Characterization of diluted mixtures of an ionic liquid in a hydrophobic natural deep eutectic solvent: Physicochemical, spectroscopic and electrochemical insights for its application in electrochemically controlled micro-extraction of heavy metals

Abstract

[EN] In this work, a hydrophobic natural deep eutectic solvent (NADES) was made by mixing up DL-menthol and decanoic acid in a 2:1 molar ratio, and its mixtures with a soluble ionic liquid (1-hexyl-3-methylimidazolium chloride, [HMIM][Cl]) were investigated. A number of physicochemical properties (ionic conductivity, viscosity, and density) were either measured or estimated by the predictive UNIFAC model over a range of temperatures and [HMIM][Cl] compositions. The addition of [HMIM][Cl] enhances ionic conductivity, while keeping high fluidity and low density. However, Walden plots and dissociation degrees suggest that significant ion pairing occurs. The analysis of excess molar volumes and viscosity deviations suggests that the introduction of the IL up to 50 wt% gives rise to liquid structure with reorganization of the NADES hydrogen-bonding network. Such structural perturbation was confirmed by FT-IR measurements. The more diluted NADES + [HMIM][Cl] system showed an electrochemical stability window of about 2.5 V. Additionally, ferrocene/ferrocenium exhibits a quasi-reversible electron transfer with a standard rate of (2.6 ± 0.3)10¿4 cm s¿1 and a diffusion coefficient of (6.16 ± 0.06)·10¿8 cm2 s¿1. To assess the viability of this medium in analytical applications, an electrochemically controlled liquid-liquid microextraction was conducted, employing the NADES+[HMIM][Cl] mixture as the extractant phase. This approach resulted in a notable enhancement (up to 8-fold gain vs the liquid-liquid microextraction in the absence of electric field) in the metal extraction efficiency from aqueous samples, underscoring the potential of the studied medium in both electrochemical and analytical applications.