The peculiar effect of water on ionic liquids and deep eutectic solvents

• Published in: Chemical Society reviews 2018, Vol.47 (23), p.8685-872
• Main Authors: Ma, Chunyan, Laaksonen, Aatto, Liu, Chang, Lu, Xiaohua, Ji, Xiaoyan
• Format: Article
• Language: English
• Subjects: Aqueous electrolytes; Binary systems; Energiteknik; Energy Engineering; Hydration; Ionic liquids; Ions; Modelling; Moisture content; New technology; Properties (attributes); Solvents; Thermodynamic models
• ISSN: 0306-0012; 1460-4744; 1460-4744
• DOI: 10.1039/c8cs00325d

Ionic liquids (ILs) and deep eutectic solvents (DESs) have been suggested as eco-friendly alternatives to organic solvents. A trace amount of water is often unavoidable as impurity, and water is also added on purpose to reduce their problematically high viscosity and lower their high price. Understanding the distinct effects of water on the properties of ILs/DESs is highly important. In this review, we collect published experimental and theoretical results for IL/DES-H 2 O systems at varied water concentrations and analyze them. Results from mechanistic studies, thermodynamic modelling and advanced experiments are collected and critically discussed. Six commonly studied IL/DES-H 2 O systems were selected to map experimental observations onto microscopic results obtained in mechanistic studies. A great variety of distinct contours of the excess properties can be observed over the entire compositional range, indicating that the properties of IL/DES-H 2 O systems are highly unpredictable. Mechanistic studies clearly demonstrate that the added H 2 O rapidly changes the heterogeneous 3D structures of pure ILs/DESs, leading to very different properties and behaviour. There are similarities between aqueous electrolytes and IL/DES solutions but the bulky and asymmetric organic cations in ILs/DESs do not conform to the standard salt dissolution and hydration concepts. Thermodynamic modelling previously assumes ILs/DESs to be either a neutral ion-pair or completely dissociated ions, neglecting specific ion hydration effects. A new conceptual framework is suggested for thermodynamic modelling of IL/DES-H 2 O binary systems to enable new technologies for their practical applications. Comprehensive review of unique effects of water on ionic liquids and deep eutectic solvents observed in experimental and theoretical studies.