Tunable Hydrophobic Eutectic Solvents Based on Terpenes and Monocarboxylic Acids

Abstract

Recently, some works claim that hydrophobic deep eutectic solvents could be prepared based on menthol and monocarboxylic acids. Despite of some promising potential applications, these systems were poorly understood, and this work addresses this issue. Here, the characterization of eutectic solvents composed of the terpenes thymol or l­(−)-menthol and monocarboxylic acids is studied aiming the design of these solvents. Their solid–liquid phase diagrams were measured by differential scanning calorimetry in the whole composition range, showing that a broader composition range, and not only fixed stoichiometric proportions, can be used as solvents at low temperatures. Additionally, solvent densities and viscosities close to the eutectic compositions were measured, showing low viscosity and lower density than water. The solvatochromic parameters at the eutectic composition were also investigated aiming at better understanding their polarity. The high acidity is mainly provided by the presence of thymol in the mixture, while l­(−)-menthol plays the major role on the hydrogen-bond basicity. The measured mutual solubilities with water attest to the hydrophobic character of the mixtures investigated. The experimental solid–liquid phase diagrams were described using the PC-SAFT equation of state that is shown to accurately describe the experimental data and quantify the small deviations from ideality.

Sustainable hydrophobic terpene-based eutectic solvents for the extraction and separation of metals

“…Sustainable HES composed of cheap naturally sourced terpenes (such as menthol and thymol) as hydrogen bond acceptor and fatty acids (from caprylic to stearic acids) as hydrogen bond donor were shown to be highly hydrophobic. 7,10 Due to the weak hydrogen bonding interactions between the two compounds, such mixtures present only a slight deviation from an ideal solution behaviour, in opposition to what is usually observed for 'deep' eutectic solvents. This lack of dominant hydrogen bond interactions results in a low viscosity HES suitable for solvent extraction applications.…”

“…Furthermore, the starting compounds are biodegradable and approved for human consumption by various food and drug agencies, turning the resulting HES of interest for a wide range of industries. In this work, sustainable HES based on menthol or its aromatic counterpart thymol combined with long chain carboxylic acids (C n H (2n+1) OOH; n = 8, 10,12,14, 16, 18) (Fig. S1 in the ESI †), are evaluated for the extraction of Cu(II) and its separation from other transition metals, namely Co(II) and Ni(II).…”

“…This lack of dominant hydrogen bond interactions results in a low viscosity HES suitable for solvent extraction applications. 10 Furthermore, the starting compounds are biodegradable and approved for human consumption by various food and drug agencies, turning the resulting HES of interest for a wide range of industries. In this work, sustainable HES based on menthol or its aromatic counterpart thymol combined with long chain carboxylic acids (C n H (2n+1) OOH; n = 8, 10, 12, 14, 16, 18) ( Fig.…”

“…The solid-liquid phase diagrams of these HES were previously reported by us along with their physico-chemical properties at compositions close to the eutectic. 10 These are summarised in solution depending on multiple factors including carboxylic acid chain length at a fixed composition, change in composition for a given carboxylic acid, pH, Cu(II) concentration and salt additive selection. A detailed description of the experimental methodology is provided in the ESI † including the extraction kinetics (cf.…”

“…S2 †). Whilst studies involving eutectics often focus on a specific molar ratio of their constituents, the complete description of the phase diagram of the studied HES 10 allows the tuning of the HES composition to maximise metal extraction. As such, Cu(II) extraction in HES was studied as function of the carboxylic acid selection and the terpene : carboxylic acid molar ratio ( Fig.…”