Lubrication Behavior of Water Molecules Confined in TiO2 Nanoslits

Article pubs.acs.org/jced

Lubrication Behavior of Water Molecules Confined in TiO2 Nanoslits: A Molecular Dynamics Study Yudan Zhu,*,† Yumeng Zhang,† Yijun Shi,‡ Xiaohua Lu,*,† Jiahui Li,† and Linghong Lu† †

College of Chemical Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 210009, P. R. China ‡ Division of Machine Elements, Luleå University of Technology, Luleå 971 87, Sweden S Supporting Information *

ABSTRACT: Titanium (Ti) metal has been widely used in orthopedic implants, such as knee replacements and fracture fixation devices, where water is the base fluid of the lubricant. In this work, a series of nonequilibrium molecular dynamics have been carried out to investigate the microstructure and lubrication of water molecules confined in TiO2 nanoslits under shearing. The effects of varying slit gap widths (0.8, 1.2, 1.6, and 2.0 nm) and shear velocities (200, 100, 50, and 10 m/s) on the friction coefficients between TiO2 and water molecules were evaluated to shed light on the role of the confined water molecules on lubrication. Simulation results showed that the friction coefficient decreased as the slit width increased. Detailed analysis of water molecules microstructure revealed that water molecules confined in the slits were layered. Typically, all the water molecules in Layer 1 and some water molecules in Layer 2 could reach the sliding velocity of the wall, which were in agreement with the reported mobility of water molecules absorbed on TiO2 nanoparticles via nuclear magnetic resonance. As the width of slit gap increased, the average lifetime of the H-bonds between water molecules within and beyond Layer 1 reduced and the amount of free water increased accordingly, which caused a decrease in the friction coefficient. This understanding can be used to explain at the molecular scale the observation in our previous atomic force microscope experiment in which the higher roughness in TiO2 reflected a lower friction coefficient.

1. INTRODUCTION The original hydrogen-bonded structure of water confined in restricted nanospace can be broken down for new microstructures, thereby leading to the anomalous properties of water molecules under nanoconfinement to differ from their bulk counterparts.1 The involvement of nanoconfined water molecules in lubrication has attracted increased attentions2−5 because of its close relevance to a wide range of applications in nanoelectromechanical systems and biolubrication. The attentions on the importance of nanoconfined water molecules on lubrication can be traced back to a dozen years ago. Initially, Zhang et al.3 evaluated the solvent polarity effect on nanoscale friction in their simulation investigations and found that friction is greatly dependent on solvent polarity. Later, Chen et al.5 experimentally observed that mica surfaces modified with polyzwitterionic brushes in water could exhibit extreme lubrication under high pressure, which suggests that © XXXX American Chemical Society

nanoconfined interfacial water molecules act as an exceptional biolubricant. With the increasing development of experimental characterization and molecular simulation techniques, considerable evidence have been pointing to the role of water molecules microstructures determined by material surface properties on the lubrication mechanism at nanoscale. For example, Sommer et al.,2 in their atomic force microscope (AFM) experiment, found that hydrogenated diamond induces a more stable interfacial water layer than non-hydrogenated diamond and its surface-anchored crystalline water layers are responsible for its superlow friction. Wang et al.4 recently proved that a specific ordered structure of water monolayer Special Issue: Proceedings of PPEPPD 2016 Received: June 28, 2016 Accepted: September 2, 2016

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DOI: 10.1021/acs.jced.6b00534 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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Figure 1. Simulation models. (a) 0.8 nm; (b) 1.2 nm; (c) 1.6 nm; (d) 2.0 nm.

molecules confined in cylindrical nanopores (1.3, 2.8, and 5.1 nm in diameter) and indicated that the first water monolayer is tightly bound at the TiO2 inner surfaces, leading to a decrease in the effective diameter of the pore. Furthermore, thin film of water confined on the TiO2 surface was also observed in previous experiments via 1H solid-state nuclear magnetic resonance (NMR),17 infrared (IR) spectroscopy,18 and backscattering neutron spectroscopy.19 Given the fact that the aforementioned studies have identified the existence of a thin film of water confined on the TiO2 surface, we speculated that the confined water molecules behaviors are associated with friction reduction. However, investigations on the lubrication behavior of water molecules confined in TiO2-restricted nanospace under shearing conditions are rare. Thus, we performed a series of nonequilibrium molecular dynamics (NEMD) in this study to provide a molecular explanation of the anomalous phenomena observed in our previous AFM experiment.13 The following text consists of three parts. Section 2 describes the simulation model and methodology in detail. We changed the width gap of TiO2 slits to model different confinement conditions for water molecules. Slits with larger gaps reflect friction on rougher TiO2 surfaces because the AFM tip does not touch the bare TiO2 surface directly. Section 3 aims to answer the following questions: Does friction reduce with increase in gap width? What is the effect of shear velocity on friction? What is the molecular explanation for the lubrication of water molecules confined in TiO2 nanoslits? Thus, Section 3.1−3.3 discusses the simulation results of spatial distribution, as well as velocity profile of nanoconfined water molecules and their correspondingly friction coefficients. Moreover, we analyzed the microstructures of confined water molecules under shearing to provide the underlying mechanism at the molecular scale in Section 3.4.

could be formed on the superhydrophilic surface with appropriate charge patterns via molecular dynamics, which could reduce the friction between confined water layers. Given that the dif ferent surface hydrophilicities under nanoconfinement could yield various interfacial water molecules microstructures,6 so it will be very interesting to study the dependences of various interfacial water microstructures on their lubrication behavior. For this reason, in this work, we focused on the microstructures of confined water molecules to shed light on the underlying mechanism of reduced friction. Titanium dioxide (TiO2)−aqueous interface, which is commonly used as a model system to investigate mineral interface, can be applied in biotechnology and environmental fields.7−9 Titanium (Ti) metal has been widely used in orthopedic implants, such as knee replacements and fracture fixation devices, because of the passivating nanolayer of TiO2 on the surface, which shows good biocompatibility. 10 Furthermore, the TiO2-based delivery of anticancer drugs has been identified as a promising approach for cancer therapy.11 Most of these applications involve the movement of TiO2 in aqueous or humid environment. Therefore, the friction reduction of TiO2 interfaces is an important research target. To reduce mineral surface friction, the classical method of chemical modification with organic molecules to form selfassembled monolayers can be used.12 However, the poor stability of the grafted thin film limits the practical, long-lasting applications of modified TiO2 as biomedical carriers or marine coatings. Our previous AFM experiment determined an anomalous phenomenon and provided a new avenue to decrease the TiO2 friction coefficient by constructing its geometrical roughness. A high geometrical roughness traditionally results in high friction. However, AFM results demonstrated that comparied to a dense TiO2, a 10 times roughness of mesoporous TiO2 results in 26-fold lower of friction coefficient than dense TiO2.13 We presumed that different geometrical roughness means different species of confined water molecules (e.g., bound and relatively free water) probed by AFM tip, which may be associated with friction variation. Numerous molecular simulations were performed to investigate confined water static and dynamic behaviors confined in various TiO2 interfaces.14−16 Koparde et al.14 reported two hydration layers around anatase and rutile nanoparticles ranging from 2.5 to 4 nm. Wei et al.15 indicated that the carbon layer modified on the inner surface of TiO2 nanoslits (1.2−2.0 nm in gap width) can enhance the diffusion of interfacial water molecules within the slit center by breaking the hydrogen bond network between bare TiO2 surfaces and interfacial water molecules. Solveyra et al.16 studied water

2. SIMULATION DETAILS The simulation model used in the NEMD is illustrated in Figure. 1a−d. Water molecules were confined between two rutile TiO2 (110) slabs, which were infinite in the X and Y directions. Each wall dimension was 2.6 × 2.4 × 1.55 nm3 (X × Y × Z).The entire simulation box was supplied with threedimensional periodic boundary conditions except for the confined water molecules in the Z direction. The top slab was made to shear along the X axis at a constant sliding velocity. In this work, effects of slit gap width (0.8, 1.2, 1.6, and 2.0 nm) and sliding velocity (200, 100, 50, and 10 m/s) on the lubrication behavior were investigated. As shown in Figure 1, the layered structure of confined water was varied with slit gap width from 0.8 to 2.0 nm, which can be explored to shed light B



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Figure 2. Density profiles of oxygen for slits with different gap widths along the Z direction under different various sliding velocities.(a) 0.8 nm; (b) 1.2 nm; (c) 1.6 nm; (d) 2.0 nm.

interfacial atoms of the wall are flexible and canonical ensemble is applied on these atoms by the temperature of 300 K (the Nosé−Hoover thermostat) whereas the rest part of wall block is kept rigid. Meanwhile, the microcanonical ensemble was used for the water molecules to make sure that they can freely motion, avoiding the appearance the velocity deviation for the water molecules in the three axial directions. In this way, water molecules and the walls had kept same temperature. In this way, the wall atoms would adsorb the viscous heat from the fluid, whereas the top wall atoms could directly move as expected. A similar method for thermostatting the system was adopted by Kannam et al.21 in their NEMD investigations. The last 9.0 ns NEMD simulation with integral step of 1 fs was performed and the coordinates were saved every 1 ps for additional analysis. The Ewald method was used for full electrostatic interactions and a cutoff distance of 1.0 nm was used to calculate the short-range van der Waals interactions. The SPC/E model22 was utilized for water molecules, which were treated as charged Lennard-Jones (12−6) sites. Buckingham potential for the Matsui−Akaogi force field23,24 was used for describing TiO2. The ab initio-derived interaction parameters25 were utilized to model the interactions between TiO2 and water molecules. Detailed force field forms and parameters adopted in this work are listed in Table S2 (Supporting Information). This set of force field parameters for TiO2 and water systems was adopted by Koparde et al.14 to study the adsorption of water molecules on the surface of anatase and rutile nanoparticles both at hydrothermal conditions and room temperature. Similarly, water molecule microstructures confined in various TiO2-based nanoslits were investigated in our previous work.15,26

on the role of confined water molecules microstructure on the lubrication. The velocities investigated in this work are the typical velocity order of microelectromechanical systems devices.3 Simulations were performed using LAMMPS.20 As shown in Table S1 (Supporting Information), the number of confined water molecules in each case is different, because slits with different width gap accommodate different water molecules under equilibrium state. For this reason, we obtained the precise density of water in slits according to the following procedure. (I) The slit was immersed in the center of a periodic bulk water box with a density of 1.0 g cm−3. (II) An isobaric− isothermal ensemble (NPT) with Lammps was utilized. Because the slit width cannot be preserved if the typical three-dimensional NPT ensemble algorithm is applied to all directions. Thus, NPT ensemble only in one direction (the X direction in this work) while placing two water reservoirs outside the slits in this direction. To save computational resources, the system was maintained at a constant temperature (300 K) and the pressure was set to 100 bar to avoid the appearance of nanobubble in the vicinity of the pore mouth, which might influence the water molecules entering into the silt pore. One ns MD simulation was performed to obtain the equilibrium configuration. (III) We retained the water molecules confined within the slit and removed those outside the slit. The obtained configurations were used as the initial configurations for NEMD simulations. Table S1 (Supporting Information) lists the retained simulation box and the number of the resulting water molecules within the slits, as shown in Figure 1. Then, 10 ns nanofriction-based NEMD was performed. Here, because there should be the thermostat to monitor the temperature, it is coupled on the wall atoms rather than the fluid atoms, so as to make water molecules move freely. The C



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Figure 3. Velocity profile for slits with different gap width along the Z direction under various sliding velocities. (a) 200 m/s; (b) 100 m/s; (c) 50 m/s; (d) 10 m/s.

Table 1. Frictional Forces and Friction Coefficient for Slits with Different Gap Widths under Various Sliding Velocities frictional force, nN velocity, m/s

0.8 nm

200 100 50 10

0.12 ± 1.0 × 10−3 0.10 ± 1.5 × 10−3 0.088 ± 1.4 × 10−4 0.065 ± 1.9 × 10−3

Velocity, m/s

0.8 nm

200 100 50 10

17 ± 4.3 × 10° 2.3 ± 1.1 × 10−1 1.3 ± 4.2 × 10−2 0.64 ± 2.5 × 10−2

1.2 nm 0.066 0.055 0.046 0.028

± ± ± ±

6.1 1.0 8.1 9.5

1.6 nm 10−4 0.048 10−3 0.039 10−4 0.033 10−4 0.017 Frictional Coefficient

× × × ×

1.2 nm 0.57 0.42 0.33 0.19

± ± ± ±

7.0 3.2 2.7 5.3

× × × ×

± ± ± ±

5.4 2.7 6.9 1.3

× × × ×

2.0 nm 10−4 10−4 10−4 10−3

0.039 0.031 0.026 0.012

10−3 10−3 10−3 10−2

0.37 ± 1.8 × 10−3 0.26 ± 3.6 × 10−3 0.20 ± 3.5 × 10−3 0.088 ± 2.0 × 10−3

1.6 nm 10−5 10−3 10−3 10−3

0.44 0.32 0.26 0.13

± ± ± ±

1.6 3.1 3.4 1.3

× × × ×

± ± ± ±

5.1 3.4 9.5 1.0

× × × ×

10−4 10−4 10−4 10−3

2.0 nm

wall. The first and second maxima of oxygen profiles appeared around 0.08 nm and 0.23 nm away from the wall, respectively. 3.2. Velocity Profile of Nanoconfined Water Molecules. The velocity profiles along the Z direction of four slits under various sliding velocities are shown in Figure 3 (data see Table S4 in Supporting Information). The water molecules in the vicinity of slit top wall reached the sliding velocity, and no water slippage phenomena were observed for all the studied cases, even at the highest sliding velocity of 200 m/s. These observations were attributed to the high affinity between TiO2 and water molecules. The strong interactions between the water layer and TiO2 surface were considered significant for the good biocompatibility of TiO2 as orthopedic implants and biomedical carriers across lipid interfaces,11 which prevented proteins from directly contacting TiO2 surfaces. Combining the peak positions derived from the spatial distribution of nanoconfined water molecules (Figure 2), we found that the velocity of all the water molecules in the first layer (less than around 0.12 nm away from the top wall) and

3. RESULTS AND DISCUSSIONS 3.1. Spatial Distribution of Nanoconfined Water Molecules. Figure 2 shows the water spatial distributions along the Z-direction when the water molecules flow in TiO2 slits with different gap widths under various sliding velocities. In this study, the density profile of the oxygen atoms along the Z direction (DPOZ) were taken as the density profile of the water molecules because the oxygen atom is quite near the mass center of the water molecule (data see Table S3 in Supporting Information). As illustrated in Figure 2, the water molecules confined in the four slits were layered, and four discernible peaks were distributed along the Z direction. In the central region, the DPOZ values turned to 1 for the slits, except the narrowest gap (0.8 nm), which indicated that the confined water density was similar to that of bulk water. The peaks sharpened with decreasing sliding velocity, whereas the velocity exerted a negligible effect on peak position. The peak positions demonstrated a dependence on the distance away from the slit D



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Figure 4. Distribution of orientation angles for slits with different gap widths (velocity = 10 m/s) for water molecules in (a) Layer 1 and (b) Layer 2; schematic image of preferential orientation in (c) Layer 1 and (d) Layer 2.

attributed to the constant slit gap in the simulations, which ensured that the water film was constantly under shearing conditions. Table 1 shows that the frictional force and friction coefficient increased as the slit width decreased. This phenomenon demonstrated our previous prediction derived from the AFM experiment.13 For the 0.8 nm slit gap, the overwhelming majority were water molecules of types I and II, and less mobility resulted in a higher friction coefficient. As the gap width increased, the amount of type III water increased and led to a decreased friction coefficient. It should be note that there is a limitation to direct compare the simulation and AFM experiments. The lowest sliding speed in our work is still too larger than the scanning speed in AFM experiments. The sliding speed below m/s is difficult to reach due to the differences in time scales involved in atomistic simulation and AFM experiment. Given the fact that friction coefficient showed a decreasing tendency with increasing thickness for all the velocities investigated in this work, the result could qualitatively explain our AFM experimental observations to some extent. We focused on the detailed microstructural analysis of nanoconfined water molecules in the following sections to further provide molecular insights for this phenomenon. 3.4. Microstructural Analysis of Nanoconfined Water Molecules. The variations in lubrication ability with slit gap widths for various sliding velocities were qualitatively similar based on the above analysis. In this section, a 10 m/s velocity was selected for investigation because it is more frequently encountered in practical applications.29 We studied the preferential orientation and hydrogen bond (H-bond) microstructures of water molecules in different layers to shed light on the underlying mechanism of reduced friction with increasing slit gap at the molecular scale. First, we defined two characteristic angles (i.e., dipole angle and OH bond angle) to investigate the preferential orientation of water molecules for different water layers. As illustrated in Figure 4c,d, the dipole angle is the angle between a vector

part of the water molecules in the second layer (less than around 0.28 nm and larger than around 0.17 nm away from the top wall) reached the sliding velocities for all the studied cases. This finding was consistent with the reported residence time of water molecules near the TiO2 surface via MD investigation. Wei et al.26 found that the residence time of the first layer at the TiO2 surface is considerably long, even much longer than that near the alkaline-earth metal ion, which is believed to be strongly hydrated at room temperature. Moreover, the different mobilities for the various water layers derived form Figures 2 and 3 were also in agreement with the reported 1H solid-state NMR results, 17 in which the adsorbed water on TiO 2 nanoparticles can be categorized into three different types: (I) the first layer is rigid water species with restricted mobility, (II) the second layer has less mobility water species weakly confined on the TiO2 surface, and (III) the high mobility water species beyond the second layer. Figure 3 also indicates that the velocity distribution fluctuations were more evident with reduced sliding velocity, which implied that statistical thermal motion became more dominant. 3.3. Friction Coefficient Analysis. We calculated the frictional force exerted on the bottom wall and the friction coefficient for the slits with different gap widths under various sliding velocities in Table 1 to quantitatively characterize the lubrication ability of confined water molecules. The friction coefficient is determined by dividing the frictional force by the normal force exerted on the bottom wall.27 Compared with zero velocity, the bottom walls for the four slits under shear conditions were subjected to the directed force along the X direction. As the sliding velocity of the wall decreased, the frictional force and the friction coefficient decreased accordingly across all the slits. The dependence of the friction coefficient on the sliding velocity was consistent with the rule of hydrodynamic lubrication regime in the well-known Stribeck curve,28 although the gap size of the studied slits was much narrower compared with the common conditions (μm) for hydrodynamic lubrication. These observations could be E



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Table 2. Mean Number of Hydrogen Bonds Per Water Molecule ⟨nHB⟩ and Hydrogen Bonds Lifetime for Slits with Different Gap Widthsa ⟨nHB⟩ 1.2 nm

1.6 nm

2.0 nm

0.8 nm

1.2 nm

1.6 nm

2.0 nm

0.01 1.46 1.31

0.01 1.46 1.49

0.01 1.44 1.52

0.01 1.44 1.56

30.40 230.54 9.25

27.07 216.98 8.42

23.00 209.28 6.94

21.19 178.79 7.07

in Layer 1 in and beyond Layer 1 beyond Layer 1 a

HBLT, ps

0.8 nm

Velocity = 10 m/s.

Table 3. Percentages of Water Molecules, f n, with na Hydrogen Bonds within Slits with Different Gap Widthsb beyond Layer 1 percentages, %

a

in and beyond Layer 1 percentages, %

fn

0.8 nm

1.2 nm

1.6 nm

2.0 nm

0.8 nm

1.2 nm

1.6 nm

2.0 nm

f1 f2 f3 f4 f5 f6

25.61 43.35 26.09 4.48 0.46 0

16.44 34.62 31.42 16.41 1.10 0

12.95 28.99 31.86 24.70 1.48 0

11.10 23.94 31.80 29.41 1.74 0

77.82 22.13 0.04 0 0 0

76.04 23.92 0.04 0 0 0

75.95 24.01 0.04 0 0 0

75.65 24.32 0.04 0 0 0

n = 1, ..., 6. bVelocity = 10 m/s.

their HBLT. The low ⟨nHB⟩ was due to the restriction of TiO2 surface to the water molecules and little H-bond was formed within Layer 1, which was also consistent with the preferential orientation analysis of water molecules(see Figures 4c). We further analyzed the H-bonds properties between Layer 1 and other layers. The extremely large HBLT of this type of H-bond demonstrated that the H-bonds connecting the water molecules between Layer 1 and other layers were stable. This phenomenon can explain that part of the water molecules in the Layer 2 reached the sliding velocity. We also observed a downward tendency in HBLT as the slit width increased, although the slit width slightly affected the H-bond number. This finding should be one of the main reasons for the reduction in friction with increasing slit width. We further analyzed the distributions of the H-bond number in Table 3. Water molecules with one to two H-bonds accounted for the majority between the water molecules in and beyond Layer 1. The slit gap width exerted minimal effects on their distributions. By contrast, as the slit gap width increased, water molecules with four to five H-bonds also increased and turned to bulk water ( f1 = 13.85%, f 2 = 30.92%, f 3 = 33.59%, f4 = 20.23%, f5 = 1.41%, f6 = 0%), thereby suggesting decreased structural defects in the H-bonded network. The formation of a 3D H-bond network among water molecules themselves beyond Layer 1 indicated that the amount of free water increased and played a lubrication effect by decreasing the force exerted on the pore wall, which may be another cause of reduced friction.

pointing from the midpoint of the H atoms to the O atom in a water molecule and the normal vector to the surface, whereas the OH bond angle is the angle between the OH bond of a water molecule and the normal vector to the surface (data see Table S5 in Supporting Information). Figure 4a presents that the OH bond angles were concentrated on 124° and dipole angles were focused at 170° for the water molecules in Layer 1, indicating that they adopted a preferential orientation in Figure 4c. The distributions of characteristic angles were virtually the same among for the four different slits, which suggested that the slit gap width played a minor role on the preferential orientation of water molecule in Layer 1. Figure 4b illustrates one obvious peak of the dipole angle and two obvious peaks of the OH bond angle, indicating that the orientation of Figure 4d was preferred for the water molecules in Layer 2. The preferential orientation was also slightly affected by the variation in slit gap width. Compared with the water molecules in Layer 1, we found that the distribution peaks of Layer 2 were broader and indicated less restriction in the TiO2 surface, resulting in more disordered of water molecules in Layer 2. The H-bonds formed between the two layers by combining preferential orientations of the two layers, with the OH bond of a water molecule in Layer 1 pointing to an oxygen atom in a water molecule. Second, we investigated the mean number of hydrogen bonds per water molecule (⟨nHB⟩) and H-bond lifetime (HBLT) for slits with different gap widths, as shown in Table 2. We divided H-bonds into three types to better reflect the detailed water microstructure variations with the slit width increased: (i) all the water molecules forming H-bond belong to Layer 1 (“in Layer 1”); (ii) one of the water molecules at least forming H-bonds belongs to Layer 1 whereas other water molecules do not belong to Layer 1 (“in and beyond Layer 1”); (iii) all the water molecules forming H-bond do not belong to Layer 1 (“beyond Layer 1”). For the water molecules beyond Layer 1, the ⟨nHB⟩ increased as the slit gap width increased and their HBLT was the shortest. This observation was in agreement with the high mobility of water molecules. Moreover, the ⟨nHB⟩ for the four slits for the water molecules in Layer 1 was extremely low and slit width slightly influenced

4. CONCLUSION In this work, we performed a series of NEMD to investigate the microstructure and lubrication of water molecules confined in TiO2 nanoslits under shearing. The effects of varying slit gap width (0.8, 1.2, 1.6, and 2.0 nm) and shear velocities (200, 100, 50, and 10 m/s) on the friction coefficients were evaluated. The spatial distribution and velocity profile of nanoconfined water molecules showed that water molecules confined in the four studied slits were all layered. The strong interactions between TiO2 and water resulted in no water slippage phenomena. All the water molecules in Layer 1 and some F



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water molecules in Layer 2 could reach the sliding velocity of the wall, which were in agreement with the reported mobility of three types of water molecules absorbed on TiO2 nanoparticles via nuclear magnetic resonance.17 Friction coefficient analysis demonstrated that the frictional force and friction coefficient decreased as the slit width increased. For the 0.8 nm slit gap, the overwhelming majority were water molecules of types (I) and (II), and less mobility resulted in a higher friction coefficient. As the gap width increased, the amount of type (III) water increased and led to a decreased friction coefficient. We previously in the AFM experiment found an abnormal phenomenon that the higher roughness in TiO2 (i.e., mesoporous TiO2) showed a lower friction coefficient than the one with lower roughness (i.e., dense TiO2).13 On the basis of the insights derived from the simulation results in this work, we could explain this phenomenon at molecular scale to some extent. Different geometrical roughness of TiO2 surface determined different types of confined water molecules (e.g., bound water, relatively free water, free water) probed by AFM tip. A denser surface of TiO2 indicates the narrower of the slits between tip and surface. Different types of water molecules confined in the mesoporous and dense TiO2 surfaces led to the abnormal phenomenon in which the higher roughness in TiO2 reflected a lower friction coefficient. Furthermore, we investigated the preferential orientation and the H-bond microstructures of water molecules in different confined water layers to shed light on the underlying mechanism at the molecular scale. The properties of H-bond between water molecules in and beyond Layer 1 demonstrated that their HBLT decreased as the slit width increased even though the slit width exerted a slight effect on the H-bond number. Moreover, the formation of a 3D H-bond network among the water molecules beyond Layer 1 suggested an increased amount of free water. Both aspects were responsible for the reduction in friction as the slit gap increased.

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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.jced.6b00534. Details of simulated systems, force field forms and parameters, density profiles, velocity profiles, distribution of orientation angles, additional references. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.Z.). Tel./Fax: +86 25 83172287. *E-mail: [email protected] (X.L.). Tel./Fax: +86 25 83588063. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (Nos. 21206070, 21576130, and 21490584), Qing Lan Project, and the State Key Laboratory of MaterialsOriented Chemical Engineering (No. KL15-03). We also thank Dr. Mingjie Wei and Prof. Luzheng Zhang for many fruitful discussions. G



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Lubrication Behavior of Water Molecules Confined in TiO2 Nanoslits

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