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Studies of volumetric and transport properties of ionic liquidwater mixtures and its viability to be used in absorption systems Pablo B Sánchez, Josefa García, Josefa Salgado, and Elisa Gonzalez-Romero ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01541 • Publication Date (Web): 16 Aug 2016 Downloaded from http://pubs.acs.org on August 20, 2016

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ACS Sustainable Chemistry & Engineering

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Studies of volumetric and transport properties of

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ionic liquid-water mixtures and its viability to be

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used in absorption systems

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Pablo B. Sáncheza, Josefa Garcíaa,*, Josefa Salgadob, Elisa González-Romeroc,*

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a

8 9

b

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Departamento de Física Aplicada, Universidad de Vigo-Campus Vigo, Lagoas-Marcosende s/n, 36310-Vigo, Spain

Departamento de Física Aplicada, Universidad de Santiago de Compostela-Campus Vida, 15782-Santiago de Compostela, Spain c

Departamento de Química Analítica y Alimentaria, Universidad de Vigo-Campus Vigo, Lagoas-Marcosende s/n, 36310-Vigo, Spain

Pablo B. Sánchez: [email protected] Josefa García: [email protected] Josefa Salgado: [email protected] E. González-Romero: [email protected]

*

Corresponding authors: Dr. J. García Tel.: +34 986 818 753; Fax: +34 986 812 322 E-mail address: [email protected] (J. García) Dr. E. González-Romero Tel.: +34 986 812 240; Fax: +34 986 812 322 E-mail address: [email protected] (E. González-Romero, responsible of submission)

KEYWORDS: Ionic Liquid; density; viscosity; electrical conductivity; proton activity; absorbent

ACS Paragon Plus Environment


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ABSTRACT: Binary systems of two ionic liquids (ILs), 1-ethylpyridinium methanesulfonate

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[C2Py][MeSO3] and choline dihydrogenphosphate [Chol][H2PO4], and water have been experi-

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mentally studied. Density, viscosity, electrical conductivity and proton activity have been meas-

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ured at several temperatures covering all the miscibility range. From density data, isobaric coef-

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ficient of thermal expansion was calculated to study the volumetric behavior of the mixtures. All

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volumetric data were fit using polynomial equations. Vogel-Fulcher-Tamman (VFT) equation

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accurately describes the temperature dependence of viscosity for all systems. Systems based on

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[C2Py][MeSO3] are less dense and viscous than those involving [Chol][H2PO4], making

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[C2Py][MeSO3] more suitable for absorption systems where pumping cost has a significant im-

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portance. Electrical conductivity data were adjusted using Casteel-Amis equation. Similar trends

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were found for both systems, although ionic conductivity is higher for [C2Py][MeSO3] + H2O

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mixtures. The relation between viscosity and electrical conductivity was also explored. Accord-

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ing to Walden plots, ILs present low ionicity, however, internal friction and ionic concentration

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does not seem to be enough to explain the behavior of ionic conductivity and IL concentration.

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Proton activity measurements show different tendencies with molar fraction of each IL,

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[C2Py][MeSO3] leads to lower paH+ values than [Chol][H2PO4] in the binary mixtures. Results of

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ionic conductivity and proton activity suggest a higher corrosive potential of [C2Py][MeSO3] +

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H2O, however, a further analysis needs to be done to evaluate this risk in absorption systems.

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Finally, a composition analysis based on ionic chromatography (IC) was carried out to obtain

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insight about its effect on their physicochemical properties.

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INTRODUCTION

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Reducing the footprint of energy production systems is one of the main challenges for scien-

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tific and industrial communities. As a consequence, environmentally friendly technologies that

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may help reduce the use of natural resources are gaining presence and visibility over recent

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years. Among the consumption sectors, the heating and cooling industry occupies a considerable

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part of the overall numbers. In this field, one alternative to traditional heat pumps based on com-

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pression devices is absorption heat pumps1. In absorption systems, thermal energy works as a

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driving force instead of electric power used for mechanical compressors, thus, it brings the op-

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portunity of recovering heat disposal or using solar energy to satisfy heating and cooling de-

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mands. Nevertheless, commercial absorption systems based mainly on two working pairs (refrig-

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erant/absorbent), H2O/LiBr and NH3/H2O, present several drawbacks2; corrosion and crystalliza-

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tion problems in case of water and lithium bromide and low relative volatility and toxicity in

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case of ammonia and water. Therefore, the use of working pairs based on water and ionic liquid

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(H2O/IL) appears as a suitable option3–5 since their properties are expected to allow an efficient

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absorption/desorption process and to overcome the disadvantages of the commercial systems. It

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is important to define the target properties for a fluid to fit the requirements of the absorption

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process. i) Wide liquid range renders flexibility for temperatures in the absorber and the genera-

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tor, liquid-solid transitions and decomposition temperature provide the lower and upper limit of

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the liquid range.2 ii) Heat and mass transfer are crucial for absorption systems to work efficiently,

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thus, low viscosities and high thermal conductivities will improve the performance of the absorp-

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tion system.6 iii) Thermodynamics of the absorption process is also a critical issue for refrigera-

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tion systems, negative enthalpies of the mixture (H2O/IL), negative deviation of Raoult´s law and



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low water vapor pressure are target properties for absorption working pairs7. In addition, a low

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corrosive potential is also important to avoid the problem caused by H2O/LiBr.

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Ionic liquids (ILs) are salts melting under 100 ºC8. Despite there being no physical significance

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at that temperature9, it has become a commonly used definition. It is also very frequent to talk

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about room temperature ionic liquids (RTILs), when IL melting temperature is about 20-30 ºC. It

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is again a diffuse definition as there is no unique room temperature and melting points cannot be

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predicted with accuracy10. Since the ILs studied in this work, [C2Py][MeSO3] (1-ethylpyridinium

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methanesulfonate) and [Chol][H2PO4] (choline dihydrogenphosphate), are solid at room tem-

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perature, a definition based on the chemical structure and the physical and chemical properties

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was found to be more appropriate.

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ILs are usually based on an organic cation and an inorganic polyatomic anion11. It leads to

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unique properties, among them, their very low (but measurable) vapor pressure12; wide liquid

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range, caused by low melting points13,14 and high thermal stabilities15; wide electrochemical

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windows16 make ILs environmental friendly candidates for different applications17–19.

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Ionic liquids melting above room temperature present some drawbacks when physical charac-

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terization needs to be done. Difficulties during the drying process or possible damages in the

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experimental equipment caused by solidification are issues to deal with. Nevertheless, since in-

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dustrial applications very often involve mixtures of ILs with different solvents instead of pure

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components, we have chosen to study the behavior of water + IL mixtures. By doing so, we

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avoid the problem of manipulating solid ILs and we obtain useful information to evaluate wheth-

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er studied systems may work satisfactorily as working pairs for absorption heat pumps.


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Although the properties of water + IL mixtures have been broadly studied20–24, there is still a

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lack of data of water mixtures for many ILs. Hereby, mixtures of water with two ILs with a rela-

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tively high melting point have been studied in the concentration range that solubility allows.

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Thus, protic and aprotic ILs and water mixtures will be assessed, analyzing their volumetric and

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dynamic properties, as well as the proton activity of these solutions.

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In this work, density and viscosity of two water + IL mixtures, [C2Py][MeSO3] and

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[Chol][H2PO4], were measured. These properties provide very useful information for the devel-

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opment of industrial applications. While high densities are usually an advantage that ILs accom-

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plish very often, viscosity values are one the main barriers for the implementation of ILs at in-

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dustrial scale, mainly for low temperature applications. Nevertheless, it does not seem reliable to

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reach organic solvent viscosities due to ILs structures and interactions, alternatives such as high

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temperature applications or mixtures base on ILs together with different solvents may help to

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reduce the viscosity problem. For that reason, effects of temperature and water concentrations

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over the aforementioned properties will be discussed. Electrical conductivity, somehow related

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with other transport properties, is also a remarkable property for ILs, since electrochemical ap-

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plications usually demand a good ionic mobility. A quantitative analysis of the electrical conduc-

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tivity and pH (proton activity, aH+) have been carried out to explore the corrosive potential of the

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studied systems in absorption heat pumps. Moreover, the relationship between all transport prop-

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erties has been analyzed. Finally, a detailed study of the ILs impurities by ion chromatography

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has been also included.

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EXPERIMENTAL SECTION

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Materials and solutions



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Reagents of maximum purity available were purchased from IoLiTec Ionic Liquids Technolo-

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gies GmbH (Heilbronn, Germany). Some details of the compounds selected for this study are

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given in Table 1 and its structures in Scheme S1. Solutions were prepared gravimetrically using a

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digital balance Sartorius CPA225D with an uncertainty of 0.00001 g. The error on the mole frac-

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tion composition of the mixtures induced due to balance uncertainty was estimated to be 5.10−5.

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All reagents used in the chromatographic assays (H2SO4 as regenerating agent, NaHCO3 and

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Na2CO3 as mobile phase) and electrolytes as KH2PO4 were provided by Sigma-Aldrich. The rea-

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gents used as standard solution for the anion quantification by ionic chromatography (analytical

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grade sodium salts of F-, Cl-, Br-, NO3-, PO43- with 12 H2O and SO42-) were obtained from Pan-

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reac and used as received. All diluted solutions were prepared by using Milli-Q grade water. The

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molar fraction range studied for each ILs and the correlation to other concentration units (Molar,

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M, and Mass Percentage, %) are given in Table 2. Notice that the wide concentration range under

130

study is for [C2Py][MeSO3] due to both higher solubility and hydroscopic properties observed in

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comparison with [Chol][H2PO4].

132 133

Table 1. Name, Abbreviation, CAS number, purity, molecular weight and melting temperature of the compounds studied in this work Name

134

Abbreviation

CAS Number

1-Ethylpyridinium [C2Py][MeSO3] 681481-41-4 methanesulfonate Choline [Chol][H2PO4] 83846-92-8 dihydrogenphosphate a Mass fraction. Information supplied by Iolitec

Puritya

MW (g mol-1)

tm (ºC)

>95%

203.26

6225

>99%

201.16

12626

135 136 137 138


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Table 2. Equivalence of the molar fraction, mass percentage and molar concentration for the systems studied. xIL 0.0039 0.0168 0.0317 0.0650 0.1842 0.3830 0.0400 0.0163 0.0316 0.0620 0.1651

%Mass [C2Py][MeSO3] 4.25 16.15 26.96 43.96 71.81 87.40 [Chol][H2PO4] 4.27 15.58 26.71 42.48 68.82

M /mol L-1 0.21 0.77 1.26 1.97 3.00 3.52 0.21 0.74 1.22 1.85 2.73

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APPARATUS AND PROCEDURES

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Density and viscosity measurements

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Density and dynamic viscosity measurements of the pure liquids and mixtures were performed

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with a rotational viscometer Anton Paar Stabinger SVM3000. The basic operating principles and

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schematic set up of the Stabinger Viscometer are described and laid down in a European Patent

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(EP 0 926 481 A2). The density cell is a glass u-tube, which is excited to produce mechanical

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resonant vibrations according to DIN 51757. The viscosity cell is based on a tube filled with the

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sample in which a hollow measuring rotor floats. The SVM 3000 uses Peltier elements for fast

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and efficient thermostatting. The temperature uncertainty (k = 2) is 0.02 K from T = (288.15 to

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378.15) K. Over the temperature range studied, the expanded viscosity uncertainty (k = 2, 95%

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confidence level) is estimated to be 2%.27 Following the methodology exposed in previous

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works28,29, these uncertainties were checked by means of the viscosity standard oil Cannon



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N350, with maximum deviations of 0.05% and 1.1%, for density and viscosity, respectively. This

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apparatus automatically corrects the influence of viscosity on the measured density.

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Electrical conductivity measurements

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The electrical conductivity was measured with a Crison CM35 conductivimeter at different

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temperatures. The electrodes (two platinum) were placed into the samples at a prefixed tempera-

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ture. Crison Standard solutions of 147 µS . cm-1, 1413 µS . cm-1 and 12.88 mS . cm-1 were used

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for calibration proposes. Further details of the calibration procedure can be found somewhere

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else.30. Electrical conductivity uncertainties are estimated to be 1. 10-2 mS . cm-1.

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Proton activity measurements

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The pH-meter 744 Metrohm was used to obtain the pH measurements. A thermopar is included

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into the electrode body. When it is not in use, the electrode is kept in KCl 3.5 M. The calibration

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with standard buffer solutions of pH 4 and 7 (Metrohm) was done prior to measurements, using

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the Nernst equation31:

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= − 0.059

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The 59.0-mV slope change in the electrode potential results from a 10-fold change in the pro-

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ton concentration of the monovalent ion, H+ (n = 1), and 2.303RT/nF where R is the universal

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gas constant, 8.134 JK−1 mol−1; F is the Faraday constant, 9.648×104 C mol−1; T is the absolute

171

temperature in Kelvin and n is the ionic charge of the proton ion.

(1)

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Notice that the Nernst ions of opposite charge tend to associate into loosely bound ion pairs in

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more concentrated solutions, thus reducing the number of ions that are free to donate or accept


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electrons at an electrode. This is because ions in solution interact with each other and with H2O

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molecules. In this way, ions behave chemically like they are less concentrated than they really

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are (or measured). For this reason, the Nernst equation cannot accurately predict half-cell poten-

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tials for solutions in which the total ionic concentration exceeds about 10–3 M, such as is the case

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here (Table 2). Then, if the Nernst equation is applied to more concentrated solutions, the term in

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the equation pH must be expressed in "effective concentrations" or activities of the electroactive

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ionic species as proton activity, being the Nernst equation31 expressed as follows:

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= + 0.059 /

182

where n is the ionic charge. The activity coefficient γ (gamma) relates the concentration of a

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(2)

proton ion to its activity in a a given solution through the relation aH+ = γ CH+ In these experiments, the “effective” proton concentration (proton activity, aH+) is given by the IL and its interactions with water.

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Measurements by Ion Chromatography (IC)

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The ion chromatographic (IC) method is based on ion-exchange separation mechanisms and

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provides detailed anionic profiles including anionic counterions and impurities. This approach

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can be used for quality assurance, impurity analysis, and trace-level residue analysis. Therefore,

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IC system with conductimetric detection was used to analyze both ionic liquid (IL) anions, [Me-

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SO3]– and [H2PO4]–, which better suited the purpose of low-level quantification of anionic pro-

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files and gave us a better knowledge of the impurities content. The chromatographic system

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(Metrohm, Germany) can be operated with chemical suppression mode (regenerating agents are

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50 mM of H2SO4 and ultrapure water) with auto step when the loop sample is filled. The follow-



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ing components are included: 792 Basic IC chromatograph equipped with a 20 µL loop Rheo-

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dyne injection valve. The column system was a 250x4 mm Metrosep A supp 4 (column packing

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of polyvinyl alcohol with quaternary ammonium groups and particle size of 9 µm), guard col-

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umns and filters. The program used to record the chromatographic data and Data Acquisition

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System was the 792 PC Software Version 1.0 (Metrohm Ltd., Switzerland).The eluent was 1.8

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mM Na2CO3 and 1.7 mM NaHCO3 (standard flow 1 mL . min-1 and pH ≈ 10), freshly prepared,

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which was microfiltered (0.45 µm filter) and degassed (30 min in ultrasonic bath) before use.

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This mobile phase determined that the system peak appeared at a retention time of 9.97 min un-

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der the experimental conditions and column used. The ionic liquids were analyzed at a concen-

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tration of around 10 ppm and its solubility in the mobile phase was previously checked to the

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injection of 20 µL of sample into the chromatographic system.

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RESULTS AND DISCUSSION

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Volumetric properties: density

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Density measurements: density of [C2Py][MeSO3] + H2O and [Chol][H2PO4] + H2O mixtures

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at different temperatures are shown in Table S1. Range of concentrations is determined by the

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miscibility range of the solution, for [C2Py][MeSO3] it goes up to 0.3830 while for

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[Chol][H2PO4] it increases 0.1651, both referred to the molar fraction of IL. Vaporization of wa-

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ter has limited the upper limit of the temperature range. In all cases, density decreases as temper-

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ature goes up, as expected (Figure 1). Data were fit to either linear or to polynomial equations.

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Linear fittings describe the relation between density and temperature very accurately when the

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mixture is concentrated in IL, nevertheless, the accuracy of linear fittings decreases as water

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concentration grows, and thus, polynomial equations of second order were used. Brennecke et al.


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have already pointed out this pattern20 caused by the nonlinear behavior of water density with

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temperature. Fitting parameters together with the R-squared of the fitting are listed in Table S2

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(Supporting Information).

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Figure 1. Experimental density values: a) x [C2Py][MeSO3] + (1-x) H2O mixtures. x=0.0039 (),

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x=0.0168 (), x=0.0317 (●), x=0.0650 (), x=0.1842 (), x=0.3830 (). b) x [Chol][H2PO4] + (1-

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x) H2O mixtures. x=0.0040 (), x=0.0163 (), x=0.0316 (●), x=0.0620 (), x=0.1651 (), fitting

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lines ().



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Despite the concentrations of IL not being identical for both systems, from Figure 1 it is possi-

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ble to state that [Chol][H2PO4] mixtures are denser than mixtures based on [C2Py][MeSO3]. Alt-

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hough, higher densities are desirable for working pairs32, refrigerant/absorbent, it is not, in gen-

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eral, the key criteria to select working fluids for absorption heat pumps.

229 230

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Volumetric behavior can also be analyzed by calculating the isobaric coefficient of thermal expansion, αp, given by:

231

= − ( )

232

Trends of αp with temperature are shown in Figure 2. It can be observed that αp increases with

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temperature for all IL concentrations. Note also that the effect of temperature over αp becomes

234

smaller as IL concentration grows. It leads to a crossover at about 310 K, the explanation yields

235

on the density dependence on temperature for different IL concentrations. Mixtures with higher

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concentrations in IL show linear dependence with temperature while those with higher water

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concentrations show a quadratic dependence. According to equation 3, the slope of trendlines

238

shown in Figure 2 is expected to be higher as water concentration of the mixture rises. Even

239

though this analysis is qualitatively consistent, to our view, further work needs to be done in or-

240

der to establish a relationship between the crossover temperature and the components of the mix-

241

ture.

(3)

242

Thermal expansion coefficient (αp) shows values lower than 1.6 10-3 K-1 for both ILs. In the

243

temperature range studied (from 293.15 K to 343.15 K), these ILs aqueous systems lead to nearly

244

identical coefficients of thermal expansion. Thus, αp should not be a criterion to choose one IL


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over the other for some particular application, in this case, as absorbent for water refrigerant in

246

an absorption cycle.

247 248

Figure 2. Isobaric expansion coefficients: a) x [C2Py][MeSO3] + (1-x) H2O mixtures at several tempera-

249

tures. x=0.0039 (), x=0.0168 (), x=0.0317 (●), x=0.0650 (), x=0.1842 (), x=0.3830 (). b) x

250

[Chol][H2PO4] + (1-x) H2O mixtures. x=0.0040 (), x=0.0163 (), x=0.0316 (●), x=0.0620 (),

251

x=0.1651 (). Fitting lines ().

252 253 254



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Transport properties: viscosity, electrical conductivity and proton activity

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Viscosity measurements: viscosity of [C2Py][MeSO3] + H2O and [Chol][H2PO4] + H2O mix-

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tures at different temperatures are shown in Table S1 (Supplementary Information). The three-

258

parameter Vogel–Fulcher–Tammann (VFT) equation (4) was used to correlate experimental vis-

259

cosities as a function of temperature:

260

= . (

!

"#

)

(4)

261

being η the dynamic viscosity, and A, B and T0 the fitting parameters reported in Table S3 (Sup-

262

porting Information) together with the R-squared of the fitting. Figure 3 illustrates that viscosity

263

decreases drastically as temperature rises. Most ionic liquids have undesirable high viscosity; to

264

deal with this problem one possible approach is lowering the viscosity by adding a molecular

265

solvent. Thus, experimental measurements of both IL + H2O systems show how viscosity de-

266

creases as the content of water increases for all temperatures.

267

However, the effect of water over viscosity is more important at lower temperatures (Figure 3).

268

This pattern was already found for [C2Py][OTf] + H2O systems 29. Dynamic studies at molecular

269

scales have pointed out the lubricating action of molecular solvents over the dynamics of ionic

270

liquids33,34 for different concentrations of water. A reliable explanation yield on the weakening of

271

the coulombic interactions between ions would lead to a higher ionic mobility, and therefore to

272

decrease the viscosity of systems.

273

Both systems, IL + H2O, present very different solubility. While the most concentrated

274

[C2Py][MeSO3] + H2O solution has a molar fraction x[C2Py][MeSO3] = 0.3830, for [Chol][H2PO4]

275

the highest molar fraction is x[Chol][H2PO4] = 0.1651 (see Table 2 for concentration equivalences).


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However, by comparing all experimental data, it is possible to establish the viscosity sequence as

277

follows, η[C2py][MeSO3] < η[Chol][H2PO4]. Thus, attending to viscosity, systems [C2Py][MeSO3] + H2O

278

are found to be more suitable than [Chol][H2PO4] + H2O for absorption systems using water as

279

refrigerant.

280

Previously, the system [C2Py][OTf] was studied by us as a potential absorbent for absorption

281

heat pumps using water as refrigerant 29. Given that, [C2Py][OTf] and [C2Py][MeSO3] share the

282

same cationic structure, [C2Py]+, a detailed comparison of both systems could provide interesting

283

information about the effect of the anionic structure apart from their suitability to work as absor-

284

bents for absorption heat pumps. Trends of density and viscosity for IL + H2O systems are

285

ρ[C2py][OTf ] > ρ[C2Py][MeSO3] and η[C2py][OTf ] < η[C2py][MeSO3]. Inverse trends are found for both proper-

286

ties, in terms of atomic interactions, the larger charge delocalization caused by the trifluorome-

287

thyl group (CF3) group compared to methyl group (CH3) could explain this effect, since it would

288

smooth the electrostatic interactions and therefore improve ionic mobility, giving place to lower

289

viscosities. It would also lead to the formation of more compacted ionic networks and therefore

290

higher densities. This statement also agrees with their solid-liquid transition temperatures, 16 0C

291

and 620C for [C2Py][OTf] and [C2Py][MeSO3], respectively25.

292 293 294



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

295 296 297

Figure 3. Experimental viscosity values: a) x [C2Py][MeSO3] + (1-x) H2O mixtures. x=0.0168 (),

298

x=0.0317 (), x=0.0650 (●), x=0.1842 (), x=0.3830 (). b) x [Chol][H2PO4] + (1-x) H2O mixtures.

299 300 301

x=0.0040 (), x=0.0163 (), x=0.0316 (●), x=0.0620 (), x=0.1651 (). Fitting lines ().

Page 16 of 36

302

Viscosity measurements show how different ionic structures lead to drastic differences in this

303

property when the mixtures are highly concentrated in IL, especially at low temperatures. These

304

differences become much smaller for diluted solutions where water concentration prevailed over

305

the IL or when temperature rises. Therefore, the addition of an organic solvent or the rising of the

306

working temperature represent two interesting strategies to overcome problems related to the

307

high viscosity of ILs.

308

Electrical conductivity measurements: Figure 4 shows dependence on electrical conductivity

309

(κ) with IL molar fraction (xIL) at several temperatures. Casteel-Amis35 equation (5) was used to

310

fit experimental data. Fitting parameters are given in Table S4 (Supporting Information).

311

$ = $%&' (

'()

'(),+,-

). /01

23

−

5 23,%&' 4

− (

'() "'(),+,'(),+,-

)6

(5)

312


Page 17 of 36

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313

being κ the electrical conductivity, and κmax, m and n the fitting parameters reported in Table S4

314

(Supporting Information) together with the R-squared of the fitting.

315

A qualitative analysis of Figure 4a and 4b indicates that ionic mobility is higher for

316

[C2Py][MeSO3] than for [Chol][H2PO4] mixtures. Therefore, it is in agreement with the viscosity

317

values, η[C2Py][MeSO3] < η[Chol][H2PO4], since a lower internal friction is expected to improve IL dy-

318

namics and therefore it would favor charge transport. The comparison of the electrical conductiv-

319

ity between the ILs with same cation, [C2Py]+, and different anion, [MeSO3]- and [OTf]-, indi-

320

cates that the trend in conductivity is κ [C2py][OTf ] < κ [C2py][MeSO3], being around 5% lower for

321

[OTf]- counterion.

322

By comparing Figures 4a and 4b again, we observe similar tendencies of ionic conductivity

323

with IL molar fraction. For dilute solutions, κ grows drastically with xIL. Then, all curves reach a

324

maximum at molar fraction about 0.05 and 0.04 for [C2Py][MeSO3] and [Chol][H2PO4] respec-

325

tively, values for each temperature are provided in Table S1 (Supporting Information). After that

326

point, ionic conductivity decreases as concentration of IL grows. This pattern was already found

327

for other binary systems involving IL + H2O systems36–40. Two mechanisms are often mentioned

328

as responsible for this behavior. On one hand the increase in the ionic concentration should lead

329

to higher ionic conductivities since more charge is available to be transported. On the other, the

330

descent in ionic mobility, since interactions (electrostatic and dispersive) between cation-anion

331

and cation-cation become stronger, will have the opposite effect. Therefore, a balance between

332

mechanisms will bring maximum conductivity. To assess this hypothesis, measurements were

333

taken at three different temperatures, from 298 to 333 K. Linear tendencies with positive slope

334

were found in all cases. The effect of temperature over conductivity is more important as IL con-



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

335

centration grows, for [C2Py][MeSO3] goes from 0.013 to 0.16 mScm-1 K-1 and for [Chol][H2PO4]

336

from 0.018 to 0.17 mScm-1 K-1 which is roughly an increase of ten times in the slope when xIL

337

moves from the most diluted solution to the most concentrated (Figure S1 in Supporting Infor-

338

mation).

339 340

Figure 4. Electrical conductivity versus xLI at several temperatures. T=298,15 K (), T=313,15 K

341 342

(), T=333.15 K (●), fitting lines (). a) [C2Py][MeSO3] + H2O. b) [Chol][H2PO4] + H2O.


Page 18 of 36

Page 19 of 36

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343

Since temperature has critical impact over ionic mobility, we will expect that κmax happens at

344

higher IL concentrations. Although this trend was found, its extent, smaller than expected, sug-

345

gests that other factors may also contribute to κ dependence with xIL33,41.

346

Viscosity and electrical conductivity relationship: The relation between these two properties is

347

often enlightened by Walden´s rule. From equation (6), where Λ is the molar conductivity, η the

348

viscosity and C is a constant; we have obtained the so-called Walden´s plot. Further details can

349

be found somewhere else 42. 8

350

Λ = 9

351

In Figure 5, IL + H2O mixtures with similar IL concentrations are represented. Deviation from

352

the ideal line21 (slope equals to 1) would provide, according to Ueno et al43, a measurement of

353

the ionicity of the IL. Both systems fall into the area of poor ILs, in terms of ionicity, no signifi-

354

cant differences between these systems were found since they show similar ratios between Λ and

355

1/η. Temperature effect is also shown in figure 5. As temperature rises, points are displaced to

356

the right. It is consistent with the fact that temperature effect over viscosity is stronger that its

357

effect over electric conductivity. Previously, several authors have analyzed the behavior of sev-

358

eral IL + H2O systems44,45, results indicate that aqueous solutions of ILs do not follow Walden´s

359

rule qualitatively, even though trends found suggest a physical background for the value of C45.

(6)



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Page 20 of 36

360 361

Figure 5. Walden plot of the [C2Py][MeSO3] (x[C2Py][MeSO3] = 0.065) + H2O () and [Chol][H2PO4]

362

(x[Chol][H2PO4] = 0.062) + H2O mixtures (●) at 298.15, 313.15, 333.15 K. The straight solid line through the

363

origin with a slope of 1 represents the so-called ideal KCl line42

364 365

Proton activity measurements: Since the solvent is pure water, the experimental proton activi-

366

ty, aH+ or “effective” proton concentration (see experimental section) of these mixtures was also

367

determined, these measurements are representative rather than exhaustive, and its effect was

368

shown in Figure 6, together with the electrical conductivity data. The proton activity (expressed

369

as paH+) depends strongly on xII. Systems show different trends, depending mainly on the anion

370

structure, since the pKa´s values of both conjugated acid of aliphatic amines or unsatured pyri-

371

dines increase with the increasing alkylsubtitution and render the corresponding cations a basic

372

character (see structures in Scheme S1), although the charge and structure of these cations will

373

influence the polarity and the solvent properties of the ionic liquids.

374

For the [C2Py][MeSO3] (Fig. 6a), paH+ is 3.42 at x[C2Py][MeSO3] = 0.0039, this value decreases

375

drastically with the increase of x[C2Py][MeSO3] up to 0.0371, thus, [C2Py][MeSO3] plays the role of

376

an acidic species, where the minimum paH+ is close to the xIL where the conductivity presents its


Page 21 of 36

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377

highest value (46.72 mS cm-1 at xIL 0.047). Afterwards, the paH+ values increase slightly with xIL,

378

reaching the value of 2.74 at xIL = 0.3830. Keeping in mind the structure for [C2Py][MeSO3],

379

that is a six-membered unsaturated ring that is more basic than the pyridinium and imidazolium

380

cations with a methanesulfonate (conjugated-base of a very strong methanesulfonic acid, Me-

381

SO3H, with a pKa = -1.8) as counteranion46,47, it should be expected to yield a “neutral” (in the

382

acidic/base sense) ionic liquid product48. This is the case of [C2Py][OTf] (note structures are sim-

383

ilar), with proton activities of paH+ 6.42 and 5.91 at xIL = 0.067 and xIL = 0.689, respectively,

384

where the weak acidity at high xIL can be attributed to the presence of free fluoride ion (conju-

385

gated-base of a weak fluorhydric, HF, with a pKa = 3.2). However, these [C2Py][MeSO3] mix-

386

tures exhibit a stronger acidity than expected, and this behavior, although we cannot yet offer an

387

conclusive explanation, seems to be, caused by some impurities from the acid used as precursor

388

in the synthesis. This hypothesis will be discussed later (next section and Fig.7a).



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Page 22 of 36

389 390

Figure 6. Electrical conductivity () and paH+ () versus xIL at 298.15 K. a) [C2Py][MeSO3] + H2O b)

391

[Chol][H2PO4] + H2O. Notice that the points are joined for better following the trends in electrical con-

392

ductivity (Casteel-Amis fitting in Figure 4) and paH+

393 394

For [Chol][H2PO4] (Fig 6b), the most diluted solution, xIL = 0.0040, has a proton activity of

395

3.22, increasing with xIL during the whole concentration range, at xIL = 0.1651 the paH+ is 4.99

396

(see Table 2 for concentration equivalences). Furthermore, paH+ trend shows two straight lines

397

with the increasing of xIL, whose equations are as follows: paH+ = (3.168±0.004) + (13.4±0.2)


Page 23 of 36

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398

x[Chol][H2PO4] for the first linear trend

(x[Chol][H2PO4] ≤ 0.0316) and paH+ = (3.29±0.05) +

399

(10.4±0.5) x[Chol][H2PO4] for the second linear trend (x[Chol][H2PO4] ≥ 0.0316), both with a good lin-

400

ear correlation (R² = 0.9999 and R² = 0.999, respectively), Fig. 6b()), and the crossing point

401

roughly coincides with the maximum of the conductivity value (28.34 mS cm-1 at xIL 0.0386).

402

According to its structure (Scheme S1), formed by an acyclic and saturated with a hydroxylfun-

403

tionalized side chain also on the cation-dependent degree of hydrogen bonding and an “ampho-

404

teric” counteranion, with the potential to both accept and donate protons depending on the other

405

substances present in the media47, these experimental results show that the dihydrogen phosphate

406

acts as Brønsted acid counteranion at diluted binary mixtures, being amphoteric at high IL con-

407

centration, where the paH+ tends to reach the theoretical value of 4.7 (semi-sum of pKa1 = 2.2

408

and pKa2 = 7.2 of phosphoric acid system) or experimental value of 4.36 for 1.845 M potassium

409

dihydrogenphosphate solution at the same concentration (Table 2) and it acts as second order

410

buffered solution.

411

Moreover, it should be noted that [Chol][H2PO4] gives a lower Brønsted acidity (higher paH+

412

values) than the observed for the [C2Py][MeSO3] at high molar fraction and probably, this is due

413

to the strong Brønsted acidity of the hydrogen sulfate counteranion or methanesulfonic acid that

414

can be present as impurities, in good agreement with what is found in the literature.49

415

Trends of electrical conductivity and proton activity for ILs + H2O systems are κ [Chol][H2PO4] < κ

416

[C2Py][OTf ]

κ ([Chol][H2PO4] + H2O)

485

•

paH+([C2Py][MeSO3]+H2O)

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