Synergistic aromatic – Hydroxyl groups influence towards physicochemical and thermal properties of choline based deep eutectic solvents

This study presents a comprehensive comparison of the thermal kinetics degradation between choline chloride based Deep Eutectic Solvents (DESs). The importance of choline chloride based DES lies in their environmentally friendly nature and potential applications in various industries due to their low toxicity and biodegradability. Additionally, characterization includes structural characterization (FTIR), physicochemical characterization (density, viscosity, and refractive index), and thermal characterization (thermogravimetric analysis and differential scanning calorimeter) with three types of DES which are choline chloride – orcinol, choline chloride – ethylene glycol and choline chloride – phenol. The density and refractive index measurements indicate that ChCl – orcinol has the highest values, followed by ChCl – phenol, with ChCl – EG having the lowest density and viscosity. The viscosity values follow a non-similar pattern, with ChCl – phenol having the highest refractive index, ChCl – orcinol being next and ChCl – EG having the lowest. Regarding melting points, ChCl – EG has the lowest melting point which is below −50 °C, ChCl – phenol has a slight higher melting point at −12.9 °C, and ChCl – orcinol has the highest melting point, which is 11.4 °C. The comparison of thermal analysis was conducted using TGA, perform on all three types of DES at various heating rates (5, 10, and 15 °C/min) and temperature range from 50 °C to 700 °C. The thermogravimetric analysis results indicated that the ChCl – EG and ChCl – phenol exhibited lower thermal stability compared to ChCl – orcinol. Through a detailed analysis of kinetic parameters such as activation energy (Ea) and pre-exponential factor (log A) using Kissinger-Akahira-Sunose (KAS), Flynn-Wall-Ozawa (FWO) and Starink methods, it was determined that the average Ea and log A values of ChCl – orcinol were the lowest compared to ChCl – phenol, followed by ChCl – EG, which could be attributed to its higher thermal stability. In conclusion, this study demonstrated a strong relationship between the onset temperature ($T_o$), decomposition temperature ($T_{max}$), log A and Ea and values of DESs and their thermal stability.