Liquid–Liquid Equilibria for the Pseudoternary Systems Water + 2,3-Butanediol with Terpene-Based Deep Eutectic Solvents at 298.15 K

Liquid–liquid equilibria (LLE) were studied for pseudoternary systems consisting of water, 2,3-butanediol, and deep eutectic solvents (DESs) at 298.15 K. The DESs were based on coumarin/thymol, menthol/decanoic acid, menthol/thymol, and thymol/10-undecenoic acid in a 1:1 molar ratio, as well as menthol/1-naphthol in a 2:1 molar ratio. The density and viscosity of the DESs were evaluated and compared with the literature data. The DESs and their compositions were determined using Fourier-transform infrared spectroscopy. The distribution coefficients and separation factors of 2,3-butanediol were examined in each pseudoternary system. Among these, the thymol-based DES, with 10-undecenoic acid as a hydrogen bond donor, exhibited the highest separation factor. A nonrandom two-liquid (NRTL) model was used to regress the LLE data. The Gibbs energy of the mixing (G^M/RT) surface was analyzed topologically to check tie-line consistency. The experimental and NRTL-modeled tie lines were tangential to the G^M/RT surface curve, indicating that the minimum Gibbs free energy of mixing was achieved at equilibrium points.