Unveiling the Molecular Interactions in an Aqueous Binary System of Guanidine Hydrochloride-Based Deep Eutectic Solvents: Physicochemical Properties and Spectroscopic Approach

Density (ρ) and speed of sound (u) of API-based DESs and their pseudo aqueous binary mixtures were investigated over an entire range of composition at T = 298.15 K – 323.15 K and 0.1 MPa. The values were used to derive excess molar volume (VmE) and excess isentropic compressibility (Ks,mE). Guanidine hydrochloride and ethylene glycol, diethylene glycol, and triethylene glycol were combined in a 1:2 molar ratio to obtain DESs. The robust interactions existing between HBA/HBD of the studied DESs and water are responsible for negative deviation of aqueous DES binary systems. The calculated negative values for VmE and Ks,mE were further reduced with an increase in the alkyl chain length of HBD and temperature. The dynamics of molecular interactions in aqueous binary systems of DES was studied using FTIR at three different mole fractions (xDES = 0.3, 0.5, and 0.7). The result showed that the slight shifting in the broad −OH band toward the higher side is an indication of the breakdown of the hydrogen bond at a higher concentration of water. The solution behavior of binary systems was investigated using the Prigogine–Flory–Patterson theory and Extended Real Association Model. The current investigation provides an opportunity to optimize process parameters for industrial applications of these solvents.