*2.5. Refractive Index (nD) Measurement*

Generally, the refractive index (*n*D) describes how fast light travels through material. It estimates the electronic polarizability of the molecules and shows the dielectric response to an external electric field produced by electromagnetic waves (light) [65]. Figure 5 shows the refractive index of ammonium-based PILs that were measured in a temperature range of 293.15 to 333.15 K at atmospheric pressure. The experimental data is tabulated in Table S3 while the plots with standard errors are presented in Figure S15 in the Supplementary Material. From the table, the *n*<sup>D</sup> values were found to be decreasing with increasing temperature. Moreover, the values of the refractive index increased with the increase in cation and anion chain length of PILs. A similar observation was also found in the literature involving PILs in which the *n*<sup>D</sup> values of the studied PILs were in the range of 1.45–1.41 [50]. The increment of refractive index values with increasing alkyl chain length in the cation is influenced by higher intermolecular interaction such as the van der Waals forces of the PILs [52].

**Figure 4.** Viscosity (*η*) values of (**a**) [EHA][C5], [EHA][C6], [EHA][C7], and (**b**) [BEHA][C5], [BEHA][C6], [BEHA][C7] as a function of temperature.

**Figure 5.** Refractive index (*n*D) values of (**a**) [EHA][C5], [EHA][C6] and [EHA][C7], and (**b**) [BEHA][C5], [BEHA][C6] and [BEHA][C7] as a function of temperature.

#### *2.6. Thermophysical Properties Correlations*

The density (*ρ*), dynamic viscosity (*η*) and refractive index (*n*D) experimental values were correlated by using the following equations [53,66]:

$$
\rho = \mathbf{A}\_1 + \mathbf{A}\_2 \mathbf{T} \tag{5}
$$

$$\mathbf{l}\lg\eta = \mathbf{A}\_3 + \mathbf{A}\_4/\mathbf{T} \tag{6}$$

$$\mathbf{M\_D = A\_5 + A\_6T} \tag{7}$$

where T is the temperature in K, and A1 through A6 are correlation coefficients using the least square method. Table 4 represents the estimation of values for correlation coefficients together with the standard deviations, *SD* which was calculated by using the Equation (8). *Zexpt* and *Zcalc* are experimental and calculated values, respectively, while *nDAT* is the number of experimental points.

$$SD = \sqrt{\frac{\sum\_{i}^{n\_{DATA}} (Z\_{exppt} - Z\_{calc})^2}{n\_{DATA}}} \tag{8}$$

**Table 4.** Fitting parameters of Equation (5) to correlate density (*ρ*) of PILs and calculated standard deviation (SD1). Fitting parameters of Equation (6) to correlate viscosity (*η*) of PILs and calculated standard deviation (SD2). Fitting parameters of Equation (7) to correlate refractive index (*n*D) of PILs and calculated standard deviation (SD3).

