Ion pair interactions were explored to design a fatty acid solvent of switchable miscibility with water. Fatty acids of medium length chains are immiscible with water but become miscible with water when ion pairs are formed with amines. The ion pairs become phase separated after bubbling CO2 into the solution due to the dissociation of the fatty acid-amine complexes. Ion pairs of caprylic acid (C8) and low toxic poly(oxypropylene) diamine (Jeffamine D-230) were characterized by FT-IR and H-1 NMR Log Kow values and surface activity were used to understand the switchable solvent mechanism in removing and recovering oily contaminants. More importantly, the ion pairs show a negligible adsorption on solid surfaces. Furthermore, both C8 and D-230 were recycled during the washing process. Thus the fatty acid as switchable solvent could be applied for oily contaminant removal from oily solid wastes. (C) 2017 Elsevier Inc. All rights reserved.