Cloud point phenomena were observed for polyoxyethylene (POE)-type nonionic surfactants in ionic liquid mixtures composed of 1-ethyl-3-methylimidazolium tetrafluoroborate (emimBF(4)) and 1-hexyl-3-methylimidazolium tetrafluoroborate (hmimBF(4)). The cloud point temperature vs. concentration curve (cloud point curve) shifts toward higher temperatures with an increase in POE chain length and with a decrease in hydrocarbon chain length of the surfactants, reflecting the fact that POE chains and hydrocarbon chains act as a solvophilic group and a solvophobic group, respectively. The analysis of H-1 NMR chemical shifts revealed that hmimBF(4) interacts with the surfactant preferentially in the ionic liquid mixture. Two liquid phases, one being surfactant rich and the other ionic liquid rich, coexist in equilibrium above the cloud point curve. In accordance with the preferential interaction of hmimBF(4) with the surfactant, hmimBF(4) is more concentrated in the surfactant-rich phase, while emimBF(4) is more concentrated in the ionic liquid-rich phase compared with the composition of the original ionic liquid mixture. The temperature dependence of H-1 NMR chemical shifts suggests that the desolvation of hmim cation from the POE chains proceeds with an increase in temperature. In other words, the solvophilicity of the surfactant POE chain is reduced with temperature, which would be responsible for the liquid-liquid phase separation occurring at elevated temperature. The present results suggest that the property of ionic liquid as a solvent to support the self-assembly of amphiphilic compounds can be tuned by mixing two ionic liquid species, since the present surfactants show no cloud point in hmimBF(4) solution and are immiscible with emimBF(4). (C) 2010 Elsevier Inc. All rights reserved.