A Proton Nuclear Magnetic Resonance (&amp;lt;sup&amp;gt;1&amp;lt;/sup&amp;gt;H NMR) Investigation of NaCl-Induced Phase Separation of Acetonitrile-Water Mixtures

Ruizhi Wen,Wenjie Deng,Bo Chen,Lili Feng,Muqian Yu,Le Jiang

doi:10.4236/ajac.2017.810048

Abstract

The microscopic properties of NaCl-induced phase separation of acetonitrile (ACN)-water mixtures have been studied by proton nuclear magnetic resonance (1H NMR). Acetonitrile-rich phase increases with increasing NaCl concentration (cNaCl) at xACN ≈ 0.25. 1H 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 cNaCl. However, 1H 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 1H 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. 1H 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 1H chemical shift, temperature and NaCl molarity, which is in good agreement with the literatures.

Highlights

We firstly investigated the effect of NaCl concentration in acetonitrile-water mixtures of 1:1 volume ratio of acetonitrile to water on chemical compositions after phase separation by atomic absorption spectroscopy, ion chromatography, moisture-determining instrument and gas chromatography
The volumes of acetonitrile-rich phase increase quickly, contrary to variety in water-rich phase. These make clear that acetonitrile molecules are rapidly exuded from the CH3CN-H2O-NaCl mixtures and the hydration of Na+ and Cl− rapidly increases with increasing the total NaCl concentration
From 1H nuclear magnetic resonance (NMR) measurements, we know that 1H chemical shift of water increases with increasing the molar concentration of NaCl in both phases, 1H chemical shift of acetonitrile for all solutions keeps relatively stable

Summary

Introduction

Acetonitrile (ACN) and water (W) are miscible at any ratio, but acetoni-. A few of studies have focused on microheterogeneity [10] of different mixtures by large angle X-ray scattering, small angle neutron diffraction, Infrared spectroscopy [10] [11] [12] [13]. Few investigations about the microscopic properties of salt-induced phase separation by nuclear magnetic resonance (NMR) were explored until now

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Results

Conclusion