Calculating the closest approach parameter for ethyl lactate-based ATPS

Abstract

Aqueous Two-Phase Systems (ATPS) is a powerful separation technique for biotechnological compounds, such as pigments, proteins and flavonoids. Nowadays, ATPS generally include dissolved salts, which cause phase immiscibility and promote solute partition. Therefore, to accurately model the liquid-liquid equilibria (LLE) of these systems, specific terms for the long-range interactions in thermodynamic models are needed. A well-known hypothesis for this purpose is the Pitzer-Debye-Huckel (PDH) term, which requires the knowledge of the closest approach parameter (rho), i.e., an adjustable parameter. The aim of this work was calculating this parameter supposing total dissociation of the salts in ethyl lactate-based ATPS. In this new approach, rho became specific for each chemical species and its physical meaning (related to the distance between ions of different charges) was preserved. This method was applied in the LLE modelling of 7 ethyl lactate + organic or inorganic salts + water systems measured by the group, using the PDH term coupled with the UNIversal QUAsi-Chemical (UNIQUAC) model, whose junction is generally known as PDH+UNIQUAC, and compared with classical UNIQUAC and with previous works. The new approach accurately described the experimental liquid-liquid equilibria (LLE) data and presented smaller composition deviations than UNIQUAC.