Structure of Water in Zn2+ Aqueous Solutions from Ambient

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Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX

Structure of Water in Zn2+ Aqueous Solutions from Ambient Conditions up to the Gigapascal Pressure Range: A XANES and Molecular Dynamics Study Valentina Migliorati,*,‡ Adriano Filipponi,† Andrea Di Cicco,§ Simone De Panfilis,# and Paola D’Angelo*,‡ ‡

Dipartimento di Chimica, Università di Roma “La Sapienza”, P.le A. Moro 5, 00185 Roma, Italy Dipartimento di Scienze Fisiche e Chimiche, Università degli Studi dell’Aquila, Via Vetoio, 67100 L’Aquila, Italy § Sezione di Fisica, Scuola di Scienze e Tecnologie, Università di Camerino, 62032 Camerino (MC), Italy # Centre for Life Nano Science - IIT@Sapienza, Istituto Italiano di Tecnologia, V.le Regina Elena 291, 00161 Rome, Italy †

ABSTRACT: The structural modifications induced on a 0.5 M Zn2+ aqueous solution by increasing the pressure to 6.4 GPa were investigated using a combination of X-ray absorption near edge structure (XANES) spectroscopy and molecular dynamics (MD) simulations. The Zn K-edge XANES experimental spectra show two different trends depending on the pressure and temperature conditions of the system. On the one hand, when the pressure is increased to 1.0 GPa while keeping the temperature at 300 K, the highly structured nature of Zn2+ second hydration shell is preserved. On the other hand, when the Zn2+ aqueous solution is simultaneously pressurized and heated to follow the melting curve above 1.0 GPa, the Zn2+ second shell loses its high degree of structuring and becomes much more disordered and unstructured. These results are confirmed by the analysis of MD simulations of Zn2+ aqueous solutions under high pressure. By combining distance and angular distribution functions it is possible to highlight the loss of water structuring in the Zn2+ second coordination shell that takes place upon pressurization and heating. A progressive crowding of the Zn2+ second shell is observed with increasing pressure; the water structure becomes remarkably different from that found at ambient conditions, and for pressure values higher than 1.0 GPa the tetrahedral arrangements of water molecules is highly distorted. Moreover, MD simulations of Zn2+ aqueous solutions performed at 1.0 GPa and at increasing temperature values have shown that the loss of water structuring in the Zn2+ second coordination shell observed by simultaneously pressurizing and heating is due to a combined effect of pressure and temperature, both producing an increase of the Zn2+ secondshell disorder.

1. INTRODUCTION Knowledge of ion solvation properties in water under pressure is of key relevance in diverse fields of science ranging from planetary modeling to geology.1−3 For instance, it is of fundamental importance to understand the behavior of aqueous solutions in the Earth mantle1 and to determine the pressure effects on the rates and mechanisms of many chemical,4−6 biological,7,8 and geological processes.2,9 Despite this broad interest, only a few studies on the properties of aqueous solutions containing salt species under pressure have been published so far.10−14 From an experimental point of view, this is mainly due to the technical challenge of accessing extreme temperature and pressure conditions. Among the experimental techniques, X-ray absorption spectroscopy (XAS) is the structural probe of choice to investigate the coordination structure of ions in solution, due to its short-range order sensitivity and atomic selectivity.15−23 For this reason, it has been widely employed in the past to gain structural © XXXX American Chemical Society

information on ion complexes in aqueous and nonaqueous solutions.24−38 Moreover, in the last years it has been shown that the low-energy region of the XAS spectrum, termed X-ray absorption near edge structure (XANES), is sensitive to both the first and the second hydration shell of a metal ion dissolved in water.39−44 On the computational side, molecular dynamics (MD) simulations are a powerful tool to provide atomic-level insights into ion−solvent interactions, allowing one to obtain a global structural picture of the solvent arrangement both in proximity of the ion and in the long-distance range.45−50 However, when performing MD simulations, it is always important to assess the reliability of the theoretical results by comparing them with some experimental data. Received: August 21, 2017

A

DOI: 10.1021/acs.inorgchem.7b02151 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Herein, we use MD simulations in combination with the XANES spectroscopy to investigate the structural properties of Zn2+ aqueous solutions in a wide range of pressure up to 6.4 GPa. In a recent MD and extended X-ray absorption fine structure (EXAFS) study we determined the pressure effects on the first coordination shell of the Zn2+ hydration complex, showing that the octahedral structure, which is characteristic of the Zn2+ aqua ion at ambient conditions, remains stable also under high pressure, while the Zn−O distance is shortened, and the solvation complex becomes more disordered.51 Here, XANES spectroscopy in conjunction with classical MD is used to provide a global structural picture of the modifications induced on Zn2+ aqueous solutions when the pressure is increased from atmospheric to 6.4 GPa, where water exists in a compressed liquid phase with very high density.52−58

2. METHODS 2.1. X-ray Absorption Measurements. Zn K-edge XAS spectra were collected at the BM29 (now moved to BM23) beamline59 of the European Synchrotron Radiation Facility using the large-volume highpressure setup based on the Paris−Edinburgh press. The storage ring was operating in uniform mode with typical currents in the 200−160 mA range between refills. The bending magnet source was monochromatized with a Si(311) energy scanning double crystal monochromator and vertically focalized to ∼50 μm using a Rh-coated bent mirror at 3.5 mrad incidence for harmonic rejection.60 The horizontal beam size was set by secondary slits to 0.8 mm to probe the central part of the sample container. The sample was a 0.5 M Zn2+ aqueous solution obtained by dissolving the proper amount of Zn(NO3)2 in water. This sample concentration was optimized to collect high-quality transmission spectra, and, due to the high absorption of the gasket material, the high beam intensity and spectral purity achievable with the mirror setup were also essential for this purpose. The sample was confined in a Teflon cylindrical container with a 1.1 mm internal cylindrical cavity inserted in a doubly conical 7 mm boron-epoxy gasket, and was compressed at the required pressure. For pressures higher than 1.0 GPa, it was simultaneously heated by passing a suitable current through the cylindrical graphite resistor surrounding the sample container using electrical cables connected with the upper and lower tungsten carbon anvils. Ancillary techniques such as sample temperature scanning capabilities61 and an image plate detector (MAR) for the collection of X-ray diffraction (XRD) from the crystalline sample powder components were also used for sample check and calibration purposes. At a fixed pressure the sample behavior of the H2O−Zn(NO3)2 mixture is understood in terms of the corresponding temperature− composition diagram.62 The sample concentration of Zn(NO3)2 is 0.5 M, and at low temperature the sample is a mixture of crystals of ice (of the stable phase) and hexahydrated salt. When the eutectic temperature is reached the salt part melts together with the required amount of water to reach the eutectic composition. When the temperature is increased the ice component progressively melts, until the melting point for the sample composition, which is just a few kelvin below the ice Tm. The sample behavior is monitored through temperature scans, as previously illustrated,63 and the MAR detector. The sample history on the (P,T) diagram is illustrated in the Results section (see panel b of Figure 1). All measurements were taken either at room temperature or (above 1 GPa) at the lowest temperature where a homogeneous fluid solution is obtained. This temperature coincides, within uncertainties, with the melting line of pure water reported in Figure 1b. The resulting uncertainty of the corresponding (P,T) values along the melting line is estimated to be ∼8%. Additional details on XAS data acquisition can be found in ref 51. 2.2. MD Simulation Details. MD simulations of the Zn2+ ion in aqueous solution at different temperature and pressure conditions were performed using an effective two-body potential to describe the Zn2+-water interactions, which was obtained by fitting the parameters

Figure 1. (a) Pressure dependence of the Zn K-edge XANES experimental spectra of the Zn2+ aqueous solutions. (b) Thermodynamic parameters of the Zn2+ aqueous solutions corresponding to the XAS experimental measurements (magenta ●) and to the MD simulations (blue ●). The phase diagram of pure water is reported for reference. (c) Zn K-edge XANES experimental spectra of the Zn2+ aqueous solutions measured at different pressure values. The spectra measured at room temperature are shown in different shades of blue, while those collected by increasing both sample temperature and pressure are reported in different shades of red. The spectra are the same shown in panel (a). The arrow indicates the structural feature at 9680 eV (see text).

of a suitable analytical function on an ab initio potential energy function as described in detail in ref 64. In particular, the Zn2+-water two-body potential has the following analytical function: V (r ) = f * +

qiqo rio

∑ ih = ih1,ih2

+

Ao B C Do + o6 + o8 + 12 + Eoe−Forio 4 rio rio rio rio

qiqh rih

+

Ah B C Dh + h6 + h8 + 12 rih 4 rih rih rih

(1) −1

−2

where f is the electric conversion factor (138.935 kJ mol nm e ), rio, rih1, and rih2 are the ion−water distances; qi, qo, and qh are the electrostatic charges of Zn2+ and of the oxygen and hydrogen in the water model used in the simulations, namely, the SPC/E65 (2.0, −0.8476, and 0.4238 au, respectively). A0,....,F0 and Ah,....,Dh are the parameters obtained by the fitting procedure (see ref 64). To include this ion−water effective pair potential in the MD force field, a modified version of the GROMACS package was employed to perform the simulations. B


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Inorganic Chemistry

0.09 Å.51 It is important to stress that the high-energy region of the XAS spectra is only sensitive to the structural contribution associated with the first coordination shell in aqueous solutions.69−71 Conversely, the second-shell water molecules were found to provide a detectable contribution to the XANES spectra of metal cations having a well-defined and stable first hydration shell such as those for Ni2+,40 Zn2+,41 Cr3+,39 Rh3+,39 Ir3+,34 for heavier cations such as Cd2+,72 Hg2+,42 and for some trivalent lanthanide ions.43,73 In agreement with the previously reported results51 the XAS spectra reported in Figure 1 are very similar for energy values higher than 9700 eV, while strong variations are evident in the low-energy region of the spectra. In particular, the white line is clearly distorted upon increasing pressure, and the structural feature at 9680 eV (arrow in Figure 1) is flattened out at high temperature. On the one hand, the shape of the white line depends on several factors: symmetry of the molecular orbitals, nature of the ligands, and structural contribution of the higher distance coordination shells. On the other hand in a recent work it has been shown that the feature at 9680 eV in the XANES spectrum of Zn2+ in aqueous solution at ambient conditions is the fingerprint of the second hydration shell.41 This feature is present in the Zn2+ aqueous solution spectrum at ambient conditions, since the second hydration sphere is highly populated and structured.41,74 Conversely, the feature at 9680 eV is not present in the XANES spectrum of Zn2+ in methanol solution despite its strong overall similarity with the water spectrum (see Figure 1 of ref 41), because in this solvent the second coordination shell is less populated and less structured. Starting from these observations it is possible to draw some conclusions on the structural organization of the second hydration shell when the pressure and temperature are increased. In particular, in all of the XANES spectra collected at 300 K with increasing pressure up to 1.0 GPa the peak at 9680 eV is clearly visible (blue spectra in Figure 1c) indicating that the high degree of structuring of the second shell is preserved also at high pressure. Conversely, when both temperature and pressure are increased to follow the melting curve, the intensity of this peak starts decreasing, and it disappears for pressure values higher than ∼3 GPa (red spectra in Figure 1c). This behavior suggests that along the melting curve the Zn2+ second hydration shell loses its high degree of structuring, becoming more disordered. As it is quite difficult to predict how the hydrogen-bond network and the water structure are modified in the compressed water region, due to the simultaneous increase of both temperature and pressure and expected interplay of several effects, we performed MD simulations that are able to provide a microscopic description of the solutions under investigation. 3.2. Radial Distribution Functions: The Zn2+ Second Hydration Shell. To characterize the structural modifications that take place in the Zn2+ second hydration sphere when the pressure is increased from atmospheric to several gigapascals, we performed MD simulations of Zn2+ aqueous solutions in different points of the phase diagram (see Figure 1b). Note that in Figure 1b we showed the phase diagram of pure water for reference. From an experimental point of view, when simultaneously increasing the system pressure and temperature, for each given pressure the measurements were performed at the lowest possible temperature to have a homogeneous liquid phase. On the theoretical side, the pressure, temperature, and density values reported in Table 1 were used to perform the simulations. In this framework, it is important to stress that the

Eleven simulations of Zn2+ in aqueous solution were performed in 11 different points of the (P,T) phase diagram. The pressure and temperature conditions used in the MD calculations are shown in Figure 1b and listed in Table 1 together with the density values of the

Table 1. Pressure, Temperature, and Density Conditionsa Used in the MD Simulations of Zn2+ Aqueous Solutions

a

P (GPa)

T (K)

density (g/L)

box edge (Å)

1.0 × 10−4 0.20 0.60 0.80 1.00 2.50 4.50 6.02 1.00 1.00 1.00

300 300 300 300 300 390 495 542 390 495 542

1002.9 1071.1 1169.2 1205.8 1271.2 1358.8 1458.9 1523.2 1192.8 1141.9 1119.2

29.0 28.4 27.6 27.3 27.1 26.3 25.6 25.3 27.4 27.8 28.0

The box edge lengths of the simulated systems are also listed.

systems. Note that the density values are those of pure water under high pressure and have been obtained by means of a modified version of the Steam tables code.66 The systems were composed by one Zn2+ ion and 819 water molecules in a cubic box, replicated by means of periodic boundary conditions. The simulations were performed in the NVT ensemble using the Berendsen thermostat67 (coupling constant of 0.1 ps), and to reproduce the chosen density, temperature, and pressure conditions, different box volumes were employed (see Table 1). Note that we decided to perform the MD simulations in the NVT ensemble, since it allows one to better reproduce the experimental conditions as compared to the use of the NPT one. Indeed, on the one hand by using NVT it is possible to perform the simulations at the system experimental densities; on the other, it is well-known that the pressure value for a simulation box oscillates significantly, since it is calculated as a microscopic pressure from the forces arising from intermolecular interactions. To give an idea of the amplitude of the fluctuations, for systems like those investigated in our work, they are typically of the order of tens of megapascals. A cutoff of 9 Å was used to deal with nonbonded interactions, with the particle mesh Ewald (PME) method to treat long-range electrostatic effects.68 A homogeneous background charge was used to compensate for the presence of the Zn2+ ion. The simulations were performed for 10 ns, after 5 ns of equilibration. The time step was 1 fs, and a configuration every 25 time steps was saved.

3. RESULTS 3.1. XANES Spectra. The Zn K-edge XAS spectra of Zn2+ aqueous solutions collected at different pressure and temperature values are shown in Figure 1a, while the corresponding sample history on the (P,T) diagram is illustrated in panel (b). The sample was first pressurized at room temperature by progressively increasing the pressure from atmospheric (0.10 MPa) to 1.0 GPa. Then both the temperature and pressure were increased, and the melting line of the solution was followed up to 6.4 GPa and 550 K. In a previous work we analyzed the evolution of the EXAFS spectra of Zn2+ in water in a wide range of pressure and temperature.51 With increasing pressure and temperature both the frequency and the amplitude of the EXAFS oscillations showed a slight variation due to a very small modification of the Zn2+ first hydration shell. In particular, along the melting curve the Zn2+ hydration complex was found to retain an octahedral coordination with a shortening of the Zn−O distance up to C


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Therefore, our results show that a significant modification of the Zn2+ second hydration shell takes place when the pressure and the temperature are simultaneously increased at 2.50 GPa and 390 K, at 4.50 GPa and 495 K, and at 6.02 GPa and 542 K. To decipher if the structural changes are due to pressure or temperature effects, we performed three MD simulations of Zn2+ aqueous solutions by keeping fixed the pressure at 1.0 GPa and increasing the temperature of the systems at 390, 495, and 542 K (Figure 1b). The Zn−O g(r) second peaks calculated from the three high-temperature simulations are shown in Figure 3a, compared with the g(r) obtained at 1.0

phase diagram of the SPC/E water model is very different from that of real water, as shown in detail in ref 75. However, this is not an issue, since our aim is to simulate the liquid phase, and at the chosen (P,T) values we are well within the liquid existence range of the SPC/E water phase diagram.75 We first focus on the results obtained by keeping fixed the temperature at 300 K and then by simultaneously increasing temperature and pressure. Figure 2a shows the Zn−O radial

Figure 2. Second peak of the Zn−O radial distribution functions g(r) values calculated from the MD simulations of Zn2+ aqueous solutions performed at different pressure values and temperature values (a) and corresponding running coordination numbers N(r) values (b). Figure 3. Second peak of the Zn−O radial distribution functions g(r)’s calculated from the MD simulations of Zn2+ aqueous solutions performed at 1.0 GPa and different temperature values (a) and corresponding running coordination numbers N(r)’s (b).

distribution functions (g(r)’s) second peak calculated from the MD simulations, and two different trends can be observed. In the pressure range between 0.10 MPa and 1.0 GPa, where the temperature is kept fixed at 300 K, the peak position slightly shifts toward lower distances, and the most evident change that takes place involves the peak minimum at ∼5.0 Å, which undergoes a shift and an increase of intensity. Conversely, when the pressure and the temperature are increased at 2.50 GPa and 390 K, at 4.50 GPa and 495 K, and at 6.02 GPa and 542 K the shape of the peak significantly changes: the Zn2+ second coordination shell shifts toward the metal ion but expands also outward, as more and more water molecules enter it. A clear picture of the significant increase of the number of water molecules at a given distance from the ion that occurs with increasing pressure can be obtained by looking at the Zn−O running coordination numbers (Figure 2b). The Zn−O short distance region (between 3.0 and 4.0 Å), where almost no water molecules are present up to 1.0 GPa, at higher pressures is able to accommodate more and more water molecules (the running coordination number at 4.0 Å is lower than 1.6 up to 1.0 GPa, and it is 2.8, 4.3, and 5.2 at 2.50, 4.50, and 6.02 GPa, respectively). At higher distances, the pressure effect is even more evident, with a progressive crowding of the Zn2+ second shell (as an example at 5.0 Å the running coordination number is 11.7, 12.4, 13.9, 14.6, 15.4, 19.0, 21.4, and 22.8 for pressure values of 0.01 MPa, 0.20, 0.60, 0.80, 1.0, 2.50, 4.50, and 6.02 GPa, respectively), since an increasing number of water molecules are pushed toward the ion.

GPa and 300 K. As expected, on the one hand, a significant broadening of the Zn−O g(r) second peak is found with increasing temperature, due to the increase of thermal disorder at high temperatures. On the other hand, the pronounced shift of the Zn2+ second coordination shell toward the metal ion observed by simultaneous pressurization and heating does not take place when only the temperature is increased. This finding is also evident from the trend of the Zn−O running coordination numbers (Figure 3b) that slightly decrease with temperature, at variance with the relevant increase of the Zn2+ second-shell coordination numbers obtained at pressure values higher than 1.0 GPa. 3.3. Combined Distribution Functions. The g(r) results show that with increasing pressure the Zn2+ second hydration shell is compressed and becomes more and more populated. However, this finding in itself does not explain the peculiar trend of the XANES spectra, which suggests a loss of water structuring upon pressurization. To corroborate this hypothesis it is necessary to inspect structural properties able to provide information on the degree of structuring of water molecules in the Zn2+ second coordination shell. We therefore decided to explore the potential of combined distribution functions (CDFs) that combine radial and angular distribution functions and are much more informative than monodimensional D


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Inorganic Chemistry functions usually employed to analyze MD trajectories. In the CDF analysis, the values of two variables (one angle and one distance in the present case) are regarded as a 2-tuple that is plotted in a two-dimensional histogram. Here, we calculated a CDF combining the g(r) between the oxygen atom of a water molecule belonging to the Zn2+ first solvation shell (O1) and the oxygen atom of all the other water molecules (O2), RO1−O2, together with the distribution function of the Zn−O1−O2 angle (ψ), which has as vertex O1 and is comprised between the Zn2+ ion and the oxygen atom of a generic water molecule (see Figure 4). The CDFs obtained

Figure 4. Schematic picture of the water arrangements found in Zn2+ aqueous solutions at room temperature and for pressure values in the range 0.10 MPa−1.0 GPa. The oxygen atom of the Zn2+ second-shell water molecule colored in blue forms a nearly linear hydrogen bond with the water molecule belonging to the Zn2+ octahedral first-shell complex colored in red. The other Zn2+ first-shell water molecules are colored orange, while those belonging to the Zn2+ second shell are ice blue. The Zn2+ ion is colored black. The RO1−O2 distance and Zn−O1− O2 angle (ψ) angle used in the CDF calculations are also shown. The configuration has been extracted from the MD simulation performed at ambient conditions as an example.

from the MD simulations are depicted in Figure 5. Very interestingly, the functions calculated for all of the simulations performed at room temperature and for pressures comprised between 0.10 MPa and 1.0 GPa show a very similar trend: a region of maximum intensity is found for an RO1−O2 distance of ∼2.8 Å and ψ angles ranging from 120° to 135°. The highintensity peak is the fingerprint of the high degree of structuring of the Zn2+ second hydration shell in this pressure and temperature range. Since water structure is dominated by the formation of hydrogen bonds at ambient conditions, this peak can be ascribed to configurations where the oxygen atoms of the Zn2+ second-shell water molecules form hydrogen bonds with the water molecules belonging to the Zn2+ octahedral firstshell complex. A representative picture of this water arrangement is shown in Figure 4. Conversely, by looking at the CDF results obtained at 2.50, 4.50, and 6.02 GPa we clearly see that the high-intensity peak found at lower pressure values disappears, and the correlation between the RO1−O2 and ψ variables becomes less and less significant. The lack of a welldefined peak indicates that the second-shell water molecules are randomly oriented with respect to the Zn2+ first hydration-shell molecules, thus showing that a significant increase of the Zn2+ second-shell disorder takes place in this pressure and temperature range. The CDF results are in perfect agreement with the XANES experimental data, whose features characteristic of the second hydration shell support the evidence that for

Figure 5. CDFs calculated from the MD simulations of Zn2+ aqueous solutions performed at different pressure values. The CDFs were obtained combining the g(r) between the oxygen atom of a water molecule belonging to the Zn2+ first solvation shell (O1) and the oxygen atom of all the other water molecules (O2), RO1−O2, together with the distribution function of the Zn−O1−O2 angle (ψ), which has as vertex O1 and is formed between the Zn2+ ion and the oxygen atom of a generic water molecule.

pressure values higher than 1.0 GPa the Zn2+ second coordination shell becomes very disordered, with the water molecules loosing the high degree of structuring that was the peculiar feature of these systems at room temperature and pressures lower than 1.0 GPa. To understand the effect of temperature on the Zn2+ secondshell disorder, it is useful to calculate the CDFs from the three MD simulations performed at 1.0 GPa and temperature values of 390, 495, and 542 K (see Figure 6). By comparing the CDFs obtained in the high-temperature conditions with the function computed at 1.0 GPa and 300 K, it can be seen that the peak at 2.8 Å and 120°−135° becomes much less intense at 390, 495, and 542 K. Therefore, the disorder of the Zn2+ second hydration shell increases with increasing temperature, as expected. Besides this strong difference, the form of the hightemperature distributions is similar to the one computed at 300 E


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Inorganic Chemistry

Figure 6. CDFs calculated from the MD simulations of Zn2+ aqueous solutions performed at 1.0 GPa and different temperature values. The CDFs were obtained combining the g(r) between the oxygen atom of a water molecule belonging to the Zn2+ first solvation shell (O1) and the oxygen atom of all the other water molecules (O2), RO1−O2, together with the distribution function of the Zn−O1−O2 angle (ψ), which has as vertex O1 and is formed between the Zn2+ ion and the oxygen atom of a generic water molecule.

Figure 7. Oxygen−oxygen radial distribution functions g(r)’s calculated from the MD simulations of Zn2+ aqueous solutions performed at different pressure and temperature values (a) and corresponding running coordination numbers N(r)’s (b). The g(r)’s were computed between the oxygen atom of a water molecule in the Zn2+ second shell and the oxygen atoms of all the other water molecules.

Clues on the structural arrangement of water molecules around a central one in the Zn2+ second shell emerge from the analysis of the O*−O−O** angle that a central oxygen forms with the oxygen atoms within a distance of 3.3 Å (see Figure 8).

K. If we compare these results with those shown in Figure 5, we can conclude that the loss of water structuring in the Zn2+ second coordination shell that occurs at 2.50 GPa and 390 K, at 4.50 GPa and 495 K, and at 6.02 GPa and 542 K is due to a combination of a pressure and a temperature effect: the temperature increase produces an increase of disorder, while the pressurization pushes the water molecules toward each other and forces them to adopt random orientations resulting in an additional increase of the Zn2+ second-shell disorder. 3.4. Water Structure in the Zn2+ Second Coordination Shell. To shed light on the perturbation induced on the water structure in the Zn2+ second coordination shell by the pressure increase it is useful to calculate the oxygen−oxygen g(r)’s between the oxygen atom of a water molecule in the Zn2+ second shell and the oxygen atoms of all the other water molecules (see Figure 7a). As it can be seen, the position of the O−O g(r) first peak shifts only slightly upon pressurization (RmaxO−O = 2.75 Å up to 1.0 GPa and RmaxO−O = 2.77, 2.79, and 2.79 Å at 2.50, 4.50, and 6.02 GPa, respectively), while remarkable differences can be observed for the peak shape, with two characteristic trends. For pressure values up to 1.0 GPa, the first peak width does not significantly change, while the intensity of the first minimum progressively increases. Conversely, at higher pressures a pronounced broadening of the O−O g(r) first peak is found, suggesting that the second hydration shell around a central water molecule collapses into the first one. The O−O running coordination numbers depicted in Figure 7b show that the number of water molecules at a given distance from a central one largely increases with pressure. In particular, by taking as O−O cutoff distance 3.3 Å, which corresponds to the O−O g(r) first minimum at ambient conditions, we obtain an O−O hydration number of 4.0, 4.3, 4.8, 5.1, 5.4, 6.7, 7.9, and 8.5 for pressure values of 0.01 MPa, 0.20, 0.60, 0.80, 1.00, 2.50, 4.50, and 6.02 GPa, respectively.

Figure 8. O*−O−O** angle distributions obtained from the MD simulations of Zn2+ aqueous solutions performed at different pressure values. In the O*−O−O** notation O is the central oxygen belonging to the Zn2+ second coordination sphere, while O* and O** are oxygen atoms of water molecules belonging to a coordination sphere of 3.3 Å radius around the central oxygen.

Note that in this analysis the central oxygen atom O belongs to the Zn2+ second coordination sphere. At ambient conditions, where there is a quite regular hydrogen-bond network, the distribution is highly peaked at 108° (1-cos(O*−O−O**) = 1.31), corresponding to a tetrahedral arrangement, and an additional low-intensity peak is detected at ∼56° (1-cos(O*− O−O**) = 0.44). The intensity of the latter peak significantly increases with pressure, while the intensity of the tetrahedral peak decreases, and the low-angle peak becomes the principal F


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hydrogen bonds is due to the very high compression of the systems subject to high pressures, which push the water molecules toward each other producing an overcrowding that hinders them to adopt the correct geometry necessary for a highly directional interaction such as the hydrogen bond.

one for pressure values higher than 1.0 GPa. Low angles in the water arrangements are formed by a central oxygen, a tetrahedral neighboring oxygen, and the oxygen of an “interstitial” water molecule, which is not hydrogen-bonded to the other two (in this sense it is purely interstitial). Therefore, the trend of the O*−O−O** angle distribution suggests that the number of interstitial water molecules significantly increases under high pressure, in line with the results previously obtained in a MD simulation study of highdensity water.57 To better characterize the pressure effect on the water hydrogen-bond network, we calculated the percentage of water molecules that engage in n hydrogen bonds in the Zn2+ second hydration shell (Figure 9). We adopted a configurational

4. DISCUSSION AND CONCLUSIONS In the present paper we investigated the structural modifications induced on Zn2+ aqueous solutions under high pressure and temperature conditions. The Zn K-edge XANES experimental data showed two different trends depending on the pressure and temperature conditions of the system. When the sample is pressurized at room temperature by progressively increasing the pressure from atmospheric to 1.0 GPa a peculiar feature in the XANES spectra is found. This feature is the fingerprint of the Zn2+ second-shell contribution,41,74 indicating that the highly structured nature of Zn2+ second hydration shell that is present at ambient conditions is preserved also when the pressure is increased to 1.0 GPa. Conversely, when the Zn2+ aqueous solutions are pressurized above 1.0 GPa and simultaneously heated, the feature in the XANES spectra disappears, suggesting that the Zn2+ second shell loses its high degree of structuring, becoming much more disordered and unstructured. These results were confirmed by performing an improved analysis of MD simulations of Zn2+ aqueous solutions at different temperature and pressure values. In particular, we explored the potential of CDFs, and, by combining radial and angular distribution functions, we were able to highlight the loss of water structuring in the Zn2+ second coordination shell that takes place upon pressurization and heating. Additional MD simulations of Zn2+ aqueous solutions performed at 1.0 GPa and at increasing temperature values showed that this loss of water structuring in the Zn2+ second coordination shell is due to a combined effect of pressure and temperature: heating produces an increase of disorder, while the pressurization pushes the water molecules toward each other and forces them to adopt random orientations resulting in an additional increase of the Zn2+ second-shell disorder. These structural changes are caused by the very high compression of the systems resulting in a progressive crowding of the Zn2+ second shell, as more and more water molecules are pushed toward the ion with increasing pressure. The water structure in the Zn2+ second shell becomes remarkably different from that found at ambient conditions: for pressure values higher than 1.0 GPa the second shell of water molecules around a central one collapses into the first shell, and the tetrahedral arrangements of water molecules is highly distorted. In conclusion, the use of XANES spectroscopy in combination with MD simulations gave us the opportunity to shed light on the modifications induced by pressure in the second coordination shell of a metal ion in aqueous solution, which is in general a nontrivial task due to the high disorder and lability of ion second hydration spheres. The modification of the Zn2+ second-shell arrangement upon heating and compressing up to the gigapascal pressure range can be an indication of the structural behavior of pure water in the compressed liquid phase.

Figure 9. Percentages of water molecules forming nHB water−water hydrogen bonds calculated for the Zn2+ second coordination shell from the MD simulations of Zn2+ aqueous solutions performed at different pressure values.

criterion where two water molecules are hydrogen bonded only if their interoxygen distance is lower than 3.5 Å and, simultaneously, the hydrogen−oxygen distance is lower than 2.45 Å and the oxygen−oxygen−hydrogen angle is less than 30°.76 At ambient conditions a dominant percentage of water molecules in our MD simulation box form four hydrogen bonds. Note that the tetrahedral structure of water and the number of hydrogen bonds formed by water molecules in the liquid phase as determined from experimental investigations have been largely debated in the literature.54,77−80 The results of our MD simulations show that the nHB = 4 configuration remains the most probable one also when the pressure is increased to 1.0 GPa. In particular, going from 0.10 MPa to 1.0 GPa, the nHB = 4 and nHB = 5 percentages increase, while the nHB = 3 probability decreases. On the contrary, at 2.50 GPa the nHB = 4 and nHB = 3 percentages become very similar, while at 4.50 and 6.02 GPa the most probable configuration corresponds to water molecules forming only three hydrogen bonds with the surrounding molecules. This partial rupture of

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*E-mail: [email protected]. (V.M.) *E-mail: [email protected]. (P.D.) G


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Inorganic Chemistry ORCID

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Valentina Migliorati: 0000-0003-4733-6188 Simone De Panfilis: 0000-0002-9428-079X Paola D’Angelo: 0000-0001-5015-8410 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Univ. of Rome “La Sapienza” (Progetti Ateneo 2015 Nos. C26N159PNB and C26H159F5B) and by the CINECA supercomputing centers through the Grant IscrC_LABILE.

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Structure of Water in Zn2+ Aqueous Solutions from Ambient

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