Ionic liquid-mediated solid–liquid extraction and separation processes for essential oils: modern trends

Abstract

Ionic liquids (ILs) have proven to be a potential medium for the extraction and separation of essential oils (EOs) from the most varied sources. This review presents a summary of the primary outcomes obtained through the application of ionic liquid-based processes for the extraction and separation of different constituents of EOs (such as terpenes, ketones, phenols, oxides, esters, aldehydes, and alcohols). ILs have been examined as solvents, co-solvents, adjuvants, electrolytes, and cosurfactants, and they have been utilized in the synthesis of IL-supported materials for separation. In addition, the application of hybrid solvents, including deep eutectic solvents (DESs), further broadens the applicability of ILs to particular EO components. A key feature of ILs is their adaptable selectivity at the molecular level. Interactions such as hydrogen bonding, e.g., imidazolium-based ILs with hydroxyl groups of phenolic compounds, π–π stacking between aromatic cations and terpene backbones, and electrostatic or hydrophobic effects that control the solubilization and dissociation of esters and aldehydes enable ILs to achieve better resolution than conventional solvents. These interactions provide a higher extraction efficiency and better purity of the target compounds. Herein, IL-based solid–liquid green extraction techniques are reviewed and compared in terms of their extraction and separation capabilities. Modern trends, including computational methods, machine learning (ML) optimization, and advanced algorithms for process optimization techniques, are used to improve results, which are often used for single-parameter modifications, such as optimizing solvent ratios or extraction conditions. These examples show that ML can help improve IL-based extraction in practice, but complete end-to-end design systems are still in the future. Combining ML with multi-scale modeling and robotic testing for automated, iterative optimization is probably necessary to realize such dramatic potential. The environmental impact of ILs is also reviewed in terms of toxicity and biodegradability, along with reuse, recovery, and purification processes. The major achievements and future challenges in this discipline are addressed, with a specific focus on the primary loopholes encountered within the IL community devoted to the separation processes, and some measures are proposed to overcome the existing limitations.