Hydrogen bonding of ethanol in supercritical mixtures with CO2 by 1H NMR spectroscopy and molecular simulation

High pressure H-1 NMR spectroscopic studies of mixtures of CO2 and ethanol (EtOH) were conducted at a wide range of temperatures (293.15, 308.15, 323.15, and 338.15 K), pressures (5-25.5 MPa) and concentrations (x(EtOH) = 0.0017-1.0 mol/mol). The relative chemical shift of the protons in the hydroxyl group was used to describe the degree of hydrogen bonding in ethanol. The dependence of the results on pressure is only weak. The quantification of hydrogen bonding was based on the assumption that the observed shift results from a superposition of the shifts of ethanol molecules differing in the way they are hydrogen bonded. Three different types were distinguished: monomer, donor and acceptor. The monomer shift was found from an extrapolation of the NMR data. Molecular dynamics simulations based on force fields from the literature were carried out for the same mixtures at the same conditions and were used to determine the distribution of differently hydrogen bonded ethanol species. Geometric cluster criteria from the literature were used for identifying the hydrogen bonding and the different types of ethanol species. Using the species distribution from the molecular simulations together with the NMR spectroscopic data, the two state independent numbers for the shifts of the hydrogen bond donor and hydrogen bond acceptor molecules were found. Even though only these two parameters are fitted the large set of experimental data is very well described. This confirms our earlier observations on the CO2-methanol system that molecular models of the simple Lennard-Jones plus point charge type as they are used here can describe both thermodynamic properties and the structural effects of hydrogen bonding in solutions. (C) 2012 Elsevier B.V. All rights reserved.