Influence of Small Quantities of Water on the Physical Properties of Alkylammonium Nitrate Ionic Liquids

International Journal of Molecular Sciences Article Influence of Small Quantities of Water on the Physical Properties of Alkylammonium Nitrate Ionic Liquids David Ausín1, Juan J. Parajó2,3 , JoséL. Trenzado 4, Luis M. Varela 2, Oscar Cabeza 1and Luisa Segade 1,*   Citation: Ausín, D.; Parajó, J.J.; Trenzado, J.L.; Varela, L.M.; Cabeza, O.; Segade, L. Influence of Small Quantities of Water on the Physical Properties of Alkylammonium Nitrate Ionic Liquids. Int. J. Mol. Sci. 2021,22, 7334. https://doi.org/ 10.3390/ijms22147334 Academic Editors: JoséS. Urieta and Ana M. Mainar Received: 8 June 2021 Accepted: 2 July 2021 Published: 8 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Departamento de Física, Facultade de Ciencias, Campus da Zapateira, Universidade da Coruña, 15071 A Coruña, Spain; [email protected] (D.A.); oscar[email protected] (O.C.) 2 Grupo de Nanomateriais, Fotónica e Materia Branda, Departamento de Física de Partículas y Departamento de Física Aplicada, Universidade de Santiago de Compostela, Campus Vida s/n, 15782 Santiago de Compostela, Spain; [email protected] (J.J.P.); [email protected] (L.M.V.) 3Departamento de Química e Bioquímica, CIQUP-Centro de Investigaçao em Química da Universidade do Porto, Universidade do Porto, P-4169-007 Porto, Portugal 4 Departamento de Física, Universidad de Las Palmas de Gran Canaria, 35017 Las Palmas Gran Canaria, Spain; [email protected] *Correspondence: [email protected]; Tel.: +34-981167000 Abstract: This paper presents a comprehensive study of two alkylammonium nitrate ionic liquids. As part of this family of materials, mainly ethylammonium nitrate (EAN) and also propylammonium nitrate (PAN) have attracted a great deal of attention during the last decades due to their potential applications in many fields. Although there have been numerous publications focused on the measurement of their physical properties, a great dispersion can be observed in the results obtained for the same magnitude. One of the critical points to be taken into account in their physical characterization is their water content. Thus, the main objective of this work was to determine the degree of influence of the presence of small quantities of water in EAN and PAN on the measurement of density, viscosity, electrical conductivity, refractive index and surface tension. For this purpose, the first three properties were determined in samples of EAN and PAN with water contents below 30,000 ppm in a wide range of temperatures, between 5 and 95 ◦ C, while the last two were obtained at 25 ◦ C. As a result of this study, it has been concluded that the presence of water is critical in those physical properties that involve mass or charge transport processes, resulting in the finding that the absolute value of the average percentage change in both viscosity and electrical conductivity is above 40%. Meanwhile, refractive index ( ≤ 0.3%), density ( ≤ 0.5%) and surface tension ( ≤ 2%) present much less significant changes. Keywords: ionic liquid; ethylammonium nitrate; propylammonium nitrate; water-free; density; viscosity; electrical conductivity; refractive index; surface tension 1. Introduction During the last decades, the study of ionic liquids (ILs) has aroused enormous interest in the scientific community because of their extraordinary physical properties and versatility, which have made them worthy of becoming a true field of research [ 1 ]. The main feature that made them attractive is that they can be tailor-made by combining the cations and anions of which they can be composed, obtaining a great variety of ILs with very different properties and applications. After a couple of decades of extensive development in the study of the potential uses of ILs, their applications are now a reality in many branches of science and industry. Thus, their uses have been demonstrated in a wide range of areas of analytical chemistry [ 2 ], organic chemistry [ 3 , 4 ], electrochemical conversion and energy storage [ 5 ], biosensing technology [ 6 ], pharmaceutical and biomedical industry [ 7 , 8 ] or recovery of industrial solvents [ 9 , 10 ], to name but a few. In this context, the characterization of these materi- Int. J. Mol. Sci. 2021,22, 7334. https://doi.org/10.3390/ijms22147334 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021,22, 7334 2 of 15 als through experimental, theoretical or computational methods plays a decisive role to promote their industrial applications [11]. In this work, we continue with the experimental study of ILs to which our groups have been devoted in the past years [12–14], and specifically, we focus on one of our lines of work dedicated to protic ionic liquids (PILs) [ 15 , 16 ], the usefulness of which has been widely tested. Within this class of materials, two representatives of the family of alkylammonium nitrates, ethylammonium nitrate (EAN) and propylammonium nitrate (PAN), have attracted great attention as a result of their multiple applications. Many examples of them can be found in fields as diverse as electrodeposition, electrochemical exfoliation, liquid–liquid extraction, organic or inorganic synthesis, biocatalysis, lubrication or biological medium solvents, among others [ 17 , 18 ]. More recent works have already explored their utility as components of smart materials [ 19 ], in hydrogen sorption processes [ 20 ], in thermoelectric generator devices [ 21 ] or in studies on the structural stability and aggregation state of proteins [ 22 ]. In general, EAN and PAN have been the subject of many studies, reported in more than 700 bibliographic references (SciFindern database), as can be seen in Figure 1. A large part of these references has been devoted to exploring the uses of these compounds, while approximately half of them have focused on the determination of their physicochemical properties. Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 2 of 15 through experimental, theoretical or computational methods plays a decisive role to promote their industrial applications [11]. In this work, we continue with the experimental study of ILs to which our groups have been devoted in the past years [12–14], and specifically, we focus on one of our lines of work dedicated to protic ionic liquids (PILs) [15,16], the usefulness of which has been widely tested. Within this class of materials, two representatives of the family of alkylammonium nitrates, ethylammonium nitrate (EAN) and propylammonium nitrate (PAN), have attracted great attention as a result of their multiple applications. Many examples of them can be found in fields as diverse as electrodeposition, electrochemical exfoliation, liquid–liquid extraction, organic or inorganic synthesis, biocatalysis, lubrication or biological medium solvents, among others [17,18]. More recent works have already explored their utility as components of smart materials [19], in hydrogen sorption processes [20], in thermoelectric generator devices [21] or in studies on the structural stability and aggregation state of proteins [22]. In general, EAN and PAN have been the subject of many studies, reported in more than 700 bibliographic references (SciFindern database), as can be seen in Figure 1. A large part of these references has been devoted to exploring the uses of these compounds, while approximately half of them have focused on the determination of their physicochemical properties. Figure 1. Increase in the number of bibliographic references related to EAN and PAN from their discovery to nowadays (SciFindern database). Considering that a thorough and accurate characterization of these materials has a direct impact on the knowledge of their structure and in the design of their potential applications, we have performed an exhaustive compilation of some of the most relevant physical properties. Thus, we have reviewed the state of the art of density, viscosity, electrical conductivity, refractive index and surface tension for EAN and PAN at different temperatures [23–89]. These properties enable us to anticipate the suitability of an IL in some applications. For instance, density and refractive index are two remarkably interesting parameters because, besides being useful to identify a substance, they allow obtaining other information such as ionic conductivities or electronic polarizabilities, respectively. Viscosity could be related to extraction processes, either facilitating dispersion (low viscosity) or by avoiding losses (high viscosity) [2]. In the case of batteries or electrochemical sensing systems, it is essential information to know thermal stability, electrochemical stability, electrical conductivity and viscosity [5]. Other studies that have analyzed the ability of certain ILs to act as surfactants have made use of the information provided by surface tension data, with the aim of developing drug delivery systems [8]. In all cases, it must be taken into account that the properties tend to depend on temperature, especially those related to mass or 0 10 20 30 40 50 60 1914 1946 1950 1953 1969 1978 1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016 2018 2020 NUMBER OF REFERENCES YEAR EAN PAN Figure 1. Increase in the number of bibliographic references related to EAN and PAN from their discovery to nowadays (SciFinderndatabase). Considering that a thorough and accurate characterization of these materials has a direct impact on the knowledge of their structure and in the design of their potential applications, we have performed an exhaustive compilation of some of the most relevant physical properties. Thus, we have reviewed the state of the art of density, viscosity, electrical conductivity, refractive index and surface tension for EAN and PAN at different temperatures [23–89]. These properties enable us to anticipate the suitability of an IL in some applications. For instance, density and refractive index are two remarkably interesting parameters because, besides being useful to identify a substance, they allow obtaining other information such as ionic conductivities or electronic polarizabilities, respectively. Viscosity could be related to extraction processes, either facilitating dispersion (low viscosity) or by avoiding losses (high viscosity) [ 2 ]. In the case of batteries or electrochemical sensing systems, it is essential information to know thermal stability, electrochemical stability, electrical conductivity and viscosity [ 5 ]. Other studies that have analyzed the ability of certain ILs to act as surfactants have made use of the information provided by surface tension data, with the aim of developing drug delivery systems [ 8 ]. In all cases, it must be taken into account that the properties tend to depend on temperature, especially those related to mass or charge transport, which could limit their applications, it being necessary to have studies that establish this dependence. Int. J. Mol. Sci. 2021,22, 7334 3 of 15 The aforementioned highlights the fact that, in order to properly design a process, the systematic and precise measurement of physicochemical properties is of the utmost importance. Despite this, we have found that the published results for a given property at the same temperature vary very significantly. According to all the data gathered in the literature, it could be assumed that water absorption from the atmosphere is one of the main reasons for the dispersion observed for a given property and temperature, since the two ILs studied here are hygroscopic substances, as are many ILs [ 90 ]. As a representative case, previously published densities ( ρ ) of EAN measured at 25 ◦ C are presented in Figure 2 , along with the experimental densities studied here. In this figure, the property as a function of the water content (w) accurately reported in the papers is represented using solid symbols, and with open ones if it was not. Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 3 of 15 charge transport, which could limit their applications, it being necessary to have studies that establish this dependence. The aforementioned highlights the fact that, in order to properly design a process, the systematic and precise measurement of physicochemical properties is of the utmost importance. Despite this, we have found that the published results for a given property at the same temperature vary very significantly. According to all the data gathered in the literature, it could be assumed that water absorption from the atmosphere is one of the main reasons for the dispersion observed for a given property and temperature, since the two ILs studied here are hygroscopic substances, as are many ILs [90]. As a representative case, previously published densities (ρ) of EAN measured at 25 °C are presented in Figure 2, along with the experimental densities studied here. In this figure, the property as a function of the water content (w) accurately reported in the papers is represented using solid symbols, and with open ones if it was not. Figure 2. Experimental and published densities, ρ, of EAN as a function of the water content, w, at 25 °C. Symbols: present work (black solid triangles) and a compilation of those published data with an accurate measure of water content (solid) or not (open): [15] (red squares), [23] (green dots), [24] (yellow dots), [27] (orange dots), [28] (cyan dots), [29] (red dots), [32] (pink dots), [36] (grey dots), [41] (dark green dots), [46] (brown dots), [48] (blue dots), [52] (dark red dots), [58] (purple dots), [60] (green squares), [65] (yellow squares), [67] (orange squares), [70] (cyan squares), [71] (pink squares), [75] (grey squares), [76] (dark green squares), [77] (brown squares), [81] (blue squares), [82] (dark red squares), [85] (purple squares), [86] (green rhombs), [87] (cyan rhombs) and [89] (orange rhombs). In view of the results, it seems critical to control and determine the water content if an accurate characterization is to be made. For this reason, in this work we have proposed as an objective to study systematically the influence of the content of small quantities of water (from about 300 to 30,000 ppm) in EAN and PAN on the measurement of density, Figure 2. Experimental and published densities, ρ , of EAN as a function of the water content, w, at 25 ◦ C. Symbols: present work (black solid triangles) and a compilation of those published data with an accurate measure of water content (solid) or not (open): [ 15 ] (red squares), [ 23 ] (green dots), [ 24 ] (yellow dots), [ 27 ] (orange dots), [ 28 ] (cyan dots), [ 29 ] (red dots), [ 32 ] (pink dots), [ 36 ] (grey dots), [ 41 ] (dark green dots), [ 46 ] (brown dots), [ 48 ] (blue dots), [ 52 ] (dark red dots), [ 58 ] (purple dots), [ 60 ] (green squares), [ 65 ] (yellow squares), [ 67 ] (orange squares), [ 70 ] (cyan squares), [ 71 ] (pink squares), [ 75 ] (grey squares), [ 76 ] (dark green squares), [ 77 ] (brown squares), [ 81 ] (blue squares), [ 82 ] (dark red squares), [85] (purple squares), [86] (green rhombs), [87] (cyan rhombs) and [89] (orange rhombs). In view of the results, it seems critical to control and determine the water content if an accurate characterization is to be made. For this reason, in this work we have proposed as an objective to study systematically the influence of the content of small quantities of water (from about 300 to 30,000 ppm) in EAN and PAN on the measurement of density, viscosity and electrical conductivity from 5 to 95 ◦ C, and the refractive index and surface tension at 25 ◦ C. To our knowledge, this comprehensive study of EAN and PAN has not been carried out over such a wide temperature range with a strict control of water content. Int. J. Mol. Sci. 2021,22, 7334 4 of 15 2. Results The characterization of EAN and PAN involves the measurement of physical properties covering a wide range of temperatures, between 5 and 95 ◦ C, when possible. This was the case for density, viscosity and electrical conductivity, which were measured every 5 ◦ C for the first two and 10 ◦ C for the latter. Thus, Figure 3a,b shows the results obtained for the density of EAN and PAN as a function of the water content, respectively. In the same way, Figure 4a,b and Figure 5a,b display the experimental results of viscosity (on a semi logarithmic scale) and electrical conductivity. Finally, the refractive index and surface tension at 25 ◦ C of both LIs are plotted in Figure 6. As can be seen, all the physical properties studied depend linearly on the composition in the concentration range studied, with the exception of the refractive index of EAN and the viscosity of PAN, whose relationship is better fitted through a polynomial of degree 2. All the experimental results obtained are shown in Tables S1–S5 in the Supporting Materials section. In all cases, the data were fitted to an equation of the type: Q=QIL + N ∑ i=1 Aiwi, (1) where Q is the property studied ( ρ , η , κ , n D or σ ) of the IL at a given temperature and content in water (w), Q IL is the corresponding property of the IL without any water content ( ρIL , ηIL , κIL , n D,IL or σIL ) and Q IL and A i are fitting parameters. All the best values for the fitting parameters, Q IL and A i , together with the coefficient of determinations, are shown in Tables 1–4. Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 4 of 15 viscosity and electrical conductivity from 5 to 95 °C, and the refractive index and surface tension at 25 °C. To our knowledge, this comprehensive study of EAN and PAN has not been carried out over such a wide temperature range with a strict control of water content. 2. Results The characterization of EAN and PAN involves the measurement of physical properties covering a wide range of temperatures, between 5 and 95 °C, when possible. This was the case for density, viscosity and electrical conductivity, which were measured every 5 °C for the first two and 10 °C for the latter. Thus, Figure 3a,b shows the results obtained for the density of EAN and PAN as a function of the water content, respectively. In the same way, Figures 4a,b and 5a,b display the experimental results of viscosity (on a semi logarithmic scale) and electrical conductivity. Finally, the refractive index and surface tension at 25 °C of both LIs are plotted in Figure 6. As can be seen, all the physical properties studied depend linearly on the composition in the concentration range studied, with the exception of the refractive index of EAN and the viscosity of PAN, whose relationship is better fitted through a polynomial of degree 2. (a) (b) Figure 3. Densities, ρ, of EAN (a) and PAN (b) as a function of the water content, w, for the temperatures studied: 5 (green dots), 10 (green squares), 15 (dark green dots), 20 (dark green squares), 25 (black dots), 30 (cyan squares), 35 (cyan dots), 40 (blue squares), 45 (blue dots), 50 (grey squares), 55 (grey dots), 60 (orange squares), 65 (orange dots), 70 (red squares), 75 (red dots), 80 (pink squares), 85 (pink dots), 90 (purple squares) and 95 °C (purple dots). Solid lines were obtained from Equation (1). Figure 3. Densities, ρ , of EAN ( a ) and PAN ( b ) as a function of the water content, w, for the temperatures studied: 5 (green dots), 10 (green squares), 15 (dark green dots), 20 (dark green squares), 25 (black dots), 30 (cyan squares), 35 (cyan dots), 40 (blue squares), 45 (blue dots), 50 (grey squares), 55 (grey dots), 60 (orange squares), 65 (orange dots), 70 (red squares), 75 (red dots), 80 (pink squares), 85 (pink dots), 90 (purple squares) and 95 ◦ C (purple dots). Solid lines were obtained from Equation (1). Int. J. Mol. Sci. 2021,22, 7334 5 of 15 Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 5 of 15 (a) (b) Figure 4. Viscosities, η, of EAN (a) and PAN (b) as a function of the water content, w, on a semi logarithmic scale for the temperatures studied: 5 (green dots), 10 (green squares), 15 (dark green dots), 20 (dark green squares), 25 (black dots), 30 (cyan squares), 35 (cyan dots), 40 (blue squares), 45 (blue dots), 50 (grey squares), 55 (grey dots), 60 (orange squares), 65 (orange dots), 70 (red squares), 75 (red dots), 80 (pink squares), 85 (pink dots), 90 (purple squares) and 95 °C (purple dots). Solid lines were obtained from Equation (1). (a) (b) Figure 5. Electrical conductivities, κ, of EAN (a) and PAN (b) as a function of the water content, w, for the temperatures studied: 5 (green dots), 15 (dark green dots), 25 (black dots), 35 (cyan dots), 45 (blue dots), 55 (grey dots), 65 (orange dots), 75 (red dots), 85 (pink dots) and 95 °C (purple dots). Solid lines were obtained from Equation (1). Figure 4. Viscosities, η , of EAN ( a ) and PAN ( b ) as a function of the water content, w, on a semi logarithmic scale for the temperatures studied: 5 (green dots), 10 (green squares), 15 (dark green dots), 20 (dark green squares), 25 (black dots), 30 (cyan squares), 35 (cyan dots), 40 (blue squares), 45 (blue dots), 50 (grey squares), 55 (grey dots), 60 (orange squares), 65 (orange dots), 70 (red squares), 75 (red dots), 80 (pink squares), 85 (pink dots), 90 (purple squares) and 95 ◦ C (purple dots). Solid lines were obtained from Equation (1). Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 5 of 15 (a) (b) Figure 4. Viscosities, η, of EAN (a) and PAN (b) as a function of the water content, w, on a semi logarithmic scale for the temperatures studied: 5 (green dots), 10 (green squares), 15 (dark green dots), 20 (dark green squares), 25 (black dots), 30 (cyan squares), 35 (cyan dots), 40 (blue squares), 45 (blue dots), 50 (grey squares), 55 (grey dots), 60 (orange squares), 65 (orange dots), 70 (red squares), 75 (red dots), 80 (pink squares), 85 (pink dots), 90 (purple squares) and 95 °C (purple dots). Solid lines were obtained from Equation (1). (a) (b) Figure 5. Electrical conductivities, κ, of EAN (a) and PAN (b) as a function of the water content, w, for the temperatures studied: 5 (green dots), 15 (dark green dots), 25 (black dots), 35 (cyan dots), 45 (blue dots), 55 (grey dots), 65 (orange dots), 75 (red dots), 85 (pink dots) and 95 °C (purple dots). Solid lines were obtained from Equation (1). Figure 5. Electrical conductivities, κ , of EAN ( a ) and PAN ( b ) as a function of the water content, w, for the temperatures studied: 5 (green dots), 15 (dark green dots), 25 (black dots), 35 (cyan dots), 45 (blue dots), 55 (grey dots), 65 (orange dots), 75 (red dots), 85 (pink dots) and 95 ◦C (purple dots). Solid lines were obtained from Equation (1). Int. J. Mol. Sci. 2021,22, 7334 6 of 15 Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 6 of 15 Figure 6. Refractive index, n D (dots), and surface tension, σ (squares), of EAN (solid symbols) and PAN (open symbols) as a function of the water content, w, at 25 °C. Solid lines were obtained from Equation (1). All the experimental results obtained are shown in Tables S1–S5 in the Supporting Materials section. In all cases, the data were fitted to an equation of the type: Q=Q+∑Aw   , (1) where Q is the property studied (ρ, η, κ, n D or σ) of the IL at a given temperature and content in water (w), Q IL is the corresponding property of the IL without any water content (ρ IL , η IL , κ IL , n D,IL or σ IL ) and Q IL and A i are fitting parameters. All the best values for the fitting parameters, Q IL and A i , together with the coefficient of determinations, are shown in Tables 1–4. Table 1. Fitting Equation (1) to density data (g·cm −3 ) of EAN and PAN as a function of water content (ppm): parameters and coefficient of determination R 2 . EAN PAN T (°C) ρ IL A 1 (10 7 ) R 2 ρ IL A 1 (10 7 ) R 2 5 1.2234 −1.4448 0.9963 1.1639 −0.66515 0.9941 10 1.2202 −1.4610 0.9971 1.1608 −0.67382 0.9941 15 1.2171 −1.4745 0.9988 1.1577 −0.70588 0.9958 20 1.2140 −1.5136 0.9939 1.1546 −0.73063 0.9913 25 1.2108 −1.5354 0.9989 1.1516 −0.76511 0.9945 30 1.2077 −1.5516 0.9978 1.1485 −0.79664 0.9938 35 1.2046 −1.5800 0.9934 1.1456 −0.83059 0.9926 40 1.2016 −1.6151 0.9988 1.1425 −0.83760 0.9920 45 1.1986 −1.6151 0.9988 1.1396 −0.89210 0.9908 50 1.1956 −1.6805 0.9977 1.1366 −0.90778 0.9920 55 1.1927 −1.6700 0.9944 1.1337 −0.96657 0.9944 60 1.1898 −1.6959 0.9969 1.1309 −1.0122 0.9956 65 1.1869 −1.7551 0.9947 1.1279 −1.0033 0.9986 70 1.1840 −1.7654 0.9913 1.1250 −1.0411 0.9980 75 1.1812 −1.8331 0.9907 1.1222 −1.0732 0.9995 80 1.1783 −1.8467 0.9922 1.1193 −1.0732 0.9995 Figure 6. Refractive index, n D (dots), and surface tension, σ (squares), of EAN (solid symbols) and PAN (open symbols) as a function of the water content, w, at 25 ◦ C. Solid lines were obtained from Equation (1). Table 1. Fitting Equation (1) to density data (g · cm −3 ) of EAN and PAN as a function of water content (ppm): parameters and coefficient of determination R2. EAN PAN T (◦C) ρIL A1(107) R2ρIL A1(107) R2 5 1.2234 −1.4448 0.9963 1.1639 −0.66515 0.9941 10 1.2202 −1.4610 0.9971 1.1608 −0.67382 0.9941 15 1.2171 −1.4745 0.9988 1.1577 −0.70588 0.9958 20 1.2140 −1.5136 0.9939 1.1546 −0.73063 0.9913 25 1.2108 −1.5354 0.9989 1.1516 −0.76511 0.9945 30 1.2077 −1.5516 0.9978 1.1485 −0.79664 0.9938 35 1.2046 −1.5800 0.9934 1.1456 −0.83059 0.9926 40 1.2016 −1.6151 0.9988 1.1425 −0.83760 0.9920 45 1.1986 −1.6151 0.9988 1.1396 −0.89210 0.9908 50 1.1956 −1.6805 0.9977 1.1366 −0.90778 0.9920 55 1.1927 −1.6700 0.9944 1.1337 −0.96657 0.9944 60 1.1898 −1.6959 0.9969 1.1309 −1.0122 0.9956 65 1.1869 −1.7551 0.9947 1.1279 −1.0033 0.9986 70 1.1840 −1.7654 0.9913 1.1250 −1.0411 0.9980 75 1.1812 −1.8331 0.9907 1.1222 −1.0732 0.9995 80 1.1783 −1.8467 0.9922 1.1193 −1.0732 0.9995 85 1.1755 −1.8688 0.9945 1.1164 −1.1194 0.9981 90 1.1727 −1.9341 0.9969 1.1136 −1.1428 0.9953 95 1.1699 −1.9799 0.9954 1.1106 −1.1107 0.9927 Int. J. Mol. Sci. 2021,22, 7334 7 of 15 Table 2. Fitting Equation (1) to viscosity data (mPa · s) of EAN and PAN as a function of water content (ppm): parameters of Equation (1) and coefficient of determination R2. EAN PAN T (◦C) ηIL A1(104) R2ηIL A1(104) A2(109) R2 5 86.5 −15.60 0.9981 201.6 −57.29 70.74 0.9990 10 69.3 −12.10 0.9983 153.7 −41.82 50.26 0.9990 15 56.3 −9.517 0.9983 119.7 −31.40 37.31 0.9989 20 46.3 −7.504 0.9984 94.8 −23.98 28.02 0.9989 25 38.7 −6.084 0.9982 76.3 −18.51 21.00 0.9988 30 32.7 −4.960 0.9989 62.3 −14.56 16.29 0.9988 35 27.9 −4.125 0.9985 51.6 −11.64 12.83 0.9987 40 24.05 −3.439 0.9987 43.2 −9.434 10.30 0.9984 45 20.92 −2.901 0.9987 36.5 −7.642 8.041 0.9987 50 18.33 −2.468 0.9987 31.2 −6.293 6.465 0.9986 55 16.19 −2.120 0.9989 26.9 −5.220 5.206 0.9983 60 14.40 −1.832 0.9989 23.39 −4.362 4.224 0.9982 65 12.88 −1.600 0.9989 20.56 −3.748 3.623 0.9989 70 11.59 −1.405 0.9990 18.15 −3.191 2.991 0.9991 75 10.48 −1.239 0.9988 16.11 −2.722 2.455 0.9993 80 9.53 −1.094 0.9987 14.40 −2.340 2.026 0.9995 85 8.69 −0.9741 0.9987 12.93 −2.021 1.673 0.9996 90 7.97 −0.8775 0.9989 11.68 −1.764 1.419 0.9998 95 7.34 −0.7932 0.9989 10.59 −1.534 1.157 0.9999 Table 3. Fitting Equation (1) to electrical conductivity data (mS · cm −1 ) of EAN and PAN as a function of water content (ppm): parameters and coefficient of determination R2. EAN PAN T (◦C) κIL A1(104) R2κIL A1(104) R2 5 10.55 1.989 0.9991 3.50 1.053 0.9982 15 15.10 2.566 0.9990 5.52 1.451 0.9987 25 20.4 3.436 0.9978 8.12 1.891 0.9986 35 27.1 3.849 0.9982 11.34 2.364 0.9984 45 33.9 4.469 0.9980 15.11 2.831 0.9990 55 41.1 5.141 0.9968 19.37 3.450 0.9954 65 48.7 5.780 0.9911 24.4 3.875 0.9927 75 56.3 6.504 0.9951 29.9 4.185 0.9984 85 64.7 6.795 0.9936 35.5 4.361 0.9981 95 73.2 7.087 0.9942 41.6 4.548 0.9974 Table 4. Fitting Equation (1) to refractive index and surface tension (mN · m −1 ) data at 25 ◦ C of EAN and PAN as a function of water content (ppm): parameters and coefficient of determination R2. EAN PAN Property QIL A1A2R2QIL A1R2 nD1.45395 −5.14135 ×10−8−3.00678 ×10−12 0.9979 1.45536 −1.08059 ×10−70.9980 σ(mN·m−1)47.75 2.008 ×10−50.9932 38.72 2.201 ×10−50.9929 3. Discussion 3.1. Experimental Measurements as a Function of Water Content Analyzing the results obtained from the perspective of the influence of a low water content on the physical characterization of EAN and PAN, it can be clearly observed that not all properties are equally affected. Thus, the quantities most significantly modified by the presence of water are those of transport. The percentage change in viscosity averaged Int. J. Mol. Sci. 2021,22, 7334 8 of 15 over all temperatures between water-free content ( ηIL ) and 30,000 ppm reaches − 39% and − 42% for EAN and PAN, respectively. Meanwhile, in the case of electrical conductivity, the corresponding percentage change is even greater, representing 43 and 56%, respectively. At the other extreme, we find refractive index ( − 0.11% and − 0.22%, respectively), density (−0.42% and −0.22%) and surface tension (1.3% and 1.7%). As can be deduced from the data obtained, the presence of small quantities of water generates a considerable increase in both the fluidity of the liquid and the mobility of the charge. Although from a volumetric and surface point of view the liquids do not undergo such a marked change, it can be sufficiently significant when a characterization of the liquid structure is pursued. 3.2. Experimental Measurements as a Function of Temperature From the fitting parameter Q IL of each property at each temperature, which corresponds to the values of the properties at water content w = 0, in Figures 7and 8we have plotted the densities, viscosities and electrical conductivities of EAN and PAN as a function of temperature. Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 8 of 15 3. Discussion 3.1. Experimental Measurements as a Function of Water Content Analyzing the results obtained from the perspective of the influence of a low water content on the physical characterization of EAN and PAN, it can be clearly observed that not all properties are equally affected. Thus, the quantities most significantly modified by the presence of water are those of transport. The percentage change in viscosity averaged over all temperatures between water-free content (ηIL) and 30,000 ppm reaches −39 and −42% for EAN and PAN, respectively. Meanwhile, in the case of electrical conductivity, the corresponding percentage change is even greater, representing 43 and 56%, respectively. At the other extreme, we find refractive index (−0.11 and −0.22%, respectively), density (−0.42 and −0.22%) and surface tension (1.3 and 1.7%). As can be deduced from the data obtained, the presence of small quantities of water generates a considerable increase in both the fluidity of the liquid and the mobility of the charge. Although from a volumetric and surface point of view the liquids do not undergo such a marked change, it can be sufficiently significant when a characterization of the liquid structure is pursued. 3.2. Experimental Measurements as a Function of Temperature From the fitting parameter QIL of each property at each temperature, which corresponds to the values of the properties at water content w = 0, in Figures 7 and 8 we have plotted the densities, viscosities and electrical conductivities of EAN and PAN as a function of temperature. Figure 7. Calculated densities, ρIL, of water-free EAN (solid dots) and PAN (open dots) as a function of the temperature, obtained from Equation (1). Solid line obtained from the best straight line. The density data follow a linear trend with temperature as follows: ρ=A,+A,T, (2) where ρIL is the density of water-free IL, T is the temperature in °C and Aρ,0 and Aρ,1 are the fitting parameters presented in Table 5. These densities for both water-free ILs present a very similar slope, differing by 1.3%. In the case of viscosity and electrical conductivity, they can be correlated by the Vogel–Tammann–Fulcher (VTF) equation, as is usual for liquids of this nature [91]. This equation can be generally written as: Q=Qe/(,), (3) 1.10 1.12 1.14 1.16 1.18 1.20 1.22 1.24 0 20406080100 ρIL (g·cm−3) T (°C) Figure 7. Calculated densities, ρIL , of water-free EAN (solid dots) and PAN (open dots) as a function of the temperature, obtained from Equation (1). Solid line obtained from the best straight line. The density data follow a linear trend with temperature as follows: ρIL =Aρ,0 +Aρ,1T, (2) where ρIL is the density of water-free IL, T is the temperature in ◦ C and A ρ,0 and A ρ,1 are the fitting parameters presented in Table 5. These densities for both water-free ILs present a very similar slope, differing by 1.3%. In the case of viscosity and electrical conductivity, they can be correlated by the Vogel– Tammann–Fulcher (VTF) equation, as is usual for liquids of this nature [ 91 ]. This equation can be generally written as: QIL =Q∞eB/(T+273,15−T0), (3) where Q IL describes ηIL or κIL , Q∞ is the limit of viscosity or electrical conductivity at infinite temperature, B is related to the activation energy of ions to flow or to the activation energy for the ion hopping and, finally, T 0 is related with the glass transition temperature in K [ 12 , 92 – 94 ] and T is the temperature expressed in ◦ C. The fitting parameters Q∞ , B Int. J. Mol. Sci. 2021,22, 7334 9 of 15 and T 0 are given in Table 5, as well as the corresponding percentual deviation, which is defined as: s=100 v u u t∑N i=1QIL, i−QVTF,i QIL, i 2 N−1, (4) where Q VTF is the value of the calculated quantity obtained from the best fit of the VTF equation. The resulting curves are shown in Figure 8, showing a very good agreement with QIL data. Int. J. Mol. Sci. 2021, 22, x FOR PEER REVIEW 9 of 15 where QIL describes ηIL or κIL, Qis the limit of viscosity or electrical conductivity at infinite temperature, B is related to the activation energy of ions to flow or to the activation energy for the ion hopping and, finally, T0 is related with the glass transition temperature in K [12,92–94] and T is the temperature expressed in °C. The fitting parameters Q, B and T0 are given in Table 5, as well as the corresponding percentual deviation, which is defined as: s=100 ∑,, ,     , (4) where QVTF is the value of the calculated quantity obtained from the best fit of the VTF equation. The resulting curves are shown in Figure 8, showing a very good agreement with QIL data. Figure 8. Calculated viscosities, ηIL, and calculated electrical conductivities, κIL, of water-free EAN (solid symbols) and PAN (open symbols) as a function of the temperature, obtained from Equation (1). Dots correspond to the left axis (viscosity) and squares to the right axis (electrical conductivity). Solid line obtained from VTF Equation (3). Table 5. Fitting of density (g·cm−3), viscosity (mPa·s) and electrical conductivity (mS·cm−1) data of water-free EAN and PAN as function of temperature: parameters of Equations (2) and (3), coefficient of determination R2 and percentual deviation (s). IL Aρ,0 A ρ,1 (104) R2ρ 𝐐,ƞ Bƞ T0,ƞ sƞ 𝐐,𝛋 Bκ T0,κ sκ EAN 1.2257 −5.9379 0.9996 0.2111 778.8 148.7 0.06 666.2 −424.9 175.8 0.7 PAN 1.1664 −5.8998 0.9998 0.1823 869.5 154.1 0.10 904.1 −620.8 166.4 0.3 3.3. Experimental Measurements as a Function of Water Content and Temperature The two properties most affected by the presence of water impurities can be expressed as a function of water content and temperature by means of a single equation. Thus, the fitting equation for the viscosity and electrical conductivity of the IL at a given water content and temperature is: Q(w,T)=Q(T)+∑A,(T) w   , (5) where Q(w,T) is η(w,T) or κ(w,T), QIL(T) is the calculated water-free viscosity or electrical conductivity of the IL given by Equation (3) and the corresponding fitting parameters can be defined as: Aƞ, (T)=(−1)e∑   , (6) 0 20 40 60 80 0 20406080100 0 30 60 90 120 150 180 210 κIL (mS·cm−1) T (°C) ƞIL (mPa·s) Figure 8. Calculated viscosities, ηIL , and calculated electrical conductivities, κIL , of water-free EAN (solid symbols) and PAN (open symbols) as a function of the temperature, obtained from Equation (1). Dots correspond to the left axis (viscosity) and squares to the right axis (electrical conductivity). Solid line obtained from VTF Equation (3). Table 5. Fitting of density (g · cm −3 ), viscosity (mPa · s) and electrical conductivity (mS · cm −1 ) data of water-free EAN and PAN as function of temperature: parameters of Equations (2) and (3), coefficient of determination R 2 and percentual deviation (s). IL Aρ,0 Aρ,1 (104) R2ρQ∞,ηBηT0,ηsηQ∞,κBκT0,κsκ EAN 1.2257 −5.9379 0.9996 0.2111 778.8 148.7 0.06 666.2 −424.9 175.8 0.7 PAN 1.1664 −5.8998 0.9998 0.1823 869.5 154.1 0.10 904.1 −620.8 166.4 0.3 3.3. Experimental Measurements as a Function of Water Content and Temperature The two properties most affected by the presence of water impurities can be expressed as a function of water content and temperature by means of a single equation. Thus, the fitting equation for the viscosity and electrical conductivity of the IL at a given water content and temperature is: Q(w, T)=QIL(T)+ N ∑ i=1 AQ,i(T)wi, (5) where Q(w,T) is η(w, T) or κ(w, T) , Q IL (T) is the calculated water-free viscosity or electrical conductivity of the IL given by Equation (3) and the corresponding fitting parameters can be defined as: Aη, i (T)=(−1)ie M ∑ j=0 CjTj , (6) Aκ,i(T)= M ∑ j=0 CjTj, (7)