general, the ζ-potential can be evaluated using the Helmholtz-Smoluchowski µη

equation, ζ = − ε0 εr , where η = 8.9 × 10–4 Pa s and εr = 79 are the experimental bulk water values of dynamic viscosity and relative permittivity, while µ represents the average bulk 19 ACS Paragon Plus Environment


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mobilities of water in electroosmosis (or using an analogous equation with a positive sign when µ represents the mobility of a solid particle in electrophoresis). Thus, the only unknown parameter that has to be evaluated is the average bulk mobility of water, which can be obtained by calculating distance-dependent electrophoretic mobilities using z

1 ⌠ Pxz ( z' ) µ( z) = d z' ,  E x ⌡0 η( z' )

(1)

where η(z') denotes distance-dependent viscosity, while Pxz(z') represents the off-diagonal component of the pressure tensor (shear stress), and is calculated as

Pxz ( z ) = E x ∫

L/2

z

ρq ( z' )dz' ,

(2)

where ρq(z') is the volume charge density of ions at a distance z above the surface (taking into account all cation and anion contributions). Eq. (2) expresses that the stress at height z is caused by the electric force along the surface acting on all the charge further from the surface – and which the immobile surface must keep at a steady mobility. Eq. (1) then integrates the slip, proportional to the local stress and inversely proportional to local viscosity η(z') from the surface up to bulk liquid region far from the surface, where constant mobility of the fluid relative the surface is observed.12 While the derivation of these equations follows the nonequilibrium approach with applied external electric field Ex, it is evident that in the linear response regime the mobility is independent of the magnitude of the field and Eqs. (1) and (2) combined can be used even in the limit of zero field, i.e. our equilibrium simulations.12 While ρq(z') can be easily obtained from the density profiles of ions (Figure 8), the determination of the distance-dependent viscosity η(z') is a challenging task.54 Here we therefore approximated, for each system studied, the distance-dependent viscosity via distancedependent self-diffusivity of water (diffusion of water in xy plane is used for this purpose, Dxy) using the Einstein-Stokes relation,

D=

k BT , 6πηa

(3)

where D denotes the diffusion coefficient, kB the Boltzmann constant, T the absolute temperature, and η the dynamic viscosity, while a corresponds to the radius of a spherical particle. To properly convert diffusivity to viscosity, it is necessary to know the parameter a. We fitted a independently for each investigated system, such that the diffusion in the bulk, i.e., more than 20 Å above the surface, yields the expected value of the viscosity in the bulk of water (ηbulk = 8.9 × 10–4 Pa s). The obtained viscosity profiles are shown in Supplementary Information (Figure S6), while the electrophoretic mobilities, calculated via Eq. (1), are 20 ACS Paragon Plus Environment

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shown in Figure S7 (see SI). The distance-dependent self-diffusion coefficients of water, used to obtain viscosity profiles in the z-direction, are calculated using the method of Předota et al.54

Figure 10. Calculated ζ-potentials of the simulated quartz (0001) systems at three investigated pH values for 0.4 mol dm–3 aqueous solution of: (•) NaCl, (♦) NaBr, (■) KCl and (▲) KBr.

The resulting ζ-potential are plotted vs. pH in Figure 10. We note that, as expected, at the neutral quartz surface (pH = pHpzc = 3), ζ-potential values for all investigated quartz/aqueous electrolyte solutions are close to zero, slightly positive (0.5–2 mV range). On the other hand, at pH = 6, we observe that all systems exhibit negative ζ-potential values, around –25 mV. QNaBr shows the most negative potential, while Q-KCl possesses the least negative value. We note that bromides give rise to more negative ζ-potential values compared to chlorides, which agrees with both SCrE and streaming current measurements (see Figure 4 and Figure 5). The difference in behavior among the investigated salts is more pronounced under basic pH in MD simulations. Thus at pH = 9 we find that sodium ions cause more negative ζ-potential values compared to their potassium counterparts, which again is in agreement with both the surface and streaming current measurements (see Figure 4). While the experimental measurements show that the effect of chlorides is somewhat more pronounced in this regime, i.e., their salts cause less negative ζ-potential values, this finding is not completely matched in the performed simulations (compare Figure 10 and Figure 4, pH = 9). However, careful inspection of experimental results (see Figure 5b) points to the relatively strong ζ-potential dependence on anion concentration, with the inversion of this phenomenon in the lower concentration range, 21 ACS Paragon Plus Environment


while the simulations are carried out at significantly larger concentrations around 0.4 mol dm– 3

. Interestingly, we observe that, in the case of all investigated quartz/aqueous electrolyte

solutions, the theoretically obtained ζ-potential becomes less negative as one goes from neutral towards basic pH values, which is somewhat unexpected. However, the same trend is reflected in the experiments, especially in the streaming current measurements (Figure 4), with all salts showing less negative potential under basic conditions (Table S1). We are now in the position to comment on the underlying reason for the behavior of ζpotentials observed in the performed MD simulations. By integrating the number density profiles of cations (Figure 8) we found that, at pH = 6, a somewhat larger number of cations resides near the negatively quartz surface in the case of Q-NaCl compared to Q-NaBr, with the same trend found when Q-KCl is compared to Q-KBr (inspect Figure 8). This implies that Q-NaCl and Q-KCl screen the negative quartz surface more successfully compared to their bromide counterparts, in turn giving rise to somewhat less negative ζ-potentials for chloride compared to bromide salts. On the other hand, at pH = 9, virtually no difference in the number of interfacial sodium ions stemming from Q-NaCl and Q-NaBr, and likewise, between potassium ions arising from Q-KCl and Q-KBr, is found. However, the number density profiles of anions at pH = 9 exhibit higher peaks of chlorides compared to bromides when salts originating the same alkali metal are compared (Q-NaCl vs. Q-NaBr, and Q-KCl vs. Q-KBr, Figure 8, bottom right panel). This indicates that chlorides actually screen the formed cation layer(s) more strongly than bromides, making the quartz surface overall more negative. This finding can be directly linked to the calculated ζ-potentials, as we observe that chloride salts indeed give rise to more negative ζ-potentials compared to bromide salts, as expected from the above analysis. A startling phenomenon at pH = 9 is the extra peak at around 4 Å in both bromide and chloride anion distributions in the presence of potassium ions. Integrating the cation distributions of potassium vs. sodium up to this distance, we observe that more potassium compared to sodium ions are adsorbed. In fact potassium cations can slightly overcompensate the negative surface charge, giving rise to the aforementioned extra peak of anions, which in turn counterbalance the charge of cations. This implies that potassium, in this pH regime, is more strongly adsorbed, giving rise to less negative ζ-potentials. In fact, the larger diameter of potassium compared to sodium may contribute to potassium being more strongly adsorbed, as it can more readily (partially) dehydrate; note that for the same conditions, a significant amount of sodium ions is still in the second inner-sphere peak (further away from the surface 22 ACS Paragon Plus Environment

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than the peak observed in the case of potassium, Figure 8, upper right panel), corresponding to less dehydrated sodium ions.

DISCUSSION The quartz (0001) surface represents the most stable plane of the crystal, and was chosen for a comprehensive study involving both experimental and theoretical techniques. Generally, in aqueous electrolyte solutions, surface silica atoms react with water and form the amphoteric ≡SiOH silanol surface sites which can in principle be protonated or deprotonated forming

positively charged ≡SiOH2+ or negatively charged siloxide ≡SiO– surface sites, respectively. The total surface concentration of silanol surface sites on quartz (0001) is 9.4 sites/nm2. Surface reactions and the distribution of ions between the interfacial region and the bulk of the solution lead to the formation of a charged interfacial layer. The inner surface potential is important parameter describing the state within the electrical interfacial layer as well as the interfacial equilibrium. Surface potential is influenced by the surface concentrations of the charged surface groups, distribution of ions and orientation of water molecules near the solid surfaces. It gradually changes from the solid surface towards the bulk of the solution. In this profile the electrokinetic ζ-potential is located at an unknown position. In this study, we explore the (0001) quartz/aqueous electrolyte solutions under different pH conditions and salt compositions, using both experimental and MD techniques. In the latter we reflect the experimental pH conditions indirectly via surface charge resulting from surface protonation/deprotonation. The inner surface and electrokinetic potentials of the (0001) quartz/aqueous electrolyte solutions were obtained by means of SCrE and via streaming current measurements, respectively. The measured ζ-potentials exhibit more negative values, with an isoelectric point (at which ζ = 0) below pH = 3 (Figure 3). Due to the small measured surface potential values, it is difficult to observe any effects of different electrolytes on surface potentials within the electroneutrality region (pH ≈ 3). Therefore, we conducted SCrE experiment at pH ≈ 2 where quartz (0001) is positively charged. As expected, the effect of the anion is pronounced, and we found stronger interaction of bromide ions, compared to chloride ions. Streaming current measurements become very difficult at the low pH values. With regard to MD simulations, for the neutral surface (pH ≤ 3) the fraction of negatively charged siloxide sites on the quartz surface is set to zero.36 The first interfacial layer of water 23 ACS Paragon Plus Environment


at the quartz/aqueous electrolyte interface exhibits the highest density under these conditions (Figure 6, full green line), but weak preferential orientation of water molecules, with water oxygen atom having a weak tendency to point towards the quartz surface (Figure 7, full green line). The interfacial water layer (z < 4.3 Å from quartz surface) is structured symmetrically, with the positions of water oxygen atoms corresponding to the positions of hydroxyl groups on (0001) quartz (Figure S3). Calculated ζ-potential for all investigated quartz/aqueous electrolyte solutions tend to be close to zero, with all of the calculated potentials being slightly positive (Figure 10). At this pH we observe relatively small differences between cation and anion distributions, respectively (Figure 8). However, different from systems at higher pH values, we do find small differences in the behavior of chloride and bromide salts, and the same tendency is observed in surface potential measurements. At pH = 6 the SiO2 surface in contact with aqueous electrolyte solutions becomes negatively charged. The experimentally measured inner surface potential confirms the existence of the siloxide groups (≡SiO–) and negative electrical charge at the surface (Figure 3). By increasing the ionic strength, the absolute values of the quartz surface potential decrease. According to the surface complexation model7 this region indicates very low values of the thermodynamic equilibrium constant for the protonation of surface siloxide sites. According to the electrical interfacial models55 the magnitude of the electrostatic potential decreases from the solid phase to the bulk of the electrolyte solution. At pH > 3 we measure a higher absolute value of the electrokinetic potential compared to the inner surface potential which indicates that the counterions and water dipoles contribute more strongly to the electrokinetic potential. It is worth noting that, in contrast to the macroscopic models that involve the interfacial potential as a monotonically increasing/decreasing function, molecular dynamics simulations point to an intrinsically oscillatory behavior dominated by damped oscillations that stem from water contributions (Figure 9).54 The experiments performed at pH = 6 show that the differences in inner surface potentials for potassium and sodium chloride and bromide are minor. Measured values are rather small, however, somewhat stronger association of bromides compared to chlorides is observed (Figure 4). On the other hand, the effect of different salts on electrokinetic potentials is more noticeable (Figure 4 and Figure 5a). In this respect, potassium ions are more strongly adsorbed than sodium ions, with anions following the sequence Br– > Cl– (in line with SCrE). To mimic the experimental conditions in simulations, namely pH = 6, a certain fraction of siloxide groups (9 %) was introduced to the otherwise neutral quartz surface, thus making it 24 ACS Paragon Plus Environment

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overall negative (surface charge density of approximately –0.13 C/m2). As in the case of the neutral quartz surface, at the negatively charged quartz interfacial water exhibits three distinct peaks, corresponding to three water layers, positioned at approximately 3, 6 and 9.5 Å above the quartz surface, respectively, with the position of layers independent of pH (Figure 6). Interfacial water shows strong orientational ordering at approximately 1.5 Å above, with hydrogen atoms pointing toward the negatively charged quartz interface (Figure 7), which correlates well with findings of Quezada et al.33 Phase measurements of Sum-frequency vibrational spectroscopy56 confirmed that water molecules at high pH are hydrogen bonded to the quartz surface with oxygens. At this pH, cations start to play the predominant role with respect to the differences observed in the behavior of the investigated sodium and potassium salts (Figure 8). Thus, we find that the smaller sodium ions tend to approach quartz surface more closely, with their first peak (inner-sphere complex) at ≈ 1 Å from the surface. Larger potassium ions strongly populate the region between 1.5 Å – 2.5 Å from the surface, characterized by two weakly separated peaks at 1.6 and 2.4 Å. These predominantly belong to the first and second inner-sphere complexes respectively, formed by K+ and ≡SiO–. While cations show pronounced differences, anions behave remarkably similar at pH = 6, regardless of the dissolved salt. Calculated ζ-potential values for all investigated quartz/aqueous electrolyte solutions at pH = 6 are negative (Figure 10), around –25 mV. However, we do observe that bromides have a tendency to generate more negative ζ-potentials compared to chlorides, which is reflected both in SCrE and in streaming current data. At pH = 9 the quartz surface is even more negatively charged. Both inner surface and electrokinetic potentials are negative, and the differences in the behavior of the investigated salts are by far most pronounced. To perform the corresponding MD simulations, the fraction of siloxide groups was set to 18 %, corresponding to an overall surface charge of –0.26 C/m2 in 0,4 M salt concentration. We find that the interfacial properties of the quartz/aqueous electrolyte solutions at pH = 9 are very similar to pH = 6 with regard to water adsorption and its distribution (Figure S3). However, water exhibits a different orientational profile at pH = 9 depending on the cation, with orientational ordering becoming significantly more pronounced when potassium salts are used instead of sodium salts (Figure 7b, compare second and third peak in the profiles of potassium and sodium salts). This suggests that, at pH = 9, the influence of electrolytes, primarily cations, plays a significantly larger role compared to systems at the lower investigated pH values. This difference between sodium and potassium 25 ACS Paragon Plus Environment


salts then causes distinct trends in both electrostatic and calculated ζ-potentials (Figures 9 and 10, respectively). We find that sodium ions yield more negative ζ-potential compared to their potassium counterparts, which is in agreement with experimental surface potential and streaming current measurements (compare Figures 5 and 10). Moreover, we observe that the electrokinetic potential becomes less negative at higher pH values, which is somewhat unexpected. This behavior has already been observed in previous simulations for strongly adsorbing cations.12 While a larger amount of cations adsorbs onto more negative quartz surfaces (pH = 9 vs. pH = 6), the local viscosity in this region is significantly larger than in the bulk, overall greatly reducing the effect of the directly adsorbed cations on the buildup of ζ-potential, as can be deduced from Eq. (1). In other words, adsorbed cations belong to the stagnant layer, effectively not contributing to the calculated mobilities, thereby not affecting the obtained ζ-potentials. The same trend is reflected in the performed surface potential measurements (Figure 4), where all examined salts show less negative potential under basic conditions. Not surprisingly, the role of anions at negatively charged surfaces is minor relative to that of cations (see Figure 8, notice the difference in scales), though strong association of anions with the surface-bound cations reamplifies their influence on the electrokinetic behavior in the investigated systems. Additionally, we may compare our findings with the absolute surface potential obtained by Xray photoelectron spectroscopy with a liquid microjet for silica nanoparticles dispersed in basic aqueous alkali metal chloride solution at 4°C. Brown et al.57 found that in the basic region the magnitude of the surface potential increases (becomes more negative) with increasing hydrated cation size. They explain this phenomenon via the larger hydrated cations that are more distant from the solid surface. This causes a larger potential drop across the interfacial layer i.e. Ψ0(Na+) > Ψ0(K+), which was proved experimentally in this paper.

Conclusion In this combined experimental and theoretical study, we analyze the influence of the potential determining ions (H+ and OH–), counterions, co-ions and water molecules on the inner surface potential and the electrokinetic potential of the hydroxylated quartz (0001) surface. We present an original approach, in which equilibrium MD simulations of aqueous electrolyte solution confined between parallel slabs of the hydroxylated quartz (0001) surfaces at three different pH values are used to calculate ζ-potentials, which are then compared with experimental results. This allows us to comment in detail on both water and counterion 26 ACS Paragon Plus Environment

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behavior and their influence on the quartz/aqueous electrolyte solution interface. In this respect, we find that water in the interfacial region forms three distinct layers, irrespective of pH. Symmetrical adsorption is most pronounced within the first layer at near neutral quartz (0001) surfaces. On the other hand, the layered profile, adsorption and orientation of water are pH dependent. We find that strong organization of water and hydrogen bonding occurs in the first water layer and influences ion penetration. The behavior of ions, as inferred from the calculated ion number density profiles, depends on the water distribution. The observed differences between anions and cations and their influence on the interfacial properties of quartz (0001) depend on pH and the fraction of the negatively charged siloxide surface sites. While we find that cations play a predominant role on the interfacial properties, at pH = 6, both calculated and experimentally obtained potentials suggest that the anion influence is becoming pronounced even in this regime. Chlorides show a tendency of shifting the electrokinetic potential towards zero. The interplay between ion-quartz and water-quartz interactions and their overall influences on interfacial properties are most apparent for basic conditions, i.e. at pH ≈ 9, where the choice of the cation greatly influences the orientations of water dipoles. This drastically changes both electrostatic (Figure 9) and electrokinetic (Figure 10) potentials, as revealed by the performed simulations. Larger cations screen more strongly the existing surface charge in this regime, thereby decreasing the absolute ζ-potential value. On the other hand, the simulations suggest that the smaller anions tend to conceal the first cation layer more strongly, thus overall increasing the absolute value of the calculated ζpotential. Overall, we find that larger cations more strongly screen the negatively charged quartz surface, whereas both MD simulations and experiments show that anions accumulate weakly. Their influence on electrokinetic phenomena varies strongly depending on their concentration, which in turn gives rise to the rich and complex multilayer nature of the quartz/aqueous solution interface. Supporting Information: Computational detail and calculated data and figures including interface-normal number density and orientational profiles of water, adsorption maps of water and ions, calculated viscosity profiles and electrophoretic mobilities of water, as well as experimental results of the influence of all analyzed ions and additionally iodides on quartz surface potential. This material is available free of charge via the Internet at http://pubs.acs.org. Acknowledgments: This work has been supported by Croatian Science Foundation under the project IP-2014-09-6972 and Czech Science Foundation project 17-10734S (M.P.). 27 ACS Paragon Plus Environment


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