Experimental investigation and thermodynamic modeling of amino acids partitioning in a water/ionic liquid system

Amino acids partitioning including phenylalanine, glutamic acid, and tryptophan in aqueous and ionic liquid phases at temperature of 298.15 K and atmospheric pressure were measured. 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide used in this work can produce two phases with water immediately. The effect of aqueous solution pH on amino acids partitioning was studied and revealed that amino acid partitioning coefficient was decreased with increasing pH. This phenomenon pertains to the electrostatic interaction between cations of amino acid and the anions of ionic liquid which is decreased when pH increases. Considering the effect of pH, liquid-liquid equilibrium data of amino acids were obtained in a pH owning maximum partitioning coefficient. It was observed that chemical structure of amino acid has an important effect on partitioning of amino acids between two phases. Experimental data indicated that glutamic acid is mostly transferred to aqueous phase while tryptophan moves to the ionic liquid phase. In the modeling section, equilibrium behavior of water, amino acid and ionic liquid system were predicted by SAFT equation of state. Necessary parameters of water and pure ionic liquid for equation of state were measured and amino acid parameters were obtained from literature. Binary interactions of components were optimized by equilibrium data. NRTL model was used in order to analyze cabapility of SAFT EoS to predict the behavior of system. The results indicate that SAFT model anticipates liquid-liquid equilibrium of the system better than NRTL model. The percentage of average absolute deviation for SAFT equiation is <1.2% while for NRTL model is >1.9%.