p-Toluene Sulfonic Acid-Based Deep Eutectic Solvents: Preparation and Characterization of Their Physical Properties and Thermal Stability

In this work, p-toluene sulfonic acid (PTSA)-based deep eutectic solvents (DESs) were prepared at different mole ratios (DESs: 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, and 1:10) by mixing of choline chloride (ChCl) and PTSA. Among them, 1:1 (DES1) and 1:2 (DES2) molar ratios show a clear and homogeneous liquid, even after 24 h. The density of DES1 and DES2 was measured at temperatures from 298.15 to 343.15 K and 101.305 kPa. The refractive index (nD) of DES1 and DES2 was measured at 298.15 K and 101.305 kPa. The density of DES1 + water and DES2 + water mixtures over the whole mole fractions was measured at temperatures from 298.15 to 343.15 K and 101.305 kPa. The refractive index of DES1 and DES2 with water over the whole mole fractions was measured at 298.15 K and 101.305 kPa. From the measured density data, the excess molar volume (VmE), apparent molar volume (Vφi), partial molar volume ((V¯l)), excess partial molar volume (V¯iE), and coefficient of isobaric expansion (αP) at constant pressure were calculated. The formation of DES was confirmed through FTIR spectra, 1H NMR spectra, and 13C spectra. Further, the thermal stability and thermal behavior of all the prepared DESs were determined using TG-DSC thermogram.