models and serve as a valuable source of basic information regarding molecular interactions[27,28].The preparation and development of PILs with tunable structures and physicochemical properties are indeed a significant challenge and an area of active research. The ability to fine-tune these properties is crucial for tailoring PILs to specific industrial applications.

ILs are indeed known for their relatively high viscosities compared to traditional solvents, which limit their commercial use to some extent. However, research and development in the field of ILs have been ongoing, and certain progresses have been made. To overcome this limitation, one important strategy involves mixing ILs with other solvents or additives to lead to the formation of IL-based binary mixtures with lower viscosity and enhanced conductivity for lowering the energy requirements for any engineering process [29–31].Thus, synthesis of novel PILs and optimization of the factors governing their unique features have attracted much attention. However, generally PILs are highly expensive and PILs based binary solvents are considered to have the potential to offer a unique way for modifying the properties of these designer solvents. To date, there are very few attempts to investigate physicochemical properties of PILs-based binary mixtures, although there are enormous studies in the case of typical aprotic ionic liquids (AILs).The properties of ILs depend on several factors; alkyl chain length has so far been the most crucial one to tune the properties[32–34].With the incredible expansion of PILs, enormous interest has been focused on the determination and understanding of the physicochemical properties, which may be significantly affected by molecular solvents such as alcohols either deliberately added or absorbed. Therefore, the knowledge of interactions between PILs and alcohols with various alkyl chain lengths is very important for the use of PILs for various applications.

In this study, neutralized a tertiary amine, diethylmethylamine (dema) with trifluoroacetic acid (CF₃COOH) to prepare a PIL, [demaH]CF₃COO^-. [demaH]CF₃COO^-was mixed with ethanol, 1- propanol, 1-butanol, and 1-pentanolat a broad range of compositions to have a series of binary mixtures. The transport properties such as density, viscosity, and ionic conductivity of the PIL and its binary mixtures were measured at 30^oC to realize the effect of alkyl chain length of alcohols on the physicochemical properties of [demaH]CF₃COO^- at a fixed temperature.

2. Materials & methods

Diethylmethylamine (dema) (>97.0%) from Sigma-Aldrich, trifluoroacetic acid (99.5%) from Scharlau, Spain, and ethanol, 1-propanol, 1-butanol, and 1-pentanol from RCI Lab scan were used without further purification.

2.1   Preparation of PIL and its binary mixtures

[demaH]CF₃COO^- was synthesized according to Scheme 1by mixing trifluoroacetic acid with the dema in equimolar amounts in a vial placedin an ice bath. The mixture was sonicated for 60 min and the resulting PIL was stored inside in an electrical desiccator. The DpK_a of the constituents, dema (10.5) and CF₃COOH (0.5) of [demaH]CF₃COO^- is 10.0 [5,7].

All the mixtures of [demaH] CF₃COO^- and alcohols were prepared in volume percentage by using a micropipette. Each of binary mixtures was sonicated for 60 min.

2.2          Characterizations

The density was measured using a small amount of PIL and its binary mixtures with alcohols (less than 2 mL), introduced into a U-shaped borosilicate glass tube and thermostated at 30^oC in each case. After each measurement, the U-shaped tube was cleaned with ethanol and ultrapure water. Ultrapure water was prepared by BOECO pure (Model:BOE-8082060, Germany).

1 mL of sample was injected into a glass capillary and placed into a capillary block at 30^oC to measure the viscosity. This block was angled in a variety of predetermined directions. Based on the approximate viscosity of the combination, the measuring angle, capillary diameter for sampling, and ball diameter were chosen. The viscosities of all binary mixtures were determined using a Lovis 2000 M/ME microviscometer with an accuracy of 10^-6 mPa.

The Autolab PGSTAT302N, a high-performance impedance analyzer with FRA32M module, was used to assess conductivity using electrochemical impedance spectroscopic (EIS) tests over a frequency range of 10 Hz to 10MHz.A glass electrode was used to measure solution resistance. The glass electrode was made up of two small Pt wires. One of which worked as the working and other as the counter electrode.0.01M aqueous solution of KCl was used to determine the cell constant at 30°C.

3. Results and discussion

3.1 Density

Fig. 1.depictsthe density of [demaH]CF₃COO^-and its binary mixtures at 30^oC.The nature of the alcohol has been found to influence density of PIL very strongly. The density of binary mixtures with alcohol increases non-linearly with the increasing mole fraction of [demaH]CF₃COO^-.

Fig.1. Density vs. mole fraction of [dema]CF₃COO^- for different alcohols in the binary mixtures at 30 ^oC.

3.2 Viscosity

The variation of dynamic viscosity of the prepared binary mixtures with the mole fraction of [demaH]CF₃COO^- is shown in Fig. 2.

Fig.2. Viscosity vs. mole fraction of [dema]CF₃COO^- for different alcohols in the binary mixtures at 30 ^oC.

The viscosities decreased with the increasing mole fraction of the alcohols at 30^oC. Addition of alcohols to PIL results in a weakened interactions between the ionic species and the hydrogen bonding might also be weakened [35]. Therefore, viscosity decreases with increasing mole fraction of alcohols.

3.3 Conductivity

The conductivity values of PIL and its binary mixtures with alcohols at 30^oC are shown in Fig. 3. The conductivity of [demaH]CF₃COO^- was 5.25 mScm^-1 at 30^oC. Conductivity of PIL increases upon the addition of alcohols at all compositions. However, the maximum value of the conductivity of [demaH]CF₃COO^--ethanol ([demaH]CF₃COO^--alcohol = 0.25:0.75) is the highest (15.76 mScm^-1 at 30^oC) compared to other [demaH]CF₃COO^--alcohol ratio.

 Fig.3. Conductivity vs. mole fraction of [dema]CF₃COO^- for different alcohols in the binary mixtures at 30 ^oC.

Generally, the conductivity of the binary mixtures decreases as the alkyl chain length of the added alcohol is increased. The profiles for conductivity of [demaH]CF₃COO^- and its binary mixtures with alcohols are shown in Fig. 3. Similar trends were observed for molar conductivity at 30^oC. In case of the PIL, the number of charge carriers might be increased upon dissolution in a polar solvent like alcohol, but PILs can also enhance the viscosity of the ternary mixtures [26]. The consequences of more charge carriers and less ion mobility are apparent in the ionic conductivity vs. concentration of PIL profiles. In the low concentration range, the number of charge carriers (ionic species that can conduct electricity) provided by the PIL becomes the primary factor influencing the overall ionic conductivity. Other factors dominate the ion mobility with changes in the PIL mole fraction in the binary mixtures ; as a result, the ionic conductivity achieves its maximum value before gradually decreasing [36].Addition of different concentrations of alcohol lowers both the charge density and viscosity of a pure PIL. Depending on the concentration of alcohol, its dilution effect becomes more noticeable which can increase or decrease the conductivity of a PIL. Addition of alcohol to PIL can enhance the movement of this charge carriers which in turn increase the electric conductivity of the solution. In case of high PIL concentration, attractive interactions among ions results in the aggregation of charge carriers. Consequently, the mobility of the charge species is lowered for a rise in viscosity. Therefore, electric conductivity of the solution tends to drop in the high concentration of PIL [26,37–43].

3.4 Excess molar volume

Realizing the nature of molecular association in binary mixtures depends greatly on excess thermodynamic parameters, which are dependent on composition and temperature. The excess molar volume, V_m^E is the deviation from the ideal behavior. The V_m^E for all mixtures was derived using the density of corresponding mixtures. Fig. 4 shows change of V_m^E of [demaH]CF₃COO^-–alcohol binary mixtures over the whole composition at 30^oC along with the profiles fitted to Redlich-Kister equation:

)ⁱ

where, A_i is the fitting coefficient and x₂ is the mole fraction of [demaH]CF₃COO^-.

The V_m^E of binary mixtures of [demaH]CF₃COO^-and alcohols were positive for the mole fraction range 0.6 to 1.0, on the other hand, [demaH]CF₃COO^-and its binary mixtures with alcohols shows negative deviation at the mole fraction range 0.0 to 0.6. At 30^oC, the V_m^E of [demaH]CF₃COO^- – alcohols and its binary mixtures changed with the alkyl chain length of the alcohol. The positive value indicates that expansion of volume occurs when two components are mixed. Positive values of V_m^E are explained by the breakdown of chemical and non-chemical interactions between the molecules.

Fig.4. Excess molar volume, V_m^E for [demaH]CF₃COO^--alcohol mixtures as a function of mole fraction of [demaH]CF₃COO^- at 30^oC.

The negative value reveals a more effective packing or attractive interaction when [demaH]CF₃COO^- and alcohol come closer in the binary mixtures. In the pure systems, the ions of [demaH]CF₃COO^- interact through hydrogen bonding though the alcohol have weak dipole-dipole interactions. Mixing of alcohol with PIL makes a mutual dissociation of dipole-dipole interaction in alcohol and breaking or weakening of hydrogen bonds in PIL encompassing ions and consequent formation of new hydrogen bonds leading to a contraction in volume, thus resulting into negative V_m^E [44,45].

It can be assumed that there is a breakdown of the mode of association in pure bulk PIL and pure alcohol by hydrogen bond formation between two components in the mixture. This is weaker than the electrostatic  interactions between the cations and anions in pure PIL to cause positive deviation of volume from the ideal behavior [46–48].

3.5 Excess Viscosity

Fig. 5. shows sigmoidal curves for deviation of viscosity vs mole fraction of [demaH]CF₃COO^- profiles over the total composition range of [demaH]CF₃COO^-–alcohols mixtures. The deviation at all compositions has been found to be negative at 30^oC.

Fig. 5. Excess viscosity as function of mole fraction of [demaH]CF₃COO^- at 30^oC.

Although temperature has a significant impact on viscosity deviation, changes in temperature have little influence on the composition at which the smallest deviation is noted. The excess viscosity of [demaH]CF₃COO^- and their binary mixtures with alcohol at 30^oC is shown in Fig. 5. The ideal viscosity of liquid mixtures can be represented by Bingham’s equation which is based on the principle of additivity.

Ƞ = X_IL ƞ_IL + X_a ƞ_a

Where, Ƞ is the viscosity deviation, X_IL and X_a are the mole fraction of pure PIL and alcohols, _IL and _a are the viscosity of PIL and alcohols. The deviation of observed viscosity from the ideal behavior is the excess viscosity of the binary mixtures. The change of excess viscosity provides a qualitative assessment of the strength of probable intermolecular interactions. Generally, the excess viscosity of all seven binary mixtures of [demaH]CF₃COO^- with alcohols was found to be negative. The viscosity attained a more negative value as the mole fraction of [demaH]CF₃COO^- in the binary mixtures increased [49]. Negative value inferred that the experimental viscosity is lower than the calculated ideal viscosity. In general, negative excess viscosity is observed due to the inclusion of alcohol molecules in the matrices of larger species. When alcohol is mixed with pure PIL, the hydrogen bonds between [demaH]⁺ and CF₃COO^- in the quasi-three-dimensional network of PIL might be weakened with the formation of a hydrogen bond between the cation and alcohol molecules [50].Therefore, mobility of the cation and anion become higher and the resistance to flow decreases which results in the lower viscosity of solution compared to the ideal value. The excess viscosity was more negative with increasing alkyl chain of the alcohols. For a further understanding, the density, viscosity, conductivity, V_m^E and Dh of binary mixtures are compared in Table 1.

Table 1. Density r, viscosity h, conductivity s, excess molar volume V_m^E and viscosity deviations Dh for various binary mixtures at 30^oC.

[demaH]CF₃COO^- + Ethanol

[demaH]CF₃COO^- + 1- Propanol

[demaH]CF₃COO^- + 1- Butanol

[demaH]CF₃COO^- + 1- Pentanol

3.6 Effect of alkyl chain length on the density

The rof [demaH]CF₃COO^- and its binary mixtures with alcohols for change in the carbon number of the alcohol is shown in Fig. 6.

Fig. 6. Density of [demaH]CF₃COO^- binary mixtures as a function of carbon number of the alcohol.

The r of [demaH]CF₃COO^- and its binary mixtures with alcohol decrease with an alkyl chain of the alcohols as well as the mole fraction of PIL. Therefore, alkyl chain length of the alcohol affects the r of the PIL strongly. PIL- alcohol binary mixtures are highly dense compared to the pure alcohols at 30^oC. When the mole fraction of the alcohol increases, aggregation between [demaH]CF₃COO^- – alcohol decreases and the volume of the binary mixtures increases so that the r of the binary mixtures decreases with the alkyl chain length of the alcohol. However, the r of pure alcohol increases with the alkyl chain length.

3.7 Viscosity of PIL and its Binary mixtures

The h of [demaH]CF₃COO^- and its binary mixtures with alcohols as a function of carbon number are shown in Fig. 7. The change in the h  with the change of the alkyl chain of the alcohol was observed in this case. At high composition ratios, such as (PIL: Alcohol = 0.35:0.65; 0.25:0.75), the effect of the alkyl chain is very large. Generally, viscosity decreases with increasing chain length of the alkyl group of the alcohols. However, the hof pure alcohol increases with the alkyl chain length.

Fig.7. Viscosity of [demaH]CF₃COO^- binary mixtures as a function of carbon number of the alcohols.

Usually, the high electrostatic interactions between the cation and anion of PIL give the distinct physicochemical properties of PIL. Furthermore, van der Waals forces and hydrogen bonds, also play an important role for the characteristic properties of PIL and alcohol binary mixtures.

3.8 Conductivity of PIL and its binary mixtures

The conductivity of [demaH]CF₃COO^- and its binary mixtures with alcohols as a function of carbon number is shown in Fig. 8.

Fig.8. Conductivity of [demaH]CF₃COO^- binary mixtures as a function of carbon number of the alcohols.

The conductivity decreases with increasing the alkyl chain length of the alcohols and increases with increasing the mole fraction of the alcohols. This is attributed both to the decrease in mobility due to the increase in mass and to the higher tendency to form clusters via hydrophobic interaction between the hydrocarbon chain of the alcohols.

3.9 Walden Plot

The ionicity of [demaH]CF₃COO^- and its binary mixtures with alcohols was investigated using Walden Plot. A plot of logL vs. log(1/h) following the Walden rule was constructed using data for a 0.01 M aqueous KCl solution used as the reference. Hence, an “ideal” line corresponds to the ionic chracteristics of an ideal electrolyte with a slope of unity [51–53].

Fig. 9. Walden plot of PIL-alcohol binary mixtures at 30^oCwith reference to 0.01 M KCl.

The vertical distance from this ideal line at log(1/h) is used to categorize PILs as super-ionic, good, poor, or non-ionic[54,55]. Fig. 9 shows the Walden plot for binary mixtures and pure PIL investigating the ionic nature of these binary mixtures in comparison with the PIL. Walden plots for binary mixtures of [demaH]CF₃COO^-– alcohols resided below the ideal line for 0.01M KCl solution. From the Walden plot, it was revealed that PIL and its binary mixtures are poor PILs.

4. Conclusion

Physicochemical properties of a PIL, [demaH]CF₃COO^- and its binary mixtures with alcohols of varying alkyl chain lengths have been investigated. A decrease in the density, viscosity, and conductivity with increasing alkyl chain length of the alcohol is observed. The experimental data for the density and viscosity of the binary mixtures of PIL with ethanol, 1- propanol, 1-butanol, and 1-pentanol at 30 ^oC were fitted to evaluate excess molar volumes, V^E_m, and viscosity deviations, Dη, of these binary mixtures. The V^E_m was found negative at low PIL concentrations (PIL< 0.50) and positive at high PIL concentrations (PIL > 0.50). The stronger association of the PIL-alcohol was probably responsible for this negative effect. The excess viscosity of all binary mixtures of [dema]CF₃COO^- -alcohol was found negative for almost all cases. From the Walden plot, it was revealed that PIL and its binary mixtures are poor PILs.