Tetrahydrofuran (THF) is fully miscible in water, and it interacts with it via hydrogen (H) bonds. We discover that the fatty acid hydroxystearic acid (HSA) separates THF from water because it preferentially H-bonds water and increases the proportion of single H-bond donors (SD) relative to double H-bond donors (DD). This change in the coordination of water molecules from DD to SD leads to phase separation between THF and water. We previously established this separation mechanism using sugars and surfactants and other water miscible solvents. Here, we use attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) to prove that this mechanism is also responsible for THF–water separation using HSA, thereby demonstrating the universality of the proposed separation mechanism. Using synchrotron small-angle x-ray scattering, we show how HSA self-assembles into reverse micelles in THF–water mixtures and determine their persistence length and periodicity using a modified Landau model. Reverse micelles host water in their interior and swell upon increasing the water content, as shown by light scattering. They then turn into droplets detectable using optical or confocal microscopy. When THF–water emulsions separate, they yield water-rich and THF-rich free phases. ATR-FTIR reveals that the top phase of THF–water mixtures separated by HSA is THF-rich. Moreover, when Cu2+ ions are in solution, HSA causes their migration into the THF-rich phase, enabling the simultaneous separation of THF and Cu2+ cations from water. This study demonstrates the potential for engineering the water structure to aid in the separation of water-miscible solvents from water with important implications for water treatment.

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