Article
pubs.acs.org/JPCC
Jumping Diffusion of Water Intercalated in Layered Double
Hydroxides
Meng Chen,†,‡ Wei Shen,§ Xiancai Lu,‡ Runliang Zhu,† Hongping He,† and Jianxi Zhu*,†
†
CAS Key Laboratory of Mineralogy and Metallogeny/Guangdong Provincial Key Laboratory of Mineral Physics and Materials,
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences (CAS), Guangzhou 510640, China
‡
State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093,
China
§
University of Chinese Academy of Sciences, Beijing 100049, China
S Supporting Information
*
ABSTRACT: Our molecular dynamics simulation study shows
water in the nanoconfined monolayer in Cl−-Mg2Al-layered double
hydroxides (Mg2Al(OH)6Cl·mH2O) diffuses in a similar way as
atoms in solid lattice. A water molecule is mostly fixed in a hydroxyl
group site, as an acceptor of hydrogen bonds donated by the upper
and lower hydroxyl groups simultaneously. Because of exchange of
acceptors, it loses hydrogen bonds from the two hydroxyl groups
and accepts hydrogen bonds from another two groups in an
adjacent site. Thus, a water molecule jumps from one site to another, which is rapid but rare. On average it takes ∼104 ps for a
jump to happen on a water molecule. The diffusion coefficient derived by the jump model is of the same order (∼10−9 cm2/s) as
that obtained by fitting the mean-square displacement, revealing water diffusion in the confined monolayer is largely contributed
by a series of jump events.
1. INTRODUCTION
Nanoconfined water appears in natural environment and
synthesized materials, e.g., in clay minerals,1,2 zeolite,3 carbon
nanotubes,4,5 reverse micelles,6 and so on. It exhibits structure
and dynamics behaviors distinctly different from bulk
water. 7−11 Whether water is confined between solid
phases3,12−15 or amphiphilic monolayers,16−19 it generally
exhibits slow dynamics and hydrogen bond (HB) exchange
rates. Liquid water confined between some solid surfaces (e.g.,
hydrophobic silica20−22 or hydrophilic mica23 surfaces) transits
into ice as the confining scale is appropriate. As physical and
chemical processes in nanoconfinement are closely correlated
to the coordination structure and hydrogen bonds rearrangement of water, understanding the behaviors of nanoconfined
water is important.
Layered double hydroxides (LDH), also known as hydrotalcite-like compounds, are a family of layered materials with
water and anions confined in nanoscale interlayers. The general
formula of LDH is [M2+(1−x)Me3+x(OH)2]x+(An‑)x/n·mH2O,
where M2+ can be Zn2+, Mg2+, Co2+, Ni2+, Ca2+, or Cu2+, Me3+
can be Cr3+, Ga3+, Fe3+, or Al3+, and An− are intercalated anions.
LDH can be synthesized with a variety of inorganic (Cl−,
SO42−, CO32−, NO3−, and so on) and organic anions (−COO−,
−PO42−, −SO4−, and so on).24 They are widely used as
catalysts, catalyst supports, adsorbing agents, electrode
modifiers, and so on.25−28 Many physical−chemical processes
in interlayers, e.g., anion exchange,29 protonic conduction,30
hydration, and dehydration,31 are correlated with structure and
dynamics of intercalated water. Molecular dynamics (MD)
© 2016 American Chemical Society
simulations showed that the water molecule in the interlayer
often locates between two hydroxyl (OH) groups from the
upper and lower layers (we call this location as an OH site
later), accepting two HBs from these OH groups and donating
two HBs to adjacent water molecules or anions, exhibiting a
tetrahedral structure very similar to that in ice Ih.32 Number of
HBs formed by per intercalated water molecule is 3.8, higher
than that in water (3.2) and close to that in ice Ih (4).33 Ab
initio34 and Raman spectroscopy studies35 showed the
stretching vibration of OH bonds of intercalated water is
close to that in ice Ih. So, it can be concluded that the structure
of intercalated water is close to that of ice Ih. As we know, the
difference between diffusion ways of atoms in liquid and solid
lattice is that atoms exhibit continuous motions in liquid while
they jump between lattice sites in solid. As water in LDH shares
some similarity with ice and generally locates in OH sites, does
it diffuse like atoms in solid? Marcelin et al. viewed the twodimensional translational motion of an intercalated water
molecule as a jumping process; i.e., it jumps from one OH site
to an adjacent one while losing and reaccepting HBs.36
Through quasi-elastic neutron scattering measurements, Mitra
et al. found the diffusion of intercalated water is best described
by the jump model.37 However, there was no direct observation
of jump to our knowledge.
Received: April 20, 2016
Revised: May 25, 2016
Published: May 26, 2016
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Table 1. Equilibrated 9 × 9 × 1 Supercells Parameters
Laage and Hynes developed the Ivanov jump model38 to
quantitatively describe HB exchanges in water.39,40 With their
model, reorientational dynamics of water in nanoconfinement3,13,14 and next to solid surfaces,41,42 which are largely
influenced by water exchanging HB acceptors, are well
described. In this study, inspired by work of Laage et al., via
molecular dynamics simulations we show OH groups of
hydroxide layers exchange HB acceptors similarly as water.
Furthermore, water jumping between adjacent OH sites
through OH groups exchanging acceptors is disclosed. The
diffusion coefficients derived by the jump model and by fitting
the mean-square displacements are compared in this article.
Disclosing the diffusive way of water sheds light on the kinetic
processes like protonic conduction, hydration, and dehydration
in the interlayer.
2. SIMULATION DETAILS AND HYDROGEN BOND
DEFINITIONS
2.1. Simulation Details. LDH we studied are of the
formula Mg2Al(OH)6Cl·mH2O (Figure 1). Nuclear magnetic
m
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
0.67
0.78
0.89
1.00
1.11
1.22
1.33
1.44
1.56
1.67
1.78
1.89
2.00
29.0
29.0
29.0
29.0
29.1
29.1
29.0
29.1
29.1
29.1
28.9
29.0
28.9
29.0
29.0
29.0
29.0
29.0
29.0
29.1
29.1
29.1
29.1
29.0
29.0
28.9
22.8
23.0
23.0
23.1
23.3
23.6
23.8
23.8
24.0
24.3
25.4
25.8
26.4
90.3
91.2
90.1
90.5
90.4
86.1
86.1
90.2
89.3
89.5
93.2
93.0
88.1
89.8
89.5
90.4
89.4
90.2
97.6
86.1
89.8
90.2
90.2
83.4
84.5
85.7
119.9
120.0
120.0
120.0
120.1
120.0
120.0
120.2
120.0
119.9
119.8
120.2
120.0
ensemble simulation for 2 ns was performed, and data were
saved every 0.1 ps.
2.2. Hydrogen Bond Definitions. In LDH, OH groups of
layers and water molecules can act as HB donors, while O
atoms of water and Cl− ions are HB acceptors. The HB
between an OH group and a water molecule (OH−H2O) or
between two water molecules (H2O−H2O) is defined
according to the widely accepted criterion: The donor−
acceptor distance, the hydrogen−acceptor distance, and the
hydrogen−donor−acceptor angle are less than 3.5 Å, 2.45 Å,
and 30°, respectively.39,40 On the other hand, the HB between
an OH group and a Cl− ion (OH−Cl) or between a water
molecule and a Cl− ion (H2O−Cl) is defined according to the
criterion: The donor−acceptor distance, the hydrogen−acceptor distance, and the hydrogen−donor−acceptor angle are less
than 3.7 Å, 2.75 Å, and 30°, respectively. Under these criterions,
the distributions of geometrical parameters exhibit clear sharp
peaks except the hydrogen−donor−acceptor angle distribution
of the OH−Cl HB (Figure 2). As Cl− ions locate in the gap
among OH groups of layers32 and probably accept 4−6 HBs
simutaneously from those groups (Figure S2), the hydrogen−
donor−acceptor angle must be larger. As a result, the
distribution is wider.
As to characterize a jumping process, we also need to define a
stable HB. A stable HB is defined according to a stricter
Figure 1. Side view (a) and top view (b) of the LDH supercell.
resonance (NMR) studies showed cations (Mg, Al) generally
exhibit an ordered arrangement while the Mg/Al ratio is 2.43,44
Therefore, we built supercells consisting of rhombohedral
45
(R3m
̅ ) unit cells with complete Mg/Al ordering (Figure 1).
Through comparing simulated structures of supercells consisting of 6 × 6 × 1, 9 × 9 × 1, and 12 × 12 × 1 unit cells in the a,
b, and c directions (section 1, Supporting Information), we
found that systems with 9 × 9 × 1 units cells are adequate to
avoid the system size effect on simulation results. Thus, 13
systems with 9 × 9 × 1 unit cells were built to do subsequent
simulations. In these systems, the number of water molecules
ranges from 18 to 54 per interlayer; i.e., m ranges from 0.67 to
2.00. Periodic boundary conditions were applied in all
directions. The ClayFF force field46 with SPC water model47
incorporated was used to describe atomic interactions. The
partial charges for O atoms in the hydroxide layers were
modified according to Wang et al.48 The cutoff radius for
Lennard-Jones potential was 10.0 Å. The PPPM method49 was
used to describe long-range electrostatic interactions.
LAMMPS50 was used to do simulations. The time step was
0.5 fs. The systems were equilibrated in isothermal−isobaric
ensemble (300 K, 1 atm) for 20 ns with the Nosé−Hoover
thermostat51,52 and the Parrinello−Rahman barostat.53,54 Each
dimension was scaled independently to achieve target stress.
The equilibrated system size parameters can be seen in Table 1.
After equilibration, the fluctuations of system size parameters
are less than 1%. In favor of calculating dynamics properties,
production runs were performed for another 20 ns in canonical
ensemble (300 K) without pressure coupling. Data were saved
every 1 ps. As to show the vibrational motion of water in
Mg2Al(OH)6Cl·1.0H2O in a short time scale, another canonical
Figure 2. Normalized donor−acceptor distance (a), hydrogen−
acceptor distance (b), and hydrogen−donor−acceptor angle distributions (c) of HBs between water (H2O−H2O), between water and Cl−
ions (H2O−Cl), between OH groups and water (OH−H2O), and
between OH groups and Cl− ions (OH−Cl) in Mg2Al(OH)6Cl·
1.0H2O. The intervals between data points of these distributions are
0.74 Å, 0.55 Å, and 0.6°, respectively.
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geometric criterion.40 The values of donor−acceptor distance,
hydrogen−acceptor distance, and hydrogen−donor−acceptor
angle corresponding to the peaks of the distributions of these
geometrical parameters are used to define a stable HB. As a
result, a stable HB donated by an OH group to a water
molecule or by a water molecule to another is defined as the
donor−acceptor distance, the hydrogen−acceptor distance, and
the hydrogen−donor−acceptor angle are less than 2.8 Å, 1.8 Å,
and 10°, respectively. On the other hand, a stable HB donated
by a water molecule or an OH group to a Cl− ion is defined as
the donor−acceptor distance, the hydrogen−acceptor distance,
and the hydrogen−donor−acceptor angle are less than 3.2 Å,
2.2 Å, and 10°, respectively.
Figure 4. Distribution of water O atoms in the interlayer. The relative
position of a water O atom is calculated according to its relative
distances to the nearest O atoms in the upper and lower hydroxide
layers, respectively.
3. RESULTS AND DISCUSSION
3.1. Diffusion of Intercalated Water. The diffusion of
water in the interlayer is characterized by mean-square
displacement (MSD) in the xy-plane (MSDxy(t)) (inset in
someplace of the interlayer. The sharp increase of D as m
increases over 1.67 is due to the appearance of water bilayer. Xray diffraction studies showed the c-axis length of Mg2Al(OH)6Cl·mH2O is less than 24 Å under all humidity
conditions,58 corresponding to the situations (m < 1.67) with
only water monolayer (Table 1). Therefore, our subsequent
study is focused on the water monolayer which exhibits
extremely slow dynamics.
Mg2Al(OH)6Cl·1.0H2O is taken as a representative to show
the structure and dynamics of water monolayer (unless
specified, the subsequent analyses are on LDH with m =
1.0). The locations of most water molecules on the xy plane
coincide with those of OH groups (Figure 5a). These water
molecules fixed in OH sites accept HBs from both upper and
lower layers (Figure 5b). They account for 76% of all water
molecules. The rest of water molecules locate in the gap
between OH sites, and they just accept HBs from one layer
(Figure 5c). The short time scale MSDxy(t) shows that water
molecules exhibit ballistic motions at first, and then they diffuse
Figure 3. Diffusion coefficients of intercalated water. Inset shows
MSDxy(t) of water.
Figure 3). MSDxy(t) is determined by diffusion coefficient D
according to the Einstein relation:
⟨(x(τ + t ) − x(τ ))2 + (y(τ + t ) − y(τ ))2 ⟩ = 4Dt
(1)
Figure 3 shows D of intercalated water is of the order 10−7
cm2/s or less, of similar magnitude as that in solid (generally
less than 10−5 cm2/s55). D with water content m less than 1.67
is of the order 10−9 or 10−10 cm2/s, surprisingly close to that in
ice Ih (10−10 cm2/s56). D does not vary obviously as water
content m increases, except when m > 1.67. The equilibrated
snapshot of Mg2Al(OH)6Cl·1.0H2O shows intercalated water
forms one monolayer, but that of Mg2Al(OH)6Cl·2.0H2O
exhibits a coexistence of monolayer and bilayer. As to clearly
show the layered structure of water, we derive the density
profiles (section 3, Supporting Information). However, due to
undulations of LDH layers,57 density profiles cannot clearly
show the existences of water monolayer or bilayer. Instead of
density profiles, we calculate the probability distribution of a
water molecule between the nearest OH group from the upper
layer and that from the lower layer (Figure 4). The
distributions of water show that as m ≤ 1.67 there is only
one sharp peak corresponding to a water monolayer. However,
as m > 1.67 another two sharp peaks appear in the shoulders of
the middle peak, reflecting water bilayer appearing in
Figure 5. (a) Atomic density contour map for an interlayer of
Mg2Al(OH)6Cl·1.0H2O. (b) A water molecule which accept HBs from
both upper and lower layers. (c) A water molecule which just accept
HBs from the lower layer. The symbols are the same as in Figure 1.
The yellow bond represents a HB.
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Figure 6. (a) MSDxy(t) (circle point) of intercalated water in Mg2Al(OH)6Cl·1.0H2O, fitted with a biexponential function (red line). Inset shows
MSDxy(t) in a large time scale. (b) Comparison between MSDxy(t) of H atoms from hydroxide layers and the fitting residue of MSDxy(t) of water O
atoms.
HB exchange, the abrupt increase of Ra and decrease of Rb
(Figure 7b) show the rapid jump of an OH group from one
acceptor to another; i.e., an old HB breaks, and a new HB
forms almost simutaneously. The time origin in Figure 7b
follows the same definition as Laage et al.,40 which is the time
when the OH group lies in the bisector plane between two
acceptors. On the other hand, the abrupt decrease of number of
HBs accepted by the original acceptor (naHB) and increase of
number of HBs accepted by the new acceptor (nbHB) (Figure
7b) also verify the jumping process. In most cases, a water
molecule accepts two HBs (Figure S2). Before a HB exchange,
water molecule a which is the current acceptor, averagely
accepts more than two HBs (naHB > 2). At the same time, water
molecule b, which is the future acceptor, averagely accepts less
than two HBs (nbHB < 2). A HB acceptor is prone to be
exchanged from an overcoordinated molecule to an undercoordinated one, consistent with the situation in liquid water.39
With respect to the origin of time (in the middle of a HB
exchange), the variations of Ra and naHB with time are symmetric
to those of Rb and nbHB, respectively. More precisely, the
coordination environment of an OH group after an HB
exchange is the same as that before the exchange. So, it is
possible that the OH group leaves the new HB acceptor and
returns to be bonded to the original one; i.e., the HB exchange
reaction is reversible. It should be noted that the HB exchange
we show is an average process. Sometimes as a HB is broken,
the OH group does not donate a HB to another acceptor
simultaneously. The donor may remain dangling for a while
until it meets an acceptor, which has been found in our
previous study.59 This is an irreversible HB exchange way.
Besides accepting HBs from OH groups, a water molecule
also donates HBs to adjacent Cl− ions or water molecules
(Figure 5b,c). It exchanges acceptors in three pathways: from a
Cl− ion to a water molecule (Cl−H2O), from a water molecule
to a Cl− ion (H2O−Cl), and from a water molecule to another
(H2O−H2O). During the HB exchange, the distance between
the donor and the original acceptor abruptly increases, while
the distance between the donor and the new acceptor abruptly
decreases (Figure 8). So, as OH groups of layers exchanging
acceptors, water molecules exchanging acceptors is also a rapid
process. Laage et al. sees a HB exchange as a chemical reaction,
which is a transition from a stable reactant (R) state to a stable
product (P) state.40 When a water molecule or an OH group is
donating a stable HB to an acceptor, it is in an R state. After a
HB exchange, the water molecule or the OH group turns to be
donating a stable HB to a new acceptor, it is in a P state. With
the stable state picture approach,60,61 the HB exchange process
is described by the cross-correlation function
with vibrational motions (Figure 6a). As to clearly show the
vibration of water, we use a biexponential function (A exp(t/τ1)
+ B exp(t/τ2) + C) to fit the short time scale trend of MSDxy(t).
The fitting residue clearly shows the vibrational motion, which
exhibits similar frequency as that of H atoms from hydroxide
layers (Figure 6b). As water embedded in lattice sites of LDH
exhibits structure close to ice Ih,32−35 its motion is strongly
connected to the solid layer.
However, water molecules diffuse almost linearly in a long
time scale (inset in Figure 6a). As most water molecules locate
in OH sites, we suppose the diffusion mainly consists of a series
of jumping processes from one OH site to another. In a
jumping process, HBs must be broken and reborn.
3.2. Hydrogen Bond Exchanges in Interlayers. Laage
and Hynes show a HB exchange in liquid water is a process in
which a water OH bond jumps from one HB acceptor to
another.39,40 In LDH, OH groups of hydroxide layers can act as
HB donors. Thus, a HB exchange appears as an OH group in
the layer jumps from one acceptor (water O atom or Cl− ion)
to another (Figure 7a). There are three HB exchange pathways
Figure 7. (a) Schematic of a HB exchange (H2O−Cl). The symbols
are the same as in Figure 5. (b) Variations of Ra, Rb, naHB, and nbHB
during HB exchanges.
concerning water molecules, i.e., an OH group jumps from a
Cl− ion to a water O atom (Cl−H2O), from a water O atom to
a Cl− ion (H2O−Cl), and from one water O atom to another
(H2O−H2O). The trajectories of three pathways of HB
exchanges are analyzed. Ra is the distance between the HB
donor and the original acceptor (atom a), and Rb is the distance
between the donor and the new acceptor (atom b). During the
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Figure 8. Variations of Ra and Rb during HB exchanges while water
molecules are HB donors. Ra is the distance between the donor and
the original acceptor, and Rb is the distance between the donor and the
new acceptor.
C RP(t ) = ⟨nR (0)nP(t )⟩
(2)
where nR(0) is 1 if the donor is in an R state at time 0 and nP(t)
is 1 if the donor is in a P state at time t; otherwise, their values
are 0. The adsorbing boundary condition is used while deriving
CRP(t). 1 − CRP(t) for HBs donated by water decays
dramatically faster than that for HBs donated by OH groups
(Figure 9), implying water molecules exchange HBs much
Figure 10. (a) Schematic of a jump. The symbols are the same as in
Figure 5. (b) Variations of RIO−O and RIIO−O during a jump. (c)
Projections of trajectories of a water O atom and H atoms of adjacent
layers. (d) 1 − CRP(t) fitted with a monoexponential function.
happen successively on the two sites. In this process, the
average distance between the water O atom and the original
donor in an OH site (RIO−O) abruptly increases, and that
between it and the new donor (RIIO−O) abruptly decreases
(Figure 10b). It is a rapid process, so that we call it as a jump. A
jump trajectory shows the water molecule hardly stays in the
gap between the two sites (Figure 10c). A jump can also be
seen as a chemical reaction like a HB exchange. In the case of a
jump, a water molecule accepting two stable HBs in an OH site
is in an R state. As it jumps to be accepting two stable HBs in
another site, it is in a P state. Equation 2 can also be used to
characterize jumping processes. 1 − CRP(t) decays monoexponentially (Figure 10d), implying a single jump time is
adequate to describe all the jumps at least during the simulation
time. We fit 1 − CRP(t) with exp(−t/τ), deriving jump time τ
(the average time for a jump to happen on a water molecule) to
be 6.2 × 104 ps. So a jump event is rare. It makes sense as a
successful jump takes place only if four successful HB
exchanges happen in a row.
As m increases over 1.11, 1 − CRP(t) deviates from a
monoexponential decay (section 4, Supporting Information).
As in these situations less water locates in OH sites (Table 2),
the transition of a water molecule from one OH site to another
does not limit to a rapid jump but also includes the situation
with a short time stay in the gap between the two sites (Figure
5c). As a result, a single jump time is not adequate to describe
Figure 9. Time correlation functions 1 − CRP(t) for HB exchanges
when OH groups and water molecules act as donors.
more frequently than OH groups. So, before a water molecule
leaves an OH group due to the OH group exchanging HB
acceptors, the water molecule has exchanged acceptors
frequently. Thus, the water molecule exhibits libration motion
around the axis perpendicular to layers, consistent with
previous observation.62
3.3. Relationship between Water Jumping and
Diffusion. A water molecule in an OH site (Figure 5b) may
lose a HB due to the OH group of that site exchanging HB
acceptors. As a result, the water molecule becomes undercoordinated. It may return accepting a HB from the original
OH site or accept a HB from an adjacent site. On the other
hand, if a water molecule originally in an OH site occasionally
accepts a HB from an adjacent site due to an OH group on that
site exchanging HBs, it becomes overcoordinated. It may lose
either HB from the new donor or the original one. If a water
molecule loses two HBs donated from an OH site and accepts
two HBs from an adjacent site, it transits from one site to
another (Figure 10a). During the transition, four HB exchanges
Table 2. Comparisons of Diffusion Coefficients Derived by
Fitting MSDxy(t) and with the Jump Model
12928
m
D (fitting MSDxy)
(10−9 cm2/s)
D (jump model)
(10−9 cm2/s)
% of water in
OH sites
0.67
0.78
0.89
1.00
1.11
1.22
10.1
11.1
10.4
7.2
2.4
9.6
9.6
11.3
8.6
4.2
10.8
12.4
85
89
79
76
63
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The Journal of Physical Chemistry C
intercalated water molecules diffuse in a jumping way, similar
to atoms in solid lattice. It is interesting to see if a jump is an
activated process, which can be revealed by disclosing the
relationship between jump time and temperature. This topic is
being studied in our ongoing project.
the heterogeneous translational motions. The transition of a
water molecule from the gap to an adjacent OH site is much
faster than that from one OH site to another (Figure S5), as it
is less stable for a water molecule locating in the gap.
If water diffusion consists of a series of jump events from one
OH site to another, the diffusion coefficient D can be derived
by τ:55
D=
d2
4τ
■
ASSOCIATED CONTENT
* Supporting Information
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.jpcc.6b04001.
Analyses of system size effect, hydrogen bond number
distributions, and density profiles (PDF)
(3)
where d is the average distance between OH sites (3.22 Å). D
derived by eq 3 are shown in Table 2, basically of the same
order as that derived by fitting MSDxy(t) with eq 1 (Figure 3).
In the situation with m = 0.78, D derived by the jump model is
closest to that derived by fitting MSDxy(t) (with error of ca.
2%). As water molecules occupying OH sites exhibit the highest
probability (89%) in that situation, the jump model well
describes diffusive motion of water between OH sites. In
situations with m = 0.89 and m = 1.00, D derived by the jump
model are smaller than those derived by fitting MSDxy(t). In
these situations, the probability of water molecules occupying
OH sites decreases to less than 80%. Some water molecule in
an OH site just loses one HB and accepts a new one from an
adjacent site during HB exchanges. It ends up locating in the
gap between OH sites (Figure 5c) in the simulation. This
scenario just happens occasionally as most water locates in OH
sites. But as it has not been considered in eq 3, it may lead to
the underestimate of D. In situations with m = 1.11 and m =
1.22, D derived by the jump model is larger than that derived
by fitting MSDxy(t). Because a single jump time is not adequate
to describe all the translational motions of water in these
situations, τ derived by a monoexponential fitting is an
underestimated value of the real jump time. As a result, D is
overestimated with eq 3. In addition, it should be reminded the
limit of simulation time leads to uncertainty in the calculation
of D, as jump events are so rare. Nevertheless, as D derived by
the two methods are of the same order, it clearly shows water
diffusion in LDH is largely contributed by water jumping
between OH sites.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail [email protected]; Tel +86-20-85290181; Fax +86-2085290181 (J.Z.).
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was financially supported by the National Natural
Science Foundation of China (41322014, 41572031,
41425009), Guangdong Provincial Youth Top-notch Talent
Support Program (2014TQ01Z249), and National Youth Topnotch Talent Support Program, CAS/SAFEA International
Partnership Program for Creative Research Teams
(20140491534). This is contribution (IS-2250) from GIGCAS.
■
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4. CONCLUSIONS
This study shows the relationship between OH groups
exchanging HB acceptors, water jumping, and diffusion in
LDH. Simulated LDH with intercalated water monolayers
exhibit c-axis lengths corresponding to X-ray diffraction study
results. Water molecules in the monolayer are mostly fixed in
OH sites, accepting strong HBs from the two OH groups from
the upper and lower layers, respectively. As a result, they exhibit
similar vibrational motions as atoms in the layers in a short time
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with time. The diffusion mainly consists of a series of jumping
processes from one OH site to another. A jump is induced by
OH groups of layers exchanging HBs. OH groups exchange
HBs much less frequently than intercalated water molecules.
Therefore, water molecules exhibit libration motions due to
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jump of a water molecule from one OH site to another takes
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slow diffusion of intercalated water. The diffusion coefficient
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by fitting the mean-square displacements. It evidences
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