Thermal decomposition kinetics of deep eutectic solvent (DES) based on choline chloride and magnesium chloride hexahydrate: New details on the reaction mechanism and enthalpy-entropy compensation (EEC)

In recent years, deep eutectic solvents (DESs) have attracted considerable attention, and they have been applied in many fields, such as dissolution and separation, electrochemistry, materials preparation, reac-tion, and catalysis. In this paper, a detailed thermal decomposition mechanism of DES-type II (consisting choline chloride (ChCl) and magnesium chloride hexahydrate (MgCl2 center dot 6H2O) in a molar ratio 2:1 (MgCl2 center dot 6H2O-[Ch]Cl)) was explained, using thermal analysis techniques. Physicochemical clarification of overall thermal decomposition mechanism and the influence of enthalpy-entropy compensation (EEC) on reactions mechanism emerging are presented for the first time, in favor of this DES type. In the kinetic analysis of the decomposition process, two approaches were used: model-free (inverse) and model-based (direct) methods. It was found that thermodynamic principles in the form of EEC are the source of kinetic compensation effect (KCE) during MgCl2 center dot 6H2O-[Ch]Cl thermal decomposition, as a con-sequence of the effects of molecular interactions. Key phenomenon in the complex multiple step process represents a parallel dehydration steps of MgCl2 center dot 6H2O in DES, leading to formation of intermediates, such as [MgCl1(H2O)5]1+ and [MgCl2(H2O)4]. It was established that formation of final products (Mg(OH)2 and MgOHCl) requires a higher expenditure of energy to overcome a high potential barrier, where reaction system compensates this energy via hydrogen bonding disruption. This was confirmed by the identifica-tion of a specific 'oscillator', flagged as H-O-H center dot center dot center dot Cl hydrogen bond donating system of the energy (heat bath). All kinetic parameters and mechanisms of individual reaction steps were confirmed by numerical optimization of the process and modulated dynamic predictions.(c) 2023 Elsevier B.V. All rights reserved.

Automated summary

Deep eutectic solvents (DESs) have gained significant attention in recent years and have been utilized in various fields. This study presents a comprehensive analysis of the thermal decomposition mechanism of a specific DES type consisting of choline chloride (ChCl) and magnesium chloride hexahydrate (MgCl2·6H2O) in a molar ratio of 2:1 (MgCl2·6H2O-[Ch]Cl). The physical and chemical aspects of the decomposition mechanism and the influence of enthalpy-entropy compensation (EEC) on the reaction mechanism are explained. Kinetic analysis of the decomposition process using both model-free and model-based approaches reveals the importance of EEC in this DES type. The formation of intermediates and final products during the complex multiple step process is also discussed. The findings highlight the significant role of hydrogen bonding disruption in the energy compensation and provide valuable insights into the reaction kinetics and mechanisms. (c) 2023 Elsevier B.V. All rights reserved.