Pressure-Dependent Structure of Methanol-Water Mixtures up to 1.2 GPa: Neutron Diffraction Experiments and Molecular Dynamics Simulations.

László Temleitner,László Pusztai,Yoichi Nakajima,Jun Abe,Takanori Hattori

doi:10.3390/molecules26051218

Abstract

Total scattering structure factors of per-deuterated methanol and heavy water, CD3OD and D2O, have been determined across the entire composition range as a function of pressure up to 1.2 GPa, by neutron diffraction. The largest variations due to increasing pressure were observed below a scattering variable value of 5 Å−1, mostly as shifts in terms of the positions of the first and second maxima. Molecular dynamics computer simulations, using combinations of all-atom potentials for methanol and various water force fields, were conducted at the experimental pressures with the aim of interpreting neutron diffraction results. The peak-position shifts mentioned above could be qualitatively reproduced by simulations, although in terms of peak intensities, the accord between neutron diffraction and molecular dynamics was much less satisfactory. However, bearing in mind that increasing pressure must have a profound effect on repulsive forces between neighboring molecules, the agreement between experiment and computer simulation can certainly be termed as satisfactory. In order to reveal the influence of changing pressure on local intermolecular structure in these “simplest of complex” hydrogen-bonded liquid mixtures, simulated structures were analyzed in terms of hydrogen bond-related partial radial distribution functions and size distributions of hydrogen-bonded cyclic entities. Distinct differences between pressure-dependent structures of water-rich and methanol-rich composition regions were revealed.

Highlights

Hydrogen-bonded liquids are indispensable parts of our lives: one can just think of liquid water and all the aqueous solutions that surroundand compose us
Number densities of the mixtures as a function of temperature were determined by the present molecular dynamics simulations, as shown in Table 1, for the calculations using the TIP4P/2005 water model [12]
To the best of our knowledge, the only experimental study on the pressure-dependent densities of methanol–water liquid mixtures is that of Kubota et al [13], which measured molar volumes up to about 0.2 GPa

Summary

Introduction

Hydrogen-bonded liquids are indispensable parts of our lives: one can just think of liquid water and all the aqueous solutions that surround (seawater, beverages, etc.)and compose (body fluids, biomolecules) us. It has to be stressed that the stability of hydrogen-bonded structures against thermodynamic variables (temperature, pressure) is a key issue from the point of view of life sciences: all living organisms are made of H-bonded constructions with an incredibly delicate balance between stability and flexibility. This is why any advance towards a better understanding of the response of. High-pressure diffraction experiments (i) can probe the validity of short-range interatomic potentials, and (ii) they provide information on the stability of the hydrogen-bonded network. Methanol–water liquid mixtures at room temperature and atmospheric pressure are among the most extensively studied hydrogen-bonded liquids: recent experiments (e.g., [1])

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Conclusion