Nuclear Magnetic Resonance (NMR) Spectroscopy for the In Situ Measurement of Vapor–Liquid Equilibria

Abstract

Vapor–liquid equilibrium (VLE) data (T, p, x, y) are vital for the development of mixture thermodynamic models. Herein, we demonstrate the use of nuclear magnetic resonance (NMR) spectroscopy as an in situ method for VLE measurements. The experiment is carried out entirely inside the NMR sample tube. The simultaneous measurement of liquid- and vapor-phase composition was achieved by the insertion of a sealed glass capillary into the NMR sample tube. In this way, a small amount of the liquid phase wicks into the wedge-shaped gap between the capillary and the inner wall of the NMR tube. The presence of a suspended liquid meniscus was confirmed by X-ray computed tomography, and its behavior was examined by computational fluid dynamics. The components of the two phases are observed as distinct signals in the NMR spectra. The temperature-dependent spectrum of methanol, contained in a sealed capillary, was used to measure the temperature. Vapor density was measured instead of pressure; this was accomplished by calibration with gas-phase ethane at known densities. With this approach, we provide proof-of-concept results at 291.2 K on binary mixtures of (R32 + R125), (R125 + R143a), (ethane + neopentane), and (ethane + benzene).

Solvent-Free Nuclear Magnetic Resonance Spectroscopy of Charged Molecules

“…Our work is motivated by the in situ measurements of vapor−liquid equilibria (VLE) by McLinden and co-workers, who simultaneously determined the NMR spectra of VLE of gravimetrically prepared mixtures of compounds like ethane and neopentane. 28 In terms of solvent-free ionization methods, we use 3-nitrobenzonitrile (3-NBN), a well-characterized MAI matrix.…”

“…Here, our work aims to determine if matrix-assisted ionization (MAI) can be used to generate charged species of small molecules with transient protons and whether the resulting products can be probed with NMR. Our work is motivated by the in situ measurements of vapor–liquid equilibria (VLE) by McLinden and co-workers, who simultaneously determined the NMR spectra of VLE of gravimetrically prepared mixtures of compounds like ethane and neopentane . In terms of solvent-free ionization methods, we use 3-nitrobenzonitrile (3-NBN), a well-characterized MAI matrix.…”