Ethanolamine-mediated microstructural transitions within terpenoid- and fatty acid-based deep eutectic solvents

Deep eutectic solvents (DESs) have emerged as solubilizing media of intense interest due partly to their easily tailorable physicochemical properties. Extensive H-bonding between the constituents in a two-constituent system is the major driving force for the formation of the DES. Addition of ethanolamine (MEA), a compound having H-bonding capabilities, to the DESs composed of a terpene [menthol (Men) or thymol (Thy)] and a fatty acid [n-decanoic acid (DA)] results in an unprecedented increase in dynamic viscosity due to the extensive rearrangement in the H-bonding network and other interactions within the system, while the liquid mixture still behaves as a Newtonian fluid. For the non-DA DES constituted of Men and Thy, this behavior is not observed. Visual color appearance, density and electrical conductivity measurements, UV-Vis and FTIR absorbance, differential scanning calorimetry, and empirical Kamlet-Taft parameters of the MEA-added DA-based DESs reveal the microstructural changes effectively. Cybotactic regions of the fluorescent microfluidity probes [1,3-bis(1-pyrenyl)propane - an intramolecular excimer forming probe, as well as perylene and 1,6-diphenylhexatriene - well-established anisotropy probes] also manifest the unprecedented increase in the viscosity of the DA-based DES system upon MEA addition. The carboxylic acid functionality of the DA plays a crucial role in bringing microstructural changes within the system as MEA is added. Physicochemical properties of DES systems can be effectively manipulated by not only changing the constituents and their compositions, but also by judicious addition of a co-solute/co-solvent. This work offers an easy and efficient way to favorably tailor the properties of interest of these environmentally-benign media.