Chemistry
The Chemistry of Natural Deep Eutectic Solvents as Green Alternatives
Quick fact
Certain mixtures of natural molecules, like choline chloride and urea, form a liquid at room temperature even though each solid melts above 50°C. This happens because they form a deep eutectic system, where hydrogen bonding between the molecules drastically reduces the melting point. This lets chemists use a non-toxic, biodegradable liquid (NADES) to dissolve everything from lignin to DNA, replacing harmful organic solvents in green chemistry.
Why this is interesting
Imagine mixing two safe solids—like sugar and water—and getting a liquid that can dissolve materials that normally need toxic solvents. What is this magical mixture, and why does it work?
Read the full explanation
Understanding The Chemistry of Natural Deep Eutectic Solvents as Green Alternatives
Think of two substances that are solid on their own. If you mix them in the right ratio, they can form a liquid that is much lower melting than either alone. This is a eutectic mixture. When the melting point drop is especially large, it's called a 'deep' eutectic solvent. NADES are a subclass where the ingredients are natural metabolites—like sugars, organic acids, or amino acids. The magic lies in the interactions between molecules: each component forms strong hydrogen bonds with its partners, creating a network that disrupts the crystalline order and prevents the individual components from freezing at their normal temperatures. So, at room temperature, you get a liquid made of safe, edible ingredients that can act as a powerful solvent. This happens because the hydrogen-bonding network lowers the energy needed to keep the components in liquid form, and the mixture's properties can be 'tuned' by choosing different components and ratios.
A deeper explanation
The key mechanism is the depression of the freezing point due to strong intermolecular interactions between the components. In a simple eutectic system, the melting point of the mixture is lower than that of the pure components because the interactions between unlike molecules are more favorable than those between like molecules, but the liquid phase is still an ideal mixture. In a deep eutectic solvent, the interactions are even stronger—often involving hydrogen bonds, van der Waals forces, or ionic interactions. For example, in the classic choline chloride:urea (1:2) mixture, the chloride ion of choline chloride hydrogen bonds with the hydrogen atoms of urea, and the hydroxyl group of choline forms hydrogen bonds with the carbonyl and amine groups of urea. This creates a low-mobility, ordered liquid with a melting point of 12°C, compared to 302°C for choline chloride and 133°C for urea. The result is a solvent that has low vapor pressure (like ionic liquids), is non-flammable, and crucially, is made from biologically compatible ingredients. Because NADES are formed from natural molecules, they are biodegradable and generally non-toxic, unlike many traditional organic solvents (e.g., dichloromethane, toluene) which are volatile, flammable, and often hazardous. This makes NADES excellent candidates for 'green' applications in extraction, biocatalysis, and electrochemistry, where they can dissolve both organic and inorganic compounds. Yet, there are challenges: they are often highly viscous, which slows mass transfer, and their 'greenness' depends on the components not being extracted from non-renewable sources or produced with high energy input. Understanding the chemistry of NADES—how intermolecular forces determine their properties—is the key to designing tailor-made green solvents that can replace petroleum-derived ones without compromising performance.