Thermophysical properties of diethylammonium (acetate + water) mixtures at different temperatures

Thermophysical properties of diethylammonium (acetate + water) mixtures at different temperatures

Journal Pre-proofs Thermophysical properties of diethylammonium (acetate + water) mixtures at different temperatures Alene D. Nascimento, Rodrigo dos ...

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Journal Pre-proofs Thermophysical properties of diethylammonium (acetate + water) mixtures at different temperatures Alene D. Nascimento, Rodrigo dos Reis, João Paulo S. Santos, Silvana Mattedi, Lilian F. Senna PII: DOI: Reference:

S0021-9614(19)30472-0 https://doi.org/10.1016/j.jct.2020.106093 YJCHT 106093

To appear in:

J. Chem. Thermodynamics

Received Date: Revised Date: Accepted Date:

20 May 2019 24 February 2020 25 February 2020

Please cite this article as: A.D. Nascimento, R. dos Reis, J.P.S. Santos, S. Mattedi, L.F. Senna, Thermophysical properties of diethylammonium (acetate + water) mixtures at different temperatures, J. Chem. Thermodynamics (2020), doi: https://doi.org/10.1016/j.jct.2020.106093

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Thermophysical properties of diethylammonium (acetate + water) mixtures at different temperatures Alene D. Nascimento†, Rodrigo dos Reis,†, João Paulo S. Santos‡, Silvana Mattedi‡, Lilian F. Senna† †

Institute of Chemistry, State University of Rio de Janeiro, Brazil, P H L C, S. 310, São

Francisco Xavier St., 524 - Maracanã - 20550-900. ‡

Department of Chemical Engineering, Federal University of Bahia, Brazil

Abstract: Density (ρ), speed of sound (w), dynamic viscosity (η), and the electrical conductivity () of the protic ionic liquid (PIL) diethylammonium acetate ([DEA][Ac]), in binary mixtures with water, were measured in order to evaluate how these thermophysical properties are affected by the variation of temperature along a wide composition range. The studied temperature range used for ρ and w was (293.15 to 323.15) K, for η was (283.15 to 343.15) K, and for  was (298.15 to 363.15) K. The pressure was 100.90 kPa. The values obtained for ρ and w were used to calculate the apparent molar volume (vϕ), excess volume (vE), the thermal expansion coefficient (α), and the isentropic compressibility (ks). It was found that temperature affects the properties of the binary mixtures differently and at different degrees, depending on the mole fraction of the PIL. It can also be noted that the electrical conductivity of the system does not depend on its dynamic viscosity only, which would be expected if the electrical conductivity was dominated by diffusion. The other known electrical conductivity mechanisms for PILs are dependent on the formation of hydrogen bonds. Therefore, based on the combined evaluation of the measured properties, [DEA][Ac] seems capable of forming hydrogen bonds with water, leading to high conductivities and making it promising for electrochemical applications.

Keywords: ionic liquid; binary mixture; thermophysical properties.



Corresponding author. Fax: +55-2334.0159; Tel: + 5521 2334 0563. E-mail: [email protected]

1. Introduction

Ionic liquids (IL) are defined as salts that have a melting point below 100 °C, which can be formed from both organic and inorganic bases and acids. Due to their unique properties, such as a wide electrochemical window, a broad temperature range in the liquid phase, high thermal and chemical resistance, high ion electrical conductivity, and charge carrier concentration, ILs have been a common research subject in many different areas [1]. These properties make them excellent for a wide range of applications, such as: electronic energy generation devices [2,3]; catalytic processes, both as solvent and/or as catalyst [4–7], separation processes (such as extraction columns, gas separation membranes, among others) [8–12], and in biological processes (as support, solvent or catalyst) [13–17]. In the literature, ILs are commonly grouped as a function of their cations: ammonium, imidazole, phosphonium, pyridinium, and pyrrolidinone [9]. The ILs where both the cation and the anion have biological origin are defined as “bio-ionic liquids.” In recent years, the need for more environmentally friendly compounds and sustainable technologies increased the investigation concerning alkylammonium-based ILs, and the main research focuses are the physicochemical and transport properties of these species [18]. ILs can be divided into two broad groups, aprotic and protic ionic liquids (AILs and PILs, respectively). AILs are formed from the combination of Lewis acids and bases. It means that, in the synthesis process, there is no transfer of protons from the acid to the base. On the other hand, PILs are characterized by proton transference, being formed from Brönsted acids and bases [19]. The proton on the PIL cation is responsible for hydrogen bonding and ensures strong interactions with polar solvents. The transfer of the proton from the acid to the base, which leads to the presence of donor and proton acceptor sites, can be used to construct a network of hydrogen bonds [20], making it possible to apply it as an electrolyte at temperatures above 100 ºC and even in anhydrous conditions [21]. Also, PILs exhibit good long-term thermal stability. These combined characteristics make these ILs promising for use in commercial applications involving electrochemical equipment such as fuel cells. In general, when compared to PILs, AILs present higher ionic electrical conductivity, as well as higher thermal and chemical resistance, which led most researchers to study the ionic liquids from this group. It is a consequence of large irregular

cations packed with small anions, leading to high mobility and ion concentration [22]. However, PILs generally have a much simpler synthesis process than the AILs, because they are produced from a direct acid-base reaction without the need for intermediates. Consequently, their cost of production is significantly lower than the aprotic ILs, with fewer compounds being used in their synthesis and without the need for post-synthesis separation processes [23]. Diethylammonium Acetate ([DEA][Ac]) is an alkylammonium -based protic ionic liquid, whose properties can be used to obtain more information about this family of PILs. Physico-chemical properties such as specific mass (ρ), speed of sound (w), and dynamic viscosity (η) are of great importance both for orientating their applications and for understanding their intermolecular interactions. The properties of these pure and mixed liquids are used not only to identify the behaviour of the molecular interactions but also to understand their role in the structure making or breaking effects for binaries systems [24]. For electrochemical applications, the charge mobility and, therefore, the electrical conductivity () is considerably affected by the dynamic viscosity of the ILs [25,26]. Also, in separation processes, solvents with low viscosities are desired, which implies lower energy costs in the transport of fluids [11,27]. Although different works evaluated these properties for binary systems of [DEA][Ac] with water [24,28,29], there is no systematic evaluation of the influence of both [DEA][Ac] mole fraction and temperature in a wide range of [DEA][Ac] concentration. It is an important task, considering that ILs may have hygroscopic behaviour and absorb moisture when exposed to humid air. Consequently, it is necessary to study at which extent the mixture of IL and water affects the properties of the IL. The presence of water is also a variable that may explain the discrepancy in the ILs properties data among different authors, as noted by Musale et al. [24]. These differences can lead to problems in the modeling of properties and the design of the equipment involving PILs, because of the quality of the data. Therefore, the measurements of PIL properties must be performed using controlled PIL and water mixtures. Additionally, it is possible to evaluate the molecular interactions between PIL and water by evaluating the variation of the macroscopic properties with temperature, whose influence was studied only for dilute solutions [24]. Zhu et al. [28] used η, ρ and к data to analyze the local composition behaviour in binary mixtures of [DEA][Ac] and water. However, all of their measurements were performed only at 298.15 K. To the best of our knowledge, there is no work showing these results at different temperatures.

Considering all the points described, the goal of this study is the systematic evaluation of density, sound velocity, dynamic viscosity, and electrical conductivity of the PIL diethylammonium acetate ([DEA][Ac]) in binary mixtures with water at different temperatures and pressure of 100.90 kPa.

2. Materials and methods

2.1. Materials. All chemicals required for the synthesis of [DEA][Ac], as well as their degree of purity, are listed in Table 1. They were used in this work without further purification. The synthesized [DEA][Ac] is also listed in Table 1. ⸱⸱ Table 1. Reactants used in the synthesis of [DEA][Ac]

Reactant

mass fraction

CAS

purity

water content/(g⸱g-

Structure

Supplier

1)

Acetic Acid

64-19-7

0.998

-

Merck

N,N-diethylamine

109-89-7

0.99

-

Synth

20726-63-0

0.997*

0.0069**

-

Diethylammonium Acetate

*obtained from

1

H NMR, water content not considered, purified by vacuum

rotoevaporation ** measured with Karl Fischer titration

2.2. Synthesis of PIL. The ionic liquid [DEA][Ac] was synthesized in a jacketed reactor connected to a thermostatic bath maintained at 283.15 K, in an inert nitrogen atmosphere. The diethylamine was placed in the reactor and stirred at 350 rpm. Quantity of acetic acid slightly higher than that stoichiometric required was added dropwise with a separation funnel. The evaporation of the reactants was prevented with a condenser

connected to the same bath (283.15 K). After all of the acid addition and the completion of the reaction, the temperature rose to room temperature (298.15 K), the stirring was elevated to 400 rpm, and nitrogen was bubbled in the product in order to evaporate the unreacted acid. The product was then vacuum rotoevaporated at 313.15 K and 60 kPa for 24h, in order to evaporate any absorbed water or unreacted compounds. The structure of the synthesized PIL was confirmed by using 1H and

13

C NMR

spectra obtained at 298K using a Brucker 400 MHz and 9.39 Tesla. The solvent used to obtain the spectra was deuterated water. To calibrate the signal at ~0 ppm 3-deuterated (trimethylsilyl)propanoic acid (TSP) was used. The spectra were processed with MesTreNova 11.0 and are shown in Appendix 1 (Figures A.1 and A.2). The spectra confirm that the [DEA][Ac] was synthesized correctly. Finally, the water content in the prepared ionic liquid was determined by KarlFischer titration (Mettler Toledo Karl Fischer Titrator, model V20, using the Karl Fischer reagent Apura Combi Titrant 5). The measurement was performed in triplicate and its average value was found to be (0.0069 ± 0.0005) g⸱g-1, which was taken into consideration during the preparation of solutions. The IL was stored in amber glass and kept in a desiccator with silica gel. All the solutions of PILs ([DEA][Ac] + water) were prepared to cover a wide range of compositions on a mass basis, using a Shimadzu (AX200 model) analytical balance with a precision of  0.1 mg. The combined mole fraction uncertainty was estimated to be in the order of 0.0001. MilliQ water (electrical conductivity = 0.05 μS cm-1) was used to prepare all solutions used in the present work. For the preparation of the solutions, the water and the IL were homogenized during a long stirring period, in order to assure complete homogenization. Vials with septa were used during the sample manipulation to avoid both absorption and loss of water. The water content of the solutions was also measured, and it was observed that the moisture absorption from the air was negligible.

2.3. Characterization

2.3.1. Density and Speed of sound Measurements

The density and speed of sound of the binary mixtures in the entire mole fraction range were obtained simultaneously using an Anton Paar densimeter (DSA 5000). This

equipment has two measuring cells; a cell with U-tube vibration for the density measurements and a pulse-echo cell for speed of sound measurements. The operating frequency for the speed of sound measurements is 3 MHz. Both cells are equipped with Peltier elements for temperature control with an accuracy of ± 0.01 K. The instrument was calibrated with deionized water (2 μS.cm-1) and air, as reported elsewhere [30]. The results were compared to the NIST standard data [31]. The density and speed of sound measurements for aqueous [DEA][Ac] solutions were carried out using a temperature range from 293.15 K to 323.15 K, with a step of 5 K. The data were determined in duplicate and the expanded uncertantites were calculated with a level of confidence of 95 % (k = 1.96); additional details about calibration and determination of standard and expanded uncertainties can be found on Paredes et al. [32]. The same procedure was used to calculate the expanded uncertainties for all the properties measured in this work. The values obtained for the binary mixtures were ± 0.002 g⸱cm-3 for density and ± 7 m⸱s-1 for speed of sound, up to a mole fraction of 0.75. The calculated uncertainties for both the density and the speed of sound data were used to determine the combined uncertainties in the derived properties, such as the apparent molar volumes and the isentropic compressibility, which are also reported in the Results section. [DEA][Ac] solidifies at high concentrations in low temperatures, which could break the capillary in the equipment. Therefore, for the mole fraction of 0.835, we first heated the sample had to be liquefied before its introduction into the capillary. The extra manipulation of the sample to ensure the integrity of the equipment could lead to extra absorption of water that could not be measured, leading to higher deviations. Due to this, we obtained a triplicate for the higher concentration and calculated the expanded deviation, which is shown in Tables 3 and 4.

2.3.2. Dynamic viscosity

The dynamic viscosity was determined by an Anton Paar viscometer model SVM 3000, using the methodology described in ASTM D 445-01 [33]. The equipment was previously calibrated by using standard oils (supplied by the manufacturer) of different viscosities. The dynamic viscosity measurements were determined from 283.15 K to 343.15 K, with a step of 2.5 K. The calculated expanded uncertainty for the calibration data was under the ± 1.0 % range. Water dynamic viscosity was also measured, and the

deviations were within this range. The temperature was controlled accurately by Peltier elements with a deviation of ± 0.01 K. For aqueous [DEA][Ac] solutions, the viscosity expanded relative uncertainty (k = 1.96) was within the ± 0.05 × .

2.3.4. Electrochemical impedance spectroscopy

The electrical conductivity of the binary mixtures was measured using electrochemical impedance spectroscopy using a Metrohm AutoLab potentiostat, model PGSTAT 220N, and a two probe method [34] using copper electrodes. The analysis was performed at the open circuit potential, with a frequency range from 100 kHz to 0.01 Hz and an amplitude of 10 mV. The calibration was carried out with a standard 0.01 mol L-1 KCl solution. The cell constant (l/A - the ratio between the distance between the electrodes, l, and the contact area between the electrodes and the medium, A) is equal to 0.33 cm-1. The measurements were repeated three times for each binary mixture, in the temperature range from 298.15 K to 363.15 K. The calculated expanded uncertainty (k = 1.96) for the electrical conductivity results were within the ± 0.1 ×  range.

3. Results 3.1. Thermophysical properties. The values of the density, ρ (Table 3), speed of sound, w (Table 4), dynamic viscosity, η (Table 5), and the electrical conductivity,  (Table 6) for the [DEA][Ac] binary mixtures with water were measured in order to understand the effect of temperature on the thermophysical properties at whole composition range. Because of equipment limitations, the temperature was varied at the range of (293.15 to 323.15) K for ρ and w, (283.15 to 343.15) K for η, and (298.15 to 363.15) K for , under atmospheric pressure. Values for experimental density, speed of sound, and dynamic viscosity data obtained for pure water at different temperatures were compared to those reported elsewhere [31]. It was observed small discrepancies among them (see Appendix 2, Figure A.3, Tables A.1-A.3), with a mean absolute deviation of 0.01 kg⸱m-3 and 0.3 m⸱s-1 for density and speed of sound, respectively, and mean relative deviation of 0.02 for viscosity.

The comparison among the data for binary mixtures of [DEA][Ac] and water in this work and the literature shows the same general trend. Even if Umapathi et al. [29] reported data for ρ, w, and η for a wide composition range at 298.15 K, the tendency observed by these authors is drastically different from those observed in this work, as well as in the studies of Musale et al. [24] and Zhu et al. [28]. As can be seen in Figure 1a, Musale et al. [24] obtained density data within a narrow [DEA][Ac] mole fraction range, that shows the same trend observed in experimental data obtained in this work and elsewhere [28] The same general trend can be seen in this work and in that of Zhu et al. [28], although a noticeable deviation between both results can be seen for higher mole fractions of [DEA][Ac]. The trend observed in Umapathi et al. [29] differs from the one observed in both this work and Zhu et al. [28]. It is worth to note that different techniques were used for the measurement of the density in both works, Zhu et al. [28] used a pycnometer method, which makes the control of the water content difficult because it maintains large sample volumes in an open system for an extended period. Additionally, Zhu et al. [28] considered a water content of less than 200 ppm for the IL, using this value as the basis for the preparation of the binary mixtures. Figure 1b shows data for pure [DEA][Ac], which is highly hygroscopic, so it is expected that the IL absorbs water from the humid air during the filling of the pycnometer and the initial water content could be higher than that reported (200 ppm). Govinda et al. [35] reported a water content from less than 70 ppm after the synthesis of [DEA][Ac]. Despite using a densimeter, the lower the water content, the higher the tendency to absorb water from the environment. Due to the water absorption from the atmosphere, the control of the water content in pure IL is tricky, making it difficult to obtain accurate data for pure [DEA][Ac]. With that in mind, the density for pure [DEA][Ac] at different temperatures was estimated by the least-squares method, with the five experimental data points with the highest [DEA][Ac] concentration, fitted by linear regression (R2 > 0.99). Small errors in mass fraction originate large deviations in mole fraction, explaining most of the variation in the data observed in Figure 1b. A deviation for higher density values is expected if a lower initial water content is considered because the density of the binary mixtures raises with the water content for high [DEA][Ac] concentrations. However, the difference among the density data for pure [DEA][Ac] from this work, Zhu et al. [28], Musale et al. [24], Umapathi et al. [29], and Govinda et al. [35], when compared to the data from Zhao et al. [36] is significant. As already noted by Musale et al. [24], the work from Zhao et al. uses the [DEA][Ac] abbreviation for two ionic liquids,

diethylammonium acetate and diethanolamine acetate. The structure for these cations differs

significantly,

as

the

diethanolamine

presents

two

hydroxyls,

while

diethylammonium presents none. This difference may explain the significant changes in the measured value of the properties.

Figure 1. Density (a) as function of mole fraction of [DEA][Ac] at 298.15 K and (b) for pure [DEA][Ac] for different temperatures. Comparison between the data reported by Musale et al. [24] (△), Zhu et al. [28] (○), Umapathi et al. [29] (▽), Govinda et al. [35] (◁), Zhao et al. [36] (◇) and this work (■). The dark squares denote estimated values obtained by the extrapolation of the linear fit of data at the five highest [DEA][Ac] compositions.

Figure 2 shows the comparison among experimental data obtained in the present work for the speed of sound and those reported by Musale et al. [24] and Umapathi et al. [29] (∇) for the studied mixture. To the best of our knowledge, there is no other work presenting the speed of sound experimental data for [DEA][Ac]-water mixture at a wide range of concentrations and different temperatures. It can be observed that the data points from Musale et al. [24] show the same initial trend of the data obtained in this work, but the range of [DEA][Ac] mole fraction is different and there is no overlap at any points. As for Umapathi et al. [29], as it was observed for the density, the trend observed differs from this work, and from those of Musale et al. [24].

Figure 2. Speed of sound as a function of the mole fraction of [DEA][Ac] at 298.15 K. Comparison between the data obtained in this work (◻) and those reported by Musale et al. [24] (△) and Umapathi et al. [29] (▽). The dark square denotes the estimated value obtained by the extrapolation of the linear fit of data at the five highest [DEA][Ac] composition.

The comparison between the dynamic viscosity data obtained in this work and those reported by Zhu et al. [28], Umapathi et al. [29], Sun et al. [37], and Zhao et al. [36] is presented in Figure 3a.

However, the data from Zhao et al. [36] differs

significantly from the other sets of data, being considerably higher than these. As mentioned earlier, Zhao et al. [36] also refer to diethanolamine acetate as [DEA][Ac]. The presence of the hydroxyls in the structure of the cation would lead to higher viscosity due to the stronger interaction between the chains, explaining the reported value. Figure 3b shows the comparison for dynamic viscosity with a zoom in the region under 70 mPa.s, for easier visualization. As it can be seen, for mole fractions of [DEA][Ac] below 0.5, there is good agreement between the data from this work and Zhu et al. [28], but it is possible to notice deviations within higher mole fractions of [DEA][Ac] range. Although the same tendency is observed for both curves, the dynamic

viscosity value obtained in the present work is higher for higher [DEA][Ac] mole fractions. These authors [28] used an Ubbelohde viscometer, which consists of a system open to the atmosphere, making the control of the water content more difficult. When comparing both sets of data with Umapathi et al. [29], it is possible to notice that this set of data differs noticeably from the other two, with much lower viscosity values and with a narrower range of values. As discussed before, the same behaviour was observed with the data from Umapathi et al. [29] for density and speed of sound. The dynamic viscosity data for pure [DEA][Ac] from Sun et al. [37] is lower than the ones from this work and Zhu et al. [28], but higher than the ones from Umapathi et al. [29], showing there is a discrepancy among the literature data. It can be explained by the fact that, at 298.15 K, [DEA][Ac] is at solid state in a water content lower than 2.5 % wt. Thus, any dynamic viscosity measurements carried out under these conditions would need the use of a metastable phase [38]. Additionally, as previously said, the water content could present high uncertainty at near pure [DEA][Ac] composition range.

Figure 3. Dynamic viscosity as function of mole fraction of [DEA][Ac] at 298.15 K. Comparison among the data obtained in this work and those reported (◻) by (a) Zhu et al. [28] (○), Umapathi et al. [29] (▽), Zhao et al. [36] (◇), and Sun et al. [37] (▷); and (b) by (a) Zhu et al. [28] (○), Umapathi et al. [29] (▽), and Sun et al. [37] (▷).

The values of ρ and w obtained in this work were used to calculate the apparent molar volume (vϕ), Uc(vϕ) = 2x10-7 m3.mol-1; the thermal expansion coefficient (α), Uc(α) = 1x10-6 K-1; and the excess molar volume (VE), Uc(VE) = 1x10-8 m3.mol-1, (Table 3), and the isentropic compressibility (ks), Uc(ks) = 4 TPa-1, (Table 4). The dynamic viscosity data is presented in Table 5. The apparent molar volume is calculated by Equation 1 [39]:

M (ρ-ρ0 ) v∅ = [ ] - [ ] , (1) ρ m∙ρ∙ρ0

where M is the molar mass of [DEA][Ac]; ρ is the density of the binary mixture; ρ0 is the density of the solvent, in this case, water; and m is the molality of the binary mixture. The thermal expansion coefficient is calculated by Equation 2 [39]:

𝛼 =−(

𝜕 𝑙𝑛(𝜌) ) , (2) 𝜕𝑇 𝑝,𝑥

where ρ is the density, T is the temperature, p is the pressure, and x is the [DEA][Ac] mole fraction. The excess molar volume is calculated by Equation 3 [32]: 2 E

V =V- ∑ xi Vi , (3) i=1

where V is the molar volume of the mixture, xi is the mole fraction of the component, and Vi is the molar volume of the pure components. The values of excess molar volume were fitted by Redlich-Kister equation (RK) using Equation 4 [32]: VE =x1 x2 [(A0 +A1 T-1 +A2 T-2 )+(B0 +B1 T-1 +𝐵2 T-2 )(x1 -x2 )+(C0 +C1 T-1 +C2 T-2 )(x1 x2 )2 ], (4)

The calculated R-K parameters are reported in Table 2. Table 2. Redlich-Kister parameters for excess molar volume, VE Parameter

Equation 4

A0

-1.79·10-5 m3·mol-1

A1

2.19·10-3 m3·mol-1·K-1

A2

1.5·10-12 m3·mol-1·K-2

B0

1.04·10-6 m3·mol-1

B1

2.07·10-6 m3·mol-1·K-1

B2

-7.05·10-9 m3·mol-1·K-2

C0

-4.39·10-6 m3·mol-1

C1

2.12·10-6 m3·mol-1·K-1

C2

-7.23·10-9 m3·mol-1·K-2

Table 3. The density (ρ), the apparent molar volume (vϕ), the thermal expansion coefficient (α), and excess molar volume (VE) of the binary solutions of [DEA][Ac] and water at T = (293.15 to 323.15) K and at a pressure of 100.90 kPa

x[DEA][Ac] 0 0.015 ± 0.001 0.055 ± 0.001 0.119 ± 0.001 0.168 ± 0.002 0.240 ± 0.002 0.350 ± 0.003 0.549 ± 0.004 0.708 ± 0.004 0.835 ± 0.005 1.000*** 0 0.015 ± 0.001 0.055 ± 0.001 0.119 ± 0.001 0.168 ± 0.002 0.240 ± 0.002 0.350 ± 0.003 0.549 ± 0.004 0.708 ± 0.004 0.835 ± 0.005 1.000*** 0 0.015 ± 0.001 0.055 ± 0.001 0.119 ± 0.001 0.168 ± 0.002 0.240 ± 0.002

10-3ρ/(kg⸱m-3) T/K=293.15 0.99820 ± 0.00002* 1.003 1.015 1.021 1.020 1.013 0.999 0.985 0.972 0.965 ± 0.006** 0.949 T/K=298.15 0.99704 ± 0.00002* 1.001 1.012 1.018 1.017 1.010 0.997 0.982 0.969 0.961 ± 0.01** 0.946 T/K=303.15 0.99564 ± 0.00002* 0.999 1.010 1.015 1.014 1.006

106vϕ/(m3⸱mol-1)

104α/K-1

106VE/(m3⸱mol-1)

133 131 130 131 132 133 135 137 138 ± 0.9**

2.214 ± 0.001* 2.53 4.96 6.02 6.30 6.42 6.39 6.28 7.06 7.36 ± 0.02**

0.00 -0.19 -0.77 -1.53 -1.94 -2.30 -2.49 -2.67 -2.16 -1.74 ± 0.09** 0.00

133 132 131 131 132 134 136 138 139 ± 1.4**

2.627 ± 0.001* 2.92 4.96 6.02 6.30 6.42 6.39 6.28 7.06 7.36 ± 0.02**

0.00 -0.19 -0.76 -1.51 -1.93 -2.28 -2.48 -2.68 -2.14 -1.71 ± 0.14** 0.00

133 132 131 131 132

3.040 ± 0.001* 3.30 4.96 6.02 6.30 6.42

0.00 -0.19 -0.76 -1.50 -1.92 -2.28

0.350 ± 0.003 0.549 ± 0.004 0.708 ± 0.004 0.835 ± 0.005 1.000*** 0 0.015 ± 0.001 0.055 ± 0.001 0.119 ± 0.001 0.168 ± 0.002 0.240 ± 0.002 0.350 ± 0.003 0.549 ± 0.004 0.708 ± 0.004 0.835 ± 0.005 1.000*** 0 0.015 ± 0.001 0.055 ± 0.001 0.119 ± 0.001 0.168 ± 0.002 0.240 ± 0.002 0.350 ± 0.003 0.549 ± 0.004 0.708 ± 0.004 0.835 ± 0.005 1.000*** 0 0.015 ± 0.001 0.055 ± 0.001 0.119 ± 0.001 0.168 ± 0.002 0.240 ± 0.002 0.350 ± 0.003 0.549 ± 0.004 0.708 ± 0.004 0.835 ± 0.005 1.000*** 0 0.015 ± 0.001 0.055 ± 0.001 0.119 ± 0.001

0.994 0.979 0.965 0.958 ± 0.014** 0.943 T/K=308.15 0.99402 ± 0.00002* 0.998 1.008 1.012 1.010 1.003 0.990 0.976 0.962 0.954 ± 0.017** 0.939 T/K=313.15 0.99221 ± 0.00002* 0.996 1.005 1.009 1.007 1.000 0.987 0.973 0.958 0.951 ± 0.016** 0.936 T/K=318.15 0.99021 ± 0.00002* 0.994 1.002 1.006 1.004 0.997 0.984 0.970 0.955 0.947 ± 0.015** 0.932 T/K=323.15 0.98803 ± 0.00002* 0.992 0.999 1.003

134 136 138 139 ± 2.0**

7.36 6.28 7.06 7.36 ± 0.02**

-2.49 -2.72 -2.16 -1.71 ± 0.20** 0.00

133 132 132 132 133 134 136 138 140 ± 2.4**

3.453 ± 0.001* 3.69 4.96 6.02 6.30 6.42 6.39 6.28 7.06 7.36 ± 0.02**

0.00 -0.19 -0.76 -1.50 -1.92 -2.29 -2.51 -2.77 -2.18 -1.73 ± 0.24** 0.00

134 132 132 132 133 135 137 139 140 ± 2.3**

3.866 ± 0.001* 4.08 4.96 6.02 6.30 6.42 6.39 6.28 7.06 7.36 ± 0.02**

0.00 -0.20 -0.75 -1.50 -1.92 -2.29 -2.53 -2.81 -2.20 -1.73 ± 0.23** 0.00

134 133 132 133 134 135 137 139 141 ± 2.2**

4.275 ± 0.001* 4.46 4.96 6.02 6.30 6.42 6.39 6.28 7.06 7.36 ± 0.02**

0.00 -0.20 -0.75 -1.49 -1.92 -2.30 -2.54 -2.85 -2.21 -1.73 ± 0.21** 0.00

134 133 133

4.692 ± 0.001* 4.85 4.96 6.02

0.00 -0.20 -0.75 -1.49

0.168 ± 0.002 1.001 133 6.30 -1.92 0.240 ± 0.002 0.993 134 6.42 -2.30 0.350 ± 0.003 0.981 136 6.39 -2.56 0.549 ± 0.004 0.966 138 6.28 -2.87 0.708 ± 0.004 0.952 140 7.06 -2.20 ** ** ** 0.835 ± 0.005 0.944 ± 0.014 141 ± 2.1 7.36 ± 0.02 -1.70 ± 0.21** *** 1.000 0.929 0.00 Expanded uncertainties (k = 1.96) are U(T) = 0.01 K, U(p) = 0.22 kPa, and U(ρ) = 2 kg.m3. The combined uncertainty for vϕ is Uc(vϕ) = 2x10-7 m3.mol-1, for α is Uc(α) = 1x10-6 K1 , and for VE is Uc(VE) = 1x10-8 m3.mol-1. The expanded uncertainty U(x[DEA][Ac]]) for the mole fraction varies with the composition, as shown in the table. * The uncertainty for pure water was calculated separately ** Due to a change in methodology described in Section 2.3.1, the expanded uncertainty for this mole fraction was calculated separately *** Estimated through linear regression.

The isentropic compressibility (𝑘𝑠 ) was calculated from the density and the speed of sound, using the Laplace Equation 5 [40]:

ks =

1 , (5) ρw2

where ρ is the density and w is the speed of sound.

Table 4. The speed of sound (w) and the isentropic compressibility (ks) of the binary solutions of [DEA][Ac] and water at T = (293.15 to 323.15) K and at pressure of 100.90 kPa x[DEA][Ac] 0.000 0.015 ± 0.001 0.055 ± 0.001 0.119 ± 0.001 0.168 ± 0.002 0.240 ± 0.002 0.350 ± 0.003 0.549 ± 0.004 0.708 ± 0.004 0.835 ± 0.005 0.000 0.015 ± 0.001

w/(m⸱s-1) T/K=293.15 1482.6 ± 0.5* 1580 1761 1844 1826 1768 1704 1597 1554 1523 ± 53** T/K=308.15 1520.2 ± 0.5* 1601

ks/TPa-1 455.8 ± 0.3* 399 312 288 294 316 344 392 426 447 ± 31** 435.3 ± 0.3* 391

w/(m⸱s-1) T/K=298.15 1496.9 ± 0.5* 1588 1756 1832 1813 1754 1691 1583 1539 1507 ± 54** T/K=313.15 1529.2 ± 0.5* 1605

ks/TPa-1 447.6 ± 0.3* 396 320 293 299 322 351 400 436 458 ± 33** 431.0 ± 0.3* 388

w/(m⸱s-1) ks/TPa-1 T/K=303.15 1509.4 ± 0.5* 440.8 ± 0.3* 1595 393 1751 323 1819 298 1800 305 1741 328 1677 358 1569 409 1524 446 ** 1492 ± 56 469 ± 36** T/K=318.15 1536.8 ± 0.5* 427.6 ± 0.3* 1608 389

0.055 ± 0.001 0.119 ± 0.001 0.168 ± 0.002 0.240 ± 0.002 0.350 ± 0.003 0.549 ± 0.004 0.708 ± 0.004 0.835 ± 0.005

1745 326 1739 329 1732 332 1807 302 1794 308 1782 313 1786 310 1772 316 1758 322 1727 334 1713 341 1699 348 1664 365 1650 372 1636 380 1555 417 1540 426 1526 435 1509 456 1494 467 1479 478 1477 ± 58** 480 ± 38** 1461 ± 59** 493 ± 41** 1446 ± 61** 505 ± 43** T/K=323.15 0.000 1543.0 ± 0.5* 425.1 ± 0.3* 0.015 ± 0.001 1610 389 0.055 ± 0.001 1725 336 0.119 ± 0.001 1768 319 0.168 ± 0.002 1744 329 0.240 ± 0.002 1685 355 0.350 ± 0.003 1622 388 0.549 ± 0.004 1512 445 0.708 ± 0.004 1464 490 ** 0.835 ± 0.005 1430 ± 62 519 ± 46** Expanded uncertainties (k = 1.96) are U(T) = 0.01 K, U(p) = 0.22 kPa, and U(w) = 7 m⸱s1 . The combined uncertainty for ks is Uc(ks) = 4 TPa-1. The expanded uncertainty U(x[DEA][Ac]]) for the mole fraction varies with the composition, as shown in the table. * The uncertainty for pure water was calculated separately ** Due to a change in methodology described in Section 2.3.1 the expanded uncertainty for this mole fraction was calculated separately Table 5. The dynamic viscosity η of the binary solutions of [DEA][Ac] and water at T = (283.15 to 343.15) K and at pressure of 100.90 kPa x[DEA][Ac] 0.000 0.015 ± 0.001 0.055 ± 0.001 0.119 ± 0.001 0.168 ± 0.002 0.240 ± 0.002 0.351 ± 0.003 0.549 ± 0.004 0.714 ± 0.004 0.840 ± 0.005 0.000 0.015 ± 0.001 0.055 ± 0.001 0.119 ± 0.001

T/K=283.15 1.306 ± 0.03x* 2.067 5.427 15.42 27.41 55.05 92.30 143.2 161.6 T/K=295.65 0.9432 ± 0.03x* 1.4374 3.3961 8.752

T/K=285.65 1.217 ± 0.03x* 1.919 4.909 13.69 24.07 47.84 79.30 122.50 138.49 T/K=298.15 0.8901 ± 0.03x* 1.346 3.124 7.909

η/(mPa⸱s) T/K=288.15 1.138 ± 0.03x* 1.777 4.456 12.18 21.15 41.61 68.30 105.0 119.0 T/K=300.65 0.8417 ± 0.03x* 1.263 2.883 7.165

T/K=290.65 1.066 ± 0.03x* 1.651 4.055 10.87 18.67 36.31 59.02 90.60 102.7 T/K=303.15 0.7974 ± 0.03x* 1.189 2.668 6.510

T/K=293.15 1.002 ± 0.03x* 1.538 3.729 9.736 16.49 31.82 51.18 78.39 89.18 T/K=305.65 0.7567 ± 0.03x* 1.121 2.477 5.943

0.168 ± 0.002 0.240 ± 0.002 0.351 ± 0.003 0.549 ± 0.004 0.714 ± 0.004 0.840 ± 0.005

14.67 28.02 44.73 68.40 77.77

13.13 24.75 39.26 59.87 68.15

11.78 21.94 34.59 52.36 59.96

10.62 19.55 30.58 46.29 53.00

T/K=310.65 T/K=313.15 T/K=315.65 T/K=318.15 0.6848 ± 0.03x* 0.6530 ± 0.03x* 0.6235 ± 0.03x* 0.5961 ± 0.03x* 1.003 0.9519 0.9043 0.8625 2.152 2.013 1.887 1.774 4.998 4.603 4.252 3.937 7.940 7.258 6.652 6.111 14.12 12.76 11.59 10.54 21.68 19.49 17.57 15.87 32.60 29.23 26.29 23.73 37.42 33.52 30.15 27.21 37.98 34.04 30.62 27.63 T/K=320.65 T/K=323.15 T/K=325.65 T/K=328.15 T/K=330.65 * * * * 0.000 0.5706 ± 0.03x 0.5469 ± 0.03x 0.5247 ± 0.03x 0.5040 ± 0.03x 0.4846 ± 0.03x* 0.8259 0.7947 0.7567 0.7280 0.6923 0.015 ± 0.001 1.670 1.576 1.489 1.411 1.339 0.055 ± 0.001 3.655 3.398 3.173 2.965 2.779 0.119 ± 0.001 5.633 5.206 4.821 4.478 4.166 0.168 ± 0.002 9.621 8.811 8.089 7.447 6.878 0.240 ± 0.002 0.351 ± 0.003 14.41 13.12 11.97 10.96 10.06 0.549 ± 0.004 21.50 19.55 17.76 16.18 14.78 0.714 ± 0.004 24.62 22.34 20.32 18.53 16.94 0.840 ± 0.005 25.00 22.67 20.60 18.78 17.14 T/K=333.15 T/K=335.65 T/K=338.15 T/K=340.65 T/K=343.15 * * * * 0.000 0.4664 ± 0.03x 0.4493 ± 0.03x 0.4333 ± 0.03x 0.4181 ± 0.03x 0.4039 ± 0.03x* 0.6658 0.6410 0.6175 0.5954 0.5743 0.015 ± 0.001 1.273 1.213 1.159 1.108 1.052 0.055 ± 0.001 2.608 2.453 2.311 2.182 2.063 0.119 ± 0.001 3.886 3.633 3.402 3.192 3.001 0.168 ± 0.002 6.366 5.907 5.492 5.119 4.779 0.240 ± 0.002 0.351 ± 0.003 9.260 8.549 7.907 7.332 6.812 0.549 ± 0.004 13.57 12.48 11.50 10.62 9.838 0.714 ± 0.004 15.52 14.25 13.11 12.08 11.16 0.840 ± 0.005 15.70 14.39 13.23 12.19 11.24 Expanded uncertainties (k = 1.96) are U(T) = 0.01 K, U(p) = 0.22 kPa, and U(η) = 0.05 x η. The expanded uncertainty U(x[DEA][Ac]]) for the mole fraction varies with the composition, as shown in the table. * The uncertainty for pure water was calculated separately 0.000 0.015 ± 0.001 0.055 ± 0.001 0.119 ± 0.001 0.168 ± 0.002 0.240 ± 0.002 0.351 ± 0.003 0.549 ± 0.004 0.714 ± 0.004 0.840 ± 0.005

T/K=308.15 0.7193 ± 0.03x* 1.059 2.305 5.442 8.712 15.67 24.21 36.57 41.87

9.605 17.47 27.16 41.07 47.01

The EIS intercept of the Nyquist diagram on high frequencies is equivalent to the solution resistance (Rs) and it can be used to calculate the electrical conductivity of the

[DEA][Ac] and water binary mixtures, considering the absence of other ohmic resistance sources, like contact resistance, using Equation 6 [41]:

κ=

l , (6) Rs .A

where κ is the electrical conductivity, l is the distance between the electrodes, Rs is the solution resistance, and A is the contact area between the electrodes and the medium. The results are shown in Table 6. Table 6. The electrical conductivity () of the binary solutions of [DEA][Ac] and water at T = (298.15 to 363.15) K and at pressure of 100.90 kPa x[DEA][Ac]

 /(mS⸱cm-1)

[DEA][Ac]+water T/K=298.15 T/K=303.15 T/K=318.15 T/K=333.15 T/K=348.15 T/K=363.15 0.007 ± 0.001 15.55 16.92 22.48 26.37 30.06 33.99 0.015 ± 0.001 23.46 32.39 47.39 73.68 100.5 129.5 0.054 ± 0.001 30.99 47.61 81.29 130.6 166.2 174.5 0.120 ± 0.001 24.82 31.99 59.08 87.66 105.8 115.8 0.168 ± 0.002 17.26 22.44 40.54 57.49 74.71 90.77 0.240 ± 0.002 9.796 15.02 25.12 36.52 49.23 54.69 0.353 ± 0.003 7.082 8.225 13.39 20.05 28.06 36.55 0.550 ± 0.004 3.651 4.727 7.675 10.70 16.82 22.37 0.714 ± 0.004 2.598 3.649 6.093 8.019 12.99 16.68 0.838 ± 0.005 2.313 2.972 4.976 6.657 11.13 13.16 0.945 ± 0.006 2.304 2.904 4.758 6.496 10.90 12.73 Expanded uncertainties (k = 1.96) are U(T) = 0.1 K, U(p) = 0.22 kPa, and U() = 0.1 x The expanded uncertainty U(x[DEA][Ac]]) for the mole fraction varies with the composition, as shown in the table. The comparison between the electrical conductivity obtained in this work and the data presented by Zhu et al. [28] at 298.15 K is shown in Figure 4. For mole fractions of [DEA][Ac] higher than 0.3, there is good agreement between the two sets of data. However, for lower [DEA][Ac] mole fractions, the electrical conductivity values measured in this work are significantly higher. It can be explained by the different methods used: EIS was used in this work, while Zhu et al. [28] used an electrical conductivity meter. The [DEA][Ac] and water binary mixture have a wide range of electrical conductivity. However, in such a broad range, the electrical conductivity meter should be calibrated using different oscillatory frequencies in order to guarantee the

accuracy of the data at all the composition range [39]. EIS is based on the imposition of a sinusoidal disturbance on the electrochemical system, and the measurement of the impedance of the system is performed over a wide range of frequencies. Due to this, EIS measurements do not have the limitations of electrical conductivity meters, because it is possible to change the frequency range and, therefore, to evaluate a broad range of electrical conductivity values with the same cell constant, just by maintaining the same contact area and the same distance between the electrodes [42,43]. Additionally, the data for pure [DEA][Ac] from Zhao et al. [36], as for the other measured properties, is in disagreement with the data from the other sources, showing a considerably lower value. It can be explained by the higher viscosity of the reported data, considering these properties are inversely proportional when the conductivity is diffusion dominated.

Figure 4. Electrical conductivity as a function of the mole fraction of [DEA][Ac] at 298.15 K. Comparison between the data obtained in this work (◻) those reported by Zhu et al. [28] (○), and Zhao et al. [36] (◇).

4. Discussion

Figure 5 shows the results of density, the calculated thermal expansion coefficient, and the excess molar volume as a function of the mass fraction of [DEA][Ac], at different temperatures. At all studied temperatures, a maximum is observed around [DEA][Ac] mole fraction of 0.15 (Figure 5a), which is equivalent to 50 % wt of [DEA][Ac]. This maximum indicates that, at this point, the mixture water and IL molecules present the most packaged structure in the whole range of proportions, leading to a higher density. The increase of the ρ values for low [DEA][Ac] compositions is originated by the strong hydrogen bonds between water and IL [29], causing more water molecules to form a packing configuration around IL molecules. However, as the IL concentration continues to rise, the IL-IL interaction prevails [28], leading to a decrease in the density values because of the difficulty in packing the ILs due to their size and shape [44].

Figure 5. Density (a), thermal expansion coefficient (b) and excess molar volume (c) of the binary mixtures as a function of [DEA][Ac] mole fraction at 293.15 K (◻); 298.15 K (○); 303.15 K (△); 308.15 K (▽); 313.15 K (◁); 318.15 K (▷); and 323.15 K (◇). Data for VE at 298.15 K obtained by Zhu et al. [28] (●). Solid lines denote RedlichKister equation (R-K)

The increase in the temperature also caused a reduction of the density values for all the binary mixtures analyzed. The rise in the temperature may lead to an increase in the average kinetic energy or even in the internal energy of the system, leading to more agitated molecules occupying a larger volume under the same pressure, resulting in a lower density of the system [45]. Furthermore, the increase in temperature has a more significant effect on this property for higher mole fractions of [DEA][Ac]. Zhu et al. [28] studied a wide range of [DEA][Ac] mole fractions in different solvents at the temperature of 298.15 K and observed that binary mixtures of [DEA][Ac] and water presented significant local heterogeneities. Local composition theory says the binary mixtures of an IL and a solvent are not wholly homogeneous in a specific local area [46]. It means that, in the vicinity of the components, the number of IL or solvent is not necessarily the same as in bulk. At high IL concentration, the interactions between the IL and the solvents are weak and there is an extensive self-association among the ILs. As the temperature rises, there is an increase in the average kinetic energy of the system, disfavoring the IL-IL organization and allowing the water molecules to enter the IL structure. Therefore, at high values of [DEA][Ac] mole fractions, the impact of the temperature is more noticeable, probably because there is a larger variation of local composition. Figure 5b shows that, for lower [DEA][Ac] mole fraction, the thermal expansion coefficient of the system is influenced by the temperature at different degrees. However, as the IL concentration rises, α becomes constant for the same mole fraction of [DEA][Ac], independent of temperature, shown by an overlap of the data points for all temperatures for [DEA][AC] mole fraction above 0.05. The general behaviour is an increase of α with the rise of [DEA][Ac] mole fraction, in which an initial raise of the α values occurs, followed by an almost constant region and, then, by another increase of the values. As reported by Umapathi et al. [29], the water molecules can be accommodated in the interstices of the IL structure, explaining the plateau in Figure 5b.

In general, there is a tendency of less effective packing of the system with the raise in [DEA][Ac] mole fraction. The excess molar volume (VE) (Figura 5c) shows a negative deviation in the whole composition range. The same behaviour was observed by Zhu et al. [28] at 298.15 K, also presented in Figure 5c. These values are related to the net creation of interaction caused by mixing. There is a minimum due to the formation of hydrogen bonds between the water and the ionic liquid ions. The negative values reflect the strong interaction between water and [DEA][Ac]. In the area of low [DEA][Ac] concentration, the water molecules create a packed solvation shell around the IL molecules, creating a configuration with a higher density than that of pure water, with the VE becoming more negative until it reaches a minimum. In the other extreme, for higher [DEA][Ac] concentrations, VE becomes more negative with the addition of water, due to how the water molecules can occupy the free volume between the IL molecules, generating a more packed structure [29]. This effect seems to become more relevant with increasing temperature, due to the rising free volume at high [DEA][Ac] content in the mixtures. As shown in Figure 6a, the speed of sound behaviour was analogous to that observed for the density (Figure 5a). While the speed of sound is inversely proportional to the density of the material, it is also directly proportional do the surface tension, which is an elastic property. A higher density value leads to more elasticity (because of the proximity of the molecules), and elasticity contributes more than density to the speed of sound values [47]. Therefore, the speed of sound trend observed in this work was expected. Figure 6b shows the behaviour of the isentropic compressibility, which presents an inverse behaviour when compared to those verified for the density and the speed of sound. It is an expected result because a higher packing leads to lower isentropic compressibility [32].

Figure 6. Speed of sound (a) and isentropic compressibility (b) of the binary mixtures as a function of [DEA][Ac] mole fraction at 293.15 K (◻); 298.15 K (○); 303.15 K (△); 308.15 K (▽); 313.15 K (◁); 318.15 K (▷); and 323.15 K (◇).

The dynamic viscosity of the binary mixtures at different temperatures and mole fractions of [DEA][Ac] can be observed in Figure 7. As well as the density and the speed of sound results, there is a pattern in the dynamic viscosity dependence on temperature, and higher temperatures led to lower dynamic viscosity values. However, the dynamic viscosity values always rise as the mole fraction of [DEA][Ac] becomes higher. Dynamic viscosity can be related to the relative motion between two molecules of a fluid [48]. Therefore, both the intermolecular forces and the shape of the molecules affect this parameter. IL-IL interaction provides much more friction and, therefore, leads to higher viscosities than water-water and water-IL interaction. Consequently, the addition of more water always leads to a decline in the dynamic viscosity data. The impact of the temperature is hugely significant for higher [DEA][Ac] mole fractions, even more than its effects on the density (Figure 5a) and the speed of sound (Figure 6a). The rise in the temperature allows the ILs to move more freely and interact with other ILs or with water. As the IL-IL interactions are more favored than IL-water ones at higher [DEA][Ac] concentration, the rise in the temperature produced such a stronger influence in the dynamic viscosity values for binary mixtures obtained under this condition, as presented in Figure 7.

Figure 7. Dynamic viscosity of the binary mixtures as a function of [DEA][Ac] mole fraction at 283.15 K (◻); 288.15 K (○); 293.15 K (△); 298.15 K (▽); 303.15 K (◁); 308.15 K (▷) ; 313.15 K (◇); 318.15 K (⬠); 323.15 K (⬡); 328.15 K (☆); 333.15 K (x); 338.15 K (◫); and 343.15 K (◐).

When analyzing the behaviour of electrical conductivity (Figure 8), it is important to note that three different types of charge transport are usually considered in protic ionic liquids. The simplest case is the proton migration by diffusion through the material. In this case, the dynamic viscosity of the system controls its electrical conductivity. In the second case, the same proton is transferred sequentially through a network of hydrogen bonds, from one carrier to the other. This mechanism differs from the third, the so-called Grotthus mechanism. Initially proposed to explain the high electrical conductivity of water, the Grotthus mechanism also gives origin to the superprotonic behaviour of many other compounds. In this case, an "excess" proton diffuses through a network of hydrogen bonds, by forming and breaking covalent bonds; thus, only with proton transfer between neighboring molecules [44]. For example, the electrical conductivity contributions from

delocalized protons (participating in the Grotthus mechanism) were observed in imidazole-based PIL [21]. As expected, higher temperatures lead to higher electrical conductivity values. The diffusion dependent electrical conductivity is directly related to the dynamic viscosity of the medium, and higher temperatures lead to lower viscosities and, consequently, to higher electrical conductivity values. However, as the formation of hydrogen bonds also participates in the conductivity of some materials, the relationship between electrical conductivity and the variation of the [DEA][Ac] mole fraction at different temperatures is not dependent solely on the dynamic viscosity of the binary mixture. The process, therefore, is very complex and the complete explanation of this phenomenon is still a work in progress.

Figure 8. Electrical conductivity of the binary mixtures as a function of [DEA][Ac] mole fraction at 298.15 K (◻); 303.15 K (○); 318.15 K (△); 333.15 K (▽); 348.15 K (◁); and 363.15 K (▷).

As it can be observed in Figure 8, there is a maximum on electrical conductivity for all temperatures in the [DEA][Ac] mole fraction around 0.05. This behaviour, in which there is an initial increase in conductivity, followed by a maximum and a subsequent decrease in this property as the IL concentration was raised, is verified for both protic [49] and aprotic [50,51] ionic liquids in different IL binary mixtures, with water [49,51] and other polar solvents [50]. Two aspects may contribute to this performance: the mobility reduction of the charge carriers, caused by the increasing viscosity, and the reduction of the number of available charge carriers due to aggregation between the anion and the cation. At 298.15 K, the electrical conductivity values for the binary mixtures found in this work are in the same range of the ones for other protic ionic liquids [49]. The maximum conductivity point, although presenting low dynamic viscosity value, differs from that shown in Figure 7 for the lowest dynamic viscosity value. If the electrical conductivity of the system was controlled solely by diffusion, the lower dynamic viscosity values should overlap with the higher electrical conductivity values, and the behaviour with temperature should be similar for both properties. However, the temperature has a significantly higher effect in the electrical conductivity for low [DEA][Ac] mole fractions, the opposite behaviour of the dynamic viscosity, suggesting a change in the electrical conductivity mechanism Additionally, the behaviour of the density (Figure 5a) indicates maximum IL-water interaction at the 0.15 [DEA][Ac] mole fraction area. The region with low viscosities and high densities is equivalent to the area with higher electrical conductivity. Above [DEA][Ac] mole fractions around 0.8, the conductivity values do not present significant variation. To further analyze this behaviour, the dependence of the electrical conductivity on the dynamic viscosity of the binary mixtures at three different temperatures (303.15 K; 318.15 K; and 333.15 K) is shown in Figure 9. The same behaviour is noted for all diagrams, which present an initial rise in electrical conductivity with the addition of more [DEA][Ac], followed by a drop in the electrical conductivity values as the dynamic viscosity rises, as observed by Zhu et al. [28] at 298.15 K. The rise in electrical conductivity can be probably attributed to the formation of hydrogen bonds between [DEA][Ac] and water. Alvarez et al. [52] observed the existence of a relatively fast chemical exchange equilibrium of the unstable protons in the alkylammonium-based IL [2-HEA][Ac] in mixtures with hydroxylic solvents, especially with water. This equilibrium indicates that the two components are close to each other, with the spatial

arrangement between them likely stabilized through the formation of hydrogen bonds. In this work, the results suggest that 0.05 mole fraction of [DEA][Ac] can be low enough to allow the water to ionize the IL, even though high enough to promote the electrical conductivity of the medium.

Figure 9. Electrical conductivity of the binary mixtures as a function of η at 303.15 K (◻); 318.15 K (○); and 333.15 K (△).

4. Conclusions

In this work, the synthesis and thermophysical properties density, speed of sound, and dynamic viscosity, as well as the electrical conductivity, of binary mixtures of the protic ionic liquid diethylammonium acetate and water are reported within the temperature range (293.15 to 363.15) K. For all the different properties measured, it was observed a more significant variation among the data for different temperatures depending on the mole fraction of [DEA][Ac] in the binary mixture. The thermal expansion coefficient showed a tendency of a less efficient packing when the ionic liquid concentration increased, which could be a consequence of the shape and size of the ionic

liquid, making packing more difficult in rich [DEA][Ac] regions. The excess molar values reflected the strong interaction between [DEA][Ac] and water. Additionally, the isentropic compressibility initially declined with a higher [DEA][Ac] concentration, reached a minimum and reached higher values than the ones for the water-rich region, corroborating the conclusion from the thermal expansion coefficient, since a higher ks indicates a lesser packing. Dynamic viscosity presented a monotonic behaviour, always rising with the increase of the mole fraction of [DEA][Ac] and decreasing up to high temperatures. However, the electrical conductivity did not follow the pattern of the dynamic viscosity, showing it is not dominated only by diffusion. This work shows evidence of a combination of effects, with the electrical conductivity of the system depending on both the hydrogen bonds between the IL and water and on the dynamic viscosity of the system. The ability to form hydrogen bonds is especially important for electrochemical use, making [DEA][Ac] a promising protic ionic liquid for such applications. Supporting Information Available: 1H and

13

C NMR spectra for [DEA][Ac];

and comparison between the density, speed of sound and viscosity data of water as a function of temperature obtained in this work with data obtained by Lemmon et al. 31

Acknowledgment

The authors acknowledge the financial support from FAPERJ and FAPESB for this work and for the scholarship of, respectively, Alene D. Nascimento and João Paulo S. Santos. Silvana S. Mattedi acknowledges CNPq (Grant 306640/2016) and FAPESB/SECTI (Project APP0075/2016). Rodrigo A. dos Reis and Lilian F. Senna, also thanks to the Prociência Program (UERJ). The authors are grateful to LABAREMN/IQUFBA for NMR spectra aquisition.

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Appendix 1 Proton and carbon NMR spectra are presented in Figures A.1 and A.2, respectively. 1H NMR shows the peaks related to [DEA][Ac] and the solvent. Using the integral values, this spectrum confirms that [DEA][Ac] was synthesized with a purity > 0.99. Additionally, 13C NRM shows solely the peaks expected for [DEA][Ac]. Therefore, the present NMR analysis indicates that [DEA][Ac] was successfully synthesized.

Figure A.1. 1H NMR spectra for [DEA][Ac]

Figure A.2. 13C NMR spectra for [DEA][Ac]

Appendix 2

Figure A.3. Comparison between the density (a), speed of sound (b) and viscosity (c) data of water as a function of temperature obtained in this work (◻) with data obtained by Lemmon et al. [31] (○)

Table A.1. Density data for water as a function of temperature obtained in this work, at a pressure of 100.90 kPa, compared with data obtained by Lemmon et al. [31] ρ/(kg⸱m-3) This work NIST Absolute deviation 283.15 999.72 999.702 2.00x10-2 285.65 999.46 999.442 1.50x10-2 288.15 999.11 999.103 1.00x10-2 290.65 998.69 998.69 3.00x10-3 293.15 998.20 998.207 3.00x10-3 295.65 997.66 997.659 2.00x10-3 298.15 997.04 997.048 8.00x10-3 300.65 996.37 996.377 7.00x10-3 303.15 995.64 995.649 6.00x10-3 305.65 994.86 994.867 2.00x10-3 308.15 994.02 994.033 9.00x10-3 310.65 993.15 993.149 1.00x10-3 313.15 992.21 992.216 4.00x10-3 315.65 991.23 991.237 3.00x10-3 318.15 990.21 990.213 3.00x10-3 320.65 989.14 989.145 0.00 323.15 988.03 988.035 5.00x10-3 Expanded uncertainties (k = 1.96) are U(T) = 0.01 K, U(p) = 0.22 kPa, and U(ρ) = 0.02 T/K

kg⸱m-3 Table A.2. Speed of sound data for water as a function of temperature obtained in this work, at a pressure of 100.90 kPa, compared with data obtained by Lemmon et al. [31] T /K

283.15 285.65 288.15 290.65 293.15 295.65 298.15 300.65 303.15

w /m⸱s⁻¹ This work

NIST

1447.8 1457.3 1466.3 1474.7 1482.6 1490.0 1496.9 1503.4 1509.4

1447.27 1456.89 1465.93 1474.41 1482.35 1489.77 1496.7 1503.16 1509.15

Absolute deviation 5.30x10-1 3.80x10-1 3.30x10-1 3.00x10-1 2.60x10-1 2.30x10-1 2.40x10-1 2.40x10-1 2.60x10-1

305.65 1515.0 1514.71 2.80x10-1 308.15 1520.2 1519.85 3.00x10-1 310.65 1524.9 1524.57 3.10x10-1 313.15 1529.2 1528.9 3.20x10-1 315.65 1533.2 1532.86 3.30x10-1 318.15 1536.8 1536.45 3.50x10-1 320.65 1540.0 1539.68 3.70x10-1 323.15 1543.0 1542.58 3.90x10-1 Expanded uncertainties (k = 1.96) are U(T) = 0.01 K, U(p) = 0.22 kPa, and U(w) = 0.5 m⸱s⁻¹ Table A.3. Viscosity data for water as a function of temperature obtained in this work, at a pressure of 100.90 kPa, compared with data obtained by Lemmon et al. [31] ƞ /mPa⸱s This work NIST Absolute deviation Relative deviation 283.15 1.306 1.3133 7.43x10-3 5.69x10-3 285.65 1.217 1.2297 1.27x10-2 1.04x10-2 -2 288.15 1.138 1.1517 1.41x10 1.24x10-2 290.65 1.066 1.0802 1.41x10-2 1.32x10-2 293.15 1.002 1.015 1.34x10-2 1.34x10-2 295.65 0.9432 0.95551 1.23x10-2 1.31x10-2 -2 298.15 0.8901 0.90114 1.11x10 1.24x10-2 300.65 0.8417 0.85139 9.74x10-3 1.16x10-2 303.15 0.7974 0.8047 7.35x10-3 9.22x10-3 305.65 0.7567 0.76249 5.79x10-3 7.65x10-3 -3 308.15 0.7193 0.72331 3.99x10 5.55x10-3 310.65 0.6848 0.68673 1.89x10-3 2.76x10-3 313.15 0.6530 0.65281 1.68x10-4 2.57x10-4 315.65 0.6235 0.62116 2.30x10-3 3.69x10-3 -3 318.15 0.5961 0.59301 3.06x10 5.13x10-3 320.65 0.5706 0.56501 5.58x10-3 9.77x10-3 323.15 0.5469 0.53846 8.39x10-3 1.53x10-2 325.65 0.5247 0.51332 1.14x10-2 2.17x10-2 -2 328.15 0.5040 0.48947 1.45x10 2.88x10-2 330.65 0.4846 0.46718 1.74x10-2 3.59x10-2 333.15 0.4664 0.44536 2.10x10-2 4.51x10-2 335.65 0.4493 0.4254 2.39x10-2 5.32x10-2 -2 338.15 0.4333 0.40562 2.76x10 6.38x10-2 340.65 0.4181 0.38647 3.17x10-2 7.57x10-2 343.15 0.4039 0.36764 3.63x10-2 8.98x10-2 Expanded uncertainties (k = 1.96) are U(T) = 0.01 K, and U(p) = 0.22 kPa, and relative T /K

expanded uncertainty is U() = 0.03 x  mPa⸱s

Highlights: 

Thermophysical properties for [DEA][Ac]-water binary mixtures were evaluated in a wide range of composition at different temperatures.



Temperature affects the properties of the binary mixtures differently depending on the [DEA][Ac] content.



The thermal expansion coefficient and the isentropic compressibility showed the packing was less effective with rise in [DEA][Ac] content.



The conductivity did not follow the pattern of the viscosity, suggesting the mechanism is not always diffusion dominated.



The network of hydrogen bonds involving [DEA][Ac] and water makes it promising for electrochemical application.

Author Contribution Statement

 Alene D. Nascimento: Conceptualization, Methodology, Software, Validation, Formal analysis, Investigation, Data Curation, Writing - Original Draft, Writing - Review & Editing.  Rodrigo dos Reis: Conceptualization, Methodology, Software, Formal analysis, Resources, Writing - Review & Editing, Supervision, Project administration, Funding acquisition.

 João Paulo S. Santos: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Data Curation.  Silvana Mattedi: Conceptualization, Resources, Writing - Review & Editing, Supervision, Project administration, Funding acquisition.

 Lilian F. Senna: Conceptualization, Resources, Writing - Review & Editing, Supervision, Project administration, Funding acquisition.