Solvatochromic Parameters of Deep Eutectic Solvents: Effect of Different Carboxylic Acids as Hydrogen Bond Donor

The physicochemical properties of carboxylic acid-based deep eutectic solvents (DESs) were measured by solvatochromic probes: Nile red, N,N-diethyl-4-nitroaniline, and 4-nitroaniline. UV–vis absorbance of these probes was computed with temperature in the range of 298.15 to 318.15 K. By increasing temperature, the normalized polarity of DESs stayed linear except for [Ch]­Cl:butyric acid (1:3) and [Ch]­Cl:caprylic acid (1:2), which had a modest shift in the maximum wavelength between T = (308.15 to 318.15) K. Changes in the normalized polarity parameter of DESs demonstrated an interesting trend, and it confirms that alkyl chain length of the hydrogen bond donor (HBD) affects the solvation microsphere of the solute–solvent interaction. Also, dipolarity/polarizability (π*) and hydrogen bond acidity (α) decrease with increasing alkyl chain length confirming the significant contribution of HBDs to Kamlet–Taft parameters. Interestingly, the π* and α parameters of DESs did not change significantly and seem to be independent of temperature. In addition, density functional theory was used to extend our understanding on how alkyl chain length of HBD affects charge transfer in the DESs. In summary, both solvatochromic molecular probes and density functional theory are effective methods for studying polarity parameters and charge transfer, respectively, for DESs with varying alkyl chain length of the hydrogen bond donor.