The densities of neat IL [QuatGly-OEt][EtOSO₃], neat PEG200, and {PEG200 (1) + [QuatGly-OEt][EtOSO₃] (2)} binary mixture over the temperature range (293.15 to 353.15) K and at atmospheric pressure are presented in Table 1, and the structure of IL is shown in Figure 1. From analysis of the data, it was found that the densities depend more strongly on the mole fraction of PEG200 in the mixture than on temperature; the density trend for the binary system with respect to temperature decreased with increasing temperature.

As can be seen from Table 1 and Figure 2, the density decreases with increasing temperature even though the variation is very small. If the variation of the density with the temperature is considered as linear, then the change in the calculated thermal expansion coefficient values becomes nearly constant. In order to have an improved accuracy, a second order polynomial of the following form was used to correlate the variation of the density with the temperature for {PEG200 (1) + [QuatGly-OEt][EtOSO₃] (2)} binary system studied in the present work.

where T is the temperature and b₀, b₁, and b₂ are the correlation coefficients. The correlation coefficients were estimated using the least square method, and the values of the coefficients are listed in Table 2 together with the standard deviations (σ) estimated using the following equation:

where n is the number of experimental points, and Z_exp and Z_cal are the experimental and calculated values, respectively.

Excess thermodynamic properties are of great importance for understanding the nature of molecular interaction in binary mixtures. The excess molar volume, V_m^E, which is the difference between the real and ideal mixing behaviors, is defined by:

where V_m represents the volume of a mixture containing one mole of (PEG200 + [QuatGly-OEt][EtOSO₃]), x₁ and x₂ are the mole fractions of components 1 (PEG200) and 2 ([QuatGly-OEt][EtOSO₃]), respectively, and V₁⁰ and V₂⁰ are the molar volumes of the pure components, respectively. The excess molar volumes of mixtures over the entire composition range were calculated from our measurements of density according to the following equation:

where ρ₁, ρ₂, and ρ are the densities of neat PEG200, [QuatGly-OEt][EtOSO₃], and their mixture, respectively; M₁ and M₂ are the molar masses of PEG200 and [QuatGly-OEt][EtOSO₃], respectively. The V_m^E values of mixtures are summarized in Table 1. The excess molar volumes of mixtures versus the mole fraction of PEG200 from 293.15 to 353.15 K are plotted in Figure 3. The excess molar volume of mixtures is mainly due to the variation in inter molecular forces between the compounds or the variation in the molecular packing due to the differences in the size and shape of the molecules forming a binary mixture with other compound. As shown in Figure 3, the excess molar volumes are asymmetric and negative for all of the systems studied over the entire composition range, the minimum of V_m^E is reached at a mole fraction of PEG200 near x₁ = 0.3, and the absolute values of the excess volume increase with increasing temperature. The negative excess molar volumes indicate that a more efficient packing and/or attractive interaction occurred when the ionic liquid and organic molecular liquids were mixed. The molar volume for [QuatGly-OEt][EtOSO₃] is 234.07 cm³ mol⁻¹, which is greater than the molar volumes of PEG200 (176.81 cm³ mol⁻¹) at T = 293.15 K. The large difference between molar volumes of the molecular liquid and [QuatGly-OEt][EtOSO₃] imply that it is possible that the relatively small organic molecules fit into the interstices upon mixing. Therefore, the filling effect of organic molecular liquids in the interstices of ionic liquids, and the ion-dipole interactions between organic molecular liquid and the glycine ester of the ionic liquids, all contribute to the negative values of the molar excess volumes. A similar phenomenon has been observed for ([bmim][PF₆] + MeCN) and ([bmim][PF₆] + MeOH) by Zafarani-Moattar and Shekaari [33].

Density results for the ILs studied in current work were also used to derive other thermodynamic properties such as the coefficient of thermal expansion. From the density-temperature dependence, the volume expansivity (coefficient of thermal expansion), α, is defined as:

where subscript p indicates constant pressure. The α values of pure [QuatGly-OEt][EtOSO₃] and PEG200, and their mixture are summarized in Table 3. The values indicate that the thermal expansion coefficient of this binary mixture (PEG200 + [QuatGly-OEt][EtOSO₃]) increases with increasing temperature and increases with increasing mole fraction of the PEG200. However, this trend also shows that the change in the thermal expansion coefficient with the temperature is not as significant as compared to the change with increasing mole fraction.

Next, the corresponding excess function was determined. The excess volume expansivity was calculated using:

where ϕ₁^id is an ideal volume fraction given by the following relation:

where V_mi is for the molar volume of pure component i.

Typical concentration dependencies of excess volume expansivity are given in Figure 4 for the {PEG200 + [QuatGly-OEt][EtOSO₃]} binary system. The values of excess thermal coefficient α^E are found to be negative over the whole composition range for the binary system at all the investigated temperatures. The curves are asymmetrical, with the minimum located at the IL mole fraction of about 0.6. In general, negative α^E values indicate the presence of a strong interaction between the components in the mixtures [34]. The trends observed in α^E values for the present mixtures suggest the formation of H-bonding between unlike molecules which is stronger in {PEG200 + [QuatGly-OEt][EtOSO₃]} binary system. The increasing negative α^E values for {PEG200 + [QuatGly-OEt][EtOSO₃]} with increase in temperature indicates more destruction of order during mixing contribute negatively to α^E.

Knowledge of electrolyte solution viscosity is needed for the design of numerous industrial processes and provides useful insights into solution structure and interactions [35]. The measured dynamic viscosities, η, for the neat PEG200, neat [QuatGly-OEt][EtOSO₃], and {PEG200 + [QuatGly-OEt][EtOSO₃]} binary mixture as a function of temperature over the whole composition range are listed in Table 4. Most ionic liquids have undesirable high viscosity; one possible way to lower their viscosities is by addition of water or other diluent. Figure 5 shows the experimental dynamic viscosities for {PEG200 + [QuatGly-OEt][EtOSO₃]} binary system, a small concentration of PEG200 significantly lowers the viscosity.

The temperature dependence becomes distinctly nonlinear, especially at high [QuatGly-OEt][EtOSO₃] content. The Vogel-Tammann-Fulcher (VTF) equation [36,37] suitably correlates the nonlinear behavior as a function of the temperature, not only the viscosities of the neat IL but also the viscosities of the mixtures for the binary systems through the composition range.

where η_o, B, and T_o are constants. The parameter T_o is the ideal glass transition temperature, which should be slightly below the experimental glass transition temperature T_g [38]. The adjustable parameters obtained from the VTF correlation were presented in Table 5. The obtained parameters η_o and B change smoothly with composition for binary mixtures.

The viscosities of the mixtures decrease rapidly when organic oligomers are added to the IL, this decrease is particularly evident in dilute solutions of organic oligomers in the IL. The strong coulomb interactions between the [EtOSO₃]⁻ anion and [QuatGly-OEt]⁺ cation are weakened upon mixing with the polar organic oligomers, which leads to a higher mobility of the ions and a lower viscosity of the mixtures [39]. This indicates that the viscosity of ILs could be therefore tuned for several applications by adding an organic solvent or by changing temperature [40].

The viscosity deviations Δη (mPa·s) were calculated using:

where x₁ and x₂ are the mole fractions of PEG200 and [QuatGly-OEt][EtOSO₃], respectively, and η, η₁, and η₂ are the viscosities (mPa·s) of the solution, the pure solvent, and the pure IL, respectively.

Figure 6 shows the viscosity deviations as a function of the PEG200 mole fraction composition, x₁, and temperature for the {PEG200 + [QuatGly-OEt][EtOSO₃]} binary mixture. The values of viscosity deviations are compared with those obtained from the Redlich–Kister polynomial equation. The viscosity deviation for the mixture show negative deviations from ideality over the whole mole fraction range. Figure 6 also shows that the negative values of viscosity deviations decrease with increasing temperature. This can be attributed to the specific interactions in mixtures, typically H-bonds, break-up as the temperature increases [41]. The minimum value of Δη is observed at about x₁ ≈ 0.4, the viscosity of a mixture depends strongly on its entropy, which is related with the liquid’s structure. Therefore, the viscosity deviation depends on molecular interactions as well as on the size and shape of the molecules [42].

The refractive index n of a material is defined as the ratio c_o/c, where c is the speed of light in the material and c_o is the speed of light in vacuo. Refractive indices n for the {PEG200 + [QuatGly-OEt][EtOSO₃]} binary mixture as a function of the composition over the whole mole fraction range at T = 293.15 K are reported in Table 6. The refractive index deviations of molar fraction, Δ_xn, from the linear additive values of the mole fractions were calculated by means of the Equation 10:

where n is the refractive index of the mixture and n₁ and n₂ are the refractive indices of components 1 (PEG200) and 2 ([QuatGly-OEt][EtOSO₃]), respectively. Values of Δ_xn for the binary mixture are shown in Table 6 and Δ_xn is plotted in Figure 7a against the mole fraction over the whole composition region.

The deviation of refractive indices Δ_xn is positive over the whole range, with the maxima lying nearly at x₁ ≈ 0.5. Moreover, the deviation of n at various volume fraction, Δ_Ψn, from ideality on volume fraction basis is given by:

where Ψ₁ and Ψ₂ are the volume fractions of component 1 (PEG200) and 2 ([QuatGly-OEt][EtOSO₃]), respectively. Values of Δ_Ψn for the binary mixture are shown in Table 6 and Δ_Ψn is plotted in Figure 7b,c against the molar fraction and volume fraction over the whole composition region, respectively. Δ_Ψn values are symmetric against the molar fraction over the entire composition range Figure 7b, whereas Δ_Ψn are asymmetric against the volume fraction over the entire composition range Figure 7c. For the comparison of Figure 7a,b, Δ_Ψn values are similar to Δ_xn and the shape of Δ_Ψn is similar to Δ_xn over the entire composition range. Furthermore, the refractive index deviation increases with the increase of temperature, indicating the decrease of unlike interaction between components of the mixture.

From the experimental densities and refractive indices, the molar refractions of the IL, R_m, were calculated at various temperatures using the Lorenz-Lorentz equation:

where n_D is the measured refractive index, and V_m is the molar volume of neat PEG200, neat [QuatGly-OEt][EtOSO₃], and {PEG200 + [QuatGly-OEt][EtOSO₃]} binary mixture. The molar refraction of the neat ILs [QuatGly-OEt][EtOSO₃] and PEG200 are 63.9 and 48.3 at 293.15 K, respectively.

Similar representation with Δn has arisen in regard to ΔR, the deviation of molar refraction R from ideality was recently noted by Fermeglia and Torriano [43]. Some papers [44,45] define it on a mole fraction basis, as

and others [46] on a volume fraction basis, as

while some [43,47] report both quantities.

Values of Δ_xR, Δ_ΨR for the binary mixture are shown in Table 6, and Δ_xR is plotted in Figure 7d against the mole fraction over the whole composition region, whereas Δ_ΨR is plotted in Figure 7e and Figure 7f against the molar fraction and volume fraction over the whole composition region, respectively. The deviation of mole fraction Δ_xR is negative over the whole range, with the minimum lying nearly at x₁ ≈ 0.3, whereas Δ_ΨR values are asymmetric against both the molar fraction Figure 7e and volume fraction Figure 7f over the entire composition range. For the comparison of Figure 7d,e, the maximum of Δ_xR values are about the half value of Δ_ΨR and the shape of Δ_ΨR is different to Δ_xR over the entire composition range.

The experimental data of conductivity of the neat ILs [QuatGly-OEt][EtOSO₃], neat PEG200, and binary system are plotted against the x₁ at various temperatures in Figure 8. The conductivities of neat ILs [QuatGly-OEt][EtOSO₃], neat PEG200, and binary system at various temperatures were fitted using the VTF equation:

where T is the absolute temperature and κ_o^,B′, and T_o are adjustable parameters. The best-fit κ_o (mS·cm⁻¹), B′ (K), and T_o (K) parameters are given in Table 5. Neat [QuatGly-OEt][EtOSO₃], neat PEG200, and binary system were very well fit by the VTF model over the temperature range studied.

As shown in Figure 8, it can be seen that the conductivity of the binary mixture increases with increasing amount of PEG200, goes through a maximum, and then goes down to the specific conductivity of the neat PEG200. This can be ascribed to two effects: (i) increase of viscosity and therefore reduction of the mobility of the charge carriers [48]; and (ii) reduction of the number of charge carriers due to aggregate formation [49]. The conductivity (κ) is related to the ion mobility and the number of charge carriers, which can be expressed by the following equation [48]:

where n_i is the number of charge carriers of species i, q_i is the charge, and u_i is the mobility, which is related to the mixture viscosity. It can be seen that an increase in the conductivity of a given system must be due to an increase in ion mobility and/or the number of charge carriers. Moreover, the aggregation becomes dominant at higher concentration, thus leading to a strong decrease of conductivity [49]. The maximum of conductivity is located at x₁ = 0.7 and corresponds to a value of 1.4 mS·cm⁻¹ at 333.15 K. It can also be observed that the conductivity increases as the temperature increases. The higher temperature may weaken the influence of aggregation to the conductivity, which suggests that a higher temperature is better for the ILs to act as electrolytes.

The binary excess property (V_m^E ) and deviations (Δη, Δ_xn, Δ_Ψn, Δ_xR, and Δ_ΨR) at several temperatures were fitted to a Redlich-Kister-type equation [50]:

where ΔY (Y^E) represents V_m^E (cm³·mol⁻¹), Δη (mPa·s), Δ_xn, Δ_Ψn, Δ_xR, or Δ_ΨR; x₁ denotes the mole fraction of PEG200, A_i represents the polynomial coefficients, and j is the degree of the polynomial expansion. The correlated results for excess molar volumes (V_m^E ), viscosity deviations (Δη), refractive index deviations (Δ_xn and Δ_Ψn), molar refraction deviations (Δ_xR and Δ_ΨR), including the values of the fitting parameters A_i together with the standard deviation σ, are given in Table 7, the tabulated standard deviation σ [51] is defined as:

where m is the number of experimental data points and n is the number of estimated parameters. The subscripts “exp” and “cal” denote the values of the experimental and calculated property, respectively. As shown in Table 7, the experimentally derived V_m^E, Δη, Δ_xn, Δ_Ψn, Δ_xR, and Δ_ΨR values were correlated satisfactorily by the Redlich-Kister equation.

Poly(ethylene glycol)s, diethyl sulfate, and N,N-dimethylglycine ethyl ester were obtained from commercial suppliers and used without further purification. The poly(ethylene glycol)s of average molar mass 200 was purchased from Showa Chemical Industry Co., Ltd., Japan, diethyl sulfate (99%) was purchased from Acros organics, N,N-dimethylglycine ethyl ester (>98%) was purchased from Tokyo Chemical Industry Co., Ltd.

Diethyl sulfate (55.5 g, 360 mmol) was added dropwise to a solution of equal molar amounts of N,N-dimethylglycine ethyl ester (47.2 g, 360 mmol) in 200 mL toluene, and then cooled in an ice-bath under nitrogen at a rate that maintained the reaction temperature below 313.15 K (highly exothermic reaction). The reaction mixture was stirred at room temperature for 1 h to 4 h. After the reaction stopped, the upper organic phase of the resulting mixture was decanted, and the lower ionic liquid phase was washed with ethyl acetate (4 × 70 mL). After the last washing, the remaining ethyl acetate was removed by rotavapor under reduced pressure. The IL obtained was dried by heating at (343.15 to 353.15) K and stirring under a high vacuum (2 × 10⁻¹ Pa) for 48 h. The IL was kept in bottles under an inert gas. In order to reduce the water content to negligible values (lower than 0.03 mass %), a vacuum (2 × 10⁻¹ Pa) and moderate temperature (343.15 K) were applied to the IL for several days. Yield: 83%. ¹H NMR (300 MHz, D₂O, ppm): 4.24 (q, 2H, O-CH₂CH₃), 4.19 (s, 2H, N-CH₂COO-), 4.03 (q, 2H, O-CH₂CH₃), 3.56 (q, 2H, N-CH₂CH₃), 3.18 (s, 6H, N-CH₃), 1.32–1.21 (m, 9H, N-CH₂CH₃, COOCH₂CH₃, CH₃CH₂-SO₄). Elem. Anal. Calcd. for C₁₀H₂₃NO₆S: C, 42.09%; H, 8.12%; N, 4.91%. Found: C, 42.18%; H, 8.07%; N, 4.81%.

The conductivity (κ) of the ionic liquids was systematically measured with a conductivity meter (LF 340, WTW, Germany) and a standard conductivity cell (TetraCon 325, WTW, Germany). The cell constant was determined by calibration after each sample measurement using an aqueous 0.01 M KCl solution. The density of the ILs was measured with a dilatometer, which was calibrated by measuring the density of deionized water at 20, 30, 40 and 50 °C. To measure the density, IL or binary mixture was placed into the dilatometer up to the mark, sealed the top of capillary tube, which was on the top of the dilatometer, and placed into a temperature bath for 10 min to allow the temperature to equilibrate. The volume of the dilatometer is 11 cm³, the inside diameter of the capillary tube is 2 mm. The main interval between two marks in capillary tube is 0.1 cm³, and the minor interval between two marks is 0.01 cm³. From the correction coefficient of deionized water in capillary tube at various temperatures, we can calculate the density of neat IL or binary system by the expanded volume of liquid in capillary tube at various temperatures. Each sample was measured at least three times to determine an average value, and the values of the density are ±0.0001 g mL⁻¹. The viscosity (η) of the IL was measured using a calibrated modified Ostwald viscometer (Cannon-Fenske glass capillary viscometers, CFRU, 9721-A50). The viscometer capillary diameter was 1.2 mm, as measured using a caliper (model No. PD-153) with an accuracy of ±0.02 mm. The viscometer was placed in a thermostatic water bath (TV-4000, TAMSON) whose temperature was regulated to within ±0.01 K. The flow time was measured using a stopwatch with a resolution of 0.01 s. For each sample, the experimental viscosity was obtained by averaging three to five flow time measurements. Measurements of the refractive index were conducted at 293.15 K with an ABBE refractive index instrument (Atago DR-A1), calibrated with deionized water with an accuracy greater than ±2 × 10⁻⁴. The water content of synthesized IL [QuatGly-OEt][EtOSO₃] was determined using the Karl-Fischer method; the content was below 100 ppm. The nuclear magnetic resonance (NMR) spectra of the synthesized ILs were recorded on a BRUKER AV300 spectrometer and calibrated with tetramethylsilane (TMS) as the internal reference.