Supramolecular Deep Eutectic Solvents and Their Aqueous Solutions: Green Carriers for Essential Oils

Abstract

In the search for alternatives to synthetic chemicals and drugs, there is growing interest in the aromatic and biological properties of essential oils. However, essential oils and their constituents are poorly soluble in aqueous systems and are highly sensitive to degradation and evaporation. Here, we evaluated the applicability of supramolecular deep eutectic solvents (SUPRADES) based on naturally occurring l-lactic acid (LA) and four cyclodextrins (CDs) as carriers for two essential oil components, 3-carene and limonene. The obtained partition coefficient in water was 1200 times higher than that in SUPRADES, which indicated that SUPRADES has a remarkable enhancing effect on the dissolution of the two essential oils. Adding water could significantly reduce the viscosity of SUPRADES and effectively control the release of essential oils. Evidence for a randomly methylated β-CD–3-carene inclusion complex was provided by molecular dynamics simulation. In addition, the formation of the inclusion complex is enhanced by increasing the water content within a specific range. Furthermore, the formation of SUPRADES is attributed to hydrogen bonds between the hydroxyl groups on CDs and LA. In particular, the van der Waals interaction plays a major role in the dissolution of 3-carene. Overall, SUPRADES and their aqueous solutions are promising carriers for essential oils due to their strong inclusion ability and controlled release.