Electrolytes under Inhomogeneous Nanoconfinement: Water

Subscriber access provided by Nottingham Trent University

Chemical and Dynamical Processes in Solution; Polymers, Glasses, and Soft Matter

Electrolytes under Inhomogeneous Nanoconfinement: Water Structuring-Mediated Local Ion Accumulation Hu Qiu, and Wanlin Guo J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b02139 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 12, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 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

The Journal of Physical Chemistry Letters

Electrolytes under Inhomogeneous Nanoconfinement: Water Structuring-Mediated Local Ion Accumulation Hu Qiu* and Wanlin Guo* State Key Laboratory of Mechanics and Control of Mechanical Structures and Key Laboratory for Intelligent Nano Materials and Devices of MOE, Institute of Nano Science, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, China AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] *E-mail: [email protected]

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry Letters 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

Page 2 of 28

ABSTRACT. The behaviors of aqueous electrolytes confined in nanoscale spaces impact a broad range of biological processes and industrial applications. Current microscopic understanding of confined electrolytes relies primarily on ideal model systems featuring homogeneous nanoconfinement. Here, we investigate the structure and dynamics of various electrolytes subject to inhomogeneous nanoconfinement, i.e., confined in two-dimensional nanochannels with gradually varying local channel height, by means of molecular dynamics simulations. Our results reveal unexpected local ion accumulation in the inhomogeneous space occurring at boundaries between coexisting structured water phases, including tri-layer, four-layer and bulk-like waters. This contrasts markedly with the intuition that hydrated ions are more favorable to weakly confined regimes due to steric exclusion effect. We further show that the location and intensity of the water structuring-mediated ion accumulation are sensitive to the nanochannel’s geometry and surface wettability. The revealed anomalous ion behaviors in inhomogeneous nanoconfinement should help to understand microscopic mechanism underlying the operation of biological ion channels and to develop functional nanofluidic devices.

TOC GRAPHIC

KEYWORDS Confined liquid; Solid-liquid interface; Wettability; Phase behavior; Molecular dynamics


2

Page 3 of 28 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


Understanding the behaviors of electrolytes in nanoconfined geometries can provide fundamental insights into a wide variety of scientific and technological processes. As an example, ion diffusion across cell membranes through nanoscale tunnels of protein channels has been investigated for several decades of intense research due to its essential role in electrical signaling in neurons.1 Alternatively, advances in nanotechnologies have allowed nanoconfined electrolytes to be routinely examined and manipulated in functional mimics of biological protein channels, namely, nanoscale channels (or nanochannels) created with artificial structures.2-9 For instance, Geim and co-authors experimentally fabricated two-dimensional (2D) nanochannels of several angstroms in height through van der Waals assembly of 2D crystals such as graphene and MoS2, and demonstrated mass transport through these narrow artificial channels.6,

10-11

Their results

revealed that hydrated ions such as K+ and Cl- could permeate through channels of ~6.7 Å in height,10 but were completely blocked when the channel height was reduced to ~3.4 Å,11 due to the steric exclusion effect. Other discoveries relating to anomalous mass transport and redistribution phenomena in nanochannels have also emerged, with diverse potential applications in water purification,12 single-molecule sensing,13 ionic current rectification and energy conversion devices.14-15 Besides experiments, molecular-level insights into such systems could be acquired by using computational protocols. For instance, suppressed ionic mobility was predicted for Na+ ions placed in a cylindrical 1D channel by molecular dynamics (MD) simulations.16 Simulations also suggested that ions had a strong tendency to retain the first hydration shell even under extremely severe confinement of 2D nanochannels with a height of 8 Å, albeit with a reduced hydration number with respect to the bulk.17 Several other groups also reported simulations results on anomalous ion behaviors inside 1D18-20 and 2D nanochannels.21-26 Despite the tremendous amount


3


Page 4 of 28

of studies, the model systems typically focus on homogeneous nanoconfinement, such as 2D nanochannels constructed usually by two parallel flat plates.21-26 By contrast, very little attention was devoted to systems under inhomogeneous nanoconfinement. In a previous report, our group examined the structure and dynamics of pure water under inhomogeneous nanoconfinement and found the coexistence of multiple liquid structures of single-, bi- and tri-layer water; each structural phase was solidified into ice nanoribbon on being compressed.27 However, how ions behave in inhomogeneously nanoconfined electrolytes remains ambiguous. In the present work, we have systematically investigated the structure and dynamics of inhomogeneously confined electrolyte solutions, particularly focusing on how ions redistribute under various confining geometries and solution conditions. Figure 1a illustrates the simulation system considered in the present work, consisting of a top convex plate with radius R and a bottom flat plate. A salt solution of 1 M NaCl is placed in the inhomogeneous space between the two plates. Other details relevant to the simulation protocols are described in Computational Methods. After a sufficiently long (usually in the range of 600 ns and 2 μs) system relaxation, the density of Na+ ions in a slab of 5 Å in thickness located at various x was recorded and averaged over all considered frames of simulation trajectories, as shown in Fig. 1b. Note that, hereafter all the results shown were obtained by averaging the data over two halves of our system, as it is symmetric about the middle plane of x = 0. It is found that, as expected, the degree of nanoconfinement can dramatically influence the Na+ distribution in the system. Under very weak inhomogeneous confinement with a large nominal channel height d0 (e.g., at d0 = 4.0 nm), there is no evident preference of ion distribution, as indicated by a nearly flat density profile (black curve in Fig. 1b). This is not surprising because no strong deformation or shedding of Na+ ions’ hydration shells occurs throughout this wide nanospace. When d0 reduced to 2.0 nm, uneven ion distribution was


4

Page 5 of 28 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


seen in the density distribution profile (blue curve in Fig.1b). At first sight we found a higher ion density at a larger x (corresponding to also a larger local channel height). This is reasonable as ions prefer to occupy weakly confined regions where the interplay of ions’ hydration shells with confining plate surfaces is relatively weak. However, this trend is not strict under inhomogeneous confinement, because the Na+ density curve is indeed not monotonic, but rather, exhibiting two local peaks (marked by red stars). This finding suggests that Na+ ions accumulate locally at certain positions in the inhomogeneous system, an observation that is not expected.

Figure 1. Lateral ion distribution in inhomogeneously confined electrolytes. (a) Simulation setup. An inhomogeneous nanochannel is made of a top convex plate with a radius R and a bottom flat plate, containing an aqueous salt solution. The resulting inhomogeneously nanoconfined solution is in spirit similar to the water meniscus condensed between scanning probe microscopy


5


Page 6 of 28

(SPM) tips and sample surfaces under ambient conditions. Ions are shown as spheres and water is represented by a semitransparent surface. The coordinate origin is placed at the geometrical center of the bottom plate. The nominal height d0 of the inhomogeneous nanochannel is defined as the gap between the flat horizontal subsections of the two confining plates. (b) Distribution of number density of Na+ ions along the x direction in 1 M NaCl electrolytes confined at various d0. The stars denote local ion density peaks, P1 and P2. As the system is symmetric about x = 0, all the results shown are obtained by averaging the data over two halves of our system. Vertical dashed line illustrates the outer boundary of the central inhomogeneous region, beyond which the system is homogeneous. (c,d) Position (xP1 and xP2; defined in inset of panel c) and local channel height (dP1 and dP2) of ion density peak P1 (c) and P2 (d) versus d0. Data were extracted from those in panel b and also Fig. S1a. Further decrease of d0 to 1.7 nm retains the trend of ion density distribution, albeit with shifted peak positions and heights (red curve in Fig. 1b). We further noted that ion density almost vanishes in the narrowest central region in the d0 = 1.7 nm system, suggesting the absence of ions due to steric repulsion. By compiling the data at d0 = 2.0 and 1.7 nm in Fig. 1b and those at other d0 values given in Fig. S1a of the Supporting Information, we plotted in Fig. 1c and 1d variation of position and local channel height (defined in Fig. 1c inset) of two ion density peaks with respect to d0. With increasing d0, the positions of the two peaks, xP1 (black curve in Fig. 1c) and xP2 (black curve in Fig. 1d), both exhibit a downward trend, namely, shifting leftward toward the channel center. By contrast, the local channel heights where the peak occurs remain almost constant for all considered d0, as indicated by the almost flat curves for dP1 and dP2 (red curves in Fig. 1c and 1d). These observations suggest that the presence of anomalous ion accumulation is ubiquitous under


6

Page 7 of 28 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


sufficiently strong inhomogeneous confinement and its position can be regulated by the nominal channel height d0. We also examined how ion distribution depends on the radius of the top confining plate as well as the solution properties. Variations in the top plate radius (Fig. S1b), nominal salt concentration (Fig. S2a), cation types (Fig. S2b) and even water models (Fig. S2c) can merely affect positions or heights of density peaks, indicating the robustness of the observed anomalous ion behavior. In particular, a test simulation at a lower salt concentration at 0.2 M persists the trend in anomalous ion accumulation (see Fig. S2a). Likewise, the anion density distribution in the solution (i.e., Cl-) also exhibits two local ion accumulation peaks (Fig. S3a), though at slightly shifted x positions with respect to Na+. Besides, the intensity of Cl- density at peak P2 is not as significant as Na+. Since the local ion accumulation occurs only at very small d0, it is likely relevant to the altered microscopic structure and dynamics of water under severe confinement. It is well known that liquids exhibit a stratified structure near solid surfaces.21,

28

Likewise, this stratified structure

persists in the present inhomogeneous nanoscale space, indicated by the well-defined peaks and valleys in transverse density profiles (TDP; along z direction) of water oxygen atoms (Fig. 2a). However, the distinct TDP curves at various x positions suggest that the inhomogeneous system features the coexistence of different liquid water structures. This observation is in line with our previous work on inhomogeneously confined pure water, namely in the absence of ions.27 To distinguish local water structures from each other, they were named after the number of peaks in the TDPs (highlighted by red ribbons): tri-layer (3L; containing three peaks), four-layer (4L; containing four peaks) and bulk-like (i.e., featuring intermediate bulk-like water in the central region of the lamellae) water. The most striking feature in these TDPs is that the aforesaid peaks (see Fig. 1b; also labeled by stars in Fig. 2a) match exactly the boundary positions between two


7


Page 8 of 28

adjacent water structures. Specifically, the peak P1 at x = 25 Å was found at the transition region between 3L and 4L water, and P2 at x = 50 Å was seen at the boundary between 4L and bulk-like water. Similar consistency persists in a system with a slightly lower d0 at 1.7 nm (Fig. S4a). Under a much larger d0 at 4.0 nm, the system contains uniformly bulk-like water (Fig. S4b) and therefore exhibits no anomalous ion distribution (see black curve in Fig. 1b). The coexistence of multiple water structural phases under inhomogeneous nanoconfinement is also supported by instantaneous configurations of the system sketched in Fig. 2b, Fig. S4c and S4d.

Figure 2. Abrupt change in water structuring induces local ion accumulation. (a) Local transverse density distribution of water oxygen atoms (normal to the bottom plate) in an inhomogeneously confined NaCl solution at d0 = 2.0 nm. The local water density inside each 5-Åthick slab centered at position x, shown in arbitrary units, was obtained by averaging instantaneous


8

Page 9 of 28 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


water transverse density distribution in this slab over considered frames of MD trajectory. The profiles are shifted horizontally to the center of each slab for clarity. All equivalent peaks in the profiles are connected by red ribbons. The number of water layers (nL) in each slab, defined as the number of peaks, are shown on top of corresponding profiles. Confined water with four transverse density peaks is treated as “4L” if the distance between two central peaks is shorter than that between a primary peak and its nearest central peak, otherwise as “bulk-like”. The star indicates the position where local ion accumulation occurs, same as in Fig. 1b. (b) In-plane (top panel) and out-of-plane views (bottom panel) of the system showing the instantaneous configuration of the inhomogeneous system. Oxygen atoms, hydrogen atoms, Na+ and Cl- ions are shown as red, white, blue and cyan spheres, respectively. The top confining plate is not shown for clearer visualization. We further probed the dynamical and energetic properties of ions under inhomogeneous nanoconfinement, which can be described by the lateral diffusion coefficient Dxy and potential of mean force (PMF), respectively. It is found that a nearly perfect inverse correlation is established between the distribution profiles of lateral diffusion coefficient (red) and local density (blue) of Na+ ions (Fig. 3a). In particular, the ionic mobility is relatively low in regions where local ion accumulation occurs (labeled as P1 and P2). However, the high diffusion coefficient of Na+ on the order of 10-5 cm2/s throughout the system (including the accumulation region) indicates that these ions are not statically adsorbed in the accumulation region, but rather, are highly mobile and can exchange freely with the rest ions or water molecules in the solution. Figure 3b shows the determined PMF profile for a Na+ ion moving along the x direction. As expected, the overall shape of the PMF profile is comparable to that of the Dxy profile, again inversely correlating with the ion density profile. In particular, the PMF energy barrier during ion movement along the channel is as low as 0.7 kcal/mol, in line with the observation of high ion mobility throughout the channel.


9


Page 10 of 28

Figure 3. Dynamics and energetics of lateral Na+ movement under inhomogeneous confinement at d0 = 2.0 nm. (a) Lateral diffusion coefficient Dxy of Na+ ions at different x positions. (b) PMF profile of a Na+ ion moving along the x direction. The ion density profile (blue curve) is shown for reference. To better understand the origin of the observed local ion accumulation, we characterized each ion density peak by comparing the microscopic structural features of hydrated ions in electrolytes around the peaks. We first examined the transverse density distribution of ions within three different regions around peak P1, namely, inside 3L region (left panel in Fig. 4a), 4L region (right panel) as well as the transition region between them (middle panel). A few common features emerge in all ion density profiles. For instance, structural layering also occurs in ion distribution,


10

Page 11 of 28 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


analogue to water layering, as indicated by the presence of Na+ density peaks (solid lines in Fig. 4a). In addition, Na+ ions tend to keep way from the channel surface. In other words, almost no Na+ ions show up around the primary (highest) peaks in the water density profile (dashed lines), which are next to the surface. This is likely because of the tendency of ions to retain their hydration shell by not approaching too close to the confining surfaces, which was also noticed in previous work.21, 26 In the 3L region leftward to the peak P1, a very high central peak and two minor side peaks exist in the Na+ density profile (left panel in Fig. 4a). In the 3L-4L transition region, the central peak decreases, while the two side peaks both significantly increase and become higher than the central peak (middle panel), resulting in the aforesaid local ion accumulation. Finally, in the 4L region rightward to P1 (right panel), the central peak completely disappears, leading to a relatively lower Na+ density compared to the transition region. Unlike P1, a quite different picture is observed around the peak P2 (Fig. 4b). It is the evident rise of the central peak (middle panel in Fig. 4b) that contributes to the local Na+ ion accumulation at P2, with respect to the mild central peak in the 4L region (left panel in Fig. 4b). In addition, no strong positional preference of Na+ occurs in the bulk-like region, as indicated by the absence of sharp peaks (right panel in Fig. 4b).


11


Page 12 of 28

Figure 4. Characterization of ion density peaks in the d0=2.0 nm system. (a,b) Transverse density distribution profiles for Na+ (solid lines) in confined electrolytes at different positions around peak P1 (a) and P2 (b). The left, middle and right panels illustrate data for electrolytes located leftward, at, and rightward each peak, respectively. The transverse water density (dotted lines) is shown for reference. Note that the units for ion density and water density are not identical. (c,d) Ion-oxygen RDFs of Na+, gNa+-O, at different positions around P1 (c) and P2 (d). The RDFs of Na+ in bulk solutions are shown as black dashed lines. Vertical dotted lines in the inset indicate the location of the outer boundary of the second hydration shell of Na+, which coincide with each other for Na+ around P2 (panel d). The distinct structural features of electrolytes surrounding P1 and P2 can also be supported by exploiting ion solvation properties through ion-water radial distribution functions (RDFs) of Na+


12

Page 13 of 28 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


around each peak, as shown in Fig. 4c and 4d. At first glance, there is no significant difference between the RDF profiles for Na+ ions residing in different regions of the confined electrolytes (solid lines) and in bulk solutions (dotted lines). This fact suggests that Na+ ions retain an almost complete solvation shell throughout our nanoconfined system. In particular, the curves almost coincide with each other in regimes corresponding to ion’s first hydration shell (i.e., r does not exceed the first minimum). Accordingly, no anomalous feature is seen in the first coordination number (defined in Fig. S5c) for ions in the transition regions at each peak (Table 1). In other words, the influence of confinement on the first solvation shell should be irrelevant to the local ion accumulation observed in our inhomogeneously confined electrolytes. In sharp contrast, we do observe abnormal features for the second solvation shell of Na+ ions at different positions around ion accumulation peaks P1 and P2. According to the zoom-in view of RDF (inset in Fig. 4c), ions in the 3L-4L transition region at P1 (i.e., x = 25 Å, red curve) exhibit the most distant boundary position of their second shells (marked by vertical red dotted line; also listed in Table 1), leading to the highest second coordination number (Table 1; defined in Fig. S5c). This trend is in exact agreement with that seen in the ion density curve (Fig. 1b). Likewise, the highest second coordination number can also be also found in the transition region between 4L and bulk-like water, namely at P2 (Table 1), although the second shell boundary positions of all considered cases are identical (inset of Fig. 4d). In this case, it is the highest peak intensity for the second solvation shell in the transition region (red curve in inset of Fig. 4d) that leads to the largest second coordination number. Despite owning different microscopic origins, the locally favored ion solvation (particularly the second solvation shell) in the transition regions between different water structural phases under inhomogeneous nanoconfinement should be responsible for the observed local ion accumulation. We have shown that Na+ ions in the transition


13


Page 14 of 28

regions do not statically bind therein, but instead, can exchange freely with other ions or water molecules, as indicated by the still high diffusion coefficients on the order of 10-5 cm2/s (see Fig. 3a) and slight energy barriers of

Electrolytes under Inhomogeneous Nanoconfinement: Water

Recommend Documents