Electric Field Induced Polarization Effects Measured by In Situ Neutron

Article pubs.acs.org/JPCC

Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX

Electric Field Induced Polarization Effects Measured by in Situ Neutron Spectroscopy Rosanna Ignazzi,† Will P. Gates,‡ Souleymane O. Diallo,§ Dehong Yu,∥ Fanni Juranyi,⊥ Francesca Natali,#,∇ and Heloisa N. Bordallo*,†,‡,○ †

Niels Bohr Institute, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø, Denmark Institute for Frontier Materials, Deakin University, Burwood, VIC 3125 Australia § Spallation Neutron Source, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Australian Centre for Neutron Scattering, Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW 2234, Australia ⊥ Laboratory for Neutron Scattering and Imaging, Paul Scherrer Institute, CH-5232 Villigen, Switzerland # Institute of Materials, Research National Council (CNR-IOM), 34149 Trieste, Italy ∇ Institut Laue-Langevin, BP 156, F-38042 Cedex 9, Grenoble, France ○ European Spallation Source ESS AB, P.O. Box 176, SE-221 00 Lund, Sweden ‡

S Supporting Information *

ABSTRACT: Despite the success of electrical stimulation in many areas, including clay or sludge dewatering, extraction of juices from fruit pulp, fracture healing, and targeted drug delivery, the induced transport mechanisms are controlled by unknown factors. While electroosmotic dewatering of clays particles 0). The short-time dynamics of the hydrogen atoms mainly determines the inelastic part of the spectrum, SIN(Q,ℏω > 0), while the dynamics of confined water can be obtained by the QE part of the spectra, SQE(Q,ℏω ≈ 0). The QE signal provides invaluable insight in how the EF changes the dynamics of water molecules. Using the direct time-of-flight spectrometer PELICAN with a wavelength of λ = 6.0 Å, the faster diffusion, originating mostly from the continuum of water populations within the interlayer, that occurs in the picosecond (ps) time regime in the sample hydrated at 58% RH was probed. In this set of measurements, the sample was oriented at 135° with respect to the incoming neutron beam, in order to have access to the low momentum transfer range (Q-range), which was between 0.4 and 1.8 Å−1. The energy resolution of the instrument in this setup is ∼70 μeV (full width half-maximum, fwhm), while the accessible dynamic range of ±2 meV varies with Q. In order to protect the high voltage supply cables, sealed EF-sample holders containing the samples were mounted in a bottom loading cryostat. To avoid possible water loss from Ca-Mt, the experiment was

hydrated in two different desiccators containing either an oversaturated solution of NaBr (58% RH; ca. 100 g NaBr salt for 50 mL of distilled water) or KCl (85% RH; ca. 100 g KCl salt for 50 mL of distilled water) for 48 h (BASIS samples) or under vacuum for at least 4 h (PELICAN samples). Sufficient quantities of each sample were equilibrated in the desiccators so that thermogravimetric analysis (TGA) could be used to determine the water content in each sample. After water vapor equilibration the EF-sample holders were removed from the desiccators and quickly sealed. Dedicated EF-Sample Holder. The EF-sample holder developed for this experiment was based on the design of a prototype sample environment developed for the backscattering spectrometer BASIS at the Spallation Neutron Source.24,25 Design of the EF-sample holder, shown in Figure 2, was

Figure 2. (a) Schematic of the assembled EF-sample holder, not to scale. (b) Picture of the EF-sample holder with the boron nitride (BN) mask fastened to it.

performed on SolidWorks at the Institute Laue-Langevin and fabricated by technical staff at the Niels Bohr Institute workshop. Aluminum alloy 5083, which contains, among other metals, 4−4.9% Mg, was used to manufacture the aluminum parts of the sample holder. The gap between the body and the lid, where the sample is placed, was 0.5 mm. In addition, in both the body and the lid, a triangular concave trench was cut along the edge of the sample window for placement of indium wire to ensure complete sealing of the cell under N2 gas (PELICAN) or under high vacuum (BASIS) conditions. Teflon gaskets were used to electrically insulate the lid of the cell from its body, while Teflon bushings insulated the screws used to seal the sample holder. Finally, a boron nitride (BN) mask was used to minimize interference with the EF created by the aluminum part. The BN mask was specifically designed in order to decrease the overall background due to neutrons scattering off the Teflon parts. A detailed description of the EF-holder is given in ref 26. The EF-sample holder was first tested and the experimental setup was optimized using the backscattering instrument IN13,27 located at the Institute Laue Langevin (ILL, Grenoble France), and the inverted geometry time-of-flight backscattering spectrometer MARS28 at the Paul Scherrer Institute (PSI, Switzerland). Subsequently, two new experiments, which are the focus of our study, were carried out at the backscattering spectrometer BASIS and the time-of-flight spectrometer PELICAN. Thermogravimetric Analysis (TGA). Using a PERSEUS TG 209 F1 Libra from NETZSCH, TGA analysis was C

DOI: 10.1021/acs.jpcc.7b08769 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 3. Room temperature QENS spectra vs Q for the Ca-Mt clay mineral equilibrated at 58% RH in the EF-sample holder measured on PELICAN (a) and on BASIS (b) at 0 V after the removal of the Bragg reflections.

oriented at 90° relative to the incoming beam. To eliminate shaded detectors and Bragg peaks, a detector mask was created by carefully evaluating the raw data as follows. The analyzer count for the Ca-Mt equilibrated at 58% and 85% RH at 0 V were plotted in a color map, where red meant high count intensity and yellow/green means low count intensity. In this way the shading in the middle of the Si(111) analyzers (between 1.3 and 1.6 Å−1), due to the edges of the sample holder, was clearly identified by intense yellow/green, while very high red intensity in the low angle analyzer crystals indicated Bragg reflections from the layers of the Ca-Mt sample (see Figure S3 in the Supporting Information). Using this mask, neutrons scattered from the shaded analyzer crystals and those affected by Bragg reflections were eliminated from further analysis. This mask was used during the data reduction processing with the MANTID software.38 Data analysis was also carried out using DAVE.32 The reduced data can be seen in Figure 3b for the Ca-Mt at 58% RH at 0 V.

performed under N2 gas at room temperature and atmospheric pressure. As under these conditions N2 has a high breakdown voltage,30 and the applied voltage (from 0 to 175 V, in steps of 25 V) was too low to originate sparks. The background was evaluated by measuring an empty EF-sample holder under the same conditions. Measurements from a vanadium slab, both in N2 atmosphere and under vacuum, were used for the detector normalization and resolution function, respectively. The data was reduced using the software LAMP31 from the Institute Laue-Langevin, and analyzed using the software DAVE.32 By summing all channels of the measured signal one can obtain the detector intensity as a function of the momentum transfer, S(Q), for the Ca-Mt (see Supporting Information Figure S2) and in this way monitor the position of the 001 Bragg reflections. Here the peak observed at about 0.45 ± 0.05 Å−1, which corresponds to a d-value of 1.39 ± 0.16 nm for the sample equilibrated at 58% RH, is intermediate to the dvalue expected for two-layer (1.52 nm) and one-layer (1.25 nm) hydrates.7 This implies that, for this sample, a 1:1 interstratification of one- and two-layer hydration states was present.33−35 In addition, Bragg reflections at 1.4 Å−1, coming from the sample holder, and other smeared reflections around 1.5 and 1.6 Å−1 are also observed. For the 85% RH sample, we obtained a similar basal spacing, 1.43 Å. While the results indicate that the interlayer spaces of both samples were occupied by interstratifications of one- and two-layers of water, in agreement with Bray et al.36 and references herein, an extra water molecule is present in the sample equilibrated at 85% RH (Table 1). To utilize the data for further analysis, the detector channels corresponding to these features were eliminated, as were the highest angle detectors that were shaded by the EFsample holder. The same detector mask was used for the data collected under applied voltage. The reduced QENS data obtained using the PELICAN spectrometer can be seen in Figure 3a for the Ca-Mt at 58% RH at 0 V. To probe the slower water dynamics in the interlayer of CaMt,37 including cation-coordinated water and tightly H-bonded interlayer water, we performed another set of experiments on the same samples using the backscattering spectrometer BASIS with wavelength of λ = 6.4 Å and an elastic energy resolution of 3.5 μeV (fwhm). This corresponds to a time scale in the nanosecond (ns) range, covering a dynamic range of ±100 μeV and a momentum transfer range between 0.2 and 2.0 Å−1. The experiment was performed under a vacuum of ∼10−3−10−4 mbar. For this experiment, the EF-sample holder plane was

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RESULTS AND DISCUSSION Picosecond Dynamics Originating Mostly from Weakly H-Bonded Interlayer and Surface Water. To further understand how the water molecules behave in these samples under EF in the ps time scale, we turned to the analysis of the evolution of the QENS response vs applied voltage of the Ca-Mt at 58% RH obtained using PELICAN, depicted in Figure 4a. From this plot, we observe a QE signal that does not change when the applied voltage is varied from 0 to 175 V in 25 V increments. We also observe that the intensity of the elastic line remains constant, thus suggesting that the mobility of the interlayer water probed in this time scale is unaffected by the EF. Note that if any intraparticle water exists in this sample, it can only be in pores no larger than ≈0.6 nm diameter. In order to analyze the origin of the QE signal, the spectra were fitted using a single Lorentzian function as follows:39 S(Q , ℏω) = A 0(Q )[p1 (Q )δ(ℏω − ℏω0) + (1 − p1 (Q )) × L(Q , ℏω − ℏω0)] ⊗ R(Q , ℏ − ℏω0) + B(Q , ℏω − ℏω0)

(1)

A0(Q) is a scaling constant (which contains the contribution from the Debye−Waller factor), p1(Q) is the fraction of elastically scattered neutrons, δ(ℏω − ℏω0) is the delta function describing the elastic scattering, centered at the energy D


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The behavior of the extracted half-width-at-half-maximum (Γ, hwhm) is shown in Figure 4c. Considering a random diffusion process, used previously7,12,21 for the samples of the same Ca-Mt, the variation of Γ vs Q can be approximated by the Singwi and Sjölander model:40 ΓT(Q ) =

Dt Q 2 2

1 + Dt Q τ0

and

Dt =

L2 6τ0

(2)

where Dt represents the self-diffusion coefficient, τ0, the average residence (or jump) time between jumps, and L, the mean jump distance. The latter corresponds approximately to the distance between proton sites or proton sites and lone pairs on the same water molecule. The fitted parameters characterizing the motion of the water molecules give an apparent translational diffusion coefficient, D = 2.22 × 10−9 m2/s, which is very close to the one of bulk water, (2.4 × 10−9 m2/s),39 and a residence time τ0 = 6.5 ps. The apparent water diffusion coefficient determined in this way carries however a degree of ambiguity due to the lack of values in the lower Q-range as well as a result of the simplistic model used. On the other hand, the observed residence time, τ0, which is 4 times higher than that for bulk water, is comparable to previously reported values.10,12−18 The large values for L (≈ 2 times that of bulk water) are incompatible with proton jumps known for bulk water, thus indicating that the water molecules are locked into an arrangement that delays their ability to jump.41 Therefore, even if we have used a simple approximation to describe the QENS data, we can identify that the water dynamics probed during the experiment on PELICAN specifically included a combination of small amount of surface water as well as weakly and tightly H-bonded interlayer water, where their relative proportions would be dependent on the water content of the Ca-Mt. Here we note that, based on previous experiments, while motions attributed to cationcoordinated water are within the resolution function and thus seen as immobile, the other types of water mobility are difficult to distinguish as their dynamics happen in very similar time scales. As observed in Figure 4a, these waters were unaffected by the applied voltage. Nanosecond Dynamics Originating Mainly from Tightly H-Bonded and Cation-Coordinated Water. Based on the results described in the previous section, the continuum of weakly H-bonded water populations within the interlayer in Ca-Mt seemed to be undisturbed by the EF. We can therefore use this information when analyzing the data collected on the backscattering spectrometer BASIS, which has sufficient energy resolution and dynamic range to differentiate the slower cation-coordinated water from the “faster” tightly Hbonded water, while the weakly H-bonded interlayer water and any intraparticle water are expected to contribute mostly to the background. During the experiments carried out on BASIS the voltage was applied to the 58% Ca-Mt sample in the following sequence: 0, 25, 50, 75, 100, 125, 150, 175, and 0 V, while data for the 85% Ca-Mt sample was only collected at 0, 10, 25, 50 and again 0 V. The spectra for both samples can be seen in Figure 5a and b, which clearly show that for both samples the QE part becomes narrower under EF and at the same time the elastic line increases in intensity. To fit the data from BASIS, assuming that the rotational and translational motions of the water molecules are decoupled,29 and considering that we only observe translational motions in

Figure 4. (a) Room temperature QENS data for the Ca-Mt at 58% RH at Q = 0.8 Å−1 collected using PELICAN at different applied voltages in log scale. (b) Representative fit (red line) of the QENS data (black dots) collected on PELICAN using eq 1. The black line represents the residual. (c) Evolution of the HWHM of the Lorentzian used to fit the QENS at 0 V (green diamonds). Error bars are the same size or smaller than the symbols. The data are fitted to the Singwi− Sjölander model (green dashed line), eq 2. The diffusion for bulk water is shown in the red.

ℏω0 = 0 and representing the particles that are seen as immobile in the time window probed by the instrument. L(Q,ℏω − ℏω0) is a Lorentzian representing the broadened energy distribution that results from neutron-nucleus collisions, corresponding to the population statistics of one relaxation process. B(Q,ℏω − ℏω0) represents a flat background term and R(Q,ℏω − ℏω0) the resolution function of the instrument. The full fitted function at 1 Å−1 can be seen in Figure 4b. E


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Figure 5. Room temperature QENS data for (a) the Ca-Mt at 58% RH and (b) the Ca-Mt at 85% RH at Q = 0.5 Å−1 collected using BASIS at different applied voltages in log scale (left side) and for better visualization at two selected voltages in linear scale−right side. Note the different x-axis between the top and bottom figures on the left side. (c) Total fit (in red) of the QENS signal measured for the Ca-Mt equilibrated at 58% RH at 0 V (black points) for Q = 0.7 Å−1. The contribution from the delta function as the elastic part is plotted in gray, from the narrow and broad Lorentzians in violet and green, respectively. The background is represented in brown. The fit residuals are plotted below the fitted data.

the spectra at low Q-range (Q < 1.1 Å−1), we distinguished again two different water dynamical contributions by fitting with two different decoupled Lorentzian functions as follows:24

LN(Q,ℏω − ℏω0) + LB(Q,ℏω − ℏω0) is the contribution from the different translational motions, where the subscript N represents the narrow component that describes the contribution from the slower cation-coordinated water, and the subscript B represents the broad component, in this case describing the tightly H-bonded interlayer water. The hwhm for this latter component is the same as that observed above from the PELICAN experiment.

S(Q , ℏω) = A 0(Q )[p1 (Q )δ(ℏω − ℏω0) + (1 − p1 (Q )) × (LN (Q , ℏω − ℏω0) + LB(Q , ℏω − ℏω0))] ⊗ R(Q , ℏω − ℏω0) + B(Q , ℏω − ℏω0)

(3) F


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Figure 6. Evolution of the hwhm of the Lorentzian used to fit the QENS at 0 V for the Ca-Mt equilibrated at 58% RH (a) and 85% RH (b). The broad component is assigned to the tightly H-bonded water and the narrower component to the cation-coordinated water. The nonvisible error bars are either the same size or smaller than the symbols. The data are fitted to the Singwi−Sjölander model (dashed line), eq 2. Obtained diffusion coefficient (c) and jump time (d) for the cation-coordinated interlayer water in both samples as a function of applied voltage. Unfortunately due to the reduced neutron measurement time the 85% RH sample was measured only up to 50 V.

Figure 7. Hemispheres show the cross sectional area where the cations may reside within the interlayer associated with the layer charge expressed at the surfaces of montmorillonite. In both cases, the extreme distance from charge (one side of the interlayer space or the other for divalent cation) is shown on the left side, the typical position is on the center, the extreme distance from surface and charge is shown on the right side. In this scenario, the volume available for water (and the amount of water within the interlayer) is unchanged under EF, but the cross sectional mobility of the cation is affected. Thus, the dynamics of water interaction with cations changes due to the EF-induced changes in cation−surface interaction.

Figure 5c shows the total fit for the Ca-Mt equilibrated at 58% RH at 0 V, as well as the contributions from the different functions used to fit the data. To decrease the number of fitting parameters, and from a priori knowledge gained from the PELICAN data, the hwhm of the broad Lorentzian fitted to the samples at 0 V was fixed in the fit of each spectrum collected under applied voltage. However, the area of this component was not fixed to monitor for possible tightly H-bonded water loss as represented by changes in the amount of H-bonded water due to the vacuum. By modeling the variation of Γ vs Q using eq 2, these two distinct water dynamics at 0 V for the Ca-Mt samples can be described as follows:

(i) For Ca-Mt at 58% RH, the broader Lorentzian component describes the contribution from water molecules that diffuse with an apparent diffusion coefficient (D) smaller than that of bulk water, D = 1.7 × 10−9 m2/s, with a residence time τ0 = 8.2 ps. In the Ca-Mt at 85% RH, these very mobile water molecules have very similar diffusion coefficient to that of bulk-water, D = 2.3 × 10−9 m2/s, while the value of τ0 is identical to that observed in the 58% RH, i.e., τ0 = 8.2 ps. (ii) The narrower Lorentzian component describes water molecules that move slower, and are therefore more constrained. This water population has very similar values for D and τ0, i.e., for the Ca-Mt at RH 58% D = 0.42 × 10−9 m2/s and τ0 = 35 ps, whereas for Ca-Mt at 85% RH, D = 0.48 × 10−9 m2/ s and τ0 = 33 ps. G


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With this in mind, and considering that the drift velocities of ions are prone to increase proportionally to EF,45 we can conclude that our results provide clear evidence that the water molecules can be highly distorted and strongly polarized by the interlayer cation when the applied voltage is increased above 50 V.

From Table 1, we inferred that the 58% RH equilibrated sample has proportionately more tightly H-bonded than the sample hydrated at 85%. This is indeed reflected by the distinct values of the apparent diffusion coefficient.17,42 Therefore, in both samples, the slower diffusing water molecules are assigned to the cation-coordinated water, which is far more strongly bound. Although translational dynamics of the cationcoordinated water has been previously discussed in similar clays minerals,13−15 to the best knowledge of the authors, it is in this work that it has been fully distinguished from the Hbonded water for the first time using QENS. Figure 6 shows the variation of the values attributed to the translational dynamics of the cation-coordinated water for both samples as a function of applied voltage. These results show that, within the Q-range accessible during this study, the diffusion coefficient of the cation-coordinated water is stable up to 175 V. On the other hand, as the cation-coordinated water molecules become more polarized, causing further distortion in the H-bond network under EF, a definite increase in τ0 (in relation to that observed for bulk water, 1 ps) is observed for the 58% RH Ca-Mt, which has proportionally more cationcoordinated water as compared to the 85% RH sample. As depicted in Figure 7, this result confirms the idea43 that without EF, water is able to interact uniformly with the cations within the interlayer, partially shielding the cation from the surface. On the other hand, when voltage is applied, the increased electrostatic attraction of the cation to the interlayer surface results in fewer water molecules available for shielding the cations, thus affecting how the interlayer water hydrates the cations. The cation-coordinated water becomes strongly Hbonded with other waters and this perturbation is extended to several H-bond linkages. We expect that at higher applied voltages similar changes as observed for the 58% RH Ca-Mt sample would become distinct also in 85% RH Ca-Mt.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b08769. Diffraction patterns obtained from the PELICAN data; details on the BASIS data reduction and on the convolution approximation used to analyze the QENS data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Will P. Gates: 0000-0001-7388-0289 Heloisa N. Bordallo: 0000-0003-0750-0553 Author Contributions

S.O.D., D.Y., and F.J. contributed equally. R.I., W.P.G., and H.N.B. conceived and carried out the neutron scattering experiments with the support from S.O.D., D.Y., F.J., and F.N. W.P.G. prepared the samples. R.I. hydrated and characterized the samples using TGA. S.O.D. developed the first EF-sample holder and shared the design with R.I. and F.N. R.I. under the supervision of F.N. designed and optimized the EF sample holder. R.I. analyzed the neutron scattering data with the input from H.N.B., S.O.D., D.Y., and F.J. R.I., W.P.G., and H.N.B. wrote the manuscript, which was approved by all coauthors in its final version.

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CONCLUSIONS In situ EF neutron spectroscopy studies of hydration water has the potential to provide unique information regarding the interplay between water and neighboring cations and/or molecules within confined environments, thus opening new possibilities in research areas as diverse as food science, medicine, and geo-environmental engineering, where EF play an important role in the water dynamics of the system. Here by combining data obtained with two different spectrometers, PELICAN and BASIS, which cover a broad dynamical range, we were able to probe the response of different water dynamics in Ca-Mt samples under EF. Our findings provide further insight into how applied voltage affects polarization and mobility of interlayer water in relatively dry (≈18−22 wt %, Table 1) Ca-Mt. Using an elastic energy resolution of ≈70 μeV in the PELICAN experiment, we observed that for the sample equilibrated at 58% RH the EF had no effect on the dynamics of the H-bonded water in the ps time scale. Based on the data from BASIS, although it is not possible to unambiguously determine whether the diffusion coefficient of the cationcoordinated water is altered with increased voltage, as may occur if ion channeling or migration occurred, a clear increase can be observed in the jump time, τ0, for the 58% RH Ca−Mt sample above 50 V applied voltage. Considering that τ0 reflects the time that water molecules remain within a given H-bonded network before jumping to another,44 the longer residence time mainly observed for the 58% RH sample can be interpreted as a clear evidence that the lifetime of the H-bonds were prolonged.

Funding

R.I.’s work was financed by an internship grant at the Institute Laue-Langevin. The Italian National Research Council (CNR) financed the development of the EF-holder and the thermal analysis apparatus was financed through Carlsbergfond grants ref: 2013-01-0589. This work is based on experiments performed at the Australian Centre for Neutron Scattering, Institute Laue-Langevin, Spallation Neutron Source and Swiss spallation neutron source SINQ. The neutron research has benefited from the support given by Danscatt funded by Danish Agency for Science, Technology and Innovation. HNB acknowledges support from the CoNext project. W.P.G. received partial travel support from the Australian Nuclear Science and Technology Organization. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.I. thanks the Institute Laue-Langevin services for supporting with the experimental setup as well as with the technical and design work. Workshop staff from the Niels Bohr Institute is deeply acknowledged by R.I. and H.N.B. for machining the sample holders with the most appropriate materials. We are grateful to the technical teams at the different neutron facilities: ILL (R.I., H.N.B., and F.N.), PSI (R.I. and H.N.B.), SNS (R.I. and H.N.B.) and ANSTO (R.I., W.P.G., and H.N.B.). H


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