María Salomé ÁLVAREZ1, Francisco Javier DEIVE1, Ana RODRÍGUEZ1, María Asunción LONGO1, Luis Miguel VARELA2, Luisa SEGADE3, Oscar CABEZA3
1Bioengineering & Sustainable Processes Group. Department of Chemical Engineering, University of Vigo, Vigo, Spain
2Nanomaterials, Photonics and Soft Matter Group, Department of Particle Physics, University of Santiago de Compostela, Santiago de Compostela, Spain
3Mesturas Group. Department of Physics and Earth Sciences, University of A Coruña, A Coruña, Spain
We present measurements of four physical properties for three deep eutectic solvents (DES) mixed with methanol. The DES were formed by a eutectic mixture of choline dihydrogen phosphate (ChDHP) with ethylene glycol in a molar proportion of 1:2 (DES1), glycerol in the same proportion (DES2) and both organic solvents in the molar proportion 1:1:1 (DES3). Ionic conductivity of these DES are very low (about 0.3, 0.04 and 0.1 mS/cm respectively), while their dynamic viscosity is very high (about 1, 8 and 3 Pa·s respectively). In order to decrease viscosity, and so improving the ionic conductivity, we have mixed the three DES with methanol, covering all composition range. We have measured electrical conductivity, viscosity, density and refractive index in the prepared mixtures from 293 to 323 K in steps of 5 K. The most unexpected results obtained were in the ionic conductivity, where a very sharp peak is observed in the three families studied at very high methanol concentration, when its molar fraction is about 0.95 (i.e., twenty methanol molecules per one unity of DES). At this concentration the conductivity values reach about 7.8, 6.1 and 7.1 mS/cm respectively for DES1, 2 and 3, which represents an increase of more than 20 times respecting the value when “pure”. At the same time, viscosity decreases faster than exponentially, being their value at that molar fraction about 1000 times lower than the “pure” DES, about few mPa·s in the three families studied. If we focus in the excess molar volume, calculated from the measured density, we observe that, when plotted as percentage of the molar volume, there appears a peak at a similar concentration of the maximum in conductivity, which gives information of an abrupt change in the mixture microstructure at that concentration. In addition, we have observed a perfect match between density and refractive index, being proportional both magnitudes for the three families studied and with the same slope. In this communication we show the obtained data for the three families and four magnitudes measured, trying to relate them and also with previous data for DES and ionic liquids in similar mixtures. In addition, we will discuss the data using the known theoretical models used to explain those magnitudes in other related systems, observing that none of them fit our data, which means that some physic-chemical transport mechanisms of the mixtures we have studied are different from the known ones.
The authors thank Xunta de Galicia and ERDF for funding through a postdoctoral grant (ED481D-2019/017) and to the Spanish Ministry of Science, Innovation and Universities for the financial support through the project RTI2018-094702-B-I00.