Hydrogen bond dynamics at vapour–water and metal–water interfaces
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Transcript of Hydrogen bond dynamics at vapour–water and metal–water interfaces
Chemical Physics Letters 386 (2004) 218–224
www.elsevier.com/locate/cplett
Hydrogen bond dynamics at vapour–water andmetal–water interfaces
Sandip Paul, Amalendu Chandra *
Department of Chemistry, Indian Institute of Technology, Kanpur 208016, India
Received 26 November 2003
Published online: 11 February 2004
Abstract
We have presented a molecular dynamics study of the dynamics of breaking and structural relaxation of hydrogen bonds at va-
pour–water and metal–water interfaces. For both interfaces, the relaxation of hydrogen bonds is found to occur at a slower rate than
that in bulk water. However, the dynamics of inter-region hydrogen bonds, where onemolecule of the bonded pair belongs to interface
and the other to its adjacent layer, are found to be faster than even the bulk hydrogen bonds. The results are explained in terms of the
energies of hydrogen bonds and the frictions that act on the rotational and translational motion of water molecules at interfaces.
� 2004 Elsevier B.V. All rights reserved.
1. Introduction
The dynamical behaviour of hydrogen bonds plays animportant role in determining the rates of many chem-
ical, physical and biochemical processes that occur in
aqueous media. In recent years, there have been a
number of experimental [1–5], theoretical [6,7] and
simulation [6–12] studies that focused on the relaxation
of hydrogen bonds in homogeneous bulk water and
aqueous solutions and now the attention is being shifted
to inhomogeneous media of aqueous interfaces [13–19].The present Letter is concerned with the dynamical
behaviour of hydrogen bonds at vapour–water and
metal–water interfaces. Both these interfaces are highly
inhomogeneous but the nature of inhomogeneity is very
different. In case of water–vapour interface, the density
decreases rather smoothly from liquid density to the
vapour density [20–23] whereas a water–metal interface
is characterized by highly oscillatory density profile dueto layered structure of such interfaces [24–27]. The ori-
entational structure of water molecules at these inter-
faces can also be quite different [20–29] and this varying
* Corresponding author.
E-mail address: [email protected] (A. Chandra).
0009-2614/$ - see front matter � 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2003.12.120
inhomogeneity and orientational structure are expected
to influence both the structure and dynamics of hydro-
gen bonds at these interfaces. In fact, recent studies onvapour–water and metal–water interfaces have shown
significant differences of the single-particle translational
and orientational diffusion of interfacial molecules as
compared to that of bulk molecules [20,23,25–27].
However, we are not aware of any study on the dy-
namics of hydrogen bonds at these interfaces. In this
Letter, we present such a study by means of molecular
dynamics simulations.The outline of the present Letter is as follows. In
Section 2, we discuss the molecular models of the va-
pour–water and metal–water systems that are employed
in the present study and the simulation details are pre-
sented in Section 3. Sections 4 and 5 deal with the
interfacial density profiles and the distribution of hy-
drogen bonds, respectively. The dynamics of hydrogen
bonds at the interfaces and in the bulk phases are dis-cussed in Section 6 and our conclusions are summarized
in Section 7.
2. Models
For both vapour–water and metal–water systems, we
have used the classical SPC/E model of water where
S. Paul, A. Chandra / Chemical Physics Letters 386 (2004) 218–224 219
each water molecule is characterized by three interaction
sites located on oxygen and hydrogen atoms [30]. The
O–H bond distances are constrained at 1.0 �A and the
bond angle between two O–H bonds is fixed at 109.47�.The interaction between atomic sites of two watermolecules is expressed as
uðri; rjÞ ¼ 4�rrij
� �12"
� rrij
� �6#þ qiqj
rij; ð1Þ
where the first term is the Lennard-Jones interaction
which is calculated only between the oxygen sites and
the second term represents the electrostatic interaction.qi is the charge of the ith atom. The values of the po-
tential parameters r, � and qi for SPC/E model are given
in [30]. For the metal–water system, the metal surface is
treated quantum mechanically by using the jellium
model. The water–surface interaction uwðzÞ is expressedin the following form [27]
uwðzÞ ¼33=2�w2
rw
z
� �9�
� rw
z
� �3�� EðzÞ � li; ð2Þ
where z ¼ jzi � z0j, �w and rw are the 9–3 Lennard-Joneswell-depth parameter and diameter which characterize
the short range part of the metal–water interaction and
EðzÞ is the electric field generated by the metallic sur-
faces. The parameters of the short range metal–surface
interaction are: rw ¼ 2:474 �A and �w ¼ 1:936 kJ/mol
[27]. In order to calculate the electrostatic field ðEðzÞÞ ofa metal surface, we note that EðzÞ ¼ �ðo=ozÞV ðzÞ, whereV ðzÞ is the metal electrostatic potential. V ðzÞ satisfies thePoisson equation
d2
dz2V ðzÞ ¼ �4pqcðzÞ; ð3Þ
where qcðzÞ is the charge density of the metal. An ex-plicit modelling of the electronic structure of the metal is
now necessary in order to calculate the charge density
and the metal field. Following earlier work [27], we
model the metal walls by semi-infinite jellium slabs of
width 2zw. The jellium model consists of a uniform
background of positive charge density qþ which repre-
sents the metal nuclei and core electrons and the asso-
ciated valence electron density qeðzÞ. The valenceelectron density is calculated by using density functional
theory [31,32]. In this approach, the electron density is
calculated by solving the effective one-electron Schro-
dinger equation
� �h2
2me
d2
dz2wnðz0Þ þ Veffðz0Þwnðz0Þ ¼ �nwnðz0Þ; ð4Þ
where wn and �n are the one-electron normalized
eigenfunction and energy eigenvalue for the nth state
and me is the mass of an electron. z0 denotes the z-coordinate with origin at the centre of the metal slab.
Veffðz0Þ is the effective potential which is given by
Veffðz0Þ ¼ Vjelðz0Þ þ Vxcðz0Þ þ Vsolðz0Þ; ð5Þ
where Vjelðz0Þ represents instantaneous interaction of
an electron with the field of the jellium, Vxcðz0Þ is the
exchange and correlation potential and Vsolðz0Þ is the
average interaction energy of the electron with the polar
solvent.In the present work, we have used the local density
approximation with Wigner�s expression for the ex-
change and correlation energy [33]. The jellium potential
is obtained through a solution of the Poisson equation
involving the electron density. Vsolðz0Þ depends on the
solvent density near the surface and its expression is
available in [27]. We note that the metal potential de-
pends on the solvent density which, in turn, depends onthe potential of the metal surface. Thus, the metal po-
tential from density functional theory and the solvent
structure from simulation are obtained self-consistently
through iteration.
3. Details of simulations
For construction of the vapour–water interfaces, we
first carried out a bulk simulation in a cubic box of 500
water molecules periodically replicated in all three di-
mensions. The box length L was adjusted according to
the experimental density of water at 298 K. After this
bulk solution was properly equilibrated, two empty
boxes of equal size were added on either side of the
original simulation box along the z-dimension and thislarger rectangular box was taken as the simulation box
in the next phase of the simulation run. The system was
reequilibrated by imposing periodic boundary condi-
tions in all three dimensions. This resulted in a lamella
of approximate width L with vapour–water interfaces on
both sides of the lamella.
The simulation of the metal–water system is done in
a rectangular box with wall origins at �11.86 �A, andperiodic boundary conditions are set at �9.27 �A in xand y directions. The simulations are carried out with
a total of 256 water molecules. Initially, the metal
potential is calculated by replacing the water by vac-
uum. The liquid was then introduced and the solvent
density was found out. After this initial calculation,
Vsolðz0Þ was evaluated and the new electron distribution
was calculated by solving the density functionalequations. The metal potential was calculated from the
new electron distribution and the corresponding metal
field was used in the next set of simulations and this
iterative process was continued until convergence was
attained.
In both the simulations, the long range electrostatic
interactions were treated by using the three dimensional
Ewald method [34]. The short range Lennard-Jones in-teractions were calculated by using a spherical cut-off at
0 4 8 12 160
0.3
0.6
0.9
1.2
Den
sity
0 2 4 6 8 10 1 2z (Å)
0
2
4
6
8
Den
sity
(a)
(b)
220 S. Paul, A. Chandra / Chemical Physics Letters 386 (2004) 218–224
distance L=2. We employed the quaternion formulation
of the equations of rotational motion and the leap-frog
algorithm for the integration over time with a time step
of 10�15 s (1 fs).
For the vapour–water interfacial system, MD runs of300 ps were used to equilibrate the system in the bulk
phase and then MD runs of 400 ps were used to equil-
ibrate the liquid–vapour interfacial system. During the
equilibration, the temperature of the simulation system
was kept at 298 K through rescaling of the velocities.
The simulations of the vapour–water interface was
then continued in microcanonical ensemble for another
600 ps for the calculation of hydrogen bond properties.For the metal–water system, we equilibrated the system
for 200 ps and then continued the simulation for another
800 ps for calculation of various equilibrium and dy-
namical quantities.
Fig. 1. The number density profiles of water molecules of (a) vapour–
water and (b) metal–water systems. The results shown are for half of
the simulation box along z-direction.
4. Inhomogeneous structure at the interfaces: the distri-bution of number density
The interfaces of both the systems were found to be
stable over the simulation time for all the systems
studied here. In order to characterize the location of the
interfacial regions, we calculated the number density
profiles of water both at vapour–water and metal water
interfaces and the results are shown in Fig. 1. In this
figure, the results are normalized by the correspondingbulk densities. It is seen that the water density at the
surfaces is highly inhomogeneous. For the purpose of
analysis of various interfacial and bulk properties, we
decompose the whole water system into two or three
regions. We define the interfacial region of a liquid–
vapour interface as the distance over which the number
density decreases from 90% to 10% of the bulk liquid
density [20,23]. We call this interfacial region as region Iand the rest of the system on the liquid side as region II.
For the metal–water system, the contact interface or
region Ia consists of molecules in the first layer near the
surfaces, the region Ib or the diffuse interface includes
water molecules in the second layer near the surfaces
and the rest of the water molecules belong to region II or
the bulk-like region.
Table 1
The percentage of water molecules having n number of hydrogen bonds and
regions of vapour–water and metal–water systems
System Region f1 f2
Vapour–water I 2.45 24.25
II 0.28 4.55
Metal–water Ia 0.11 2.6
Ib 0.16 3.05
II 0.24 4.12
5. Hydrogen bond distribution at interfaces
In order to define a hydrogen bond, we have used a
set of geometric criteria where two water molecules
are taken to be hydrogen bonded if their interoxygen
distance is less than 3.5 �A and simultaneously hydrogen–
oxygen distance is less than 2.45 �A and the oxygen–
oxygen–hydrogen angle is less than a cut-off value hc.We note that the critical distances of 3.5 and 2.45 �A are
essentially the positions of the first minimum in the ox-ygen–oxygen and oxygen–hydrogen radial distribution
functions, respectively. The angular criterion reflects the
directional character of hydrogen bonds. We have used a
value of 45� for hc which corresponds to a �less strict�definition of the hydrogen bonds [7]. The quantities of
interest are the percentages fn of water molecules that
engage in n hydrogen bonds and the average number of
hydrogen bonds per water molecule nHB. The values offn (n ¼ 1; . . . ; 5) and nHB are included in Table 1 for the
interfacial and bulk regions of both the systems.
In the bulk phase of water, majority of water mole-
cules participate in four hydrogen bonds whereas, in the
liquid–vapour interfacial region, most of the molecules
the average number of hydrogen bonds per water molecule in different
f3 f4 f5 nHB
49.9 19.18 4.0 2.97
25.6 58.6 10.8 3.74
23.6 58.45 14.6 3.83
21.65 58.45 15.5 3.82
24.8 58.05 12.2 3.76
-3
-2
-1
0
ln S
HB(t
)
-3
-2
-1
0
ln S
HB(t
)
(a)
(b)
S. Paul, A. Chandra / Chemical Physics Letters 386 (2004) 218–224 221
are found to have either three or two hydrogen bonds.
The average number of hydrogen bonds per water
molecule is also significantly smaller at the liquid–
vapour interface than that in the bulk phase. This
smaller number of hydrogen bonds at the vapour–waterinterfaces is likely due to the lower density and the
presence of vapour (essentially vacuum) on one side of
the liquid. In case of metal–water system, however, the
distributions of hydrogen bonds in the interfacial and
bulk regions are found to be not very different from each
other. The interfacial molecules near the metal surfaces
are found to participate in slightly more number of
hydrogen bonds which might be due to higher density ofwater near the metal surfaces.
0 1 2 3Time (ps)
Fig. 2. The time dependence of the continuous hydrogen bond corre-
lation function SHBðtÞ in different regions of (a) vapour–water and (b)
metal–water systems. In (a), the solid, dashed and the dotted-dashed
curves correspond to I–I, I–II and II–II hydrogen bonds and, in (b),
the solid, dashed and dotted-dashed curves correspond to Ia–Ia, Ia–Ib
and II–II hydrogen bonds.
6. Dynamics of hydrogen bonds
The dynamics of hydrogen bonds in the interfacial
and bulk regions are investigated by calculating the
average lifetime, the rate constant of hydrogen bondbreaking and also the structural relaxation time of hy-
drogen bonds in these regions. We have calculated these
dynamical properties for hydrogen bonds between two
molecules of the same region, interface or bulk, and also
for inter-region hydrogen bonds where one molecule of
the bonded pair is in the interface and the other in the
adjacent layer on the bulk side.
The calculations of the above dynamical quantitiesare done by means of different hydrogen bond time
correlation functions. In order to define these functions,
we first define two hydrogen bond population variables
hðtÞ and HðtÞ: hðtÞ is unity when a particular tagged pair
of water molecules is hydrogen bonded at time t, ac-cording to the adopted definition as described in Section
5 and zero otherwise. The function HðtÞ is unity if the
tagged pair of water molecules remains continuouslyhydrogen bonded from t ¼ 0 to time t and it is zero
otherwise. We define the continuous hydrogen bond
time correlation function SHBðtÞ as [9,11–14]SHBðtÞ ¼ hhð0ÞHðtÞi=hhi; ð6Þwhere h� � �i denotes an average over all hydrogen bonds
that are present at t ¼ 0. Clearly, SHBðtÞ describes the
Table 2
The dynamical properties and the energies of hydrogen bonds between wate
System Region sHB
Vapour–water I–I 1.50
I–II 0.80
II–II 1.35
Metal–water Ia–Ia 2.0
Ia–Ib 1.15
II–II 1.3
Region A–B means one molecule of the hydrogen bonded pair is in region
rate constant are expressed in units of ps and the hydrogen bond energies a
probability that a water pair, which was hydrogen
bonded at t ¼ 0, remains continuously bonded up totime t. The time integral of this function describes the
average time that a hydrogen bond survives after it is
chosen at time t ¼ 0. We denote the integral by sHB and
call it the average hydrogen bond lifetime. However,
strictly speaking, the time constant sHB corresponds to
the lifetime of a hydrogen bond if only those hydrogen
bonds are chosen which are created at time t ¼ 0. In the
present study, the hydrogen bonds are chosen randomlywithout keeping any condition on when they were cre-
ated and, therefore, the integral of the present SHBðtÞshould better be called the average persistence time of a
randomly chosen hydrogen bond [7].
In Fig. 2, we have shown the decay of SHBðtÞ for
different regions of the vapour–water and metal–water
systems and the corresponding results of sHB are in-
cluded in Table 2. We note that both fast librational andslower diffusional motion can contribute to the decay of
SHBðtÞ. Since the librational motion occurs on a faster
r pairs
EHB sR 1=k
)19.74 10.5 3.2
)18.65 4.5 1.1
)18.72 7.1 2.4
)17.82 22.0 4.1
)16.94 7.45 2.0
)18.60 7.6 2.4
A and the other one is in region B. The relaxation times and the inverse
re expressed in units of kJ/mol.
-3
-2
-1
0
ln C
HB(t
)
0 3 6 9 1 2Time (ps)
-3
-2
-1
0
ln C
HB(t
)
(a)
(b)
Fig. 3. The time dependence of the intermittent hydrogen bond cor-
relation function CHBðtÞ in different regions of (a) vapour–water and
(b) metal–water systems. The different curves are as in Fig. 2.
222 S. Paul, A. Chandra / Chemical Physics Letters 386 (2004) 218–224
time scale and since the correlation function SHBðtÞ doesnot allow any reformation event, the dynamics of SHBðtÞprimarily reveal the dynamics of hydrogen bond
breaking due to fast librational motion. The relaxation
of SHBðtÞ is found to be slower at both vapour–waterand metal–water interfaces compared to the corre-
sponding relaxation in the bulk phases. An insight into
this different relaxation behaviour of interfacial and
bulk hydrogen bonds can be obtained from the ener-
getics of these hydrogen bonds which are also included
in Table 2. For the metal–water system, the interfacial
hydrogen bonds are found to be of relatively higher
energy than those of the bulk and this would lead to afaster breaking of interfacial hydrogen bonds. But there
is a second effect which is the additional friction that is
exerted by the metal surface on the rotational and
translational motion of water molecules in its vicinity.
For metal–water system, this frictional effect wins over
the hydrogen bond energetic effect and the net result is a
slowing down on the hydrogen bond dynamics in the
vicinity of the metal surface. For vapour–liquid inter-faces, however, there is no such external field that gives
rise to an additional friction on the motion of water
molecules. The hydrogen bond energies are )19.7 and
)18.7 kJ/mol at vapour–liquid interface and in the bulk
phase, respectively. Thus, although, the number of hy-
drogen bonds in the vapour–water interfacial region is
less as reported in Table 1, the hydrogen bonds in this
region are found to be relatively stronger and hence livelonger.
A different way to analyze the hydrogen bond dy-
namics is to calculate the intermittent hydrogen bond
correlation function [6–15,35]
CHBðtÞ ¼ hhð0ÞhðtÞi=hhi; ð7Þ
which describes the probability that a hydrogen bond is
intact at time t, given it was intact at time zero, inde-
pendent of possible breaking in the interim time.
Clearly, bonds which were briefly �broken� by fast li-
brational motions would continue to contribute to thecorrelation function at later times and this leads to a
much slower decay of CHBðtÞ at longer times. The re-
laxation time sR of this function is usually called the
structural relaxation time of hydrogen bonds. In Fig. 3,
we have shown the results of CHB and the values of sR,which are obtained by assuming an exponential decay of
CHBðtÞ as described in [11], are included in Table 2.
Again, the relaxation of hydrogen bonds at interfaces(i.e. regions I and Ia) is found to be slower than that
in the bulk for both vapour–water and metal–water
systems.
We note that after a hydrogen bond is broken, the
two water molecules can remain in the vicinity of each
other for some time before either the bond is reformed
or the molecules diffuse away from each other. We de-
fine NHBðtÞ as the time dependent probability that a
hydrogen bond is broken at time zero but the two
molecules remain in the vicinity of each other i.e. asnearest neighbours but not hydrogen bonded at time t.Following previous work [6–8,35], we write a simple rate
equation for the �reactive flux� �dCHB=dt in terms of
CHBðtÞ and NHBðtÞ
� dCHBðtÞdt
¼ kCHBðtÞ � k0NHBðtÞ; ð8Þ
where k and k0 are the forward and backward rate
constants for hydrogen bond breaking. The inverse of kcan be interpreted as the average lifetime of a hydrogenbond. The probability function NHBðtÞ can be calculated
from the simulation trajectories through the following
correlation function approach [8]
NHBðtÞ ¼ hhð0Þ½1� hðtÞ�h0ðtÞi=hhi; ð9Þwhere h0ðtÞ is unity if the interoxygen distance of the pair
of water molecules is less than 3.5 �A at time t and it is
zero otherwise. The results of NHBðtÞ are shown in Fig. 4.
We calculated the derivative of the intermittent hy-drogen bond correlation of Eq. (7) from the simulation
results of CHBðtÞ that are presented in Fig. 3 and we
used a least-squares fit of Eq. (7) to the simulation re-
sults of the CHBðtÞ, its time derivative and NHBðtÞ to
produce the forward and backward rate constants. The
inverse of the corresponding forward rate constant,
which correspond to the average hydrogen bond life-
time, is included in Table 2. The values of 1=k alsoreveal a slower dynamics of the hydrogen bonds at
both vapour–water and metal–water interfaces. We
note that, for all the regions, the values of 1=k are found
to be somewhat longer than sHB obtained from the
continuous hydrogen bond correlation function SHBðtÞ.This is not unexpected because SHBðtÞ primarily cap-
tures the hydrogen bond �breaking� dynamics due to fast
0
0.05
0.1
0.15N
HB(t
)
0 3 6 9 12Time (ps)
0
0.05
0.1
0.15
NH
B(t
)(a)
(b)
Fig. 4. The time dependence of the probability function NHBðtÞ, definedby Eq. (9), in different regions of (a) vapour–water and (b) metal–water
systems. The different curves are as in Fig. 2.
S. Paul, A. Chandra / Chemical Physics Letters 386 (2004) 218–224 223
librational and rotational motion whereas the quantity
1=k includes contributions from fast librational, rota-tional and also from slower translational diffusional
motion of water molecules. In fact, when k is obtained
only from the short time part of Eq. (8), the value of 1=kdoes appear to be close to sHB obtained from SHBðtÞ. We
also note that the value of 1=k for bulk water as re-
ported in Table 2 agree well with the results of [8]
considering the fact that the fitting of simulation data to
Eq. (8) was done over a longer time (25 ps) in [8] thanthat in the present work (12 ps).
Till now we have discussed the dynamics of hydrogen
bonds where both molecules of a bonded pair either
belong to the interface (regions I or Ia) or to the bulk
(region II). An interesting dynamical behaviour is found
for the dynamics of those hydrogen bonds where one
molecule of the bonded pair belongs to the interface
(I or Ia) and the second one to its adjacent region, i.e.region II for vapour–water or region Ib for metal–water
systems. Both the lifetime and the structural relaxation
time of these inter-region I–II or Ia–Ib hydrogen bonds
are found to be faster than even those in the bulk phase.
It is seen from Table 1 that the hydrogen bond envi-
ronment of different regions are different to some extent.
The energetics data of Table 2 show that the energies of
these inter-region or inter-environment hydrogen bondsare higher than the corresponding intra-region hydrogen
bonds for the bulk or interfacial zones for both the va-
pour–water and metal–water systems. Thus, the hydro-
gen bonds that connect water molecules of two different
regions or environments are found to be relatively
weaker and hence relax at a faster rate than those be-
longing to a single region, either interface or the bulk.
We note that, for the vapour–water system, the some-what lower energy of the interfacial (I–I) hydrogen
bonds as compared to that of the bulk phase was also
found in an earlier study of hydrogen bonding at va-
pour–water interfaces [36].
7. Conclusion
In this Letter, we have investigated the dynamics of
water–water hydrogen bonds at vapour–water and me-
tal–water interfaces by means of molecular dynamics
simulations. The inhomogeneous density and also thedistribution of hydrogen bonds in interfacial regions are
also calculated which help to understand the structural
aspects of these interfaces and their effects on the dy-
namical properties. For both interfaces, the relaxation
of hydrogen bonds is found to occur at a slower rate
than that in bulk water. However, the dynamics of those
hydrogen bonds which connect interfacial molecules to
its adjacent layer on the bulk side is found to be fasterthan even the bulk hydrogen bonds. The results are
explained in terms of the energies of hydrogen bonds
and also the frictions that act on the rotational and
translational motion of water molecules in different re-
gions of the vapour–water and metal–water systems.
Acknowledgements
We gratefully acknowledge the financial support from
BRNS, Department of Atomic Energy and Council of
Scientific and Industrial Research, Government of In-
dia.
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