A Proton Nuclear Magnetic Resonance (¹H NMR) Investigation of NaCl-Induced Phase Separation of Acetonitrile-Water Mixtures

Abstract

The microscopic properties of NaCl-induced phase separation of acetonitrile (ACN)-water mixtures have been studied by proton nuclear magnetic resonance ( 1 H NMR). Acetonitrile-rich phase increases with increasing NaCl concentration ( c NaCl ) at x ACN ≈ 0.25. 1 H chemical shift of water for acetonitrile-rich phase rapidly decreases with decreasing NaCl mole concentration and that for water-rich phase quickly increases with increasing c NaCl . However, 1 H chemical shift of acetonitrile has nothing to do with the molar concentration of NaCl, and it keeps relatively stable for all solutions (±0.002). These results reveal that Na + and Cl - are rapidly hydrated by water, not by acetonitrile. The change of 1 H chemical shift of water has shown that the number of hydrogen bond increases or hydrogen bond strengths with increasing NaCl molarity in mixtures. But hydrogen bond is broken or weaken with the temperature rising. 1 H chemical shifts of pure water and the water in acetonitrile-rich phase have been investigated at 293 K, 298 K and 303 K. The hydration number of Na + (6.05) in water-rich phase is determined by an empirical equation involving 1 H chemical shift, temperature and NaCl molarity, which is in good agreement with the literatures.