Home
Search
Collections
Journals
About
Contact us
My IOPscience
The amorphous silica–liquid water interface studied by ab initio molecular dynamics (AIMD): local organization in global disorder
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys.: Condens. Matter 26 244106 (http://iopscience.iop.org/0953-8984/26/24/244106) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.137.5.42 This content was downloaded on 13/11/2014 at 18:41
Please note that terms and conditions apply.
Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 244106 (10pp)
doi:10.1088/0953-8984/26/24/244106
The amorphous silica–liquid water interface studied by ab initio molecular dynamics (AIMD): local organization in global disorder Álvaro Cimas1, Frederik Tielens2, Marialore Sulpizi3, Marie-Pierre Gaigeot1,4 and Dominique Costa5 1
Laboratoire Analyse et Modélisation pour la Biologie et l’Environnement, LAMBE UMR CNRS 8587, Université d’Evry val d’Essonne, Blvd F Mitterrand, Bat. Maupertuis, 91025 Evry, France 2 Sorbonne Université, UPMC Univ. Paris 06, UMR 7574, Laboratoire Chimie de la Matière Condensée, Collège de France, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France 3 Johannes Gutenberg University Mainz, Staudinger Weg 7, 55099 Mainz, Germany 4 Institut Universitaire de France IUF, 103 Blvd St Michel, 75005 Paris, France 5 Institut de Recherches de Chimie de Paris, Chimie Paristech CNRS, UMR8247, 11 rue P et M Curie, 75005 Paris, France E-mail: [email protected] and [email protected] Received 13 November 2013, revised 07 February 2014 Accepted for publication 24 February 2014 Published 27 May 2014 Abstract
The structural organization of water at a model of amorphous silica–liquid water interface is investigated by ab initio molecular dynamics (AIMD) simulations at room temperature. The amorphous surface is constructed with isolated, H-bonded vicinal and geminal silanols. In the absence of water, the silanols have orientations that depend on the local surface topology (i.e. presence of concave and convex zones). However, in the presence of liquid water, only the strong inter-silanol H-bonds are maintained, whereas the weaker ones are replaced by H-bonds formed with interfacial water molecules. All silanols are found to act as H-bond donors to water. The vicinal silanols are simultaneously found to be H-bond acceptors from water. The geminal pairs are also characterized by the formation of water H-bonded rings, which could provide special pathways for proton transfer(s) at the interface. The first water layer above the surface is overall rather disordered, with three main domains of orientations of the water molecules. We discuss the similarities and differences in the structural organization of the interfacial water layer at the surface of the amorphous silica and at the surface of the crystalline (0 0 0 1) quartz surface. Keywords: amorphous silica, water, ab initio molecular dynamics, geminal silanols (Some figures may appear in colour only in the online journal)
1. Introduction
number of situations, such as in geochemistry, prebiotic chemistry or biomineralization. These two compounds can also be related to human activity, and their interactions are typically of interest in the fields of biomedical devices, sensors or chromatography. Microscopic details of the water–silica interactions are not easily captured by experiments, especially in the case of an amorphous silica surface. The two major experimental data that shed light on the influence of the hard matter (silica) on the soft one, even at short length scales (1 nm or less), are the sum frequency generation (SFG) spectroscopy experiments from Shen and co-workers [2] which for instance reported that water at the
Silica (silicon dioxide, SiO2) is a solid compound present in many different crystalline and amorphous allotropic forms. In nature, it is usually found as quartz, the second most abundant mineral on earth. Silica features and its interaction with water and biomolecules have been recently reviewed [1]. The present introduction summarizes briefly some facts, and more details can be found in [1]. Of particular interest here are the interactions of silica with water. As water and silica are the two most common terrestrial compounds, interactions between these two species occur in a great 0953-8984/14/244106+10$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
(a)
(b)
0
120-150 0
5-25
H
0
30-60
O H Figure 1. Schematic representations of isolated (a), geminal (b) and vicinal (c) silanols encountered at silica surfaces. (Adapted with permission from [1].)
H O
interface with (0 0 0 1) quartz has spectroscopic signatures of both liquid-like and ice-like types (with a certain pH dependence), and that of Engemann et al [3] who evidenced that amorphous silica induces a liquid-like, amorphous ice layer at the silica–ice interface at T lower than the ice melting temperature. Most forms of divided silicas are structurally amorphous and, therefore, few techniques can provide atomic resolution information. It is then clear that experiments alone cannot cope with the complexity of the silica–water interactions. Computer simulation techniques are ideal to provide the missing information about the atomistic interfacial organization. Silica surface functional groups are mainly the surface silanol groups (SiOH). We schematically review below the three main kinds of silanols encountered at the silica surfaces (a schematic picture is also shown in figure 1). A single or terminal SiOH group is called an isolated silanol when the distance to the closest SiOH groups is such that they cannot be involved in H-bond interactions. Silanol groups which are separated by more than ~3.3 Å can be considered as unable to establish mutual hydrogen bond, and therefore are isolated. The isolated silanol groups are then free to establish H-bond interactions with adsorbate molecules, and can possibly act as both H-bond donors and acceptors. Pairs of silanols belonging to tetrahedra that share a common oxygen vertex are normally called vicinals. The two silicon atoms of vicinals are separated by one single oxygen so that the two hydroxyl groups are separated by less than 3 Å. On the disordered surface, it is also possible that silanols that do not belong to directly connected tetrahedra, but are closer than ~3.3 Å, establish H-bonds. They are called interacting or H-bonded silanols. The optimum O···O distance between the two OH involved in a H-bond lies between ~2.5 and ~2.8 Å. Two OH groups linked to the same surface silicon atom to give the Si(OH)2 moiety, are called geminals. Even though they are very close, the two geminal OH groups are oriented in such a way that they cannot be involved in mutual H-bonds. See schemes in figure 1 for illustrations.
O
(c)
H
H 5-250
HO
Figure 2. (a) Top view of the quartz 0001 dry surface. (b), (c) Organization of in-plane and out-of-plane silanols at the interface of (0 0 0 1) quartz with water and the H-bonds formed (intra-silanol H-bonds and silanol–water H-bonds), together with the interfacial water average orientations, taken from [19, 20]. (b) Snapshot of the AIMD simulation box used in [19, 20]. (c) A scheme summarizing the main repeated motif built at the quartz–water interface. (a) is reprinted with permission from [19].
Crystalline silica surfaces are expected to exhibit a regular distribution of the hydroxyls, even if the upper layers can present an amorphous character with a loss of the expected order [4, 5]. A number of ab initio theoretical investigations have been performed on the silica–water interactions [6–36]. Among them, some works have been performed to clarify the (0 0 0 1) quartz hydroxylated surface [16, 19, 20, 36] and the interactions with discrete water molecules [16, 17, 31], water layers [16, 18] and bulk water [19–21, 25, 27, 29]. Ab initio molecular dynamics (AIMD) simulations have been performed on the (0 0 0 1) α-quartz surface (0 0 0 1) [19, 25, 29]. Adeagbo et al showed that the originally clean quartz surface is rapidly hydroxylated after chemical reaction with one water layer and that geminal silanol groups are predominantly formed. In the absence of water, the fully hydroxylated (0 0 0 1) α-quartz surface exhibits a zig-zag hydrogen bond network formed within the quartz surface, with alternating strong and weak H-bonds [18, 19, 21] (figure 2(a)). Once the surface is wetted, this in-plane H-bond network is partially destroyed: two types of silanols, i.e. in-plane and out-of-plane, appear at the quartz surface (figure 2(b)). The in-plane silanols are H-bond donors to an out-of-plane silanol (forming strong H-bonds), while the out-of-plane silanols are H-bond donors to water molecules belonging to the first layer of water at the interface 2
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Table1. Density of geminal silanol groups for the crystalline
(0 0 0 1) quartz dry model surface and for the present amorphous silica dry surface model (upper/lower face). Polymorph and Total OH density surface (OH nm−2) Geminal (0 0 0 1) quartz Amorphous
9.5 5.8
100% 35%
H-bonded 100% 46 % (upper), 54% (lower)
surface–liquid water interface. Namely, are strong interactions between surface silanols and the first interfacial layer of water molecules also present at the interface with amorphous silica? To our best knowledge, no AIMD investigation has been reported until now on an amorphous silica–water interface, although such interfaces are of high relevance in many fundamental and technological areas. Two reasons can explain the lack of such investigations to date: models of amorphous silica sufficiently small to be used in DFT calculations and AIMD simulations are rare [15, 35, 37], while AIMD simulations of oxide–water interfaces are somehow still emerging [19–21, 23, 25, 26, 29, 30, 38–44]. In some works the interaction of the surface with discrete water molecules has been described while the bulk liquid has been included in the calculation through thermodynamics [14, 45–47]. An explicit atomistic description of the solvent–surface organization is however still lacking at the ab initio level. Furthermore, such atomistic local description of the amorphous silica–liquid water interface has not been given with force-field classical molecular dynamics simulations either (see [1] and references therein). In the present work we extend our previous theoretical framework [14, 15, 19, 20] to the AIMD characterization of the amorphous silica–liquid water interface. We benefit from the amorphous silica model developed in [15], a model derived from an amorphous silica bulk originally proposed by Garofalini [48], which has been used until now in dry or microsolvated conditions [15, 49, 50]. The distribution of tetrahedral (SiO4) rings in the solid is mainly of 4, 5 and 6 membered rings, in good agreement with the experimental data [50]. The starting slab model is about 10 Å thick (vertical spacing), with an in-plane cell size of 13 Å×17.5 Å, which resulted in Si27O67H26 composition (120 atoms as a whole). The slab is about three layers thick, with thickness locally varying between 5 and 8 Å, depending on the localization at the surface, see figure 3. The OH density of 5.8 OH nm−2 is the same for the upper and the lower surfaces of our surface model. Our amorphous surface model thus suitably mimics the behaviour of a fully hydroxylated amorphous silica surface (4.5/4.9 OH nm−2). A relatively high population of geminal silanols is present in our model, with an uneven distribution between the two surfaces (23% and 46% for the upper/lower face, respectively). Due to the high OH density, the population of H-bonded SiOH groups at the surface is nearly half of the total (46% and 54% for the upper/lower face, respectively). Gas phase deprotonation energy [15, 51] and lutidine adsorption [51] calculations did not reveal any intrinsic acidity difference between
Figure 3. Surface amorphous silica model used in the present study, showing (a) the unit cell and the rugosity of the surface, and zooms over the surface that highlight specific sites (in blue): (b) a convex geminal silanol pair, (c) a concave geminal silanol pair and (d) an isolated silanol. (a) is reprinted with permission from [1].
(figure 2(b)). Due to the crystalline nature of quartz, these in-plane and out-of-plane silanols alternate at the surface, as schematically represented in the pattern in figure 2(c). One pivotal result from [7, 19, 20] is that the intra-silanols H-bond network is a determining factor of the silanols’ acidity. In particular out-of-plane silanols are about 3pKa units more acidic than the in-plane silanols. The key question of interest is how this picture of the water organization changes when going from the crystalline (0 0 0 1) quartz–liquid water interface to an amorphous 3
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 4. Distribution of the silanols (Si)O–H orientation with respect to the normal to the surface represented by the z-vertical axis (left panel). The angle that characterizes the silanols’ orientation is schematically shown in the right panel. Scheme in right panel: grey = solid, blue = liquid water, O and H are surface silanols (whatever their nature), z-vertical axis is taken as perpendicular to the surface. Full line: all silanols taken into account; dashed line: convex geminal; dotted line: isolated; dotted–dashed line: concave geminal; dotted–dotted–dashed line: vicinals.
and one of them also accepts one weak H-bond (2.3 Å), from neighbour silanols. In contrast, in a geminal pair located in a concave zone, both silanol groups are H-bond donors (H-bond distance 1.8–1.9 Å). We did not observe any trend in H-bond strengths dependent on the nature of the silanols; here the local topology seems the dominant factor for the acceptor/ donor and strong/weak H-bonds. In this contribution we now report and discuss the organization of liquid water at the amorphous silica–liquid water interface once the surface is wetted. The influence of the solvent on the pre-existing surface silanol–silanol H-bond network and the formation of H-bonds between silanols and water molecules are reported. This is then discussed in comparison to our previous results on the crystalline (0 0 0 1) quartz–liquid water interface [19, 20]. 2. Computational details
Figure 5. The OW–OW (continuous line) and the OW–HW (dashed line) RDF for the liquid water between the silica slabs.
Our AIMD simulations follow the same general set-ups used in our previous investigations of solid–liquid interfaces [19, 20, 38, 39]. DFT-based Born–Oppenheimer molecular dynamics (BOMD) simulations have been performed with the CP2K/Quickstep package, using the hybrid Gaussian and plane wave method [52]. The Perdew–Burke–Ernzerhof (PBE) exchange-correlation density functional was used. Goedecker–Teter–Hutter pseudopotentials [53], a double zeta plus polarization (DZVP) Gaussian basis set for the orbitals and a density cutoff of 400 Ry were used. Only the gamma point was considered in a supercell approach. The box dimensions are 12.77 × 17.64 × 25.17 Å3. Periodic boundary conditions are applied in all directions of space. Dynamics were conducted in the microcanonical NVE ensemble (after 5 ps equilibration) with a time step of 0.4 fs. One 22 ps trajectory has been accumulated and analysed (after equilibration), providing the average views discussed hereafter.
geminal and terminal silanols. With the rather high density of geminal silanols in our model, we will indeed be able to compare results from this model to the (0 0 0 1) quartz surface from our previous investigations [19, 20] composed only of geminal silanols. Table 1 summarizes the silanols’ density and nature on both surfaces. As seen in figure 3, the dry amorphous silica surface is not flat (contrary to the crystalline (0 0 0 1) crystal surface) and exhibits concave and convex zones, and the local topology has an influence on the silanols’ directionality. Silanols on convex zones indeed tend to point out of the surface (called out-of-plane silanols to keep our nomenclature from [19, 20]), whereas silanols in concave zones may point towards other silanols that are located in the local cavity. Hence for a geminal pair located in a convex zone of the surface, each SiOH of the pair accepts one strong H-bond (H-bond distance of 1.80 Å), 4
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 6. RDFs for convex surface geminal silanols, between the H/O atoms and the oxygen (OW)/hydrogen (HW) atoms of water molecules that belong to the topmost interfacial water layer above the surface. The black and red lines correspond to the two distinct silanol groups. (a) H…OW RDF; (b) O…HW RDF; (c) (Si–)O…H(–O–Si) RDF between the geminals and all possible surface silanols. Dashed lines correspond to the integration of RDFs thus giving the number of neighbours.
3. Results
interval), and another fraction (16 out of 26) lay within the surface (50°–90° angles).
3.1. Orientation of silanols at the amorphous silica–liquid water interface
3.2. Surface silanols are hydrogen bonded with interfacial water molecules
Figure 4 reports the distribution of angles formed between the O–H silanol vectors and the vector normal to the amorphous surface (here the z-axis is used as the normal to the surface); see scheme in figure 4. It results in a broad distribution of silanol orientations with respect to the surface, with angles varying between 0° and 140°. Most of the silanol orientations with respect to the surface normal are contained within the 30°–80° interval, a trend already observed under vacuum conditions, and thus primarily due to the amorphous silica surface topology. Looking in more detail at the geminal silanols’ orientations we find that the two main angles are 24° and 49° for the convex geminal (dashed line in figure 4), and 61° for the concave geminal (dotted–dashed line in figure 4). Note also that the silanols of the convex geminal pair display two different orientations (two peaks in the distribution), while the silanols of the concave geminal pair are oriented with the same mean angle (one peak in the distribution). These angles are to be compared to the two systematic orientations of the geminals at the crystalline (0 0 0 1) quartz–water interface [19, 20], i.e. in-plane silanols (30°) and out-of-plane silanols (100°). The small bump seen above 110° is related to one vicinal silanol that displays an average angle of 125° during 5 ps of the trajectory and then changes to an orientation of 55° on average for the rest of the trajectory. This result suggests that a silanol has a certain freedom to rotate/bend in the presence of the solvent, a property that was not observed for the silanols H-bonded to their neighbour silanols at the dry surface (vide infra). Most of the vicinal silanols adopt orientations within the interval 30°–80° (dotted–dotted–dashed line in figure 4). The isolated silanol has a mean orientation about 40° (dotted line in figure 4). Silanols at the amorphous silica–liquid water interface thus form a very diverse crowd. A fraction of them, namely 10 out of the total 26 in our model, are pointing out of the amorphous surface (roughly with angles in the 0°–50°
In the following the organization of water at the interface is discussed through the radial pair distribution functions (RDFs). As a preliminary, we report in figure 5 the OW–OW and OW–HW RDFs of the water box above the amorphous silica, where we show that they are identical to bulk water, giving us confidence that the system size is large enough and well equilibrated to simulate liquid water above the surface. Figures 6 and 7 display RDFs between selected surface silanols H/O atoms and oxygen (OW)/hydrogen (HW) atoms of water molecules that belong to the topmost interfacial water layer above the surface (first layer of water molecules above the amorphous surface). The surface convex geminal H…OW and O…HW RDFs are respectively presented in figures 6(a) and (b), together with the resulting number of neighbours (integral of RDF). We can see that the two geminal Si–O–H groups point towards the solvent and form strong H-bonds with interfacial water molecules as shown by the first peak in the H…OW RDFs located at quite short distances (1.56 and 1.66 Å). The peaks are also rather intense with a first minimum around 2.50 Å close to zero, which suggest a strong water organization and thus low mobility. Each geminal silanol forms one H-bond with one water molecule on average (dashed line in figure 6). There is a subsequent second peak in the RDFs located around 3.5–4.0 Å showing that the second layer of water molecules around the two geminal silanols is also well structured. While the two H atoms of the geminal silanols are strong H-bond donors to water molecules, the oxygens of these Si–O–H are not simultaneously H-bond acceptors with water. This can be inferred from the first peaks positions in the (Si–)O…HW RDFs in figure 6(b), respectively located at 3.0 Å and 3.5 Å for each Si–O–H of the geminal group. These distances are too long for hydrogen bonds, and the closest water molecules 5
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 7. RDFs between the isolated surface silanol H/O atoms and oxygen (OW)/hydrogen (HW) atoms of water molecules that belong to the topmost interfacial water layer above the surface. (a) (Si–O)H…OW RDF; (b) (Si–)O…HW RDF. Dashed lines correspond to the integration of RDFs thus giving the number of neighbours.
on the dynamical behaviour of these H-bonds. The stronger H-bond has a lifetime comparable to the trajectory timescale, while the weaker oxygen H-bonds can form and break a few times over the same timescale. The situation is slightly different for the concave geminal where each silanol is giving a strong H-bond to a neighbour silanol in the vacuum situation. In the presence of interfacial liquid water, we observed that one strong H-bond was conserved (at a distance of 1.75 Å), whereas the other initial H-bond has been rapidly broken. Instead a strong H-bond with a water molecule was formed (at the average distance of 1.6 Å) and is found stable for the whole trajectory. It thus appears that in a concave region, water can be trapped and strongly stabilized in interaction with the surface, if sterically allowed. The solvation structure for the isolated silanol (isolated refers to the state of the silanol not H-bonded at the dry surface) is given in figure 7. In the RDF of (Si–O)H…OW one can see a first peak located at 1.50–1.60 Å, with peak intensity around 4.0, then a subsequent minimum located at around 2.5 Å whose intensity is close to zero, and two subsequent peaks respectively around 3.5 and 6.5 Å. This means that the isolated silanol from the dry surface now is donating, on average, one H-bond to one water molecule once the surface is wetted, and cannot thus be strictly speaking denoted as isolated anymore. The strength of such a H-bond is similar to that of the H-bond donated from the geminal silanols to water (as estimated by the ratio of intensities of first peak and the subsequent minimum in the RDFs). As already observed for the convex geminal group, the zero intensity for the minimum subsequent to the first peak in all RDFs suggests that there is no water diffusion during the length-scale of our dynamics. Simultaneously to being H-bond donor to an interfacial water molecule, this silanol is also accepting one H-bond from one interfacial water molecule, as can be observed from the first peak position at 1.76 Å in the silanol (Si–)O…HW RDF displayed in figure 7(b).
Figure 8. RDFs between vicinal silanols O/H atoms and HW/ OW water molecules. Black: O…HW, red: H…OW. Dashed lines correspond to the integration of RDFs thus giving the number of neighbours.
to the oxygens of these Si–O–H silanols belong to the second hydration shell of the geminal group. In figure 6(c), the (Si–)O…H(–O–Si) RDFs between each oxygen of the two Si–O–H silanols of the convex geminal group and any H atom that belongs to the rest of the silanols at the amorphous silica surface are reported. We observe that the two geminal silanols are not equivalent. One silanol displays a first peak at 1.71 Å corresponding to 0.65 H-bond accepted on average from a neighbouring vicinal silanol. The second geminal oxygen displays an even more intense first peak in the (Si)O…H(O–Si) RDF located at a slightly shorter distance of 1.65 Å, and forming an average 1.9 H-bond with H atoms from neighbouring vicinal silanols. Note that these H-bonds are reinforced as compared to those formed in vacuum, where the H-bond distances were 1.80 Å and 2.00 Å respectively. The same tendency was already observed in quartz (1 0 0) by Musso et al [21] where the strong inter-silanols H-bonds were maintained and even reinforced whereas the weak ones were broken in the presence of solvent. We furthermore comment 6
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 9. Distribution of orientation of the water molecules’ dipole vector with respect to the vector normal to the amorphous surface (taken as the z-vertical axis, see scheme on the right-hand side). Solid line: no distinction among the nature of silanols taken into account; Dotted–dashed line: water molecules H-bonded to the isolated silanol only; dashed line: water molecules H-bonded to geminal silanols only; dotted line: water molecules H-bonded to vicinal silanols only.
Interestingly, the water molecules H-bonded to the geminal silanols have their dipole oriented outwards these silanols, with angles in the 20°–80° range only (dashed line in figure 9). The two water molecules H-bonded to the isolated silanol, one donor and one acceptor, adopt an orientation roughly between 20° and 70°. The water acceptor dipole points outward away from the surface, while the water donor dipole points more directly towards the surface (dotted–dashed line in figure 9). Also as expected, the two populations of water molecules H-bonded to the vicinal silanols (donor/acceptor of H-bonds) adopt two general orientations of their dipoles, either outward from the surface (angle below 60°) or towards the surface (angle above 60°) (dotted line in figure 9).
All but six vicinal silanols are H-bond donors to one water molecule, and half of them simultaneously receive one H-bond from another water molecule. See figure 8 for the RDFs between vicinal O/H atoms and HW/OW atoms of the water molecules, and associated H-bond distances with the first peak of these RDFs. The six remaining vicinals are either H-bonded to another vicinal silanol (two of them), or H-bonded to geminal silanols (four of them). The vicinalwater H-bonds are dynamical in essence, as can be observed from time evolutions along the AIMD trajectory: H-bonds can be formed and broken rather easily over the 22 ps dynamics, and exchange of the water molecule involved can also be observed over this period of time. We have also verified that the H-bond donor and H-bond acceptor water molecules are different molecules, for all vicinals. We have also observed some interchange of H-bonded water molecules only for the H-bond donor waters to these silanols, never for the H-bond acceptor water molecules.
3.4. The geminal silanol groups are a source of local water H-bond networks
The previous investigations have shown the extent of the delocalized H-bonding network between the amorphous silica surface and the interfacial water molecules within the first layer over the top of the surface. As it can be seen from the snapshots illustrated in figures 10(a) and (b), there are also well-defined local H-bond networks at the interface organized around the geminal groups (illustration here around the convex geminal): there is a ring of H-bonded water network organized between the pair of silanols of the convex geminal group. We have identified for this convex geminal two networks over the 22 ps trajectory: a five-membered ring in which three water molecules are forming a H-bonded network bridge from one silanol of the geminal group to the other silanol, and a six-membered ring involving four water molecules in the H-bonded network bridge. The two rings are illustrated by the snapshots (figures 10(a) and (b)) and by the plots of the evolution with time of H2O…H2O distances (figure 10(c). The change from the five-membered to the six-membered ring occurs around 4 ps along the trajectory. Similarly, a five-membered ring based on three water molecules exists between the pair of silanols of the concave
3.3. Orientation of the water molecules at the interface
The distribution of orientation of the water dipole vector with respect to the vector normal to the amorphous surface (taken as the z-vertical axis) is presented in figure 9, see also scheme in the figure, for the water molecules located at the interfacial layer. Three main domains can be seen. Domain 1 corresponds roughly to 0°–60° orientation angles and therefore represents water molecules whose dipoles are on average pointing out from the amorphous silica surface. They are the water molecules accepting H-bonds from out-of-plane silanols at the interface. Domain 2 roughly goes from 70° to 140°, and thus represents water molecules whose dipoles point towards the amorphous surface. On average, one water O–H group is donating an H-bond to one surface silanol. Domain 3 represents orientations in the 140°–180°, therefore water molecules pointing their two O–H groups towards the surface. 7
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 10. Local H-bond networks around the convex geminal group. (a) A five-membered ring in which three water molecules bridge
from one silanol of the geminal group to the other silanol (five oxygen vertices), and (b) a six-membered ring involving four water molecules and the two OH groups from the geminal (six oxygen vertices); (c) time evolution of the H2O…H2O distances for a fourmembered ring that changes into the five-membered (change occurs around t = 4 ps).
geminal. Here, no opening of the ring and incorporation of supplementary water molecule(s) is observed during the length of the simulation, presumably because of steric hindrance. It is interesting to notice that such pre-organized water networks could have an important impact on proton mobility along the silica–water interface, providing e.g. special channels for proton transfer, a property also found on boehmite [39].
therefore respectively acting as H-bond acceptors to water molecules and as H-bond donors to water molecules on average. This results in an alternating motif within the interfacial quartz–water layer, schematically illustrated in figure 2(c). No hydrogen bonds between these interfacial water molecules have been found to exist for a sufficient amount of time. At the amorphous silica–water interface, we found that all silanol groups donate strong H-bonds to the water molecules in the first adsorbed layer at the amorphous silica interface. The strength of such H-bonds is overall comparable to that of the H-bonds donated by the geminal silanols of the (0 0 0 1) crystalline quartz surface in contact with water. We hence found that geminal silanols and (most of the) vicinal silanols are involved in hydrogen bonds with water molecules, similarly to the quartz– water interface. There are however no privileged directionalities of the silanol–water H-bonds, contrary to the two systematic orientations observed at the quartz–water interface. We have found that the broad distribution of orientations of the silanols with respect to the normal to the amorphous surface is maintained once the surface is wetted (in the 30°–80° range). As a consequence, the water molecules can be roughly grouped into three categories, either pointing their dipole moment towards silanols and thus acting as H-bond donors to the surface, or having their dipole moment roughly lying parallel to the surface and acting as H-bond donors to the surface, or pointing their dipole moment out of the surface and acting as H-bond acceptors to the surface. The average amorphous silica–water H-bond network view obtained from our simulations is schematically illustrated in figure 11. On average, the vicinal silanols form two simultaneous H-bonds with two separate water molecules, i.e. one H-bond donor, one H-bond acceptor (left side of the scheme). The same is obtained for the ‘isolated silanol’ (isolated at the dry surface) that becomes involved into two H-bonds with two water molecules when the surface is wetted. Roughly the same average distances as for vicinals are obtained. The two silanols of the convex geminal simultaneously donate one H-bond to one water molecule. Again strong H-bonds are formed, according to the very short H-bond distances of 1.56
3.5. Discussion of amorphous versus crystalline silica at the interface with water
We are now in a position to discuss similarities and differences between the amorphous silica–water interface and the (0 0 0 1) crystalline quartz–water interface (see [19, 20]) from the point of view of interfacial structure (surface and water). Before discussing the interface with water, one striking remark has to be made for the dry surfaces. At the dry planar quartz surface, all geminal silanols are H-bonded within the surface, thus forming a regular in-plane H-bond zig-zagging network, as illustrated in figure 2(a). At the non-planar dry amorphous surface, the non-crystalline local topology dictates the structural arrangement between the three types of silanols (isolated, geminal, vicinal, figure 1). This results in geminals forming H-bonds with neighbouring vicinal silanols at the surface. Also due to the non-planar topology and thus the existence of concave/convex zones at the surface, the silanols at the amorphous silica surface display varied orientations with respect to the surface, and are found mostly pointing out of the surface. This is at odds with the dry (0 0 0 1) crystalline quartz surface. Once liquid water is added to the (0 0 0 1) crystalline quartz surface, we have shown [19, 20] that the geminal silanols are distributed into two populations, i.e. one that keeps in-plane orientations, one that adopts out-of-plane orientations, with average angle values with respect to the normal to the surface respectively of 30° and 100°. This results in the breakage of the regular in-plane silanol H-bond network that was observed at the surface of dry quartz. These two silanol populations are 8
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 11. Scheme of the main H-bond networks between geminal/isolated/vicinal silanols at the surface of amorphous silica and interfacial
water molecules. Grey = solid, blue = liquid water. Dashed lines represent H-bonds. Numbers are average H-bond distances in Ångstroms.
and 1.66 Å. These silanols are acting only as H-bond donors towards water. These two silanols are simultaneously H-bond acceptors with neighbour vicinal silanols, forming H-bonds of 1.65–1.75 Å distances on average. For the concave geminal, both silanols are H-bond donors to water, and one silanol of the pair is an H-bond acceptor from a neighbour vicinal. We have also observed local nests of water–water H-bonds linking the silanol pairs of these geminals: four- and five-membered H-bond rings have been observed. This is strikingly different from our observations at the crystalline quartz–water interface.
and convex and concave geminal silanols. A local molecular organization occurs around geminal silanols with water bridges being persistent during the 22 ps time of the dynamics. Convex geminals have water rings that can open and accommodate one supplementary water molecule, while concave geminals represent a more ‘fixed’ situation with water molecules trapped in the local cavity (over the time scales of our dynamics). The arrangement of interfacial water molecules at the amorphous silica surface is thus in essence very much different from the one at the crystalline quartz (0 0 0 1) surface. This does not only reflect a very different topological surface, thus dictating some structural organization of water, but this should also reflect the different chemical nature of the amorphous silica silanols. This can be inferred by comparing pKa values of the silanols at the amorphous and crystalline surfaces. Silanols’ pKa values have been calculated at the quartz– water interface in our previous works [19, 20], with a bimodal behaviour of the in-plane and out-of-plane silanols, nicely mirroring the bimodal behaviour recorded in SHG experiments. We have also shown how the silanols’ acidities at the quartz–water interface dictate the arrangement of water molecules [19, 20]. Our next step in the analysis of the amorphous silica–water interface is to calculate silanols’ pKa’s, and the relationship with the water structural arrangement obtained in the present work.
4. Conclusions and perspectives The amorphous silica–water interface has been investigated by means of DFT-based molecular dynamics simulations, providing a very detailed knowledge of the interfacial structural organization. Using a unique model of the amorphous silica surface, we have described the structure of the interface of two disordered systems in contact with each other, one liquid, one solid, at the DFT ab initio level. To the best of our knowledge, this is a pioneering work on the amorphous silica–liquid water system at this level of calculation. The interface model is built as a relevant and representative amorphous silica surface containing isolated, H-bonded vicinal and geminal silanols. The silanols donate strong H-bonds to the first adsorbed layer of water, as already observed on the (0 0 0 1) quartz surface in contact with water. In contrast to the previously observed organization of interfacial water at crystalline quartz (0 0 0 1) [19, 20], the interfacial water layer, albeit exhibiting some degree of favourite orientation towards the surface, is on average very much disordered. This probably represents a less dipolar water layer at the interface with amorphous silica, when compared to the water layer at the crystalline silica interface. The layer of water molecules at the amorphous silica surface is not mobile, to such an extent that an adsorbed water molecule would probably not be easily replaced by adsorbate organic molecules. The average disorder of the first water layer at the interface is complemented by local organization. Three specific zones were scrutinized, with increasing local order: vicinal silanols,
Acknowledgments GENCI under the project c2013082217 at IDRIS and CINES supercomputer centers in France are acknowledged. N Folliet is thanked for providing initial configurations to the present AIMD simulations. References [1] Rimola A, Costa D, Sodupe M, Lambert J F and Ugliengo P 2013 Chem. Rev. 113 4216 [2] Ostroverkhov V, Waychunas G A and Shen Y R 2004 Chem. Phys. Lett. 386 144 9
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
[28] Fernandez-Cata G, Perez-Gramatges A, Alvarez L J, Comas-Rojas H and Zicovich-Wilson C M 2009 J. Phys. Chem. C 113 13309 [29] Adeagbo W A, Doltsinis N L, Klevakina K and Renner J 2008 Chem Phys Chem 9 994 [30] Argyris D, Tummala N R, Striolo A and Cole D R 2008 J. Phys. Chem. C 112 13587 [31] Wander M C F and Clark A E 2008 J. Phys. Chem. C 112 19986 [32] Yang J J and Wang E G 2006 Phys. Rev. B 73 035406 [33] Lu Z Y, Sun Z Y, Li Z S and An L J 2005 J. Phys. Chem. B 109 5678 [34] Yang J J, Meng S, Xu L F and Wang E G 2005 Phys. Rev. B 71 035413 [35] Masini P and Bernasconi M 2002 J. Phys.: Condens. Matter 14 4133 [36] Musso F, Sodupe M, Corno M and Ugliengo P 2009 J. Phys. Chem. C 113 17876 [37] Ugliengo P, Sodupe M, Musso F, Bush I J, Orlando R and Dovesi R 2008 Adv. Mater. 20 4579 [38] Motta A, Gaigeot M P and Costa D 2012 J. Phys. Chem. C 116 23418 [39] Motta A, Gaigeot M P and Costa D 2012 J. Phys. Chem. C 116 12514 [40] Argyris D, Cole D R and Striolo A 2009 Langmuir 25 8025 [41] Argyris D, Ho T, Cole D R and Striolo A 2011 J. Phys. Chem. C 115 2038 [42] Cheng J and Sprik M 2010 J. Chem. Theory Comput. 6 880 [43] Raybaud P, Digne M, Iftimie R, Wellens W, Euzen P and Toulhoat H 2001 J. Catal. 201 236 [44] Sulpizi M and Sprik M 2010 J. Phys.: Condens. Matter 22 284116 [45] Lodziana Z, Norskov J K and Stoltze P 2003 J. Chem. Phys. 118 11179 [46] Hemeryck A, Motta A, Swiatowska J, Pereira-Nabais C, Marcus P and Costa D 2013 Phys. Chem. Chem. Phys. 15 10824 [47] Chiche D, Chizallet C, Durupthy O, Channeac C, Revel R, Raybaud P and Jolivet J-P 2009 Phys. Chem. Chem. Phys. 11 11310 [48] Garofalini S H 1990 J. Non-Cryst. Solids 120 1 [49] Folliet N, Gervais C, Costa D, Laurent G, Babonneau F, Stievano L, Lambert J-F and Tielens F 2013 J. Phys. Chem. C 117 4104 [50] Folliet N et al 2011 J. Am. Chem. Soc. 133 16815 [51] Galeener F L 1982 J. Non-Cryst. Solids 49 53 [52] Leydier F, Chizallet C, Chaumonnot A, Digne M, Soyer E, Quoineaud A-A, Costa D and Raybaud P 2011 J. Catal. 284 215 [53] The cp2k Developers Group. http://cp2k.berlios.de [54] Goedecker S, Teter M and Hutter J 1996 Phys. Rev. B 54 1703 [55] Ong S W, Zhao X L and Eisenthal K B 1992 Chem. Phys. Lett. 191 327
[3] Engemann S, Reichert H, Dosch H, Bilgram J, Honkimaki V and Snigirev A 2004 Phys. Rev. Lett. 92 205701 [4] Fubini B 1998 The Surface Properties of Silicas ed A P Legrand (Chichester: Wiley) p 415 [5] Li I, Bandara J and Shultz M J 2004 Langmuir 20 10474 [6] Musso F, Ugliengo P and Sodupe M 2011 J. Phys. Chem. A 115 11221 [7] Criscenti L J, Kubicki J D and Brantley S L 2006 J. Phys. Chem. A 110 198 [8] Kubicki J D, Solo J O, Skelton A A and Bandura A V 2012 J. Phys. Chem. C 116 17479 [9] Tosoni S, Civalleri B, Pascale F and Ugliengo P 2008 Ab Initio Simulation of Crystalline Solids: History and Prospects— Contributions in Honor of Cesare Pisani (IOP Conf. Series vol 117), ed R Dovesi et al (Bristol: IOP Publishing) pp 12026 [10] Tosoni S, Civalleri B and Ugliengo P 2010 J. Phys. Chem. C 114 19984 [11] Guo X Y, Watermann T, Keane S, Allolio C and Sebastiani D 2012 Z. Phys. Chem.—Int. J. Res. Phys. Chem. Chem. Phys. 226 1415 [12] Bandura A V, Kubicki J D and Sofo J O 2011 J. Phys. Chem. C 115 5756 [13] Minibaev R F, Zhuravlev N A, Bagatur’yantz A A and Alfimov M V 2009 Russ. Phys. J. 52 1164 [14] Costa D, Tougerti A, Tielens F, Gervais C, Stievano L and Lambert J F 2008 Phys. Chem. Chem. Phys. 10 6360 [15] Tielens F, Gervais C, Lambert J F, Mauri F and Costa D 2008 Chem. Mater. 20 3336 [16] Yang J and Wang E G 2006 Phys. Rev. B 73 035406 [17] Musso F, Ugliengo P and Sodupe M 2011 J. Phys. Chem. A 115 11221 [18] Chen Y-W, Chu I-H, Wang Y and Cheng H-P 2011 Phys. Rev. B 84 155444 [19] Sulpizi M, Gaigeot M-P and Sprik M 2012 J. Chem. Theory Comput. 8 1037 [20] Gaigeot M-P, Sprik M and Sulpizi M 2012 J. Phys.: Condens. Matter 24 124106 [21] Musso F, Mignon P, Ugliengo P and Sodupe M 2012 Phys. Chem. Chem. Phys. 14 10507 [22] Zhao Y L 2012 J. Theor. Comput. Chem. 11 155 [23] Skelton A A, Fenter P, Kubicki J D, Wesolowski D J and Cummings P T 2011 J. Phys. Chem. C 115 2076 [24] Chen Y W, Chu I H, Wang Y and Cheng H P 2011 Phys. Rev. B 84 155444 [25] Leung K, Nielsen I M B and Criscenti L J 2009 J. Am. Chem. Soc. 131 18358 [26] Argyris D, Cole D R and Striolo A 2009 J. Phys. Chem. C 113 19591 [27] Tilocca A and Cormack A N 2009 ACS Appl. Mater. Interfaces 1 1324
10
Search
Collections
Journals
About
Contact us
My IOPscience
The amorphous silica–liquid water interface studied by ab initio molecular dynamics (AIMD): local organization in global disorder
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 J. Phys.: Condens. Matter 26 244106 (http://iopscience.iop.org/0953-8984/26/24/244106) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 129.137.5.42 This content was downloaded on 13/11/2014 at 18:41
Please note that terms and conditions apply.
Journal of Physics: Condensed Matter J. Phys.: Condens. Matter 26 (2014) 244106 (10pp)
doi:10.1088/0953-8984/26/24/244106
The amorphous silica–liquid water interface studied by ab initio molecular dynamics (AIMD): local organization in global disorder Álvaro Cimas1, Frederik Tielens2, Marialore Sulpizi3, Marie-Pierre Gaigeot1,4 and Dominique Costa5 1
Laboratoire Analyse et Modélisation pour la Biologie et l’Environnement, LAMBE UMR CNRS 8587, Université d’Evry val d’Essonne, Blvd F Mitterrand, Bat. Maupertuis, 91025 Evry, France 2 Sorbonne Université, UPMC Univ. Paris 06, UMR 7574, Laboratoire Chimie de la Matière Condensée, Collège de France, 11 place Marcelin Berthelot, 75231 Paris Cedex 05, France 3 Johannes Gutenberg University Mainz, Staudinger Weg 7, 55099 Mainz, Germany 4 Institut Universitaire de France IUF, 103 Blvd St Michel, 75005 Paris, France 5 Institut de Recherches de Chimie de Paris, Chimie Paristech CNRS, UMR8247, 11 rue P et M Curie, 75005 Paris, France E-mail: [email protected] and [email protected] Received 13 November 2013, revised 07 February 2014 Accepted for publication 24 February 2014 Published 27 May 2014 Abstract
The structural organization of water at a model of amorphous silica–liquid water interface is investigated by ab initio molecular dynamics (AIMD) simulations at room temperature. The amorphous surface is constructed with isolated, H-bonded vicinal and geminal silanols. In the absence of water, the silanols have orientations that depend on the local surface topology (i.e. presence of concave and convex zones). However, in the presence of liquid water, only the strong inter-silanol H-bonds are maintained, whereas the weaker ones are replaced by H-bonds formed with interfacial water molecules. All silanols are found to act as H-bond donors to water. The vicinal silanols are simultaneously found to be H-bond acceptors from water. The geminal pairs are also characterized by the formation of water H-bonded rings, which could provide special pathways for proton transfer(s) at the interface. The first water layer above the surface is overall rather disordered, with three main domains of orientations of the water molecules. We discuss the similarities and differences in the structural organization of the interfacial water layer at the surface of the amorphous silica and at the surface of the crystalline (0 0 0 1) quartz surface. Keywords: amorphous silica, water, ab initio molecular dynamics, geminal silanols (Some figures may appear in colour only in the online journal)
1. Introduction
number of situations, such as in geochemistry, prebiotic chemistry or biomineralization. These two compounds can also be related to human activity, and their interactions are typically of interest in the fields of biomedical devices, sensors or chromatography. Microscopic details of the water–silica interactions are not easily captured by experiments, especially in the case of an amorphous silica surface. The two major experimental data that shed light on the influence of the hard matter (silica) on the soft one, even at short length scales (1 nm or less), are the sum frequency generation (SFG) spectroscopy experiments from Shen and co-workers [2] which for instance reported that water at the
Silica (silicon dioxide, SiO2) is a solid compound present in many different crystalline and amorphous allotropic forms. In nature, it is usually found as quartz, the second most abundant mineral on earth. Silica features and its interaction with water and biomolecules have been recently reviewed [1]. The present introduction summarizes briefly some facts, and more details can be found in [1]. Of particular interest here are the interactions of silica with water. As water and silica are the two most common terrestrial compounds, interactions between these two species occur in a great 0953-8984/14/244106+10$33.00
1
© 2014 IOP Publishing Ltd Printed in the UK
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
(a)
(b)
0
120-150 0
5-25
H
0
30-60
O H Figure 1. Schematic representations of isolated (a), geminal (b) and vicinal (c) silanols encountered at silica surfaces. (Adapted with permission from [1].)
H O
interface with (0 0 0 1) quartz has spectroscopic signatures of both liquid-like and ice-like types (with a certain pH dependence), and that of Engemann et al [3] who evidenced that amorphous silica induces a liquid-like, amorphous ice layer at the silica–ice interface at T lower than the ice melting temperature. Most forms of divided silicas are structurally amorphous and, therefore, few techniques can provide atomic resolution information. It is then clear that experiments alone cannot cope with the complexity of the silica–water interactions. Computer simulation techniques are ideal to provide the missing information about the atomistic interfacial organization. Silica surface functional groups are mainly the surface silanol groups (SiOH). We schematically review below the three main kinds of silanols encountered at the silica surfaces (a schematic picture is also shown in figure 1). A single or terminal SiOH group is called an isolated silanol when the distance to the closest SiOH groups is such that they cannot be involved in H-bond interactions. Silanol groups which are separated by more than ~3.3 Å can be considered as unable to establish mutual hydrogen bond, and therefore are isolated. The isolated silanol groups are then free to establish H-bond interactions with adsorbate molecules, and can possibly act as both H-bond donors and acceptors. Pairs of silanols belonging to tetrahedra that share a common oxygen vertex are normally called vicinals. The two silicon atoms of vicinals are separated by one single oxygen so that the two hydroxyl groups are separated by less than 3 Å. On the disordered surface, it is also possible that silanols that do not belong to directly connected tetrahedra, but are closer than ~3.3 Å, establish H-bonds. They are called interacting or H-bonded silanols. The optimum O···O distance between the two OH involved in a H-bond lies between ~2.5 and ~2.8 Å. Two OH groups linked to the same surface silicon atom to give the Si(OH)2 moiety, are called geminals. Even though they are very close, the two geminal OH groups are oriented in such a way that they cannot be involved in mutual H-bonds. See schemes in figure 1 for illustrations.
O
(c)
H
H 5-250
HO
Figure 2. (a) Top view of the quartz 0001 dry surface. (b), (c) Organization of in-plane and out-of-plane silanols at the interface of (0 0 0 1) quartz with water and the H-bonds formed (intra-silanol H-bonds and silanol–water H-bonds), together with the interfacial water average orientations, taken from [19, 20]. (b) Snapshot of the AIMD simulation box used in [19, 20]. (c) A scheme summarizing the main repeated motif built at the quartz–water interface. (a) is reprinted with permission from [19].
Crystalline silica surfaces are expected to exhibit a regular distribution of the hydroxyls, even if the upper layers can present an amorphous character with a loss of the expected order [4, 5]. A number of ab initio theoretical investigations have been performed on the silica–water interactions [6–36]. Among them, some works have been performed to clarify the (0 0 0 1) quartz hydroxylated surface [16, 19, 20, 36] and the interactions with discrete water molecules [16, 17, 31], water layers [16, 18] and bulk water [19–21, 25, 27, 29]. Ab initio molecular dynamics (AIMD) simulations have been performed on the (0 0 0 1) α-quartz surface (0 0 0 1) [19, 25, 29]. Adeagbo et al showed that the originally clean quartz surface is rapidly hydroxylated after chemical reaction with one water layer and that geminal silanol groups are predominantly formed. In the absence of water, the fully hydroxylated (0 0 0 1) α-quartz surface exhibits a zig-zag hydrogen bond network formed within the quartz surface, with alternating strong and weak H-bonds [18, 19, 21] (figure 2(a)). Once the surface is wetted, this in-plane H-bond network is partially destroyed: two types of silanols, i.e. in-plane and out-of-plane, appear at the quartz surface (figure 2(b)). The in-plane silanols are H-bond donors to an out-of-plane silanol (forming strong H-bonds), while the out-of-plane silanols are H-bond donors to water molecules belonging to the first layer of water at the interface 2
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Table1. Density of geminal silanol groups for the crystalline
(0 0 0 1) quartz dry model surface and for the present amorphous silica dry surface model (upper/lower face). Polymorph and Total OH density surface (OH nm−2) Geminal (0 0 0 1) quartz Amorphous
9.5 5.8
100% 35%
H-bonded 100% 46 % (upper), 54% (lower)
surface–liquid water interface. Namely, are strong interactions between surface silanols and the first interfacial layer of water molecules also present at the interface with amorphous silica? To our best knowledge, no AIMD investigation has been reported until now on an amorphous silica–water interface, although such interfaces are of high relevance in many fundamental and technological areas. Two reasons can explain the lack of such investigations to date: models of amorphous silica sufficiently small to be used in DFT calculations and AIMD simulations are rare [15, 35, 37], while AIMD simulations of oxide–water interfaces are somehow still emerging [19–21, 23, 25, 26, 29, 30, 38–44]. In some works the interaction of the surface with discrete water molecules has been described while the bulk liquid has been included in the calculation through thermodynamics [14, 45–47]. An explicit atomistic description of the solvent–surface organization is however still lacking at the ab initio level. Furthermore, such atomistic local description of the amorphous silica–liquid water interface has not been given with force-field classical molecular dynamics simulations either (see [1] and references therein). In the present work we extend our previous theoretical framework [14, 15, 19, 20] to the AIMD characterization of the amorphous silica–liquid water interface. We benefit from the amorphous silica model developed in [15], a model derived from an amorphous silica bulk originally proposed by Garofalini [48], which has been used until now in dry or microsolvated conditions [15, 49, 50]. The distribution of tetrahedral (SiO4) rings in the solid is mainly of 4, 5 and 6 membered rings, in good agreement with the experimental data [50]. The starting slab model is about 10 Å thick (vertical spacing), with an in-plane cell size of 13 Å×17.5 Å, which resulted in Si27O67H26 composition (120 atoms as a whole). The slab is about three layers thick, with thickness locally varying between 5 and 8 Å, depending on the localization at the surface, see figure 3. The OH density of 5.8 OH nm−2 is the same for the upper and the lower surfaces of our surface model. Our amorphous surface model thus suitably mimics the behaviour of a fully hydroxylated amorphous silica surface (4.5/4.9 OH nm−2). A relatively high population of geminal silanols is present in our model, with an uneven distribution between the two surfaces (23% and 46% for the upper/lower face, respectively). Due to the high OH density, the population of H-bonded SiOH groups at the surface is nearly half of the total (46% and 54% for the upper/lower face, respectively). Gas phase deprotonation energy [15, 51] and lutidine adsorption [51] calculations did not reveal any intrinsic acidity difference between
Figure 3. Surface amorphous silica model used in the present study, showing (a) the unit cell and the rugosity of the surface, and zooms over the surface that highlight specific sites (in blue): (b) a convex geminal silanol pair, (c) a concave geminal silanol pair and (d) an isolated silanol. (a) is reprinted with permission from [1].
(figure 2(b)). Due to the crystalline nature of quartz, these in-plane and out-of-plane silanols alternate at the surface, as schematically represented in the pattern in figure 2(c). One pivotal result from [7, 19, 20] is that the intra-silanols H-bond network is a determining factor of the silanols’ acidity. In particular out-of-plane silanols are about 3pKa units more acidic than the in-plane silanols. The key question of interest is how this picture of the water organization changes when going from the crystalline (0 0 0 1) quartz–liquid water interface to an amorphous 3
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 4. Distribution of the silanols (Si)O–H orientation with respect to the normal to the surface represented by the z-vertical axis (left panel). The angle that characterizes the silanols’ orientation is schematically shown in the right panel. Scheme in right panel: grey = solid, blue = liquid water, O and H are surface silanols (whatever their nature), z-vertical axis is taken as perpendicular to the surface. Full line: all silanols taken into account; dashed line: convex geminal; dotted line: isolated; dotted–dashed line: concave geminal; dotted–dotted–dashed line: vicinals.
and one of them also accepts one weak H-bond (2.3 Å), from neighbour silanols. In contrast, in a geminal pair located in a concave zone, both silanol groups are H-bond donors (H-bond distance 1.8–1.9 Å). We did not observe any trend in H-bond strengths dependent on the nature of the silanols; here the local topology seems the dominant factor for the acceptor/ donor and strong/weak H-bonds. In this contribution we now report and discuss the organization of liquid water at the amorphous silica–liquid water interface once the surface is wetted. The influence of the solvent on the pre-existing surface silanol–silanol H-bond network and the formation of H-bonds between silanols and water molecules are reported. This is then discussed in comparison to our previous results on the crystalline (0 0 0 1) quartz–liquid water interface [19, 20]. 2. Computational details
Figure 5. The OW–OW (continuous line) and the OW–HW (dashed line) RDF for the liquid water between the silica slabs.
Our AIMD simulations follow the same general set-ups used in our previous investigations of solid–liquid interfaces [19, 20, 38, 39]. DFT-based Born–Oppenheimer molecular dynamics (BOMD) simulations have been performed with the CP2K/Quickstep package, using the hybrid Gaussian and plane wave method [52]. The Perdew–Burke–Ernzerhof (PBE) exchange-correlation density functional was used. Goedecker–Teter–Hutter pseudopotentials [53], a double zeta plus polarization (DZVP) Gaussian basis set for the orbitals and a density cutoff of 400 Ry were used. Only the gamma point was considered in a supercell approach. The box dimensions are 12.77 × 17.64 × 25.17 Å3. Periodic boundary conditions are applied in all directions of space. Dynamics were conducted in the microcanonical NVE ensemble (after 5 ps equilibration) with a time step of 0.4 fs. One 22 ps trajectory has been accumulated and analysed (after equilibration), providing the average views discussed hereafter.
geminal and terminal silanols. With the rather high density of geminal silanols in our model, we will indeed be able to compare results from this model to the (0 0 0 1) quartz surface from our previous investigations [19, 20] composed only of geminal silanols. Table 1 summarizes the silanols’ density and nature on both surfaces. As seen in figure 3, the dry amorphous silica surface is not flat (contrary to the crystalline (0 0 0 1) crystal surface) and exhibits concave and convex zones, and the local topology has an influence on the silanols’ directionality. Silanols on convex zones indeed tend to point out of the surface (called out-of-plane silanols to keep our nomenclature from [19, 20]), whereas silanols in concave zones may point towards other silanols that are located in the local cavity. Hence for a geminal pair located in a convex zone of the surface, each SiOH of the pair accepts one strong H-bond (H-bond distance of 1.80 Å), 4
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 6. RDFs for convex surface geminal silanols, between the H/O atoms and the oxygen (OW)/hydrogen (HW) atoms of water molecules that belong to the topmost interfacial water layer above the surface. The black and red lines correspond to the two distinct silanol groups. (a) H…OW RDF; (b) O…HW RDF; (c) (Si–)O…H(–O–Si) RDF between the geminals and all possible surface silanols. Dashed lines correspond to the integration of RDFs thus giving the number of neighbours.
3. Results
interval), and another fraction (16 out of 26) lay within the surface (50°–90° angles).
3.1. Orientation of silanols at the amorphous silica–liquid water interface
3.2. Surface silanols are hydrogen bonded with interfacial water molecules
Figure 4 reports the distribution of angles formed between the O–H silanol vectors and the vector normal to the amorphous surface (here the z-axis is used as the normal to the surface); see scheme in figure 4. It results in a broad distribution of silanol orientations with respect to the surface, with angles varying between 0° and 140°. Most of the silanol orientations with respect to the surface normal are contained within the 30°–80° interval, a trend already observed under vacuum conditions, and thus primarily due to the amorphous silica surface topology. Looking in more detail at the geminal silanols’ orientations we find that the two main angles are 24° and 49° for the convex geminal (dashed line in figure 4), and 61° for the concave geminal (dotted–dashed line in figure 4). Note also that the silanols of the convex geminal pair display two different orientations (two peaks in the distribution), while the silanols of the concave geminal pair are oriented with the same mean angle (one peak in the distribution). These angles are to be compared to the two systematic orientations of the geminals at the crystalline (0 0 0 1) quartz–water interface [19, 20], i.e. in-plane silanols (30°) and out-of-plane silanols (100°). The small bump seen above 110° is related to one vicinal silanol that displays an average angle of 125° during 5 ps of the trajectory and then changes to an orientation of 55° on average for the rest of the trajectory. This result suggests that a silanol has a certain freedom to rotate/bend in the presence of the solvent, a property that was not observed for the silanols H-bonded to their neighbour silanols at the dry surface (vide infra). Most of the vicinal silanols adopt orientations within the interval 30°–80° (dotted–dotted–dashed line in figure 4). The isolated silanol has a mean orientation about 40° (dotted line in figure 4). Silanols at the amorphous silica–liquid water interface thus form a very diverse crowd. A fraction of them, namely 10 out of the total 26 in our model, are pointing out of the amorphous surface (roughly with angles in the 0°–50°
In the following the organization of water at the interface is discussed through the radial pair distribution functions (RDFs). As a preliminary, we report in figure 5 the OW–OW and OW–HW RDFs of the water box above the amorphous silica, where we show that they are identical to bulk water, giving us confidence that the system size is large enough and well equilibrated to simulate liquid water above the surface. Figures 6 and 7 display RDFs between selected surface silanols H/O atoms and oxygen (OW)/hydrogen (HW) atoms of water molecules that belong to the topmost interfacial water layer above the surface (first layer of water molecules above the amorphous surface). The surface convex geminal H…OW and O…HW RDFs are respectively presented in figures 6(a) and (b), together with the resulting number of neighbours (integral of RDF). We can see that the two geminal Si–O–H groups point towards the solvent and form strong H-bonds with interfacial water molecules as shown by the first peak in the H…OW RDFs located at quite short distances (1.56 and 1.66 Å). The peaks are also rather intense with a first minimum around 2.50 Å close to zero, which suggest a strong water organization and thus low mobility. Each geminal silanol forms one H-bond with one water molecule on average (dashed line in figure 6). There is a subsequent second peak in the RDFs located around 3.5–4.0 Å showing that the second layer of water molecules around the two geminal silanols is also well structured. While the two H atoms of the geminal silanols are strong H-bond donors to water molecules, the oxygens of these Si–O–H are not simultaneously H-bond acceptors with water. This can be inferred from the first peaks positions in the (Si–)O…HW RDFs in figure 6(b), respectively located at 3.0 Å and 3.5 Å for each Si–O–H of the geminal group. These distances are too long for hydrogen bonds, and the closest water molecules 5
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 7. RDFs between the isolated surface silanol H/O atoms and oxygen (OW)/hydrogen (HW) atoms of water molecules that belong to the topmost interfacial water layer above the surface. (a) (Si–O)H…OW RDF; (b) (Si–)O…HW RDF. Dashed lines correspond to the integration of RDFs thus giving the number of neighbours.
on the dynamical behaviour of these H-bonds. The stronger H-bond has a lifetime comparable to the trajectory timescale, while the weaker oxygen H-bonds can form and break a few times over the same timescale. The situation is slightly different for the concave geminal where each silanol is giving a strong H-bond to a neighbour silanol in the vacuum situation. In the presence of interfacial liquid water, we observed that one strong H-bond was conserved (at a distance of 1.75 Å), whereas the other initial H-bond has been rapidly broken. Instead a strong H-bond with a water molecule was formed (at the average distance of 1.6 Å) and is found stable for the whole trajectory. It thus appears that in a concave region, water can be trapped and strongly stabilized in interaction with the surface, if sterically allowed. The solvation structure for the isolated silanol (isolated refers to the state of the silanol not H-bonded at the dry surface) is given in figure 7. In the RDF of (Si–O)H…OW one can see a first peak located at 1.50–1.60 Å, with peak intensity around 4.0, then a subsequent minimum located at around 2.5 Å whose intensity is close to zero, and two subsequent peaks respectively around 3.5 and 6.5 Å. This means that the isolated silanol from the dry surface now is donating, on average, one H-bond to one water molecule once the surface is wetted, and cannot thus be strictly speaking denoted as isolated anymore. The strength of such a H-bond is similar to that of the H-bond donated from the geminal silanols to water (as estimated by the ratio of intensities of first peak and the subsequent minimum in the RDFs). As already observed for the convex geminal group, the zero intensity for the minimum subsequent to the first peak in all RDFs suggests that there is no water diffusion during the length-scale of our dynamics. Simultaneously to being H-bond donor to an interfacial water molecule, this silanol is also accepting one H-bond from one interfacial water molecule, as can be observed from the first peak position at 1.76 Å in the silanol (Si–)O…HW RDF displayed in figure 7(b).
Figure 8. RDFs between vicinal silanols O/H atoms and HW/ OW water molecules. Black: O…HW, red: H…OW. Dashed lines correspond to the integration of RDFs thus giving the number of neighbours.
to the oxygens of these Si–O–H silanols belong to the second hydration shell of the geminal group. In figure 6(c), the (Si–)O…H(–O–Si) RDFs between each oxygen of the two Si–O–H silanols of the convex geminal group and any H atom that belongs to the rest of the silanols at the amorphous silica surface are reported. We observe that the two geminal silanols are not equivalent. One silanol displays a first peak at 1.71 Å corresponding to 0.65 H-bond accepted on average from a neighbouring vicinal silanol. The second geminal oxygen displays an even more intense first peak in the (Si)O…H(O–Si) RDF located at a slightly shorter distance of 1.65 Å, and forming an average 1.9 H-bond with H atoms from neighbouring vicinal silanols. Note that these H-bonds are reinforced as compared to those formed in vacuum, where the H-bond distances were 1.80 Å and 2.00 Å respectively. The same tendency was already observed in quartz (1 0 0) by Musso et al [21] where the strong inter-silanols H-bonds were maintained and even reinforced whereas the weak ones were broken in the presence of solvent. We furthermore comment 6
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 9. Distribution of orientation of the water molecules’ dipole vector with respect to the vector normal to the amorphous surface (taken as the z-vertical axis, see scheme on the right-hand side). Solid line: no distinction among the nature of silanols taken into account; Dotted–dashed line: water molecules H-bonded to the isolated silanol only; dashed line: water molecules H-bonded to geminal silanols only; dotted line: water molecules H-bonded to vicinal silanols only.
Interestingly, the water molecules H-bonded to the geminal silanols have their dipole oriented outwards these silanols, with angles in the 20°–80° range only (dashed line in figure 9). The two water molecules H-bonded to the isolated silanol, one donor and one acceptor, adopt an orientation roughly between 20° and 70°. The water acceptor dipole points outward away from the surface, while the water donor dipole points more directly towards the surface (dotted–dashed line in figure 9). Also as expected, the two populations of water molecules H-bonded to the vicinal silanols (donor/acceptor of H-bonds) adopt two general orientations of their dipoles, either outward from the surface (angle below 60°) or towards the surface (angle above 60°) (dotted line in figure 9).
All but six vicinal silanols are H-bond donors to one water molecule, and half of them simultaneously receive one H-bond from another water molecule. See figure 8 for the RDFs between vicinal O/H atoms and HW/OW atoms of the water molecules, and associated H-bond distances with the first peak of these RDFs. The six remaining vicinals are either H-bonded to another vicinal silanol (two of them), or H-bonded to geminal silanols (four of them). The vicinalwater H-bonds are dynamical in essence, as can be observed from time evolutions along the AIMD trajectory: H-bonds can be formed and broken rather easily over the 22 ps dynamics, and exchange of the water molecule involved can also be observed over this period of time. We have also verified that the H-bond donor and H-bond acceptor water molecules are different molecules, for all vicinals. We have also observed some interchange of H-bonded water molecules only for the H-bond donor waters to these silanols, never for the H-bond acceptor water molecules.
3.4. The geminal silanol groups are a source of local water H-bond networks
The previous investigations have shown the extent of the delocalized H-bonding network between the amorphous silica surface and the interfacial water molecules within the first layer over the top of the surface. As it can be seen from the snapshots illustrated in figures 10(a) and (b), there are also well-defined local H-bond networks at the interface organized around the geminal groups (illustration here around the convex geminal): there is a ring of H-bonded water network organized between the pair of silanols of the convex geminal group. We have identified for this convex geminal two networks over the 22 ps trajectory: a five-membered ring in which three water molecules are forming a H-bonded network bridge from one silanol of the geminal group to the other silanol, and a six-membered ring involving four water molecules in the H-bonded network bridge. The two rings are illustrated by the snapshots (figures 10(a) and (b)) and by the plots of the evolution with time of H2O…H2O distances (figure 10(c). The change from the five-membered to the six-membered ring occurs around 4 ps along the trajectory. Similarly, a five-membered ring based on three water molecules exists between the pair of silanols of the concave
3.3. Orientation of the water molecules at the interface
The distribution of orientation of the water dipole vector with respect to the vector normal to the amorphous surface (taken as the z-vertical axis) is presented in figure 9, see also scheme in the figure, for the water molecules located at the interfacial layer. Three main domains can be seen. Domain 1 corresponds roughly to 0°–60° orientation angles and therefore represents water molecules whose dipoles are on average pointing out from the amorphous silica surface. They are the water molecules accepting H-bonds from out-of-plane silanols at the interface. Domain 2 roughly goes from 70° to 140°, and thus represents water molecules whose dipoles point towards the amorphous surface. On average, one water O–H group is donating an H-bond to one surface silanol. Domain 3 represents orientations in the 140°–180°, therefore water molecules pointing their two O–H groups towards the surface. 7
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 10. Local H-bond networks around the convex geminal group. (a) A five-membered ring in which three water molecules bridge
from one silanol of the geminal group to the other silanol (five oxygen vertices), and (b) a six-membered ring involving four water molecules and the two OH groups from the geminal (six oxygen vertices); (c) time evolution of the H2O…H2O distances for a fourmembered ring that changes into the five-membered (change occurs around t = 4 ps).
geminal. Here, no opening of the ring and incorporation of supplementary water molecule(s) is observed during the length of the simulation, presumably because of steric hindrance. It is interesting to notice that such pre-organized water networks could have an important impact on proton mobility along the silica–water interface, providing e.g. special channels for proton transfer, a property also found on boehmite [39].
therefore respectively acting as H-bond acceptors to water molecules and as H-bond donors to water molecules on average. This results in an alternating motif within the interfacial quartz–water layer, schematically illustrated in figure 2(c). No hydrogen bonds between these interfacial water molecules have been found to exist for a sufficient amount of time. At the amorphous silica–water interface, we found that all silanol groups donate strong H-bonds to the water molecules in the first adsorbed layer at the amorphous silica interface. The strength of such H-bonds is overall comparable to that of the H-bonds donated by the geminal silanols of the (0 0 0 1) crystalline quartz surface in contact with water. We hence found that geminal silanols and (most of the) vicinal silanols are involved in hydrogen bonds with water molecules, similarly to the quartz– water interface. There are however no privileged directionalities of the silanol–water H-bonds, contrary to the two systematic orientations observed at the quartz–water interface. We have found that the broad distribution of orientations of the silanols with respect to the normal to the amorphous surface is maintained once the surface is wetted (in the 30°–80° range). As a consequence, the water molecules can be roughly grouped into three categories, either pointing their dipole moment towards silanols and thus acting as H-bond donors to the surface, or having their dipole moment roughly lying parallel to the surface and acting as H-bond donors to the surface, or pointing their dipole moment out of the surface and acting as H-bond acceptors to the surface. The average amorphous silica–water H-bond network view obtained from our simulations is schematically illustrated in figure 11. On average, the vicinal silanols form two simultaneous H-bonds with two separate water molecules, i.e. one H-bond donor, one H-bond acceptor (left side of the scheme). The same is obtained for the ‘isolated silanol’ (isolated at the dry surface) that becomes involved into two H-bonds with two water molecules when the surface is wetted. Roughly the same average distances as for vicinals are obtained. The two silanols of the convex geminal simultaneously donate one H-bond to one water molecule. Again strong H-bonds are formed, according to the very short H-bond distances of 1.56
3.5. Discussion of amorphous versus crystalline silica at the interface with water
We are now in a position to discuss similarities and differences between the amorphous silica–water interface and the (0 0 0 1) crystalline quartz–water interface (see [19, 20]) from the point of view of interfacial structure (surface and water). Before discussing the interface with water, one striking remark has to be made for the dry surfaces. At the dry planar quartz surface, all geminal silanols are H-bonded within the surface, thus forming a regular in-plane H-bond zig-zagging network, as illustrated in figure 2(a). At the non-planar dry amorphous surface, the non-crystalline local topology dictates the structural arrangement between the three types of silanols (isolated, geminal, vicinal, figure 1). This results in geminals forming H-bonds with neighbouring vicinal silanols at the surface. Also due to the non-planar topology and thus the existence of concave/convex zones at the surface, the silanols at the amorphous silica surface display varied orientations with respect to the surface, and are found mostly pointing out of the surface. This is at odds with the dry (0 0 0 1) crystalline quartz surface. Once liquid water is added to the (0 0 0 1) crystalline quartz surface, we have shown [19, 20] that the geminal silanols are distributed into two populations, i.e. one that keeps in-plane orientations, one that adopts out-of-plane orientations, with average angle values with respect to the normal to the surface respectively of 30° and 100°. This results in the breakage of the regular in-plane silanol H-bond network that was observed at the surface of dry quartz. These two silanol populations are 8
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
Figure 11. Scheme of the main H-bond networks between geminal/isolated/vicinal silanols at the surface of amorphous silica and interfacial
water molecules. Grey = solid, blue = liquid water. Dashed lines represent H-bonds. Numbers are average H-bond distances in Ångstroms.
and 1.66 Å. These silanols are acting only as H-bond donors towards water. These two silanols are simultaneously H-bond acceptors with neighbour vicinal silanols, forming H-bonds of 1.65–1.75 Å distances on average. For the concave geminal, both silanols are H-bond donors to water, and one silanol of the pair is an H-bond acceptor from a neighbour vicinal. We have also observed local nests of water–water H-bonds linking the silanol pairs of these geminals: four- and five-membered H-bond rings have been observed. This is strikingly different from our observations at the crystalline quartz–water interface.
and convex and concave geminal silanols. A local molecular organization occurs around geminal silanols with water bridges being persistent during the 22 ps time of the dynamics. Convex geminals have water rings that can open and accommodate one supplementary water molecule, while concave geminals represent a more ‘fixed’ situation with water molecules trapped in the local cavity (over the time scales of our dynamics). The arrangement of interfacial water molecules at the amorphous silica surface is thus in essence very much different from the one at the crystalline quartz (0 0 0 1) surface. This does not only reflect a very different topological surface, thus dictating some structural organization of water, but this should also reflect the different chemical nature of the amorphous silica silanols. This can be inferred by comparing pKa values of the silanols at the amorphous and crystalline surfaces. Silanols’ pKa values have been calculated at the quartz– water interface in our previous works [19, 20], with a bimodal behaviour of the in-plane and out-of-plane silanols, nicely mirroring the bimodal behaviour recorded in SHG experiments. We have also shown how the silanols’ acidities at the quartz–water interface dictate the arrangement of water molecules [19, 20]. Our next step in the analysis of the amorphous silica–water interface is to calculate silanols’ pKa’s, and the relationship with the water structural arrangement obtained in the present work.
4. Conclusions and perspectives The amorphous silica–water interface has been investigated by means of DFT-based molecular dynamics simulations, providing a very detailed knowledge of the interfacial structural organization. Using a unique model of the amorphous silica surface, we have described the structure of the interface of two disordered systems in contact with each other, one liquid, one solid, at the DFT ab initio level. To the best of our knowledge, this is a pioneering work on the amorphous silica–liquid water system at this level of calculation. The interface model is built as a relevant and representative amorphous silica surface containing isolated, H-bonded vicinal and geminal silanols. The silanols donate strong H-bonds to the first adsorbed layer of water, as already observed on the (0 0 0 1) quartz surface in contact with water. In contrast to the previously observed organization of interfacial water at crystalline quartz (0 0 0 1) [19, 20], the interfacial water layer, albeit exhibiting some degree of favourite orientation towards the surface, is on average very much disordered. This probably represents a less dipolar water layer at the interface with amorphous silica, when compared to the water layer at the crystalline silica interface. The layer of water molecules at the amorphous silica surface is not mobile, to such an extent that an adsorbed water molecule would probably not be easily replaced by adsorbate organic molecules. The average disorder of the first water layer at the interface is complemented by local organization. Three specific zones were scrutinized, with increasing local order: vicinal silanols,
Acknowledgments GENCI under the project c2013082217 at IDRIS and CINES supercomputer centers in France are acknowledged. N Folliet is thanked for providing initial configurations to the present AIMD simulations. References [1] Rimola A, Costa D, Sodupe M, Lambert J F and Ugliengo P 2013 Chem. Rev. 113 4216 [2] Ostroverkhov V, Waychunas G A and Shen Y R 2004 Chem. Phys. Lett. 386 144 9
Álvaro Cimas et al
J. Phys.: Condens. Matter 26 (2014) 244106
[28] Fernandez-Cata G, Perez-Gramatges A, Alvarez L J, Comas-Rojas H and Zicovich-Wilson C M 2009 J. Phys. Chem. C 113 13309 [29] Adeagbo W A, Doltsinis N L, Klevakina K and Renner J 2008 Chem Phys Chem 9 994 [30] Argyris D, Tummala N R, Striolo A and Cole D R 2008 J. Phys. Chem. C 112 13587 [31] Wander M C F and Clark A E 2008 J. Phys. Chem. C 112 19986 [32] Yang J J and Wang E G 2006 Phys. Rev. B 73 035406 [33] Lu Z Y, Sun Z Y, Li Z S and An L J 2005 J. Phys. Chem. B 109 5678 [34] Yang J J, Meng S, Xu L F and Wang E G 2005 Phys. Rev. B 71 035413 [35] Masini P and Bernasconi M 2002 J. Phys.: Condens. Matter 14 4133 [36] Musso F, Sodupe M, Corno M and Ugliengo P 2009 J. Phys. Chem. C 113 17876 [37] Ugliengo P, Sodupe M, Musso F, Bush I J, Orlando R and Dovesi R 2008 Adv. Mater. 20 4579 [38] Motta A, Gaigeot M P and Costa D 2012 J. Phys. Chem. C 116 23418 [39] Motta A, Gaigeot M P and Costa D 2012 J. Phys. Chem. C 116 12514 [40] Argyris D, Cole D R and Striolo A 2009 Langmuir 25 8025 [41] Argyris D, Ho T, Cole D R and Striolo A 2011 J. Phys. Chem. C 115 2038 [42] Cheng J and Sprik M 2010 J. Chem. Theory Comput. 6 880 [43] Raybaud P, Digne M, Iftimie R, Wellens W, Euzen P and Toulhoat H 2001 J. Catal. 201 236 [44] Sulpizi M and Sprik M 2010 J. Phys.: Condens. Matter 22 284116 [45] Lodziana Z, Norskov J K and Stoltze P 2003 J. Chem. Phys. 118 11179 [46] Hemeryck A, Motta A, Swiatowska J, Pereira-Nabais C, Marcus P and Costa D 2013 Phys. Chem. Chem. Phys. 15 10824 [47] Chiche D, Chizallet C, Durupthy O, Channeac C, Revel R, Raybaud P and Jolivet J-P 2009 Phys. Chem. Chem. Phys. 11 11310 [48] Garofalini S H 1990 J. Non-Cryst. Solids 120 1 [49] Folliet N, Gervais C, Costa D, Laurent G, Babonneau F, Stievano L, Lambert J-F and Tielens F 2013 J. Phys. Chem. C 117 4104 [50] Folliet N et al 2011 J. Am. Chem. Soc. 133 16815 [51] Galeener F L 1982 J. Non-Cryst. Solids 49 53 [52] Leydier F, Chizallet C, Chaumonnot A, Digne M, Soyer E, Quoineaud A-A, Costa D and Raybaud P 2011 J. Catal. 284 215 [53] The cp2k Developers Group. http://cp2k.berlios.de [54] Goedecker S, Teter M and Hutter J 1996 Phys. Rev. B 54 1703 [55] Ong S W, Zhao X L and Eisenthal K B 1992 Chem. Phys. Lett. 191 327
[3] Engemann S, Reichert H, Dosch H, Bilgram J, Honkimaki V and Snigirev A 2004 Phys. Rev. Lett. 92 205701 [4] Fubini B 1998 The Surface Properties of Silicas ed A P Legrand (Chichester: Wiley) p 415 [5] Li I, Bandara J and Shultz M J 2004 Langmuir 20 10474 [6] Musso F, Ugliengo P and Sodupe M 2011 J. Phys. Chem. A 115 11221 [7] Criscenti L J, Kubicki J D and Brantley S L 2006 J. Phys. Chem. A 110 198 [8] Kubicki J D, Solo J O, Skelton A A and Bandura A V 2012 J. Phys. Chem. C 116 17479 [9] Tosoni S, Civalleri B, Pascale F and Ugliengo P 2008 Ab Initio Simulation of Crystalline Solids: History and Prospects— Contributions in Honor of Cesare Pisani (IOP Conf. Series vol 117), ed R Dovesi et al (Bristol: IOP Publishing) pp 12026 [10] Tosoni S, Civalleri B and Ugliengo P 2010 J. Phys. Chem. C 114 19984 [11] Guo X Y, Watermann T, Keane S, Allolio C and Sebastiani D 2012 Z. Phys. Chem.—Int. J. Res. Phys. Chem. Chem. Phys. 226 1415 [12] Bandura A V, Kubicki J D and Sofo J O 2011 J. Phys. Chem. C 115 5756 [13] Minibaev R F, Zhuravlev N A, Bagatur’yantz A A and Alfimov M V 2009 Russ. Phys. J. 52 1164 [14] Costa D, Tougerti A, Tielens F, Gervais C, Stievano L and Lambert J F 2008 Phys. Chem. Chem. Phys. 10 6360 [15] Tielens F, Gervais C, Lambert J F, Mauri F and Costa D 2008 Chem. Mater. 20 3336 [16] Yang J and Wang E G 2006 Phys. Rev. B 73 035406 [17] Musso F, Ugliengo P and Sodupe M 2011 J. Phys. Chem. A 115 11221 [18] Chen Y-W, Chu I-H, Wang Y and Cheng H-P 2011 Phys. Rev. B 84 155444 [19] Sulpizi M, Gaigeot M-P and Sprik M 2012 J. Chem. Theory Comput. 8 1037 [20] Gaigeot M-P, Sprik M and Sulpizi M 2012 J. Phys.: Condens. Matter 24 124106 [21] Musso F, Mignon P, Ugliengo P and Sodupe M 2012 Phys. Chem. Chem. Phys. 14 10507 [22] Zhao Y L 2012 J. Theor. Comput. Chem. 11 155 [23] Skelton A A, Fenter P, Kubicki J D, Wesolowski D J and Cummings P T 2011 J. Phys. Chem. C 115 2076 [24] Chen Y W, Chu I H, Wang Y and Cheng H P 2011 Phys. Rev. B 84 155444 [25] Leung K, Nielsen I M B and Criscenti L J 2009 J. Am. Chem. Soc. 131 18358 [26] Argyris D, Cole D R and Striolo A 2009 J. Phys. Chem. C 113 19591 [27] Tilocca A and Cormack A N 2009 ACS Appl. Mater. Interfaces 1 1324
10
