(462j) Solvatochromic Evaluation of Hydrophobic Deep Eutectic Solvents

Deep eutectic solvents (DES) are attractive for separation based applications due to the ability to design a solvent with a particular set of properties. Type III DES are formed by a hydrogen bond donor (HBD), typically an acid, and a hydrogen bond acceptor (HBA) such a quaternary ammonium halide salt. The hydrogen bond sharing results in a deep melting point depression, or eutectic point. Even if both HBA and HBD are solids at room temperature, the correct ratio of the two can form a liquid. By varying HBA and HBD species and ratio it is possible to adjust properties such as melting point, density, conductivity, hydrophobicity, viscosity, etc. Numerous studies have investigated a small selection (typically 2 to 8 solvent combinations) of DES and a relatively fewer number of studies have taken a broader take on how DES work. Thus far, a predictive model to estimate DES properties that does not use computationally intensive methods has not been described.

Solvatochromic methods of analysis are useful for measurements of a solvent’s physical properties such as dipolizability, hydrogen bond donating ability, and hydrogen bond accepting ability. H-bond characteristics will be measured using betaine dyes in a UV-VIS. The solvatochromic method was developed in the 1980s and has been applied for IL and DES. Reichardt’s dye or betaine dye 30 (2,6-diphenyl-4-(2,4,6-triphenylpyridinium- 1-yl)phenolate) is one of the most widely used solvatochromic probes, however, acidic conditions will strongly affect its solvatochromic nature. Since, the goal of this project is to characterize HDES, where HBAs are long chain acids, Reichardt’s dye 33 (2,6-dichloro-4-(2,4,6-triphenylpyridinium1-yl)phenolate) will be chosen for this project instead of dye 30. Kamlet-Taft (KT) parameters will be evaluated for hydrophobic DES. The KT β parameter provides H-bond accepting ability of HDES. This parameter will be obtained by solvatochromic method by comparing solvent-induced shifts of the absorption bands of probes. Kamlet and Taft proposed probes, 4-nitroaniline/N,N-diethyl-4-nitroaniline will be used in this project, since they are the most commonly used KT probes for ILs and DES. A lower value of β is expected as the formation of HDES through establishment of H-bonds increases the capacity of the HDES to engage in further H-bonds network with HBD. In this paper, effects of anion, aliphatic chain length, and acidic hydrogen bond donor will be evaluated for β parameter. Solvatochromic parameters will then be used to compare solvent physical properties with that of the HBA and HBD components.