Green Solvents and Microwaves Unlock Antioxidant Treasures Hidden in Eucalyptus Leaves

Chemists in Pakistan have shown that the secret to extracting powerful antioxidant compounds from the leaves of the lemon-scented eucalyptus tree lies in matching the polarity of the solvent to the chemistry of the target molecules. In a study published in Discover Green Chemistry, Mariyam Asif and Dildar Ahmed of Forman Christian College in Lahore combined microwave-assisted extraction with two natural deep eutectic solvents, one water-loving and one water-repelling, to systematically compare how each performs when pulling phenolic and flavonoid compounds out of dried Eucalyptus citriodora leaves. Their results reveal that there is no single best solvent; instead, the optimal choice depends entirely on whether the goal is to harvest polar phenolic acids, less polar flavonoids, or specific classes of antioxidant activity.

Natural deep eutectic solvents, or NaDES, have emerged in recent years as one of the most promising tools in green chemistry. These liquids are formed when certain natural compounds, such as sugars, alcohols, and organic acids, are mixed together in specific ratios, causing them to melt into a stable liquid through an extensive network of hydrogen bonds. Unlike many conventional organic solvents, NaDES are typically non-toxic, biodegradable, inexpensive, and tunable in polarity, which makes them attractive replacements for the volatile organic solvents that have long dominated phytochemical extraction. The United Nations has explicitly called for a transition away from traditional organic solvents as part of its 2030 Agenda for Sustainable Development, and NaDES chemistry is one of the leading answers to that call.

The research team selected two contrasting NaDES for their investigation. The first was a hydrophilic mixture of glycerol and choline chloride, abbreviated GCC, prepared by heating the two components in a one-to-three molar ratio at 70 degrees Celsius until a clear viscous liquid formed. The second was a hydrophobic blend of camphor and thymol, abbreviated CT, which liquefied within ten minutes at 35 to 40 degrees Celsius in a one-to-one ratio. Because deep eutectic solvents are inherently viscous and struggle to penetrate plant tissue on their own, the researchers diluted the GCC system with water and the CT system with acetonitrile, lowering viscosity and improving the solvents’ ability to reach the phytochemicals locked inside the leaf matrix.

To squeeze the maximum performance out of each solvent system, the team turned to microwave-assisted extraction, a technique that heats the solvent and plant material from the inside out. Microwaves interact with polar molecules by inducing dipole rotation, generating heat directly within the extraction medium. This localized heating builds internal pressure inside plant cells, rupturing cell walls and releasing intracellular compounds far faster than conventional stirring or reflux methods. The approach conserves energy, slashes extraction times to mere seconds, and reduces the risk of degrading heat-sensitive molecules because the total exposure window is so short. In this study, powdered leaves were combined with 30 milliliters of the solvent-diluent mixture in a conical flask and irradiated in a domestic microwave oven at controlled power and duration settings.

Optimizing the process required a statistical framework capable of mapping how multiple variables interact simultaneously. The researchers employed response surface methodology using a Box-Behnken design, which generated 17 experimental runs per solvent system with five replicates at central points. Three factors were varied across three levels each: the concentration of the NaDES in its diluent, the microwave power ranging from 220 to 440 watts, and the irradiation time ranging from 20 to 40 seconds. Five responses were measured for every run: total phenolic content expressed as gallic acid equivalents, total flavonoid content expressed as rutin equivalents, DPPH radical scavenging activity expressed as ascorbic acid equivalents, ABTS radical cation scavenging activity expressed as Trolox equivalents, and metal iron chelating activity expressed as EDTA equivalents, all normalized to grams of dry leaf powder.

The results exposed a striking polarity-driven divide between the two solvent systems. The hydrophilic GCC system proved dramatically superior for phenolic recovery, achieving a predicted optimal total phenolic content of 137.49 milligrams of gallic acid equivalents per gram of dry weight, nearly three times the 48.35 milligrams obtained with the hydrophobic CT system. The GCC extracts also dominated in ABTS radical scavenging, reaching 11.57 milligrams of Trolox equivalents per gram, while the CT extracts managed only 0.17 milligrams. This makes chemical sense: the ABTS assay operates through single-electron transfer and preferentially detects highly polar antioxidant molecules, which dissolve readily in the hydrogen-bonding, water-compatible environment that glycerol and choline chloride provide.

The story reversed for flavonoids. The hydrophobic camphor-thymol system extracted 61.72 milligrams of rutin equivalents per gram, roughly five times the 12.17 milligrams achieved by GCC. Many flavonoids possess substantial hydrophobic aromatic ring systems, and the non-polar character of camphor and thymol enables favorable hydrophobic and pi-pi interactions with these structures. The CT system also edged out GCC slightly in DPPH radical scavenging, at 28.38 versus 27.08 milligrams of ascorbic acid equivalents per gram, and in metal chelating activity, at 23.39 versus 21.38 milligrams of EDTA equivalents per gram, suggesting that a meaningful fraction of the leaf’s lipophilic antioxidants and metal-binding ligands are better captured in the hydrophobic medium. Both systems produced comparable DPPH responses overall, indicating that radical-scavenging compounds were extracted efficiently across most processing conditions.

The statistical models underlying these findings proved highly reliable, with coefficients of determination exceeding 0.91 for every response and non-significant lack-of-fit tests confirming that the quadratic equations adequately described the experimental data. Solvent concentration emerged as the most influential variable, followed by microwave power and extraction time. For the GCC system, moderate DES concentration around 75 percent paired with 220 watts and 40 seconds produced optimal results with a desirability value of 1.0, and experimental validation showed relative standard deviations between 0.15 and 5.10 percent. For the CT system, the optimum landed at 60 percent DES concentration in acetonitrile, 220 watts, and 30 seconds, with validation deviations between 0.16 and 6.25 percent. The researchers explain that too much water in the GCC mixture dilutes its solubilizing power for moderately polar polyphenols, while too little water leaves the solvent too viscous to penetrate the biomass, so an intermediate composition balances fluidity and polarity. Similarly, acetonitrile lowers the viscosity of the hydrophobic CT solvent while adding just enough polarity to assist extraction.

Beyond simple solubility, the study highlights a deeper mechanism by which NaDES enhance recovery: direct attack on the plant cell wall. The chloride ions of the GCC mixture can form hydrogen bonds with the hydroxyl groups of cellulose, disrupting the wall’s architecture, while camphor and thymol interact with cell wall components in their own way. Combined with the internal pressure buildup from microwave dielectric heating, this dual assault on cellular structure accelerates the desorption and release of target compounds from the plant matrix. The authors conclude that microwave-assisted extraction coupled with carefully chosen NaDES offers a sustainable, rapid, and selective strategy for recovering antioxidant phytochemicals from Eucalyptus citriodora leaves, and that the choice between hydrophilic and hydrophobic solvents should be guided by the specific class of biomolecules a manufacturer or researcher wishes to target, whether for nutraceutical, pharmaceutical, or food applications.

Subject of Research: Microwave-assisted extraction of phenolic and antioxidant compounds from Eucalyptus citriodora leaves using hydrophilic and hydrophobic natural deep eutectic solvents

Article Title: Microwave-assisted extraction of phenolic and antioxidant compounds from Eucalyptus citriodora leaves using hydrophilic and hydrophobic natural deep eutectic solvents

Article References: Asif, M., & Ahmed, D. (2026). Microwave-assisted extraction of phenolic and antioxidant compounds from Eucalyptus citriodora leaves using hydrophilic and hydrophobic natural deep eutectic solvents. Discover Green Chemistry, 1(1), Article 21. https://doi.org/10.1007/s44509-026-00025-z

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00025-z

Keywords: Eucalyptus citriodora, natural deep eutectic solvents, microwave-assisted extraction, green chemistry, polyphenols, flavonoids, antioxidant activity, response surface methodology, glycerol-choline chloride, camphor-thymol, DPPH, ABTS