Dynamics of Water Monolayers Confined by Chemically

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Dynamics of Water Monolayers Confined by Chemically Heterogeneous Surfaces: Observation of Surface-Induced Anisotropic Diffusion Mehdi Karzar Jeddi, and Santiago Romero-Vargas Castrillón J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b07454 • Publication Date (Web): 22 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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The Journal of Physical Chemistry

Dynamics of Water Monolayers Confined by Chemically Heterogeneous Surfaces: Observation of Surface-Induced Anisotropic Diffusion Mehdi Karzar Jeddi and Santiago Romero-Vargas Castrillón

Department of Civil, Environmental, and Geo- Engineering, University of Minnesota-Twin Cities, Minneapolis, MN 55455, USA The Journal of Physical Chemistry B

Revision submitted

09/19/2017



Present address: Department of Mechanical Engineering, Vanderbilt University, Nashville, TN 37235 USA 

Corresponding author. Electronic mail: [email protected]

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ABSTRACT: Water present in confining geometries plays key roles in many systems of scientific and technological relevance. Prominent examples are living cells and nanofluidic devices. Despite its importance, a complete understanding of the dynamics of water in nanoscale confinement remains elusive. In this work, we use molecular dynamics (MD) simulation to investigate the diffusive dynamics of water monolayers confined in chemically heterogeneous silica slit pores. The effect of chemical heterogeneity is systematically investigated through the fraction 𝑓SiOH of randomly distributed surface sites that possess hydroxyl functional groups. Partial hydroxylation results in heterogeneous surfaces comprising nanoscale hydrophobic and hydrophilic regions. We find that the in-plane diffusivity of water increases monotonically with 𝑓SiOH ; at low surface hydroxylation (𝑓SiOH ≤ 50%) slow water dynamics arise due to the formation of ice-like structures in the hydrophobic regions, while at 𝑓SiOH ≥ 75%, surface-water H-bonds in the hydrophilic regions result in faster dynamics. We show that surface patterning with ordered hydrophobic and hydrophilic “stripes” can be used to induce 1-D diffusion, with water diffusing through the slit pore preferentially along the direction of the hydrophilic surface patterns.


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I. Introduction Water transport in nano-scale geometries is ubiquitous in systems of scientific and technological relevance. Examples include water permeation through the cell membrane1, water transport in micro- and nano-fluidic devices2,3, transport in polymeric membranes for water treatment4, and the recovery of water in shale nano-pores5. It is well understood that the thermodynamic and structural properties of nanoscopically confined water are dependent on the nature of the interactions between water molecules and the confining media, the state variables (temperature and pressure or density), and the length scale of confinement.6–8 Moreover, the dynamic properties of water in confinement have been shown to strongly deviate from those of the bulk liquid due to the influence of the confining surfaces.9,10 Previous computational and experimental studies have provided insights into the properties of water confined by chemically and structurally homogenous surfaces.11–14 These include the appearance of phase transitions, such as crystallization15–17 and cavitation18, not observed in the bulk under the same thermodynamic conditions, and surface-induced perturbations of the density, hydrogen bond network and orientational structure of confined water films.19 Further, studies focusing on chemically heterogeneous surfaces have revealed that the length scale and distribution of hydrophobic and hydrophilic domains and functional groups have a profound impact on the hydration layer structure and dynamics.20–22 The structure and dynamics of water in the hydration shell of proteins and micelles6,7,23 have also been investigated using molecular simulation24–26; the chemical and topological heterogeneity of these complex interfaces has been shown to significantly influence their solvation27,28, resulting in slower dynamics compared to the bulk liquid6. Nevertheless, understanding the origin of slow dynamics has proven difficult in biological systems, given the inherent topological and chemical complexity of biomolecular hydration shells.


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Moreover, biological systems preclude the systematic investigation of the effect of surface heterogeneity, since changing the spatial distribution of functional groups will inevitably modify other variables, such as nanoscale topography or charge. This work aims to provide a more complete understanding of water’s diffusive dynamics in confinement by planar, chemically heterogeneous solid surfaces. To this end, we use equilibrium molecular dynamics (MD) simulation to systematically investigate the dynamics of a water monolayer confined by silica surfaces with various distributions of hydrophilic silanol groups, leading to the formation of hydrophobic and hydrophilic subdomains (both patterned and randomly distributed) on each surface. Variation of 𝑓SiOH , the fraction of hydroxylated (i.e., silanol) surface sites, between 𝑓SiOH = 0% (corresponding to hydrophobic apolar surfaces) and 100% enables us to systematically investigate the effect of surface heterogeneity on water dynamics. Our results show that the self-diffusion coefficient of water increases monotonically with 𝑓SiOH . The slow dynamics observed at low 𝑓SiOH are due to slow-diffusing water molecules that form ice-like configurations in the hydrophobic subdomains of the surfaces. Further, we investigate the influence of subdomains comprising patterned hydrophobicity and hydrophilicity, and show that nanoscale patterning can be used to induce anisotropy in diffusive dynamics. This paper is organized as follows. In section II we provide details of the systems and MD simulations. The results, presented in section III, are structured as follows: in sections III.1 and III.2, we study the influence of surface chemical heterogeneity on the interfacial hydration structure and diffusive dynamics of confined water monolayers. In section III.3, the observed dependence of the diffusion coefficient on 𝑓SiOH is explained by the slow dynamics of ice-like water within hydrophobic regions, and more mobile water dynamics in hydrophilic regions. In


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section III.4, we show how surfaces with ordered nano-scale patterning induce 1-dimensional diffusion. Concluding remarks are provided in section IV. II. Simulation and Computational Details We perform MD simulations in the microcanonical ensemble (constant N, V, and E). The simulation system consists of a water monolayer with mean density 𝜌0 = 0.97 g cm-3 (Nw = 1089 water molecules) confined between two atomistic silica surfaces, with lateral dimension 10.25 × 10.35 nm2, representing the structure of β-cristobalite.15,19 The two surfaces are positioned parallel to each other and held at a fixed separation ℎ = 0.32 nm, measured from the planes containing the surface H atoms. Periodic boundary conditions are applied in the x and y directions, parallel to the surfaces, rendering a slit pore geometry that is macroscopic along x and y and confined along the z direction. Water molecules are modeled using the SPC/E pair potential29, known to provide a quantitative description of the diffusion coefficient of bulk water at ambient conditions30. In the SPC/E model, the O-H bond length is constrained to 0.1 nm and the H-O-H angle is fixed to the tetrahedral value, cos-1(-1/3)  109.47º.29 Water-surface interactions are modeled with the pair potential of fully hydroxylated silica developed by Lee and Rossky19. This potential affords a fully atomistic description of water-surface interactions, including hydrogen bonding between water and silanol functional groups.9,31 The Lee-Rossky potential includes contributions due to LennardJones (L-J) interactions between water O atoms and surface Si and O atoms, and electrostatic Coulombic interactions between water atoms and charged silanol (-Si-O-H) surface groups. The Si and O surface atoms remain fixed and in registry during the simulation, while the surface H atoms can reorient with fixed bond lengths and angles in a circular trajectory. Further details about the surfaces can be found elsewhere.15,19


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We investigate four classes of surfaces: hydrophobic apolar surfaces, hydrophilic silica surfaces, randomly hydroxylated surfaces, and surfaces with patterned hydrophobicity and hydrophilicity. Representative structures of each surface type are shown in Figure 1. 𝑓SiOH = 100%

(a)

𝑓SiOH = 50%

𝑓SiOH = 50%

𝑓SiOH = 0%

(b)

(c)

(d)

Figure 1. Renderings of representative silica surfaces used in this study. The fraction of hydroxylated surface sites, 𝑓SiOH , is indicated in the legend: a) fully hydroxylated, b) randomly hydroxylated, c) surface with patterned hydrophobicity/hydrophilicity, and d) hydrophobic apolar. The color coding for the molecular structure is: O (red), Si (grey), and H (yellow). Beginning with fully hydroxylated silica surfaces (Figure 1(a)), randomly hydroxylated silica surfaces are obtained by removing surface hydrogen atoms from a fraction of randomly selected silanol (i.e., non-bridging) oxygen atoms. Removing the silanol H atom renders the Coulombic charges on the remaining Si and O atoms equal to zero. As illustrated in Figure 1(b), the resulting non-hydroxylated regions form nanoscopic subdomains in which the surface Si and O atoms interact with water solely through L-J interactions. A surface such as that shown in Figure 1(b) thus possesses randomly distributed hydrophobic (i.e., non-hydroxylated, non-H-bonding) and hydrophilic nanoscale regions. We quantify the degree of surface hydroxylation through the parameter 𝑓SiOH , defined as the fractional surface density of silanol groups (i.e., the fraction of non-bridging surface O atoms that are part of a silanol group). By varying 𝑓SiOH , randomly hydroxylated surfaces enable the systematic investigation of the effect of chemical heterogeneity (derived from hydrophobic and hydrophilic functional groups) on water dynamics. We vary 𝑓SiOH


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between 0% (Figure 1(d), corresponding to hydrophobic apolar surfaces that interact with water solely via L-J interactions) and 100% (Figure 1(a), corresponding to fully hydroxylated hydrophilic surfaces). Randomly hydroxylated surfaces were generated with 𝑓SiOH = 12.5%, 25%, 37.5%, 50%, and 75%, corresponding to 126, 252, 378, 504, and 756 silanol groups attached to the surfaces. Figure 1(b) shows the randomly hydroxylated surface with 𝑓SiOH = 50%. For a given value of 𝑓SiOH , each of the two randomly hydroxylated surfaces is independently generated (i.e., one surface is not a replica of the other). Further, we investigate a class of surfaces with patterned hydrophobicity and hydrophilicity. These systems are constructed with alternating hydrophobic and hydrophilic nanoscale “stripes” spanning the entire surface, as shown in Figure 1(c). We investigate striped patterns with 𝑓SiOH = 25% and 50%. All simulations are performed in LAMMPS.32 The equations of motion were solved using the Verlet algorithm29, constraining bond lengths and angles using the SHAKE algorithm.33 The longrange electrostatic interactions are evaluated using the particle-particle particle-mesh scheme with a convergence tolerance of 10-6.34 A cutoff radius of 0.9 nm was applied to L-J and real-space electrostatic interactions. To prepare the computational domain, a monolayer comprising 1089 SPC/E water molecules, obtained from a pre-equilibrated MD simulation of bulk water (𝜌 = 1 g cm-3, 𝑇 = 300 K), is placed between the silica surfaces. The system is then equilibrated for 0.2 ns in the canonical ensemble (constant N, V, T) at 300 K using a Nosé-Hoover35,36 thermostat with coupling constant of 100 fs and a time step of 1 fs. Next, the confined monolayer is equilibrated for an additional 0.8 ns in the NVE ensemble with a time step of 1 fs, before beginning the NVE production run with a time step of 0.5 fs to ensure adequate energy conservation (< 0.02% energy drift per ns). The systems were propagated for 200 ns for surfaces with 𝑓SiOH ≤ 50% and 100 ns for 𝑓SiOH ≥ 75%, saving configurations every 2.5 ps. The average kinetic temperature of the system


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during the production run ranged from 308 to 328 K depending on the surface, with a maximum standard deviation of 5.5 K. III. Results and Discussion III.1. Diffusion of water between randomly hydroxylated surfaces To investigate water dynamics between randomly hydroxylated surfaces, we first consider the inplane mean-squared displacement (MSD, 〈Δ𝑟∥2 (𝑡)〉 = 〈Δ𝑥 2 (𝑡) + Δ𝑦 2 (𝑡)〉), computed from the coordinates of water O atoms. Figure 2 presents results for systems with various 𝑓SiOH .

0.50 fSiOH = 37.5% 25% 12.5% 0%

0.38 0.25

r2II(t) [nm2]

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0.13 0.00 100% 75% 50%

5.4

3.6

1.8

0.0

0

50

100

150

200

t [ns] Figure 2. In-plane mean-squared displacement (MSD) of water molecules confined by surfaces with various degrees of surface hydroxylation, 𝑓SiOH , noted in the legend. Note the different y-axis scales in the upper and lower panels.


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We note that water confined by hydrophobic apolar surfaces (𝑓SiOH = 0%) exhibits a root-meansquared displacement (RMSD) 〈Δ𝑟∥2 〉1/2  0.07 nm at t  200 ns, indicating that water diffusion is severely hindered in this system, with molecular motion being limited to local vibrations. As explained below, the dramatic slowdown of dynamics observed when 𝑓SiOH = 0% (Figure 2) is due to the formation of an ice-like monolayer in hydrophobic apolar confinement, in agreement with previous computational studies showing confinement-induced crystallization of water films near hydrophobic surfaces.15,37,38 Figure 3 shows the in-plane diffusion coefficient, 𝐷∥ , computed from the long-time limit (𝑡 ≥ 150 ns for 𝑓SiOH ≤ 50%, 𝑡 ≥ 50 ns otherwise) of the slope of the MSD vs. 𝑡 plots (Figure 2), for systems with 𝑓SiOH > 0 (𝑅 2 > 0.99 for all fits).

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D|| [10-5 cm2s-1]

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0.01 confined water bulk water

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1E-4 0

20

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60

80

100

fSiOH Figure 3. In-plane diffusion coefficient (𝐷∥ ) of water molecules in confinement, as a function of the degree of surface hydroxylation, 𝑓SiOH . The diffusion coefficient was computed from the meansquared displacement data of Figure 2. The blue dash line shows the diffusion coefficient of bulk SPC/E water, obtained from an NVE MD simulation of bulk SPC/E water at 1 g cm-3 (N = 1500, T = 301.32 ± 3.72 K).


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The data in Figures 2 and 3 show that translational dynamics of water speed up with increasing 𝑓SiOH , i.e., with increasing surface density of H-bonding silanol groups. To investigate the connection between H-bonding and dynamics, we computed the number of hydrogen bonds (HBs) per water molecule following a standard geometric definition39,40: two water molecules (or a water molecule and a silanol group) are hydrogen bonded if the distance between oxygen atoms, 𝑟O′ O , is less than 0.35 nm (the location of the first minimum of the O-O pair correlation function)41 and simultaneously the angle formed between 𝐫O′ O and 𝐫OH , the O-H donor vector, is less than 30º (the amplitude of librations that break HBs)39. On increasing 𝑓SiOH from 0 to 100%, the number of silanol-water HBs increases monotonically from 0 to 1.88 per water molecule, while the number of water-water HBs monotonically decreases from 2.66 to 1.7 per water molecule. Thus, surfacewater HBs play a crucial role in promoting dynamics of confined monolayers. To discern the roles of water-surface and water-water HBs, we examined the dynamics of water molecules near and far from H-bonding surface groups. Figure 4 shows representative single-molecule trajectories of water molecules in confinement, recorded over a 3-ns observation time. In Figure 4(a), corresponding to hydrophobic apolar confinement (𝑓SiOH = 0%), molecules do not translate over the 3-ns observation time, their dynamics being limited to vibration and rotation in discrete locations of the confined domain (cf. movie M1 in the Supporting Information). Moreover, we observe that molecules arrange themselves into hexagonal structures, in which molecules participate in ~3 water-water HBs, consistent with the ensemble average value noted above (2.66 HBs per molecule). These ice-like structures have been reported previously.15,37,38,42 Two different dynamic behaviors are observed in systems comprising partially hydroxylated surfaces. These are illustrated considering the system with 𝑓SiOH = 50% (Figures 4(b) and (c), movies M2 and M3 in the Supporting Information), though the observations apply similarly to the other systems with


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partial hydroxylation (12.5% ≤ 𝑓SiOH ≤ 50%, results not shown). When the selected molecule is found at a distance 𝑟O′O > 0.35 nm from surface silanol oxygen atoms (which precludes Hbonding39), translational dynamics are absent (cf. movie M2), similar to those of the hydrophobic apolar system (𝑓SiOH = 0%, Figure 4(a)). Thus, water molecules at 𝑟O′O > 0.35 nm from silanol O atoms reside in regions characterized by slow dynamics. Hereinafter, we refer to these regions as hydrophobic subdomains. Conversely, when water molecules are near surface silanol groups (𝑟O′O < 0.35 nm, Figure 4(c)), the single-molecule trajectory exhibits large displacements (>0.25 nm), due to H-bonding interactions with vicinal silanols. H-bonding silanol groups can reorient in a circular trajectory as they interact with and exchange water molecules (see movie M3 in the Supporting Information), thereby speeding up water dynamics.9 The fast dynamics observed in this region, hereinafter designated the hydrophilic subdomain, are consistent with the increase in 𝐷∥ with 𝑓SiOH (i.e., increasing number of silanols per surface), as shown in Figure 3. 𝑓SiOH = 0%

𝑓SiOH = 50% (hydrophobic subdomain, rO’O > 0.35 nm)

𝑓SiOH = 50% (hydrophilic subdomain, rO’O < 0.35 nm)

(a)

(b)

(c)

Figure 4. Single-molecule trajectories (in-plane projection shown as blue lines) recorded over 3 ns for: (a) fully hydrophobic surfaces (𝑓SiOH = 0%); (b) surfaces with 𝑓SiOH = 50%, showing the trajectory of a single molecule located at 𝑟O′ O > 0.35 nm from silanol oxygens (i.e., in the hydrophobic subdomain); (c) surfaces with 𝑓SiOH = 50%, showing the trajectory of a single molecule located at 𝑟O′ O < 0.35 nm from silanol oxygens (in the hydrophilic subdomain). The color coding of surface atoms is the same as in Figure 1. In (a), the distance to the nearest and next-nearest neighbor sites are shown. In panels (b) and (c), all water molecules except for the tagged trajectory were omitted for clarity. See also movies M1-M3 in the Supporting Information.


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III.2. Interfacial hydration structure of water confined by randomly hydroxylated surfaces Additional insights into the influence of surface chemical heterogeneity on dynamics can be gleaned from the hydration structure of water. In Figure 5, we present time-averaged water surface density contours for systems with 𝑓SiOH = 0%, 25%, and 100% (the complete set of contour maps for all 𝑓SiOH is given in Figures S1-S7). The contour plots presented are averages of water density over 100 ns (Equation S1). 𝑓SiOH = 0%

𝑓SiOH = 25%

𝑓SiOH = 100%

(a)

(b)

(c)

Figure 5. Time-averaged density contours of water molecules 𝜌(𝑥, 𝑦)/𝜌0 (𝜌0 = 0.97 g cm-3) computed over 100 ns. Results are shown for 𝑓SiOH = 0% (a), 25% (b), and 100% (c). The figure shows 2  2 nm2 representative regions of the confined domain (~10  10 nm2). In Figure 5(c), the preferential locations of water molecules in hydrophilic confinement, equidistant to nearestneighbor silanol groups, are noted by crosses in the inset.

Under hydrophobic apolar confinement (Figure 5(a), 𝑓SiOH = 0%), the density contour plot shows a hydration structure characterized by well-defined six-member rings, confirming the long-time persistence of the ice-like structure of water presented in Figure 4(a). In the ice-like configurations formed in hydrophobic confinement, water is found at two discrete locations, determined by the crystalline morphology of the silica surfaces: at the center of the (distorted) silica hexagons, or in cavities formed by Si atoms that point away from the monolayer. The distance between these two locations, 0.28 nm, is noted in Figure 4(a). The structure of water is highly localized: the density


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of water is ~0 around the hexagonal vertices (Figure 5(a)). It is worth pointing out that formation of the hexagonal structures depicted in Figure 5(a) is likely to be dependent on the silica surfaces being in registry; we surmise that surface misalignment will hinder the formation of ice-like configurations, a phenomenon akin to the shear melting transition observed in epitaxial solids 43. The ice-like structure of the monolayer in hydrophobic confinement explains the vanishing diffusion coefficient and MSD at 𝑓SiOH = 0%: water remains in the residence sites noted in Figure 4(a) and 5(a) for ~100 ns, with translations to neighboring sites occurring with low probability (see section III.3). On the other hand, a different hydration structure is observed for water in hydrophilic confinement (𝑓SiOH = 100%, Figure 5(c)), where the preferential location of water molecules, as determined by the local maxima in the contour plots, is equidistant to two neighboring silanol groups. The location of these sites, and the distance between them, 0.25 nm, is indicated in Figure 5(c). The remarkable effect of surface chemical heterogeneity is shown in Figure 5(b), corresponding to the system with 𝑓SiOH = 25%. For this and other partially hydroxylated surfaces (12.5% ≤ 𝑓SiOH ≤ 50%, cf. Figures S2 - S5 of the Supporting Information), the density contour plots show that the hydration structure is a combination of that observed when 𝑓SiOH = 0% and 100%, i.e., hydrophobic subdomains of ordered water interspersed with hydrophilic subdomains where the density contours evidence less structured water. In Figure 5(b), the contour plots show both the highly localized, hexagonally arranged water molecules characteristic of the hydrophobic subdomain, adjacent to delocalized density contours of the hydrophilic subdomain. Water molecules in hydrophilic subdomains exhibit the same preferential location as described for 𝑓SiOH = 100% (see the inset to Figure 5(c)), i.e., in sites equidistant from two proximal surface silanol groups. We note that the system with 𝑓SiOH = 75% does not possess hydrophobic subdomains. Given that each of the confining surfaces is independently hydroxylated, the resulting system does


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not possess regions in which water adopts an ice-like structure (i.e., 𝑟O′O < 0.35 nm for all molecules), the density contour plot being similar to that of the 𝑓SiOH = 100% system (see Figure S6). This is consistent with the diffusion coefficient for 𝑓SiOH = 75%, which is close to that observed for 𝑓SiOH = 100% (cf. Figure 3). The interfacial hydration structure can also be examined in terms of the in-plane pair correlation function, presented in Figure 7 for water O atoms at 𝑓SiOH = 0, 50 and 100% (data for all 𝑓SiOH is given in Figure S8).

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3 2 1 0 0.0

0.2

0.4

0.6

0.8

1.0

r [nm] Figure 6. Water O-O in-plane pair correlation function for systems with different degree of hydroxylation (𝑓SiOH ) indicated in the legend. The strong position correlations of the ice-like configuration observed when 𝑓SiOH = 0% is evidenced by the sharp peaks in the 𝑔∥ (𝑟). Further, the locations of the three peaks nearest the origin (centered at 0.28 nm, 2 × 0.28 cos(π/6) nm = 0.49 nm, and 2 × 0.28 nm = 0.56 nm) correspond to the observed hexagonal configuration shown previously in Figures 4(a) and 5(a). The peak at the origin in Figure 6 for 𝑓SiOH = 0% is due to the presence of two water molecules in


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the same site of opposing surfaces, leading to nearly overlapping 𝑥𝑦 projections of their coordinates (this overlap vanishes as 𝑓SiOH increases to 100%, cf. Figure S8 at r = 0). The positional correlations observed when 𝑓SiOH = 0% are in contrast with the structure of the monolayer in fully hydrophilic confinement (Figure 6 for 𝑓SiOH = 100%). We observe that, when 𝑓SiOH = 100%, the smaller and broader peaks of the pair correlation function are consistent with weaker position correlations as compared to hydrophobic confinement (compare the diffuse density contours in Figure 5(c) with the localized density in Figure 5(a)), and with liquid-like behavior that manifests itself in faster translational dynamics (cf. Figures 2 and 3). The location of the nearest-neighbor peak, at 0.25 nm, corresponds to the distance between the water sites noted in the inset to Figure 5(c). The pair correlation function of the partially hydroxylated surface (Figure 6, shown for 𝑓SiOH = 50%) exhibits features intermediate between those of the 0% and 100% systems, e.g., the nearest-neighbor peak is found at a location between that observed in fully hydrophobic and fully hydrophilic confinement. Figure S8, showing 𝑔∥ (𝑟) for all 𝑓SiOH , shows that the radial position of the first peak shifts monotonically to lower r as the degree of hydroxylation increases. For the next-nearest and far-field peaks, the trends are less obvious; nonetheless, in all systems shown in Figure S8, the structure-breaking effect as 𝑓SiOH increases is made evident by the weaker positional correlations. The density profile as a function of z, normalized by the mean water density 𝜌(𝑧)/𝜌0 , is presented in Figure 7 for 𝑓SiOH = 0, 50 and 100% (Figure S9 of the Supporting Information summarizes the data for all values of 𝑓SiOH ). We observe a prominent peak at 𝑧 = 0 in hydrophobic apolar confinement (𝑓SiOH = 0%), corresponding to the hexagonally arranged ice-like monolayer described in Figures 4(a) and 5(a). Two smaller peaks appear at 𝑧 = ±0.13 nm, due to water molecules that occupy the same sites on opposing surfaces. Conversely, in hydrophilic


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confinement (𝑓SiOH = 100%), strong surface-water H-bond interactions9 result in a trilayer liquid structure, characterized by prominent peaks near the walls (at 𝑧 = ±0.075 nm), and a smaller central peak. For partially hydroxylated surfaces, Figure 7 and Figure S9 show that the density profile is intermediate between the monolayer ice and the trilayer liquid, and that the evolution from an icelike monolayer to a trilayer liquid occurs monotonically as 𝑓SiOH increases.

Figure 7. Normalized density of water molecules in the 𝑧-direction. The density profile was calculated from the 𝑧-coordinates of oxygen atoms. Dash lines denote the location of the plane containing surface H atoms (if any) in each plate. The density profile was normalized by the bulk water density, 𝜌0 = 0.97 g cm-3.

III.3. Slow and fast dynamics of water in chemically heterogeneous confinement In this section, we examine the different dynamics exhibited by water molecules in the hydrophobic and hydrophilic subdomains (cf. Figures 4(b) and (c), movies M2 and M3 in the Supporting Materials), using this information to gain insight into the monotonic increase of 𝐷∥


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with 𝑓SiOH . We first turn to the van Hove distribution function, which provides the probability distribution of single-molecular displacements (i.e., the probability of finding a water molecule at time 𝑡 > 0 a distance 𝑟 away from its position at 𝑡 = 0),44–46 𝑁𝑤

1 〈∑ 𝛿[𝐫 − 𝐫𝑖 (𝑡) + 𝐫𝑖 (0)]〉 𝐺𝑠 (𝑟, 𝑡) = 𝑁𝑤

(1)

𝑖=1

In Equation 1, 〈… 〉 indicates ensemble average, 𝐫𝑖 (𝑡) is the position vector of the oxygen atom of water molecule 𝑖 at time 𝑡, 𝛿 is the Dirac delta function, and 𝑟 = |𝐫|. Figure 8 shows the van Hove correlation function computed at 𝑡 = 0.01 – 30 ns for systems with 𝑓SiOH = 0%, 50% and 100%. 𝑓SiOH = 0% 10

0

10

t = 0.01 ns 0.1 ns 1 ns 10 ns 20 ns 30 ns

-1

10-2 10

𝑓SiOH = 50%

-3

10

101

101

0

100

10-1

10-1

10

10-4

10

0.1

0.2

0.3

r [nm]

(a)

0.4

0.5

-2

10-3 10

-4

10

-5

-5

0.0

𝑓SiOH = 100%

Gs(r,t)

10

1

Gs(r,t)

Gs(r,t)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

10-2 10-3 10-4

0.1

0.2

0.3

0.4

0.5

10-5 0.0

0.1

0.2

0.3

r [nm]

r [nm]

(b)

(c)

0.4

0.5

Figure 8. van Hove function 𝐺𝑠 (𝑟, 𝑡) of single-molecular displacements of water molecules, computed at different times (𝑡) indicated in the inset of (a). Results are shown for confinement between (a) hydrophobic silica plates (𝑓SiOH = 0%), (b) randomly hydroxylated silica surfaces (𝑓SiOH = 50%), and (c) hydrophilic silica surfaces (𝑓SiOH = 100%).

For 𝑓SiOH = 0% (Figure 8(a)), the long-time (𝑡 = 30 ns) persistence of the prominent peak centered at r = 0 shows that molecules remain at their original locations for long times, with translations to the neighboring sites (reflected in the smaller peaks that appear at 𝑡 ≥ 1 ns) occurring with low probability. The location of the smaller peak at 0.28 nm in Figure 8(a) is due to molecules that jump to the nearest-neighbor site, while the small peak at 0.49 nm reflects molecules that translate to the next-nearest-neighbor site on the hydrophobic apolar surface (cf. Figure 4(a)). Figure 8(a)


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Page 18 of 33

thus shows that molecules in hydrophobic apolar confinement exhibit long waiting times in discrete residence sites imposed by the hydrophobic silica, with low-probability of translation to neighboring sites, resulting in a vanishing diffusion coefficient. On the other hand, in hydrophilic confinement (𝑓SiOH = 100%, Figure 8(c)) the van Hove distribution function approaches a uniform (constant probability) distribution at long times, reflecting the weaker positional correlations imposed by fully hydrophilic surfaces (cf. Figure 5(c)), and the significantly faster dynamics observed (cf. the diffusion coefficient at 𝑓SiOH = 100% in Figure 3). Moreover, Figure 8(c) shows that the peak at 𝑟 = 0 observed at 𝑡 = 0.01 ns decays rapidly, in contrast with its long-time persistence observed in hydrophobic apolar confinement (Figure 8(a)). Water molecules therefore exhibit significantly shorter residence times in hydrophilic confinement, in agreement with the speedier dynamics reported in Figures 2 and 3. Further, we note that the broad, secondary peaks observed in Figure 8(c) at 0.25 nm, 0.43 nm, and 0.49 nm (when 𝑡 > 1 ns) are consistent with the preferential locations of water molecules observed in Figure 6 for 𝑓SiOH = 100%. In partially hydroxylated confinement (𝑓SiOH = 50%, Figure 8(b)), the van Hove function exhibits signatures of both slow and fast dynamics. Namely, at long times, we note a non-vanishing peak at 𝑟 = 0, due to molecules that persist in their residence sites in hydrophobic subdomains, as well as the more uniform distribution of molecular displacements (when t ≥ 10 ns) of molecules in the hydrophilic subdomains. Consequently, the picture that emerges from Figure 8(b) is that the observed dynamic behavior in partially hydroxylated confinement reflects both the slow dynamics in the hydrophobic subdomain, in which molecules are trapped in discrete, non-H-bonding sites; and the speedier dynamics near hydrophilic regions, where surface-water H-bonding promotes translations. The coexistence of two populations of molecules showing slow and fast dynamics is common to all partially hydroxylated systems (12.5% ≤ 𝑓SiOH ≤ 50%), as shown in the van Hove functions


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presented in Figures S10 and S11 of the Supporting Information. The monotonic increase of 𝐷∥ with 𝑓SiOH is due to the fact that the fraction of mobile water molecules increases with the number of silanol groups that promote water translations through H-bonds. Stated another way, translational dynamics slow down as the fraction of the surfaces occupied by hydrophobic regions increases. Through formation of ice-like water configurations, hydrophobic subdomains trap water molecules, a phenomenon akin to caging observed in supercooled liquids and water.47 III.4. Anisotropic diffusion through surface patterning The results presented in Figures 4 and 5 demonstrate that diffusion is promoted by the exchange of water molecules between vicinal surface H-bonding groups. The diffusion mechanism is therefore similar to that observed in non-equilibrium MD simulations of water molecules in Couette flow48, where slip (thus surface diffusion) is reported on hydrophilic MgO surfaces provided the distance between sorption sites is < 0.21 nm48. In this section, we explore how surface patterning with H-bonding silanol groups can induce anisotropic diffusion along one of the inplane directions. To investigate this hypothesis, we consider a slit pore as that studied above, setting 𝑓SiOH = 25% and 50%. Unlike the randomly hydroxylated surfaces, however, silanol groups are placed along stripes parallel to the y-axis of the simulation domain, as illustrated in Figures 1(c) and 9. The distance between hydrophilic stripes is 1.71 nm and 1.29 nm (measured from SiOH oxygen atoms) for 𝑓SiOH = 25% and 𝑓SiOH = 50%, respectively (cf. Figure 9). MD simulations are run for 200 ns in the NVE ensemble. The mean water density, system size, and other simulation parameters are the same as those considered above.


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𝑓SiOH = 25%

Page 20 of 33

𝑓SiOH = 50%

(a)

(b)

Figure 9. Structure of patterned hydroxylated surfaces, showing (a) 𝑓SiOH = 25% and (b) 𝑓SiOH = 50% (b) hydroxylation. The color coding for atoms is as per Figure 1.

Figure 10 shows the in-plane MSD of water O atoms computed along the directions parallel and orthogonal to the striped patterns (〈𝑦 2 (𝑡)〉 and 〈𝑥 2 (𝑡)〉, respectively) for 𝑓SiOH = 25% (Figure 10(a)) and 50% (Figure 10(b)). In both systems, the in-plane motion orthogonal to the surface patterns (〈𝑥 2 〉) is highly restricted, the MSD at 𝑡 = 200 ns being nearly one order of magnitude lower than that computed in the direction parallel to the stripes. For 𝑓SiOH = 50%, the RMSD observed in the perpendicular direction appears to saturate at an upper bound value, 〈𝑥 2 (𝑡)〉1/2  0.4 nm, determined by the width of each stripe (i.e., 0.42 nm, as depicted in Figure 9(b)). Similarly, for 𝑓SiOH = 25%, we observe 〈𝑥 2 (𝑡)〉1/2  0.16 nm at 𝑡 = 200 ns, comparable to the OH bond length of surface silanol groups (0.1 nm). The corresponding diffusion coefficients parallel to the stripes are 𝐷yy = 2.0 × 10-9 cm2 s-1 and 𝐷yy = 2.2 × 10-8 cm2 s-1 for 𝑓SiOH = 25% and 50%, respectively. The data presented in Figure 10 therefore show that Brownian motion of water molecules in 2-D confinement can be “rectified”, and rendered effectively one dimensional, through surface patterning.


19

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fSiOH = 50%

fSiOH = 25%

1.00

x2(t) y2(t) x2(t)+y2(t)

0.10

0.50

0.05

0.00

x2(t) y2(t) x2(t)+y2(t)

MSD(t) [nm2]

MSD(t) [nm2]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.00 0

100

200

0

100

200

t [ns]

t [ns] (b)

(a)

Figure 10. Mean-squared displacement (MSD) of water molecules in confinement between surfaces with hydrophilic and hydrophobic surface patterns (shown in Figure 9). The MSD of water O atoms computed in the directions parallel and orthogonal to the hydroxylated stripes (〈𝑦 2 (𝑡)〉 and 〈𝑥 2 (𝑡)〉, respectively) is shown for 𝑓SiOH = 25% and 50%. Observe the different yaxis scales in the panels shown in the left and right.

To explain the origin of preferential translational dynamics, we computed the time-averaged density contours in patterned systems (Figure 11). For 𝑓SiOH = 50% (Figure 11(b)), we observe delocalized density contours characteristic of water molecule exchanges between silanol groups. 𝑓SiOH = 25%

𝑓SiOH = 50%

(a)

(b)


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Page 22 of 33

Figure 11. Time-averaged density contours of water molecules 𝜌(𝑥, 𝑦)/𝜌0 computed over 100 ns. Results are shown for patterned surfaces (cf. Figure 9) with 𝑓SiOH = 25% (a), and 50% (b). Representative 2  2 nm2 regions of the confined domain (10  10 nm2) are shown. The silanol O atoms are placed along the red dash lines shown in both panels.

Thus, the hydration structure along the silanol patterns locally resembles that of fully hydroxylated surfaces (cf. Figure 5(c)). On the other hand, in the hydrophobic regions between hydrophilic patterns, water adopts the ice-like configurations previously observed for fully hydrophobic confinement (observe the highly localized density contours, akin to those shown in Figure 5(a) for 𝑓SiOH = 0%). For the 𝑓SiOH = 25% system (cf. Figure 11(a)), the contours reflect a local density that is strongly localized, even near silanols. Thus, for this silica morphology, patterns comprising a minimum of two rows of nearest-neighbor silanols are necessary to observe a fluid-like hydration structure and dynamics along the stripes.

Figures 12 and 13, showing the 1-D density profile along the hydrophobic and hydrophilic patterned regions, provide further information on the interfacial water structure. For comparison, the 1-D density profiles in the fully hydrophobic (𝑓SiOH = 0%) and fully hydrophilic (𝑓SiOH = 100%) systems are also shown. When 𝑓SiOH = 25% (see Figure 12), 𝜌(𝑦)⁄𝜌𝑜 reveals a water structure reminiscent of the hydrophobic apolar confined system. In Figures 12(a) and (b), we note that the peak centers coincide with those of the 𝑓SiOH = 0%, both along the hydrophobic (Figure 12(a)) and hydrophilic patterns (Figure 12(b)). On the other hand, the structure revealed by 𝜌(𝑦)⁄𝜌𝑜 in the 𝑓SiOH = 50% system shows the fluid-like broad peaks of the fully hydrophilic systems (cf. Figure 13(b), computed along the hydrophilic stripes), while along the hydrophobic patterns, strong positional correlations, indicative of ice-like water, are observed (Figure 13(a)).


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(a)

(b)

Figure 12. Density profile 𝜌(𝑦)⁄𝜌𝑜 (𝜌𝑜 = 0.97 g cm-3) of water molecules in confinement between hydrophobic apolar surfaces (𝑓SiOH = 0%), hydrophilic surfaces (𝑓SiOH = 100%), and surfaces with patterned surface hydroxylation (𝑓SiOH = 25%). The inset depicts the region within which 𝜌(𝑦)⁄𝜌𝑜 was computed in the 25% system, showing the density profile along the hydrophobic pattern (a), and hydrophilic pattern (b).

In closing, the existence of ordered hydrophobic and hydrophilic subdomains explains the emergence of enhanced diffusion along the Cartesian direction with patterned hydrophilicity. Only those water molecules H-bonded to SiOH groups can diffuse, doing so preferentially in the direction parallel to the stripes. Water molecules translating in the in-plane direction normal to the stripes can at most diffuse the distance between two silanol groups (i.e., the width of the hydrophilic patterns, which for 𝑓SiOH = 50% is ~ 0.4 nm). The observed anisotropy of the in-plane translational dynamics is similar to that reported in a recent MD simulation study of supercritical fluids confined by calcite slit pores49, where molecule sorption and the crystalline morphology of calcite were shown to enhance surface diffusion along one of the in-plane directions of the slit. Anisotropy in methane transport within water-filled calcite pores was also recently reported50; in this system, however, methane diffusion is enhanced along the direction of low free energy


22


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Page 24 of 33

pathways that depend on the water structure, rather than on interactions with the confining surface atoms.

(a)

(b)

Figure 13. Density profile 𝜌(𝑦)⁄𝜌𝑜 (𝜌𝑜 = 0.97 g cm-3) of water molecules in confinement between hydrophobic apolar surfaces (𝑓SiOH = 0%), hydrophilic surfaces (𝑓SiOH = 100%), and surfaces with patterned surface hydroxylation (𝑓SiOH = 50%). The inset shows the region over which 𝜌(𝑦)⁄𝜌𝑜 was computed in the 50% system, showing the hydrophobic pattern (a), and hydrophilic pattern (b).

IV. Conclusions Using equilibrium MD simulations, we characterized the effect of heterogeneous surface chemistry on the dynamics and interfacial structure of confined water monolayers. Distinct water dynamics and structural features were observed in hydrophobic and hydrophilic regions that were randomly distributed on the confining silica surfaces: within hydrophobic regions, water was shown to form ice-like water configurations characterized by slow translational dynamics, whereas more mobile and disordered water was observed within hydrophilic regions, within which surfacewater H-bond interactions speed up water translations. The role played by surface-water Hbonding was evidenced by the monotonic increase of the diffusivity of water with 𝑓SiOH , the


23

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fraction of randomly distributed surface sites that bear hydroxyl (silanol) groups. Further, we showed that the fast dynamics observed in hydrophilic regions can be exploited to “rectify” diffusion (i.e., render diffusion approximately 1-dimensional) through surface patterning. Diffusion of water molecules was shown to occur preferentially along the direction of the surface patterns, owing to H-bond exchanges between surface silanol groups and water molecules. V. Supporting Information Details on the calculation of density contour plots (Equation S1); density contour plots for all systems investigated (Figures S1-S7); plots of the radial distribution, density profile, and van Hove distribution function for all systems (Figures S8-S11). Movies M1-M3. VI. Acknowledgements Support from 3M Co. (St. Paul, MN) is gratefully acknowledged. This work was carried out in part using hardware provided by the Minnesota Supercomputing Institute at the University of Minnesota. We thank Fábio Aarão Reis (Universidade Federal Fluminense) and Nicolás Giovambattista (Brooklyn College of CUNY) for commenting on an earlier version of the manuscript. VII. References (1)

de Groot, B. L.; Grubmüller, H. Water Permeation Across Biological Membranes: Mechanism and Dynamics of Aquaporin-1 and GlpF. Science 2001, 294, 2353–2357 DOI: 10.1126/science.1062459.

(2)

Qiu, T.; Meng, X. W.; Huang, J. P. Nonstraight Nanochannels Transfer Water Faster than Straight Nanochannels. J. Phys. Chem. B 2015, 119, 1496–1502 DOI: 10.1021/jp511262w.


24


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(3)

Page 26 of 33

Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Water Conduction through the Hydrophobic Channel of a Carbon Nanotube. Nature 2001, 414, 188–190 DOI: 10.1038/35102535.

(4)

Luo, Y.; Harder, E.; Faibish, R. S.; Roux, B. Computer Simulations of Water Flux and Salt Permeability of the Reverse Osmosis FT-30 Aromatic Polyamide Membrane. J. Memb. Sci. 2011, 384, 1–9 DOI: 10.1016/j.memsci.2011.08.057.

(5)

Hu, Y.; Devegowda, D.; Striolo, A.; Phan, A.; Ho, T. A.; Civan, F.; Sigal, R. The Dynamics of Hydraulic Fracture Water Confined in Nano-Pores in Shale Reservoirs. J. Unconv. Oil Gas Resour. 2015, 9, 31–39 DOI: 10.1016/j.juogr.2014.11.004.

(6)

Pizzitutti, F.; Marchi, M.; Sterpone, F.; Rossky, P. J. How Protein Surfaces Induce Anomalous Dynamics of Hydration Water. J. Phys. Chem. B 2007, 111, 7584–7590 DOI: 10.1021/jp0717185.

(7)

Bagchi, B. Water Dynamics in the Hydration Layer around Proteins and Micelles. Chem. Rev. 2005, 105, 3197–3219 DOI: 10.1021/cr020661+.

(8)

Fayer, M. D.; Levinger, N. E. Analysis of Water in Confined Geometries and at Interfaces. Annu. Rev. Anal. Chem. 2010, 3, 89–107 DOI: 10.1146/annurev-anchem-070109-103410.

(9)

Romero-Vargas Castrillón, S.; Giovambattista, N.; Aksay, I. A.; Debenedetti, P. G. Effect of Surface Polarity on the Structure and Dynamics of Water in Nanoscale Confinement. J. Phys. Chem. B 2009, 113, 1438–1446 DOI: 10.1021/jp809032n.

(10)

Cerveny, S.; Mallamace, F.; Swenson, J.; Vogel, M.; Xu, L. Confined Water as Model of Supercooled

Water.

Chem.

Rev.

2016,

116,

7608–7625

DOI:


25

Page 27 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


10.1021/acs.chemrev.5b00609. (11)

Bergman, R.; Swenson, J.; Börjesson, L.; Jacobsson, P. Dielectric Study of Supercooled 2D Water in a Vermiculite Clay. J. Chem. Phys. 2000, 113, 357–363 DOI: 10.1063/1.481800.

(12)

Hu, J.; Xiao, X.-D.; Ogletree, D. F.; Salmeron, M. Imaging the Condensation and Evaporation of Molecularly Thin Films of Water with Nanometer Resolution. Science (80. ). 1995, 268, 267–269 DOI: 10.1126/science.268.5208.267.

(13)

Odelius, M.; Bernasconi, M.; Parrinello, M. Two Dimensional Ice Adsorbed on Mica Surface. Phys. Rev. Lett. 1997, 78, 2855–2858 DOI: 10.1103/PhysRevLett.78.2855.

(14)

Hu, J.; Xiao, X. -d.; Ogletree, D. F.; Salmeron, M. Structure of Molecularly Thin Films of Water on Mica in Humid Environments. Surf. Sci. 1995, 344, 221–236 DOI: 10.1016/00396028(95)00858-6.

(15)

Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G. Effect of Pressure on the Phase Behavior and Structure of Water Confined between Nanoscale Hydrophobic and Hydrophilic Plates. Phys. Rev. E 2006, 73, 41604 DOI: 10.1103/PhysRevE.73.041604.

(16)

Giovambattista, N.; Rossky, P. J.; Debenedetti, P. G. Phase Transitions Induced by Nanoconfinement in Liquid Water. Phys. Rev. Lett. 2009, 102, 50603 DOI: 10.1103/PhysRevLett.102.050603.

(17)

Ferguson, A. L.; Giovambattista, N.; Rossky, P. J.; Panagiotopoulos, A. Z.; Debenedetti, P. G. A Computational Investigation of the Phase Behavior and Capillary Sublimation of Water Confined between Nanoscale Hydrophobic Plates. J. Chem. Phys. 2012, 137, 144501


26


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 33

DOI: 10.1063/1.4755750. (18)

Leung, K.; Luzar, A.; Bratko, D. Dynamics of Capillary Drying in Water. Phys. Rev. Lett. 2003, 90, 65502 DOI: 10.1103/PhysRevLett.90.065502.

(19)

Lee, S. H.; Rossky, P. J. A Comparison of the Structure and Dynamics of Liquid Water at Hydrophobic and Hydrophilic Surfaces—a Molecular Dynamics Simulation Study. J. Chem. Phys. 1994, 100, 3334 DOI: 10.1063/1.466425.

(20)

Giovambattista, N.; Debenedetti, P. G.; Rossky, P. J. Hydration Behavior under Confinement by Nanoscale Surfaces with Patterned Hydrophobicity and Hydrophilicity. J. Phys. Chem. C 2007, 111, 1323–1332 DOI: 10.1021/jp065419b.

(21)

Argyris, D.; Cole, D. R.; Striolo, A. Dynamic Behavior of Interfacial Water at the Silica Surface. J. Phys. Chem. C 2009, 113, 19591–19600 DOI: 10.1021/jp906150n.

(22)

Argyris, D.; Cole, D. R.; Striolo, A. Hydration Structure on Crystalline Silica Substrates. Langmuir 2009, 25, 8025–8035 DOI: 10.1021/la9005136.

(23)

Ball, P. Water as an Active Constituent in Cell Biology. Chem. Rev. 2008, 108, 74–108 DOI: 10.1021/cr068037a.

(24)

Brovchenko, I.; Geiger, A.; Oleinikova, A.; Paschek, D. Phase Coexistence and Dynamic Properties of Water in Nanopores. Eur. Phys. J. E - Soft Matter 2003, 12, 69–76 DOI: 10.1140/epje/i2003-10028-4.

(25)

Brovchenko, I.; Krukau, A.; Smolin, N.; Oleinikova, A.; Geiger, A.; Winter, R. Thermal Breaking of Spanning Water Networks in the Hydration Shell of Proteins. J. Chem. Phys.


27

Page 29 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2005, 123, 224905 DOI: 10.1063/1.2121708. (26)

Oleinikova, A.; Brovchenko, I.; Smolin, N.; Krukau, A.; Geiger, A.; Winter, R. Percolation Transition of Hydration Water: From Planar Hydrophilic Surfaces to Proteins. Phys. Rev. Lett. 2005, 95, 247802 DOI: 10.1103/PhysRevLett.95.247802.

(27)

Giovambattista, N.; Debenedetti, P. G.; Rossky, P. J. Effect of Surface Polarity on Water Contact Angle and Interfacial Hydration Structure. J. Phys. Chem. B 2007, 111, 9581–9587 DOI: 10.1021/jp071957s.

(28)

Giovambattista, N.; Lopez, C. F.; Rossky, P. J.; Debenedetti, P. G. Hydrophobicity of Protein Surfaces: Separating Geometry from Chemistry. Proc. Natl. Acad. Sci. 2008, 105, 2274–2279 DOI: 10.1073/pnas.0708088105.

(29)

Berendsen, H. J. C.; Grigera, J. R.; Straatsma, T. P. The Missing Term in Effective Pair Potentials. J. Phys. Chem. 1987, 91, 6269–6271 DOI: 10.1021/j100308a038.

(30)

van der Spoel, D.; van Maaren, P. J.; Berendsen, H. J. C. A Systematic Study of Water Models for Molecular Simulation: Derivation of Water Models Optimized for Use with a Reaction Field. J. Chem. Phys. 1998, 108, 10220–10230 DOI: 10.1063/1.476482.

(31)

Romero-Vargas Castrillón, S.; Giovambattista, N.; Aksay, I. A.; Debenedetti, P. G. Structure and Energetics of Thin Film Water. J. Phys. Chem. C 2011, 115, 4624–4635 DOI: 10.1021/jp1083967.

(32)

Plimpton, S. Fast Parallel Algorithms for Short-Range Molecular Dynamics. J. Comput. Phys. 1995, 117, 1–19 DOI: 10.1006/jcph.1995.1039.


28


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(33)

Page 30 of 33

Ryckaert, J.-P.; Ciccotti, G.; Berendsen, H. J. C. Numerical Integration of the Cartesian Equations of Motion of a System with Constraints: Molecular Dynamics of N-Alkanes. J. Comput. Phys. 1977, 23, 327–341 DOI: 10.1016/0021-9991(77)90098-5.

(34)

Pollock, E. L.; Glosli, J. Comments on P3M, FMM, and the Ewald Method for Large Periodic Coulombic Systems. Comput. Phys. Commun. 1996, 95, 93–110 DOI: 10.1016/0010-4655(96)00043-4.

(35)

Nosé, S. A Unified Formulation of the Constant Temperature Molecular Dynamics Methods. J. Chem. Phys. 1984, 81, 511–519 DOI: 10.1063/1.447334.

(36)

Hoover, W. G. Canonical Dynamics: Equilibrium Phase-Space Distributions. Phys. Rev. A 1985, 31, 1695–1697 DOI: 10.1103/PhysRevA.31.1695.

(37)

Koga, K.; Zeng, X. C.; Tanaka, H. Freezing of Confined Water: A Bilayer Ice Phase in Hydrophobic

Nanopores.

Phys.

Rev.

Lett.

1997,

79,

5262–5265

DOI:

10.1103/PhysRevLett.79.5262. (38)

Koga, K.; Tanaka, H. Phase Diagram of Water between Hydrophobic Surfaces. J. Chem. Phys. 2005, 122, 104711 DOI: 10.1063/1.1861879.

(39)

Luzar, A.; Chandler, D. Effect of Environment on Hydrogen Bond Dynamics in Liquid Water. Phys. Rev. Lett. 1996, 76, 928–931 DOI: 10.1103/PhysRevLett.76.928.

(40)

Lawrence, C. P.; Skinner, J. L. Vibrational Spectroscopy of HOD in Liquid D2O. III. Spectral Diffusion, and Hydrogen-Bonding and Rotational Dynamics. J. Chem. Phys. 2003, 118, 264–272 DOI: 10.1063/1.1525802.


29

Page 31 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41)

Soper, A. K.; Phillips, M. G. A New Determination of the Structure of Water at 25°C. Chem. Phys. 1986, 107, 47–60 DOI: 10.1016/0301-0104(86)85058-3.

(42)

Koga, K.; Tanaka, H.; Zeng, X. C. First-Order Transition in Confined Water between HighDensity Liquid and Low-Density Amorphous Phases. Nature 2000, 408, 564–567 DOI: 10.1038/35046035.

(43)

Diestler, D. J.; Schoen, M.; Cushman, J. H. On the Thermodynamic Stability of Confined Thin Films under Shear. Science. 1993, 262, 545–547 DOI: 10.1126/science.262.5133.545.

(44)

Van Hove, L. Correlations in Space and Time and Born Approximation Scattering in Systems

of

Interacting

Particles.

Phys.

Rev.

1954,

95,

249–262

DOI:

10.1103/PhysRev.95.249. (45)

Hansen, Jean-Pierre; McDonald, I. R. Theory of Simple Liquids; Academic Press: London, U.K., 1990.

(46)

Hopkins, P.; Fortini, A.; Archer, A.; Schmidt, M. The van Hove Distribution Function for Brownian Hard Spheres: Dynamical Test Particle Theory and Computer Simulations for Bulk Dynamics. J. Chem. Phys. 2010, 133, 224505 DOI: 10.1063/1.3511719.

(47)

Starr, F. W.; Sciortino, F.; Stanley, H. E. Dynamics of Simulated Water under Pressure. Phys. Rev. E 1999, 60, 6757–6768 DOI: 10.1103/PhysRevE.60.6757.

(48)

Ho, T. A.; Papavassiliou, D. V.; Lee, L. L.; Striolo, A. Liquid Water Can Slip on a Hydrophilic Surface. Proc. Natl. Acad. Sci. 2011, 108, 16170–16175 DOI: 10.1073/pnas.1105189108.


30


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(49)

Page 32 of 33

Franco, L. F. M.; Castier, M.; Economou, I. G. Anisotropic Parallel Self-Diffusion Coefficients near the Calcite Surface: A Molecular Dynamics Study. J. Chem. Phys. 2016, 145, 084702 DOI: 10.1063/1.4961408.

(50)

Bui, T.; Phan, A.; Cole, D. R.; Striolo, A. Transport Mechanism of Guest Methane in WaterFilled

Nanopores.

J.

Phys.

Chem.

C

2017,

121,

15675–15686

DOI:

10.1021/acs.jpcc.7b02713.


31

Page 33 of 33

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Dynamics of Water Monolayers Confined by Chemically

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