The phase behavior and microstructures of the aqueous two-phase systems (ATPS) composed of a cationic/an anionic surfactant (3-p-nonylphenoxy-2-hydroxypropyltrimethylammonium bromide, NPTAB/sodium dodecylbenzenesulfonate, SDBS) and an ionic liquid (1-hexyl-3-methylimidazolium tetrafluoroborate, [Hmim][BF4]) were studied by using cryogenic transmission electron microscopy (Cryo-TEM) observations, nuclear magnetic resonance (NMR) experiments and electric conductivity. The concentration effects of the surfactants and the ionic liquid on the phase separation were systematically investigated. The experimental outcomes indicated that the formation of ATPS can be attributed to the coexistence of different micelles in the upper and lower phases. The ATPS here investigated were further characterized for the first time through their application to the extractive potential for amino acids, where L-tryptophan was selected as a model biomolecule. (C) 2013 Elsevier B.V. All rights reserved.