Molecular dynamics study on the structure and dynamics of NaCl

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Molecular Dynamics Study on the Structure and Dynamics of NaCl Solution Transport in the Nanometer Channel of CASH Gel Dongshuai Hou,* Tao Li, and Pan Wang

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Department of Civil Engineering, Qingdao University of Technology, 11 Fushun Road, Qingdao 366033, China ABSTRACT: The transport of water molecules and ions in the nanopores of calcium aluminosilicate hydrate (CASH) influences the durability and sustainability of environmentally friendly cement-based materials with industrial waste substitution. In this study, molecular dynamics was utilized to study aqueous NaCl solution capillary transport through the calcium silicate hydrate (CSH) and calcium aluminosilicate hydrate (CASH) gel pore with pore size of 3.2 nm. The invading depth for the NaCl solution advancing frontier with meniscus shape follows a parabolic relation as a function of time, consistent with the classic Lucas−Washburn equation in capillary adsorption theory. As compared with the solution transport in the CSH pore, both water molecules and ions migrate more slowly in the gel pore of CASH, and sodium ions accumulate in the entrance region of the gel pore. The incorporation of Al atoms in the silicate substrate resists the ingress of ions and water. The Al−Si substitution on the CASH interface enhances the charge negativity of solid oxygen atoms, which polarizes the dipole moment of surface water molecules to a larger extent, strengthens the interfacial hydrogen bond, and elongates the residence time of water near the aluminate substrate. In addition, the silicate− aluminate chains in the CASH substrate provide plenty of oxygen sites to associate with the sodium ions by forming a stable Na−OS bond, immobilizing the cations deeply in the vacancy region of the aluminate−silicate channel. The inner sphere adsorption of Na ions on the CASH surface further contributes to the secondary outer sphere adsorption of the Cl ions by forming the Na−Cl ionic pairs. Hopefully, the transport and adsorption mechanism of the ions and water in the CASH gel can help guide the cementitious material substituted by Al-rich industry waste with sustainability and durability. KEYWORDS: Al incorporation, CASH gel, Molecular dynamics, Capillary adsorption, Ion ingress

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“dreierketten” silicate chains grafted on the both sides of the sheet. The crystal structure and polymerization degree of the CASH have been studied by many experimental techniques such as X-ray diffraction (XRD), nuclear magnetic resonance (NMR), and thermogravimetric analysis (TGA).9−12 It is found that the addition of aluminum species significantly increases the degree of polymerization of silicate chains in CSH gels.10,11 The ab initio study also confirmed that the aluminum is energetically more favored in the bridging site compared to in the pairing site of the silicate chains.13,14 In the long term, avoiding the deterioration of the concrete structure is also an applicable way to reduce the CO2 emission.15 CSH gel is a mesoporous material that contains pores of diameters ranging from 0.5 to 10 nm scale,16,17 which can provide a channel for the transport and adsorption of aggressive ions. This significantly influences the durability and sustainability of the cement paste. In the nanometer pores of CSH, the capillary adsorption of NaCl solution frequently happens because of the humidity changes and external loading.18,19 The transport of water and ions in the gel pore of CSH influences the physical

INTRODUCTION Worldwide, cement production accounts for approximate 6−8% of yearly man-made CO2 emissions,1 which is closely relevant to global warming. To reduce the carbon footprint of the construction industry, people have developed alternative binders to partially replace the cement clinker in concrete preparation.2 Industrial wastes [e.g., pulverized fly ash (PFA) and granulated ground blast furnace slag (GBFS)] and calcined clay [e.g., metakaolin (MK)] have been widely used as supplementary cementitious materials (SCMs). While Portland cement (PC) manufacture produces carbon dioxide of about 0.85 tonne/ tonne (t/t),3 that of PFA, GBFS, and MK only results in ∼0.009, ∼0.02, and 0.175 t/t of CO2 emission, respectively.4,5 Hence, the replacement of cement clinker with these materials shows great environmental advantages. On the other hand, the substitutes of SCMs, often containing aluminum-rich minerals,6 also change the cement chemistry. With respect to the calcium silicate hydrate (CSH) gel, as the main hydration product of the cement paste, it occupies 60− 70% volume fraction. Some aluminum is incorporated into the CSH gel, leading to the transformation from CSH gel to calcium aluminosilicate hydrate (CASH) gel. Studies have shown that the CSH gel has a layered tobermorite-like structure.7,8 The principal layer is composed of a calcium sheet with © XXXX American Chemical Society

Received: May 9, 2018 Revised: June 1, 2018 Published: June 5, 2018 A

DOI: 10.1021/acssuschemeng.8b02126 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering and chemical properties of the cement-based material. A lot of experimental and simulating work has been devoted to the investigation of water and ion transport in the CSH gel nanopores. Experimental techniques, like nuclear magnetic resonance (NMR)20 and quasielastic neutron scattering (QENS),21 have been performed to study the water confined in the gel pore of CSH, and categorize water molecules according to their different mobilities. Utilizing the molecular dynamics (MD) simulation, Youssef22 revealed the glassy nature of water molecules that were confined in the nanopores of CSH gel. Qomi et al.23 explored the dual nature of aluminum species in CSH gels through atomic simulations. They found that the aluminate species are Al−O octahedrons in the interlayer region of CSH gel, and are tetrahedron structures in the silicate−aluminate chains. Hou et al. further simulated the CASH gels with different aluminum contents through the reaction force field and found that the addition of aluminum, healing the weak interlayer region and defective silicate chains, enhanced the mechanical properties of CASH gels.24 Additionally, the transport of water and ions in mineral analogues of CSH gel and CSH of different Ca/Si ratios were studied.16,25−29 The results have shown that, different from diffusion of bulk water and ions, the transport of water and ions in the nanopores is greatly influenced by the CSH substrate. Zhou et al.29 studied the adsorption of water and ions by CSH gels with different Ca/Si ratios (C/S = 0.66, 1, 1.5) and found that the surface calcium to silicon ratio greatly affected the adsorption of water and ions. The charge-balancing Ca2+ ions, hydroxyl groups, and nonbridging oxygen atoms embedded in the CSH surface can form hydrogen bonds or ionic bonds with the water molecules and ions, and immobilize them. With an increasingly common presence of CASH gel in the cement blend, aluminosilicate chains more frequently participate in the interaction with restricted water molecules and ions. However, the influence of Al−Si substitution on the transport and adsorption of water molecules and ions in the nanopores remains an enigma. In this study, molecular dynamics is utilized to model the capillary transport of NaCl solution in the nanometer pore of CASH gel. On the basis of the model, the fluid transport rate in different confinements is first studied by analyzing the timedependent penetration depth and solution concentration distribution. Subsequently, the influence on the capillary transport process from the aluminate−silicate substrate is further studied by the interfacial local structure and dynamics properties of the solution species. Upon systematic investigation of the atomic intensity distribution, radial distribution function (RDF), and mean square displacement (MSD), the transport behavior of water and ions and the corresponding ion adsorption characteristics in different channels are revealed.

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Figure 1. (a) Constructed CSH/CASH system. (b) Schematic representation of the pore channel for CASH, in which different colored spheres represent different atoms or ions. 22.3 Å × 64 Å × 100 Å, and 4886 water molecules were inhaled by Monte Carlo method to reach a saturated solution (1.0 g/cm3).27,33−35 The ion concentration is also 1.0 mol/L. The relatively large ion concentration is to avoid the random effects of individual ions and make the results have more statistical significance. The CSH gel pores consisting of two parallel CSH substrates with thicknesses of 16.2 and 17 Å were placed perpendicular to the NaCl solution surface to study the unsaturated transport of the solution, and the pore width was 32 Å. The silicon chain extends in the y direction and has a length of 89 Å. Periodic boundaries are set in the x, y, and z directions to eliminate unnecessary errors caused by boundary effects.25,26,36 Two vacuum layers of at least 30 Å thickness were placed at the bottom of the NaCl solution in the y direction and above the CSH gel hole, respectively, to prevent water or ions from flying out. The resulting nonsaturated CSH model parameters are a = 22.3 Å, b = 289 Å, c = 64 Å, α = 90°, β = 90°, and γ = 90°. The unsaturated CASH model was obtained by replacing the silicon on the surface of the CSH silicon chain with aluminum, in which the small charge loss caused by the substitution of atoms was balanced by the surface calcium ions. Force Field and Molecular Dynamics Procedure. The ClayFF force field was used in this work, and this force field has been proven to be accurate to describe the interatomic potentials of Ca, Si, Al, O, H, Na, and Cl atoms in NaCl solution and the CSH (CASH)

SIMULATION METHOD

Model Construction. In this work, all of the model construction and molecular dynamics (MD) simulations were conducted using LAMMPS series software.30 On the basis of the ideal hydrated calcium silicate model, tobermorite, with low calcium to silicon ratio, a partial chain scission, and protonation were used to obtain a more realistic CSH structural model.31,32 The constructed CSH system consists of NaCl solution and CSH gel pores. Na+ and Cl− are highly concentrated in marine environments. The chloride ingress through the gel pore of CASH can destroy the passivation film of steel bars, and induce the corrosion of steel, which seriously reduces the durability of reinforced concrete structures. Hence, NaCl solution is selected in the model construction. As shown in Figure 1a, 86 Na+ and 86 Cl− were uniformly distributed in a box with dimensions of B


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RESULTS AND DISCUSSION

Water and Ion Transport in the CSH and CASH Nanopores. Figure 2 shows the unsaturated capillary adsorption process of NaCl solution in the CSH/CASH gel pore. It can be observed in Figure 2 that Na+ and Cl− ions gradually migrate from the bulk solution into the 3.2 nm pore during 2000 ps simulations. The advanced frontier of NaCl solution exhibits an obvious meniscus shape, and the contact angle between the advancing solution and the substrate is less than 90°, indicating that the CASH and CSH surface are hydrophilic. It should be noted that the contact angle gradually decreases with the rising of the liquid surface, which is consistent with previous reports,36 and this phenomenon can be explained by molecular−kinetic theory.42,43 The change of dynamic contact angles exhibited in Figure 2c also clearly implies a potentially significant impact on the capillary transport behavior of fluid in the nanometer channel. This is consistent with a previous study from Joos et al.44 Because of the speed dependence of the contact angle and the high initial velocity, the dynamic contact angle may be close to 90° during the early stage of capillary penetration, which reduces the capillary driving force as the liquid penetrates further into the gel pore. On the basis of general principles,45 it is expected that the imbibition velocity and contact angle selfregulate to minimize the dissipation. Hou et al. studied the transport of NaCl solution in different-pore-diameter CSH gels (0.5−10 nm). It was found that, at very small pore sizes

Figure 2. Snapshots of capillary flow of water and Na+ and Cl− ions at 0, 100, 350, 1000, and 2000 ps in the (a) CSH gel pore and (b) CASH gel pore. (c) Dynamic contact angle diagram. gel system. Because of its good transferability and reliability, this force field has also been widely used to explore the interactions of water molecules and ions with oxide or hydroxide surfaces.37,38 The single point charge (SPC) water model was used to parametrize the water molecules,39 which could accurately predicted water dynamics and structural properties. The Nose−Hoover thermostat method with relaxation time of 0.1 ps was used to control the simulation temperature.40 The Verlet velocity algorithm was used to obtain accurate integration of the dynamics equations and statistical ensembles.41 All of the simulations were performed in NVT ensemble with the time step of 1 fs. The temperature was maintained at 300 K. The specific simulation process is as follows: First, a rigid body algorithm was used to “freeze” the CSH substrate, and meanwhile an invisible wall was placed between the inlet of the CSH gel hole and the NaCl solution to prevent water molecules and ions from going into the gel pores. A 1 ns simulation was conducted to relax the system. Then, the CSH (CASH) substrate is released, and another 1 ns simulation was conducted to ensure that the substrate structure was optimized and that the NaCl solution reached a thermodynamic equilibrium state. Finally, the interface wall at the inlet of the CSH gel pores was removed (water molecules and ions in the solution could freely diffuse into the pores), so that the entire system could move freely for 2 ns. Throughout the entire process, thermodynamic information such as pressure, temperature, and energy of the system was monitored to verify the stability of the system and the rationality of the model. The results of the MD simulations were derived from analysis of the final 2 ns atomic trajectory information. Through the analysis of the two models, we could obtain the transport mechanism and adsorption mechanism of water and ions in the capillary adsorption process in CSH (CASH) gel pores, and discuss the effects of aluminum substitution of the CSH interface on its transport process.

Figure 3. Penetration depth of water, and Na+ and Cl− ions evolution with time in the (a) CSH and (b) CASH nanopore. C


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Figure 4. Concentration distribution of (a) water, (b) Na+ sodium ions, and (c) Cl− ions.

(1 nm), the transport of water and ions within the pores becomes significantly slower because of the “restricted effect” from the interface.36 Upon comparison of the screenshots of the two models at the same time, in the CASH gel pores, the capillary adsorption process of the NaCl solution is quite slower than that in the CSH gel pores. For instance, at 1000 ps, the filling percentage of the solution in the pore of the CSH model is as high as 80%, while the NaCl solution only occupies less than 60% of gel pore space of the CASH. This indicates that incorporation of aluminate species in the silicon chains retards the ingress of water molecules and ions in the nanometer pore. It is worth noting that, because of the attraction of the CASH interface, clustering and retention of ions occur near the interface, which could affect the transport of water molecules and ions in the pores. The specific transport inhibition mechanism will be further demonstrated in the structural and dynamic parts. Upon monitoring of the frontier boundaries of water molecules and ions, the migration of NaCl solution in the nanopore

is quantitatively explored. As shown in Figure 3a, the penetration depth curves of water and ions exhibit a rising parabolic change as a function of time. The parabolic relation between invading depth and simulation time follows the Lucas−Washburn equation in the traditional capillary adsorption theory.46,47 In both the CSH and CASH model, the penetration depth of ions is always about 8 Å behind water molecules, which suggests that the transport of ions is slower than that of water molecules. This reflects that the moving forward of ions is carried from water molecules by forming the ionic hydration shell. In addition, the different transport mechanisms of ions and water molecules are also due to different interactions between solution species and solid substrate. The complex interfacial chemical environment will be discussed in the following section. At 1500 ps, water molecules penetrate into the CSH nanopore with a depth of 72 Å, whereas, in the CASH pore, the intrusion depth is only 63 Å. This indicates that the aluminum substitution at the CSH interface delays the unsaturated transfer motion of D


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Figure 5. Atomic intensity distribution perpendicular to the (a) CSH channel and (b) CASH channel (Ca, HO, OW, and HW). (c, d) Corresponding atomic intensity distribution details.

On the other hand, in the CASH model, it can be observed that sodium ions with concentration higher than 1.6 mol/L are located at the entrance region of the gel pore. This means that the CASH substrate immobilizes part of the sodium ions, resulting in the local accumulation of ions. The different transport rates of NaCl solution in the CSH model and CASH model imply that the substrate has great influence on the mobility of water and ions in the nanometer gel pores. In previous experimental studies, the capillary transport process of water and ions was retarded in the cement-based material as the cement matrix is partially substituted by the Al-rich furnace slag.48−50 The transport experiments mainly attributed the slow migration of water and ions to the fact that the supplementary cementitious material contributes to the hydration process and densifies the microstructure, inhibiting the ion transport. Recent molecular dynamics study provides valuable insight that the chemical composition change in the substrate also plays an essential role in resisting the ingress of ions and water. This is related with different energy, disjoining pressure, structure, and dynamic properties of water and ions at the CASH surface. Local Structure of Water Confined in the CSH/CASH Gel Pore. Water Intensity Distribution. The intensity profile of the water molecules perpendicular to the CSH/CASH

the solution in its nanopore. At the same time, it also resists the ingress of ions in the nanopore of CASH. For a better understanding of the spatial distribution, Figure 4 exhibits the evolution of water density and ion concentration profile along the solution penetration direction. As shown in Figure 4a, the time-dependent density maps of water molecules show that water molecules gradually occupy the gel pores, and there is an obvious transition zone of density gradient between water and vacant areas. This is caused by its crescent-shaped advanced frontier zone, as observed in Figure 2. In the early stage of simulation, the transition area is relatively narrow, but the transition area gradually becomes wider with time. This is consistent with the reduction of the contact angle of the invading NaCl solution. It can be observed that a majority of the entered water molecules in the pores have a density around 0.9 g/cm3. This means that the capillary absorption process is so rapid that the incoming water molecules cannot reach the saturation state in time. As compared with the water in the CSH model, the water confined in the CASH gel with density from 0.7 to 0.8 g/cm3 occupies a relative high percentage. This implies the slow capillary condensation process in the gel pore of CASH. As shown in Figure 4b, in the CSH nanopore, the sodium ions are uniformly distributed over the solution’s advancing area, middle area, and channel entrance area in the gel pore. E


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ACS Sustainable Chemistry & Engineering substrate can well reflect the distribution of water molecules throughout the pore, and thus explore the influence of different interfaces on the water molecules. As shown in Figure 5a,b, the intensity curves of the water molecules in the entire channel are symmetrically distributed, indicating that the two interfaces have similar properties. In the water profile of CSH model, there are four obvious peaks of OW atoms (oxygen in a water molecule) at 15.2, 16.5, 19, and 21.5 Å with the highest intensity at the third peak. Extending 7 Å outward from the boundary line, the intensity peaks gradually disappear. Multiple peaks near the interface indicate the layered packing of the surface water molecules. In the CASH model, the OW intensity curves show obvious peaks at 15.2, 16.5, 18.7, and 21 Å. As compared with the water distribution in the CSH interface, the position of the peak in the CASH model shifts toward the substrate. This indicates that the CASH surface exhibits a stronger affinity on the water molecules. Al−Si substitution in the substrate, increasing the negativity of the charge, can allow more water molecules to enter the channel region of the silicate−aluminate chains. Furthermore, the HW (hydrogen in water molecules) curve is distributed closer to the substrate than that of the OW curve. This means that the water molecules adsorbed at the interface have their hydrogen atoms pointing to the substrate. The silicate−aluminate chains provide plenty of oxygen sites to accept the hydrogen bonds from the surface water molecules.51 This also confirms the hydrophilic characteristic of the interface. For a better understanding of the water distributions, snapshots of water molecules near the interface are shown in Figure 6a,b. It can be observed that the O−H bond of the water molecules points to the substrate and

Figure 7. Interfacial dipole moments of different models.

Figure 8. Atomic intensity distribution perpendicular to the (a) CSH and (b) CASH nanopore.

forms a hydrogen-bonded structure with the exposed oxygen atoms in the silicon chains, while the aluminum−silicon chains of the CASH substrate provide more oxygen atoms to form hydrogen bonds. Layering and orientation preference of the water molecules are greatly dependent on the H-bond structure formed between CASH substrate and surface water molecules.51

Figure 6. Snapshot of water confined in the (a) CSH gel pore and (b) CASH gel pore. F


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ACS Sustainable Chemistry & Engineering Dipole Moment. The single water dipole moment distribution can be utilized to characterize the interaction between water molecules and the calcium aluminate−silicate hydrate substrate. As shown in Figure 7, the average dipole moment at the CSH interface is 2.48 D. Compared with the magnitude distribution of bulk water dipole moment (2.44 D),22 the upshift of water magnitude distribution indicates the hydrophilic properties of the surface. This is consistent with the small contact angle discussed in the previous section. Furthermore, the mean dipole moment at the CASH interface is 2.52 D, indicating that the hydrophilicity of the CASH interface after aluminum replacement is enhanced. It should be noted that incorporation of Al species in the silicate chain increases the charge negativity of the interfacial oxygen atoms. The enlargement of the dipole moment magnitude is due to the large electric field caused by the silicate−aluminate skeleton that polarizes the water molecules to some extent.52 The strong H-bond interaction from the surface oxygen site can further stretch the O−H bond and open the H−O−H angle in the water molecule, disturbing the self-polarization in the bulk water solution. Local Structure of Ions Confined in the CSH/CASH Gel Pore. Ionic Intensity Distribution. The intensity profile of ions can well reflect the interaction between ions and calcium aluminate silicate substrate. As shown in Figure 8, in both models, the peaks of Na+ ions with higher intensity are distributed approximately 5 Å closer to the interface than the Cl− ion peaks with broadening distribution. This indicates that the negatively charged CSH/CASH interface exhibits an affinity to cations and is repulsive to the anions. As compared to the CSH model, the peak intensity of the Na+ ions near the CASH interface is higher, and part of the Na+ ions are distributed deeply

inside the CASH substrate. This means that the aluminum substitution of the silicon chain can improve the adsorption capacity of the Na+ ion at the interface. This is mainly because the interface has different microscopic adsorption mechanisms for anions and cations. As shown in Figure 9, during the unsaturated transport process, Na+ ions are attracted by the exposed oxygen atoms in the silicon chain to form Na−OS ion bonds, which contributes to stable adsorption. Especially in the CASH

Figure 9. (a) Ion distribution in the nanopores. Local structure of (b) Cl− ion and (c) Na+ ion adsorption on the CASH surface.

Figure 10. Radial distribution function of (a) Na−OS, (b) Na−Cl, and (c) Cl−Ca in the CSH and CASH model. G


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first RDF peak of ion-solution species and ion-solid atoms. Tables 1 and 2 list the CN of Na+ and Cl− ions at the interface or in solution, respectively. The representative hydration structure of ions at the interface and in the solution is also exhibited in Figure 11 for better understanding. The coordination

Table 1. Coordination Number of Na+ Ions in Different Models interface CSH CASH

solution

Na−OW

Na−OS

Na−Cl

Na−OW

Na−OS

Na−Cl

4.9 4.48

0.41 0.95

0.13 0.11

5.42 5.48

0 0

0.14 0.12

model, Al−Si substitution increases charge negativity of oxygen atoms in silicate and aluminate tetrahedron. Thereby, the enhanced electric environment allows more oxygen atom adsorption sites for the adsorption of Na+ ions. This is the main reason that the CASH model has more Na+ ions entering the interface. On the other hand, the weak adsorption of Cl− ions at the interface mainly depends on the electric attraction from surface cations (Na+, Ca2+) by forming a Na−Cl or Ca−Cl ionic pair. This conclusion is consistent with the previous 35Cl NMR study of the cement hydrate suspension.53 The adsorption of Cl− ions only depends on the limited cation adsorption sites near the interface, so the adsorption is quite weak. Radial Distribution Function (RDF) and Coordination Number (CN). To investigate the local chemical environments and interactions of ions with the interface in CSH/CASH gel pores, we calculated their RDF and coordination number (CN). Figure 10a shows the spatial correlation between Na+ ions and the structural oxygen atoms. The RDF curves of Na−OS have the first sharp peak at 2.35 Å, which represents the length of the Na−OS bond. This means that Na+ ions form a stable interaction with the structural oxygen atoms. The Na−OS connection allows the Na ion inner sphere adsorption on the aluminate− silicate skeleton. In addition, the broadened weak peak near 4.5 Å represents the lengths of the Na−OW−OS connection. This represents the solid oxygen atoms forming the H bond with hydrated shells around Na+ ions. The hydrated ions could contact the interface by outer sphere adsorption, which is relatively weak. Figure 10b shows the RDF of Na−Cl. The first sharp peak at 3 Å represents the formation of a Na−Cl ion pair, and the second broadening peak at 5.2 Å indicates the hydration structure of Na−OW−Cl. This means that Na+ ions and Cl− ions form ion clusters in the solution, which also is the main component of the electrostatic double layer near the interface. From Figure 10c, the RDF curve of Ca−Cl in the CSH gel pore has a strong correlation near 2.9 Å, which corresponds to the formation of Ca−Cl ion clusters. Another broadening peak near 5.4 Å suggests the existence of the Ca−OW−Cl connection. Furthermore, in the CASH gel pores, the weakened peak of the RDF curve of Cl−Ca could be explained by the following two reasons. First, because of the capillary water-absorbing effect of the hydrophilic interface, the flow rate of the solution during the unsaturated transport process is relatively high, which enhances the difficulty for the Cl− ions to contact the surface calcium ions to form ion clusters. Second, the Ca2+ ion adsorption sites on the surface are relatively scant, and the formation of Ca−Cl clusters is random. The coordinate number (CN) of ions is obtained by calculating the number of nearest neighbors in the corresponding

Figure 11. Snapshots for coordination neighbors of (a) Na+ ions at the interface, (b) Na+ ions in solution, and (c) Ca2+ ions at the interface.

neighbors around Na+ ions include water molecules (OW), chloride ions (Cl), and oxygen atoms (OS) in the silicate− aluminate chains. The Cl− ions have water molecules and cations surrounding them. In the solution, the average CN of Na and Cl is about 5.5 and 7.5, respectively, which is consistent with the results from the neutron diffraction studies of NaCl solutions.54,55 In the solution of the CSH gel pore, the number of OW around Na+ ions is 5.42, and the number of Cl is 0.14. Near the CSH interface, the coordination number of OW surrounding Na+ ions decreases to 4.9, and the CN of OS increases to 0.41.

Table 2. Coordination Number of Cl− Ions in Different Models interface CSH CASH

solution

Cl−OW

Cl−OS

Cl−Na

Cl−Ca

Cl−OW

Cl−OS

Cl−Na

Cl−Ca

7.16 7.22

0.05 0.09

0.10 0.11

0.05 0.04

7.45 7.46

0 0

0.13 0.11

0 0

H


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ACS Sustainable Chemistry & Engineering This indicates that solid oxygen atoms substitute part of the water molecules near the Na+ ions. On the other hand, as compared with the CSH model, the CN of the OW around the Na+ ion decreases to 4.48, and the CN of the OS increases to 0.95 in the interfacial region of CASH. This indicates that the Na+ ions have more stable coordination with the oxygen atoms in the aluminum−silicon chain structure. This is also the main reason for the better adsorption of Na+ ions at the CASH interface. Dynamic Properties of Water and Ions Invading the Nanopores. Time Correlation Function (TCF). In the previous section, the silicate−aluminate chains provide oxygen sites to accept H bonds from the neighboring water molecules, which associate with the cations by forming the Na−OS connection. The chloride ions can secondarily adsorb on the CASH surface by forming ionic pairs with surface-adsorbed cations. The stability of the hydrogen-bonded network between the water molecule and the interface structure and the bond strength of the ion with atoms in the solid substrate are evaluated by the time correlation function (TCF). The specific formula is as follows: C(t ) =

δb(t ) δb(0) δb(0) δb(0)

(1)

δb(t) = b(t) − ⟨b⟩, where ⟨b⟩ is the average of b during the simulation; b(t) is 0 or 1 depending on the bonding condition. The value of TCF (from 0 to 1) could be used to estimate the stability of the bonds. For instance, the TCF remaining at a constant value indicates the stable connection, and otherwise, the rapid degrading of the TCF implies the frequent breakage of the chemical bonds. Also, the residence time of water and ions remaining in the surface can be calculated by integrating the TCF in the time domain. Figure 12a shows the TCF curves of hydrogen bonds formed between water molecules and the oxygen atoms in silicate and aluminate chains. It shows the TCF curves decrease very quickly from 1 to 0.2 during 200 ps. During the rapid capillary adsorption process, water molecules, frequently escaping from the restriction of the solid substrate, move forward quickly in the gel pore. This results in the quite short residence time of water near the substrate. In particular, at very beginning of the capillary adsorption, the dragging force from the substrate plays a predominant role in pushing fluid forward. It should be noted that the TCF of OS−HW in the CASH system is always higher than that in the CSH system. The residence time of the interfacial H bond in CSH and CASH is 31.6 and 35.5 ps, respectively. This indicates that the hydrogen bonds formed between water molecules and structural atoms in the CASH system are more stable. As compared with fluid transport in the CSH gel pore, the relatively stronger interfacial H bond in the CASH, exerting a resistant force on the fluid in the gel pore, retards the capillary adsorption process. Furthermore, in Figure 12b, the TCF value of Na−OS slowly decreases from 1 to 0.75 during 200 ps. This means that the calcium aluminate−silicate substrate can immobilize the cations stably, and the ionic Na−OS bond is quite stronger than the interfacial H bond. Meanwhile, the TCF in the CASH model is slightly higher than that of CSH, implying that the sodium ions can remain in the CASH surface for a longer time, as compared with the CSH surface. Considering that the CASH substrate provides more oxygen sites to form the Na−OS connection than the CSH substrate, the Na−OS bond with higher strength can explain the resistance of ions ingressing in the gel pore of CASH. On the other hand, the TCF of Na−Cl rapidly

Figure 12. (a) Time correlation function of sodium and other atoms at the interface in different models. (b) Time correlation function of structural oxygen atoms and water molecules in different models.

decreases to 0.1, suggesting the weak connection of the ionic pairs in the gel pore solution. The weak Na−Cl bonds also result in the unstable secondary chloride adsorption on the calcium silicate aluminate surface. MSD of Water Molecules and Ions. The MSD profiles can reflect the moving ability of various atoms (ions) in the systems, and thus can be utilized to explore their transport behavior. The specific equation for MSD calculation is as follows: MSD(t ) = ⟨|rn(t ) − rn(0)|2 ⟩ I

(2)


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Figure 14. Atomistic displacement distribution in the yz plane for a water molecule in the (a) CSH model and (b) CASH model. The color map represents the absolute value of displacement (unit: Å).

Figure 13. MSD curves of water and Na+ and Cl− ions in the (a) CSH nanopore and (b) CASH nanopore.

Here, rn(t) is the coordinate position of atom n at time t, and rn(0) is the initial position of atom n. As shown in Figure 13, the MSD curves of water and ions increase linearly with time in the first 500 ps, and their curves are basically coincident. After 500 ps, the movement of ions is significantly slower than that of water molecules. The MSD values after 500 ps rank in the following order: MSD(OW) > MSD(Cl) > MSD(Na). This shows that the overall mobility of water molecules is faster than that of ions. This is consistent with the time-dependent water and ion penetration process discussed in the first section. The slowest transport of Na+ ions is mainly due to the adsorption of Na+ ions on the interface by forming stable Na−OS ionic bonds with long residence time. Additionally, the MSD values of water and ions in the CASH nanopore are smaller than those in the CSH nanopore. This reflects the resistant effect of the CASH gel on the fluid transport. For a further understanding of the atomic mobility in different regions, the displacement for an individual water molecule is calculated, and the displacement profile at 400 ps in the yz plane is shown in Figure 14. During the capillary transport of the solution, a crescent-shaped frontier is observed in the gel pore. From the center of the nanopore toward the interface, there is a clear transition zone of a velocity gradient. In particular, at the entrance of the pore, there is an area where the velocity decreases significantly. The slow mobility for the water near the CSH surface of the channel entry is mainly attributed to the restriction from two CSH surfaces both normal and

parallel to the transport pathway. The transition zone with low mobility results in narrowing the width of the channel entry. This necking effect is more pronounced in the CASH gel pore. It has been known from the transport part that, in the CASH model, there is ion aggregation in the entrance area of the channel. The accumulated ions and the corresponding ionized hydration film make the entrance narrower and hinder the further transport of water and ions. This is an important reason for the slower water and ion transport rate in the gel pore of CASH.

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CONCLUSIONS In this work, using MD simulation, we explore the unsaturated transport process of water and ions in CSH/CASH gel pores, and the adsorption mechanism of ions is revealed. The following conclusions can be drawn: (1) In the CSH/CASH gel nanopores, the solution is subjected to the capillary action of the hydrophilic interface, and the penetration depth is parabolic, which is consistent with the LW equation. The aluminum substitution of the interface silicon chains could slow water and ion intrusion. (2) The delamination phenomenon of the water molecules near the CASH interface is more obvious, mainly because the aluminum enhances the activity of the bridging oxygen atoms in the silicon chain, and more structural oxygen J


Research Article


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atoms form a hydrogen-bonding network with the water molecules. (3) The aluminum substitution of the silicon chains enhances the adsorption of Na+ ions at the interface. This is because the increased oxygen atom sites in the interface can easily form Na−OS bonds, and this linkage is more stable in the CASH model. The adsorption of Cl− ions mainly depends on the cations (Na+, Ca2+) near the interface, but the limited cation adsorption sites and the instability of Na− Cl and Ca−Cl clusters make the interface exhibit a weak adsorption capacity to Cl− ions. (4) In terms of kinetics, more Na+ ions are adsorbed and immobilized in the CASH interface, and thus limits their movement. Near the entrance area of the CASH model, because of the accumulation of ions, the necking phenomenon that could hinder the transport of water and ions is more pronounced. This is an important reason for the slow unsaturated transport of the solution in the CASH channel.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +8618562624208. Fax: +8653285071509. ORCID

Dongshuai Hou: 0000-0002-1252-2987 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from National Natural science foundation of China under Grant 51508292, 51678317, 51420105015; the China Ministry of Science and Technology under Grant 2015CB655100; Natural science foundation of Shandong Province under Grant ZR2017JL024; and Qingdao Research Program 16-5-1-96-jch are gratefully acknowledged.

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Molecular dynamics study on the structure and dynamics of NaCl

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