Decanoic Acid Based Deep Eutectic Solvents: Constituent-Controlled Physicochemical Properties and Solvatochromic Probe Behavior

The search for environmentally benign sustainable replacements for volatile organic compounds (VOCs) has resulted in advent of deep eutectic solvents (DESs) that have exhibited enormous application potential. Apart from the fact that their constituents could be inexpensive and naturally occurring, the physicochemical properties of the DESs can be easily tailored by judicious and systematic alteration of their constituents. Physicochemical properties and solute solvation environment afforded by five so-called "hydrophobic" natural DESs (NADESs) prepared by mixing n-decanoic acid (DA) with terpenoids thymol (Thy), geraniol (Ger), linalool (Lin), eugenol (Eug), and beta-citronellol (Cit), respectively, in 1:1 mol ratio are investigated. Significantly low water miscibility in the DA-based DESs renders them viable nonaqueous media in lieu of currently used organic solvents. Their densities are lower than that of water and higher than most of the common "hydrophilic" DESs. The refractive index of the DESs scale with the density. The dynamic viscosity of these DESs are relatively lower as compared to the common tetralkylammonium salt- and metal-salt-based DESs thus enhancing their utility due to faster diffusion and dynamics. Their surface tension values are similar to those of several common organic solvents but lower than those of common "hydrophilic" DESs. Kamlet-Taft parameters, obtained using UV-vis absorbance probes, reveal the dipolarity/polarizability (pi*) to be much higher for phenyl group containing DESs (DA with Eug and Thy) in comparison to the others, whereas H-bond-donating acidity (alpha) does not vary much for the DESs. The H-bond-accepting basicity (beta) spans wide range depending on the position of the hydroxyl group on the constituents. The solvation milieu of the ratiometric fluorescence probe pyrene is more polar for the phenyl-containing DESs as compared to the others. Five different fluorescence wavelength-shift solvatochromic probes reveal the solvation environment afforded by these DA-based DESs to be considerably nonpolar in nature. The outcomes reported here establish the DA-based NADESs as possible replacements for nonpolar organic solvents.