Adsorption of Singly Charged Ions at the Hydroxylated (0001) α

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Adsorption of Singly Charged Ions at the Hydroxylated (0001) α‑Quartz/Water Interface Morgane Pfeiffer-Laplaud‡ and Marie-Pierre Gaigeot*,‡,† ‡

Institut Universitaire de France, 103 Boulevard St Michel, 75005 Paris, France LAMBE CNRS UMR8587, Université d’Evry val d’Essonne, Boulevard F. Mitterrand, Bât Maupertuis, 91025 Evry, France

†

ABSTRACT: Individual alkali (Na+, K+) and halide (Cl−, I−) ion effects have been characterized at the fully hydroxylated (0001) αquartz water interface by means of ab initio molecular dynamics simulations in the framework of the electronic DFT representation (DFT-MD). We particularly focus our analyses on the ion adsorption and solvation structures (made by water and by surface silanols), as well as on perturbations undergone by the silanol surface sites when comparing the charged interfaces (present work) to the neat interface (our previous works, J. Chem. Theory. Comput. 2012, 8, 1037; J. Phys.: Condens. Matter 2012, 24, 124106). Both sodium and potassium cations are found adsorbed in an inner-sphere configuration, while chloride and iodide are found in between inner- and outer-sphere. Cation adsorption at the interface is found to induce more perturbation on interfacial properties than anions do. In particular, we show in details how and why the orientation of out-of-plane and in-plane surface silanols found at the neat interface are modified by inner-sphere cations at the charged interfaces, with also consequences on the silanol−silanol intrasurface hydrogen bond network. All this detailed analysis provides a clear picture of a reduction of acidity of the surface silanols at the quartz/water interface in the presence of the alkali/halide salts.

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INTRODUCTION The knowledge of the distribution of charges at solid/liquid interfaces, known as the electric double layer (EDL), is of pivotal importance for interfacial chemical reactivity. This is typically relevant in electrochemistry for reactions taking place at electrode/electrolyte interfaces, in heterogeneous catalysis, for instance at oxide/water interfaces in the petrochemical and refining industry with issues in rational design for improving chemical reactions, or in geochemistry where chemical reactions of ions and/or (in)organic molecules in the underwater as well as their adsorption properties can contribute to the transport of pollutants, to name a few areas of fundamental and industrial importance. Specific knowledge of the EDL at silica/liquid water interfaces is of interest in several fundamental and industrial communities, e.g., oil industry for applications in enhanced oil recovery,1−3 nuclear industry where the issue of silica dissolution around nuclear waste storage is a hot topic,4,5 environment with issues related to geological capture and sequestration of carbon dioxide,6 biomedicine with drug delivery purposes where mesoporous silica nanoparticles7,8 can be used to selectively encapsulate molecular drugs and control their release in certain physiological conditions, or biomimetics9 where silica interfaces are relevant for biotechnological applications, to name a few. How the presence of aqueous electrolytic species influence the structural properties of silica/water interfaces, and conversely, how the interfacial chemical composition and structure influence the adsorption of the ions at the interface (e.g., inner- versus outer-sphere adsorption), are the general © XXXX American Chemical Society

questions to be addressed. Our goal is to unravel the complex and intricate effects of electrolyte adsorption at silica/water interfaces. It is certainly obvious, though necessary, to state that the intrinsic structural and chemical properties (e.g., surface chemical nature of sites, acido-basic character of these sites, etc.) of the surface at the interface with the liquid will influence the adsorption capabilities of the interface, properties that in turn will possibly be modified by the presence of the electrolytes. It is also obvious to state as is now well-recognized that the structure of the liquid at the interface, i.e. the hydrogen bond network between surface and liquid, and in the liquid, within the few layers on top of the surface, will influence the electrolyte interfacial organization, and that this organization will possibly in turn influence the interfacial final structural and chemical properties. Hence, the SHG titrations from the Gibbs−Davis group10−13 demonstrated that alkali/halide electrolytes affect the intrinsic surface acidities of fused silica, both cations and anions being responsible. A complete rationalization of these results has not been achieved yet, however desirable for the understanding of reactivity at this interface. Alkali ions, like sodium, are known to be involved in surface degradation of silicate glasses,14 facilitating not only water penetration into the glass network but also the kinetics of silica dissolution. Alkali chlorides have also been shown to enhance rates of dissolution (see ref 15 and references therein): Received: November 8, 2015 Revised: January 8, 2016

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date on solid/liquid interfaces has been generated by SFG and SHG experiments,30,31,34 as these second-order non linear optical techniques are surface specific (i.e., not probing a too deep surface layer), forbidden in bulk liquids while being allowed at surfaces and interfaces. We refer the reader to reviews13,30,31,36−39 to assess how much developments have been achieved in the past decade on SFG and SHG experimental set-ups, in order to probe solid/liquid and liquid/air interfaces at the atomistic level and, in particular, to go beyond the conventional “static” SFG experiments. Let us mention the advent of phase-resolved SFG,34,40,41 two-dimensional SFG (2D-SFG),42 pump−probe time-dependent SFG, 42−45 which provide not only (static) structural information at the interface but also intra- and intermolecular couplings between molecules at the interface, as well as timedependent dynamical properties of the interfacial liquid. SFG and SHG have been widely employed for the characterization of neat oxide-water interfaces, see for instance refs 24, 31, 39, 40, and 46−48. Although the EDL at the water/ air interface has been the topic of quite a large number of investigations,36,37,49 electrolytes at solid/liquid water interfaces have received to date less attention. Surface charge effects investigations through pH modulation at silica/water or alumina/water interfaces have been conducted,30,31 with characterization of interfacial water (re)orientation due to the change in surface electric field (surface protonation/deprotonation).46,50 These experiments mainly provide structural information on the surface and defects arising from chemical reactions, or information on the interplay between the liquid and molecular ordered monolayers at the surface: a direct knowledge on the aqueous electrolyte organization at the surface (EDL) has not yet been obtained from these experiments alone. Such microscopic information can be obtained from molecular dynamics simulations. Classical molecular dynamics simulations have contributed to some understanding of EDL’s structures, dynamics, and charge storage mechanisms. See, for instance, refs 51 and 52 and references therein, in the context of electrochemical interfaces, or ref 53 for classical MD of charged silica/water interfaces and refs 54 and 55 at rutile/water interfaces. However ideal classical MD might seem, ab initio molecular dynamics (AIMD) simulations of small scale systems are necessary,51,52,56 as they do not rely on parametrized force fields (and all related troubles of parametrization and transferability), but more importantly as they naturally include charge transfers and chemical reactivities that take place at interfaces. Reactive force fields have been developed to overcome such issues, but to the best of our knowledge, they have only been applied to silica/ water interfaces without electrolytes.57−60 AIMD therefore remains the method of choice for investigating complex inhomogeneous electrolytic interfaces; see, for instance, ref 61. We focus our present work on the fully hydroxylated (0001) α-quartz/water interface with alkali/halide ions at the interface, characterized by ab initio molecular dynamics simulations in the framework of the DFT electronic representation. We chose to investigate how this quartz-water interface, that we fully characterized in previous works62,63 by AIMD (interfacial structure, spectroscopy, and pKa values of surface sites), is modified by the presence of single ions, either alkali cations (Na+, K+) or halide anions (Cl−, I−). α-quartz consists of the most stable polymorph of quartz at room temperature,64 and first-principles molecular dynamics evidenced that the (0001) surface is the most stable, particularly when it is fully

potentiometric measurements evidenced that their presence increased the surface charge at a given pH, at relevant temperatures for geochemical groundwater. Besides, a correlation was obtained between the hydration radius of the cations and surface charge densities enhancements: weakly hydrated cations exhibit better charge screening properties, the surface charge thus increases according to Li+ < Na+ < K+. DFT-based molecular dynamics simulations from Kubicki and co-workers16,17 at the (101) α-quartz/water interface (Q3 type silanols) investigated ion-induced mechanisms for the protonation of surface bridging oxygens, which consists of a hypothetical rate-limiting state of quartz dissolution. Sodium and chloride were found to increase the number of intrasurface hydrogen bonds, while reorienting some of the interfacial water molecules so that they can bind to the surface. Sodium also appears to increase the basicity of bridging oxygens (Si−Obr− Si), its adsorption leading to a significant elongation of the covalent Si−Obr bonds. The formation of stable and direct hydrogen bonds between surface silanols and chloride in ref 18 was also found responsible for silanol rotations toward the bridging sites. However, no critical change in the water-surface hydrogen bonding patterns induced by the presence of Cl− could be observed, e.g., bond-length, number of H-bonds, coordination numbers). The role of the interfacial solvent in these results has been highlighted. Dove et al.,19 for instance, showed that the more a cation had structure-changing effects on the liquid (structure-breakers for alkali, structure-makers for alkali-earth cations), the more negatively charged the surface of silica colloids. In a different context related to organic charged molecules of interest to prebiotic chemistry, where hydroxylated silica surfaces are known to enable the adsorption of model biomolecules,20 the complex interplay between surface properties and interfacial liquid organization has also been highlighted. In the work by Walsh and co-workers21 presenting firstprinciples molecular dynamics simulations, NH 4 + and CH3COO− peptide terminals are found to bind the fully hydroxylated (100) α-quartz/water interface covered by geminal silanols (Q2 type), forming weak hydrogen bonds for the former (H → OHSi) and strong ones (O → HOSi) for the latter. This is in agreement with the DFTB simulations of zwitterionic glycine at another geminal-terminated silica/water interface.22 However, other ab initio simulations suggest outersphere adsorption for zwitterionic alanine at the interface with surface Q3 silanols (site composed of one hydroxyl and three siloxane bridges).23 From second harmonic generation (SHG) spectroscopic experiments the adsorption behavior of some pollutants at the fused quartz/water interface has been investigated.24 Surface activity of nitrate was highlighted, though it only physisorbs onto quartz, and cations like strontium and cadmium were found to adsorb in outer-shell configurations.25 As reviewed in refs 26−34, the majority of experimental works shedding light on the EDL structure are based on thermodynamics, spectroscopies including linear and non linear vibrational spectroscopies such as infrared reflection absorption spectroscopy (IRRAS), subtractively normalized Fourier transform infrared spectroscopy (SNIFTIRS), second harmonic generation (SHG), sum frequency generation (SFG), and Xrays scattering. Atomic force microscopy (AFM) experiments have also more recently contributed, with high-resolution AFM being capable of providing more direct knowledge at the atomic level.35 Admittedly, most of our microscopic understanding to B

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The Journal of Physical Chemistry C hydroxylated.65,66 Several theoretical62,63,67−70 and experimental40,50 works have focused on the neat fully hydroxylated (0001) α-quartz/water interface. From sum frequency generation (SFG)50 and phase-sensitive SFG40 experiments, Shen and co-workers showed that “ice-like” and “liquid-like” water molecules at the origin of the two O−H stretch bands in the spectra belong to the very first layer on top of the solid, and that, in acidic conditions when the surface is neutral (i.e., mostly hydroxylated) the ”ice-like” water molecules are hydrogen-bond acceptors and their dipoles oriented toward bulk water. Besides, two distinct surface pKAs have been found at the silica/water interface from second harmonic generation (SHG) titration curves.71 In recent first-principles molecular dynamics simulations, going farther than the studies of monomers or a few monolayers on top of the surface,69,70 we calculated the values of pKA surface sites at the (0001) αquartz/water interface.62 We found two distinct values that we have shown to be correlated with the actual hydrogen bonding patterns occurring between surface silanols and first layer interfacial water molecules.62,63 Hence, the most acidic surface silanols (pKA = 4.571 and 5.662 for SHG and AIMD simulations respectively) have their hydrogens protruding toward the solvent, denoted out-of-plane silanols, and are H-bonded to the oxygen atoms of water molecules acting as hydrogen bond acceptors to the surface with their dipoles oriented toward the bulk. The second silanol species lies in the plane of the surface, thus denoted in-plane silanols, and has a basic pKA of 8.5 obtained by both SHG experiments71 and AIMD simulations.62 These in-plane surface silanols receive one hydrogen-bond from one water molecule. See Figure 1 in the Computational

arrangement,48,72 but some works have detected at least cationspecific trends in the spectra,73 i.e., potassium decreasing more the SFG intensity than sodium. Here, we chose to consider single ions located at the interface in order to characterize individual effects: adsorption sites of cations versus anions, possible charge effect, and/or size effect, and/or polarizability effect of the ions, solvation of the ions (geometry, orientation of solvating water molecules with respect to bulk solvation), changes undergone by surface silanols, and how these silanols might be involved in solvating the ions. Our work will help refining the interpretation of SFG spectra. This paper is the first in a series; ion pairing effects will be discussed in a subsequent paper. This paper is organized as follows: structural properties on how the cations/anions are adsorbed at the interface with the aqueous (0001) α-quartz surface are given first, the structural organization of solvation of these ions at the interface is reported next, how the surface silanols participate to the adsorption of the interfacial ions and the consequences on their intrinsic structural properties (as observed previously at the neat interface62,63) is reported afterward. Our presentation of results is closed with a specific description of acido-basic properties of the surface silanols once the adsorption of ions has occurred. This is based on simple covalent-bond and intermolecular H-bond distances consideration. We then conclude and provide perspectives to that work.

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COMPUTATIONAL METHOD First-principles molecular dynamics simulations (FPMD or AIMD for ab initio MD) are performed with the CP2K/ Quickstep package.74,75 The fully hydroxylated (0001) αquartz/liquid water interface from our previous works is used,62,63 corresponding to the point of zero charge (PZC). This neat interface has been characterized in details for its structure, the surface sites acidities (pKa), and vibrational spectroscopy.62,63 The interface is now charged by the presence of ions, either cations (e.g., Na+, K+) or anions (e.g., Cl−, I−), one single monovalent ion at a time.The electrostatic treatment of charged simulation boxes (+|e| or −|e| depending on the ion) is performed via the Ewald summation as implemented in CP2K. The DFT (density functional theory) electronic representation is BLYP with the Grimme D2 dispersion correction.76 Goedecker−Teter−Hutter (GTH) pseudopotentials77 are used in conjunction with a plane-wave basis set defined by a density cutoff of 280 Ry and Gaussian basis sets of the triple-ζ polarized type (TZV2P for O, H, Si, Na, Cl atoms) and double-ζ type for potassium and iodide for which the DZVP-MOLOPT-SR functions are chosen.78 We present four FPMD simulations of the ionic (0001) αquartz/liquid water interface, where the quartz surface is fully hydroxylated, and composed of silanols of the Q2 type (geminals). Each simulation box is composed of 6 O−Si−O layers, 8 surface Si−OH silanols, 63 water molecules, and one ion, either an alkaline cation (Na+ or K+) or an halide anion (Cl− or I−). This corresponds to a bulk concentration of 1 M. Because of the computational cost of FPMD simulations, we only selected two cations and two anions in our investigations. Na+, K+, and Cl− represent the most widely used singly charged ions in experimental and theoretical studies published to date. We furthermore chose I− because of its relevance in geochemistry, i.e. this ion has indeed a larger natural abundance than Br− in the earth crust. F− has not been considered because of the lack of related SHG-based titration experiments. The

Figure 1. Interfacial repeated motif over the 2D surface of the fully hydroxylated surface of aqueous (0001) α-quartz, from refs 62 and 63. One in-plane Si−OH (IP) is an H-bond donor to its neighbor out-ofplane Si−OH (OP), while the IP is simultaneously HB acceptor to one interfacial water molecule, and the OP is HB donor to one interfacial water molecule.

Method section, where we will provide more details on the alternating 2-dimensional interfacial hydrogen-bond network hence formed. Most importantly, the silanol-water hydrogen bonds were found to be much stronger when formed with the out-of-plane silanols, giving rise to covalent Ow−Hw stretching frequencies within the range of the “ice-like” band in the SFG spectrum. The water H-bond donors to in-plane silanols on the other hand give rise to Ow−Hw stretching frequencies within the ”liquid-like” region of the SFG spectrum. The present paper is thus a follow up from our previous theoretical work on the neat (0001) α-quartz/water interface.62,63 We are interested in how the geologically relevant alkali−halide ions can perturb the interfacial arrangement initially observed at the neat (0001) α-quartz/water interface, and whether there are cation- or anion-specific interfacial effects. SFG experiments based on fused quartz have mainly focused on ionic-strength effects on the interfacial water C

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The Journal of Physical Chemistry C total volume of the super cell is 9.820 × 8.504 × 32.165 Å3. Periodic boundary conditions are applied in the three directions of space. The dynamics are performed in the NVE microcanonical ensemble, and are separated into two steps, i.e., an initial 9 ps of thermalisation (where the atomic velocities can be rescaled) followed by 12 ps of trajectory purely conducted in the NVE ensemble. Analyses are conducted on this latest part of the trajectory. Timestep is 0.4 fs, and the average temperature is ∼330 K. Only the Γ point is taken into account in the electronic representation. We have shown in refs 62 and 63 that the neat fully hydroxylated interface between the adsorbed water molecules and the surface silanols is composed of one repeated motif over the 2D surface (note that such motif might be partly enforced by the lateral periodic boundary conditions), based on two alternating hydrogen-bonded (HB) configurations, as illustrated in Figure 1. The repeated motif is built on two consecutive Si−OH surface silanols, one located in-plane of the surface (hereafter denoted IP), one located out-of-plane of the surface (hereafter denoted OP), the IP Si−OH being hydrogen bonded to its neighouring OP through a donor (IP)O−H··· O(OP) hydrogen bond, see Figure 1. Each of these silanols is in turn hydrogen bonded to one water molecule each, the OP being HB donor to water, the IP being HB acceptor to water; see Figure 1. In order to build the initial configuration of the ionic (0001) quartz/liquid water interface of interest in the present work, one of these H-bonded water molecule has been replaced by an ion, typically Na+ for our preliminary tests. Two independent trajectories were constructed, i.e., the first one replacing one water HB donor to an IP silanol by the cation (Figure 2, left),

solvation shells of interfacial ions. Water···water H-bonds are found whenever the Ow···Ow distance is less than 3.2 Å and the angle ∠Ow H w Ow is larger than 140°. Anion···hydroxyl (water or silanol) H-bonds are found whenever the anion···O (Ow or OH−Si) distance is contained within the first RDFanion−O peak and the angle ∠M−HO is larger than 140°. Note, however, that we found that the directionality criterion is never fulfilled between anions and silanols; angle values in the range 35−127° are found instead.

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RESULTS AND DISCUSSION Ions Adsorption. Two distinct adsorption behaviors have been identified depending on the ion charge. On the one hand, Na+ and K+ cations show a strong affinity for the surface since they both adsorb in an inner-sphere configuration, including two (Na+) and three (K+) oxygens from surface silanols within their solvation shells. Parts a and b of Figures 3 show

Figure 3. Snapshots extracted from the FPMD simulations involving one alkaline ion (a and b) or one halide ion (c and d) adsorbed at the (0001) α-quartz/liquid water interface. Only the water molecules and silanols from the solvation shell of the ions appear in these figures, the rest of the liquid is hidden for the sake of clarity.

representative snapshots for both systems; only the water molecules and silanols located within the ion solvation shell have been highlighted in these figures. On the other hand, Cl− and I− anions avoid forming direct hydrogen bonds with the surface hydroxyls, nonetheless they roughly are located within the first water layer located above the quartz slab. We thus find an intermediate adsorption of these anions in between inner and outer sphere, as depicted in Figure 3, parts c and d, respectively, for Cl− and I−. No water molecule is found located between the surface and the anion(s), so as to possibly screen the anions from the aqueous quartz surface. The anions can furthermore laterally diffuse along a few angstroms, mostly within a hexagon made of surface silanol oxygens. See, for instance, Figure 4 taken from the dynamics with iodide. This diffusion ability is not observed for cations. It clearly appears that anions try to minimize some electrostatic repulsion arising from the surface, while cations do the opposite. So as to explain these ions adsorption properties at the (0001) fully hydroxylated quartz/water interface, we have calculated the surface charge arising from the hydroxyls at the neat interface (ions are not present in this

Figure 2. Scheme showing the strategy of insertion of one ion (cation/ anion) at the interface, for the start of the FPMD trajectories.

the second one replacing one water HB acceptor to an OP silanol by the cation (Figure 2, right). After a few picoseconds, each of these trajectories provided the same preliminary conclusion about the adsorbing behavior of Na+ at the interface, i.e., inner-sphere adsorption of Na+, as will be described in details in the first part of the Results for Na+ and for the other ions (cations, anions). These preliminary trajectories ensured us that the initial position of the ion at the interface, replacing a H-bonded water either donor or acceptor to surface silanols, is of no consequence to the final result. We consequently chose to use the last snapshot of one of these two preliminary trajectories as an identical starting point for all trajectories performed here. All trajectories thus start with an inner-sphere adsorption of the cation or the anion. Analyses are presented in the Ions Adsorption section. To define hydrogen bonds, the criteria based on distance and directionality angle defined by Galli et al. for water···water hydrogen bonds (see, for instance, ref 79) were used in the present work and adapted to the presence of anions and to the presence of surface silanol Si−OH groups that might be part of

Figure 4. Top and side views of the hexagon of oxygens in which iodide laterally diffuses at the (0001) α-quartz/liquid water interface. Similar geometries can be found for chloride. D

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The Journal of Physical Chemistry C calculation). Using the density-derived atomic point charges (DDAPC) method of Bloechl80 for periodic systems on 30 decorrelated snapshots taken from the trajectory of the neat interface, we find an average negative surface charge of −0.21|e| per surface hydroxyl. This negative surface charge is therefore responsible for the direct attraction and adsorption of Na+ and K+ at the interface, while it sufficiently repells Cl− and I− from the interface. The strong adsorption of cations at the fully hydroxylated (0001) quartz/water interface is therefore driven by the electrostatic attraction of the negatively charged surface hydroxyls. As a summary, the average vertical distance between each ion and the surface (defined as the plane formed by the topmost oxygens, i.e., all the silanol oxygens at our fully relaxed interface) can be found in Table 1. Results obtained here with

Figure 5. Schematic views of the distinct environments of a cation (Na+ and K+) or an anion (Cl− and I−) at the (0001) fully hydroxylated quartz/water interface. Only the solvation shell of the ions is represented in these schemes, water molecules from the liquid are not represented.

Table 1. Average Vertical Distance between Each Ion and the Quartz Surfacea ion height (Å) a

Na+ 1.9 ± 0.3

K+ 2.4 ± 0.4

Cl− 3.3 ± 0.7

I− 2.7 ± 0.6

The surface is defined as the plane formed by the topmost oxygens.

the alkali cations can be compared to recent structural data obtained by a joint experimental (X-ray reflectivity) and theoretical (DFT) study on Rb+ adsorption at the (101) quartz/water interface.61 They revealed dominant inner-sphere adsorption on two surface silanols, consistent with our results for Na+ and K+ at the (0001) α-quartz/water interface. Discussing relative heights (Rb+: 1.9−2.3 Å depending on experiments/calculations) is however not straightforward because of the distinct surface terminations between (101) and (0001) quartz slabs. Despite this issue and the difference in ionic radius between the three cations, we obtain comparable results concerning adsorption heights. We can also reasonably anticipate that, due to its larger ionic radius, Rb+ would locate slightly farther than K+ at the (0001) quartz/water interface. Our results are also corroborated by classical MD simulations by Kroutil et al.81 who show that, independently of the pH (PZC or higher), Na+ comes closer to the (101) quartz surface by 0.3 Å with respect to Rb+ because of its smaller ionic radius and distinct solvation behavior. We also present in Figure 5 schematic views of the adsorption configurations of cations and anions at the (0001) fully hydroxylated quartz/water interface. These are developed further in the next section. We performed one additional simulation of the charged interface with potassium in order to definitely make sure that the initial configurations of the trajectories do not bias the final inner-sphere adsorption result. Hence, an initial position of K+ was chosen located within the second water layer on top of the solid surface, as shown in Figure 6a. The idea was to investigate whether the first water layer would screen the surface charge and prevent the cation from approaching the (negatively charged) surface and directly interact with it. As can be seen in Figure 6b where the evolution with time of the vertical position of K+ with respect to the surface plane (taken as the average position of the topmost oxygens) is plotted, K+ remains close to its initial location (above 4 Å) for about 30 ps, before diffusion takes over and the cation is driven toward the quartz surface, where it stays for the rest of the 45 ps trajectory, as previously observed. The final configuration is found with K+ lying on top of 2−3 (2.6 from RDF integration, see Table 2) surface silanols,

Figure 6. Evolution with time of the vertical distance between K+ and the quartz surface (curve in panel b), when K+ is initially located in the second water layer above the surface slab (a). After 30 ps, it finally diffuses toward the fully hydroxylated surface and steadily adsorbs in an inner-sphere configuration on top of 2.6 SiOH’s (c).

as illustrated in Figure 6c, and as already described above. Note that the two simulations of the adsorption of K+ at the quartz/ water interface perfomed here, and referred in Table 2 as “simulation 1” (Figure 3b) and ’”simulation 2” (Figure 6), respectively, show a certain flexibility of the solvation shell of K+ once adsorbed at the interface. There is indeed a variability in the number of Si−OH surface sites that can be part of the solvation shell of K+, between average values of 2.6 and 3.2 (values extracted from the radial distribution functions and commented later in the text and presented in Table 2) that shows a dynamical solvation of this cation by surface silanols that has not been observed for Na+. Ions Solvation. This section now demonstrates that the solvation structures of the ions at the (0001) fully hydroxylated quartz/water interface bear a strong resemblance to the ones that exist in bulk liquid water, typically in terms of number of oxygens (cations) or hydrogens (anions) within the ions solvation shells, and in terms of general solvation shell structure (ion−Ow/Hw distances and angles for ion-shell symmetry). We find that each ion is thus able to adapt to the (0001) αquartz/water interface so as to build a stabilizing environment very similar to the one favored in the liquid phase. To demonstrate these results, we have investigated in details the solvation structures around each ion by means of (1) radial distribution functions (RDF), i.e. cation−Ow, cation−O(Si), anion−Hw and anion−H(OSi), where Ow and Hw are respectively oxygen and hydrogen atoms of water, and O(Si)/H(OSi) are respectively the oxygen/hydrogen atoms of surface silanols; (2) angular distributions of Ow and Hw within these solvation shells, in order to describe the average geometry of the solvation shells and more specifically the orientation of the water molecules within these solvation shells. E

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Table 2. Positions (in Å) of the First Maximum (R) and First Minimum (r) Peaks of Radial Distribution Functions of the Type X−O and X−H, Where X = Any Ion (Na+, K+, Cl−, I−) Investigated Herea ROX Na −quartz Na+−water tot. coord of Na+ at the interface Na+ (bulk water, our work) Na+ (bulk water82) Na+ (bulk water79) K+−quartz (simulation 1) K+−water (simulation 1) tot. coord of K+ at the interface (simulation 1) K+−quartz (simulation 2) K+−water (simulation 2) tot. coord of K+ at the interface (simulation 2) K+ (bulk water, our work) K+ (bulk water82) K+ (bulk water87) Cl−−quartz Cl−−water tot. coord of Cl− at the interface Cl− (our work) Cl− (ref 88) Cl− (ref 89) I−−quartz I−−water tot. coord of I− at the interface I− (bulk water, our work) I− (bulk water90) I− (bulk water91) +

rOX

2.44 2.46

2.93 3.07

2.47 2.40 2.47 2.79 2.84

3.00 3.0 3.2 3.54 3.58

2.85 2.83

3.47 3.59

2.81 2.85 2.81 4.16 3.12

3.54 3.6 3.75 5.38 3.61

3.16 3.2 3.15 3.66 3.71

3.76 3.76 3.83 5.00 4.89

3.59 3.55 3.66

4.2−4.9 4.1 4.1

NO

RHX

rHX

NH

2.0 4.1 6.1 5.7 5.2 5.2 3.2 5.1 8.3 2.6 6.0 8.6 6.9 7.1 6.8 5.4 6.3 6.3 6.5 5.8 6.3 5.9 12.6 18.5 8.4−15.2 6.6 7.0

2.85 2.93

3.45 3.95

2.95 3.0 3.0 3.41 3.33

4.01 3.9 3.6 3.99 4.35

3.21 3.31

4.09 4.35

3.40 3.4 3.44 4.01 2.17

4.13 4.3 4.31 5.04 2.80

2.22 2.3 2.19 3.48 2.58

2.86 2.93 2.90 5.32 3.03

2.64 2.61 2.70

3.18 3.2 3.3

2.0 13.6 15.6 19.0 17 14 4.0 16.5 19.5 4.1 17.3 21.4 19.7 21 18 4.7 5.1 5.1 5.4 5.2 5.1 6.0 4.2 4.2 5.5 5.1 6.5

a NO and NH are the associated number of O and H neighbors (first solvation shell) of the X ion (including water and surface atoms in the count). For each ion: the first line corresponds to our DFT-MD simulations at the (0001) quartz−water interface with only the silanol O/H atoms taken into account, the second line corresponds to our DFT-MD simulations at the (0001) quartz−water interface, taking into account only the oxygens/ hydrogens of the water molecules, the third line corresponds to a summary of the interfacial coordination in the f irst solvation shell only, the fourth line corresponds to our DFT-MD simulations of the ion immersed in bulk liquid water (our own reference), and the fifth and sixth lines are AIMD data from the literature for the ion immersed in bulk liquid waterCPMD/PBE,79 CPMD/HCTH,82 CPMD/BLYP,88,90 and CP2K/BLYP+D.91 In the case of interfacial K+, two simulations have been performed, see text, and are referred below as “simulation 1”/“simulation 2”. “Simulation 2” corresponds to K+ initially immersed in the bulk liquid and diffusing toward the surface (see Figure 6). For the values presented in the table, only the last part of this trajectory is considered, i.e., those with the actual surface adsorption of the cation.

Figure 7. Radial distribution functions of cations Na+ and K+ at the (0001) α-quartz/water interface. The black curves are related to the M+−water interactions at the interface, the red ones to M+-surface hydroxyl interactions at the interface, and the green lines are the references obtained here for the cations immersed in bulk liquid water. Dashed lines are the integration curves providing the number of neighbors at a given distance, i.e. (running) coordination numbers.

molecules of volume 15.64 × 15.64 × 15.64 Å3, and FPMD simulations were performed at 330 K for 15 ps. RDFs for the two Na+ and K+ cations are presented in Figure 7, and Table 2 reports relevant numerical values and comparisons with the literature. In the plots and table, the references for the ions immersed in the bulk liquid water (this

We furthermore performed additional FPMD simulations of each ion immersed in bulk liquid water, using the exact same setup as for the dynamics at the interface, in order to provide a reference in bulk liquid water on which to directly compare the results obtained at the interface. To that end, each ion was put inside a cubic box of liquid water containing 128 water F

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Figure 8. Radial distribution functions of anions Cl− and I− at the (0001) α-quartz/water interface. The black curves are related to the M−−water interactions at the interface, the red ones to M−−surface hydroxyl interactions at the interface, and the green lines are the references obtained here for the anions immersed in bulk liquid water. Dashed lines are the (running) coordination numbers.

work) are also presented. In the figures, the black curves are related to the M+−water interactions at the quartz interface, the red ones to M+−surface hydroxyl interactions at the quartz interface, and the green lines are our references obtained for the cations immersed in bulk liquid water. The very nice matching of the first peaks positions for the RDF cation−O w and RDF cation−O(Si) on the one side and RDFcation−Hw and RDFcation−H(OSi) on the other side shows that indeed both sodium and potassium cations steadily adsorb in an inner-sphere configuration at the (0001) fully hydroxylated quartz/water interface. The surface oxide lattice dimensions thus allow nearly perfect matches of quartz oxygens and hydrogens inside the first solvation shells of the cations. The atoms from the surface are located slightly closer to the central cation than the water molecules, see Table 2 where all the distances are reported, and one can see shorter cation−O(Si) distances than cation−Ow, again highlighting a strong affinity of the cation for the α-quartz surface. In more detail, we find that the solvation shell of sodium is composed of four Ow atoms from four water molecules and two silanol oxygens from the surface. Added together, these six O atoms (water + silanols) in the first shell of Na+ allow to recover the coordination number of Na+ obtained in pure bulk liquid water (5.7 on average from our reference simulations). Note that our reference for the number of closest oxygens in liquid water, denoted NO in Table 2, slightly overestimates recent ab initio findings by 0.5 (5.279,82), presumably due to the different choices in the FPMD setups. For instance, in ref.,79,82 the authors perform Car−Parrinello molecular dynamics instead of Born−Oppenheimer MD with, respectively, the PBE and HCTH exchange correlation functionals, not taking into account dispersion corrections. Besides, our result is in very good agreement with experimental measurements (4−6 from X-ray diffraction,83 4−8 from neutron diffraction83,84) and classical force field MD (5.684), all these reference data concerning ions in pure bulk liquid water. Our RDFNa+−Ow at the interface can moreover be compared to the work by Hassanali et al.85 using their newly developed model for dissociated amorphous silica/water interface as well as AIMD simulations (BLYP, CP2K), where they obtain first maxima located at 2.45 and 2.40 Å respectively, to be compared to our value of 2.46 Å. Besides, Na+ is found at a distance of 2.3 Å from SiO− groups, testifying of inner-sphere adsorption. Our result is also in agreement with the atomic profiles obtained from classical MD by Heinz et al.86 where sodium is observed at around 2 Å on top of the silica surface. Importantly, Heinz

and co-workers show that this vertical position is not affected by the amount of deprotonated sites. The solvation shell of potassium is composed of five Ow from five water molecules and three surface silanol oxygens, for a total of eight oxygens. This corresponds to one first shell oxygen neighbor more than in bulk liquid water (our reference), see Table 2 and references. As already mentioned in the text, note however that the number of surface adsorption sites for potassium was found to oscillate between 2.6 (see simulation presented in Figure 6, denoted “simulation 2”) and 3.2 (this simulation, denoted “simulation 1”), see values in Table 2. This shows a nonrigid adsorption behavior over silanol surface sites. Note also that the water/K+ RDF’s nicely match in both simulations. Oscillating between bidentate and tridentate conformations of the cation interacting with surface silanols has also been observed in classical MD of the monovalent Rb+ cation at the (101) quartz aqueous interface,81 bidentate/ tridentate forms observed for 58%/21% of the time. In the same reference,81 Na+ was found to form monodentate (60%) and bidentate (34%) structures, while we only observe bidentate adsorption at the (0001) quartz/water interface. We have already observed that Cl− anion does not form direct hydrogen bonds with surface silanols, as it is located farther away from the surface than the two cations described above. This is confirmed by the RDFs of Figure 8a (Cl−−O(Si) and Cl−−H(OSi), red lines), where one can observe that the first maximum of the Cl−−H(OSi) curve is located at ∼4 Å instead of 2.2 Å for Cl−−Hw. Furthermore, the Cl−−Hw and Cl−−Ow RDF curves obtained in the bulk liquid (green lines) or at the interface (black lines) are almost identical in terms of peaks positions and intensities, and the number of Hw atoms building Cl− first solvation shell is identical in bulk water and at the aqueous quartz interface. See also Table 2. As a consequence, even though chloride is maintained close to the solid surface, i.e., around 3.3 Å on top of the topmost plane of the surface oxygens, it is able to conserve a bulk-like environment in its first solvation shell, at least from the point of view of number of water neighbors and Cl−−Ow/Cl−−Hw distances within the first solvation shell (RDFs). Interfacial iodide differs from chloride. We previously mentioned the shorter vertical distance separating this larger anion from the quartz surface, i.e. 2.7 Å (Table 1), when compared to Cl−. It is then reasonable to possibly expect another type of solvation shell for I−. Indeed, we find an identical position of the RDF first peak of I−−Ow (black line in Figure 8b) with I−−O(Si) (red line in Figure 8b) at the aqueous surface, and with I−−Ow (green line in Figure 8b) in G

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The Journal of Physical Chemistry C bulk liquid water, as well as identical intensity of this first peak. There are hence six oxygen silanols and 13 oxygen waters building the first solvation shell around I− at the (0001) quartzwater interface. The six oxygen silanols provide the above picture already depicted in Figure 4b, where we already mentioned that I− has a lateral motion over a hexagon made of surface oxygens. Note that the first peak in the RDF of I−− H(Si) (red curve, Figure 8b) starts around 2.3 Å and is peaked at ∼3.5 Å.This could suggest that hydrogen bonding may occur between I− and the surface. However, we find average ∠I−HO(Si) angles of 92° (35−121° fluctuations), showing that hydrogen bonds can not be hence defined from the point of view of directionality. Note also that surface hydrogens are closer to I− than surface oxygens by 0.2 Å, which might locally screen the negative global surface charge. It is interesting to note that the minima of the RDF first peak of I−−Ow (and I−−O(Si)) at the quartz interface are much deeper than the I−−Ow reference obtained in bulk liquid water. This shows that the HB strength for the water molecules located in the first shell around I− at the interface is different from the pure liquid. As a rough estimate, the ratio of intensity between first maximum and first minimum of any RDF provides an estimate of the H-bond strength formed between the pair of atoms.92 It is therefore clear that H-bond strength of I−−Ow and I−−O(Si) at the interface is twice the H-bond strength of I−−Ow in pure bulk liquid water. It is also fair to stress that the exact location of the first minimum in our reference RDFI−−Ow curve is difficult to read as it is not welldefined (see the green curve in Figure 8b), reflecting a fluctuating and dynamical solvation sphere around I− in the liquid phase. We therefore only provide ranges of (minimum) positions in Table 2: the first minimum is found between 4.2 and 4.9 Å, which would then lead to coordination numbers of I− in bulk liquid water ranging between 8.4 and 15.2. As Table 2 shows, including our own reference data for the solvation of ions in bulk liquid water, we find a very good agreement in the RDFI−−Hw between our results for I− in bulk water and those with other ab initio MD from refs 90 and 91 for first maximum and minimum as well as for the hydrogen coordinations. However, the agreement between our RDFI−−Ow and the ones from literature is less obvious. Even though the position of the first maximum is in excellent agreement with previous studies (3.59 vs 3.55 Å91 or 3.66 Å90), the exact location of the subsequent first minimum is not well-defined, while this is not the case in previous simulations. Due to this not well-defined position of the first miminum in RDFI−−Ow in pure liquid water, i.e., 4.2−4.9 Å, we get a number of closest Ow atoms NO = 8.4−15.2 instead of 6.6−7.0 in refs 90 and 91. This may be due to the differences in the FPMD setups, and maybe more importantly, this may be due to the number of water molecules in the simulation box. Both refs 90 and 91 indeed consider 64−95 water molecules for bulk water modeling, while we have 128, for comparable dynamics lengths (15−18 ps). Size issues might indeed be important for the rather unstructured solvation of such a bulky anion. However, our lowest value NO = 8.4 belongs to the accessible range of I− coordination number obtained in ref 90, where values from 4 to 9 were obtained. Our 8.4 value is also in agreement with several X-ray diffraction experiments, where coordination numbers are spread over the 4.2−9.6 range,83 again emphasizing the unstructured solvation shell around I− in pure liquid water.

Coming back to the interface, we note that the water coordination around I− is different from the one in bulk water: there are less direct H-bonds formed with iodide (4.2 instead of 5.5) but they are stronger since the first I−−Hw RDF peak is located at a shorter distance. If we compare the coordination of I− by water oxygen atoms at the interface and in our reference bulk simulation at a given cutoff distance, namely 4.9 Å, we find a high and comparable amount of water molecules in both cases, i.e., 12−15. This can be surprising regarding other AIMD simulations.90,91 We furthermore propose to have a look at the snapshot in Figure 9, where the number of closest water neighbors reaches

Figure 9. Iodide at the interface surrounded by water molecules and silanols belonging to the first solvation shell. We hightlighted the hexagon of silanols in red and white. The blue water molecules are involved in direct H-bonding with I−. The grey ones form one HB with silanols from the hexagon whereas the orange molecules bind either the blue or the grey water molecules, making HB chains.

13. In this snapshot, we have highlighted only water and silanols belonging to the first solvation shell around iodide. The three dark blue water molecules donate one hydrogen to I−, like in bulk liquid water. All six SiOH’s are visible in red (O) and white (H). Five of them are interacting with one water molecule each, shown in grey in the figure, and these water molecules form H2O···H2O hydrogen bonds inside the first water layer on top of quartz (this was not observed at the neat interface). This already amounts to 8 closest Ow neighbors around I−. The five others can be considered as H-bond bridges between or inside the above-mentioned water groups. They appear in orange in the Figure. They enable the formation of HB chains, here three chains composed of four water molecules. This specific adsorption geometry for I−, intermediate between inner- and outer-sphere, is consistent with the well-known weak hydration of iodide with respect to chloride. In fact, this nicely agrees with the solvation found in our reference simulation in bulk water, see Figure 10 where the first solvation shell of I− (4.9 Å used as distance cutoff) containing 15 water molecules is represented with a similar color code as in Figure 9 and where Hbond chains can be observed. Together with some sort of “hydrophobic solvation” observed in the hexagon made of surface silanols, it leads to an encapsulation of iodide close to the quartz surface, with however attempts to screen the global negative charge of the surface hydroxyls with a reorientation of the six (positively charged) H−OSi (see before for more discussions on that aspect). In order to get further insights on the interfacial solvation of these simple ions and particularly to compare more deeply with H

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Figure 10. Iodide surrounded by water molecules from its first solvation shell in pure bulk liquid (128 H2O). The blue water molecules are involved in direct H-bonding with I−. The orange water molecules bind either the blue or each other, making HB chains.

Figure 13. ∠OMO angular distributions (purple lines) in the first solvation shell of each single M± ion, and ∠HM−H angular distributions (green lines) for M− ions. Plain curves correspond to the interfacial systems and dashed lines to pure bulk solvation.

respect to their bulk solvation, we now analyze angular distributions in the first solvation shells of each ion, as described in Figures 11 and 12. The knowledge of the

crystal lattice. Once involved in the solvation shell of interfacial Na+, they will contribute to make the shell around the ion slightly more rigid than when the ion is only solvated by (the more flexible) water molecules from the liquid, thus the bands in the ∠ONa +O distribution that are slightly more well-defined at the interface (purple plain line) than in bulk water (pink dashed line). Potassium is surrounded by a total of 8 oxygens in its first solvation shell, and no symmetry can be clearly distinguished from Figure 13b, neither in the liquid phase nor at the solid−liquid interface. This is in agreement with X-ray diffraction experiments in liquid water.83 The ∠OK+O distribution is however more randomly distributed in the interval 40−130° at the interface, where a rather ”flat” distribution is observed, while 80° angles seemed more privileged in liquid water. We have already observed that K+ solvation shell at the interface is built with one more oxygen atom than in liquid water, thus providing an enhanced local congestion around the cation. This is presumably the reason for the observed increase of angles smaller than 70° at the interface. As already highlighted, the direct neighborhood of chloride at the interface is very similar to the one in bulk water. This is again verified in Figure 13c for both ∠OCl−O and ∠HCl−H distributions. Like Meijer et al.,88 we can conclude that the symmetry of the first solvation shell around Cl− is made of a distorted octahedron, both in liquid water and at the interface. Larger changes occur for the solvation shell symmetry of iodide at the interface, as already expected from the analysis of RDFs of this anion. Even though the ∠OI−O distributions in Figure 13d nicely show the same features as in ref 90 for I− in liquid water (namely peaks around 40°, 80°, and 130−150°), it is not the case for the ∠HI−H distribution. Contrary to the behavior in bulk water, where a main peak in the distribution is observed around 80° (green dashed line), the main peak of ∠HI−H at the interface is centered at a lower value of 65° (green plain line) and the shape of that peak is much higher and thinner. The first solvation shell of iodide is thus partly perturbed at the interface, but there is no stabilization within a given symmetry, similarly to the behavior in bulk water and in agreement with Xray studies.83

Figure 11. Definition of the α = ∠OMO angle used to characterize the angular distribution of oxygen atoms belonging either to water or to SiOH groups in the first shell around the M± ion. Same definition for ∠HMH .

Figure 12. Definition of the θ angle used for the orientation of the HH bisector of water molecules located in the first shell around the M± ion.

distribution of the α = ∠OMO angle is used to characterize the symmetry of the first solvation shell around the M± ion. These distributions are reported in Figure 13a−d, for Na+, K+, Cl−, and I−. In the figures, results obtained at the (0001) quartz− water interface (purple and green plain lines, respectively for ∠OMO and ∠HMH angles) are compared to the ones obtained in pure liquid water (dashed lines). The influence of the interface on the ion solvation is also manifested in the orientation of water molecules surrounding each ion. Our indicator is the orientation of the HH bisector (i.e., roughly the dipole moment) from water molecules inserted in the first solvation shells as defined in Figure 12. For sodium, surrounded by six oxygens in its first solvation shell (four from water; two from surface silanols), we observe in Figure 13a two distinct bands centered around 80° and 150− 160°. These angles correspond to an octahedral nest of oxygens around Na+. Silanol oxygens have rather rigid positions in the I

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The Journal of Physical Chemistry C Results for the orientation of the water molecules dipole moment (see definition in Figure 12) are now gathered in Figure 14a−d, where again the results at the interface (brown

Figure 16. Angular distributions of the out-of-plane silanols, in dark blue, and in-plane silanols, in purple, at the neat fully hydroxylated (0001) α-quartz/water interface.

surface (blue line in Figure 16) and forming strong hydrogen bonds with interfacial water oxygens. From Figure 16, one can also note that there is almost no dynamical exchange between IP (purple line) and OP (dark blue line) silanols at the neat interface. The small tail in the purple curve is due to a limited period of time of incursion of an IP into an OP configuration, and apart from this marginal event, no other IP/OP exchange has been attempted. We also remind the reader that in-plane and out-of-plane silanols belong to the Q2 type, i.e., the topmost surface silicon atoms carry two hydroxyls and two siloxane bridges. The IP/OP ratio is 4:4 in our neat simulation box. We have furthermore shown62 that the OP/IP orientations of the surface silanols are associated with two distinct pKA’s at the neat (0001) α-quartz-water interface: out-of-plane silanols were found acidic, with a pKA of 5.6, whereas in-plane silanols were found more basic, with a pKA of 8.5. The values of the covalent (Si)O−H bond lengths were found consistent with the acido-basic characters: the longer the covalent bond, the more acidic the surface site, since its proton is better shared with the environment. See the first column of Table 3 where it is clear that the basic in-plane silanols have shorter (Si)O−H bonds than the acidic out-of-plane silanols. We will now focus on the impact of the presence of our selected singly charged ions on the structural properties of these silanols in terms of the repartition between the in-plane (IP) and out-of-plane (OP) groups, the orientations of hydroxyls with respect to the normal to the surface, and the (Si)O−H covalent bond lengths. This will emphasize the rather large surface perturbations induced by the adsorbed cations and the negligible effects arising from the anions in their intermediate adsorption configurations. One important consequence of the cations adsorption at the interface is that nearby OP surface hydroxyls are pushed toward IP orientations. This means that, at this high ionic coverage (one ion for eight surface sites), the initial 4:4 IP/OP ratio of the neat interface is modified, with an increase in the number of IP groups: it hence evolves to a 6:2 average ratio for Na+ adsorbed at the interface and 5.5:2.5 for adsorbed K+. On the contrary, as anions are located farther away from the surface hydroxyls, they either do not significantly modify the silanol orientation populations or only slightly modify these populations. IP:OP ratios of 4:4 and 5:3 are found, respectively for Cl− and I−. The adsorption of the ions at the interface also leads to changes in the absolute values of the angular orientations of the hydroxyls with respect to the normal to the surface. At the neat interface, the IP and OP silanols have average angles of

Figure 14. Angular distributions of the HH bisector of water molecules in the first solvation shell of each single M± ion. Brown plain curves correspond to the interfacial systems and red dashed lines to our reference in bulk liquid water.

plain lines) are compared to the ones in pure liquid water (our own references, red dashed lines). For cations directly adsorbed onto two or three surface hydroxyls, a more linear coordination with water is found (main peak for cos θ < −0.8), i.e. the water dipoles can align more easily with the M+···O direction at the interface than in bulk water Figure 14a,b. The water orientation around both chloride and iodide, presented in Figure 14c,d, shows some loss of specific orientation once the anion is at the interface. Hence, the distributions become broader (Cl−) and the ratio of the band amplitudes tends to decrease (I−, see the two bands at cos θ = −0.4 and 0.7). It is remarkable to note that, despite the very different coordination around iodide at the interface and in bulk liquid water, I− is still able to polarize and orient water molecules so that the distribution of HH bisectors still resembles as much as possible the one in bulk water. Silanols: Populations, Orientations, and Covalent Bonds. We have previously shown62 that the neat fully hydroxylated (0001) α-quartz surface at the interface with liquid water is covered by two silanol species according to their orientations with respect to the normal to the surface, as schematically illustrated in Figures 1 and 15 and seen in the angular distribution of Figure 16: in-plane silanols (IP), found orthogonal to the surface normal (peak at 100°, pink line in Figure 16), with their hydrogens involved in intrasurface hydrogen bonds; out-of-plane silanols (OP) protruding toward the liquid with an orientation of 30−35° from the normal

Figure 15. Schematic view of the out-of-plane (OP) and in-plane (IP) silanols covering the quartz surface in the presence of liquid water. J

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Table 3. Averaged Distances in Å of the Covalent (Si)O−H Bonds According to the Different Interfaces (Reference Arising from the Neat Interface) ⟨dIP⟩ ⟨dOP⟩ a

neat

Na+

Na+,a

K+

K+,a

Cl−

I−

1.006 ± 0.031 1.022 ± 0.036

1.001 ± 0.029 1.012 ± 0.035

1.003 ± 0.033 −

1.000 ± 0.032 1.014 ± 0.036

0.994 ± 0.026 −

1.002 ± 0.031 1.012 ± 0.036

1.000 ± 0.031 1.015 ± 0.036

Na+,a and K+,a refer to hydroxyls located in the first solvation shell of the cation.

distribution induced by anions seen in Figure 17 is the broadening of the OP peaks, which now display a nonzero intensity tail for angles in between 40 and 80°, where the neat interface showed a much smaller amplitude. This is especially observed at the Cl−-containing interface. There is thus less distinction between the IP and OP silanols at the anionic Cl− interface than at the neat interface. Figure 18 shows interesting dynamical exchanges between IP and OP silanols orientations at ionic interfaces, that were not observed at the neat interface, and that typically explain the angular population in between 40 and 80° at the Cl−-containing interface. One can indeed observe the blue and pink tails in the OP and IP distributions in this region, showing interchanges between these two silanol populations during the dynamics. This is not observed at the I−-containing interface. The IP/OP distributions obtained at the cations containing interfaces also show dynamical exchanges between the two populations. Interestingly, these interchanges occur for all silanols at the sodiated interface, i.e. whether they are part of Na+ solvation shell or not, while they are related solely to silanols in the solvation shell of K+ at the potassiated interface. Also striking, the OP 60° peak obtained at the Na+-containing interface is due to silanols located away from Na+ first shell, while the nonzero tail of the OP peak in the 40−80° is solely due to silanols from K+ first shell. The 10−20° central peak at the K+-containing interface results from silanols not belonging to the cation first solvation shell. One last comment. As mentioned above, the IP angular distribution is broadened at the two anionic interfaces, especially toward lower angular values than observed at the neat interface. This means that the intrasurface hydrogen bonding network strength formed by the IP silanols at the neat interface is weakened at these anionic interfaces, possibly enabling easier reorientation toward the liquid. Consequences on the surface− interfacial water H-bond network will be discussed in the next section. Hints on the Acidity Character of Silanols at (0001) αQuartz/Water Ionic Interfaces. Conclusion from the previous section is that interfacial cations have a perturbing power onto the surface silanol groups orientations. When cations are present at the interface, there is indeed an enhanced flexibility of the silanol hydroxyls, with more fluctuations of the OH silanols with respect to the normal to the surface, and IP/ OP flipping, in comparison to the observations made at the neat interface. The natural question now to be addressed is whether these observed structural and dynamical properties have any possible relevance to possible changes of the acidobasic character of the IP and OP surface silanols at the charged interfaces. The analysis of the hydroxyl O−H average covalent-bond lengths and their evolution from the neat interface to the charged interfaces are of direct relevance for providing hints on the possible changes in the intrinsic pKA acidity of the surface IP and OP groups. We base our reasoning on the fact that the acido-basic character of the Si−OH groups can be simply correlated to the covalent bond-lengths:62 the more acidic the

respectively 100° and 30−35°, as shown from Figure 16 and schematically indicated in Figure 15. Figure 17 now presents

Figure 17. Comparison of angular distributions of silanols between the neat interface (black) and the interfaces containing one single ion.

the silanol angular distributions for all ionic interfaces investigated here, while Figure 18 reports the details of these distributions in terms of IP and OP silanols for each ionic interface, and possible dynamical exchanges between the two silanol populations.

Figure 18. Detailed angular distributions of OP silanols, in dark blue, and IP groups, in purple, for each ionic interface. Plain lines stem from the hydroxyls away from the first solvation shells of the ions whereas dashed lines represent surface OH groups from the first shell of cations. Note that no OP silanol belongs to the first shell of Na+ (no dashed blue line). No distinction has been made around chloride or around iodide.

First comment from Figure 17 is that the silanols angular distribution observed at the neat interface is modified by the presence of cations while this distribution is barely modified by the presence of anions at the interface. The interfacial cations induce a change in the orientation of the OP silanols. They are now oriented ∼60° with respect to the surface normal at the sodiated interface (i.e., more in-plane than at the neat interface) and ∼10−20° at the potassiated interface (i.e., more out-ofplane), while the IP silanol distribution observed at the neat interface is only affected at anionic interfaces in the peak intensity without showing any change in the central value of the distribution (100°). The only noticeable change in the silanol K

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Table 4. Averaged Intra-Surface Hydrogen Bonds for the Different Charged Interfaces Investigated Here, To Be Compared to the Result at the Neat (0001) α-Quartz Interfacea interface ⟨dHB intra⟩ (Å) a

neat 1.69 ± 0.13

Na+ 1.79 ± 0.24

K+ 1.85 ± 0.36

Cl− 1.73 ± 0.18

I− 1.73 ± 0.17

HB intra refers to HBs where an in-plane silanol acts as a donor to a neighboring silanol.

ours (from the perspective of the surface structure and from the perspective of single ions present at our interfaces while ionpairs are present in Gibbs−Davis work), we will still consider that the “more acidic” sites correspond to out-of-plane silanols and that the “less acidic” sites correspond to in-plane silanols, maintaining the assignments that we have found at the crystalline (0001) α-quartz/water neat interface.62 All measured pKA’s for the more acidic site are 4.0−4.5, i.e., the well-known experimental SHG value of the neat interface71 (pKA = 4.5) and ab initio (VASP/PBE) results93 where the ionic concentration was equal to zero (pKA = 4.5). The uncertainties arising from both theoretical and experimental works prevent from concluding about a higher acidity or basicity of the “more acidic” sites upon addition of ions. However, the less acidic silanols with a pKA between 8 and 971,93 experienced a systematic increase of 1 or 2 pK units, reaching pKA’s of 10−11 when adding electrolytes. At least for these sites, one can thus conclude that the intrinsic basicity of the “less acidic” silanols is enhanced by the presence of singly charged ions. We suggest that we probably recover this behavior at our (0001) α-quartz/water interface since the surface perturbations induced by alkali and halide ions shorten the (Si)O−H covalent bonds of in-plane silanols with respect to the neat interface, consistently with the weakening of intrasurface hydrogen bonding. We would also expect a pKA increase for the out-ofplane groups at our interface, as all (Si)O−H covalent bonds get shorter upon addition of one ion. Here, we carefully stop the discussion with experimental data from the Gibbs−Davis group since they look highly dependent on the selected interfacial ion pairs. We will come back to that particular point in a forthcoming paper where ion pairs have been included at the interface. Assuming that the length of the silanol covalent bonds is directly correlated to the intrinsic acidity of the surface sites, we thus find the following trends for the fully hydroxylated (0001) α-quartz/water charged interfaces. OP silanols are less acidic at a ionic interface than at the neat interface. However, given the small differences between the four ions investigated here, no clear ranking appears and one can think that the actual calculation of the pKA would lead to close enough values. For IP silanols, the conclusion is very similar: an overall increase in basicity with the inclusion of ions at the surface and no clear distinction between the ions, with the exception of the surface sites within the first solvation shell of potassium for which the (Si)O−H covalent bonds seem particularly shortened. Shortrange polarizability effects could be behind this behavior: the more polarizable as well as the closer the cation to the surface, the more basic the in-plane silanol. As one last comment, one can wonder whether the number of silanols belonging to the first solvation shell of a monovalent cation can be sufficient to affect macroscopic experiments like titrations. Here, within our small simulation box, around 25% of the whole silanol population is concerned by the immediate vicinity of potassium, which is quite a lot. Our bulk concentration is of the same order of magnitude as the one chosen in experimental works13,15 (around 1 M). Moreover,

surface site, the longer the O−H covalent bond-length, and the easier for the proton to be shared in any hydrogen bond (i.e., the longer the H-bond). From Table 3, it is clear that the OP Si−OH, which were found acidic at the neat interface, remain acidic at charged interfaces, but the shorter ⟨dOP⟩ average values found at the charged interfaces (roughly by −0.010 Å) show that these sites are however found less acidic at the charged interfaces. Only the direct calculation of these pKA’s62 will tell us what is the quantitative change in the OP pKA in going from the neat interface (where the average ⟨dOP⟩ was 1.022 Å) to the charged interfaces (where the ⟨dOP⟩ averages are found in the range 1.012−1.015 Å), and if a difference of 0.003 Å in the average values between the sodiated and the iodiated interfaces provides any significant change in the acidic intrinsic character of the OP SiO−H sites. The IP Si−OH sites were found basic at the neat interface with a pKA = 8.5,62 and an average ⟨dIP⟩ of 1.006 Å. The average ⟨dIP⟩ distances found at the charged interfaces vary from 0.994 Å (potassiated interface) to 1.003 Å (sodiated interface). The IP SiO−H covalent bonds are thus found shorter than at the neat interface by 0.012−0.003 Å. The IP sites are thus found more basic at the charged interfaces than at the neat interface, with interfacial Na+, Cl−, and I− ions inducing very similar shortening of the Si−OH bond-lengths, while interfacial K+ induces a stronger shortening of the Si−OH distance, especially if the measure is done for IP silanols that belong to the K+ solvation shell. There is furthermore a very interesting disordering of the intrasurface hydrogen-bond network that was initially observed at the neat interface. In Table 4, we have reported the averaged HB distances between in-plane silanols acting as donors to a neighboring silanol (OP in the case of the neat interface), corresponding to all the Hbonds lying within the quartz surface plane. The intrasurface HB network is clearly distorted by the presence of interfacial ions, with a large increase of 0.10 Å (Na+)−0.16 Å (K+) in the averaged HB distances from the reference values at the neat interface. Fluctuations of these distances is also doubled (Na+) and almost tripled (K+) when the interface is charged with cations. Anions have a far less distorting influence, with intrasurface HB distances elongated by 0.04 Å, and their fluctuations found very close to the ones observed at the neat interface. The largest distortions from the neat interface are thus due to cations, especially potassium. This is a first hint that IP silanols, found of basic character at the neat interface (pKA of 8.5 calculated in our previous investigation62), will be found even more basic at charged interfaces, as the intrasurface HBs formed by these IPs are found systematically longer than the ones at the neat interface. Interfaces charged with cations (Na+ and K+ in the present work) will certainly have the more basic IP silanol groups. Only the direct calculation of these pKA’s following the method of our previous work62 will provide a quantitative acidity ranking. This is work in progress. Recent SHG experiments from the Gibbs−Davis group demonstrated that alkali halide electrolytes modify the instrinsic pKA’s of the two silanol species at the fused silica/water interface.11−13 Note that, at this interface somehow different to L

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We would like to make a final comment. We have highlighted here cation induced changes on the surface silanol structural and acidity properties, at the fully hydroxylated (0001) αquartz/water interface. Cation effects have definitely been seen from SFG (sum frequency generation) spectroscopic experiments,11,73,95 while specific halide effects are still subject of debate.12,73,96 We have in particular shown here that the presence of the inner-sphere interfacial cations induces a decrease in the number of out-of-plane (OP) silanols at the (0001) α-quartz aqueous surface. In our previous papers,62,63 the OP silanols have been shown indirectly responsible for the “ice-like” band in the SFG spectrum of the neat interface: they indeed are responsible for the presence of interfacial water molecules being H-bond acceptors to surface OP silanols. These specific water molecules were shown to be solely responsible for the “ice-like” band in the SFG spectrum of the neat interface.62,63 Reducing the occurrence of OP silanols, as it occurs at the charged interfaces, will certainly reduce the number of such H-bond surface-acceptor water molecules, that in turn would provide one good reason to explain the shrinking of the SFG “ice-like” band upon addition of ions observed in experiments.

surface coverage of monovalent cations can be non negligible at low bulk concentration: 10% for instance for a strontium monovalent complex at ∼10−3 M at the silica/water interface.94 Since increasing bulk ionic concentration would certainly increase the surface coverage, the adsorption SiOH sites would become more representative in titrations. Their influence should thus be considered in the hypotheses to explain the surface acidity changes upon addition of electrolytes.

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CONCLUSION Monovalent ion-induced perturbations at the fully hydroxylated (0001) α-quartz/water interface have been analyzed via firstprinciples molecular dynamics simulations. We find that cation (Na+, K+) adsorption occurs via an inner-sphere mechanism due to the electrostatic attraction arising from the negatively charged surface hydroxyls. These alkali ions adapt their solvation shell to their environment: their solvation shell found at the interface is indeed similar to the one observed in pure liquid water, with two or three surface silanol oxygens now replacing water molecules oxygens. On the contrary, simple halides like chloride and iodide mostly remain within the first water layer, without forming any hydrogen bond with the surface, nor with water molecules that could be located in between surface and anion. We thus observe an intermediate inner-/outer-sphere adsorption of the anions. Globally, a bulklike solvation of cations and anions is obtained, with conserved coordination numbers, once surface silanols replace water molecules within the first shell of cations. Water orientation around each ion at the interface is also kept similar to the one in pure bulk water. More importantly, adding ions at the (0001) α-quartz interface has a strong impact on the surface silanols properties, with larger perturbations arising from the inner-sphere adsorbed cations. Indeed, the intrasurface Si−OH···(Si)−OH H-bond network evidenced at the neat interface is weakened once ions are present at the interface, and silanols can now undergo several dynamical exchanges between in-plane (IP) and out-of-plane (OP) orientations. Such IP/OP flipping was not observed at the neat interface. Interestingly, the IP/OP ratio depends on the nature of the ion with, however, an overall trend toward a decrease in the number of out-of-plane silanols once cations adsorb to the surface. Anions, located away from the surface, without forming direct Hbonds with surface silanols, have negligible effect on the relative IP/OP silanol populations. The inner-sphere adsorption of cations leads to a general effect of “pushing OP” hydroxyls toward the quartz surface (thus becoming “more IP”). Surface silanol acidities have been shown modulated by both anions and cations at the interface. From our previous investigation of the fully hydroxylated (0001) α-quartz/water neat interface,62,63 OP silanols were found acidic with a pKa = 4.5 (calculated from the ab initio MD trajectories) and IP silanols were found basic with a pKa = 8.5. From the detailed analysis of the structural properties of surface silanols and Hbond networks they form (intrasurface and with the interfacial water) at the here investigated charged interfaces, we have found that OP silanols become less acidic and IP silanols become more basic. Overall, our theoretical results provide trends of acidity/basicity of surface silanols that agrees well with experimental results from the Gibbs−Davis group.11−13 Actual pKA calculations of these sites will be the subject of a forthcoming paper, including the full ion-pair electrolytes at the interface.

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AUTHOR INFORMATION

Corresponding Author

*(M.-P.G.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Georges Wipff, Riccardo Spezia, and Marialore Sulpizi are acknowledged for fruitful discussions. This work was performed using HPC resources from GENCI-[CINES/IDRIS] (Grant 2014-15[082484]).

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REFERENCES

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DOI: 10.1021/acs.jpcc.5b10947 J. Phys. Chem. C XXXX, XXX, XXX−XXX