Phase Behavior of Green Aqueous Biphasic Systems Based on Ethyl Lactate and Choline Salts

Aqueous biphasic systems (ABS) have recently emerged as an economic and sustainable solution for the separation and isolation of biomolecules. Ethyl lactate (EL) is an attractive phase-forming component, as it is a biorenewable, biodegradable, and nontoxic solvent. In this study, cloud points and tie-line data for ethyl lactate (EL)-based aqueous biphasic systems (ABS) with four choline salts─choline bicarbonate (ChHCO3), choline chloride (ChCl), choline bitartrate (ChBitar), and choline dihydrogen citrate (ChH2Cit)─were experimentally determined at 298.2 and 328.2 K. For both temperatures, three models were used to fit the data: the three-parameter Merchuk’s equation, a two-parameter correlation, and the effective excluded volume. The molecular-level interactions and dynamic behavior within the ABS systems were investigated using diffusion nuclear magnetic resonance. EL–ChH2Cit showed the most significant changes in diffusion coefficients and water shifts, indicating increased viscosity and altered water structuring. In contrast, ChCl effects were primarily viscosity-driven, ChBitar exhibited complex, nonlinear trends suggestive of solvation or aggregation phenomena, whereas ChHCO3 uniquely displayed peak splitting, pointing to multiple EL environments. This work provides novel insights into the design of green solvent systems and contributes to the development of alternatives to hazardous organic solvents, with potential applications across biotechnology, pharmaceuticals, and the green chemistry industries.