Understanding the Influence of the Electrochemical Behavior of

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Understanding the Influence of the Electrochemical Behavior of Potassium Hexacyanoferrate(II/III) on Acetonitrile/Water Binary Mixtures. An Experimental and Molecular Dynamics Study Fabio Henrique Ferreira Batista, Leonardo José Amaral Siqueira, Luciano T. Costa, and Tiago Luiz Ferreira J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10989 • Publication Date (Web): 11 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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The Journal of Physical Chemistry

Understanding the Influence of the Electrochemical Behavior of Potassium Hexacyanoferrate(II/III) on Acetonitrile/Water Binary Mixtures. An Experimental and Molecular Dynamics Study Fábio Henrique Ferreira Batista,1 Leonardo José Amaral Siqueira,1 Luciano T. Costa2 and Tiago Luiz Ferreira*1 1

Departamento de Química, Instituto de Ciências Ambientais, Químicas e Farmacêuticas, UNIFESP, Diadema, SP, Brazil

2

Instituto de Química, Universidade Federal Fluminense, Niteroi, RJ, Brazil

E-mail: [email protected] Fax number: +55 11 33193472

*To whom correspondence should be addressed

1

ACS Paragon Plus Environment

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Abstract

Microelectrode voltammetry was employed to gain information on the behavior of potassium hexacyanoferrate(II/III) in acetonitrile/water binary mixtures in order to understand the bulk properties of the mixture and the electrode / binary solution interface properties. The first analysis of the obtained data demonstrated that the diffusion coefficient values of hexacyanoferrate(III) apparently do not respect the Stokes-Einstein equation, considering the hydrodynamic radius does not change with acetonitrile content. Another interesting observation is the adsorption of electrogenerated potassium hexacyanoferrate(II) onto electrode surface in binary mixtures with 50% mol of acetonitrile or higher. Molecular dynamics simulations showed that acetonitrile molecules accumulate on the surface of an electrode model and also that ionic species tend to form clusters in acetonitrile-rich mixtures. Since the potassium hexacyanoferrate(II) is not soluble in acetonitrile, the adsorption of iron complex onto the electrode is related to the formation of the acetonitrile layer on the surface of the electrode. The diffusion coefficient of hexacyanoferrate(III) in the mixtures decreases because of the ion clustering and the formation of acetonitrile layer on the surface of the electrode, which is an obstacle that the ions have to overcome to reach and leave the electrode.

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1. Introduction Water and acetonitrile (ACN) binary mixtures are widely used in many fields of chemistry such as molecular biology1, organic synthesis2, chromatography3, electrochemistry4 and solvent extraction5. Water is a polar protic solvent able to provide H+ by autoprotolysis and ACN is a polar aprotic solvent. The intermolecular interactions are different for those solvents: water molecules form hydrogen bonds; ACN molecules form weaker interactions, the dipoledipole interactions. In binary mixtures, despite being an aprotic solvent, ACN can interact with water forming one hydrogen bond. Although water molecules can form highly ordered structures, in ACN/water mixtures one water molecule interacts with four other water molecules to form ordered structures with very few ACN molecules involved6,7. That way, there are regions very rich in water and poor in ACN and vice-versa6,7. This kind of system is called microheterogeneous: macroscopically the mixture is totally homogeneous, but, at molecular level, those regions (microdomains) are present6,8,9. At high water concentrations (xACN < 0.2), the ACN molecules fill the cavities in water structure and at intermediate concentrations (0.2 < xACN < 0.75) ACN breaks down the water structure. Above xACN 0.75, the ACN molecules would be weakly structured surrounded by isolated water molecules6. The ACN/water binary mixture has been extensively studied to confirm its microheterogeneity. These studies agree that microheterogeneity occurs at intermediate acetonitrile concentrations6,7,9. At low ACN concentrations, the vast majority of ACN molecules develop ACN-water hydrogen bonds. Above xACN = 3



0.5, the microheterogeneity tends to vanish because the water molecules would be hydrogen bonded to acetonitrile molecule7. Computer simulations are also employed to study ACN/water binary mixtures6,10-12. In mixtures with acetonitrile mole fraction around 0.30, simulations show the coexistence of water-rich and ACN-rich domains10-12. In simulations of water and ACN at silica interface, Mountain11 showed that a 1.5 nm thick layer of water is formed on the surface of hydrophilic silica. Simulations performed by Fileti13 showed that the accumulation of solvent at graphene surfaces is also observed for pure solvents. Some other methods use solute probes to investigate ACN/water mixtures14,15. These methods, even at low concentrations of probe, are more commonly employed to gain information on preferential solvation of probe by one of the components of the mixture6. Regarding the use of the electrochemical methods, Gritzner et al.16,17 showed that the half-wave potential (E1/2) values for the redox pair tetra-n-butylammonium hexacyanoferrate(II)/(III) was related to the acceptor number for many pure solvents such as ACN, dimethyl sulfoxide, dimethylformamide, ethanol and methanol. Acceptor and donor numbers are parameters that quantify the hardness or softness of acids and bases. These studies showed a linear relation between solvent acceptor number and E1/2, which indicates the possibility to determine solvents polarity using voltammetry. Gritzner describes the transfer properties of ions between water and acetonitrile is also dependent of the principle of hard and soft acids and bases. Cations act as acceptors in solution, interacting with nucleophilic sites of solvent molecules and anions as donors, interacting with electrophilic 4


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regions17. Hard cations interact strongly with solvents of hard donor properties and weakly with solvents of soft donor properties. On the other hand, soft cations interact much stronger with soft donor than the hard donor solvents. The same principle is applicable to anions and solvents17,18. All the previous articles regarding investigations on ACN / water binary mixtures employing electrochemical probes, describe essentially bulk effects as, for example, the preferential solvation of these probes for one of the solvents in the mixture. In this work, our efforts are devoted to understand not only the bulk characteristics, but the properties of electrode / binary mixture interface using the pair potassium hexacyanoferrate(II/III) as electroactive probe in binary mixtures of ACN/water, in different proportions. This electroactive probe has a very well-known electrochemical behavior in aqueous solutions19,20 and has been used in previous works to investigate binary mixtures of solvents16,17. Microelectrode and ring-disk rotating voltammetry have been used to gain information on the diffusion coefficients values of the probe and collection efficiencies of the reduced probe, respectively. Electrochemical quartz microbalance experiments were performed to verify the adsorption of the probe onto electrode surface. Molecular dynamics simulations of mixtures have been employed to describe the electrode/mixture interface structure to shed some light on voltammetric behavior for those species.

2. Experimental 2.1 . Chemicals

5



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All reagents were of analytical grade and no further purification was performed.

Potassium

hexacyanoferrate(II)

(99,5%)

and

potassium

hexacyanoferrate(III) (98%) were obtained from Merck S. A. (Rio de Janeiro, Brazil). KCl (99,5%), KI (99,5%) and acetonitrile (99,5%) were obtained from Sigma-Aldrich Co. (Steinheim, Germany). All solutions were prepared with deionized water purified by a Milli-Q system (Millipore®, Darmstadt, Germany). 2.2 . Voltammetry measurements All voltammetric measurements were carried out with an Autolab PGSTAT 302N potentiostat (Eco Chemie, Utrecht, the Netherlands) with data acquisition software from the manufacturer (GPES 4.9 version) using the threeelectrode electrochemical cell. The experiments were performed in a conventional electrochemical cell using a home-constructed Ag/AgCl (saturated KCl) reference electrode21 and platinum wire was used as auxiliary electrode. The working microelectrode was a carbon fiber microelectrode (6 µm diameter) constructed by attaching the fiber to glass capillaries. To ensure reproducible measurements, microelectrodes were polished with alumina slurry and thoroughly washed with water prior to use. Values of diffusion coefficient of hexacyanoferrate(III) in binary mixtures were calculated from Equation 1, applicable to disk microelectrodes under steady-state conditions19 = 4 (1), where IL is the limiting current value for a diffusion controlled system, n is the number of electrons participating in the electrochemical reaction, F is the 6


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Faraday constant, C is the concentration of electroactive species in mol cm-3, D is the diffusion coefficient in cm2 s-1 and r is the radius of the disk microelectrode in cm.

Equation 1 was also employed to calculate the radius of the microdisk electrode from the limiting currents measured for standard solutions of hexacyanoferrate(III) in 0.1 mol L-1 KCl20. Voltammograms of hexacyanoferrate(III) 10 mmol L-1 in ACN/water binary mixtures were registered at different solvent proportions, from pure water to 65% mol of acetonitrile. Binary mixtures with more acetonitrile content have presented phase separation and salt precipitation. Potential values were generally scanned from 0.5 to -0.1 V versus Ag|AgCl reference electrode, but this range was changed for some voltammograms

registered

in

binary

mixtures

to

accommodate

the

hexacyanoferrate(III) reduction signal as limiting current. Half-wave potentials (E1/2) were obtained collecting the potential values where I = IL/2 from the registered voltammograms.

2.3 . Electrochemical quartz crystal microbalance For

the

electrochemical

quartz

crystal

microbalance

(EQCM)

experiments, the working electrode was a 6-MHz AT-cut piezoelectric quartz crystal with a 7 mm diameter and a piezo-active electrode area of 0.385 cm2 7



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manufactured by Autolab. The frequency resonance shift (∆f) was used to calculate mass change (∆m) by using the Sauerbrey equation22 ∆f = –K∆m (2), where the integral sensitivity constant is, K = 0.0815 ng-1 cm2 Hz.

2.4 . Rotating ring-disc voltammetry An Autolab PGSTAT 302N bipotentiostat was used for Rotating RingDisc

experiments.

The

working

electrode

was

a

Pine®

Research

Instrumentation Rotating ring-disc electrode (RRDE) model E7R9-VC/Pt RRDE comprising a glassy carbon disk and platinum ring. The rotation rate was controlled using a Pine® rotation controller model AFMSRCE. Reference and auxiliary electrodes were the same used in voltammetric experiments, the aqueous supporting electrolyte was 0.1 mol L-1 KCl. 2.5 . Molecular dynamics simulations All

molecular

dynamics

simulations

were

performed

using

the

GROMACS software23. The starting configurations were generated with PACKMOL software24. Water and ACN molecules were inserted into tetragonal boxes with the following acetonitrile mole fraction (xACN) 0.10, 0.20, 0.35 and 0.50. Two kinds of boxes were built: One containing only water and acetonitrile molecules (bulk) and another one containing the solvents molecules and two graphene sheets (xy-plane) on each border (z side) of the box as a model of the 8


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working electrodes used on voltammetric studies. To simulate the effect of salts in these water-acetonitrile mixtures, the force field for [Fe(CN)6]3- was built and it is based on the intramolecular parameters derived from Universal Force Field (UFF)25 and it was generated by the OBGMX tool26, which uses the GROMACS package topology format as a template and the open babel libraries to build the connectivity between the atoms.27 The UFF based force field has been proven to be suitable for the metal complexes28,29. The charges for the [Fe(CN)6]3anion were obtained from quantum chemistry calculations at the M06L/def2TZVP level of theory, using the Hirshfeld method implemented in the ORCA package. The topology for the ions is available in the Supporting Information. Table 1 shows the number of species for each system. A typical snapshot of the interface simulation box used is presented in Figure 1. For mixtures systems without K3[Fe(CN)6], the equilibration procedure was 10 ns long at 298 K. After the equilibration, longer simulations lasted for 20 ns in NVT ensemble. When the ions were present, the simulations were 60 ns long. In all the simulations, the Berendsen thermostat was employed with a relaxation time of a 5 ps. The equations of motion were integrated using the leapfrog algorithm with a time step of 2 fs. Lennard-Jones and Coulomb interactions were considered up to 1.5 nm. Coulomb interactions beyond this radius were treated by the Particle Mesh Ewald (PME) method with interpolation order 6 and 0.08 nm mesh spacing. All bonds and angles were kept fixed. Intermolecular interactions were described by typical Coulomb and Lennard-Jones potentials

9



= ∑$ %

+ 4

Page 10 of 59

!

+ "# (3),

where e is the electron charge, ε0 is the vacuum permittivity, qi and qj are the atoms charges i and j, rij is the distance between the pairs, εij is the parameter that measures the depth of the potential well and σij is the point where the potential curve is zero. The Lennard-Jones parameters and the partial atomic charges used in the simulations are presented in Table 2.

3. Results and Discussion 3.1.

Electrochemical

behavior

of

potassium

hexacyanoferrate

in

water/acetonitrile mixtures Potassium hexacyanoferrate is a well-known electroactive probe, its redox process is frequently used to evaluate electrodes performance in voltammetry19,20,31.

Cyclic

voltammograms

of

10

mmol

L-1

potassium

hexacyanoferrate(III) solutions were registered at xACN varying from 0.0 to 0.65. The experiments performed at higher content of ACN were not carried out due to the formation of precipitate and phase separation. The voltammograms registered for mixtures with xACN = 0 – 0.30 are presented on Figure 2. The intensity of current for hexacyanoferrate(III) cathodic reduction gradually decreases as xACN increases. As the limiting current is proportional to diffusion coefficient (D) of the electroactive species19 (Equation 1), the D values for hexacyanoferrate(III) also decrease with the increase of ACN content.

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According to Stokes-Einstein relation (Equation 4), this observation indicates a decrease on diffusion coefficient (D) of the probe caused by increasing of dynamic viscosity of the mixture or increasing of hydrodynamic radius of probe as the content of ACN increases. &'

= !() (4), *

where k is the Boltzmann constant, T is temperature, η is the dynamic viscosity of solution and RH0 is the hydrodynamic radius of the solute. According to Gritzner17, at high water concentrations (xACN < 0.2), the ACN molecules fill the cavities in water structure at the bulk of the mixture. The behavior observed in Figure 2 could be caused by ACN molecules filling water cavities in solvated hexacyanoferrate(III), increasing the value of RH, and consequently, decreasing the value of D and the observed limiting current. Another explanation could be the increasing of viscosity with the increase of xACN in the mixture, but the effect is the contrary in this xACN range32. Additionally, a shift on the E1/2 to more negative/less positive values is observed as the xACN increases (Figure 3). Voltammetric studies16,17 show that the E1/2 values for tetra-n-butylammonium hexacyanoferrate(II) and (III) shift to less positive values with the increase of the mole fraction of ACN into the binary mixture. This behavior was assigned to a more effective stabilization of the Fe(II) complex in electron-acceptor solvents (the acceptor number of water is 54.8, of ACN is 18.9 and of a 33% mol ACN/water mixture is 46.3)33.

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For mixtures with values of xACN among 0.50 and 0.65, the intensity of current increases as xACN increases and reduction and oxidation peaks are observed (Figure 4). These observations suggest that hexacyanoferrate(II) species accumulate onto microelectrode surface in mixtures with xACN > 0.5. Experiments employing ring-disk rotating electrode were carried out to investigate the electrode/solution interface. The intention is verifying whether hexacyanoferrate(II) species generated in disk electrode can be collected on ring electrode in binary mixtures. 3.2. RRDE study for potassium hexacyanoferrate(III) in water/acetonitrile mixtures Experiments with rotating ring-disc electrode (RRDE) were performed to understand why the voltammograms were distorted in binary mixtures with xACN > 0.5. Cyclic voltammograms of hexacyanoferrate(III) were registered in water (Figure 5A) and in a xACN = 0.50 water/ACN mixture (Figure 5B) at different rotation rates. For both cases, the potential on disk electrode was scanned from 0.5 V to negative values and returning to 0.5 V, while the ring electrode was polarized at 0.5 V in order to collect the hexacyanoferrate(II) generated on disk. The collecting efficiency (N), the fraction of hexacyanoferrate (II) which reaches the ring electrode is defined by the Equation 5. +=

,-./ ,012

(5),

where Iring is current on ring electrode and Idisk the current on disk electrode.

12


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The values of collecting efficiency were estimated from the ring and disk current values collected from the voltammograms presented in Figure 5 after both signals have achieved limiting currents. These values are presented on Table 3. Comparing

the

voltammograms

registered

(Figure

5),

relevant

differences can be noticed: (a) the collection efficiencies in binary mixture are very low when compared to those calculated in aqueous solution (Table 3), indicating that a considerable amount of generated hexacyanoferrate(II) accumulates onto disk electrode surface (or do not reach the ring electrode) and only a small amount of these species achieve the ring electrode as soluble species; (b) the oxidation peak in disk voltammogram is still present at rotation rates value of 400 rpm (Figure 5B – black curve) but at higher rotation rates this peak disappears, suggesting that the hexacyanoferrate(II) species accumulate on disk electrode even at hydrodynamic conditions and; (c) the current values for all ring voltammograms registered in binary mixture achieve limiting values around 0.1 V while, in generator electrode, the cathodic current still rising up to 0.1 V. 3.3. Adsorption study for potassium hexacyanoferrate in water/acetonitrile mixtures using the EQCM Experiments with an EQCM were performed to gain information about the accumulation of hexacyanoferrate(II) species electrogenerated onto electrode surface.

13



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In Figure 6 the voltammograms and the vibration frequency of the quartz crystal for K3[Fe(CN)6] solutions in water (A) and in a xACN = 0.50 water/ACN mixture (B). Note that there is no change in vibration frequency on the experiment performed in water, indicating that the electrode process does not involve adsorption. Otherwise, there is a decrease in vibration frequency on experiment performed in the water/acetonitrile mixture, evidencing an adsorptive process in the electrode surface. The careful analysis of Figure 6B provides important information: a) while the reduction signal of hexacyanoferrate(II) appears around 0.2 V, the consecutive decrease of frequency is observed around 0.1 V, i.e., the generation of hexacyanoferrate(II) and its adsorption onto electrode surface are not concomitant, indicating that a given amount of hexacyanoferrate(II) needs to be generated to initiate the adsorptive process and; b) in the reverse scan, after the register of the oxidation peak, the frequency returns to a value very near to the initial value, indicating that a very small amount of hexacyanoferrate(II) stays onto electrode surface after one potential cycle. Some more experiments were performed changing the potential scan rate to estimate the molar weight of the adsorbed species. However, the identification of the adsorbed species using the EQCM was not possible because there is no variation in the amount of hexacyanoferrate(II) adsorbed onto electrode surface with potential scan rate.

14


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3.4. Computational study 3.4.1. Bulk properties Inorganic hexacyanoferrate(III) salts are insoluble in acetonitrile and slightly soluble in mixture of water and acetonitrile. To evaluate the effect of acetonitrile content to the local structure and the diffusion coefficients of ions, molecular dynamics simulations of K3[Fe(CN)6] model were performed. Figure 7 shows the radial distribution functions (rdf) calculated for the correlations Fe-Fe and Fe-K+ in the mixtures with mole fraction of acetonitrile equals to 0.00, 0.10, 0,20 and 0.35 at 298 K. The proper integration of the rdf provides the coordination number, n(r), in this case, the number of anions and cations surrounding an [Fe(CN)6]3-. The Fe-K+ correlations arise around 0.4 nm, which is consistent with the distance found in ab initio molecular dynamics simulation of K4[Fe(CN)6] water solution34. As the acetonitrile mole fraction increases in the mixtures, the Fe-K+ correlations increase, indicating the number of cations surrounding the [Fe(CN)6]3-. For instance, in water solution the average number of K+ close to the anions is 2 at 0.75 nm (black line of the bottom panel on the right). In mixture with xACN = 0.35, the average number of K+ in the first neighborhood of anion reaches 7.5, which indicates the formation of ion clusters in the mixtures with high acetonitrile mole fraction. In fact, the correlations between iron atoms of anion increase in the mixtures with high content of acetonitrile. Note that, the average number of anion close to one another in the mixture with xACN = 0.35 is around 6 (blue line of bottom panel on the left). 15



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The ion clustering observed in the simulations might affect the diffusion coefficient of the ions, D, which can be estimated from slope of the mean

square displacement of the ions in its linear regime,

35

.

Figure 8 presents the mean square displacement (MSD) calculated for the ions in four mixtures from bulk simulations whose box lengths span from 6.24 (xACN = 0.0) to 7.40 nm (xACN = 0.35). In the presence of acetonitrile, the slope of curves decreases taking water solution (xACN = 0.00) as reference. Table 4 shows the diffusion coefficients calculated for the ions in four solvent systems. Although smaller, the calculated and experimental diffusion coefficients of [Fe(CN)6]3- in water (xACN = 0.00) have the same order of magnitude, indicating that the model used for the iron complex is fair to access dynamics properties. The diffusion coefficients of the ions decrease as more acetonitrile is used in mixture, similarly to what happen experimentally obtained by the cyclic voltammetry, section 3.1. The viscosity of water-acetonitrile mixtures decreases with the increase of acetonitrile content in the mixture, as depicted by Moreau32. By the StokesEinstein relation (Equation 4), one would expect an increase of ion diffusion coefficients, if the ion hydrodynamic radii are similar in the mixtures. Therefore, the lowering of anion diffusion coefficients with the increase of acetonitrile in the mixture is, in part, due to larger anion hydrodynamic radii that are likely because of the ion clustering increase.

3.4.2. Interface Properties 16


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Molecular dynamics of electrolytes comprising acetonitrile and ionic liquids accounts for the accumulation of ions and acetonitrile on the surface of graphite electrodes with and without an application of potential difference on the electrodes36. To characterize the structure of water-acetonitrile mixtures at interface with a model of electrode, molecular dynamics simulations were performed for the water-acetonitrile mixtures at interface with graphene sheets as depicted in Figure 1. Figure 9 shows the density profiles of water and acetonitrile close to the graphene sheets. Given the hydrophobicity of graphene sheets, there is an accumulation of acetonitrile on the surfaces of uncharged graphene with the displacement of water molecules toward the bulk region. The effect of increasing the mole fraction of acetonitrile is the appearance of solvent layers close to each graphene surface. In this work, the cyclic voltammograms were obtained by applying low potentials. Therefore, molecular dynamics simulations of these wateracetonitrile mixtures were performed with charged graphene sheets to evaluate the effect of the applied potential to the structure of liquids at interfaces, Figure 10. In these simulations, all carbon atoms have the same charge (|2.41x10-3q|), yielding a graphene sheet on the left-hand side with -0.1q/nm2, whereas the other one on the right has +0.1q/nm2. No remarkable difference is observed in the interface structure of acetonitrile in comparison with that observed in the uncharged graphene interface, i.e., even at low charge regime the accumulation of acetonitrile prevails.

17



Figure 11 shows the density profiles of water, acetonitrile, [Fe(CN)6]3-, and K+ at charged graphene sheets. The acetonitrile molecules remain adsorbed on the surface of graphene sheets with 0.10q/nm2 charge density. In the xACN=0.20 solution, the ions are located farther from the charged surface. As the mole fraction of ACN decreases, the ions can be closer the charged graphene sheets. It is worth mentioning that in aqueous solution, xACN=0.00, there are small peaks of ions close to the graphene sheets, indicating the adsorption of ions on the charged surfaces. The accumulation of acetonitrile on the surface of charged graphene represents an obstacle for the arriving of electroactive species on the electrode, considering that the [Fe(CN)6]3- has lower affinity for acetonitrile than for water. Thus, to have the electrochemical event in the mixtures with high acetonitrile mole fraction, the [Fe(CN)6]3- must disrupt the acetonitrile barrier close to the electrode, delaying the arrival of electroactive species on the electrode, which might be also responsible for decrease of D as the xACN increases. In fact, according with Karyakin et al.37, a reduction-oxidation reaction involving organic electroactive species when a thin layer of organic solvent is present on the electrode surface depends on the counter ion migration from aqueous to organic phase. In the present case, both electroactive species and counter-ion are hydrophilic, so the migration of K+ to the acetonitrile-rich electrode surface is not favored. Turning back to the adsorption of [Fe(CN)6]4- species on the electrode when the xACN is 0.5 (Figure 6B), the accumulation of ACN on the electrode surface could be the driving force to the electrogenerated [Fe(CN)6]4- adsorbs 18


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on the electrode surface. As the anion has even less affinity by the ACN accumulated on the surface and also have to overcome this ACN barrier in order to diffuse toward bulk, they adsorb on the surface. The layer of ACN on the surface of carbon electrodes may also affect electrochemical behavior of species with low affinity for water. A well-known example is the anodic oxidation of iodide producing iodine (Reaction 1) that adsorbs onto electrode surface in aqueous solution due to its low solubility (around 1 x 10-3 mol L-1) (Figure 12A). The adsorption of iodine blocks the formation of triiodide, a water-soluble complex (Reaction 2).

23 ⇌ 2 + 25 3 (Reaction 1) 2 + 3 ⇌ 33 (Reaction 2) 233 ⇌ 32 + 2 3 (Reaction 3) Voltammograms registered in binary mixtures of water and acetonitrile (Figure 12B) shows that this adsorption does not occur, i.e, there is no reduction peak in the voltammogram, a detailed description about the mechanism of these electrode processes can be found elsewhere38. Probably, the presence of acetonitrile barrier on electrode surface hinders the adsorption of iodine because the solubility of iodine in acetonitrile is larger than that in pure water. Under these conditions, the chemical reaction between iodine and iodide produces triiodide (Reaction 2), allowing the anodic oxidation of triiodide to occur, generating iodine (Reaction 3).

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4. Conclusion The

electrochemical

behavior

of

the

redox

pair

potassium

hexacyanoferrate(II)/(III) in ACN/water binary mixtures was affected by two factors: The E1/2 values are dependent of the ACN content in binary mixture, E1/2 acquires less positive/more negative with the increasing of ACN amount in the mixture, these observations were already known and according to Gritzner and collaborators17,

they

are

dependent

on

specific

interactions

among

hexacyanoferrate species and ACN molecules. The D values of these electroactive species decrease as the xACN increases, contrary to what was expected considering only the dependence of viscosities of these mixtures on the xACN. Computational simulations demonstrate that the ion clustering of hexacyanoferrate(III) results in an increase of hydrodynamic radius of these species. The other point of discussion is about the adsorption of potassium hexacyanoferrate(II) onto electrode surface when the cathodic reduction of hexacyanoferrate(III) were processed in a binary mixture with 50% mol of ACN. The adsorption of material was evidenced by RRDE and EQCM. Computer simulations demonstrate that the solvated hexacyanoferrate species become larger with the increasing of ACN content and that ACN molecules accumulate onto graphene surface. This accumulation did not depend on the surface polarization. As [Fe(CN)6]4- has a larger charge than [Fe(CN)6]3-, the lack of water molecules near to the electrode results on the adsorption of the material on electrode surface. 20


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5. Supporting Information All intramolecular and intermolecular parameters for the acetonitrile, hexacyanoferrate and potassium ions, and the water molecule are described in the Supporting Information at the original format of the Gromacs package, the ITP files used in all MD simulations.

6. Acknowledgements Financial support by the Brazilian agency Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, 2011/17857-9) is gratefully acknowledged. The authors also thank Drs. Mauro Bertotti and Thiago Regis Longo César da Paixão for the use of RRDE and EQCM, respectively.

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7. References [1] Park, H.-R.; Oh, C.-H.; Lee, H.-C.; Lee, J.-K.; Yang, K.; Bark, K.-M. Spectroscopic Properties of Fluoroquinolone Antibiotics in Water–methanol and Water–acetonitrile Mixed Solvents. Photochem. Photobiol., 2002, 75(3), 237– 248. [2] Salmar, S.; Vaalma, M.; Vider, H.; Tenno, T.; Kuznetsov, A.; Järv; Tuulmets, A. Reaction Kinetics and Solubility in Water-Organic Binary Solutions Are Governed by Similar Solvation Equilibria. J. Phys. Org. Chem., 2016, 29, 118– 126. [3] Uchiho, Y.; Goto, Y.; Kamahori, M.; Koda, K. New Baseline Correction Method Using Near-Infrared Absorption of Water in Water/Acetonitrile Gradient High-Performance Liquid Chromatography with Far-Ultraviolet Absorbance Detection. Chromatographia, 2017, 80, 329–333. [4] Du, C.; Hu, Y.; Li, Y.; Fan, L.; Li, X. Electrochemical Detection of Benzo(a)pyrene in Acetonitrile–Water Binary Medium. Talanta, 2015, 138, 46– 51. [5] Valente, I. M.; Gonçalves, L. M.; Rodrigues, J. A. Another Glimpse over the Salting-Out Assisted Liquid–Liquid Extraction in Acetonitrile/Water Mixtures. J. Chromatogr. A, 2013, 1308, 58–62. [6] Marcus, Y. The Structure of and Interactions in Binary Acetonitrile + Water Mixtures J. Phys. Org. Chem., 2012, 25, 1072-1085.

22


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[7] Mountain, R. D. Microstructure and Hydrogen Bonding in Water-Acetonitrile Mixtures. J. Phys. Chem. B, 2010, 114, 16460-16464. [8] Huang, N.; Nordlund, D.; Huang, C.; Bergmann, U.; Weiss, T. M.; Pettersson, L. G. M.; Nilsson, A. X-Ray Raman Scattering Provides Evidence for Interfacial Acetonitrile-Water Dipole Interactions in Aqueous Solutions. J. Chem. Phys., 2011, 135, 164509(1-6). [9] Reimers, J. R.; Hall, L. E. The Solvation of Acetonitrile J. Am. Chem. Soc.,

1999, 121, 3730-3744. [10] Melnikov, S. M.; Höltzel, A.; Seidel-Morgenstern, A.; Tallarek, U. Composition, Structure, and Mobility of Water-Acetonitrile Mixtures in a Silica Nanopore Studied by Molecular Dynamics Simulations. Anal. Chem., 2011, 83, 2569-2575. [11] Mountain, R. D. Molecular Dynamics Simulation of Water−Acetonitrile Mixtures in a Silica Slit J. Phys. Chem. C, 2013, 117, 3923-3929. [12] Melnikov, S. M.; Höltzel, A.; Seidel-Morgenstern, A.; Tallarek, U. Adsorption of Water−Acetonitrile Mixtures to Model Silica Surfaces J. Phys. Chem. C,

2013, 117, 6620-6631. [13] Fileti, E. E.; Dalpian, G. M.; Rivelino, R. Liquid Separation by a Graphene Membrane. J. App. Phys., 108, 2010, 113527. [14] Sigalov, M. V.; Kalish, N.; Carmeli, B.; Pines, D.; Pines, E. Probing Small Protonated Water Clusters in Acetonitrile Solutions by 1H NMR. Z. Phys. Chem., 2013, 227, 983-1007.

23



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[15] Rounaghi, G. H.; Deiminiat, B. Study of Complexation Process Between Nphenylaza-15-crown-5 with Yttrium Cation in Binary Mixed Solvents. J. Incl. Phenom. Macrocycl. Chem., 2012, 72, 113-119. [16] Gritzner, G.; Danksagmüller, K.; Gutmann, V. Outer-Sphere Coordination Effects on the Redox Behaviour of the [Fe(CN6)]3-/ [Fe(CN6)]4- Couple in NonAqueous Solvents. J. Electroanal. Chem., 1976, 72, 177-185. [17] Gritzner, G.; Danksagmüller, K.; Gutmann, V. Solvent Effects on the Redox Potentials

of

Tetraethylammonium

Hexacyanomanganate(III)

and

Hexacyanoferrate(III). J. Electroanal. Chem., 1978, 90, 203-210. [18] Pearson, R. G. Hard and Soft Acids and Bases. J. Am. Chem. Soc., 1963, 85(22), 3533-3539. [19] Baur, J. E.; Wightman, R. M. Diffusion Coefficients Determined with Microelectrodes. J. Electroanal. Chem., 1991, 305, 73-81. [20] Pons, S.; Fleischmann, M. The Behavior of Microelectrodes. Anal. Chem.

1987, 59(24), 1391A-1399. [21] Pedrotti, J.; Angnes, L.; Gutz, I. G. R. Miniaturized Reference Electrodes with Microporous Polymer Junctions. Electroanalysis, 1996, 8, 673-675. [22] Sauerbrey, G. Messung von Plattenschwingungen sehr kleiner Amplitude durch Lichtstrommodulation. Zeitschrift für Physik, 1964, 178, 457-471. [23] Hess, B.; Kutzner, C.; van der Spoel, D.; Lindahl, E. GROMACS 4: Algorithms for Highly Efficient, Load-Balanced, and Scalable Molecular Simulation. J. Chem. Theory Comput., 2008, 4, 435-447.

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[24] Martínez, L.; Andrade, R.; Birgin, E. G.; Martínez, J. M. Packmol: A Package for Building Initial Configurations for Molecular Dynamics Simulations. J. Chem. Theory Comput., 2009, 30, 2157-2164. [25] Rappe, A. K.; Casewit, C. J.; Colwell, K. S.; Goddard III, W. A.; Skiff, W. M. UFF, A Full Periodic Table Force Field for Molecular Mechanics and Molecular Dynamics Simulations. J. Am. Chem. Soc., 1992, 114, 10024-10035. [26] Garberoglio, G. OBGMX: A Web-Based Generator of GROMACS Topologies for Molecular and Periodic Systems Using the Universal Force Field. J. Comput. Chem., 2012, 33, 2204-2208. [27] O’Boyle, N.; Banck, M.; James, C. A.; Morley, C.; Vandermeersch, T.; Hutchison, G. R. Open Babel: An Open Chemical Toolbox. J. Cheminformatics,

2011, 3:33. [28] Vela, J.; Vaddadi, S.; Kingsley, S.; Flaschenriem, C. J.; Lachicotte, R. J.; Cundari, T. R. Holland, P. L. Bidentate Coordination of Pyrazolate in LowCoordinate Iron(II) and Nickel(II) Complexes. Angew. Chem. Int. Ed., 2006, 45, 1607-1611. [29] Knops-Gerrits, P. P.; Goddard III, W. A. Methane Partial Oxidation in Iron Zeolites: Theory versus Experiment. J. Mol. Catal., 2001, 166, 135-145. [30] Méndez-Morales, T.; Carrete, J.; Pérez-Rodriguez, M.; Cabeza, O.; Gallego, L. J.; Lynden-Bell, Ruth; Varela, L. M. Molecular Dynamics Simulations of the Structure of the Graphene-Ionic Liquid/Alkali Salt Mixtures Interface. Phys. Chem. Chem. Phys., 2014, 16, 13271-13278.

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Page 26 of 59

[31] Pitman, K.; Raud, M.; Scotti, G.; Jokinen, V. P.; Franssila, S.; Nerut, J.; Lust, E.; Kikas, T. Electrochemical Characterization of the Microfabricated Electrochemical Sensor-Array System. Electroanalysis, 2017, 29, 249-258. [32] Moreau, C.; Douhéret, G. Thermodinamic Behaviour of Water-Acetonitrile Mixtures. Thermochim. Acta, 1975, 13, 385-392. [33] Mayer, U.; Gerger, W.; Gutmann, V. NMR-Spectroscopic Studies on Solvent Electrophilic Properties, Part II: Binary Aqueous - Non Aqueous Solvent Systems. Mh. Chem, 1977, 108, 489-498. [34] Tirler, A. O.; Persson, I.; Hofer, T. S.; Rode, B. M. Is the Hexacyanoferrate(II) Anion Stable in Aqueous Solution? A Combined Theoretical and Experimental Study. Inorg. Chem., 2015, 54, 10335-1034. [35] Allen, M. P.; Tildesley D. Computer Simulation of Liquids; Clarendon Press: Oxford, U. K., 1987. [36] Lagoutte, S.; Aubert, Pierre-Henri; Pinault, M.; Tran-Van, F.; MayneL’Hermite, M.; Chevrot, C. Poly(3-methylthiophene)/Vertically Aligned MultiWalled Carbon Nanotubes: Electrochemical Synthesis, Characterizations and Electrochemical Storage Properties in Ionic Liquids. Electrochim. Acta, 2014, 130, 754-765. [37] Karyakin, A. A.; Vagin, M. Y.; Ozkan, S. Z.; Karpachova, G. P. Thermodynamics of Ion Transfer Across the Liquid|Liquid Inteface at a Solid Electrode Shielded with a Thin Layer of Organic Solvent. J. Phys. Chem. B,

2004, 108, 11591-11595. [38] Rogers, E. I.; Silvester, D. S.; Aldous, L.; Hardacre, C.; Compton, R. G. Electrooxidation of the Iodides [C4min]I, LiI, NaI, KI, RbI and CsI in Room 26


Page 27 of 59 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


Temperature Ionic Liquid [C4min][NTf2]. J. Phys. Chem. C, 2008, 112, 65516557.

27



Page 28 of 59

Tables Table 1. Compositions of the simulated systems and box length. Bulk simulations xACN

H 2O

ACN

Box-x

Box-y

Box-z

(nm)

(nm)

(nm)

K3[Fe(CN)6]

0

8000

0

40

6.2438

6.2438

6.2438

0.10

7200

800

40

6.5769

6.5769

6.5769

0.20

6400

1600

40

6.9319

6.9319

6.9319

0.35

5200

2800

40

7.3989

7.3989

7.3989

Box-x

Box-y

Box-z*

(nm)

(nm)

(nm)

Interface simulations xACN

H 2O

ACN

K3[Fe(CN)6]

0

5050

0

20

3.2660

3.0744

20.000

0.10

3870

430

-

3.2660

3.0744

20.000

0.20

2920

730

20

3.2660

3.0744

20.000

0.35

1800

1170

-

3.2660

3.0744

20.000

* The graphene sheets were located at z=0 and z=15 nm.

28


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Table 2. Lennard-Jones potential parameters and partial atomic charges. Molecule

Atom

ε (kJ mol-1)

σ (nm)

q (e)

Ref.

H

0

0

0.4238

11

O

0.65

0.31657

-0.8476

11

Me

0.8167

0.375

0.2690

11

C

0.50

0.355

0.1290

11

N

0.50

0.295

-0.3980

11

Fe

0.0544

0.2594

0.0720

26

C

0.4396

0.3431

-0.0720

26

N

0.2889

0.3261

-0.4400

26

K+

0.0014

0.4736

1.0000

C0

0.2930

0.3550

0.000

30

C-0.1

0.2930

0.3550

-2.41x10-3

30

C+0.1

0.2930

0.3550

2.41x10-3

30

H2O

ACN

[Fe(CN)6]3-

K+

Graphene

Note: C0 C-0.1 and C+0.1 atoms refer to the carbon atoms of the graphene sheets when the charge densities were 0, -0.1q nm-2 (left side) and +0.1q nm-2 (right side), respectively.

Table 3. Collecting efficiency (N) for 10.0 mmol L-1 K3[Fe(CN)6] in water and in

29



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a xACN = 0.50 water/ACN mixture. ω1/2 (rpm1/2)

N (water)

N (mixture)

20

0.36

0.06

30

0.36

0.06

50

0.35

0.06

60

0.33

0.06

30


Page 31 of 59 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


Table 4. Diffusion Coefficients of [Fe(CN)6]3- and K+ in different mixture obtained from bulk simulations. [Fe(CN)6]3-

K+

(10-6 cm2 s-1)

(10-6 cm2 s-1)

0

4.1 (0.1)

9.6 (0.8)

0.10

2.1 (0.7)

6.8 (0.3)

0.20

3.2 (0.5)

3.6 (0.3)

0.35

0.5 (0.3)

1.4 (0.2)

xACN

31



Page 32 of 59

Captions Figure 1. Snapshot of a random configuration of water-ACN mixture, x = 0.10, ACN

at the surface of the electrode model, where H (white), O (red), Me (cyan), C (cyan) and N (blue). Figure 2. Cyclic voltammograms recorded with a carbon fiber microelectrode (r = 3.2 µm) of ACN/water binary mixture solution containing 10.0 mmol L-1 K3[Fe(CN)6], where xACN = 0.00 (—), 0.05 (—), 0.10 (—), 0.20 (—) and 0.30 (—). Scan rate = 50.0 mV s-1 Figure 3. Dependence of half-wave potential (E1/2, red) and diffusion coefficient (D, blue) with xACN for 10 mmol L-1 K3[Fe(CN)6] Figure 4. Voltammograms recorded with a carbon fiber microelectrode (r = 3.2 µm) in a solution containing 10.0 mmol L-1 K3[Fe(CN)6]. Scan rate = 50.0 mV s-1. Where xACN = 0.00 (—), 0.50(—), 0.60 (—) and 0.65 (—). Figure 5. Voltammograms recorded at the disk electrode in (A) aqueous solution and (B) water/ACN mixture (xACN = 0.50), both containing 10.0 mmol L-1 K3[Fe(CN)6] and 0.1 mol L-1 KCl. v = 50.0 mV s-1. Rotation rate = (—) 400, (—) 900, (—) 1600 and (—) 2500 rpm. Ering = +0.5 V. Figure 6. Voltammograms (—) recorded with the EQCM gold electrode in (A) aqueous solution and (B) water/ACN mixture (xACN = 0.50), both containing 10.0 mmol L-1 K3[Fe(CN)6] and 0.05 mol L-1 KCl and respective quartz crystal vibration frequency (—).Scan rate = 50.0 mV s-1.

32


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Figure 7. Radial distribution function between hexacianoferrate(III) and K+ ions for 10-90ions (left) and 35-65ions (right) systems at 298K; n(r) are presented in red. Figure 8. Mean square displacement calculated for the ions in four mixtures from bulk simulations whose box lengths span from 6.24 (x=0.0) to 7.40 nm (x=0.35).

Figure 9. Density profiles of water and acetonitrile along z axis after a 20 ns long simulation for all systems in a box with a graphene sheet in each border of z axis. Figure 10. Density profiles of water and acetonitrile along the z-axis for the mixtures in a box with a graphene sheet charged. Figure 11. Density profiles of water, acetonitrile, [Fe(CN)6]3-, and K+ at different acetonitrile mole fraction at graphene sheets with a density charge of 0.1q/nm2. Figure 12. Cyclic voltammograms recorded with a carbon fiber microelectrode (r = 3.2 µm) in (A) aqueous solution and (B) water/ACN mixture (xACN = 0.40), both containing 10.0 mmol L-1 KI. Scan rate = 50.0 mV s-1.

33



Page 34 of 59

Figure 1

34


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Figure 2

35



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Figure 3

36


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Figure 4

37



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Figure 5

38


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Figure 6

39



Page 40 of 59

Figure 7

40


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Figure 8

41



Page 42 of 59

Figure 9.

42


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Figure 10.

43



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Figure 11.

44


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Figure 12.

45



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TOC graphic

46


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0

-4

I / nA

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

Page 48 of 59

-8

-0.2

0

0.2

E/V vs Ag/AgCl ACS Paragon Plus Environment

0.4

Page 49 of 59

0.2 0.15

8 0.1 6

0.05 0

0.2

xACN


0.4

D / 10 cm s

2 -1

10

-6

E1/2 / V

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



40

I / nA

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

Page 50 of 59

20 0

-20 -0.2

0

0.2

E/V vs Ag/AgCl ACS Paragon Plus Environment

0.4

Page 51 of 59

A

B

0.5

I / mA

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


0

-0.5 -1

-1.5 0

0.2

0.4

E / V vs Ag/AgCl

-0.2

0

0.2

0.4

E / V vs Ag/AgCl



A

B -20.1

200

-20.1

0

-200

0

0.2

∆f / kHz

400

∆f / kHz I / µA

I / µA

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

Page 52 of 59

-20.4

0

-20.4

-20.7

-400

-20.7

0.4

0 0.2 0.4 E / V vs Ag/AgCl

E / V vs Ag/AgCl ACS Paragon Plus Environment

Page 53 of 59

Fe - Fe

40

100

g(r)

x = 0.00 x = 0.10 x = 0.20 x = 0.35

60

20

40

10 0 0 14 12 10 8 6 4 2 0 0

Fe - K

80

30

n(r)

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


20 0.5

0.5

1

1.5

1 1.5 r / nm

0 2 0 30 25 20 15 10 5 0 2 0 ACS Paragon Plus Environment

1

1.5

1 0.5 r / nm

1.5

0.5


30 x = 0.00 x = 0.10 x = 0.20 x = 0.35 20

MSD / nm

2

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

Page 54 of 59

10

0 0

2500

5000 t / ps


7500

10000

Page 55 of 59


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 56 of 59

Page 57 of 59

H2O

2000

1500

1000

1000

500

500

Density / kg m

0 0

3

6

9

[Fe(CN)6]

200

12 15

50 6 9 Z / nm

3

6

9

12 15

6 9 Z / nm

12 15

+

100

3

0 0

xACN=0.00 xACN=0.10 xACN=0.20

-3

150

0 0

ACN

2000

1500

-3

Density / kg m

-3

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


12 15

100 80 60 40 20 0 0


K

3


B

100

100

50

50

0

0 0

0.2

0.4

0.6

0.8

1

0

Evs Ag/AgCl / V

0.2 0.4 0.6 0.8

E vs Ag/AgCl / V ACS Paragon Plus Environment

1

I / nA

A

I / nA

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

Page 58 of 59

Page 59 of 59 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


TOC graphic 390x152mm (96 x 96 DPI)


Understanding the Influence of the Electrochemical Behavior of

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