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Molecular dynamics modelling of hydrated

mineral interlayers and surfaces: structure and dynamics

R. J. KIRKPATRICK*, A. G. KALINICHEV AND J. WANG

Department of Geology, 1301 W. Green St., University of Illinois, Urbana, Il 61801, USA

ABSTRACT

This paper reviews the results of recent molecular dynamics (MD) modelling studies of the interaction

of water and solute species with mineral surfaces and their behaviour in mineral interlayers. Emphasis

is on results for single and double hydroxide phases. Computational results are presented for water and

anions in the interlayers of the Ca2Al, Mg2Al, and LiAl2 layered double hydroxides and on the surfaces

of the Ca2Al phase. Detailed results for water on the (001) surface of brucite (Mg(OH)2) are presented

and compared to published results for other phases. In all these cases, hydrogen bonding and the

development of a hydrogen-bond network involving the H2O molecules and the solid substrate play

very significant roles. The MD methods are especially effective for investigating the structure and

dynamics of mineral-fluid interfaces and mineral interlayers, because they can be applied to systems

containing hundreds to thousands of atoms and for extended durations of the order of nanoseconds.

KEYWORDS: molecular modelling, mineral interlayers, hydroxide phases, brucite, hydrogen bonding, mineral-

¯uid interfaces.

Introduction

THE interaction of water with mineral surfaces and

its intercalation in structural cavities and inter-

layer spaces are among the most important

geochemical and mineralogical processes.

Despite considerable experimental and computa-

tional effort, many aspects of the structure,

dynamics, and energetics of water-mineral inter-

a c t i on a r e i n comp l e t e l y unde r s t ood .

Computational chemistry using both ab initio

quantum mechanical and semi-empirical poten-

tial-based methods has made signi®cant contribu-

tions to addressing these issues. This paper

reviews the results of our recent computational

MD modelling studies of water and ionic species

in mineral interlayers and on mineral surfaces,

and comparison is made with experimental,

mostly spectroscopic, results. Our main focus is

on studies of hydrophillic single and double

hydroxides, but comparison is made with results

for other hydrophillic and hydrophobic surfaces.

A key point of these results is that evaluation of

molecular-scale dynamical effects are essential to

understanding these interactions, and the structure

of mineral-associated water cannot be understood

independently of dynamical behaviour over a

wide range of frequencies. The structural,

dynamical and energetic effects of hydrogen

bonds (H bonds) and H-bond networks involving

water molecules, surface species (principally OH

groups for the hydroxide phases described here),

and solute species play key roles in controlling

these interactions. The MD methods provide an

effective method to investigate H-bond networks,

because relative to quantum chemical methods the

useable system sizes are larger and the simulation

durations longer, and because well tested

potentials are readily available (e.g. Kalinichev,

2001; Guillot, 2002). Spectroscopic methods,

including infrared (IR), nuclear magnetic reso-

nance (NMR), X-ray absorption (XAS), neutron

and sum-frequency generation (a non-linear

optical method that probes interfacial regions;

Miranda and Shen, 1999) and neutron and X-ray

scattering methods have provided important

insights into local structure and dynamics of

* E-mail: [email protected]

DOI: 10.1180/0026461056930251

Mineralogical Magazine, June 2005, Vol. 69(3), pp. 287±306

# 2005 The Mineralogical Society

mineral-solution interfaces and mineral inter-

layers. These chemical environments are,

however, statically and dynamically disordered,

and many details are dif®cult to probe experi-

mentally. Quantum chemical approaches,

including quantum MD, are limited to relatively

small systems and relatively short simulation

times, despite the rapid growth of computational

power (e.g. Odelius et al., 1997; Marx, 2004).

Several papers in the recent `Reviews in

Mineralogy and Geochemistry, volume 42

(Cygan and Kubicki, editors, 2001)' discuss

many important aspects of molecular modelling

theory and methods applied to aqueous solutions

and mineral-solution interfaces, and Kirkpatrick

et al. (2005) discussed the details of calculating

vibrational dynamics of surface and interlayer

species.

Our approach has been to use classical MD

methods and the CLAYFF force ®eld (Cygan et

al., 2004), which is speci®cally optimized for

low-temperature hydrous minerals and that does

not require a priori de®nition of most chemical

bonds. This non-bonded (pseudo-ionic) approach

allows the study of large, complex and disordered

systems containing thousands of atoms in solid

and ¯uid phases and at solid-¯uid interfaces. It is

intrinsically less accurate than ab initio and

quantum MD methods, but it is able to capture

the complex and cooperative interactions that are

critical in these situations. The absence of de®ned

chemical bonds for most interatomic interactions

allows effective and relatively simple treatment of

solids, ¯uids and interfaces and proper accounting

of energy and momentum transfer between the

¯uid phase and the solid. It also keeps the number

of interaction parameters small enough to allow

modelling of large and highly disordered systems.

The only de®ned bonds in CLAYFF are O-H in

H2O, OH-groups in the solid, and the bonds in

aqueous oxyanions (e.g. SO4

2ÿ). The ¯exible SPC

(simple point charge) water model (Berendsen et

al., 1981; Teleman et al., 1987) is used to describe

the H2O and OH behaviour. This model has been

well tested in many situations (e.g. Jorgensen et

al., 1983; Kalinichev, 2001; Guillot, 2002; Head-

Gordon and Hura, 2002). Cygan et al. (2004),

Kalinichev and Kirkpatrick (2002), and

Kirkpatrick et al. (2005) provide examples that

demonstrate the effectiveness of this overall

approach in modelling complex mineralogical

systems.

There are many other effective approaches to

molecular modelling of minerals and mineral-

¯uid systems, and each has its own particular

advantages. For example, treating the atoms as

®xed on a rigid lattice while still allowing for the

degrees of freedom associated with swelling and

lateral displacement of the lattice as a whole can

save substantial amounts of computer time,

because the degrees of freedom associated with

the motion of the atoms in the solid are excluded

from the calculations (Delville, 1995; Boek et al.,

1995; Chang et al., 1995, 1998; Bridgeman et al.,

1996; Karaborni et al., 1996; Desiqueira et al.,

1997; Greathouse and Sposito, 1998; Smith, 1998;

Greathouse et al., 2000; Sutton and Sposito,

2001). Such models obey all fundamental

conservation laws, but due to the immobility of

the lattice atoms, the exchange of energy and

momentum among the interacting atoms of the

substrate and the molecules of the pore ¯uid

(inelastic interactions) is not possible. For

hydrous phases, the characteristic time scales for

vibrational (~3600 cmÿ1

) and librational

(~450 cmÿ1) motions of surface OH groups are

comparable to those of similar motions of H2O

molecules and hydrated ions in the aqueous phase.

Thus, accurate representation of the dynamics of

such processes as hydrogen bonding, adsorption,

surface hydration and complexation may be

limited if the atoms of the substrate layer are

considered completely immobile. Surface diffu-

sion rates of ions and water molecules may also

be overestimated, and the computed structure of

the aqueous layers at the interface may be

distorted.

In a different approach using force ®elds with

bonded interaction terms, Teppen et al. (1997)

and Bougeard et al. (2000) modelled ionic

complexes in clays with all atoms in the system

movable. The force-®eld parameters are based on

charge assignment from quantum chemical

calculations. In this approach, bonds must be

identi®ed and evaluated for all possible metal-

oxygen coordinations. Thus, this approach is quite

accurate but limited to relatively small-scale

simulations of relatively well known mineral

structures, because of the large number of force-

®eld parameters needed to describe the bonded

states. The numerous parameters of a bonded

force ®eld are not easily transferred from

relatively simple and well known materials to

systems with complex and ill-de®ned structures,

and application of such a force ®eld can lead to

signi®cant over-parameterization due to lack of

experimental data to constrain all the necessary

terms.

288

R. J. KIRKPATRICK ET AL.

In the CLAYFF force ®eld, the partial atomic

charges are derived from periodic DFT (Density

Functional Theory) quantum chemical calcula-

tions for simple oxide, hydroxide, and oxyhydr-

oxide model compounds with well known

structures, and the empirical parameters

describing the Lennard-Jones attractive and

repulsive terms are optimized based on known

mineral structures. Thus, in contrast to the work

of Teppen et al. (1997) and Bougeard et al.

(2000), this approach incorporates a set of

experimental crystal-structure re®nements of

model phases to parameterize the empirical

force ®eld, rather than rely on quantum-mechan-

ical calculations alone. Oxygen and hydroxyl

charges vary depending on their occurrence in

water molecules or hydroxyl groups and on the

nearest-neighbour cations. For instance, oxygens

in Si-O-Si, Si-O-Al[4], Si-O-2Al[6] linkages have

different partial charges. All metal-oxygen inter-

actions are based on a simple Lennard-Jones

(12-6) potential combined with electrostatics.

Only harmonic terms are included to describe

the bond-stretch and bond-angle bending terms

associated with water molecules, hydroxyls and

polyatomic anions.

Because the OÿH bonds of water molecules

and OH groups are de®ned in the CLAYFF

approach, models using this force ®eld cannot

account for reactions involving ligand exchange

at mineral surfaces, and we thus limit our

applications to problems where such reactions

are not signi®cant. Development of generally

applicable reactive force ®elds capable of

addressing these situations is a signi®cant need

in geochemistry (e.g. Rustad, 2001; Rustad et al.,

2003).

Methods

MD calculations are performed by building the

desired structure in the computer, assigning the

individual atoms or molecules initial positions

and velocities, and then allowing the system to

evolve according to the laws of classical

Newtonian mechanics and the imposed force

®eld. The techniques and algorithms are well

developed, and the details of the methods are well

established (e.g. Allen and Tildsley, 1987;

Heinzinger, 1990). The modelled structure can

consist of up to many thousands of atoms,

depending on the needs of the problem and

available computer resources. These structures

can be built atom-by-atom, but for crystalline

phases they are more typically based on the

positional parameters of known structures. These

structures can be modi®ed as needed to account

for positional disorder over a crystallographic site,

for instance. Three-dimensional periodic

boundary conditions are applied and the Ewald

summation is used to account for long-range

Coulombic interactions. Mineral-¯uid interfaces

are generated by cleaving the mineral model

structures, often in the middle of an interlayer,

and ®lling all or part of the remainder of the

simulation box with water with or without

dissolved solute atoms (Fig. 1). In most cases,

the number of H2O molecules in this layer is

chosen to give a ¯uid density of ~1 g/cm3. In our

simulations, the thickness of the water layer is

typically >30 AÊ to minimize interaction of one

surface with another in the periodic layered model

structure. Typically, the time step is 0.001 ps, and

the `dynamic trajectory' of the simulated system

in its `phase space' (an ideal multidimentional

space in which the 6N coordinate dimensions

represent the positions and velocities of all N

atoms constituting the system) is recorded for

analysis every 0.004 ps. As in all computational

approaches to molecular-scale problems, care

must be taken to adequately sample the phase

space of the system to adequately ensure that it is

not trapped in a local energy minimum. For

crystalline phases this is not usually a dif®cult

problem, because the starting con®guration is

normally a known structure. For aqueous ¯uids

and solid-¯uid interfaces without solute, the

reorientational and diffusional correlation times

of water molecules are relatively short, and the

system properties typically converge to their

thermodynamic equilibrium values over a few

10s of ps. For systems containing a solid-¯uid

interface and dissolved solute, we position the

ions in the aqueous phase at distances not less

than 8ÿ10 AÊ (~3 molecular diameters of H2O)

from the solid surface and carefully monitor their

dynamic evolution and any adsorption onto the

surface. A typical simulation normally consists of

a pre-equilibration stage in which the atoms move

under only an energy minimization algorithm, a

further pre-equilibration period of MD simulation

lasting 50ÿ500 ps during which the system

reaches its equilibrium thermodynamic state, and

a ®nal equilibrium MD period of typically 100 ps

to 1 ns during which the trajectories of all atoms

are recorded for further statistical analysis. For

solid-¯uid systems containing solute species,

those atoms that become associated with the

MOLECULAR DYNAMICS OF WATER-MINERAL INTERACTION

289

interface typically move to it during the pre-

equilibration stage but often undergo exchange

with the solution during the MD runs. This allows

evaluation of surface-site lifetimes. Quantitative

results for structural parameters such as radial

distribution functions (RDFs), interatomic

distances and angles, and H-bond con®gurations;

dynamic parameters such as diffusion coef®-

cients, adsorption-site lifetimes, and power

spectra of atomic motion; and energetic para-

meters such as bulk system energy and energies of

adsorption are obtained only from analysis of the

equilibrium stage of the MD trajectories. The

power spectra (total dynamical density of states)

of the entire system, individual species and even

the motion of individual species in particular

directions are calculated by Fourier transforma-

tion of the velocity autocorrelation function

(Wang et al., 2003; Kirkpatrick et al., 2005).

The criteria for the existence of an H bond used

here are those often used for bulk liquid water:

intermolecular O_H distances <2.45 AÊ and

angles b, between O_O and O-H <30ë (Luzar,

2000). Surface OH groups are treated in the same

way as O-H of water molecules for the purpose of

HB calculations. The threshold of RO_H

4 2.45 AÊ is used because it corresponds to the

®rst minimum in the O-H radial distribution

function for SPC water at ambient conditions,

and b 4 30ë includes 90% of the angular

distribution of H bonds in water under the same

conditions (Teixeira et al., 1990; Luzar, 2000).

Results and discussion

Interlayer and surface structure and dynamics of

hydroxide phases

Many mineral surfaces are hydroxylated under

low-temperature geochemical conditions, and

much of our work is, thus, focused on hydroxide

phases. The single hydroxides brucite (Mg(OH)2),

gibbsite (Al(OH)3) and portlandite (Ca(OH)2)

have structures containing charge-neutral hydro-

xide sheets with the metals in octahedral

coordination and provide examples of trioctahe-

dral (brucite and portlandite) and dioctahedral

(gibbsite) structures. Layered double hydroxides

(LDHs) are a diverse group of phases with

positive structural charges that interact quite

differently with H2O and solute species than do

single hydroxides. Their layered structures can be

thought of as based on those of brucite,

portlandite, or gibbsite and consist of metal

hydroxide octahedral sheets and interlayer

galleries containing anions and associated water

molecules (Fig. 2). The hydroxide sheets develop

permanent positive structural charge due to

heterovalent substitution of, e.g. Al3+

for Mg2+

or Ca2+

, or Li+for vacancies, that is charge-

FIG. 1. A typical MD modelling cell used in computa-

tions of surface interactions. The top and bottom crystal

structures are hydrocalumite and the central region is

water containing Clÿ

, SO4

2ÿ, and Na

+. The water layer is

~30 AÊ thick in this model.

290


balanced by the interlayer anions. Thus, these

phases are sometimes known as anionic clays.

The LDHs have a wide range of applications in

catalysis, environmental remediation and medi-

cine (e.g. Miyata, 1983; Cavani et al., 1991;

Kagunya et al., 1996, 1998; Newman and Jones,

1998; Choy et al., 1999; Basile et al., 2001) and

are being increasingly recognized as important

phases in many low-temperature natural and

anthropogenic geochemical environments (e.g.

Bish, 1980; Trolard et al., 1997; Ford and

Sparks, 1998; Ford et al., 1999; Thompson et

al., 1999; Gade et al., 1999, 2000; Genin et al.,

2001). Many tens of LDH phases are known (Hou

et al., 2003; Braterman et al., 2004), and they

offer a wide range of opportunities to investigate

the effects of hydroxide layer composition and

anion charge, size and conformation on the

development of interlayer H-bond networks and

dynamics.

The LDH phase with the best known structure

is the mineral hydrocalumite, [Ca2Al(OH)6]

Cl´2H2O, also known as Friedel's salt, and MD

modelling of its interlayer and surface structure

and the diffusional, translational and librational

dynamics of its interlayer and surface species

illustrates well the capabilities of MD methods.

The MD results demonstrate that our techniques

described above reproduce the experimentally

determined hydroxide layer and interlayer struc-

tures well, that librational dynamics of the

interlayer water molecules play a key role in its

experimentally observed structural phase transi-

tion, and that the low-frequency translational

dynamics of interlayer and surface species are

similar in many ways to that in bulk aqueous

solutions (Kalinichev et al., 2000). This phase is

unique among well known LDHs, because it has

both an ordered Ca,Al distribution in the

hydroxide layer and a well ordered Clÿ

and

water structure in the interlayer space (Fig. 1;

Terzis et al., 1987). The interlayer order is due to

coordination of the water molecules to Ca in the

hydroxide layer, which results in an unusual

7-coordinate Ca environment (Fig. 3d). The

interlayer Clÿ

ions form almost regular triangles

of a 2-D hexagonal net, and the water molecules

are located at the centre of each triangle. Each

Clÿ

has six nearest neighbour (NN) H2O, and

each water molecule has three NN Clÿ

, but is

frustrated because it can only donate two H bonds

at any instant. Each Clÿ

is coordinated through H

bonds to six OH-groups: three one each side of

the interlayer (Fig. 3c).35Cl NMR spectroscopy

(Kirkpatrick et al., 1999; Andersen et al., 2002)

shows the presence of a phase transition at which

the symmetry at Clÿ

changes from triaxial to

uniaxial or nearly so due to reorientation of the

electrical ®eld gradient at35Cl at frequencies

>~105Hz.

MD modelling shows that in the low-temp-

erature phase the water molecules are statically

disordered among three possible positions

donating H bonds to Clÿ

, whereas in the high-

temperature phase they librate (hop) among these

positions at frequencies that result in a strong IR

band near 500 cmÿ1. The observed phase

transition is due to the onset of this libration,

which causes the time-averaged symmetry of the

Clÿ

to change from triaxial with four static H

bonds from water molecules to uniaxial with six

2/3-occupied H bonds from water molecules.

FIG. 2. View of the Clÿ

hydrotalcite, (Mg2,Al)

(OH)6Cl´2H2O, parallel to the layering showing the

single Mg2,Al octahedral sheets and interlayer Clÿ

and

H2O. As in hydrocalumite, the interlayer water in this

phase occupies two sublayers due to H-bond donation

from the OH-groups to interlayer Clÿ

and H2O and from

interlayer H2O to Clÿ

.


291

Each Clÿ

anion is also coordinated by six OH

groups (three from each adjacent hydroxide layer)

under all conditions. In our simulations, these H

bonds immobilize the Clÿ

on the ns timescale,

even at temperatures as high as 300ëC.

Experimental35Cl NMR results suggest that

similar dynamics occurs in the less ordered

interlayer of the Mg,Al LDH hydrotalcite with

interlayer Clÿ

(Fig. 2; Kirkpatrick et al., 1999).

The structural environments of water and Clÿ

on the hydrocalumite basal surface are similar in

some ways to those in the interlayer but are more

disordered both statically and dynamically

(Kalinichev et al., 2000; Kalinichev and

Kirkpatrick, 2002). MD simulations show that

Clÿ

is associated with the surface principally as

inner sphere complexes due to their large

Coulombic interaction with the hydroxide

sheets. In contrast to the highly ordered interlayer,

however, the Clÿ

and H2O are disordered over

sites comparable to the `Clÿ

'and `H2O' sites in

the interlayer (Fig. 3). The `H2O' site is directly

coordinated to Ca, whereas the `Clÿ

' site is

coordinated to OH groups by H bonds.

Dynamically, the mean residence time of a Clÿ

on a surface site is ~50 ps, resulting in computed

diffusion coef®cients of ~1.6610ÿ6

cm2sÿ1

for

inner sphere Clÿ

and 7.5610ÿ6

cm2sÿ1

for outer

sphere Clÿ

. These values are intermediate

between interlayer diffusion coef®cients that are

too small to compute (<<10ÿ7

cm2sÿ1) and a bulk

s o l u t i o n d i f f u s i o n c o e f ® c i e n t o f

2.3610ÿ5

cm2sÿ1. The interlayer Cl

ÿ

and H2O

do not hop among sites during 100 ps MD runs.

FIG. 3. Computed positions of water molecules and Clÿ

on mineral surfaces. (a) Water located above the vacant

tetrahedral sites of the octahedral sheets of Mg(OH)2 or Ca(OH)2 and receiving three H bonds from the surface

hydroxyls. (b) Water located near the OH sites of the octahedral sheets of Mg(OH)2 or Ca(OH)2 and receiving one H

bond from it. These waters also typically have four NN H2O. (c) Clÿ

occupying the `Cl' position on the surface of

hydrocalumite. This position receives three H bonds from the surface and is located above a Ca that is displaced

downwards from the centre of the octahedral sheet in this view. (d) Clÿ

occupying the `H2O' position on the surface

of hydrocalumite. This position coordinates a Ca that is displaced upwards from the centre of the octahedral sheet in

this view.

292


The vibrational dynamics of surface Clÿ

and H2O

and the librational dynamics of the surface H2O

are also quite different from those species in the

interlayer or bulk water. In the interlayer,

cooperative motion of the Clÿ

and H2O leads to

low-frequency translational bands centred near

50 cmÿ1

for motion dominantly parallel to the

hydroxide sheets (comparable to H-bond bending

in bulk water) and near 150 cmÿ1

for motion

dominantly perpendicular to the hydroxide sheets

(comparable to H-bond stretching in bulk water).

On the surface, the relative intensity of the band

due to motion perpendicular to the hydroxide

sheets is greatly reduced due to the absence of the

second wall of the interlayer and is more similar

to the power spectra in bulk solution (Fig. 4). The

computed band for surface H2O libration is

signi®cantly broader than that of interlayer H2O

due to the structural disorder and is quite similar

to that of bulk water. The MD calculations can

also be used to calculate the full vibrational power

spectrum of a phase, and recent work has shown

that the computed frequencies for hydrotalcite and

other LDHs in the far infrared region correlate

well with the observed band positions and that

MD simulations can effectively assist with band

assignment in this frequency range (Wang et al.,

2003; Kirkpatrick et al., 2005).

The computed results for the structure and

dynamics of H2O and Clÿ

associated with the

charge neutral (001) surface of portlandite

(Ca(OH)2) are quite different from the positively

charged hydrocalumi te (001) sur face .

Structurally, the water molecules are associated

with the portlandite surface both via donation of

H bonds to the O of surface OH-groups and

acceptance of H bonds from the surface OH

groups (Fig. 3a,b; Kalinichev and Kirkpatrick,

2002). Surface OH groups also bend towards the

Clÿ

to form inner sphere sorption sites. Such sites

are consistent with35Cl NMR T1 relaxation rate

data, which show signi®cant Clÿ

association with

the portlandite surface (Yu and Kirkpatrick,

2001). In the simulations, however, only a

fraction of the dissolved Clÿ

is associated with

the portlandite surface. The computed mean Clÿ

FIG. 4. Computed low-frequency power spectra for the motion of Clÿ

on the surface of Ca(OH)2 (inner-sphere and

outer-sphere sites), on the surface and in the interlayer of hydrocalumite (Ca/Al LDH), and in bulk aqueous solution.

The bands near 50 cmÿ1

involve mostly H-bond bending and are due to motion parallel to the layers in

hydrocalumite interlayers. The bands near 150 cmÿ1

involve mostly H-bond stretching and are due to motion

perpendicular to the layers in hydrocalumite interlayers.


293

surface site residence time is only ~20 ps, and the

computed diffusion coef®cients are signi®cantly

greater than for hydrocalumite, 3.7610ÿ6

cm2sÿ1

for inner sphere Clÿ

and 1.5610ÿ5

cm2sÿ1

for

outer sphere Clÿ

. The computed low-frequency

translational power spectrum of surface-asso-

ciated Clÿ

on portlandite is dominated by the H-

bond bending band near 50 cmÿ1

and is quite

similar to that in bulk solution (Fig. 4).

The LiAl2(OH)6Aÿ

´nH2O LDH phases can be

thought of as being derived from gibbsite by Li+

for vacancy substitution and in many ways have

different interlayer structures than the Mg2Al or

Ca2Al LDHs because of the small Li+

ionic

radius. As for hydrocalumite, MD methods have

played an important role in understanding their

interlayer structure and dynamics (Hou et al.,

2002). The LiAl2 LDHs have a highly ordered

octahedral Li,Al distribution, and in the anhy-

drous Clÿ

phase the interlayer Clÿ

lies directly

above and below the octahedral Li and is also

highly ordered as shown by XRD and35Cl NMR

data (Besserguenev et al., 1997; Hou et al., 2002).

The MD simulations reproduce this structure very

well. For the hydrated phase, the maximum water

content, n = 1, and the room temperature35Cl

NMR spectra of substantially hydrated samples

show a broad component similar to that of the

anhydrous sample and a much narrower compo-

nent representing disordered and dynamically

averaged Clÿ

(Hou et al., 2002). At temperatures

above 70oC, all of the interlayer Cl

ÿ

undergoes

dynamical averaging. Diffraction data show that

the interlayer structure of the hydrated phase is

disordered but the details are poorly known. The

MD results provide a quite detailed picture of this

statically and dynamically disordered interlayer

structure and demonstrate the high degree of

similarity between the most common NN Clÿ

environments in the interlayer and in bulk

aqueous solution (Fig. 5). The Clÿ

and H2O are

located at the centre of the interlayer along the c

direction, and the H-O-H plane of the water is

parallel to the hydroxide layers. The H2O are

located near the OH groups and receive two H

bonds from them, one from each side of the

interlayer. The Clÿ

receive H bonds from OH-

groups or H2O, and there is a well developed H-

bond network in the interlayer. A few of the Clÿ

are located near the cross-layer Li-Li vectors at

sites comparable to those in the anhydrous phase

and also in trigonal prisms of OH groups above

and below the vacant tetrahedral sites on the

octahedral sheet. In both these con®gurations,

they receive H bonds principally from OH groups.

Most Clÿ

, however, are located on distorted

octahedral sites above and below the vector

connecting nearest neighbour OH groups. These

FIG. 5. Computed interlayer structure of the LDH LiAl2(OH)6Cl´H2O. The interlayer H2O/Clÿ

ratio is limited to 1/1,

because the horizontally oriented water molecules are in stable, tetrahedral, ice-like NN coordination with two

accepted and two donated H bonds, and the Clÿ

are in 6-fold coordination receiving 6 H bonds.

294


Clÿ

receive two H bonds from OH-groups of one

side of the interlayer, two from the other side, and

two from H2O (Fig. 5b). This arrangement allows

each H2O to be in a highly stable tetrahedral

H-bond environment very similar to that in bulk

solution and in ice Ih. The mean ClÿH distance is

2.16 AÊ for H of H2O and 2.12 AÊ for H of OH,

compared to ~2.22 AÊ in bulk solution. The

stability of this structure explains the maximum

1:1 Clÿ

to H2O ratio for the LiAl2 LDH, because

there are no additional sites where H2O can be in

stable, H-bonded tetrahedral coordination. This

contrasts with the Ca2Al and Mg2Al LDHs, in

which the water molecules can form two

sublayers, and the Cl/H2O ratio can approach 2

(Fig. 2). Dynamically, the interlayer H2O under-

goes restricted librational motion (hindered

rotational hopping) among H-bonded con®gura-

tions at approximately 1011

Hz (near 500 cmÿ1),

as observed in other LDH phases. The site

hopping frequencies are ~107Hz for H2O and

36108Hz for Cl

ÿ

, consistent with the observed

narrowing of the35Cl NMR resonances. The MD

computed power spectrum of this phase is in

particularly good agreement with the observed

far-IR spectrum, which shows bands for motion of

Li, Al and OHÿ

as well as the interlayer species

(Kirkpatrick et al., 2004).

Water structure at mineral surfaces

It has long been known that the structure and

physical properties of water near mineral surfaces

can be substantially different from those of bulk

water, that surfaces can perturb the ¯uid structure

and properties up to several molecular diameters

from the surface, and that these differences can be

key to understanding mineral surface chemistry

(Packer, 1977; Israelachvili and Pashley, 1983;

Hochella and White, 1990; Israelachvili and

Wennerstron, 1996; Brown et al., 1999;

Criscenti and Sverjensky, 1999; Nandi et al.,

2000; Raviv et al., 2001; Zhu and Granick, 2001;

Brown, 2001 Michot et al., 2002). Despite

decades of study, the structure, dynamics and

physical properties of this near-surface water

remains incompletely understood (e.g. McCarthy

et al., 1996; Bridgeman and Skipper, 1997; Spohr

and Hartnig, 1999; StoÈckelmann and Hentschke,

1999; Kalinichev et al., 2000; Greathouse et al.,

2000; Dore, 2000; Fenter et al., 2000a,b; Cheng et

al., 2001; Teschke et al., 2000; Bellissent-Funel,

2001, 2002; Fouzri et al., 2002; Park and Sposito,

2002; Sakuma et al., 2003). Computational

methods are playing an important role in

advancing understanding of near-surface water

(Lee and Rossky, 1994; McCarthy et al., 1996;

Bridgeman and Skipper, 1997; Hartnig et al.,

1998; Spohr and Hartnig, 1999; StoÈckelmann and

Hentschke, 1999; Kalinichev et al., 2000;

Greathouse et al., 2000; Gordillo and MartõÂ,

2000; Cygan, 2001; Gallo et al., 2002; Park and

Sposito, 2002; Michot et al., 2002; Kalinichev

and Kirkpatrick, 2002; Rustad et al., 2003;

Sakuma et al., 2003; Wang et al., 2004), and

MD methods can be especially useful, because

they can be applied to systems large enough to

capture the complex correlations of molecular

motions at the surface over much longer time- and

length-scales than is currently possible with ab

initio calculations, as described above. Here we

illustrate the capabilities of MD methods in this

regard with recent results concerning water

structure at the brucite (Mg(OH)2) (001) surface

(Wang et al., 2004) and compare these results

with published computational results for water at

other surfaces.

Solid surfaces can perturb the water structure

and dynamics by affecting its molecular packing,

orientation, rotation and translation. These effects

arise due to the `hard wall' effect, the presence of

electrostatic ®elds, and local surface-speci®c H-

bonding donor and acceptor sites (e.g. Odelius et

al., 1997; Cheng et al. 2001; Wang et al., 2004).

The `hard wall' or `excluded volume' effect

creates near-surface layering due to the spatial

geometric constraint that no part of an atom or

molecule at the surface can penetrate it. This

effect occurs for all con®ned ¯uids (e.g. Abraham,

1978). Thus, the structure of water in an

interfacial region re¯ects a delicate balance of

the ordering due to `hard wall' or `excluded

volume' effects of packing H2O molecules at a

solid surface, surface-speci®c H bonding and

orientational ordering of the water molecules,

and disordering due to thermal motion. This

structure is typically different and more disor-

dered than the tetrahedral, H-bonded structure of

ice Ih, the evidence of which is still prominently

present in the more disordered short-range

tetrahedral H-bonding molecular arrangements

in bulk liquid water (e.g. Eisenberg and

Kauzmann, 1969; Soper, 2000; Errington and

Debenedetti, 2001; Head-Gordon and Hura,

2002).

For hydrophyllic phases such as hydroxides, H

bonding between the surface and water molecules

and among water molecules both play central


295

roles, whereas for hydrophobic phases such as

carbon nano-tubes and talc, H bonding among the

water molecules is important, but H bonding

between the surface and water is much less

signi®cant. Many studies of surface-water struc-

ture rely on computed pro®les of atomic density

variations with distance from the surface,

comparable to radial distribution functions, some-

times supplemented by pro®les of molecular

water orientation. We have found that computed

statistics for NN coordination, H bonding, and

order parameters related to NN structure (Chau

and Hardwick, 1988; Errington and Debenedetti,

2001) provide important additional information

that leads to a greatly improved, atomistically

detailed understanding.

The MD results for water at the brucite and

portlandite surfaces show that these hydrophyllic

substrates signi®cantly in¯uence the near-surface

water structure, with both H-bond donation to the

surface oxygen atoms and H-bond acceptance

from the surface hydrogen atoms in the ®rst

surface layer of H2O molecules playing key roles

(Kalinichev and Kirkpatrick, 2002; Wang et al.,

2004). The oxygen and hydrogen atomic densities

deviate from those of bulk water to distances as

large as 10 AÊ (Fig. 6). The distances between

maxima in the O-density pro®les are not equally

spaced, as would be expected from hard wall

effects alone, clearly demonstrating that surface

structure, charge distribution and H bonding must

play important roles in controlling the near-

surface water structure. The H2O dipole orienta-

tions show structuring to as far as 15 AÊ (~5

molecular water layers) from the surface (Fig. 7).

The average number of H bonds per H2O

molecule changes from 3.8 in the near-surface

layer to 3.5 (approximately the value for bulk

FIG. 6. Computed atomic density pro®les for water con®ned between brucite layers. Curves are displaced vertically

by 0.03 AÊÿ3

(oxygen atomic density) and 0.07 AÊÿ3

(hydrogen atomic density) to avoid overlap. The position of the

surface (0.0 in these plots) is computed as the average position of the brucite surface oxygen atoms. The system size

labels are the increases in the brucite c-axis dimension used to generate the systems, and the actual water layer

thicknesses vary somewhat from these values.

296


SPC water) at ~10 AÊ from the surface, and there

are signi®cant oscillations in this value (Fig. 8).

The MD simulations show that nm-scale

con®nement in slit-like pores at least 15 AÊ thick

and less leads to signi®cant overlap of the

structural effects of the two surfaces, as described

in detail by Wang et al. (2004). For thin pores, the

structure of the entire water volume is substan-

tially perturbed compared with bulk water, and

the effects of the surface depend signi®cantly on

pore thickness.

For the uncon®ned brucite surface, the varia-

tion in atomic density re¯ects the presence of

three important, well de®ned layers. These are a

high atomic density, highly structured, near-

surface layer centred near 2.5 AÊ that contains

molecules that are directly coordinated to the

surface; a low atomic density, transitional layer

centred near 4.0 AÊ from the surface; and a region

extending from ~5 to 15 AÊ from the surface in

which the structure becomes progressively more

similar to that of bulk water. Figure 9 illustrates

schematically the most common orientations of

water molecules in these layers. The layer nearest

the surface contains two principal types of water

molecules, both of which are directly coordinated

FIG. 7. Computed angular distributions of the orientations of water molecules con®ned in 30 AÊ thick pores in brucite.

This thickness is large enough that the two surfaces do not perturb each other. jD (left) and jHH (right), are the

angles between the water dipole (H-end = positive) and brucite [001], and between the water H-H vector (no sign

convention) and brucite [001] respectively. Each curve was normalized, rescaled, and displaced vertically by 1.0 to

®t the ®gure. The values listed in scales a to j are the distances from the surface.


297

to surface OH groups. These two types have

different orientations, NN coordinations and

H-bond con®gurations. They are, on average,

located at slightly different distances from the

surface but are intimately mixed with each other

in the plane parallel to the surface. Large domains

of one structural type cannot form, because

H-bond formation between neighbouring water

molecules prevents each type of environment

from extending more than three molecules in the

plane parallel to the surface. Type 1 molecules are

on average slightly closer to the surface (mean

distance ~2.3 AÊ ) and are predominantly oriented

with the positive (hydrogen) end of their

molecular dipole towards the surface, making an

average angle of ~130ë with the surface normal

(Fig. 7). These molecules typically have NN

coordinations of six, three surface OH-groups

and three H2O. On average, they accept ~0.5 H

bonds from surface OH groups, donate 1.0 H bond

FIG. 8. (a) Variation of the average total number of H bonds per water molecule and the contributions of various H

bond types to this number with distance from the brucite surface for the system with 30 AÊ of water. (b) Variation of

the average fraction of water molecules with different numbers of H bonds with distance from the brucite surface for

the system with 30 AÊ of water.

298


to surface OH-groups, accept ~1.3 H bonds from

other water molecules, and donate ~0.8 H bonds

to other water molecules. Type 2 molecules are on

average somewhat further from the surface

(~2. 6 AÊ ) and are predominantly oriented with

the positive ends of their dipoles oriented away

from the surface, making angles of ~30ÿ80ë with

the surface normal. These molecules typically

have NN coordinations of ®ve, one surface

OH-group and four water molecules. On

average, they accept ~0.5 H bonds from surface

OH-groups, donate none to surface OH groups,

accept ~1.4 H bonds to other water molecules and

donate ~1.7 H bonds to other water molecules.

The (type 1)/(type 2) abundance ratio is ~5/4.

These two types of H2O occur on very different

surface sites. Type 1 molecules are preferentially

located above the vacant tetrahedral sites of the

trioctahedral sheet (triangles formed by surface

OH-groups; Fig. 3a,b). They form a reasonably

well ordered and dynamically averaged 2-dimen-

sional hexagonal net that re¯ects the underlying

brucite structure (Fig. 10). There appear to be

four local potential energy minima on which they

occur. One of these is at the centre of the OH

triangle, and three are near the middle of the line

connecting two nearest-neighbour OH sites. The

water molecules spend, on average, about half of

the time at the OH-triangle centre, where they

accept 1 HB, and 1/6 of the time at each of the

other sites, where they donate 1 and accept 1 HB.

This site hopping, libration, and formation and

breaking of HBs result in the computed average of

1.5 HBs with surface OH groups. The type 2

water molecules are preferentially located above

the surface OH-groups, and their distribution is

much less ordered than for the type 1 molecules

(Fig. 10). Only half accept one H bond from

surface OH groups at any instant.

Further from the surface, the low-density

region between 3 and 5 AÊ from the surface

provides the essential transition between the

near-surface layer, with a structure largely

controlled by the substrate surface, to a more or

less bulk-like water structure (Fig. 6). This

transition occurs by gradual adjustment of the

second neighbour con®guration in a distorted, but

locally tetrahedral structure. The NN coordination

in this region is ~4.4, similar to that of bulk liquid

water at the same temperature and density.

Molecules in this region have two different

preferred orientations, and as in the ®rst layer,

these types are mixed on a molecular scale across

the surface. The orientational order is, however,

much less than in the ®rst layer. Type 3 molecules

have their positive ends generally oriented

towards the surface, making angles between 90

and 180ë with the surface normal (Fig. 7). Type 4

molecules have their positive ends generally

oriented away from the surface, making angles

between 40 and 140ë with the surface normal. The

(type 3)/(type 4) abundance ratio varies from 2/1

at 3 AÊ from the surface, to 4/1 at 4 AÊ , and back to

2/1 at ~5 AÊ .

The greatest degree of the tetrahedral (ice-like)

ordering occurs at ~4 AÊ from the surface, where

the O-density has its minimum value. Indeed, at

this distance the water molecules are locally more

ordered (more similar to ice Ih) than in bulk

water. The fraction of molecules with exactly 4 H

bonds reaches a maximum at this position and the

average orientational order parameter, q, as

de®ned by Errington and Debenedetti (2001) has

a value greater than bulk water. Such structuring

is in agreement with the notion that the number of

water molecules with four H bonds increases with

decreasing density at liquid-like densities (Geiger

FIG. 9. Schematic diagram illustrating the orientations

and H bonding of water molecules in the different near-

surface layers for brucite. Black balls and sticks are

water molecules or surface OH groups. Dotted balls and

sticks are water molecules with different orientations,

which schematically show the orientational ranges in the

different layers. The dotted lines connecting water

molecules are H bonds.


299

and Stanley, 1982; Kalinichev, 2001; Paulo et al.,

2002), and similar structuring is also observed for

water con®ned in pores in hydroxylated silica

glass (Gallo et al., 2002).

Beyond ~6 AÊ from the surface, the water

structure is generally similar to that of bulk water,

with an average of ~3.5 H bonds/molecule

(Jorgensen et al., 1983) and an average NN

FIG. 10. Atomic density maps for oxygen of water in the ®rst atomic density maximum near the brucite surface. The

upper map is for type-1 water molecules, and the lower map for type-2 water molecules. The ®lled squares and

circles are surface Mg and O atoms respectively, and the thin lines are contours of water oxygen probability density.

The most probable positions of the type-1 molecules re¯ect the underlying brucite structure.

300


coordination of ~4.5. The atomic density pro®les

(Fig. 6) show statistically meaningful variation to

~10 AÊ from the surface, and small but statistically

meaningful variations in the molecular orienta-

tions occur out to ~15 AÊ from the surface (Fig. 7).

These changes of angular distribution are due to

adjustment of the orientations of individual water

molecules to ®t their local environments, which

are perturbed indirectly by the surface through its

effects on H2O molecules next to it.

The water structure in the layer closest to the

brucite surface does not resemble those of bulk

water at ambient conditions, ice Ih, or water at

low temperatures, as has been proposed for water

con®ned in Vycor glass and silica gel based on

neutron diffraction studies (Dore, 2000;

Bellissent-Funel, 2001). This is clearly shown

by the 5- and 6-fold NN coordinations of the near-

surface molecules and by this coordination being

as much as 2.2 larger than the H bond number. In

ice Ih, the NN coordination and H bond number

are both ~4.0 (Eisenberg and Kauzmann, 1969),

and in bulk SPC liquid water at ambient

temperature and pressure the NN coordination is

~4.4 and the H bond number is ~3.5. Cooling of

bulk liquid water causes the average number of H

bonds to increase with the NN coordination

remaining more or less constant (Rapaport,

1983; Paulo et al., 2002). The structure of the

®rst layer of water does share some similarities

with those of high-pressure ice phases and liquid

water at elevated pressure. With increasing

pressure, the average NN coordination for liquid

water increases more rapidly than the average

number of H bonds (Kalinichev et al., 1999;

Paulo et al., 2002), and the structural changes can

best be interpreted in terms of an increasing

number of interstitial (non-H bonded) water

molecules in the NN coordination sphere

(Bagchi et al., 1997; Kalinichev et al., 1999;

Saitta and Datchi, 2003). In the crystalline ice

phases, there are always four H-bonded nearest

neighbours at intermolecular distances of

2.7ÿ2.9 AÊ , but the number and intermolecular

distances of the non-H-bonded molecules are

different for different phases. There are zero non-

H-bonded molecules in ice Ih (stable up to

0.3 GPa), 3.75 at 3.1ÿ3.3 AÊ in ice IV (a

metastable phase at 0.4ÿ0.55 GPa, Engelhardt

and Kamb, 1981), and 4 at 2.74 AÊ in ice VIII

(stable above 2.1 GPa, Kuhs et al., 1984). The

latter distance is shorter than the H-bond distance

(2.88 AÊ ) in that phase, paralleling a similar trend

for liquid water under pressure (Schwegler et al.,

2000). At the brucite surface, the coordination

number of type 1 molecules is 6 and number of H

bonds is ~3.8, qualitatively following the trends

for liquid water and the crystalline phases with

increasing pressure.

The MD modelling for water at the brucite

surface adds to a growing body of computational

and experimental studies that demonstrate that

different types of surfaces can have substantially

different effects on surface water structure and

that this structure should not be thought of as

simply `ice-like'. For instance, previously

published MD simulations for water at a variety

of oxide and hydroxide surfaces show the

presence of molecules with two different and

well de®ned orientations in the ®rst layer and that

the local structural environments or orientations

are different for different phases. The coexistence

of water molecules with different orientations

mixed and interconnected in the plane parallel to

the surface appears to allow the development of

an interconnected H-bond network involving the

water molecules and surface atoms. The inter-

facial water on the portlandite, Ca(OH)2, (001)

surface is similar to that for brucite due to the

similarity in their structures and involves H-bond

donation and acceptance to/from the solid surface

(Kalinichev and Kirkpatrick, 2002). MD simula-

tions for water at the magnetite (001) surface

using a potential model that allows the surface

protonation state to change during the MD

simulation show that H-bond donation and

acceptance between the surface and H2O are

important in this situation also (Rustad et al.,

2003). On this surface, interfacial water mole-

cules accept H bonds from several surface

functional groups, of which about a half are[6]FeOH2 sites (doubly protonated O-atoms

coordinated to one octahedral Fe). About three

quarters of the H bonds donated by H2O

molecules to surface sites go to[4]FeOH (singly

protonated O-atoms coordinated to tetrahedral

Fe). These results suggest that different surface

functional groups can play different roles in

developing interfacial H-bonding networks. In

contrast, our model brucite (001) surface contains

only one type of surface functional group

([6]Mg3OH) that serves as both an H-bond donor

and acceptor.

Lee and Rossky (1994) have proposed two

idealized H-bond structures for water in the ®rst

hydration layer of a hydroxylated silica surface,

and these are quite different from those for

brucite. One type has the positive end of its


301

dipole oriented away from the surface and accepts

one H bond from and donates one H bond to

surface OH groups. The other type has the

positive end of its dipole oriented towards the

surface and accepts one H bond from and donates

two H bonds to surface OH groups. The

differences between water orientations at the

brucite and hydroxylated silica surfaces may be

caused by the differences in the substrate surface

structure. On the silica surface modelled by Lee

and Rossky, the Si-OH groups are 5.0 AÊ apart,

whereas for brucite the shortest MgOHÿMgOH

distance is only 3.1 AÊ , approximately the

diameter of a water molecule. MD simulations

for water con®ned in pores in hydrated Vycor

glass show that the ice Ih-like NN and H-bond

geometry of bulk water is destroyed near these

surfaces (Gallo et al., 2002). MD simulations of

water at the NaCl (100) surface show a lattice-like

2-D distribution parallel to the surface, and as for

brucite this 2-D structure re¯ects the underlying

NaCl crystal structure (StoÈ ckelmann and

Hentschke, 1999).

The presence of both donating and accepting

H-bond con®gurations at hydroxylated surfaces is

not universal. H2O on the surface of hydro-

calumite described above has the positive ends of

its dipole pointing only away for the surface due

to the positive structural layer charge of this

phase, and these waters accept H bonds from the

surface OH-groups but do not donate any to them

(Kalinichev and Kirkpatrick, 2002).

The extensive H bonding between the surface

and near-surface H2O computed for brucite,

portlandite and other hydrophyllic surfaces

contrasts signi®cantly with the water structure

near hydrophobic surfaces. For instance, at the

surfaces of carbon nano-tubes H bonding to the

surface is not signi®cant and the ice Ih-like NN

and H-bond geometry of bulk water is destroyed.

The water molecules nearest to the surface

participate on average in only ~2.5 H bonds,

although this number increases to ~3.5 at

distances >4 AÊ from such surfaces (Gordillo and

MartõÂ, 2000). At the talc (001) surface, water

dipoles have two predominant orientations that

appear to correspond to molecules that either

accept H bonds from OH groups in the talc

octahedral sheet or donate H bonds to the surface-

bridging oxygens (Bridgeman and Skipper, 1997).

Our recent calculations for the talc (001) surface

show only weak H-bond donation to the bridging

oxygens at ambient conditions and essentially

none at elevated temperatures (Wang, 2004).

Acknowledgements

This research was supported by DOE Basic

Energy Sciences Grant DEFGO2-00ER-15028.

Computation was partially supported by the

National Computational Science Alliance (Grant

EAR 990003N) and utilized NCSA SGI/CRAY

Origin 2000 computers and the Cerius2-4.6

software package from Accelrys. J. Wang also

acknowledges a fellowship from the University of

Illinois at Urbana-Champaign. We gratefully

acknowledge detailed discussion of the experi-

mental data with Xiaoqiang Hou and Ping Yu and

fruitful discussion of molecular modelling with

R.T. Cygan and J.D. Kubicki.

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