Mechanism of tetrahydrofuran separation from water by stearic acid

[Display omitted] •Stearic acid separates tetrahydrofuran (THF) from water.•Stearic acid competes against H2O for hydrogen (H) bonding with the same THF species.•Stearic acid self-assembles into reverse micelles that host water in their interior.•Acetic acid resembles the head of stearic acid but does not yield separation.•The tail of stearic acid does not H bond THF, but plays a role in separation. Stearic acid separates tetrahydrofuran (THF) and water into either bulk phase or emulsions, above 30 % THF (relative to water, v/v). The oxygen on the THF ring interacts with either water or stearic acid through hydrogen (H) bonding, as probed using attenuated total reflectance – Fourier transform infrared spectroscopy. Upon mixing with water, the COC band of THF splits into four peaks (at 1020 cm−1, 1040 cm−1, 1050 cm−1 and 1070 cm−1). These correspond to different THF species, i.e., THF molecules which coordinate in different ways with water. In THF-water mixtures containing >30 % THF (v/v), the dominant THF species yields a peak at 1050 cm−1·THF also interacts with stearic acid through H bonds, splitting the ν(COC) vibration into three peaks, at 1030 cm−1, 1050 cm−1 and 1065 cm−1. Similar to THF-water mixtures, the peak at 1050 cm−1 is dominant in THF-stearic acid mixtures. Therefore, we propose that stearic acid induces separation by competing against water for interactions with the same THF species. Synchrotron X Ray diffraction in the small angle X ray scattering region shows that stearic acid self assembles into reverse micelles. Water likely partitions inside them, yielding either kinetically stable emulsions or rapidly separating into bulk layers. Upon separation, stearic acid resides in the THF-rich phase. This is because its hydrophobic tail prevents it from mixing with water, thereby rendering THF-stearic acid interactions more favorable. Acetic acid resembles the head of stearic acid and H bonds THF, without yielding separation. This demonstrates the importance of the stearic acid tail.