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Optimization of the extraction of phenolic compounds from walnut leaves using DES.

Publication . Vieira, Vanessa; Prieto Lage, Miguel A.; Barros, Lillian; Coutinho, João A.P.; Ferreira, Olga; Ferreira, Isabel C.F.R.

Deep eutectic solvents (DES) are a new generation of alternative solvents resulting from the mixture of two (or more) starting materials where the eutectic temperature of the mixture is considerably lower than the melting point of the individual components [1]. DES can be considered “designer solvents” due to the possibility of combining different hydrogen bond acceptors and donors, to obtain solvents with specific affinity to the target molecules. Among their many applications is the potential use of DES as both solvents and formulation media of extracts rich in phenolic compounds [2]. In this work, the heat assisted extraction of phenolic compounds from walnut leaves was optimized, using DES based on choline chloride (CC) and carboxylic acids. To evaluate the response, the main phenolic compounds present in the extract (acid 3-O-caffeyloquinic acid, quercetin 3-O-glucoside and quercetin O-pentoside) were determined by HPLC-DAD. From a preliminary solvent screening, butyric acid (BA) and phenylpropionic acid (PPA) were selected as hydrogen bond donors. The extraction conditions (time, temperature and water content) were then optimized by an experimental design, assisted by response surface methodology. Water content was the most relevant extraction variable, followed by temperature and, lastly, extraction time. Under the optimized conditions, it was possible to obtain a response of 37.9±4.0 mg/g dw for CC:BA and 31.7±4.2 mg/g dw, for CC:PPA. Compared to the traditional water + ethanol reference solvent, similar or higher extraction yields were obtained using the selected DES.

2017Conference object

Open access

Densities, viscosities and derived thermophysical properties of water-saturated imidazolium-based ionic liquids

Publication . Martins, Mónia A.R.; Neves, Catarina M.S.S.; Kurnia, Kiki A.; Carvalho, Pedro J.; Rocha, Marisa A.A.; Santos, Luís M.N.B.F.; Pinho, Simão; Freire, Mara G.

In order to evaluate the impact of the alkyl side chain length and symmetry of the cation on the thermophysical properties of water-saturated ionic liquids (ILs), densities and viscosities as a function of temperature were measured at atmospheric pressure and in the (298.15 to 363.15) K temperature range, for systems containing two series of bis(trifluoromethylsulfonyl)imide-based compounds: the symmetric [C n C n im][NTf2] (with n = 1-8 and 10) and asymmetric [C n C1im][NTf2] (with n = 2-5, 7, 9 and 11) ILs. For water-saturated ILs, the density decreases with the increase of the alkyl side chain length while the viscosity increases with the size of the aliphatic tails. The saturation water solubility in each IL was further estimated with a reasonable agreement based on the densities of water-saturated ILs, further confirming that for the ILs investigated the volumetric mixing properties of ILs and water follow a near ideal behaviour. The water-saturated symmetric ILs generally present lower densities and viscosities than their asymmetric counterparts. From the experimental data, the isobaric thermal expansion coefficient and energy barrier were also estimated. A close correlation between the difference in the energy barrier values between the water-saturated and pure ILs and the water content in each IL was found, supporting that the decrease in the viscosity of ILs in presence of water is directly related with the decrease of the energy barrier.

2016Journal article

Restricted access

Insights into the nature of eutectic and deep eutectic mixtures

Publication . Martins, Mónia A.R.; Pinho, Simão; Coutinho, João A.P.

A stricter definition of a deep eutectic solvent (DES) is urgent, so that it may become a sound basis for further developments in this field. This communication aims at contributing to deepening the understanding of eutectic and deep eutectic mixtures concerning their definition, thermodynamic nature and modelling. The glut of literature on DES applications should be followed by a similar effort to address the fundamental questions on their nature. This hopefully would contribute to correct some widespread misconceptions, and help to establish a stringent definition of what a DES is. DES are eutectic mixtures for which the eutectic point temperature should be lower to that of an ideal liquid mixture. To identify and characterize them, their phase diagrams should be known, in order to compare the real temperature depression to that predicted if ideality is assumed, and to define composition ranges for which they are in the liquid state at operating temperatures. It is also shown that hydrogen bonding between the DES components should not be used to define or characterize a DES, since this would describe many ideal mixtures. The future of deep eutectic solvents is quite promising, and we expect that this work will contribute to the efficient design and selection of the best DES for a given application, and to model properties and phase equilibria without which the process design is impractical.

2018Journal article

Open access

Indirect assessment of the fusion properties of choline chloride from solid-liquid equilibria data

Publication . Fernandez, Luis; Silva, Liliana P.; Martins, Mónia A.R.; Ferreira, Olga; Ortega, Juan; Pinho, Simão; Coutinho, João A.P.

The temperature and enthalpy of fusion of choline chloride -[Ch]Cl- are not directly measurable since this compound decomposes upon melting. Yet, given the wide use of this compound in the preparation of deep eutectic solvents (DES), its thermophysical fusion properties are very important for a better understanding of these mixtures and the thermodynamic description of their solid-liquid phase diagrams. In this work, the fusion properties of choline chloride were estimated using the solubility curves of choline chloride in ten different ionic compounds, forming simple binary eutectic mixtures with quasi-ideal liquid phases. Experimental solid-liquid equilibria data for these systems -[Ch] Cl + ionic compounds- were measured, and the ideality of the systems assessed through the quantification of the activity coefficients and their comparison in each pair of binary solutions. The values estimated for the fusion properties of choline chloride are T fus,[Ch]Cl = 597 ± 7 K and Δ fus H [Ch]Cl = 4300 ± 600 J mol −1 . These were additionally checked by thermodynamic consistency tests and by the prediction of the solid-liquid curves with COSMO-RS model. The results obtained with both procedures allow us to guarantee the usefulness and robustness of the estimated data.

2017Journal article

Open access

Tunable Hydrophobic Eutectic Solvents Based on Terpenes and Monocarboxylic Acids

Publication . Martins, Mónia A.R.; Crespo, Emanuel A.; Pontes, Paula V.A.; Silva, Liliana P.; Bülow, Mark; Maximo, Guilherme J.; Batista, Eduardo A.C.; Held, Christoph; Pinho, Simão; Coutinho, João A.P.

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.

2018Journal article

Restricted access