DENSITY , VISCOSITY AND EXCESS PROPERTIES OF BINARY MIXTURES OF PROTIC IONIC LIQUID ( 2-HDEAF , 2-HDEAA ) + WATER AT DIFFERENT TEMPERATURES

Over the last decade, ionic liquids (IL) have attracted considerable attention for their potential to replace traditional volatile organic solvents due to their well-known physico-chemistry properties. This is an important topic for industrial applications today, and have been the subject of extensive literature studies. Nevertheless, there is still a lack of experimental thermodynamics data on aqueous mixtures, i.e., ionic liquids + water, especially volumetric and transport properties. These data could provide essential information about interactions and molecular phenomena in such mixtures. To investigate volumetric and transport properties for ionic liquids + water, density and viscosity data of the protic ionic liquids 2-hydroxydiethanolamine formate (2-HDEAF), 2-hydroxydiethanolamine acetate (2-HDEAA), water and their binary mixtures were measured at T = (293.15 to 343.15) K and atmospheric pressure. Excess molar volumes and deviation of viscosity were computed from experimental density and viscosity values and were fit to the Redlich-Kister equation. We observed a negative behavior for both ionic liquids + water mixtures that can be attributed to a shrinkage of the mixtures.


INTRODUCTION
Ionic liquids are defined as salts that have a low melting point (Kurnia et al., 2011).Historically, ethylammonium nitrate was the first ionic liquid studied, in the beginning of the 20th century.Ionic liquids are considered as potential environmentally friendly solvents (Xuemei et al. 2013), presenting low vapor pressure in comparison to traditional volatile organic solvents widely used in chemical industries.There are other important characteristics that take advantage of their use, as follows: low melting temperature, favorable solvation behavior, high ionic conductivity, stability in air, and modification of selectivity in chemical reactions, which open new possibilities in various industrial fields such as catalysis, separation techniques, and electrochemical device building (Iglesias et al., 2008).
Ionic liquids are produced from the combination of cations, that includes compounds based on substituted amines, and organic acids with different numbers of carbon atoms as anions in order to adapt more effectively to a specific process, e.g., separation processes and catalytic reactions (Peric et al., 2014).Generally, ionic liquids are divided into two large families, aprotic and protic.
Aprotic ionic liquids (AIL's) are constituted primarily of imidazolium cations.Nevertheless, their use becomes difficult in industrial process applications due to their high cost (Bourbigou-Olivier et al., 2010).On the other hand, protic ionic liquids (PIL's) are produced by proton transfer of a Brønsted acid to a Brønsted base (cation with amino group).For this reason, their production could be characterized as low cost.In addition, their syntheses are less complex and, depending on the process, can be recycled and reused (Alvarez, 2010).
Thermodynamic and transport properties are very important experimental data for design and from a theoretical point of view.The addition of a solvent in a liquid mixture can dramatically increase or decrease the density and viscosity in these systems (Alvarez et al., 2011).Various technological processes involving IL's need the knowledge of these mixture properties with the addition of other solvents.To understand the interactions of their constituent cations and anions with water, the behavior of IL's when mixed with various solvents is of utmost importance.The mixing deviation from ideality is useful in the study of molecular interactions and is of great interest for industrial applications, e.g., polar solvent extraction processes and Aqueous Biphasic Systems (ABS).The data of densities and viscosities for 2-hydroxydiethanolamine formate (2-HDEAF), 2-hydroxydiethanolamine acetate (2-HDEAA), and water and their binary mixtures (2-HDEAF + water and 2-HDEAA + water) at temperatures ranging between (293.15 and 343.15)K have been experimentally determined.It is important to note that these data were not found in the literature.

Materials
Formic acid (purity ≥ 0.85 in mass fraction), acetic acid (purity ≥ 0.99 in mass fraction), diethanolamine (purity ≥ 0.99 in mass fraction) were used without further purification and purchased from VETEC.Deionized water was used.
The system for the production of PIL's was constituted of a glass three-necked flask equipped with thermocouple, a funnel and a condenser.The acid was added dropwise to the flask with diethanolamine.After complete addition of the acid, the reaction remained under agitation for at least 24 hours for the formation of a viscous and slightly yellow product.The resulting liquids were dried under a vacuum pump for 48 h and 20 kPa in order to reduce the water content as much as possible, and were stored in a dark bottle (Alvarez et al., 2011).It is important to mention that ionic liquid densities have been measured before and after a drying procedure, in order to have a roughly idea of the effect of water content.It has been found that the amount of water in the ionic liquids produced has negligible effect.Figure 1 shows the chemical structures of the ionic liquids studied.

Methods
The binary mixtures containing ionic liquid and water were prepared by mass weight percent and they were kept in sealed glass bottles to avoid contamination and evaporation of the components.2-HDEAF and 2-HDEAA ionic liquids were completely solubilized in water.All samples were prepared using an electronic scale (Shimadzu, model AY220) with uncertainty of ± 1 × 10 −4 g.All mixtures were prepared in a composition range from 0.1 to 0.9.The uncertainty in experimental mole fraction was estimated to be approximately ± 1,7 × 10 −3 .
Densities and viscosities of the pure liquids (2-HDEAF, 2-HDEAA and water) and their binary mixtures (2-HDEAF + water, 2-HDEAA + water) were experimentally determined by using a viscodensimeter (SVM 3000, Anton Paar).A 5 mL sample was injected into the equipment by using a syringe in the experimental measurements.Both measurements were realized simultaneously and in duplicate by the equipment itself.The viscodensimeter presented an accuracy of ± 0.01 K for temperature, ± 0.0005 g•cm -3 for density, and for relative viscosity ± 0.35 %.After analyses performed in triplicate, it was observed that the temperature measurement had an experimental uncertainty of ± 0.01 K for temperature, ± 0.00002 g•cm -3 for density, and ± 0.0006 mPa•s for viscosity.

Thermodynamics correlation
The temperature dependence of densities of the pure components and binary mixtures were fitted using the linear equation: where η is the dynamic viscosity of the mixture; η i and x i denote viscosities and mass fractions of pure components, respectively.
Density (ρ) and viscosity (η) data were used to derive the excess Gibbs free energy of activation (G *E ) by: where ρ (g•cm −3 ) is the density and A 0 and A 1 are the coefficients.
The excess molar volume (V E ) was calculated from the density of pure component, ρ i and of binary mixtures ρ, by: where x i and Mi are the mole fraction and molar mass of pure component i respectively.
Experimental viscosity data were used to calculate the deviation of viscosity (∆η) of the binary mixtures, by: where R is the universal gas constant; T is the absolute temperature; V and V i are the molar volumes of the binary blends and pseudo-pure compounds, respectively.
The excess properties, V E , ∆η and G *E were correlated by the Redlich-Kister polynomial equation: where M E is the excess property or deviation, x 1 is the mole fraction, Aj is a parameter, and k is the degree of the polynomial expansion.A j values were obtained using a nonlinear least-squares fitting procedure.
) ( The corresponding standard deviations σ (M E ) were calculated by: where M exp is the experimental excess property and M cal is the calculated (adjusted) excess property, n is the number of experimental points, and p is the number of parameters retained in the respective equation.

Density and excess molar volume
Table 1 presents the experimental density data of pure protic ionic liquid from this work and the literature at 293.15 K. To proving the chemical structures was been realized a NMR of the studied ionic liquids.  Santos et al., 2016)  Table 2 and Figures 2 a and 2 b depict density data for pure 2-HDEAF + water and 2-HDEAA + water from T = (293.15to 343.15) K and atmospheric pressure.From these data it is possible to state that 2-HDEAF + water and 2-HDEAA + water binary systems present a volumetric behaviour of regular liquids, i.e., decreasing with increasing temperature, in a linear behaviour.Observing various compositions at a single temperature, it can be seen that increasing water composition in the binary mixture causes a decrease in density.
Table 3 and Figures 3 a and 3 b show the excess molar volumes for the systems 2-HDEAF + water and 2-HDEAA + water, from T = (293.15to 343.15) K.It could be observed that, for both binary systems (2-HDEAF + water and 2-HDEAA + water) in the whole range of composition and for both temperature studied, a negative behavior is characterised.This negative excess molar volume can be attributed to strong interactions between ionic liquids (2-HDEAF and 2-HDEAA) and water and, as a result, a volume contraction.The same conclusion was obtained by Alvarez et al. (2011) by studying mixtures of 2-hydroxyethylammonium acetate with low molecular weight polar solvents.This behavior can be explained by the high dielectric constant of water.The maximum value in V E occurs between 0.7 and 0.8 mol fraction of water.

Viscosity, deviation of viscosity and excess Gibbs free energy
The experimental viscosities values of pure ionic liquids, water and binary mixtures at the various temperatures are listed in Table 4.           4, the greater the amount of water in the mixtures the lower the viscosity.It can be observed that the 2-HDEAF + water mixture showed viscosity values lower than the values of the 2-HDEAA + water mixture; this difference may be due to the fact that the ionic liquid 2-HDEAF was produced from a lower carbon chain acid.
Deviation of viscosity for the binary mixtures of (2-HDEAF + water and 2-HDEAA + water) at different mole fractions from T = (293.15to 343.15) K and atmospheric pressure are listed in Table 5.
Figures 5(a) and 5(b) show that the Δη values are negative for all temperatures and composition range for the mixtures studied.From these figures it can also be observed that, the higher the temperature, the less negative the deviations of viscosity values in both systems analyzed.According to Prausnitz (1986), Δη of a mixture depends on molecular interactions, as well as on the size and shape of the molecules.For both figures, the minimum deviation in Δη occurs between 0.4 and 0.5 mole fraction of water.
The excess Gibbs free energy of activation for the binary systems studied is reported in Table 6.
Figures 6(a) and 6(b) show that G *E presents positive values in all the temperature and composition range.According to Smith et al., (1996), positive values of G *E indicate specific interactions leading to complex formation through intermolecular hydrogen bonding interaction between unlike molecules compared to like molecules.For both figures, the largest excess molar Gibbs free energy of activation (G* E ) is at 0.65 mole fraction of water.
The adjustment parameters, A j , obtained with the Redlich-Kister polynomial equation and the standard deviation for the systems studied in T = (293.15to 343.15) K are presented in Table 7.

Figure 1 .
Figure 1.Structures and abbreviations of the analyzed PILs.

CONCLUSIONS 2 -
HDEAF and 2-HDEAA binary mixtures with water are completely miscible and exhibit a liquid ideal linear density-mole fraction and exponential viscosity-molar isotherms[T = (293.15and 343.15)K].They present negative values for excess molar volume (V E ) and viscosity deviation (∆η) throughout the composition range, and present a minimum position around x ≈ 0.4-0.5.Excess Gibbs free energy of activation presented positive values.All excess properties were fitted by the Redlich-Kister polynomial equation.This behavior suggests a shrinkage mixture with no symmetric isotherms, characteristics of a mixture that does not represent a regular solution, with no significant structural change takes place during the mixture.

Table 2 .
Densities (ρ) of the pure components (2-HDEAF, 2-HDEAA and water) and binary mixtures in the temperature range (293.15 to 343.15) K and at atmospheric pressure.

Table 3 .
Excess molar volumes (VE) of the studied binary mixtures in the temperature range (293.15 to 343.15) K and at atmospheric pressure.
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Table 4 .
Viscosity (η) of the pure components (2-HDEAF, 2-HDEAA and water) and binary mixtures in the temperature range (293.15 to 343.15) K and at atmospheric pressure.

Table 5 .
Deviation of viscosity (∆η) of the studied binary mixtures in the temperature range (293.15 to 343.15) K and at atmospheric pressure.
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Table 6 .
Excess Gibbs free energy of activation (G *E ) of the studied binary mixtures in the temperature range (293.15 to 343.15) K and at atmospheric pressure.

Table 7 .
Adjustment parameters of excess properties of (2-HDEAF + water ) and (2-HDEAA + water) binary mixtures as function of temperature with the Redlich-Kister equation together with the standard deviation (σ).

Excess Gibbs free energy of activation
Density, viscosity and excess properties of binary mixtures of protic ionic liquid (2-hdeaf, 2-hdeaa) + water at different temperatures