Electric-Field Effects on Ionic Hydration: A Molecular Dynamics Study

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B: Liquids, Chemical and Dynamical Processes in Solution, Spectroscopy in Solution

Electric-Field Effects on Ionic Hydration: A Molecular Dynamics Study Zhongjin He, Haishuai Cui, Shihua Hao, Liping Wang, and Jian Zhou J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.8b02773 • Publication Date (Web): 11 May 2018 Downloaded from http://pubs.acs.org on May 14, 2018

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The Journal of Physical Chemistry

Electric-Field Effects on Ionic Hydration: A Molecular Dynamics Study Zhongjin He†*, Haishuai Cui†, Shihua Hao†, Liping Wang† and Jian Zhou*‡ †

School of Chemical Engineering, Xiangtan University, Xiangtan, Hunan 411105, China

‡

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China

*Corresponding Author: E-mail: [email protected]; [email protected]

Abstract In this work, we report the electric-field effects on ionic hydration of Cl-, Na+ and Pb2+ using molecular dynamics simulations. It is found that the effect of weak fields on ionic hydration can be neglected. Strong fields greatly disturb the water orientation in the hydration shells of ions, though ion coordination number remains almost unchanged. Under strong fields, the first hydration shell of ions is significantly weakened and the ion-water interaction energy is dramatically reduced; surprisingly, the second hydration shells of Cl- and Na+ are slightly structured because of the optimal water orientation; moreover, ionic hydration structures become asymmetrical along the field direction due to the uniformly aligned water dipoles. Compared with Na+ and Pb2+, the hydration of Cl- is less disturbed by external fields, probably ascribed to the different water reorientation around anions and cations as well as the different structure-maker/breaker nature of the ions. Additionally, strong fields significantly enhance ion mobility and remarkably shorten the water residence time in the hydration shell. This work demonstrates that applying strong fields is an effective way to weaken ion hydration.

1. Introduction Electric fields are ubiquitous in biological systems and play an important role in dictating many biological functions.1 For example, the rapid transport of ions across cell membranes via ion channels is conducted under transmembrane electric fields; the internal electric fields generated by the residues in protein channels are crucial to the selective conduction of ions and water. Moreover, electric fields are of great interest for a wide range of technological applications,2-7 owing to the ease of application and fast control. Electric fields are indispensable in the processes of electro-deionization, electrodialysis, electroosmosis and electrophoresis.8 Employment of electric fields in chemical catalysis is important to some chemical reactions.9-12 In the separation of ions by membranes and nanopores, electric fields are usually applied to drive the transport of ions.13-15 Electrolyte solutions in ion-batteries and electrochemical capacitors are under the influence of electric fields during charge and discharge cycles.16 In addition, recent experimental and theoretical studies17-18 have demonstrated that voltage-gated hydrophobic nanopores can be designed via electrowetting, i.e. applying electric fields to reversibly switch the nanopores between dewetting (closed) and wetting (open) states. A detailed understanding of electric-field effects on water property and ion hydration can facilitate the development of electric-field-based innovative applications and may help to interpret the mechanisms for mass transport in biological channels. Previous studies19-28 have observed some peculiar changes in pure water and electrolyte solutions induced by electric fields. It is reported that applying electric fields can significantly increase water density near nonpolar surfaces or in narrow nanochannels, a phenomenon termed electrostriction.20-21

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English et al. have systematically investigated electric-field effects on the property of liquid water with molecular dynamics (MD) simulations, and observed that electric fields can significantly alter molecular mobility, hydrogen bonding patterns and induce dipolar alignment.22-23 The simulation results of Murad show that applying electric fields can enhance ion mobility and drift velocity in electrolyte solutions.24 Wang and co-workers performed MD simulations on the concentrated NaCl solutions under electric fields, and revealed that under strong fields, ion clusters become smaller and the structural and dynamic properties of the solution become anisotropic.25 Cassone et al performed ab initio simulations to explore water dissociation in aqueous NaCl solution by applying electric fields, and found that the dissociation threshold is lower in NaCl solution than in pure water.26-27 Lu and co-workers found that applying a weak electric field can enhance Na+-water interaction energy in carbon nanotubes.28 Despite these achievements, it remains unclear how ion hydration changes with external electric fields. Given that Cl-, Na+ and Pb2+ carry different charges/valences and are ubiquitous in seawater desalination and wastewater purification, we perform MD simulations of the hydration of these ions in bulk solution under external uniform electric fields to systematically elucidate the electric-field effects on the ion hydration.

2. Simulation Details The simulation system contained a pair of Na+ and Cl- (or Pb2+ and 2 Cl-) and 2136 water molecules in a cubic box with an initial length of 4 nm, i.e. approaching infinite dilution. After energy minimization, the system was equilibrated for 1 ns under NPT ensemble at 300 K and 1 atm to obtain proper water density in the system. In the subsequent 3-ns simulation, NVT ensemble was implemented and a uniform electric field was applied along the positive direction of the z-aixs. The field strength ranged from 0 to 3 V/nm. The last 2 ns were used for data analysis, while the first 1 ns was excluded as equilibration. SPC/E model29 was used for water. The parameters for Na+, Cl- and Pb2+ were taken from Joung30 and Li.31 The settle algorithm was adapted to constrain the rigid geometry of water molecules. The temperature and pressure were regulated by the Nosé-Hoover thermostat32 and the Parrinello-Rahman barostat,33 respectively. The short-range van der Waals interactions were cut off at 1 nm. The electrostatic interactions were computed with particle mesh Ewald method.34 Periodic boundary conditions were imposed in all directions. Trajectories were integrated using the leapfrog scheme with a time step of 2 fs. Coordinates were stored every 0.1 ps. All the simulations were performed with GROMACS4.5.7 software.35

3. Results and Discussions 3.1 Water Structuring under External Fields To better understand the effects of external fields on ion hydration, we first analyzed their influences on water structuring. Note that the disturbance of ions to water structuring can be neglected, as the ion concentrations are approaching infinite dilution. Radial distribution functions g(r) of oxgen-oxgen and oxgen-hydrogen for water molecules have been analyzed and are shown in Figure 1. It is found that the g(r) curves are almost unaltered by external fields. Under weak fields (≤1V/nm), the peak locations and heights almost do not change. Under strong fields of 2 and 3 V/nm, the heights for the first and second peaks slightly increase, while that for the first trough slightly decrease, implying that strong fields may slightly strengthen the water-water interaction. This agrees well with the results of the recent ab initio MD simulations.23, 26 Furthermore, external fields have little effects on the water coordination numbers and the average number of hydrogen bonds per water (Figure 2A), consistent with previous ab initio simulation results.26

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Figure 1. Radial distribution functions g(r) of (A) oxgen-oxgen and (B) oxgen-hydrogen for water molecules under external fields. Note that the curves for fields between 0 and 1 V/nm almost overlap that for 0 V/nm, thus their legends are not shown in the figure. The inset in panel (A) magnifies the first trough and second peak. 7

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Figure 2. (A) The coordination numbers for the first and second shells of water molecules and the average number of hydrogen bond (HB) per water and (B) the lifetime for HB under external fields. To explore the effect of external fields on water dipole orientation, the distribution of φ (defined as the angle between a water dipole and the field) has been calculated and depicted in Figure 3. The angle range of 0 to 180° splits into bins with equal bin width (using 2°), and the probability distribution of φ is defined as the ratio of the number of angles in each bin to the total number of angles. It shows that the distribution of φ progressively shifts towards 0° with the increasing field strength, indicating that water dipoles tend to align with the field direction, as shown in the snapshot in Figure 3. It seems that these uniformly aligned water dipoles can stabilize the hydrogen bonds formed in water, as the lifetime is prolonged when field strength increases (Figure 2B). Such reorienting of water dipoles by external electric fields has also been reported in previous studies,18, 26 which can profoundly disturb ion hydration in bulk solution, as discussed below.

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Figure 3. Probability distributions of water dipole orientation under different external fields; φ is the angle between a water dipole and the external field (the positive direction of z-axis). The insets illustrate the definition of angle φ and the water dipole orientation under the external field. 3.2 Changes in the Ion-Water Interaction Energy under External Fields Effects of external field on ion hydration were first explored in terms of the ion-water interaction energy, which reflects the hydration strength. We compute the change of the interaction energy (∆E0) between ions and the water molecules within 1 nm of each ion under external fields with respect to the field-free case (0 V/nm), and decompose it into the contributions from the changes for the first hydration shell (∆E1), the second hydration shell (∆E2) and the remaining region (∆E3) (i.e. beyond the second hydration shell but within 1 nm of the ion). The results are plotted in Figure 4. Positive values indicate that the ion-water interaction is weakened, while negative values imply that the ion-water interaction is strengthened. In the absence of external fields, the interaction energy of Cl-, Na+ and Pb2+ with the water molecules within 1 nm are -300.8±0.4, -329.4±0.5 and -869.2±0.9 kJ/mol, suggesting that the ionic hydration strength of Pb2+ is the strongest due to its bicharge, which is consistent with other simulation results.30-31 It is found that only applying strong fields (> 0.5 V/nm for Cl-, > 0.2 V/nm for Na+ and > 0.3 V/nm for Pb2+) can alter the ion-water interaction energy, probably due to that the local radial electric fields of ions can resist the disturbance of the external weak fields. As the field strength increases, the interaction energy of Cl-, Na+ and Pb2+ with the first hydration shell is significantly weakened, whereas the interaction energy of Cl- and Na+ with the second hydration shell are slightly enhanced, which partially compensates for the reduction of ion-water interaction energy in the first hydration shell. At 3 V/nm, ∆E1 for Cl-, Na+ and Pb2+ are 27.6, 57.0 and 51.4 kJ/mol; ∆E2 for Cl-, Na+ and Pb2+ are -4.8, -12.9 and -1.1 kJ/mol; ∆E3 for Cl-, Na+ and Pb2+ are close to 0. Consequently, at 3V/nm, the interaction energy of Cl-, Na+ and Pb2+ with the water molecules within 1 nm is reduced to -278.5, -286.1 and -820.9 kJ/mol. These energy changes for ion hydration under external fields may be caused by the changes of ion hydration structures, which are explored in detail below. 60

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(B) Na+ and (C) Pb2+, including the changes for the water molecules within 1 nm of ions (∆E0), the first hydration shell (∆E1), the second hydration shell (∆E2) and in the remaining region (∆E3) (i.e. beyond the second hydration shell but within 1 nm of the ion). 3.3 Ion Hydration Structures under External Fields Ion-oxygen radial distribution functions g(r) of Na+, Cl- and Pb2+ under external field have been analyzed and are shown in Figure 5. We observe that g(r) curves are almost not affected by weak fields (≤1V/nm), but become slightly flatted under strong fields of 2 and 3 V/nm, i.e. the peak heights decrease and the trough height increase, implying a weakening of ion hydration. This is just opposite to the effects of strong fields on the g (r) curves for water (see the inset in Figure 1A). Figure S1 displays the coordination numbers for the first shell (N1) and second shell (N2) of ions, which are computed by the numeric integration of the g(r) curves in Figure 5. It shows that N1 is almost unchanged under external fields, while N2 is slightly reduced by strong fields. Specifically, at 2 and 3 V/nm, N2 is decreased by 0.30 and 0.49 for Cl-, 0.25 and 0.43 for Na+, 0.81 and 1.41 for Pb2+. Compared with ion-oxygen g(r) in Figure 5, the field-induced structural changes are less severe for the oxygen-oxygen g(r) in Figure 1. Moreover, the variations in water coordination numbers (Figure 2A) under fields are sizably lower than those of ion coordination numbers (Figure S1). These observations suggest that external fields seem to affect much more the ion-water interaction than the water-water interaction. 4

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Figure 5. Ion-oxygen radial distribution functions g(r) under external fields for (A) Cl-, (B) Na+ and (C) Pb2+. Note that the curves for external fields between 0 and 1 V/nm almost overlap that for 0 V/nm, thus their legends are not shown in the figure. The g(r) for Cl- is based on the NaCl solution, and the result is almost the same for Clin the PbCl2 solution. The orientation of water molecules surrounding ions is another key factor in ion hydration.36-37 Previous studies38-39 showed that the confinement effect and the unique ice-like water structures in narrow carbon nanotubes can profoundly affect water orientation in ion hydration shells. To investigate this orientation, we defined α for Cl- and θ for Na+ and Pb2+. α is the angle between O-H bond and the line joining the water oxygen to the ion, and θ is the angle between the water dipole and the line joining the water oxygen to the ion, as illustrated in the insets of Figures 6A and 6B, respectively. When an anion is hydrated by a water molecule, one of the OH groups points toward it. For a cation, the water oxygen atom points toward the ion, while both hydrogen atoms point away. Thus, the optimal value for α and θ is 0o and 180o, respectively. The actual value for α and θ generally deviates from these optimal values due to the perturbation of the hydrogen bonds formed among the water molecules in the hydration shell. If the values of α and θ are closer to the optimal values, the hydration shell yields a more favorable ion-water interaction energy.

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Figure 6. Probability distributions of the orientation angle α and θ of water molecules in the first hydration shells of (A) Cl-, (B) Na+ and (C) Pb2+ under strong fields. The insets show the definition of angle α and θ. Note that the curves for weak fields (< 0.5 V/nm for Cl-, < 0.2 V/nm for Na+ and < 0.3 V/nm for Pb2+) almost overlap that for 0 V/nm (see Figure S2). As shown in Figure S2, weak fields (< 0.5 V/nm for Cl-, < 0.2 V/nm for Na+ and < 0.3 V/nm for Pb2+) almost do not perturb the orientation of water molecules in the first hydration shell of ions. The distribution of α for the first hydration shell of Cl- presents two sharp peaks with the maximal located at around 10o and 110o respectively, suggesting that one O-H bond of a water points to Cl- (corresponding to 10o) and the other points away (corresponding to 110o). The distribution of θ for the first hydration shell of Na+ shows a wide peak with the maximum located at around 150o, while that of Pb2+ displays a sharp peak with the maximum located at around 165o. The θ for Pb2+ is closer to the optimal value 180o than Na+, due to the stronger electrostatic interaction between Pb2+ and water molecules in the hydration shell. Figure 6 displays the distribution of α and θ for the first hydration shells of Cl-, Na+ and Pb2+ under external fields. For Cl- under field strength ranged from 0.5 to 1 V/nm, the position and height of the peak around 10o almost do not change compared to the field-free case (0 V/nm), while the position of the other peak shifts toward slightly higher values (Figure 6A). Under 2 and 3 V/nm, the height of the two peaks decreases and the peak around 110o shifts to 115o. For Na+, the peak gradually shifts to lower values with the increasing field strength (Figure 6B). Such peak shift caused by external fields is also observed for Pb2+ (Figure 6C), though the shift is much smaller than that of Na+. These results indicate that strong fields make the orientation of the first hydration shell less favorable for ion hydration. Similarly, weak fields almost do not perturb the water orientation in the second hydration shells, as shown in Figure S3. Under strong fields, for the second hydration shell of ions, the probability of α for Cl- falling in the range of 40o-60o increases, while the distribution of θ for Na+ and Pb2+ is more concentrated in the range of 60o-130o. Ion hydration structures become asymmetrical in the field direction, as water dipoles tend to uniformly align with the direction of external fields. Figure 7 shows the distributions of α and θ for the water molecules in the left and right parts of the first hydration shells along the direction of external fields. In the absence of external fields, the curves for water in the left and right parts of ions overlap with each other (Figure 7A), indicating a symmetrical hydration structure, due to the symmetrical radial electric fields of the ions. As illustrated in the inset of Figure 7B, for the water molecules on the left side of Cl-, aligning water dipoles with the field direction is consistent with the favourable water orientation for Cl- hydration; while it is just opposite to the favourable orientation for Clhydration for the water molecules on the right side of Cl-. Thus, the orientation of water molecules in the left part of the first hydration shells of Cl- are only slightly affected by external fields (Figures 7B and Figure S4A), and the distribution of α for the water molecules in the left part of the second hydration shell and remaining region shift to smaller values with the increasing electric field (Figures S4C,E), i.e. more favourable orientations for Clhydration. In contrast, the orientation of water molecules in the right part of the first hydration shells of Cl- are

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significantly disturbed (Figure 7B and Figure S4B), and the distribution of α for the water molecules in the right part of the second hydration shell and remaining region shift to larger values (Figures S4D,F), implying less favourable orientations. For Na+ and Pb2+, the case is just opposite to that for Cl-, as shown in the inset in Figure 7C. Electric fields make the orientations of water molecules in the left part less favourable for Na+ hydration, as evidenced by the shifting of θ to smaller values (Figure 7C and Figures S5A,C,E). For the water molecules in the right part of Na+, θ for first hydration shell shift to slightly lower value under 2 and 3 V/nm (Figure S5B), while θ for the outer hydration shells gradually shift to high values (Figures S5D,F), indicating more favourable orientations for Na+ hydration. Similar phenomena are observed for Pb2+ (Figure S6). The asymmetry of ion hydration structures is also reflected in the different number of water molecules in the left and right parts of ion hydration shells. As shown in Figure S1, at 2 and 3 V/nm, the number of water molecules in the left part of the first and second hydration shells of Cl- is slightly larger than the right counterpart; there are more water molecules in the right part of the second hydration shells of Na+ and Pb2+ than in the left part. 0.08 -

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Figure 7. Probability distributions of the orientation angle α and θ of water molecules in the left and right parts of the first hydration shells along the field direction (A) for Cl-, Na+ and Pb2+ under 0 V/nm, (B) for Cl- under 0.4 V/nm and (C) for Na+ under 0.2 V/nm. The insets show the water dipole orientation around ions under external field. For comparison, the curves for the whole first hydration shells of Cl- and Na+ under 0 V/nm are shown in panel (B) and (C), respectively. In the hydration shells of ions, there are two competing interactions: water-water interactions (i.e., hydrogen bonding) and ion-water interaction (i.e., ionic hydration). When more hydrogen bonds are formed, the orientation of water molecules in the hydration shell of ions is more severely perturbed, leading to a less favorable ionic hydration. Effects of external field on water oxgen-oxgen radial distribution functions g(r) and the number of hydrogen bonds in the first hydration of ions are analyzed, and the results are shown in Figure 8. It is found that under strong fields, the locations of the first peaks are slightly shifted to smaller values and the peak heights slightly decrease (Figure 8A-C), implying that water-water distance becomes closer in the first hydration shell of ions. It seems that such structural change under strong fields can facilitate hydrogen bond formation among the water molecules in the first hydration shells of ions, as the number of hydrogen bond per water increases with the filed strength (Figure 8D). This observation suggests that apart from the direct perturbation to water dipole orientation, strong fields may further disturb the hydration shell orientation by inducing water molecules therein to form more hydrogen bonds.

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Figure 8. (A-C) Oxgen-oxgen radial distribution functions g(r) and (D) the average number of hydrogen bond (HB) per water for water molecules in the first hydration shell of Cl-, Na+ and Pb2+ under external fields. Ion-water interaction change under external fields can be interpreted by the change of coordination number and hydration shell orientation. Our previous study39 has demonstrated that if the orientation of hydration shell is more favorable for ion hydration, the average interaction between an ion and a single water molecule is stronger. Thus, the interaction between ions and water will be stronger, when ions are coordinated by more water molecules with more favorable orientation. Under strong fields, the interaction energy between ions and water molecules in the first shell of Cl-, Na+ and Pb2+ is weakened due to the less favorable orientation of the hydration shell, though the coordination numbers almost remain unchanged. The slightly enhanced interactions of Cl- and Na+ with the second hydration shell is due to that more water molecules are in the left and right parts of the hydration shell (Figure S1), where the orientation is more favorable for ion hydration (Figures 7B,C). Such interaction enhancement is not seen for Pb2+, as the coordination number for its second hydration shell is slightly reduced. Some different hydration features for Cl-, Na+ and Pb2+ under external fields can be observed. The critical field strength (0.5 V/nm) to perturb Cl- hydration is much higher than that for Na+ (0.2 V/nm) and Pb2+ (0.3 V/nm), as shown in Figure 4. Moreover, the reduction in Cl--water interaction energy is much smaller than that of Na+ and Pb2+. We speculate that these differences may arise from the different water reorientation37 around cations and anions: Cl- is coordinated by one O-H group of a water molecule, which leaves the other O-H group to freely rotate to adjust the water dipole to align with the field direction; Na+ and Pb2+ are coordinated by water oxygen atom with both hydrogen atoms pointing away, i.e. with specific water dipole orientation, which can be easily disturbed by external fields. This can be confirmed from Figure 6A: the orientation of the O-H group directly coordinating Cl- (peak around 10o) remains almost unchanged till under 2 and 3 V/nm, while the other O-H group (peak around 110o) slightly adjusts its orientation. The weakening of the interaction of Pb2+ with the first hydration shell is less severe than that for Na+, mainly attributed to its stronger electrostatic interaction with water. Such different orientation change of water molecules around cations and anions leads to different variation in the

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distance between ions and oxygen or hydrogen atoms of water molecules in the first hydration shell. Under strong fields, both average distances of Cl--oxygen and Cl--hydrogen atoms slightly increase (Figure 9A), whereas the average distances of Na+- hydrogen and Pb2+-hydrogen atoms are slightly shortened (Figure 9B,C). This indicates that the electrostatic interactions between water molecules and Na+ or Pb2+ are less favourable than that for Cl-. The least disturbed hydration of Cl- by external fields may be also related to the fact that Cl- is a structure-breaker species, whereas Na+ and Pb2+ are typical structure-maker ions. Under external fields, compared with Na+ and Pb2+, water molecules in the first hydration shell of Cl- show larger structural changes (Figure 8A), and form more hydrogen bonds (Figure 8D) and exhibit shorter residence times in the shell (Figure 10D). These observations may account for the less disturbed Cl- hydration under external fields. 0.33

0.33 0.30 Average distance (nm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-

-

(A) first shell of Cl 0.31

Cl -O Cl -H

0.30 0.29 0.0

0.32

0.29

0.32

0.5


0.31

+

0.28

+

(B) first shell of Na

Na -O + Na -H

0.27

0.29

0.26

0.28

0.25

0.27

0.24 3.0 0.0

0.5


2+

2+

0.30

3.0

0.26 0.0

(C) first shell of Pb

0.5

Pb -O 2+ Pb -H


3.0

Figure 9. Average distance between ions and oxygen or hydrogen atoms of water molecules in the first hydration shell of (A) Cl-, (B) Na+ and (C) Pb2+ under external fields. Coordination number and hydration shell orientation have been demonstrated as the most important factors for ion hydration.38-39 Previous studies40-42 have reported that ion dehydration, i.e. reducing ion coordination number by stripping some water molecules around ions is an important approach to weaken ion hydration and generate a free energy barrier to block ion transport in nanochannels, which are often seen in the gating of biological ion channels and widely used in membranes for seawater desalination. This study demonstrates that applying strong electric fields can also weaken ion hydration mainly by perturbing water orientation in ion hydration shells, which may be used to control ion transport, such as in nano-fluidic systems and membranes. To evaluate the effects of external fields on the dynamic properties of ionic hydration, we analyze the mean square displacement (MSD) of ions along the field direction (z-axis) and the residence time of water molecules in the first hydration shell of ions, and show the results in Figure 10. The residence time is calculated according to the numeric integration of the residence time correlation function. When the field strength is larger than 0.1 V/nm, the MSD of ions along the field direction is significantly enhanced and increases monotonically with the field strength (Figure 10A-C). In contrast, the MSD of water along the field direction is slightly enhanced under external fields (Figure S7), due to the electro-neutrality of water molecules. Meanwhile, the residence time of water in the first shell decrease rapidly with increasing field strength (Figure 10D). These changes mean that under strong fields, ions diffuse much faster in solution and it becomes easier for ions to escape from their original hydration shells. In the field-free case (0 V/nm), ions and their first hydration shells can move together as an entity for a short time during the diffusion of ions in solution. Under strong fields, the very short residence time of water in the first hydration shell suggests that the concerted motions of ions and their first hydration shells can decouple soon, probably ascribed to the weakened ion-water interaction and the relative slow diffusion of water.

9



2500

2500

(A) Cl

-

+

(B) Na 2

z-axis MSD (nm )

2000

2

z-axis MSD (nm )

2000 1500 1000 500

1500 1000 500 0

0 0

200

400

600

800

0

1000

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400

time (ps)

600

800

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(V/nm) 3 2 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.07 0.05 0.03 0.01 0

time (ps)

2500 160

2+

(C) Pb

(D)

140

2000

Residence Time

2

z-axis MSD (nm )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1500 1000 500

-

120

Cl + Na 2+ Pb

100 80 60 40 20

0 0

200

400

600

800

0 0.0

1000

time (ps)

0.5

1.0

1.5

2.0

2.5

3.0


Figure 10. (A-C) Mean square displacement (MSD) of ions along the field direction (z-axis) and (D) the residence times of water molecules in the first hydration shell of ions under electric fields.

4. Conclusions Molecular dynamics simulations have been performed to study the hydration of Cl-, Na+ and Pb2+ under external uniform electric fields in the range of 0 - 3 V/nm. The results show that with the field strength increasing, water dipoles are reoriented along the field direction and the lifetime for water hydrogen bonds is prolonged, while the water coordination numbers and the average number of hydrogen bonds per water remains almost unchanged. Such uniformly aligned water dipoles induced by external fields disturb ion hydration profoundly. The local radial electric fields of ions orient surrounding water molecules with specific orientations and resist the disturbance of weak fields. Under strong fields, the interaction energy between ions and the first hydration shell is greatly weakened due to the severely distorted hydration structure in spite of the unchanged coordination number. Moreover, strong fields induce water molecules in the first hydration shell to form more hydrogen bonds and further disturb the hydration shell orientation. Ion hydration structures become asymmetrical along the field direction: water molecules are well oriented for ion hydration in one direction, while those are badly oriented in the opposite direction. Interestingly, for Cl- and Na+, their interaction energy with the second hydration shell is even slightly enhanced under external fields, as more water molecules are optimally oriented for ion hydration. This phenomenon is not observed for Pb2+ due to the slight dehydration. The hydration of Cl- is less severely disturbed by external fields than that of Na+ and Pb2+, probably ascribed to the different water reorientation around anions and cations as well as the different structure-maker/breaker nature of the ions. The critical field strength to perturb Cl- hydration is much higher than that for Na+ and Pb2+. Under strong fields, ion mobility is significantly enhanced and the water residence time in the hydration shell is dramatically shortened. This work demonstrates that perturbing water orientations in ion hydration shells by applying strong electric fields is an effective way to weaken ion hydration, which may be used to control ion transport in nano-fluidic systems or membranes.

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Supporting Information The Supporting Information is available free of charge on the ACS Publication website. Ion coordination numbers, distributions of the orientation angle α and θ for water molecules in the hydration shells of Cl-, Na+ and Pb2+, and the mean square displacement of water molecules along the field direction.

Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 21506178).

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Electric-Field Effects on Ionic Hydration: A Molecular Dynamics Study

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