Role of hydrogen bonding on solvent separation using amphiphilic sorbitan ester

Abstract

Tetrahydrofuran (THF) and water are miscible and interact trough hydrogen (H) bonds. Span 80 (sorbitan ester) is soluble in THF, but not water. Nonetheless, Span 80 H bonds with water, as shown by attenuated total reflectance – Fourier transform infrared spectroscopy. In pure water, species are single (SD) or double (DD) donors, and single (SA) or double (DA) acceptors. In pure water, SD-SA and DD-DA are dominant and have similar abundance. Span 80 and THF alter the distribution of water species. When THF and Span 80 compete for the same water species, THF separation from water is most effective. Span 80 induces a marked shift of SD-SA to higher wavenumbers, which are close to DD-DA. This species intermediate between DD-DA and SD-SA is dominant, indicating that Span 80 mainly interacts with it. This same species is also dominant in THF-water mixtures containing 50–70% THF. Instead, DD-DA is dominant up to 40% THF, while DD-SA dominate at the highest THF percentages. Bottle tests show that Span 80 separates THF and water into bulk phases with 50–70% THF within 1 hr. In contrast, outside this THF range, emulsions are stable for more than 1 hr, as observed by either light scattering or optical microscopy. In mixtures with 50–70% THF, bulk phase separation occurs within 1 hr, because Span 80 competes with THF for the same water species. Separation is poorer outside of this THF range, where Span 80 and THF interact with different water species.