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.
Aiming at providing an extensive characterization of the solid−liquid equilibria (SLE) of deep eutectic solvents (DESs), the phase diagrams of nine eutectic mixtures composed of choline chloride ([Ch]Cl) and (poly)carboxylic acids, commonly reported in the literature as DESs, were measured experimentally. Contrarily to the behavior reported for eutectic mixtures composed of [Ch]Cl (hydrogen-bond acceptor, HBA) and monofunctional hydrogen-bond donors (HBD) such as fatty acids and fatty alcohols, which have recently been shown to be almost ideal mixtures, a significant decrease of the melting temperature, at the eutectic point, was observed for most of the systems studied. This melting temperature depression was attributed to a pronounced nonideality of the liquid phase induced by the strong hydrogen-bond interactions between the two mixture components. Perturbed-chain statistical associating fluid theory (PC-SAFT) was used to describe these interactions physically. PC-SAFT allowed accurately modeling the experimental phase diagrams over the entire concentration and temperature ranges. Depending on the kind of mixture, up to two temperature-independent binary interaction parameters between HBA and HBD were applied. The PC-SAFT approach was used to provide trustworthy information on the nonideality of the liquid phase (expressed as the activity coefficients of HBA and HBD) as well as to estimate the eutectic points coordinates. The experimental data along with the modeling results allowed us to infer about the importance of the HBD's chemical structure on the formation of [Ch]Cl-based DESs.
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