Influence of Hydrophilic Surface Specificity on the ... - ACS Publications

J. Phys. Chem. B 2009, 113, 13825–13839

13825

Influence of Hydrophilic Surface Specificity on the Structural Properties of Confined Water† Ateeque Malani,‡ K. G. Ayappa,*,‡ and Sohail Murad§ Department of Chemical Engineering, Indian Institute of Science, Bangalore, India, and Department of Chemical Engineering, UniVersity of Illinois at Chicago, Chicago, Illinois 60607 ReceiVed: March 21, 2009; ReVised Manuscript ReceiVed: June 23, 2009

The influence of chemical specificity of hydrophilic surfaces on the structure of confined water in the subnanometer regime is investigated using grand canonical Monte Carlo simulations. The structural variations for water confined between hydroxylated silica surfaces are contrasted with water confined between mica surfaces. Although both surfaces are hydrophilic, our study shows that hydration of potassium ions on the mica surface has a strong influence on the water structure and solvation force response of confined water. In contrast to the disrupted hydrogen bond network observed for water confined between mica surfaces, water between silica surfaces retains its hydrogen bond network displaying bulklike structural features down to surface separations as small as 0.45 nm. Hydrogen bonding of an invariant contact water layer with the surface silanol groups aids in maintaining a constant number of hydrogen bonds per water molecule for the silica surfaces. As a consequence, water depletion and rearrangement upon decreasing confinement is a strong function of the hydrophilic surface specificity, particularly at smaller separations. An oscillatory solvation force response is only observed for water confined between silica surfaces, and bulklike features are observed for both surfaces above a surface separation of about 1.2 nm. We evaluate and contrast the water density, dipole moment distributions, pair correlation functions, and solvation forces as a function of the surface separation. 1. Introduction The study of water confined to nanoscale dimensions has implications in areas ranging from biology1-3 to geology.4 Elucidating the role of confined water is important in seeking a molecular mechanism that governs friction,5,6 adhesion,7 swelling of clays,8-10 transport in ion channels,11,12 and the nature of forces between surfaces that are mediated by water.13-15 In this manuscript, we are interested in understanding the structure of water confined between hydrophilic surfaces of different chemical specificity. Water structure is analyzed in two systems: water confined between hydroxylated silica (HS) surfaces and water between mica surfaces, as a function of surface separation. The key difference between the two surfaces is the presence of K+ ions on mica which alter its hydrophilic characteristics in contrast to the HS surface. Due to its atomically smooth surface, mica has been widely used in a variety of experiments which seek to probe the structure of confined water, and silica is extensively used in the manufacturing of micro- and nanoelectromechanical systems (MEMS and NEMS). There are a number of experimental and simulation studies performed to understand the structure and dynamics of water in confined geometries. The interaction between two mica surfaces in the presence of pure water and electrolyte solutions as a function of surface separation, H, has been measured using the surface force apparatus (SFA).16-19 At larger surface separations (H > 20 Å), the interaction between mica surfaces is dominated by van der Waals interaction and electrostatic double layer repulsion which is well described by the DLVO theory.20 However, for smaller separation distances, additional hydration †

Part of the “H. Ted Davis Special Section”. * Corresponding author. ‡ Indian Institute of Science. § University of Illinois at Chicago.

forces which are short ranged and repulsive in nature are observed.16,17,21 The magnitude as well as the force-distance (H) profile changes as a function of electrolyte concentration.14,19,21 The measured force-distance profile is nonoscillatory in nature for pure water14 and dilute aqueous salt solutions (less than 10-4 M21) confined between mica surfaces, and the surfaces jump into adhesive contact for surface separations below 2 nm. An increase in the salt concentration introduces oscillations in the force-distance profile below surface separations of 1 nm.21 Although the origin of the oscillation with increasing salt concentration are not completely understood, it is associated with the formation of hydration shells with the increased K+ ion density on the mica surface.21 The oscillatory force profile is also observed for pure water confined between either mica or glass surfaces and a silicon nitride tip measured using atomic force microscopy (AFM).22 In a separate study, a force-distance profile is obtained for two surfaces modified with hydrophilic or hydrophobic self-assembled monolayers (SAM) at different relative humidity.23 The force-distance profile obtained for two hydrophilic SAMs is characterized by a nonoscillatory behavior with a single attractive minima at smaller distances. The strength of the attractive minima was found to increase with increasing relative humidity.23 Complementary to experimental investigations, molecular simulations have been widely used to probe the confined fluid structure and have proved to be a powerful tool in providing a molecular interpretation of the physics of confined fluids. Although a wide body of literature addresses the influence of confinement on nonpolar fluids24-27 and water in hydrophobic pores,28 there have been relatively few simulations which probe the structure of water confined between mica surfaces. Simulations have been performed to understand water structure and K+ hydration in an isothermal-isobaric ensemble at p ) 1 bar and T ) 298 K for H ) 6.1-24.4 Å.29 The deviation in the

10.1021/jp902562v CCC: $40.75  2009 American Chemical Society Published on Web 07/22/2009

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J. Phys. Chem. B, Vol. 113, No. 42, 2009

density of confined water from bulk density occurs for H < 9.2 Å. At large surface separations of H ) 24.4 Å, water shows density oscillations adjacent to the mica surface,29 consistent with the previous experiments30 and simulations.31 In a grand canonical Monte Carlo (GCMC) simulation of water confined between mica surfaces,32 water is modeled using the ab initio based Matsuoka-Clementi-Yoshimine (MCY) potentials,33 and the parameters of the water-mica interaction potentials were used from the MINDO force field.34 Two types of surfaces were constructed by exposing water molecules to different cleavage positions to elucidate the role of K+ ions on the structure of water. In one case, referred to as the charged mica, K+ ions are present on the mica surface, and in another case referred to as neutral mica, the K+ ions are buried and the silica layer is exposed to water. For water confined between neutral mica, an oscillatory force profile and swelling is observed.32 On the contrary, for water confined between charged mica, a nonoscillatory force profile with a single minimum was observed.32 In contrast, a later study of confined water between mica surfaces using molecular dynamics (MD) simulations revealed an oscillatory force behavior and strong layering of confined water.35 The origin of this oscillatory force behavior could be attributed to the force field parameters used in this particular study.35 The water charges are half the value of the original SPC36 model (even though the geometry and other parameters were kept constant) thereby altering the hydrogen bonding and hydration characteristics. This reduction in charge increases the tendency to form layers and produce solvation force oscillations similar to the features observed in confined nonpolar fluids.27,37 Molecular dynamics simulations report an order of magnitude decrease in self-diffusivity22,29,38 and increase in the viscosity38,39 for subnanometer confined water when compared with bulk water. A shear thinning of confined water for H ) 9.12 Å was observed in MD simulation studies.38 However, the shear rate over which this shear thinning occurs is 109 s-1 which is much larger than shear rates applied in experiments, 102-104 s-1. Further simulation studies reveal that these shear rates are associated with an order of magnitude higher orientational relaxation time compared to relaxation in bulk water.38,39 In contrast to the mica surface where hydration of the surface K+ ions influences the structural properties of water, silica surfaces are devoid of this specific feature. Water confined in silica pores has been widely investigated using both Monte Carlo and molecular dynamics simulations. These studies, primarily carried out in cylindrical silica pores,40-42 to understand water structure in Vycor pores, focus on the effect of pore radius and hydration levels on hydrogen bonding characteristics and the dynamical relaxation of water in the proximity of a silica surface. Simulations have also been carried out to investigate the effects of pore size on the adsorption isotherms and capillary condensation in silica pores.43,44 In the molecular dynamics of water confined between hydroxylated silica surfaces45 placed 5 nm apart, confined water was found to be disrupted to only within 10 Å from the surface, and strong water-silica hydrogen bonding was observed. The influence of surface hydroxyl density on water structure and dynamics adjacent to crystalline silica surfaces has recently been studied using molecular dynamics simulations.46 The behavior of water adjacent to flat crystalline silica surfaces differs from water confined in amorphous (Vycor) silica pores. In a combined experimental and simulation study, significant disruption in the hydrogen bonding was observed for water confined in 4 nm cylindrical and spherical Vycor pores.42 In a recent molecular dynamics

Malani et al. study where the silica surfaces are in contact with a water reservoir to simulate an open system, water density distributions and solvation force oscillations were obtained for water confined between two silica surfaces.22 In this manuscript, our primary focus is on understanding the role of confinement and surface chemical specificity on the structure of water confined between two extended hydrophilic surfaces. Mica and hydroxylated silica (HS) provide two commonly occurring hydrophilic surfaces, and this study elucidates the manner in which hydrogen bonding and structural changes occur in water as a function of confinement, with particular emphasis in the nanometer and subnanometer regimes. Simulations using SPC/E water are carried out in the grand canonical ensemble where water loading between the surfaces is determined by the chemical potential of the bulk water reservoir in equilibrium with the pore. By analyzing water properties, such as layer density distributions, pair correlation functions, hydrogen bond distributions, and solvation force, we contrast the differences in water structure in these two different, albeit hydrophilic, confining environments. Our findings are discussed in light of the interpretations of confined water structure from recent experiments and simulations. 2. Simulation Details 2.1. Surface Structure. We have used two types of surfaces, namely, hydroxylated silica (HS) and muscovite mica. To construct the HS surface the (-1,0,1) Miller plane of the R-cristobalite crystal47 is exposed to water. Figure 1a shows the unit cell with a repeat unit in the positive z direction. The blue plane is the plane formed by the Miller indices (-1,0,1). The atoms below the Miller plane are removed, and the rest of the atoms are used to construct the HS surface used in the simulations. The dangling oxygens are hydroxylated with hydrogen atoms, such that the Si-O-H angle is 109.5° with a distance of 0.9572 Å between surface oxygen and hydrogen.45 The simulation cell is constructed by repeating the unit cell to form the HS surface with dimensions Lx ) 42.78 Å and Ly ) 38.87 Å containing 3040 atoms. The top view of the surface constructed using this procedure is shown in Figure 1b. The resulting silanol surface density is 4.69 nm-2, which is in good agreement with the values of 4.62 and 4.71 nm-2, reported in simulations45 and experiments,48 respectively. The plane passing through the dangling surface oxygens is defined as the basal plane. As shown in Figure 1c, muscovite mica KAl2(AlSi3)O10(OH)2 consists of tetrahedral silicon and octahedral aluminum layers, which are connected through a bridging oxygen. The charge deficiency in each silicon layer created by the substitution of one of the silicon atoms with an aluminum ion is balanced by the presence of potassium ions at the surface. A freshly cleaved mica surface has an oxygen surface layer with a random distribution of K+ ions on it. In this study, we have used X-ray structure data of muscovite mica.49 The z coordinates of the surface oxygen atoms differ within 0.23 Å. In our simulation, we cleave a single unit cell (Figure 1c) which is then repeated in x and y directions to construct a mica surface. The mica surface consists of 7 × 4 unit cells making up a periodic box of dimensions Lx ) 36.43 Å and Ly ) 36.07 Å containing 2352 atoms. Using this procedure, the K+ ions are arranged in a regular manner on the mica surface as shown in Figure 1d. The plane passing through the innermost surface oxygen layer is defined as the basal plane. The simulation cell is created by placing two surfaces separated by a distance of H, which is the distance between basal planes of opposing surfaces. The x-y

Structural Properties of Confined Water

J. Phys. Chem. B, Vol. 113, No. 42, 2009 13827 TABLE 1: Potential Parameters Used in the Simulation ε, kJ · mol-1

atoms a

O (water) H (water)a H-surface (silica)b O-surface (silica)b Si-surface (silica)b O (silica)b Si (silica)b K (mica)c O1 (mica, bridging oxygen between Si layer and K)c O2 (mica, bridging oxygen between Si and Al layer)c O of OH (mica)c H of OH (mica)c Si (mica)c Al-substituted (mica)c Al (mica)c a

Figure 1. (a) Two unit cells of R-cristobalite.47 The plane formed by the Miller indices (-1,0,1) is shown by a blue line. The atoms below the Miller plane are removed to create the silica surface. (b) The top view of the hydroxylated silica (HS) surface used in simulation. (c) The cleaved unit cell of mica used to make the mica surface. (d) The top view of mica surface showing the ordered arrangement of potassium ions. Red, O; Yellow, Si; White, H; Green, Al; and Blue, K+.

plane of the surface coincides with the x-y plane of the simulation cell. An extra vacuum of length Lv on each side is kept above the surface for the treatment of long-range electrostatic forces. The length of vacuum is Lv ) 2Hs + H, where Hs is the height of the surface in the z direction. The values of Hs for HS and mica surfaces are 22.3 and 18.13 Å, respectively. The opposing surfaces are placed in registry for both mica and HS surfaces. 2.2. Interatomic Potentials. The interactions between atomic water-water sites and water-surface sites are modeled with the 12-6 Lennard-Jones (LJ)

UijLJ

) 4εij

[( ) ( ) ] σij rij

12

σij rij

(1)

In the above equations, rij is the distance between two atoms i and j. The cross interaction parameters (εij, σij) in the LJ potential

-0.8476 0.4238 0.4 -0.71 0.31 0.0 0.0 1.0 -1.05

0.6502

3.166

-1.16875

0.6502 0.0 7.007 × 10-6 7.007 × 10-6 5.5639 × 10-6

3.166 0.0 3.302 3.302 4.2714

-0.95 0.425 2.1 1.575 1.575

(3)

C C where Uww is the water-water interaction energy and Uws is the water-surface contribution to the energy. For a system containing N water molecules

C Uww )

N

]

3

i)1 j)i+1 R)1 Mj N 3

q q erfc(κriRjβ) + riRjβ β)1

j)1 R)1 N 3

∑∑

i)1 R)1

[

[∑ ∞

e-k /4κ cos(k · riRjβ) + k2 k*0 2 3 qiRqiβ erf(κriRiβ) κ qiR + (4) 4πε0 riRiβ √π 4πε0 β)R+1

qiRqjβ 4πε0 β)1

∑∑∑ ∑

ziRzjβ -

and

3

iR jβ ∑ ∑ ∑ ∑ 4πε 0 N

(2)

3.166 0.0 0.0 3.795 3.154 3.795 3.154 3.334 3.166

C C UC ) Uww + Uws

2π Vc i)1 qiqj 4πε0rij

0.6502 0.0 0.0 0.5336 0.6487 0.5336 0.6487 0.4184 0.6502

are obtained using the Lorentz-Berthelot mixing rules εij ) (εiiεjj)1/2 and σij ) (σii + σjj)/2, where, εii and σii are the interaction energy parameter and molecular diameter of atom i, respectively. In eq 2, qi is the charge on atom i, and ε0 is the vacuum permittivity. In the case of water, only the oxygen atoms interact with the LJ potential. The parameter values used in these simulations are listed in the Table 1. Water-water interactions are modeled using the Extended Simple Point Charge (SPC/E)50 model; the HS surface is modeled with the parameters described by Lee and Rossky;45 and the CLAYFF51 potential is used for mica atoms. The CLAYFF potential has been used to successfully predict the adsorption isotherm and structure of water on muscovite mica52 and has been used widely to study the swelling of clays.51 The LJ potential is evaluated with a cylindrical cutoff (Rcut ) Ly/2), whereas Coloumbic interactions are treated using 3D Ewald sum with an extra vacuum above the surface53 along with an appropriate correction for the slab geometry.54 The total Coloumbic interaction for the system is

and Coloumbic interactions

UijC )

q, e

Berendsen et al.50 b Lee et al.45 c Cygan et al.51

N

6

σ, Å

2

∑

2

]

13828

J. Phys. Chem. B, Vol. 113, No. 42, 2009 N

C Uws

)

Ns

3

Mj

q q erfc(κriRjβ) + riRjβ β)1

iR jβ ∑ ∑ ∑ ∑ 4πε 0

i)1 j)1 R)1 Mj N Ns 3

q q

[

∞

-k2/4κ2

iR jβ e ∑ ∑ ∑ ∑ 4πε ∑ k2 0

4π Vc i)1

Malani et al.

j)1 R)1 β)1

i

]

cos(k · riRjβ) + ziRzjβ

k*0

(5)

where κ and k are Ewald parameters; Vc ) LxLyLz is the volume of the simulation cell; riRjβ ) |riR - rjβ| is the distance between site “R” of molecule i and site “β” of molecule j; ziR is the z coordinate of site R of molecule i; and Mj is the total number of atomic sites on surface j, which is 3040 for the HS surface and 2352 for the mica surface. Since we are considering a slitpore geometry, the number of surfaces Ns ) 2. In the water-water interaction (eq 4), the first term is the short-range interaction evaluated in real space. The second term consists of the long-range interaction evaluated in Fourier space and followed by the dipole correction term for slab geometry.54 The last term is the self-correction term for molecules. For the water-surface interaction (eq 5), we have only short-range, long-range, and dipole correction contributions. 2.3. GCMC Simulations. GCMC (µVT) simulations of water confined between two surfaces (held at a fixed surface separation, H) are performed by equilibrating the gap with a bulk reservoir at fixed temperature, T, and chemical potential, µ. The excess chemical potential, µex, and bulk density, Fb, are related to the activity, z, in the following manner

( )

z ) Fb exp -

µex RT

∆z ∆z N (z ,z + 〈 2 2 )〉 F (z) )

(6)

where R is the gas constant. To obtain bulk activity corresponding to a bulk density, Fb ) 1000 kg · m-3 and temperature 300 K, a series of GCMC simulations at various water activities were performed. From these simulations, z ) 0.256 × 10-6 Å-3 yields Fb ) 997 kg · m-3 and µex ) -29.37 kJ · mol-1. The µex obtained from these simulations is in excellent agreement with the reported µex ) -29.2 kJ · mol-1 at T ) 298.15 K for SPC/E water.55,56 The pressure of bulk water is found to be P ) 154.04 bar, which is within the range of values P ) 40-170 bar reported in the literature.57-59 The GCMC simulations of confined water between the surfaces are performed for various values of the surface separations, H. The atoms that make up the surfaces are held rigid during the simulation. We have compared our results with simulations where the surface atoms are flexible, and these comparisons are discussed later in the text. GCMC trial moves consist of insertion, deletion, and displacement accompanied by rotation of the water molecule. The moves were randomly chosen with equal probabilities to ensure microscopic detailed balance. The maximum rotation and displacement of a molecule is modified with a probability of 0.16 (half of displacement probability) every 100 GCMC moves. The rotation of water molecules is achieved using the standard rotation matrices for rigid molecules.60 Simulations involve 6 × 108 equilibration moves followed by 4 × 108 production moves during which ensemble averaged properties are evaluated. 2.4. Structural Properties. 2.4.1. Density Distribution. The structure of confined water perpendicular to the surface is evaluated using density distribution defined as

i

(7)

A∆z

where i is either a hydrogen or oxygen atom; (Ni(z - (∆z/2), z + (∆z/2))) is the ensemble averaged number of atoms in a bin of thickness ∆z in the z direction; and A ) LxLy. 2.4.2. Hydrogen Bonding. We have used the geometric criteria61 to define a hydrogen bond (HB) between two water molecules i and j. The criteria used involve both distances and angles. Hydrogen bonding between the two water molecules c c , rOH,ij e rOH,ij , and φ e φc, where rOO,ij ) occurs if rOO,ij e rOO,ij |rO,i - rO,j| is oxygen-oxygen distance between two water molecules, rOH,ij ) |rO,i - rH,j| is oxygen-hydrogen distance of the HB, and φ is the angle between the vectors rOO,ij and rOH,ii. c c and rOH,ij are 3.6 and 2.4 Å based on the The values of rOO,ij positions of the first minima in the oxygen-oxygen and oxygen-hydrogen PCFs (gOO(r) and gOH(r)) for bulk SPC/E water, respectively. The critical value for φc is based on the distribution of φ in bulk water,and is found to be 30°.61 Using these values, the HB distribution calculated in bulk water was in good agreement with previous simulation results.62 To define the HB between water molecules and the surface, only two c c and rOH,ij e rOH,ij , are distance criteria, i.e., rOO,ij e rOO,ij considered. 2.4.3. In-Plane Pair Correlation Function. The site-site inplane pair correlation function (PCF) of confined water within each layer is calculated using63

gl,Rβ(r) )

〈

NR,l Nβ,l

∑ ∑′

1 NR,lNβ,l i)1

j)1

δ(r + ri - rj) 2πr∆r

〉

(8)

where NR,l and Nβ,l are the number of sites R and β, respectively, in layer l. The prime indicates that the i ) j terms are omitted when R ) β. The bounds for each layer are obtained from the minima in the corresponding density distribution defined in eq 7. 2.4.4. Dipole Orientation Distribution. The orientation of confined water between the surface for each layer is analyzed by understanding the probability distribution of angle between the surface normal and water dipole, calculated using

P(cos θ) )

〈

Nl

∑

〉

1 δ(cos θ) Nl i)1

(9)

where Nl is the number of water molecules in layer l; cos θ ) nD · ez; θ is the angle between the surface normal, ez, and the unit vector of the water dipole, nD ) D/|D|. 2.5. Disjoining Pressure. The force acting normal to the surface is described as the solvation force, and the corresponding pressure is termed as the solvation pressure. The solvation force acting on surface j consisting of Mj atoms is the sum of the normal force (z-component) acting on each surface atom and is evaluated using Mj

fj,sf )

LJ C + fjβ,z ) ∑ (fjβ,z

β)1

(10)


J. Phys. Chem. B, Vol. 113, No. 42, 2009 13829

Figure 2. Density profile of oxygen (solid) and hydrogen (dashed) atoms of water confined between HS surfaces for various values of surface separations, H. The density profile illustrates the formation of layers as the surface separation is varied. The contact layer L1 is present for the entire range of surface separations H, and as the surface separation is decreased, water density in the inner layer is affected. The central regions where bulklike water densities are observed are denoted by CR. LJ C where fjβ,z and fjβ,z are the LJ and Coloumbic contributions of the force acting on atomic site β of surface j, respectively. The forces are evaluated using

N

LJ f jβ,z )-

∑ 48εiOjβ i)1

N

C f jβ,z )N

3

q q

iR jβ ∑ ∑ 4πε 0 i)1 R)1 3

q q

[

[

[( ) ( ) ] σiOjβ riOjβ

R)1

-

1 σiOjβ 2 riOjβ

6

]

ziOjβ 2 riOjβ

(11)

erfc(κriRjβ) ziRjβ 2 2κ -κ2riRjβ + e 2 riRjβ riRjβ √π

∞

-k2/4κ2

iR jβ e ∑ ∑ 4πε ∑ k2 0

4π Vc i)1

12

k*0

]

sin(k · riRjβ)kz + ziR

(12)

The deviation of solvation pressure from the bulk pressure is termed as the disjoining pressure Π ) Psf - Pb, where Psf ) (1/2A)[f1,sf + f2,sf] is the mean solvation pressure and Pb is the bulk pressure of the water in equilibrium with the confined water. 3. Results and Discussion 3.1. Water Structure in Silica Surfaces. The oxygen and hydrogen density distributions for a range of surface separations are illustrated in Figure 2. A common feature that is observed is the presence of a contact layer (L1), whose intensity is relatively invariant for separations ranging from 13 to 4.5 Å. At H ) 13 Å, in addition to the contact layer, an additional inner layer L2 is also present. Beyond L2, the density distributions reveal the presence of bulklike water in the central regions

Figure 3. (a) In-plane pair correlation function (PCFs), gl,OO(r) and (b) dipole orientation distribution (P(cos θ)) of water in the L1 layer adjacent to the hydroxylated silica (HS) surface. Corresponding properties for L2 water are depicted in (c) and (d), respectively. For clarity, the PCFs and P(cos θ) distributions are displaced along the y axis by 1.5 and 0.1 units, respectively, and the P(cos θ) data follow a similar sequence for H as shown for the corresponding PCFs. The PCF (a) illustrates the invariant structure of L1 water over a wide range of surface separations. The peak at cos θ ) -0.5 and the broader peak at cos θ ) 0.5 in P(cos θ) distributions represent distinct hydrogen bond environments in the contact layer L1 (Figure 4). Both the PCFs (c) and P(cos θ) distributions (d) reveal bulklike features of water in the L2 layer at larger H, and the uniform nature of the P(cos θ) distributions changes only for H < 7 Å.

denoted by CR in Figure 2. At H ) 13 Å, the layers corresponding to L1 and L2 are observed at 0.85 and 3.3 Å, respectively, from the basal plane of the HS surface. At the larger surface separations, the hydrogen density distribution indicates that the z coordinates of the hydrogen atoms are not significantly different from those of the oxygen atoms. The in-plane oxygen-oxygen PCF of the contact L1 layer illustrated in Figure 3a indicates that the extent of ordering of the oxgyen atoms is also similar over the entire range of separations. The corresponding dipole orientation distribution of the layer L1 (Figure 3b) has a broad distribution between cos θ ) -0.5 and 0.5 with specific changes occurring at the smaller surface separations below H ) 6 Å. The in-plane oxygen-oxygen PCF of the layer L2 as a function of surface separation illustrated in Figure 3c indicates the liquid-like arrangement of water molecules in this layer. Interestingly, the extent of ordering in the oxgyen atoms is relatively invariant down to surface separations of H ) 6.5 Å although the environment for L2 water changes as the surface separation is decreased. For H below 8 Å, L2 water occupies the central regions (Figure 2) between the surfaces, and water exists only in the form of layers. The corresponding dipole orientation distribution (Figure 3d) illustrates a uniform distribution at the larger H similar to bulk water, with distinct changes occurring below H ) 6.5 Å. To understand the in-plane structure and dipole orientation distributions, we examined the snapshot (Figure 4) for the L1 molecules at H ) 13 Å. The snapshots reveal two distinct environments for the surface water molecules. Water is able to HB with the hydrogen as well as oxygen of the OH groups that form the HS surface, and examples of this situation are illustrated with circles in Figure 4. In this situation, each water molecule is able to hydrogen bond with three surrounding OH groups on the HS surface and clearly indicates the manner in

13830


Malani et al. TABLE 2: Percentage of Water Molecules (pn) in Layers L1 and L2 and the Central Region of Hydroxylated Silica Surfaces Forming n ) 1...5 HB and Average Number of HBs per Water Molecule (nHB) for Various Surface Separations, Ha

Figure 4. Snapshot of water molecules in the L1 layer adjacent to the HS surface for H ) 13 Å. Blue, surface oxygen; Red, water oxygen; and White, hydrogen in the surface hydroxyl (OH) groups and water. The dashed line represents the HB between water-OH or water-water. Water molecules which form HBs with the dangling surface OH, with other water molecules within the layer, and both (dangling surface OH and water molecules within the layer) are enclosed by circles, squares, and triangles, respectively. The configurations represented by circles contribute to the narrow peak at cos θ ) -0.5 in the P(cos θ) distribution (Figure 3b).

which dangling OH of the HS silica surface participates with the water HB network. Due to their preferential orientation and bonding with the HS surface, these water molecules give rise to a narrower peak in the dipole orientational distribution corresponding to cos θ ) -0.5 (Figure 3b). The other distinct environments for the water molecules (marked by squares and triangles) consists of water that HBs with other water molecules (squares) as well as water that HBs with both surface OH and neighboring water molecules (triangles) giving rise to a broader range of orientations within the layer. These water molecules contribute to the broader peak corresponding to cos θ ) 0.5 (Figure 3b). The reason for these different populations of water orientations is partly due to the dual rhombic sublattice structure of the OH groups identified by circles (Figure 4), and the water molecules in the larger sublattice give rise to the water identified by squares. The increased density of hydrogen bonds adjacent to the silica surfaces has also been observed in previous simulations with both the SPC/E46 and TIP4P45 models of water adjacent to HS silica with similar silanol densities used in the current study. 3.2. Hydrogen Bonding in Silica Surfaces. Table 2 summarizes the percentage of water molecules, pn having n hydrogen bonds for the different layers L1, L2, and central regions (CR) for the larger separations. The data for bulk SPC/E water is also shown for comparison. For water in the contact L1 layer, we also obtain the partitioning to the HBs from adjacent water ww ) as well as the number of hydrogen bonds molecules (nHB formed with the silica surface (nws HB) using the criterion described earlier. The HB distribution obtained for the water molecules at H ) 13 Å in layer L1 including HBs within the layer as well as with the adjacent (L2) layer shows that ∼ 28% and 5.3% of water molecules form 3 and 4 HBs per water molecule (HBWM), respectively (Table 2). The water molecules form 1 HBWM within the layer and 1 HBWM with the L2 layer, which

ws nHB

t nHB

2.07 2.18 2.12 2.01 2.17 2.09 1.97

1.48 1.46 1.47 1.52 1.36 1.47 1.52

3.55 3.63 3.59 3.53 3.53 3.56 3.49

3.39 3.41 3.35 3.33 3.10

-

3.39 3.41 3.35 3.33 3.10

Central Region (CR) 9.43 34.12 49.94 5.37 3.49 9.28 33.59 50.79 5.23 3.49 8.31 32.60 51.89 6.22 3.54 -

3.49 3.49 3.54

H, Å

p0

p1

p2

p3

p4

p5

∞ (bulk)

0.0

1.12

9.58

13.0 9.0 7.5 6.5 5.5 4.5 4.0

3.28 2.87 3.53 2.49 4.74 1.54 2.77

24.67 21.35 22.76 28.64 18.56 23.40 26.11

Layer 1 (L1) 38.61 28.00 5.31 37.02 31.78 6.80 37.65 29.59 6.22 38.81 25.58 4.32 38.63 30.63 7.32 45.03 24.44 5.44 45.60 21.87 3.59

0.11 0.16 0.22 0.14 0.09 0.13 0.03

13.0 9.0 7.5 6.5 5.5

0.05 0.04 0.06 0.11 0.24

1.48 1.36 1.64 2.01 3.25

Layer 2 (L2) 11.80 36.60 45.66 11.34 36.21 46.25 12.94 38.40 42.40 13.02 37.62 43.33 18.65 44.27 30.78

4.34 4.72 4.50 3.85 2.75

13.0 11.5 10.5

0.03 0.03 0.03

1.03 1.02 0.86

ww nHB

33.91 49.87 5.40 3.49

a ww ws t ww , water-surface HB; and nHB ) nHB nHB , water-water HB; nHB ws + nHB .

gives rise to an average of 2 HBWM. Whereas in bulk water, we observe ∼34% and 50% of water molecules forming 3 and 4 HBWM giving rise to an average of 3.49 HBWM (Table 2). To compensate for the reduced HBs, water molecules in layer L1 form 1.48 HBWM with the HS surface giving rise to a total of 3.55 HBWM which is close to that of bulk water. This HB compensation from the surface silanol groups where both the oxygen and hydrogen of the silanols participate is observed in the snapshots shown in Figure 4 and helps in maintaining the total number of hydrogen bonds close to that observed in bulk water. As a result of the hydrogen bonding compensation from the HS surface, the number of HBs in the water layer adjacent to the HS surface remains invariant over the entire range of separations, down to H ) 4.0 Å (Table 2). Although the HBWM is close to that of bulk in the L1 layer, the distribution of hydrogen bonds (pn) is shifted downward and much broader than the distribution observed for bulk water. In contrast to the trends observed for water in the L1 layer, the distributions for L2 and the central regions are similar to bulk water (Table 2). From the analysis of the HB distribution for water in the L2 layer, we observe that ∼36.6% and 45.6% of water molecules form 3 and 4 HBWM giving rise to an average of 3.39 HBWM at H ) 13 Å (Table 2). This upward shift in the distribution is similar to the trends observed in bulk water. The water molecules in L2 do not form hydrogen bonds with the surface, and the average number of HBs in the L2 layer retain bulklike features for H g 8 Å. For H < 8 Å, central bulklike regions of free water are no longer present (Figure 2), and molecules in the L2 layer do not benefit from forming HBs with the central regions. Despite this loss of free water in the central regions, the number of HBs in the L2 layer remain close to that of the bulk (Table 2), with a decrease occurring only below H ) 6 Å which corresponds to the surface separation below which the L2 layer begins to get expelled (Figure 3c). Despite these structural changes, the deviation from bulklike features is not significant.



Figure 5. Variation in the number of water molecules in layers L1, L2, and central regions (CR) as a function of surface separation, H, for the HS surfaces. Note that the x-axis is in decreasing order of H. In the range of H > 9.5 Å, water from CR depletes, after which the L2 layer depletes for H < 9.0 Å. Water loading in the layer L1 is relatively invariant over the entire range of H. The complete depletion in CR at H ) 9.5 Å results in an increase in the contribution to L2 water.

The HB analysis of water in the central regions for H g 10.5 Å is given in Table 2, where both the average number of HBs as well as the distributions for the number of HBWM are similar to that of bulk water. This analysis includes HBs with water in the adjacent L2 layers as well. The uniform dipole orientation distribution for the central regions (not shown) is also consistent with the bulklike features observed in both the HB distributions as well as the in-plane structural characteristics. The HB analysis for L2 and the central regions signifies that surface effects in the inner layers are weak, and beyond the L1 layer, water shows structural characteristics similar to that of bulk water. 3.3. Water Depletion in Silica Surfaces. The density distributions (Figure 2) illustrate the manner in which water is depleted as the surface separation, H, is decreased. For H g 9.5 Å, we observe that the peak intensity corresponding to layers L1 and L2 in the density distributions does not change, whereas the central region shrinks with decreasing H. To understand the water depletion mechanism in detail, we analyzed the changes in the number of water molecules within each region. We observed that for H > 9.5 Å water molecules from the central region deplete with decreasing H as shown in Figure 5. In addition to depletion, we also observe redistribution of water in the range of H ) 9.5-13 Å where along with depletion of water from the central regions a small increase of water in the L2 regions is observed. However, the loading of water molecules in the L1 layer remains relatively constant over the entire range of surface separations. This is consistent with the invariant nature of the PCF (Figure 3a), dipole orientational distribution (Figure 3b), and HB distribution (Table 2) for water in layer L1. Further, when water depletes from the central regions (13 < H < 9.5 Å), both the PCF as well as the HB distributions (Table 2) for water in the central layers are relatively invariant. This suggests that as the central layer is depleted, water molecules are able to reorient themselves to retain their average number of hydrogen bonds. Below H ) 9.5 Å, water from the central regions is completely depleted, and water molecules from the L2 layer now occupy the central region between the HS surfaces (Figure 2). With further decrease in H ( 9.5 Å, with both the in-plane structure and HB network of water molecules preserved. However, to facilitate the HB with the relatively ordered structure of the L1 layer, water molecules show changes in the dipole orientational distribution for H ) 5.5-7.5 Å (Figure 3d). A further decrease in the surface separation (H < 5.5 Å) leads to the expulsion of water from the L2 layer leaving only two layers of water adjacent to the each surface. Water in the layers from opposing surfaces reorient to retain the average number of HBs as shown in the dipole orientational distribution (Figure 3b). 3.4. Water between Mica Surfaces: Large Separation, H ) 40 Å. Water between mica surfaces at a large surface separation of H ) 40 Å can be considered equivalent to having bulk water adjacent to the mica surface as revealed in the layer density distribution shown in Figure 6. The oxygen density distribution near the mica surface reveals four distinct peaks at 1.86, 2.86, 3.96, and 6.4 Å (Figure 6b), respectively. The peak positions are measured from the innermost oxygen atoms that form the basal oxygen plane of the mica surface. The layer associated with each of the peaks is referred to as L1, L2, L3, and L4, respectively. The peak intensities for these maxima compare very well with the recent simulation studies of interfacial water,64 and the peak positions are similar if comparisons are made with respect to a similar reference plane. The lower intensity of the L1 oxygen peak observed in the MD simulations of Wang et al.64 is due to the fully flexible mica

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Figure 7. Snapshot depicting the arrangement of water molecules in layer L1 for H ) 40 Å: (a) top view, (b) side view. Red, O; Yellow, Si; White, H; Green, Al; and Blue, K+. Water molecules in the contact L1 layer are localized in the ditrigonal cavities between the K+ ions and are uniformly oriented with their hydrogen atoms directed toward the mica surface.

lattice used in their simulations. The results indicate that the differences between a rigid lattice used in our simulations and a fully flexible lattice are small as far as the structural features are concerned. The density distributions also compare very well with recent experimental data from X-ray reflectivity studies.30 We point out that in the X-ray reflectivity experiments, K+ ions are completely exchanged with hydronium ions, and the reasons for the agreement with the simulations, despite this difference, are not completely understood. Similar agreement with experiments has been obtained in other molecular simulations investigations29,31 of water adjacent to mica with K+ ions. This general agreement with experiments has been attributed to the dominant role played by the basal oxygens on the mica surface.29 We also investigated the effect of keeping the K+ ions rigid by carrying out independent molecular dynamics simulations with unconstrained K+ ions. The peak positions and layering were similar to those observed in Figure 6a, and a small decrease in the first peak height was observed, indicating that keeping the K+ ions rigid is a reasonable assumption. The water molecules from the L1 layer occupy the ditrigonal cavities formed by the surface K+ ions, adsorbing in the center of the hexagonal ring formed by the surface oxygens of the mica surface as illustrated in the snapshot in Figure 7a. The tendency of water molecules to adsorb in the center of a hexagonal ring or ditrigonal cavities is also observed in other systems such as uncharged clays,65,66 smectites,67,68 and alkali-cation montmorillonite clays.9 However, the amount adsorbed, orientation, and distance from the surface depends on the type of clays as well as interaction potentials used. For the water-mica system, the location of the L1 layer occurs at 1.86 Å from the basal oxygens and corresponds to the plane of surface K+ ions (Figure 7b). As shown in Figure 8, the dipole orientation of water in the L1 layer at H ) 40 Å reveals that the plane of the water molecules is oriented parallel to the surface normal with hydrogen atoms pointing toward the oxygen atoms on the mica surface to form a hydrogen bond. This orientation of water is clearly visible in the snapshot shown in Figure 7b. In our simulations, the number of water molecules

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Figure 8. In-plane pair correlation function (left column) gl,OO(r) and dipole orientation distribution P(cos θ) (right column) for different layers (L1, L2, L3, and L4) adjacent to the mica surface for surfaces separated by 40 Å. The in-plane PCF, gl,OO(r), indicates the presence of strongly localized water in the ditrigonal cavities with surface normal orientation of the dipole vector in the L1 layer (see Figure 7). Bulklike features are observed in L4 water.

per ditrigonal cavity is found to be 0.98 ( 0.02 which is in excellent agreement with 1 and 1.3 observed in experiments30 and simulation29,31 results, respectively. The in-plane PCF reveals that water molecules in the L1 layer form an ordered lattice (Figure 8). However, unlike ice or bulk water, the first nearest neighbor distance is ∼5 Å, which is greater than 2.76 Å observed for bulk SPC/E water.50 Water molecules along with K+ ions form a slightly distorted hexagonal lattice as shown in the snapshot (Figure 7a), due to preferential adsorption in the ditrigonal cavities. As a result, L1 water does not contribute to the first hydration shell of the K+ ion. Since the first neighbor distance between oxygens in the layer L1 is ∼5 Å, which is c (used for the HB criterion), water much higher than rOO molecules in the L1 layer do not form HBs within the layer. However, they are able to form 2.17 HBWM with the L2 layer and 2 HBWM with the basal oxygens of the mica surface. The in-plane structure and HB distribution of water in the L1 layer is consistent with the previous MD simulation studies.29 Water molecules in the L2 layer are ∼1 Å away from the K+ ions and the L1 layer occupying positions between the oxygen of the L1 layer and the K+ ions, arranging themselves to form the first hydration shell around the K+ ions with hydrogen pointing toward the oxygen in the L1 layer. Similar structural arrangements were observed in our recent study on the adsorption isotherms of water on mica where the K+ ions were randomly positioned on the mica surface.52 This gives rise to the in-plane structuring of water molecules in the L2 layer observed in the PCF (Figure 8). Unlike the L1 layer, the nearest neighbor distance of water molecules in the L2 layer is 2.77 Å, which is close to the value observed for bulk water (2.76 Å50). The corresponding dipole orientation distribution shows two peaks at cos θ ) -0.84 and -0.26 (Figure 8). A careful examination of the snapshots reveals that two different orientations correspond to water molecules in different adsorption sites. The water molecules adsorbed above the substituted aluminum have a dipole orientation of cos θ ) -0.26, whereas cos θ ) -0.84 corresponds to water molecules adsorbed above the silicon atoms. The orientation is governed by the electrostatic interactions as well as the ability to complete the HB network around the K+ ion. The water molecule above the silicon atom



Figure 10. Variation in the ensemble averaged number of water molecules in the various water layers confined between mica surfaces as a function of surface separations, H. Note that the x-axis is in decreasing order of H and the right ordinate of (a) corresponds to the number in L4 + CR. Water in contact layer L1 begins to deplete only below H ) 7 Å. Bulk regions of water in L4 and the central region (CR) are absent for H below 10 Å.

Figure 9. Density distributions of oxygen (solid) and hydrogen (dashed) atoms of water molecules confined between mica surfaces for various values of surface separations, H.

experiences increased repulsion due to the higher electric charge when compared with the substituted aluminum (Table 1). Water in the L2 layer forms 0.51, 0.24, and 1.47 HBWM with water in the L1, L2, and L3 layer, respectively. L2 water also forms 0.75 HBWM with the mica surface giving rise to a total of 2.98 HBWM which is 85% of that observed in bulk water. We observe that despite the high loading of water in layer L2 when compared with water in the L1 layer and a nearest neighbor distance of 2.77 Å the HB network of water in the L2 layer does not attain bulk values. Unlike the HS silica, hydration of K+ plays a strong role in determining the water structure at the mica surface. For water molecules in the L3 layer, we observe a weak longrange structure as seen in the PCF (Figure 8). From the analysis of the HB network, water in the L3 layer forms an average of 0.91, 0.96, and 1.02 HBWM with water in L2, L3, and L4 layers, respectively. Water in the L3 layer does not form any HBs with the mica surface. This gives rise to a total of 2.88 HBWM which is 82.57% of total HBWM observed in bulk. The in-plane PCF and dipole orientation distribution of water in the L4 layer show bulklike characteristics (Figure 8) and form 3.48 HBWM. At these large separations, water in the central regions exhibits bulklike characteristics as seen in the density distribution (Figure 6). 3.5. Water Depletion between Mica Surfaces: Small Separations, H < 19 Å. The variation in the oxygen and hydrogen density distributions with decreasing H is illustrated in Figure 9. To quantify the changes in each layer, water loading within each layer is evaluated as a function of H and is shown in Figure 10. The number of water molecules in layers L1, L2, and L3 remains invariant for 11 e H e 19 Å, and water from L4 (Figure 6b) depletes up to H ) 11.5 Å. This depleting trend of L4 with a fixed water loading in L1, L2, and L3 layers is clearly illustrated in Figure 10a for 11 e H e 19 Å. At about H ) 12 Å, bulk water from the central regions is completely depleted,

giving rise to a single central L4 layer as seen in the corresponding density distribution in Figure 9. For H ) 9-10 Å, there is further depletion in the central regions, now referred to as L3, while loading in L1 and L2 remains unchanged (Figure 10b). We observe that the complete deletion of an inner layer is accompanied by a rearrangement of water into the adjacent layers. This situation is clearly observed in Figure 10a at H ) 11 Å where depletion in the L4 layer increases the loading in the adjacent L3 layers. The rearrangement of water molecules in this range of H is complete at H ) 8.5 Å giving rise to a single L3 layer between the surfaces (Figure 9), and an increased water loading in the L2 layer is observed (Figure 10c). The snapshots (Figure 11) clearly indicate increased order in the central L3 layer attributed to the redistribution effect as the surface separation is decreased from H ) 8.5 to 7.5 Å. In this range, the surface-planar orientation of molecules in the L3 layer is clearly observed. The rationale behind these rearrangements is better understood in light of potassium hydration discussed in the next section. For H < 8.5 Å, water depletion no longer takes place from the central L3 layer, and depletion occurs from the L2 layer with water redistributing from the L2 to L3 layers (Figures 10c and 11). The loading of water in the L1 layer remains constant (Figure 10a-c). At H ) 6.5 Å, the L2 layer is completely depleted leaving the adsorbed L1 and central layer between the two mica surfaces (Figure 9 and Figure 12). This central layer is referred to as L2 for H ) 4.5-6.5 Å (Figure 10d and Figure 12), and in this range of H (4.5-6.5 Å), although the loading in both L1 and L2 layers decreases with decreasing H, the depletion of water occurs more rapidly in the contact L1 layer (Figure 10d). At H ) 4.5 Å, only a central layer is retained between the two mica surfaces as seen in the density distribution in Figure 9 and snapshot in Figure 12. To our knowledge, this is the first simulation result confirming the presence of a monolayer around H ) 4 Å as suggested in the experiments of Raviv et al.14 The changes that occur at these smaller surface separations are clearly evident from an examination of the water configurations between the mica surfaces as illustrated in Figure 11 and Figure 12. We note that the rearrangement of water molecules at the smallest separations are likely to be influenced

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Figure 11. Snapshots illustrating the locations of water molecules between mica surfaces. At H ) 9 Å the hydration shells that form around the K+ ion are clearly visible. Below H ) 9 Å, the L2 layer gets progressively depleted, and water in the L3 layer contributes to the hydration shell that is shared by opposing K+ (H ) 8.5 Å). Red, O; Yellow, Si; White, H; Green, Al; and Blue, K+.

Figure 12. Snapshots illustrating the locations of water molecules between mica surfaces. Below H ) 7.5 Å, only two water layers L1 and L2 are present, and at H ) 4.5 Å, only a single layer of water occupies the central regions of water between mica surfaces. Red, O; Yellow, Si; White, H; Green, Al; and Blue, K+.

by the registry of the K+ ions on the opposing mica surfaces. In our simulations, we have only investigated the situation when the ions are in complete registry as indicated in the snapshots for, e.g., Figure 11. 3.6. Potassium Hydration. For the water-mica system, hydration of the surface K+ ions plays a dominant role in

Figure 13. K+-O pair correlation function for water confined between mica surfaces. The dashed line corresponds to the peak position for K+-O in bulk SPC/E water. Deviation from bulk hydration features occurs for H < 9 Å.

determining the layering and structuring of water as the surface separation is decreased. The PCF between the K+ ion and oxygen of the water molecule is investigated to understand the structure of the hydration shell around the K+ ion. While computing the ion-water (K+-O) PCF (Figure 13), normalization is carried out with a hemispherical volume. For large surface separations, the position of the first maxima is at 2.83 Å (shown in Figure 13 by the vertical dashed line), which compares very well with that observed in bulk ionic solutions.69,70 From the running integrals, evaluated at a distance of 3.65 Å, 3.75 water



Figure 14. Variation of the hydration number, Nh, for K+ ions as a function of H. The data illustrate the contribution from individual layers. Variation from the contribution to the first hydration shell occurs below H ) 9 Å (c) and (d). Note that the x-axis is in decreasing order of H. Below H ) 6.5 Å, only the L2 layer contributes to the hydration shell, and deviation from Nh occurs below H ) 5.5 Å (d).

molecules form the first hydration shell around a K+ ion. A hydration number of 7.1-8.1 is reported in the literature for K+ ions hydrated in bulk water.69,70 Assuming that a hemispherical hydration shell for the surface K+ ion will reduce the hydration number by a factor of 2, the hydration number obtained at large H compares very well with the values reported in the literature. This indicates that K+ ions experience a level of hydration similar to in bulk solutions at large surface separations. For H > 9 Å, the K+-O PCF does not show any changes indicating that the first hydration shell remains intact (Figure 13). This is consistent with the nearly invariant number of water molecules in L1, L2, and L3 for H > 9 Å (Figure 10a). For H < 9 Å, the position of the first maximum begins to deviate from the bulk value indicating rearrangement of water due to increasing confinement (Figure 13). The complete formation of the hydration shell at H ) 9 Å around the K+ ion can be observed in the snapshots (Figure 11). In our recent study of water adsorption on a single mica surface,52 we observed that the contribution of water toward solvating the K+ ion depends both on the layer location (L1, L2, or L3) as well as on the amount of water adsorbed on the surface.52 Water molecules from the L1 layer primarily adsorb in the ditrigonal cavities and do not contribute to the first hydration shell of the K+ ion. However, water molecules in the L2 and L3 layers contribute to the first hydration shell of the K+ ion.52 To quantify and compare the number of water molecules from various layers contributing to the first hydration shell of the K+ ion, we define the first hydration shell up to a distance of 4 Å based on the K+-O PCF obtained in the single mica study.52 This definition is slightly larger than the value used for defining the first hydration shell in bulk water. The variation in the hydration number contribution from the different layers, as a function of surface separation, is illustrated in Figure 14. We observe that the total number of water molecules in the first hydration shell remains nearly constant for H g 6 Å. For H g 9 Å, water in L3 has the largest contribution toward the hydration shell (Figure 14). The drop in the hydration contribution from the L3 layer at H ) 8.5 Å is due to the formation of a well-defined L2 layer, and the subsequent increase in the hydration contribution from L3 is due to the squeezing out of the L2 layer to develop a single central layer, L2, at H ) 6.5

Figure 15. Variation of average number of water molecules, (N), ww ws number of hydrogen bonds, nHB (water-water), nHB (water-surface), t and total number of hydrogen bonds, nHB , confined between (a) hydroxylated silica surfaces and (b) mica surfaces. The line with slope FbA where the bulk water density Fb ) 0.0334 Å-3 and A ) LxLy ) 1662.9 Å2 for the HS surface and 1314.1 Å2 for the mica surface is plotted for comparison. In the case of the HS surfaces (a) the HBs and water density remain close to that of bulk water; however, a significant lowering in the HBWM particularly at the smaller H values is observed for the mica surface.

Å. These changes that occur in the hydration shell are illustrated in the snapshots of Figure 11. The only contribution to the hydration shell of K+ ions for surface separations between 4.5 e H e 6.5 occurs from the central layers. Hence, the depletion of water layers adjacent to the mica surface, in contrast to the depletion of water from the inner layers for the HS surface, is dictated by the presence of the K+ ions and the ability of water to benefit from hydrating the K+ ion. The snapshots (Figure 11 and Figure 12) also reveal that water molecules begin to occupy adsorption sites directly above the K+ ions. However, below H ) 7 Å, these atop sites are no longer populated due to steric considerations. We note that this hydration picture which emerges at the smaller separations (H e 6.5 Å) is also determined by the registry of K+ ions on opposing faces of the mica surfaces. 3.7. Water Loading and Hydrogen Bonding: Silica vs Mica Surfaces. The total water loading, and the hydrogen bonding as a function of surface separation, H, are summarized in Figure 15 for both the silica and mica pores. For the silica pores (Figure 15a) the first feature is the linear variation of the ensemble averaged number of water molecules, (N), as a

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function of H. The slope of this line is accurately represented by FbA (where A is the area of the periodic surface used in the simulations) indicting that the averaged density of confined water is similar to that of bulk water down to surface separations of H ) 4.5 Å. The total number of HBs per water molecule, t ww (which includes HB within water-water, nHB , as well as nHB ws water-surface, nHB), also reveals that hydrogen bonding characteristics are close to that of bulk water across the entire range of surface separations. Weak steplike features are observed ww ws ) and water-surface (nHB ) in both the water-water (nHB contribution to the hydrogen bonds below H ) 8 Å (Table 2). This could be attributed to the depletion of inner layers as the surface separation is decreased, causing water molecules in L1 to reorient (Figure 3b) to retain their HB network. The propensity of water to retain its hydrogen bonding network within the confined space plays a crucial role in determining the structure, orientation, and depletion of confined water. Similar to water confined between HS surfaces, the average number of water molecules decreases monotonically with decreasing H for the mica surfaces (Figure 15b). The density variations are greater when compared with that observed for the HS surfaces. The data points in the range of H ) 4.5-9 Å are more scattered indicating the greater variation of density in these surface separations. This is consistent with the behavior ww t t ww and nHB . We observed that both nHB and nHB decrease of nHB monotonically with a decrease in H for H > 9 Å. This region corresponds to the depletion of water molecules from the central regions between the surfaces (Figure 10a and b). In this region, the L1 and L2 layers remain invariant, and hence the HBs with the mica surface are relatively constant. For H < 9 Å, major redistribution occurs, disrupting the loading as well as the HB network. This is confirmed by the observation of large changes t ww and nHB for H ) 4.5-6 Å and H ) 6-9 Å where in the nHB depletion of L1 and L2 layers occurs. The changes in the HB network which occur below H ) 9 Å coincide with the point at which a sharp decrease in the confined water density was observed in the constant pressure MD study of Leng and Cummings.29 At this point, it is useful to contrast the depletion of water molecules between the two hydrophilic surfaces investigated in this work with that observed for confined nonpolar fluids. In the case of nonpolar fluids, the pore averaged density deviates significantly from the bulk, and the number of particles confined between the two surfaces shows distinct steplike features associated with the depletion of a new layer of fluid molecules as the surface separation is decreased27,71 In the case of water, distinct layering transitions are absent, and water depletion is close to monotonic for both the hydrophilic surfaces investigated in this study. The strong tendency of water to successfully retain its hydrogen bond network for the silica surface results in bulklike water densities down to confinements as small as 4.5 Å. In the case of HS, the surface layer compensates for the loss of HBs by forming HBs with the OH groups on the surface, thereby enabling the water to maintain its bulklike structural features. In contrast to the mica surfaces, hydration of potassium ions is a major driving force, and the HB network is disrupted to a greater extent when compared with water confined between HS surfaces. This is reflected in the deviation of the pore density from the bulk density and the lowering of the number of HBs. Despite the strong influence of ions on the mica surface, water density approaches the bulk density beyond a surface separation of H ) 10 Å. 3.8. Disjoining Pressure: Silica vs Mica Surfaces. The solvation pressure is measured in experiments using the SFA

Malani et al.

Figure 16. Disjoining pressure and excess grand potential as a function of H for (a) hydroxylated silica (HS) surfaces and (b) mica surfaces. An oscillatory response is observed for the HS surface, and a single attractive minima is observed for the mica surface.

and AFM. From the experiments, the measured solvation force includes contribution to the force from both fluid-surface and surface-surface interactions. In the present study, we have computed only the forces due to fluid-surface interactions as a function of H. From the thermodynamics of the grand ensemble, we can calculate the excess grand potential of the system using24,71,72

∆Ωex ) A

∫HH

∞

ΠdH for H > Hc

(13)

where ∆Ωex(H) ) Ω(H) - Ωb; Ω(H) is the system free energy for a surface separation H; Ωb is the free energy of bulk fluid in equilibrium with the pore; Π the disjoining pressure is the difference between the solvation pressure and the bulk liquid presure, A ) LxLy; and Hc is the critical surface separation below which the density of confined fluid is zero (F ) 0). Figure 16a and b contrasts the disjoining pressure versus surface separation H for water confined between HS surfaces and mica surfaces, respectively. Also shown are the corresponding excess free energies (∆Ωex) calculated by integrating the disjoining pressure for both systems. Despite both surfaces being hydrophilic in nature, the disjoining pressures have distinctly different characteristics. The deviations in the disjoining pressure characteristics arise in the range of H e 10 Å, where prominent

Structural Properties of Confined Water oscillatory behavior is observed for HS surfaces. Similar oscillations have been observed in recent AFM experiments as well as MD simulations.22 In contrast, a single minimum is observed at lower H values only for water confined between mica surfaces, and the oscillations are clearly absent. These observations of single minima are consistent with a previous simulations32 study. At surface separations greater than 11-12 Å, the disjoining pressure is zero, indicating that water rapidly approaches its bulklike structure at separations greater than 10 Å at these conditions. The presence of oscillatory solvation forces and depletion of fluid from the center in the water-silica system are qualitatively similar to the trends observed in nonpolar fluids confined between two surfaces.27,71 However, the origins of the oscillatory forces observed in water which is a network forming liquid are quite different. It is well understood that force oscillations observed in confined nonpolar fluids are intimately related to the formation and disruption of layers as the distance between the two surfaces is decreased and atomistic simulations reveal distinct steplike features in the number of fluid particles that are accommodated between the surfaces as a function of the surface separation.27,71 However, the number of water molecules as a function of the surface separation, H, for the HS system displays a monotonic variation. Although the density distributions reveal the formation of layers at surface separations that correspond to maxima in the disjoining pressure at H ) 6 and 8 Å, the connection between layering and force oscillations is not obvious. At H ) 6 Å, three fluid layers are formed, and at H ) 8 Å two broadened peaks are observed in the central regions of the pore (Figure 2). Unlike the situation for confined nonpolar fluids, the force oscillations are not accompanied by abrupt changes in the loading (layering) between the surfaces when a new layer is formed, and the total number of average HBWM maintains its bulk value resulting in a continuous decrease in the number of water molecules as a function of the ww , in Figure surface separation. A closer look at the HB data, nHB 15a reveals the presence of weak plateaus at H ) 6 and 8 Å in the HBWM as a function of surface separation. Interestingly, the location of these plateaus (although weak) appears to correlate with the maxima in the disjoining pressure at both H ) 6 and 8 Å. The resulting small positive deviations in the HBWM from the bulk values appear to result in the positive pressure exerted between the two silica surfaces. A decrease in the HBWM, from the bulk values, results in an attractive force between the surfaces. While carrying out SFA experiments with water confined between mica sheets, the measured force is a strong function of the K+ ion density at the surface. It is well established that experiments with deionized water or low concentration salt solutions show a long-range repulsion and a strong attraction leading to mica-mica adhesive contact at short distances. This is consistent with DLVO theory.19,20 At higher salt concentrations, the K+ ion concentration at the mica surface increases, leading to force oscillations below separations of 2 nm. These short-range hydration forces occur with a periodicity of about 0.2 nm. There are some key differences between the experiments and the simulation results reported in this manuscript that should be kept in mind while attempting to make comparisons with experiments. The solvation pressure reported in our simulations does not include the interaction between the two mica surfaces, as measured in the SFA experiment, and hence provided a direct measure of the forces exerted by the confined fluid. The K+ ions are bound to the mica surface in their native crystalline state, and importantly, unlike in the experiments, the confined

J. Phys. Chem. B, Vol. 113, No. 42, 2009 13837 water does not contain any ions. Finally, the K+ ions on opposing mica surfaces are in complete registry. The absence of the oscillatory hydration forces could be due to some of these differences. Additional simulations with different salt concentrations would be required to reconcile these differences. Hence the simulations correspond to a situation where the surface cation concentration is high despite a low salt concentration. This situation is closest to the SFA experiments with 5 × 10-4 M KBr solutions17 where the surface cation density is 0.63 × 1014 ions per cm2 (in comparison, the surface density of the K+ ion in our simulations is 2.12 × 1014 ions per cm2). In this case, a single minimum is consistently observed at a separation of about 1.6 nm and provides a qualitative signature of the single minimum observed in our simulations. In the Monte Carlo simulations of Delville,32 the solvation pressure also reveals a weak minimum at 1.1 nm; however, the range of the pressures reported is significantly larger than the ones obtained in this work. This quantitative difference is attributed to the stronger surface-water interaction potential used in their model.32 In particular, the force field used in our simulations assigns a low value to the Si-O interaction.51 The accuracy of the force field used in this work has been recently tested by studying the adsorption of water on bare mica surfaces, where excellent agreement with experiments was observed.52 We observe that the solvation force behavior depends on the structure and depletion trends of confined water. For water confined between mica surfaces, the hydration of the K+ ion plays an important role in the structure, depletion mechanism, and HB network formation of confined water. At large surface separations, H > 10 Å, where the first hydration shell of the K+ ion remains intact, the structure and HB network of water molecules near the surface remain unaltered. The variation in the density distribution occurs primarily in the central regions with decrease in surface separation (Figure 9). Hence in this range the disjoining pressure does not show any variation remaining close to zero. For H < 10 Å where the depletion of water molecules in the layer hydrating K+ ions occurs, water molecules rearrange themselves to solvate the K+ ions leading to the observed solvation force characteristics. At H ) 9 Å, water molecules of two opposing L3 layers merge and contribute to the first hydration shell of the K+ ion on both surfaces. The sharing of water molecules from the L3 layer between surfaces (see snapshots, Figure 11) correlates with the attractive minima in the solvation force profile. With further decrease in H, a continuous rearrangement of the hydration shell gives rise to a repulsive region in the solvation force. The corresponding excess free energy of the system also shows a single minima at 7.45 Å. The relation between the HBWM and the features in the disjoining pressure characteristics is less obvious. However, we observe that the attractive regions in the disjoining pressure are accompanied by a relatively sharp decrease in the HBWM below H ) 11 Å, and the minima in the disjoining pressure at H ) 9 Å coincide with the minima in the HBWM (Figure 15). 4. Summary and Conclusion In this manuscript, we have examined the structural variations of water confined between two hydrophilic surfaces as a function of surface functionality with particular emphasis on the subnanometer regime. Two types of hydrophilic surfaces have been examined to study the effect of surface chemistry on the properties of confined water. In one case, water is confined between hydroxylated silica (HS) surfaces, and in the other case water is confined between mica surfaces. Although both surfaces are hydrophilic, the key difference is the ionic nature of the

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mica surface due to the presence of K+ ions. GCMC simulations are carried out over a wide range of surface separations, and we compute the changes in layering, hydrogen bond (HB) characteristics, dipole orientation distributions, and the resulting solvation forces as a function of surface separation. In the case of HS, confined water is remarkably bulklike in all the structural features examined, down to surface separations as small as 4.5 Å. Above H ) 9.5 Å, in addition to a contact layer, central regions of bulk water are present, and below H ) 9.5 Å, water only exists in the form of layers. Despite this layered configuration, the average number of hydrogen bonds per water molecule is remarkably close to that of bulk water over the entire range of surface separations (13 e H e 4.5 Å). As a consequence, the density of confined water is similar to that of free water. The presence of the OH groups on the HS surface enables water in the contact layer to complete its hydrogen bond network. When water is confined between mica surfaces, layering is more pronounced when compared with the HS surface. Hydration of potassium ions on the mica surface plays a dominant role in structuring and rearranging water adjacent to the surface. Hence, the HB characteristics are strongly effected, and unlike the HS surfaces, large deviations in the HBWM are observed below H ) 10 Å. As the surface separation is decreased, water depletion occurs from the central regions of confinement for both HS and mica surfaces. In case of the HS surface, the ability of the contact layer to HB with the OH groups leaves it structurally unperturbed down to H ) 4.5 Å. In the case of mica, the contact layers get disrupted for surface separations below H ) 6.5 Å, and significant rearrangement of water between layers is observed for 8.5 e H e 7 Å. The solvation force also shows distinct differences between the two surfaces. An oscillatory response below H ) 12 Å is observed for water between HS surfaces with a period of about 2 Å. Unlike confined nonpolar fluids where layering transitions are intimately related to the oscillations in the solvation force, the origins of the oscillations observed for water confined between HS surfaces are not directly related to the formation and disruption of layers since water depletes in a more or less continuous manner retaining its HB network within the confined space. For HS confinement, although the number of HBs is close to that of the bulk, small deviations from the bulk values are related to the oscillations in the solvation force with positive deviations giving rise to a repulsive force and negative deviations leading to an attractive force between the surfaces. These deviations from the bulk values, albeit small, appear to be related to the water rearrangements that occur due to confinement. In the case of mica, however, force oscillations are absent, and a strong attractive regime is observed below H ) 11 Å. Although a microscopic explanation for this force response is absent, we observe that this negative regime is connected with the overlap between hydration shells of K+ ions on opposing surfaces which are assumed to be in registry in our simulations. We have not investigated the effect of registry of ions from the opposing surface on the structuring and force response. It is likely that the response would be modulated between two extremes corresponding to ions being in full registry (as considered here) and ions completely out of registry. The effect of registry is expected to play a role in ion hydration and water rearrangements at the smaller values of H, and further studies would be required to completely appreciate this effect. Acknowledgment. This work was carried out under a grant from the Department of Science and Technology, India. This

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