Quartz/Aqueous Electrolyte Solution Interface: Molecular DynamicSimulation and Interfacial Potential MeasurementsZlatko Brkljaca,† Danijel Namjesnik,† Johannes Lutzenkirchen,‡ Milan Predota,§

and Tajana Preocanin*,†

†Department of Chemistry, Faculty of Science, University of Zagreb, Horvatovac 102A, HR-10000 Zagreb, Croatia‡Institut fur Nukleare Entsorgung, Karlsruher Institut fur Technologie, P.O. Box 3640, 76021 Karlsruhe, Germany§Institute of Physics, Faculty of Science, University of South Bohemia, Branisovska 1760, 37005 Ceske Budejovice, Czech Republic

*S Supporting Information

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

■ INTRODUCTION

Quartz is one of the most common minerals that occur in theenvironment. The quartz (0001) crystal face is the most stableplane with the lowest surface energy and is often considered asa “model surface”, convenient for modeling SiO2 materials andhydrophilic surfaces in general.1 In aqueous electrolytesolution, surface silica atoms react with water and formamphoteric SiOH silanol surface sites. The extent of thesurface protonation and deprotonation of these silanol groupsdepends on pH and the composition of the aqueous electrolytesolution. Surface concentrations of positively and negativelycharged surface groups determine the overall surface chargeand ion distributions as well as the orientation and diffusion ofwater molecules within the interfacial layer. Surface chargingand formation of the electrical interfacial layer (EIL) arecomplex and mutually related processes. The electrostaticsurface potential is determined by the charge distribution atthe quartz/electrolyte solution interface, resulting from aninterplay of electrostatic and van der Waals interactions withkey roles of surface charge and interfacial structure of thesolvent.The inner surface potential, Ψ0, is the electrostatic potential

at the solid plane exposed to the liquid medium. Because thispotential markedly affects the state of charged species bound tothe surface, it plays a dominant role in surface equilibration.The expressions for the inner surface potential depend on theassumed surface complexation model.2 However, irrespectiveof the model, the inner surface potential depends on the bulkconcentration of the potential determining ions (H+/OH− in

the case of quartz), the thermodynamic equilibrium constantsof surface complexation, and the ratio of surface concentrationsof the charged groups.3 The measurement of the inner surfacepotential, enabled by construction of single crystal electrodes(SCrE’s),4 provides important information on the equilibriumat the interfacial layer and enables a critical examination of thetheoretical models describing the interfacial equilibrium.5−7 ASCrE consists of a single crystal mounted to a poly(methylmethacrylate) holder. Ideally, one specific crystal plane isexposed to the aqueous electrolyte solution and measurementsof the electrode potential with respect to a reference electrodeprovides information about surface complexation and dis-tribution of ions within the EIL. A few limitations of thismethod make its application slightly difficult. This includes therequired calculation of an absolute inner surface potential fromthe measured relative electrode potential,5 the high resistanceof the single crystal, and titration hysteresis.8 The electro-kinetic potential, often called ζ-potential, is assumed to occurat the hypothetical slip (or shear) plane that divides thestagnant from the mobile part of the EIL. The position of theslip plane distance has often been estimated, by fittingexperimental data,9 to be about 1 nm from the metal oxidesurface. Molecular dynamic studies have attempted to explainthe molecular origins of the electrokinetic potential and thelocation of the slip plane.10−12

Received: April 29, 2018Revised: October 2, 2018Published: October 2, 2018

Article

pubs.acs.org/JPCCCite This: J. Phys. Chem. C 2018, 122, 24025−24036

© 2018 American Chemical Society 24025 DOI: 10.1021/acs.jpcc.8b04035J. Phys. Chem. C 2018, 122, 24025−24036

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Experimental methods for the determination of electro-kinetic potential are based on electrokinetic properties ofsurfaces,13 such as mobilities of colloid particles in an appliedelectrical field (electrophoresis). Streaming potential andstreaming current are electrokinetic phenomena14 caused byan aqueous electrolyte driven by a pressure gradient through amicrochannel with electrically charged solid walls. The chargeseparation at the solid−liquid interface results in the motion ofthe charge and therefore a net electrical current. The measuredvalue of streaming potential difference or current is related tothe ζ-potential of the charged surface.15,16

Molecular dynamics (MD) simulations represent a powerfultheoretical tool enabling the study of atomistic details ofcomplex systems, thereby complementing and helping toexplain at the molecular scale the nature of the experimentallyobserved macroscopic phenomena. With respect to theinteraction between surfaces and aqueous solutions, MDtechniques have been widely applied in studies of both solidsurface/aqueous17−20 and soft surface/aqueous interfaces.21−23

Quartz/water interfaces have also been extensively studiedusing MD techniques. In this respect, Du and de Leeuw24

investigated the interactions of bare and hydroxylated quartz(0001) surfaces with water, observing several orderedmonolayers 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 otherhand, Adeagbo et al.26 studied the behavior of water confinedbetween two (0001) α-quartz surfaces using Car−ParrinelloMD simulations and found that water molecules rapidlyreacted with the Si-terminated quartz surface, leading tohydroxylation of both surfaces. Yang and Wang showed usingclassical MD techniques that a monolayer of water on thehydroxylated quartz (0001) surface adopts a flat two-dimensional (2D) structure, where the water molecules areoriented with water hydrogens toward the surface to satisfyhydrogen bonding between water and the surface hydroxyls.27

Skelton et al.28 performed a comparison of differentparameterizations for quartz/water simulations. They testeddifferent classical descriptions (force fields) against ab initioMD, concluding that the original force field for neutral claymaterials, namely ClayFF,29 outperforms other parameter-izations.The major problem in investigating quartz/water and even

more so, quartz/aqueous solution interfaces by MD, was theinability to cover a wide pH range, as force fields incorporatingdeprotonated silanol groups, necessary to study the systemunder pH conditions above the point of zero charge (pHpzc),were quite rare. In this respect, Hassanali et al.30 expandedtheir force field for a neutral amorphous silica surface with newsiloxide (Si−O−) parameters. However, the use of bothBuckingham and Lennard-Jones potentials limited thecompatibility of their force field with common biomolecularforce fields.31 Kroutil et al.,31 following previous work,28,32

modified the ClayFF force field to describe the negativelycharged (101) quartz surface above its pHpzc, allowing theevaluation of the influence of negative surface charge oninterfacial water and to study the adsorption of Na+, Rb+, andSr2+.31 Very recently, in a detailed MD study the adsorption ofmonovalent alkali metal cations (Li+, Na+, K+, Rb+, and Cs+)on the quartz (101) surface was investigated, employing theClayFF force field.33 On the other hand, ab initio MDsimulations attempted to explain the effect of dissolved cationsin the vicinity of the (101)34 quartz/water surface and the

dependence of the acidity of silanol sites on (001)35 quartz/water interfaces, providing valuable insight in the reactivity ofthese surfaces, also aiding in parameterization of classical forcefields. Significant progress with respect to the parameterizationof quartz/aqueous solutions and, more generally, silica/aqueous solution interfaces, was achieved in the work ofEmami et al.36 These authors introduced a silica force field,thus resolving a number of deficiencies in computed interfacialproperties, which in turn enables accurate computationalpredictions of aqueous interfacial properties for all types ofsilica in a wide pH range.36

In this study, we aim to investigate the behavior of water andions at the (0001) quartz/aqueous electrolyte interface as wellas their influence on the interfacial properties, for 2 ≤ pH ≤ 9.For this purpose, we chose to investigate different electrolytesin contact with quartz, namely aqueous solutions of NaCl, KCl,NaBr, and KBr, by means of surface potential measurementsvia SCrE and streaming current experiments, on the one hand,and using classical MD techniques on the other hand, utilizingthe aforementioned Emami et al.36 force field. We validate ourchoice of the force field by first calculating the ζ-potential forthe investigated systems and by comparing the obtained resultswith experiments. ζ-potentials have been previously calculatedusing MD techniques for rutile nanoparticles37 and wereelucidated by means of electro-osmotic flow simulations for ageneric surface with five different charge densities.38 Only veryrecently, Predota et al.12 have connected this macroscopicmeasure to the microscopic realm through both non-equilibrium MD simulations of electro-osmotic flow andequilibrium MD (EMD) simulations, showing that EMDperforms reasonably well in the case of monovalent cationspecies.12 Motivated by these results, we decided to use thelatter technique, namely EMD, to calculate ζ-potentialfollowing the methodology presented in Predota et al.12

Having validated our simulations by comparing withexperimental results, we explore in detail the behavior ofboth the investigated electrolytes and water at the (0001)quartz surface for 2 ≤ pH ≤ 9 and their influence on theinterfacial properties of quartz/aqueous electrolyte solutions.

■ MD METHODOLOGYProper propagation of MD simulations necessitates the choiceof a suitable force field (parameters), governing all interactionsin the investigated systems. To describe the (0001) quartzsurface, we decided to employ the force field of Emami et al.,36

which enables modeling the influence of variable pHconditions on the surface of the quartz crystal. This isaccomplished indirectly by changing the fraction of siloxidegroups (SiO−) on the quartz surface (0, 9 and 18% ofsiloxide groups corresponding to pH ≈ 3 (pHpzc), pH ≈ 6 andpH ≈ 9, respectively).36 Ions in the system were describedusing the standard Amber force field,39 which is compatiblewith the Emami et al.36 parameter set, whereas water wasdescribed using the standard TIP3P model. We propagatedfully atomistic simulations of aqueous electrolyte solution ofthe aforementioned salts of alkali metals confined between twoidentically charged (0001) quartz surface using GROMACS,40

setting the bulk concentration of dissolved salts to approx-imately 0.4 mol dm−3. Overall, we simulated 12 systems,covering four salts (NaCl, NaBr, KCl, and KBr) and threedifferent pH conditions. An example of the preparedsimulation box is shown in Figure 1. The correspondingdistribution of deprotonated siloxide sites is shown in Figure 2.

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To prepare the systems, we first employed semi-isotropicNPT simulations (p = 1 bar), in which the x and y directions(Lx = 34.5 Å, Ly = 35.5 Å), corresponding to the force-fieldoptimized (0001) quartz crystal lattice, were kept constant,while the z direction (perpendicular to the surface) wasallowed to change. The atoms were allowed to move freely inall directions at this stage. In this way, we obtained systems inwhich bulk water possesses the correct bulk density, namely≈0.98 g cm−3, irrespective of the dissolved salt, which is theexpected density for TIP3P water model. A large vacuum gapin the z direction was added (end slab to slab distance ismeasured at approximately 12 nm, with a vacuum gap ofapproximately 88 nm imposed), making the simulationeffectively periodic in two dimensions, although periodicboundary conditions in three dimensions are applied. This isbecause 3D Ewald summation is significantly faster comparedto 2D Ewald. After obtaining equilibrated system dimensions,we replaced the crystal geometries obtained during semi-isotropic NPT with minimized (perfect) crystal geometry. Inthe subsequent canonical (NVT) simulations, the majority ofquartz atoms were kept frozen to achieve proper interfacialproperties leaving only the first two layers free to move. Thetemperature in the simulations was controlled by a Nose−Hoover thermostat (T = 300 K). After the initial relaxation andequilibration of the prepared system, each MD simulation waspropagated for 100 ns, constituting production runs.

■ EXPERIMENTAL METHODOLOGYQuartz (0001) single crystals for surface potential measure-ments (10 mm × 10 mm × 0.5 mm; polished on one side)were obtained from SurfaceNet GmbH (Germany). Toremove organic contamination prior to the measurements,the crystals were soaked in acetone overnight and subsequentlywashed with ethanol and finally with MilliQ water.All solutions were prepared using deionized and decarbon-

ized water (>18 MΩ cm) and analytical grade chemicals. The

aqueous electrolyte solutions were prepared by dilution ofstandard acid and base solutions (NaOH, KOH, HCl, HBr:Fluka, Fixanal, c = 0.1 mol dm−3) and dissolution of weightedamount of salt (NaCl, KCl, NaBr, KBr: Fluka, puriss p.a.).A quartz (0001) SCrE 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 electrodeThe measuring system was thermostated (t = 25 ± 0.1 °C)

and was kept under an argon atmosphere during titration. Thedata were continuously collected and plotted against time sothat the stability of the reading could be verified in real time.The pH was measured by a combined glass electrode using aMetrohm 826 pH meter. The pH-electrode was calibratedusing standard buffers. The surface potential values7,42,43 werecalculated from the measured SCr electrode potentialsfollowing the standard procedure, namely assuming that thepoint of zero potential for quartz (0001)/aqueous electrolytesolution interface is equal to the isoelectric point.The streaming current measurements,44 on the quartz

(0001) crystal plane, were carried out using a commercialdevice from Anton Paar (Graz) with platinum electrodes, atroom temperature. The conductivity electrode was calibratedusing 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 = 0.001mol dm−3) and the electrokinetic potential of quartz (0001) in0.001 mol dm−3 sodium chloride for acidimetric titration wereobtained as a function of pH, Figure 3.

For the surface potential measurements, contrary to astandard titration where the electrode is continuouslyimmersed in a solution of variable composition, we decidedto employ a batch method, in which multiple solutions withconstant and well-defined compositions are used. This methodhas advantages over standard titration, such as betterreproducibility, precision, and shorter overall experimentalduration, because the time needed to equilibrate the solution ata certain pH is avoided. The main disadvantage of this methodlies in the fact that rinsing of the SCrE with deionized waterbetween two measurements might change the crystal surface.This effect was tested by repeatedly switching solutions ofdifferent pH and different salt compositions. From those tests,

Figure 1. Simulation box of an aqueous NaCl electrolyte solutionconfined between two (0001) quartz surfaces (pH ≈ 6). Red linesdenote the position of oxygens of inner hydroxyl groups, which wetake to define the surface of the quartz (position z = 0).

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 ofcharged siloxide groups are depicted as red spheres.

Figure 3.Measured inner surface potentials Ψ0 (blue filled circle; 0.01mol dm−3 and blue filled diamond; 0.001 mol dm−3) andelectrokinetic potentials ζ (blue open circle; 0.001 mol dm−3) ofquartz (0001)/sodium chloride solution at t = 25 °C.

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it was concluded that in the case of the quartz SCrEequilibrium is achieved within 30 min and that the systemwas completely reversible with regard to switching betweendifferent solutions. The influence of different salts (NaCl, KCl,NaBr, and KBr) on the inner surface potentials the solutions ofdifferent pH (2, 6 and 9) was studied by dissolving therequired amount of each salt in deionized water and adjustingthe pH by addition of acid or base (HCl, HBr, NaOH, orKOH). 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 SCrEpotential was measured after equilibration (approximately 30min). Between two measurements, the quartz electrode wasthoroughly rinsed with deionized water, but without wiping.The reversibility of the measured electrode potential wastested. The quartz (0001) inner surface potentials wereevaluated from the measured electrode potentials, beingshifted according to the reference value, which correspondsto the obtained Ψ0 of Q-NaCl at identical pH and Icconditions of 0.01 mol dm−3 (Figure 4).

To perform electrokinetic measurements, a certain amountof salt solution (NaCl, NaBr, KCl, and KBr) was added to theaqueous solution at different pH whereupon the streamingcurrent of the quartz (0001) plane was measured. The effect ofsalt concentrations on the electrokinetic potential is presentedon Figure 5. As expected, by addition of electrolyte, due toelectrostatic interactions, the magnitude of the electrokineticpotential decreases. The effect of pH on the electrokineticpotential is presented on Figure 4. The presented values ofelectrokinetic potentials are obtained by interpolating theexperimental data from Figure 5 and extracting the values at Ic= 10 mmol dm−3. Overall, at pH 6 and 9, where the quartzsurface is negatively charged, potassium ions more stronglyshield the quartz surface compared to sodium ions. The effectof the anion follows the sequence Br− < Cl− for all examinedpH values. The stronger binding of chlorides compared withbinding of bromides is consistent with the SCrE results at thesame electrolyte concentration 0.01 mol dm−3 (Figure 4).The effects of iodide salts (KI and NaI) on the quartz

surface were also examined by SCrE and streaming currentmeasurements. The quartz electrode potential in contact withaqueous iodide salts changed by more than 100 mV duringequilibration (electrode potential raw data are given in

Supporting Information). The presence of iodine (I2) on thequartz surface was qualitatively confirmed by UV−vis spec-troscopy (see Supporting Information). The adsorbed speciescannot be removed from contaminated quartz surface bysimply rinsing with deionized water, only wiping with ethanol,followed by rinsing with deionized water returns the electrodeto the initial state. The specific adsorption of iodine on quartzis documented in the literature.45−53 Because the presence of I2in the concentrated aqueous iodide solutions could not beavoided, the results obtained by SCrE cannot be comparedwith results obtained for chlorides and bromides (for the samereason MD simulations for iodides were not performed) andwe excluded them from further discussion. The effect of iodideon the quartz surface during streaming current measurementswas significantly less pronounced. This probably stems fromthe fact that streaming current measurements are performed onshorter timescales compared to SCrE measurements.

■ RESULTSStructure of Water Perpendicular to the Surface. We

start by exploring the structure of water molecules in thevicinity of the quartz surface. The interface−normal numberdensity profiles of water from the MD simulations are shown inFigure 6 for Q-NaCl, where the calculated number densityprofiles of water remain virtually identical when otherelectrolytes, i.e., NaBr, KCl, and KBr, are used (see FigureS1). This suggests that the surface charge, stemming fromvarying the number of siloxide groups at the (0001) quartzsurface (see Molecular Dynamics Methodology), and not theelectrolyte type, plays the predominant role with respect to thebehavior of water at quartz/aqueous electrolyte solution

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

Figure 5. Electrokinetic potential of quartz (0001) in different saltsolutions at (a) pH ≈ 6, and (b) pH ≈ 9, obtained by means ofstreaming current measurements at t = 25 °C. The values presented inFigure 4 are obtained by interpolating the experimental data(corresponding curves) and extracting the values at Ic = 10 mmoldm−3.

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interfaces. The point of origin, z = 0, is taken to be at theaverage position of hydroxyl oxygen atoms on the quartzsurface, which we consider as the end of the quartz slab. Wefind that water shows three distinct peaks, corresponding tothree water layers, positioned at approximately 3, 6, and 9.5 Åabove the quartz surface, respectively, with the positions oflayers being independent of pH. While profiles at different pHvalues visibly differ, the overall number of water molecules inits first layer, obtained by integrating the profiles up to the firstminimum, is very similar in all investigated cases. Waterapproaches the quartz surface closer at higher pH, while at pH= 3 the first peak of water density is highest. Overall, we findthat the layering profile of water in the interfacial region tendsto decrease with the increase in pH (adsorption maps of waterat the (0001) quartz surface are given in SupportingInformation, Figure S3).Water Orientation at the Interface. We have also

investigated the quartz/aqueous electrolyte solution interfaceby analyzing the orientational preference of water close to thesurface. Similarly to the interface−normal number densityprofiles of water (Figure 6) the orientation of water moleculestypically does not depend on the type of the electrolyte used,with pH again playing the key role. Here, we presentorientational profiles of water for the Q-NaCl system (Figure7a). Orientational profiles for other electrolytes are presented

in Figure S2 (see Supporting Information). Interfacial waterclosest to the quartz surface, at approximately 1.5 Å from thesurface, shows only weak preferential orientation at pH = 3,with water oxygens having a weak tendency to point towardthe surface. On the other hand, at pH = 6 and pH = 9,interfacial water shows strong orientational ordering withhydrogen atoms pointing toward the negatively charged quartzinterface (Figure 7a). It is important to notice that, whileinterfacial water shows either weaker (pH = 3) or stronger (pH= 9) orientational ordering, the molecules in the center of thefirst water layer, situated at ≈3 Å from the surface (Figure 6),show very weak ordering. We inspected this region in detail, aseven a mixture of well-defined orientations of water toward andaway from the surface can yield nearly zero average dipole.However, by inspecting the distribution of dipole angles ofwater molecules in the aforementioned region we find that itactually closely corresponds to the one found in the bulk, withwater molecules showing no preferred orientation toward oraway 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 thequartz for pH = 6 or 9, implying (as might be expected)slightly stronger influence of negatively charged quartzsurfaces.While we find that the behavior of water does not depend on

the type of dissolved electrolyte, with virtually the same profilesfor all quartz/aqueous electrolyte systems at pH = 3 and pH =6 (Figure S2), respectively, we observe noticeable differencesin 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 topotassium salts (Figure 7b). This finding is the first indicationof 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 numberdensity profiles of cations and anions, for all investigatedquartz/aqueous electrolyte systems.

IonNumber Density Profiles. We have furtherinvestigated the quartz/aqueous electrolyte solution propertiesby analyzing ion distributions at the interface. Figure 8presents interface−normal number density profiles of bothcations and anions for all investigated pH values. In general, weobserve relatively small differences between potassium andsodium distributions already at pH = 3. As the surface becomesmore negatively charged, i.e., at pH = 6 and especially at pH =

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 simulationsof Q-NaCl system. Black dashed lines denote positions of three peaks.

Figure 7. Average cosine of the angle between water dipole and surface normal (denoted by θ) as a function of distance from quartz (0001) surfacefor (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 andsurface normal are depicted as red and blue arrow, respectively. Again, z = 0 is taken to be the average position of hydroxyl oxygen atoms on thequartz surface.

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9, the differences in the behavior of the two cations becomemore evident. More precisely, we find that at the neutralsurface (pH = 3), rather small differences are found betweencation and anion distributions, respectively, regardless of thesalt. However, even at the neutral surface, sodium ions tend tobehave differently from potassium ions. In this respect,potassium salts exhibit more pronounced peaks in theirinterface−normal number density distributions in the firstwater layer (first peak in the upper leftmost panel, 3 Å from thesurface). At the same time, we find that sodium ions are shiftedby approximately 0.5−1 Å toward the quartz surface comparedto potassium ions (Figure 8, upper left panel, compare the firstthree peaks for sodium and potassium ion distributions).Interestingly, both sodium and potassium ions tend to occupyminima in the number density distribution of water, namely ataround 4.5 and 7.5 Å from the surface (compare the positionsof the two most dominant peaks in potassium and sodium ionprofiles, Figure 8, with Figure 6). This tendency is morepronounced in the case of sodium ions. Such a behavior ofcations, i.e., the tendency to avoid entering the three interfacialwater layers (Figure 6), can be tentatively explained by the factthat strong organization of water due to hydrogen bondingmakes these positions energetically more favorable for waterbut less favorable for cations. The same phenomenon isobserved for anions (Figure 8, lower leftmost panel) butsignificantly less pronounced.A first noticeable difference between the different cations

occurs at pH = 6 (Figure 8, upper middle panel), wheresodium ions, both in Q-NaCl and Q-NaBr, strongly occupythe region at approximately 2 Å from the surface, denoting thesecond inner-sphere complex between Na+ and exposed SiO−

groups, i.e., complexes in which the majority of cationsinteracts with the SiO− groups directly. The smaller of the twoinvestigated cations, i.e., sodium, tends to approach the quartzsurface more closely, with the first peak (first inner-spherecomplex 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 thatpotassium ions also strongly populate the region between 1.5and 2.5 Å from the surface. However, different from sodiumions, they exhibit two weakly separated peaks (positioned at1.6 and 2.4 Å). It is worth noting that the third peak, foundboth in potassium and sodium ion distributions, lies betweenthe first and second interfacial water layer, as was observed intheir profiles at pH = 3 (Figure 8, upper leftmost panel, secondpeak, lying at ≈4.5 Å). Unlike cations, anions do not showpronounced differences at pH = 6, regardless of the dissolvedsalt.Under basic conditions (pH = 9), we observe the most

pronounced differences in the behavior of the two alkali metalcations. Sodium cations exhibit a significantly stronger firstpeak in their distribution compared to pH = 6. Specifically, weobserve predominantly the first inner-sphere SiO− Na+

complex (at approximately 1 Å from the surface). The secondstrong peak belongs to the second inner-sphere SiO− Na+

complex, with sodium ions residing in the region ≈2.3 Å abovethe quartz surface. Qualitatively, a completely different profileis obtained for potassium ions, with a single strong peakbetween the two sodium peaks at approximately 1.8 Å. Forboth pH = 6 and pH = 9, the behavior of the cations is virtuallyindependent of the anion. This should be expected because theinterfaces 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 andbromides in sodium salts show only a rather small peak atapproximately 3.7 Å above the quartz surface, the anionprofiles in potassium salts are significantly better defined,particularly at pH = 9. Here, we observe two strong peaks ofanions at around 4.3 and 6.8 Å from the quartz surfaceshowing their preference toward regions in which theinterface−normal number density profiles of water reachminima (see Figure 6).Overall, we find that cations play the predominant role with

respect to the differences observed in the behavior of the

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.

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investigated sodium and potassium salts, with the distinctionsbecoming more pronounced under 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 differencesin the behavior of chloride and bromide salts, respectively(Figure 8, bottom leftmost panel). At higher pH values, theirbehavior is virtually identical, irrespective of the dissolved salt(pH = 6). At pH = 9, chloride and bromide anions, originatingfrom the salt of the same alkali metal, possess very similarprofiles, with the profile of the same anion being bothqualitatively and quantitatively completely different when thatanion species originates from a different cation−anioncombination (Figure 8, bottom rightmost panel).Adsorption maps showing the distribution of cations in their

first and second interfacial layers are discussed in SupportingInformation (Figures S4 and S5). We now turn our attentionto two important system properties enabling direct comparisonof simulations with experiments, namely electrostatic potentialand ζ-potential.Electrostatic Potentials. The profiles of the electrostatic

potentials, shown in Figure 9, were calculated by doubleintegration of the charge distribution, taking into accountcontributions from the entire system. The electrostaticpotential is generally dominated by damped oscillations aroundzero, which originate from the water contributions due topreferred orientation of water dipoles and inhomogeneouswater density. As expected, at the neutral quartz surface, theprofiles essentially do not depend on the combination ofcation−anion pairs. In general, because of their excess at theinterface compensating the negative surface charge, the effectof cations, rather than anions, increases with negative surfacecharge, but is limited mostly to the region of inner-sphere andfirst outer-sphere adsorption peaks, i.e., around 3−7 Å, mostpronounced at around 5 Å above the quartz surface.While electrostatic potentials show very similar profiles for

pH = 3 and pH = 6, under basic conditions (pH = 9), adifference is observed when Q-NaCl and Q-NaBr arecompared to Q-KCl and Q-KBr systems. This differencestems from two effects, namely different sodium and potassiumion distributions at the negatively charged quartz surface at pH= 9 (Figure 8, upper rightmost panel) and the concomitantorientational behavior of water (Figure 7).From the electrostatic potentials, it should be in principle

possible to infer ζ-potential, traditionally described as the valueof the electrostatic potential at the slipping plane. However, itwas recently found12 that it is not possible to locate a clearly

defined slipping plane that can be linked to the macroscopi-cally observable ζ-potential, thereby preventing direct deter-mination of this property from the electrostatic potential. Wethus decided to calculate a ζ-potential from two axial profilesaccessible from the performed EMD simulations, namely usingcharge density and the viscosity profile in the directionperpendicular to the solid surface, following the proceduredescribed in the Supporting Information of ref 12 andelaborated in more details in the following section.

ζ-Potentials. In general, the ζ-potential can be evaluatedusing the Helmholtz−Smoluchowski equation, ζ = − μη

ε ε0 r,

where η = 8.9 × 10−4 Pa s and εr = 79 are the experimentalbulk water values of dynamic viscosity and relative permittivity,while μ represents the average bulk mobilities of water inelectro-osmosis (or using an analogous equation with apositive sign when μ represents the mobility of a solid particlein electrophoresis). Thus, the only unknown parameter thathas to be evaluated is the average bulk mobility of water, whichcan be obtained by calculating distance-dependent electro-phoretic mobilities using

∫μη

=′

′′z

EP z

zz( )

1 ( )( )

dx

zxz

0 (1)

where η(z′) denotes distance-dependent viscosity, whilePxz(z′) represents the off-diagonal component of the pressuretensor (shear stress) and is calculated as

∫ ρ= ′ ′P z E z z( ) ( ) dxz xz

L/2

q (2)

where ρq(z′) is the volume charge density of ions at a distancez above the surface (taking into account all cation and anioncontributions). Equation 2 expresses that the stress at height zis caused by the electric force along the surface acting on all thecharge further from the surfaceand which the immobilesurface must keep at a steady mobility. Equation 1 thenintegrates the slip, proportional to the local stress and inverselyproportional to local viscosity η(z′) from the surface up to bulkliquid region far from the surface, where constant mobility ofthe fluid relative to the surface is observed.12 While thederivation of these equations follows the non-equilibriumapproach with applied external electric field Ex, it is evidentthat in the linear response regime the mobility is independentof the magnitude of the field and eqs 1 and 2 combined can beused even in the limit of zero field, i.e., our equilibriumsimulations.12 While ρq(z′) can be easily obtained from the

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.

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density profiles of ions (Figure 8), the determination of thedistance-dependent viscosity η(z′) is a challenging task.54

Here, we approximated, for each system studied, the distance-dependent viscosity via distance-dependent self-diffusivity ofwater (diffusion of water in xy plane is used for this purpose,Dxy) using the Einstein−Stokes relation

πη=D

k Ta6

B

(3)

where D denotes the diffusion coefficient, kB the Boltzmannconstant, T the absolute temperature, and η the dynamicviscosity, while a corresponds to the radius of a sphericalparticle. To properly convert diffusivity to viscosity, it isnecessary to know the parameter a. We fitted a independentlyfor each investigated system, such that the diffusion in the bulk,i.e., more than 20 Å above the surface, yields the expectedvalue of the viscosity in bulk water (ηbulk = 8.9 × 10−4 Pa s).The obtained viscosity profiles are shown in SupportingInformation (Figure S6), while the electrophoretic mobilities,calculated via eq 1, are shown in Figure S7 (see SupportingInformation). The distance-dependent self-diffusion coeffi-cients of water, used to obtain viscosity profiles in the z-direction, are calculated using the method of Predota et al.54

The resulting ζ-potentials are plotted versus pH in Figure10. 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 observethat all systems exhibit negative ζ-potential values, around −25mV. Q-NaBr shows the most negative potential, whereas Q-KCl possesses the least negative value. We note that bromidesgive rise to more negative ζ-potential values compared tochlorides, which agrees with both SCrE and streaming currentmeasurements (see Figures 4 and 5). The difference inbehavior among the investigated salts is more pronouncedunder basic pH in MD simulations. Thus, at pH = 9, we findthat sodium ions cause more negative ζ-potential valuescompared to their potassium counterparts, which again is inagreement with both the surface and streaming currentmeasurements (see Figure 4). While the experimental

measurements show that the effect of chlorides is slightlymore pronounced in this regime, i.e., their salts cause lessnegative ζ-potential values, this finding is not completelymatched in the performed simulations (compare Figures 10and 4, pH = 9). However, careful inspection of experimentalresults (see Figure 5b) points to the relatively strong ζ-potential dependence on anion concentration, with theinversion of this phenomenon in the lower concentrationrange, while the simulations are carried out at significantlylarger concentrations around 0.4 mol dm−3.Interestingly, we observe that, in the case of all investigated

quartz/aqueous electrolyte solutions, the theoretically obtainedζ-potential becomes less negative as one goes from neutraltoward basic pH values, which is rather unexpected. However,the same trend is reflected in the experiments, especially in thestreaming current measurements (Figure 4), with the bromidesystems 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 ζ-potentials observed in theperformed MD simulations. By integrating the number densityprofiles of cations (Figure 8), we found that, at pH = 6, aslightly larger number of cations resides near the negativelyquartz 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-KClscreen the negative quartz surface more successfully comparedto their bromide counterparts, in turn giving rise to slightly lessnegative ζ-potentials for chloride compared to bromide salts.On the other hand, at pH = 9, virtually no difference in thenumber of interfacial sodium ions stemming from Q-NaCl andQ-NaBr and, likewise, between potassium ions arising from Q-KCl and Q-KBr, is found. However, the number densityprofiles of anions at pH = 9 exhibit higher peaks of chloridescompared to bromides when salts originating from the samealkali metal are compared (Q-NaCl vs Q-NaBr, and Q-KCl vsQ-KBr, Figure 8, bottom right panel). This indicates thatchlorides actually screen the formed cation layer(s) morestrongly than bromides, making the quartz surface overall morenegative. This finding can be directly linked to the calculated ζ-potentials, as we observe that chloride salts indeed give rise tomore negative ζ-potentials compared to bromide salts, asexpected from the above analysis.A startling phenomenon at pH = 9 is the extra peak at

around 4 Å in both bromide and chloride anion distributionsin the presence of potassium ions. Integrating the cationdistributions of potassium versus sodium up to this distance,we observe that more potassium compared to sodium ions areadsorbed. In fact, potassium cations can slightly over-compensate the negative surface charge, giving rise to theaforementioned extra peak of anions, which in turn counter-balance the charge of cations. This implies that potassium, inthis pH regime, is more strongly adsorbed, giving rise to lessnegative ζ-potentials. In fact, the larger diameter of potassiumcompared to sodium may contribute to potassium being morestrongly adsorbed, as it can more readily (partially) dehydrate;note that for the same conditions, a significant amount ofsodium ions is still in the second inner-sphere peak (furtheraway from the surface than the peak observed in the case ofpotassium, Figure 8, upper right panel), corresponding to less-dehydrated sodium ions.

Figure 10. Calculated ζ-potentials of the simulated quartz (0001)systems at three investigated pH values for 0.4 mol dm−3 aqueoussolution of: (blue circle) NaCl, (green diamond) NaBr, (red square)KCl, and (blue triangle) KBr.

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■ DISCUSSION

The quartz (0001) surface represents the most stable plane ofthe crystal and was chosen for a comprehensive study involvingboth experimental and theoretical techniques. Generally, inaqueous electrolyte solutions, surface silica atoms react withwater and form the amphoteric SiOH silanol surface siteswhich can in principle be protonated or deprotonated formingpositively charged SiOH2

+ or negatively charged siloxideSiO− surface sites, respectively. The total surface concen-tration of silanol surface sites on quartz (0001) is 9.4 sites/nm2. Surface reactions and the distribution of ions between theinterfacial region and the bulk of the solution lead to theformation of a charged interfacial layer. The inner surfacepotential is an important parameter describing the state withinthe EIL as well as the interfacial equilibrium. Surface potentialis influenced by the surface concentrations of the chargedsurface groups, distribution of ions, and orientation of watermolecules near the solid surfaces. It gradually changes from thesolid surface toward the bulk of the solution. In this profile, theelectrokinetic ζ-potential is located at an unknown position.In this study, we explore (0001) quartz/aqueous electrolyte

systems under different pH conditions and salt compositions,using both experimental and MD techniques. In the latter, wereflect the experimental pH conditions indirectly via surfacecharge resulting from surface protonation/deprotonation.The inner surface and electrokinetic potentials of the (0001)

quartz/aqueous electrolyte solutions were obtained by meansof SCrE and via streaming current measurements, respectively.The measured ζ-potentials exhibit more negative values, withan isoelectric point (at which ζ = 0) below pH = 3 (Figure 3).Because of the small measured surface potential values, it isdifficult to observe any effects of different electrolytes onsurface potentials within the electroneutrality region (pH ≈ 3).Therefore, we conducted the SCrE experiment at pH ≈ 2,where quartz (0001) is positively charged. As expected, theeffect of the anion is pronounced, and we found strongerinteraction of bromide ions, compared to chloride ions.Streaming current measurements become very difficult at thelow pH values.With regard to MD simulations, for a neutral surface

(pH ≤ 3), the fraction of negatively charged siloxide sites onthe quartz surface is set to zero.36 The first interfacial layer ofwater at the quartz/aqueous electrolyte interface exhibits thehighest density under these conditions (Figure 6, full greenline), but weak preferential orientation of water molecules,with water oxygen atom having a weak tendency to pointtoward the quartz surface (Figure 7, full green line). Theinterfacial water layer (z < 4.3 Å from quartz surface) isstructured symmetrically, with the positions of water oxygenatoms corresponding to the positions of hydroxyl groups on(0001) quartz (Figure S3). Calculated ζ-potential values for allinvestigated quartz/aqueous electrolyte solutions tend to beclose to zero, with all of the calculated potentials being slightlypositive (Figure 10). At this pH, we observe relatively smalldifferences between cation and anion distributions, respectively(Figure 8). However, different from systems at higher pHvalues, we do find small differences in the behavior of chlorideand bromide salts, and the same tendency is observed insurface potential measurements.At pH = 6, the SiO2 surface in contact with aqueous

electrolyte solutions becomes negatively charged. Theexperimentally measured inner surface potential confirms the

existence of the siloxide groups (SiO−) and negativeelectrical charge at the surface (Figure 3). By increasing theionic strength, the absolute values of the quartz surfacepotential decrease. According to the surface complexationmodel,7 this region indicates very low values of thethermodynamic equilibrium constant for the protonation ofsurface siloxide sites. According to the electrical interfacialmodels,55 the magnitude of the electrostatic potentialdecreases from the solid phase to the bulk of the electrolytesolution. At pH > 3, we measure a higher absolute value of theelectrokinetic potential compared to the inner surfacepotential, which indicates that the counterions and waterdipoles contribute more strongly to the electrokineticpotential. It is worth noting that, in contrast to themacroscopic models that involve the interfacial potential as amonotonically increasing/decreasing function, MD simulationspoint to an intrinsically oscillatory behavior dominated bydamped oscillations that stem from water contributions(Figure 9).54

The experiments performed at pH = 6 show that thedifferences in inner surface potentials for potassium andsodium chloride and bromide are minor. Measured values arerather small; however, slightly stronger association of bromidescompared to chlorides is observed (Figure 4). On the otherhand, the effect of different salts on electrokinetic potentials ismore noticeable (Figures 4 and 5a). In this respect, potassiumions are more strongly adsorbed than sodium ions, with anionsfollowing 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, thusmaking it overall negative (surface charge density ofapproximately −0.13 C/m2). As in the case of the neutralquartz surface, at the negatively charged quartz interfacial waterexhibits three distinct peaks, corresponding to three waterlayers, positioned at approximately 3, 6, and 9.5 Å above thequartz surface, respectively, with the position of layersindependent of pH (Figure 6). Interfacial water shows strongorientational ordering at approximately 1.5 Å above, withhydrogen atoms pointing toward the negatively charged quartzinterface (Figure 7), which correlates well with findings ofQuezada et al.33 Phase measurements of sum-frequencyvibrational spectroscopy56 confirmed that water molecules athigh pH are hydrogen bonded to the quartz surface withoxygens.At this pH, cations start to play the predominant role with

respect to the differences observed in the behavior of theinvestigated sodium and potassium salts (Figure 8). Thus, wefind that the smaller sodium ions tend to approach the quartzsurface more closely, with their first peak (inner-spherecomplex) at ≈1 Å from the surface. Larger potassium ionsstrongly populate the region between 1.5 and 2.5 Å from thesurface, characterized by two weakly separated peaks at 1.6 and2.4 Å. These predominantly belong to the first and secondinnersphere complexes, respectively, formed by K+ andSiO−. While cations show pronounced differences, anionsbehave remarkably similar at pH = 6, regardless of thedissolved salt. Calculated ζ-potential values for all investigatedquartz/aqueous electrolyte solutions at pH = 6 are negative(Figure 10), around −25 mV. However, we do observe thatbromides have a tendency to generate more negative ζ-potentials compared to chlorides, which is reflected both inSCrE and in streaming current data.

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At pH = 9, the quartz surface is even more negativelycharged. Both inner surface and electrokinetic potentials arenegative, and the differences in the behavior of the investigatedsalts are by far most pronounced. To perform thecorresponding MD simulations, the fraction of siloxide groupswas set to 18%, corresponding to an overall surface charge of−0.26 C/m2 in 0.4 M salt concentration. We find that theinterfacial properties of the quartz/aqueous electrolytesolutions at pH = 9 are very similar to pH = 6 with regardto water adsorption and its distribution (Figure S3). However,water exhibits a different orientational profile at pH = 9depending on the cation, with orientational ordering becomingsignificantly more pronounced when potassium salts are usedinstead of sodium salts (Figure 7b, compare second and thirdpeak in the profiles of potassium and sodium salts). Thissuggests that, at pH = 9, the influence of electrolytes, primarilycations, plays a significantly larger role compared to systems atthe lower investigated pH values. This difference betweensodium and potassium salts then causes distinct trends in bothelectrostatic 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 isin agreement with experimental surface potential and stream-ing current measurements (compare Figures 5 and 10).Moreover, we observe that the electrokinetic potentialbecomes less negative at higher pH values, which is ratherunexpected. This behavior has already been observed inprevious simulations for strongly adsorbing cations.12 While alarger amount of cations adsorbs onto more negative quartzsurfaces (pH = 9 vs pH = 6), the local viscosity in this region issignificantly larger than in the bulk, overall greatly reducing theeffect 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 notcontributing to the calculated mobilities, thereby not affectingthe obtained ζ-potentials. The same trend is reflected in theperformed surface potential measurements (Figure 4), whereall examined salts show less negative potential under basicconditions. Not surprisingly, the role of anions at negativelycharged surfaces is minor relative to that of cations (see Figure8, notice the difference in scales), though strong association ofanions with the surface-bound cations re-amplifies theirinfluence on the electrokinetic behavior in the investigatedsystems.Additionally, we may compare our findings with the absolute

surface potential obtained by X-ray photoelectron spectrosco-py with a liquid microjet for silica nanoparticles dispersed inbasic aqueous alkali metal chloride solution at 4 °C. Brown etal.57 found that in the basic region the magnitude of the surfacepotential increases (becomes more negative) with increasinghydrated cation size. They explain this phenomenon via thelarger hydrated cations that are more distant from the solidsurface. This causes a larger potential drop across theinterfacial layer, i.e., |Ψ0(Na

+)| > |Ψ0(K+)|, which was

experimentally confirmred in this paper for quartz (0001).

■ CONCLUSIONSIn this combined experimental and theoretical study, weanalyze the influence of the potential determining ions (H+ andOH−), counterions, co-ions, and water molecules on the innersurface potential and the electrokinetic potential of thehydroxylated quartz (0001) surface. We present an originalapproach, in which EMD simulations of aqueous electrolyte

solution confined between parallel slabs of the hydroxylatedquartz (0001) surfaces at three different pH values are used tocalculate ζ-potentials, which are then compared withexperimental results. This allows us to comment in detail onboth water and counterion behavior and their influence on thequartz/aqueous electrolyte solution interface. In this respect,we find that water in the interfacial region forms three distinctlayers, irrespective of pH. Symmetrical adsorption is mostpronounced 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 thatstrong organization of water and hydrogen bonding occurs inthe first water layer and influences ion penetration. Thebehavior of ions, as inferred from the calculated ion numberdensity profiles, depends on the water distribution. Theobserved differences between anions and cations and theirinfluence on the interfacial properties of quartz (0001) dependon pH and the fraction of the negatively charged siloxidesurface sites. While we find that cations play a predominantrole on the interfacial properties, at pH = 6, both calculatedand experimentally obtained potentials suggest that the anioninfluence is becoming pronounced even in this regime.Chlorides show a tendency of shifting the electrokineticpotential toward zero. The interplay between ion-quartz andwater−quartz interactions and their overall influences oninterfacial properties are most apparent for basic conditions,i.e., at pH ≈ 9, where the choice of the cation greatly influencesthe orientations of water dipoles. This drastically changes bothelectrostatic (Figure 9) and electrokinetic (Figure 10)potentials, as revealed by the performed simulations. Largercations screen more strongly the existing surface charge in thisregime, thereby decreasing the absolute ζ-potential value. Onthe other hand, the simulations suggest that the smaller anionstend to conceal the first cation layer more strongly, thus overallincreasing the absolute value of the calculated ζ-potential.Overall, we find that larger cations more strongly screen thenegatively charged quartz surface, whereas both MDsimulations and experiments show that anions accumulateweakly. Their influence on electrokinetic phenomena variesstrongly depending on their concentration, which in turn givesrise to the rich and complex multilayer nature of the quartz/aqueous solution interface.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.jpcc.8b04035.

Computational detail and calculated data and figuresincluding interface−normal number density and orienta-tional profiles of water, adsorption maps of water andions, calculated viscosity profiles and electrophoreticmobilities of water, and experimental results of theinfluence of all analyzed ions and additionally iodides onquartz surface potential (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: tajana@chem.pmf.hr.ORCIDDanijel Namjesnik: 0000-0002-8963-0169Milan Predota: 0000-0003-3902-0992Tajana Preocanin: 0000-0002-6670-6503

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NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work has been supported by Croatian Science Foundationunder the project IP-2014-09-6972 and Czech ScienceFoundation project 17-10734S (M.P.).

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The Journal of Physical Chemistry C Article

DOI: 10.1021/acs.jpcc.8b04035J. Phys. Chem. C 2018, 122, 24025−24036

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