Molecular dynamics simulations of the interactions between water andinorganic solids

Sebastien Kerisit, David J. Cooke, Dino Spagnoli and Stephen C. Parker*

Received 8th October 2004, Accepted 24th December 2004

First published as an Advance Article on the web 2nd February 2005

DOI: 10.1039/b415633c

Molecular dynamics simulations of three solid surfaces, namely, the (00.1) and (01.2) hematite

surfaces and the (10.4) calcite surface, in contact with an aqueous solution have been performed

and the structure of water near the interface investigated. We initially calculated the hydration

and hydroxylation energies of the two hematite surfaces using static calculations to determine the

adsorbed state of water on these surfaces before studying hydration using molecular dynamics.

The dynamics simulations show that, in each case, the water density exhibits a damped oscillatory

behaviour up to a distance of at least 15 A from the surface. Next, we investigated the adsorption

of ions on the (10.4) calcite surface by calculating their free energy profile. These profiles show a

strong correlation with the water structure at the interface. This implies that the adsorption of

water at the surface of the solid causes the density fluctuations, which in turn control further

adsorption. Further analysis revealed that, in each case, the solid surface had a strong effect on

the self-diffusion coefficient and the orientation order parameter of water near the interface.

Finally, to consider the effect of the crystal size on the solid/water interface, we modelled a calcite

nanoparticle in vacuum and immersed in water. We found that the nanoparticle undergoes a

phase change in vacuum but that, in the presence of water, the calcite structure was stabilised.

Also, the water residence time in the first hydration shell of the surface calcium ions suggested

that the dynamics of water in the vicinity of the nanoparticle resemble that around an isolated

calcium ion.

1 Introduction

Many material properties and synthetic routes involve the

transfer of ions and molecules between the aqueous phase and

the solid surface. Thus to comprehend and control processes

such as corrosion, trace metal incorporation, crystal growth,

or dissolution, we require an understanding of the molecular

scale interactions between the solid surface, the adsorbing or

reacting species, and water. Hence, many experimental and

computational studies have concentrated on characterising the

structure and reactivity of the aqueous phase in the vicinity of

a solid surface. For example, X-ray reflectivity measurements

of Fenter et al.1 revealed the presence of a monolayer of

hydroxyl species (OH or OH2) adsorbed on the (10.4) calcite

surface. Computational studies have also identified the

presence of a strongly bound water layer on calcite2,3 and on

other solids.4–6 In addition, there has been considerable work

investigating the adsorbed state of water at surfaces. Many

studies have shown that water dissociatively adsorbs onto the

surfaces of hematite.7–9 For example, Liu et al.8 used X-ray

photoemission and found that water vapour reacts readily

with the (00.1) surface to form hydroxyls on the surface. The

adsorbed state of water on calcite surfaces has also been

investigated by, amongst others, Stipp and Hochella,10 who

used X-ray photoelectron spectroscopy (XPS) and low-energy

electron diffraction (LEED) to examine calcite surfaces and

found evidence of the presence of CaOH+ and HCO32 surface

species, hence suggesting that water dissociates on the surfaces

of calcite in ultra-high vacuum. Finally, recent studies have

started to probe the structure of the aqueous phase in the first

few Angstroms away from the solid surface. For example,

Fenter and co-workers11 observed oscillations of the water

density at the mica(001)/water interface under ambient condi-

tions up to 12 A. They made similar observations at the

orthoclase(001)/ and (010)/water interfaces,12 albeit only up to

5 A. These observations can be directly compared with mole-

cular dynamics simulations such as those presented in this paper.

Although the studies mentioned above show that the solid

surface affects the structure and chemistry of water near the

interface, it is still not clear whether this phenomenon is

strongly dependent on the nature of the solid surface,

particularly its structure and composition. Moreover, little is

known of the effect of the solid surface on the dynamics of

water near the interface. Finally, we need to investigate to

what extent these findings are dependent on the size of the

crystals studied. In the case of nanoparticles, a large pro-

portion of the crystal is exposed to the solvent and thus one

would expect water to have a direct influence on the structure

of the particle.

In this paper, we describe our recent work on the investiga-

tion of the hematite/water and calcite/water interfaces. They

were chosen because both materials are very abundant in

nature and are important in many fields of chemistry. We

focused on the most stable surfaces of each material, namely,

the (00.1) and (01.2) surfaces of hematite and on the (10.4)

surface of calcite. In addition, as a way of investigating the*s.c.parker@bath.ac.uk

PAPER www.rsc.org/materials | Journal of Materials Chemistry

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influence of the interface on ion transport we studied the effect

of the modified water structure near the interface on the

adsorption of ions. Finally, we considered a calcite nano-

particle immersed in water to probe the effect of the solvent on

the nanoparticle’s structure. However, before describing the

results we give a brief summary of the methodology employed.

2 Methods

All the calculations in this work are based on the Born model

of solids,13 in which the interactions between the atoms of a

system are divided into long-range electrostatic interactions

and short-range forces. The latter include electron cloud

repulsion and van der Waals attraction forces and, where

appropriate, an angle-dependent term to reproduce covalent

effects. The short-range interactions are described with

parameterised functions. The model also employs a simple

mechanical shell model introduced by Dick and Overhauser14

to account for the ionic polarisability. In this model, an ion

consists of two particles, namely, a negatively charged

shell and a positively charged core, connected by a harmonic

spring. In our calculations, the cation polarisability was

assumed to be negligible and hence the shell model was only

used to simulate oxygen ions.

We first performed preliminary static calculations, using

the computer code METADISE,15 to determine the favoured

mode of adsorption (dissociative or associative) of a mono-

layer of water on the mineral surfaces. The addition of mole-

cular water was initiated by placing a water molecule 1.7 A

above each surface cation. The hydrogen atoms were

positioned so the H–O–H bond angle was approximately

108u and the O–H bond distance was 1 A. Surface hydroxyla-

tion was initiated by adding an OH2 group 1.7 A above each

surface cation and an H+ above the same number of surface

oxygen atoms. After energy minimisation, the surface energies

of the dry, hydroxylated, and hydrated surfaces were

calculated according to the following equation:

c 5 (Us 2Ub)/A (1)

where Us is the energy of the surface, Ub is the energy of the

same number of bulk atoms and A is the surface area. For the

hydrated surface an additional term is added to Ub equivalent

to the energy of the appropriate number of isolated water

molecules. The calculation of the hydroxylated surface energy

requires an additional term to take into account the change in

oxidation state of the surface oxygen atoms. This is done using

a Gibbs Cycle and considering the general solid state reaction

AO(s) + H2O(l) A A(OH)2(s).16 The energies of hydration and

hydroxylation are also calculated in a similar way.

Once the energetically favoured mode of adsorption has

been identified the computer code DL_POLY17 was used to

perform molecular dynamics simulations of the solid/water

interfaces. Trajectories were generated in the NVT ensemble

(i.e., constant number of particles, constant volume, and con-

stant temperature) by means of the Verlet leapfrog algo-

rithm18,19 with a timestep of 0.1 or 0.2 fs and the temperature

was kept constant by the Nose–Hoover thermostat.20 The

shells were given a small mass of 0.2 a.u. following the

approach introduced by Mitchell and Fincham.21 The electro-

static interactions were calculated using the smooth particle

mesh Ewald method (SPME),22 which is a modification of the

Ewald summation scheme.23

The parameters used to model hematite and its interactions

with water were based on the potentials developed by Lewis

and Catlow,24 where the potential parameters for the oxygen–

oxygen interaction were taken from the work of Catlow.25

These parameters are well established and have been used in a

variety of studies.16,26–28 The potential parameters of Baram

and Parker29 were used to model the hydroxyl ion. The

potentials parameters for calcite were derived by Pavese et al.30

to reproduce the thermal dependence of the structural and

elastic properties of calcite. In a recent paper,31 we showed that

the relative surface energies of the main low-index surfaces of

calcite calculated by the Pavese potential are in very good

agreement with electronic structure calculations. The intra-

and intermolecular interactions of water were described using

the de Leeuw and Parker32 model with the modified hydrogen

bond potential of Kerisit and Parker.33 The potential para-

meters for the interactions between calcite and water were

taken from previous studies on the adsorption of water on the

surfaces of calcite.3,34

3 Water structure at the solid/water interface

Molecular dynamics is a technique that lends its self naturally

to simulating the solid/water interface. This is true for two

reasons. Firstly, much time and effort in recent times has been

engaged in developing efficient codes, which scale readily to a

large number of processors and to the large systems sizes

required if water is to be considered explicitly. Secondly, the

dynamic nature of the method means it can be applied to

both solids and liquids. The study of solid/water interfaces is

not new. However, much of the work previously undertaken,

either uses continuum models35–37 to describe the aqueous

phase or uses a rigid body description of the water

molecules,5,38,39 if they are described explicitly. Calculations

by ourselves3,32,33 and others5,40 as well as experimental

studies11,12 have shown that water shows considerable ordering

close to the solid surface and consequently we believe that an

explicit representation of the water molecules in the simulation

is essential. As described in the previous section, our model of

water also uses the shell model to describe the polarisability of

the oxygen ion of a water molecule, thus making our model

chemically more realistic.

The calculations discussed below all consisted of a slab of

solid sufficiently thick as to prevent interaction between the

two faces of the slab. About 500 water molecules were then

added on each face hence forming two water slabs about 30–

40 A thick, above which there was a large vacuum gap of at

least 60 A. This enabled the water slabs to relax to an appro-

priate density and ensured there was no interaction between

the two interfaces. Therefore, this means our calculations

effectively consisted of two isolated systems.

3.1 (01.2) Hematite surface

The surface, hydration, and hydroxylation energies are

summarised in Table 1. These calculations indicate that it is

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energetically favourable for water to adsorb as molecular

water on the (01.2) surface. Hence, the hematite (01.2)/water

interface was generated using the methodology described

above and the surface was not hydroxylated. The water

density as a function of distance from the surface was

calculated from a 1 ns molecular dynamics simulation and is

shown in Fig. 1. For arbitrary reasons the surface was defined

as the plane passing through the centres of the uppermost

iron atoms. As in our previous studies,3,32,33 the water shows

ordering close to the surface. This effect diminishes as a

function of distance from the surface; however, the water

density has still not completely converged to its bulk value by

20 A. The first hydration layer consists of three different water

molecules at 1.9, 2.3 and 2.7 A, which corresponds to a

coverage of 1.5 water molecules per surface cation. Fig. 2(a)

shows a snapshot of the first hydration layer.

Next, we calculated the free energy profile of a water mole-

cule as it approaches the surface. The free energy of a water

molecule relative to its free energy in the bulk is calculated by

integrating the average force in the direction perpendicular to

the surface, fz, from the centre of the water slab to the mineral

surface:41

DA zað Þ~A zað Þ{A z0ð Þ~ðza0

Sfz zð ÞTdz (2)

where z is the distance perpendicular from the surface.

Another approach is to use the water density perpendicular

to the surface, r, as an approximation for the partition

function, to give the free energy difference at a particular

height.41 DAz 5 2RT ln(rz/r0)

Both approaches give the same free energy of adsorption for

the three modes, 23.0, 22.5 and 22.9 kJ mol21, respectively.

Effectively these are only estimations, however, it does give

good qualitative analysis. These free energies are considerably

smaller than the energies of hydration obtained from the

static calculations, as shown in Table 1. This suggests that the

adsorption process results in a large change in entropy and

Table 1 Dry, hydroxylated, and hydrated surface energies of the (00.1) and (01.2) surfaces of hematite and hydration energies of the hydrated andhydroxylated surfaces

Surface cdry/J m22 cdissoc./J m22 Ehydroxyl./kJ mol21 cassoc./J m22 Ehydration/kJ mol21

(00.1)Fe 2.41 1.65 271.5 1.75 287.5(00.1)O 3.74 1.22 2209.5 2.89 289.4(01.2) 2.10 1.74 228.8 1.29 264.6

Fig. 1 Water density and free energy profiles as a function of distance

from the (01.2) hematite surface. The MD simulation was performed at

300 K and zero pressure. The mineral slab contained 120 Fe2O3 units

and the water slabs 960 water molecules. The cell dimensions were

20.28 6 16.00 6 200.00 A.

Fig. 2 Snapshots showing the first hydration layer of the (01.2)

hematite surface (a), (00.1) hematite surface (b), and (10.4) calcite

surface (c). Iron and calcium ions are black, carbon ions are dark grey,

oxygen ions are light grey, and hydrogen ions are white.

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that the entropic term of the free energy of adsorption is of

the same order of the enthalpy of adsorption. The three

adsorption sites seem to be separated by small free energy

barriers; however, the analysis of the water residence time

at each level shows that there was no exchange between the

three modes. The lack of spontaneous exchange between the

water molecules in the hydration layer during the simulation

means that the energy barriers are poorly constrained, and

would need further work to obtain quantitative values for

these free energy barriers. Part of the reason for the lack of

exchange is that the surface cations are not hydroxylated

and hence the water molecules are strongly bound to the

iron atoms.

To investigate the effect of the solid surface on the structural

and dynamical properties of water further, we calculated the

water self-diffusion coefficient41 in the directions normal and

parallel to the surface and the water orientation order para-

meter42 near the interface. Details on the method used to

calculate these two quantities can be found in a previous

publication.33 The self-diffusion coefficient, in Fig. 3, shows

a clear correlation with the water density in the direction

normal to the surface. In addition, in this direction, the effect of

the surface persists up to 20 A, whereas in the direction parallel

to the surface the diffusion coefficient starts decreasing around

8 A from the surface. The diffusion coefficient in both direc-

tions converges to a value of about 3.26 1029 m2 s21, which is

slightly higher than the bulk value of 2.3 6 1029 m2 s21

published previously. One possible explanation for this

surprising result is that the disruption of the hydrogen bond

network of water, caused by the interface, facilitates diffusion

at large distances. A final point to note from Fig. 3 is the

very small value of the diffusion coefficient of the three

water molecules of the first hydration layer (less than 1 61029 m2 s21), which clearly shows the ice-like behaviour of

water in this layer. Fig. 4 shows that the orientation of the

water molecules near the interface is affected to a distance of

about 12 A. In the first 6 A, which correspond to the first two

hydration layers, the water orientation is greatly influenced by

the surface as the sharp peaks indicate. Beyond 6 A, the

orientation order parameter shows slight oscillations that have

the same period as those of the water density.

3.2 (00.1) Hematite surface

The results from the static calculations, given in Table 1, show

that the most stable form of the (00.1) surface in the presence

of water is a hydroxylated surface based on the O terminated

system and, hence, this surface was considered for the mole-

cular dynamics simulation. A hematite (00.1)/water interface

was generated following the method described above and was

run for 300 ps after an equilibration period of 50 ps. A density

profile of the water – perpendicular to the surface – calculated

over the entire production run is shown in Fig. 5. Layering is

once again evident to about 15 A from the surface. The first

peak represents the layer of hydroxyl groups, which consists of

dissociated water molecules and nominally terminal oxygen

atoms. Perhaps the most striking feature of the density plot is

the two sharp peaks seen beyond the hydroxyl layer, the first at

2.4 A and the second at 3.2 A away from the surface, which

form the second hydration layer. The first peak of the two

represents water molecules bonded to hydroxyl groups,

whereas the second peak represents the water molecules

interacting with the surface hydroxides and iron atoms (see

Fig. 2(b)). This second peak corresponds to a density of one

water molecule per surface cation and accounts for twice as

many water molecules than the first peak. The next hydration

layer also shows evidence of splitting with a main peak at 5.2 A

Fig. 3 Water diffusion coefficient in the directions normal and

parallel to the (01.2) hematite surface.

Fig. 4 Water orientation order parameter as a function of distance

from the (01.2) hematite surface.

Fig. 5 Water density and free energy profiles as a function of distance

from the (00.1) hematite surface. The MD simulation was performed at

300 K and zero pressure. The mineral slab contained 192 Fe2O3 units

and the water slabs 960 water molecules. The cell dimensions were

20.25 6 17.54 6 155.00 A.

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and a shoulder peak at 5.9 A, which is probably due to

hydrogen bonding to the two water molecules in the second

hydration layer.

The free energy profile of water, also shown in Fig. 5,

indicates that the free energy of adsorption of a water molecule

in the second hydration layer is about 23.0 kJ mol21 for both

modes of adsorption, which is similar to the values obtained

for the first and second hydration layers on the (0.12) surface.

The free energy barrier for adsorption in the second hydration

layer is 8.8 kJ mol21. In comparison, the free energy barriers

for adsorption in the first and second hydration layers of the

(01.2) surface are 16.0 and 3.4 kJ mol21, respectively. This

implies that, due to the dissociatively adsorbed water layer

on the (00.1) surface, the water molecules of the second layer

can adsorb closer to the surface than those in the second

layer of the (01.2) surface and thus interact with the surface

more strongly, which results in higher energy barriers to escape

to the bulk.

The analysis of the water self-diffusion coefficient and

orientation order parameter gives similar results to those

obtained with the (01.2) surface. In particular, the self-

diffusion coefficient converges to almost the same value, i.e.,

3.6 6 1029 m2 s21, which is, as mentioned before, slightly

larger than the bulk value.

3.3 (10.4) Calcite surface

In a recent paper, we showed that, unless it has reacted with

the surface, water is associatively adsorbed on the (10.4) calcite

surface.3 Therefore, only molecular water was considered for

the molecular dynamics simulation. The calcite (10.4)/water

interface was generated using the method described at the start

of this section and the water density and free energy profiles,

obtained from a 1 ns dynamics simulation, are reported in

Fig. 6. As shown in this figure, the calcite hydration layer

consists of two water molecules at a height of 2.2 and 3.2 A.

The first peak corresponds to water molecules directly bonded

to surface calcium atoms and the second peak to those forming

hydrogen bonds with surface oxygen ions, as shown in

Fig. 2(c). They both account for a density of one water

molecule per surface cation. These results are in excellent

agreement with a recent X-ray scattering study of the calcite/

water interface by Fenter and co-workers.43 Indeed, their study

suggested the presence of two water molecules at 2.3¡ 0.1 and

3.2 ¡ 0.2 A above the surface. However, they did not observe

any layering beyond the hydration layer, which differs from

the density profile in Fig. 6, where the water density converges

to its bulk value after 15 A.

The free energy of adsorption in the first hydration layer is

about 22.0 kJ mol21 for both water molecules. One would

expect the water molecules in the first hydration layer of calcite

to be less strongly bound to the surface than in the hydration

layer of hematite. The lower free energy of adsorption reflects

the slightly lower packing density of water with respect to

bulk water. In addition, the free energy barriers for adsorption

and desorption are 6.8 and 6.1 kJ mol21, respectively. These

values are much smaller than those obtained for the adsorp-

tion and desorption on the (01.2) hematite surface (16.0 and

16.4 kJ mol21, respectively). Therefore, these results suggest

that the strength of the interactions between water and the

solid surface has little influence on the free energy of adsorp-

tion but clearly shows a correlation with the size of the free

energy barriers involved.

The calculations of the water self-diffusion coefficient

and orientation order parameter indicate that water shows

a similar behaviour near the calcite/water interface to that

previously reported for the hematite surfaces. Indeed, the self-

diffusion coefficient in the first hydration layer is lower than

1 6 1029 m2 s21 and then converges to 3.7 6 1029 m2 s21

beyond 15 A, as found on the hematite surfaces.

4 Ion adsorption at the solid/water interface

In this section, the effect of the water structure, described in

the previous section, on the adsorption process is investigated

focusing on the adsorption of a calcium ion and a carbonate

molecule on the (10.4) calcite surface as an example. To

achieve this, we used the thermodynamic integration method,44

which allows us to compute the free energy change associated

with the adsorption of an ion or a molecule on the surface.

Fifty independent molecular dynamics calculations were

performed where in each calculation the incoming species

was fixed at a height above the surface between 10 and 2.5 A.

The free energy profiles, shown in Fig. 7 together with the

water density, were obtained by integration of the force acting

on the centre of mass of the molecule in direction normal to

the surface. The free energy profiles are strongly correlated

with the water density profile. Indeed, as the two species

approach the surface, the change in free energy follows the

oscillations in water density. In addition, the position of the

free energy barrier between the inner- and outer-sphere

complexes is the same for the two species and coincides with

the region of lowest water density. In the case of calcium, the

profile shows the presence of two outer-sphere complexes

where the calcium is coordinated to one or two water mole-

cules of the hydration layer at 5.7 and 4.8 A above the surface,

respectively. The carbonate molecule shows a clear preference

for adsorption as an outer-sphere complex, whereas for

calcium the inner- and outer-sphere complexes have similar

Fig. 6 Water density and free energy profiles as a function of distance

from the (10.4) calcite surface. The MD simulation was performed at

300 K and zero pressure. The mineral slab contained 180 CaCO3 units

and the water slabs 1080 water molecules. The cell dimensions were

24.12 6 23.98 6 200.00 A.

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free energies of adsorption. The size of the free energy barrier

for adsorption is 13.9 and 15.1 kJ mol21 for calcium and the

carbonate molecule, respectively. The free energy barrier

for desorption is 16.3 and 9.3 kJ mol21 for calcium and the

carbonate molecule, respectively. Hence we expect the

residence time of the carbonate molecule to be shorter than

that of calcium. However, the free energy required to desorb

from an outer-sphere complex position is 2.2 and 12.8 kJ mol21

for calcium and the carbonate group, respectively.

These calculations enable us to start to consider the rates of

adsorption of these species on the surface. If we define the

reaction coordinate as being the difference in height between

the surface and the adsorbing species, we can apply the

transition state theory to determine the rate of exchange

between the inner- and outer-sphere complexes. The rate

constant is given by

k 5 kkTST (4)

where kTST is the transition state rate constant,

kTST~

ffiffiffiffiffiffiffiffiffikBT

2pm

se{bDA(r�)

Ðr�0

e{bDA rð Þdr

(5)

where m is the reduced mass of the adsorbing species-mineral

slab pair where the mass of the slab is taken to be infinite, b is

1/kBT, and r* is the transition state distance.

The transmission coefficient, k, gives an indication of the

fraction of successful jumps weighted by the velocity of the

adsorbing species at the transition state position. Details on

the computation of k are given elsewhere.33 The transition state

rate constants for the adsorption and desorption of a calcium

ion and a carbonate group on the surface are given in Table 2.

This table shows that the calcium ion is expected to desorb

from the surface at a much lower rate than the carbonate

molecule. In addition, the rate of adsorption on the surface is

slightly higher for calcium than for the carbonate molecule.

Therefore, these calculations suggest that calciumwould adsorb

more easily as an inner-sphere complex than the carbonate

molecule and that it would remain adsorbed for a longer

period of time. In a previous paper,33 the transmission

coefficient for calcium was calculated to be 0.024 and thus

the adsorption and desorption rate constants were found to be

3.3 6 108 and 1.9 6 108 s21, respectively, although it is worth

noting at this point that the uncertainty on the transmission

coefficient was relatively large and thus these results should be

taken with caution. These rates correspond to an adsorption

event every 3 ns and a residence time on the surface of about

5 ns. A similar transmission coefficient for the carbonate

molecule would result in an adsorption event every 5 ns and a

residence time on the surface of 0.5 ns. To conclude, the free

energy difference between the inner- and outer-sphere com-

plexes of both species as well as the estimated rates of

adsorption and desorption strongly suggest that calcium ions

would preferentially adsorb on the surface as inner-sphere

complexes and have relatively long residence times on the

surface, whereas carbonate ions would have shorter residence

times as inner-sphere complexes but are more likely to be

found as outer-sphere complexes.

5 Effect of water on the structure of a calcitenanoparticle

The previous sections described how the solid surface can alter

the structure and dynamics of water in the first 10 A away

from the surface and how this has a clear effect on the adsorp-

tion of ions and molecules on the solid surface. However, the

surfaces considered so far are ideal surfaces with effectively

infinite surface areas. Therefore, the aim of this section is to

begin to consider the effect of the size of the crystal on the

water structure near the interface. Moreover, we investigate

whether water can have an influence on the structure of a

nanoparticle.

A 1.5 nm calcite nanoparticle was submerged in water and a

molecular dynamics simulation was performed in the NPT

ensemble (i.e., constant number of particles, constant pressure,

and constant temperature) at 300 K and zero pressure. The

cubic simulation cell had a side length of 36.5 A on average

and contained just under 2000 water molecules. The simulation

was run for 400 ps with a timestep of 0.1 fs.

The calcium-carbonate oxygen radial distribution function

(RDF) obtained from this simulation is shown in Fig. 8. For

comparison, the calcium-carbonate oxygen RDF obtained

from the simulation of bulk calcite is given in Fig. 9. Fig. 8

shows a loss in the crystalline structure compared to the bulk

crystal. Indeed, the peaks in the RDF of the nanoparticle are

not as clearly separated as in the bulk RDF. However, their

position changes by 0.05 A at most and thus remains almost

the same.

In order to consider the effect of water on the crystalline

structure of the nanoparticle, we modelled the same particle

in vacuum. The calcium-carbonate oxygen RDF of the

Fig. 7 Free energy profiles of adsorption of calcium ion and

carbonate molecule on the (10.4) calcite surface.

Table 2 Transition state rate constants of the adsorption anddesorption of a calcium ion and a carbonate group on the (10.4)calcite surface

Ion/molecule Adsorption/s21 Desorption/s21

Carbonate 8.5 6 109 7.6 6 1010

Calcium 1.4 6 1010 7.9 6 109

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nanoparticle in vacuum is shown in Fig. 10. Although the first

peak is still present and at the same distance of 2.33 A, the next

three peaks have merged into one at 3.88 A. This suggests a

change in the structure and that the nanoparticle in vacuum is

more amorphous than that in water. Fig. 11(a)–(c) compare

snapshots of the calcite nanoparticle in water and vacuum

with the initial configuration, which corresponds to bulk

positions. Fig. 11(a) shows the clear rhombohedral structure

with relatively sharp corners. In Fig. 11(b), however, the

nanoparticle has lost its rhombohedral shape and the corners

have smoothed out. When immersed in water, the nanoparticle

adopts an intermediate structure as shown in Fig. 11(c).

Similar results were obtained for ZnS by Banfield and co-

workers,45 who used molecular dynamics simulation to study

the phase stability of ZnS nanoparticles in vacuum and in

the presence of water. They found that the adsorption of water

on the ZnS nanoparticle stabilises the sphalerite structure,

whereas in vacuum the ZnS particles adopt a different struc-

ture based on wurtzite, in contrast to calcium carbonate which

favours a more disordered phase in these conditions.

Another property of the nanoparticle–water system that can

be determined is the residence time of a water molecule in the

hydration layer of the nanoparticle. We first define the four

types of calcium ions present in the nanoparticle. There are

four calcium ions on the corners of the nanoparticle, eight

at the edges, five on the faces, and one in the inside of the

nanoparticle. Table 3 shows the average residence time of a

water molecule in the first hydration shell of each type of

calcium atoms together with the average calcium coordination

number. The first point to note from Table 3 is that the change

in coordination number follows the expected trend. Secondly,

Fig. 8 Calcium-carbonate oxygen RDF of the nanoparticle in water.

Fig. 9 Calcium-carbonate oxygen RDF in bulk calcite.

Fig. 10 Calcium-carbonate oxygen RDF of the nanoparticle in

vacuum.

Fig. 11 Snapshots from the molecular dynamics simulations of a nanoparticle in vacuum and in water at 300 K and NPT ensemble: (a) initial

configuration (i.e., bulk positions), (b) in vacuum (no periodic boundary conditions), and (c) in water. Water density at 1.27 g cm23 and cell length

at 36.5 A. Calcium ions are black, carbon ions are dark grey, and oxygen ions are light grey.

Table 3 Water residence time at the nanoparticle surface and calciumcoordination number with water

Calcium position Water residence time/ps Coordination number

Side 30 1.1Edge 34 2.6Corner 33 3.6

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the water residence time only deviates a little from ion to ion

and is much lower than that obtained in the first hydration

layer of a perfect (10.4) surface.33 However, these residence

times are very similar to that in the hydration shell of an

isolated calcium ion in solution.33 This could imply that the

nanoparticle has considerably less effect on the water structure

and dynamics that the ideal infinite surface and, hence, the

behaviour of the nanoparticle in water resembles that of an

isolated calcium ion.

6 Conclusions

In this paper, we considered three solid/water interfaces to

probe the effect of the solid surface on the structure and

dynamics of water. As previously seen in both experimental

and computational studies, our calculations showed an

oscillation of the water density at the interface. Moreover,

the first hydration layer was found to consist of different

numbers of water molecules depending on the geometry

of the surface. In the case of calcite, the heights at which

the water molecules of the first hydration layer can be found

are in very good agreement with a recent X-ray scattering

study.43

Analysis of the water self-diffusion coefficient and orienta-

tion order parameter showed that water adopts very specific

orientations in the first few Angstroms away from the surface

and, in this region, its diffusion coefficient is reduced by about

one order of magnitude. Further away from the interface,

the effect of the surface is not as strong but the diffusion

coefficient in the direction normal to the surface and the

orientation order parameter still show some correlation with

the water density.

The free energy profile of a water molecule adsorbing on the

surface was also calculated and we found that the free energy

of adsorption is not strongly dependent on the solid surface.

However, a general trend is that there is a correlation between

the strength of water–surface bond and the size of the free

energy barriers of adsorption and desorption.

Next, we investigated the adsorption of ions on the (10.4)

calcite surface and found that there was a correlation between

the water density and the free energy profile of the adsorbing

species. Therefore, the solid–water interactions alter the water

structure near the interface, which in turn affects the adsorp-

tion of ions on the surface. The calculations showed that

calcium ions are more likely to adsorb as inner-sphere com-

plexes and that they have long residence times on the surface

compared to carbonate ions, which preferentially adsorb as

outer-sphere complexes.

Finally, we considered a 1.5 nm calcite nanoparticle in water

and in vacuum, and we found that the nanoparticle exhibits

a much more amorphous structure in vacuum but that

the calcite structure is stabilised by the presence of water. In

addition, it was found that the residence time of water in the

first hydration shell of surface calcium ions is of the same

order of that around an isolated calcium ion in water. These

calculations suggest that the effect of the nanoparticle on the

structure and dynamics of the water molecules surrounding it

resembles more that of an isolated calcium ion than that of an

infinite surface.

In the future, we wish to consider nanoparticles of different

sizes to investigate the solid/water interface as a function

of crystal size. In addition, we would like to employ the

same methods to investigate the adsorption of ions and

molecules at steps and defects where the material growth

occurs preferentially.

Acknowledgements

The authors thank Dr A. Marmier for useful discussions,

EPSRC Grants No. GR/H0185 and GR/H0413 for funding,

and the Materials Chemistry Consortium and the NERC

funded Mineral Physics Consortium for the provision of

computer time.

Sebastien Kerisit, David J. Cooke, Dino Spagnoli andStephen C. Parker*Chemistry Department, University of Bath, Claverton Down, Bath, UKBA2 7AY. E-mail: s.c.parker@bath.ac.uk

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