Jumping Diffusion of Water Intercalated in Layered Double Hydroxides

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) © XXXX 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

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DOI: 10.1021/acs.jpcc.6b04001 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

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 (R3̅m) unit cells45 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. B


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The Journal of Physical Chemistry C 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. C


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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 D


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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

E

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 57


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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

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ASSOCIATED CONTENT

S Supporting Information *

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.

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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.

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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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REFERENCES

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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 scale. However, in a long time scale, they exhibit linear diffusion 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 exchanging HB acceptors while fixed in OH sites. A successful jump of a water molecule from one OH site to another takes place only if four successful HB exchanges happen in a row on corresponding OH groups. So, a jump is rare, leading to the slow diffusion of intercalated water. The diffusion coefficient derived by the jump model is of the same order as that derived by fitting the mean-square displacements. It evidences F


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Jumping Diffusion of Water Intercalated in Layered Double Hydroxides

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