Nanophase separation in aqueous dilutions of a ternary DES as revealed by Brillouin and NMR spectroscopy

Abstract

Liquid binary mixtures of deep eutectic solvents (DESs) with H2O are lately attracting the interest of many research works because of the improved performance that these dilutions demonstrated in number of application – e.g., catalysis, separations, or biochemistry, among others – as compared to their respective neat DES counterparts. Interestingly, molecular simulations have revealed that, despite the macroscopically homogeneous appearance of the DES/H2O binary mixtures, phase segregation may occur at the nanoscale. Spectroscopic studies corroborating this event in this sort of mixtures are basically limited to NMR-based ones determining diffusion coefficients and some recent Brillouin spectroscopic studies performed for a quite short list of binary DESs – i.e., those composed of one single hydrogen bond donor (HBD) and one single hydrogen bond acceptor (HBA), being urea-choline-chloride-based DES the most widely studied. Interestingly, NMR data such as chemical shifts and relaxation times have also proved quite useful for the study of liquid binary mixtures different than DES-based ones – e.g., ionic-liquid-based ones. Herein, we have obtained a full set of NMR data – chemical shifts, diffusion coefficients and relaxation times – from liquid binary mixtures of H2O and a ternary DESs, in this particular case one composed of two HBD – e.g., resorcinol and hexylresorcinol – and one HBA – tetraethyl ammonium bromide. Good correlation among different NMR data as well as between NMR and Brillouin data was found in the determination of the DES content at which phase segregation occurs.