Aqueous biphasic systems (ABS) based on ionic liquids (ILs) are considered as promising separation and extraction media for replacing conventional extraction systems consisting of volatile organic solvents. In the present work, set of ILs consisting of tetrabutylammonium cation and carboxylate anions were synthesised and studied for their ability to form ABS in presence of strong salting-out agent K3PO4 at 298.15 K and atmospheric pressure. The influence of alkyl chain length of ILs' anions on the phase splitting ability has been systematically investigated. Experimentally determined binodal curve data has been fitted to Merchuk's equation and corresponding fitting parameters have also been determined. Further, selected ternary systems have been analysed for their extraction efficiencies for an amino acid, Tryptophan. Solvatochromic parameters of aqueous solutions of ILs have been determined to further scrutinize their influence on extraction efficiencies of studied ABS. The results indicated that phase behaviour and extraction efficiencies are governed by multiple factors prevailing at microscopic level like hydrophobicity, hydrogen bond accepting ability etc. The combination of suitable cation and anion entities of ILs, in turn, decides the dominant physical phenomenon and consequent phase behaviour and extraction aptitudes. In addition, this work addresses the effect of temperature on ABS formation and temperature dependence similar to that of ABS consisting of polymers and inorganic salts have been observed. This work gives insights into multifaceted effects of longer alkyl chains of IL's anion on phase behaviour and extraction aptitudes of IL based ABS. (c) 2018 Elsevier Ltd.