Ab Initio Molecular Dynamics Study of Methanol-Water Mixtures under External Electric Fields

Giuseppe Cassone,Jiri Sponer,A Marco Saitta,Adriano Sofia,Franz Saija

doi:10.3390/molecules25153371

Abstract

Intense electric fields applied on H-bonded systems are able to induce molecular dissociations, proton transfers, and complex chemical reactions. Nevertheless, the effects induced in heterogeneous molecular systems such as methanol-water mixtures are still elusive. Here we report on a series of state-of-the-art ab initio molecular dynamics simulations of liquid methanol-water mixtures at different molar ratios exposed to static electric fields. If, on the one hand, the presence of water increases the proton conductivity of methanol-water mixtures, on the other, it hinders the typical enhancement of the chemical reactivity induced by electric fields. In particular, a sudden increase of the protonic conductivity is recorded when the amount of water exceeds that of methanol in the mixtures, suggesting that important structural changes of the H-bond network occur. By contrast, the field-induced multifaceted chemistry leading to the synthesis of e.g., hydrogen, dimethyl ether, formaldehyde, and methane observed in neat methanol, in 75:25, and equimolar methanol-water mixtures, completely disappears in samples containing an excess of water and in pure water. The presence of water strongly inhibits the chemical reactivity of methanol.

Highlights

Alcohol-water mixtures have extensively been investigated due to their academic and industrial relevance [1]
We used the software package CP2K [40,41], based on the Born-Oppenheimer approach, to perform ab initio molecular dynamics (AIMD) simulations of five samples (i.e., neat methanol, 75:25 (X025), 50:50 (X050), 25:75 (X075) methanol-water mixtures, and neat water) under the action of static and homogeneous electric fields applied along a given direction.AIMD simulations do not employ specific parametrizations for mimicking the interactions between atoms or molecules but explicitly solve the electronic problem within a rigorous Density Functional Theory (DFT) framework
The location of the first peak is independent of the amount of solvated water, falling at a distance of ∼2.71 Å both in the neat methanol sample and in all the other methanol-water mixtures

Summary

Introduction

Alcohol-water mixtures have extensively been investigated due to their academic and industrial relevance [1]. Investigations on these solutions were focused on the role of the hydrophobic headgroups of alcohols in determining the physical properties of water and vice versa [2]. Methanol (CH3 OH)—the simplest alcohol—can classically be considered as a water molecule where a hydrogen-donor site is replaced by a hydrophobic methyl group. Such a substitution produces several differences between these two. From a topological point of view, water exhibits its peculiar tetrahedrally-coordinated structure, whereas methanol contains linear and irregular chains of H-bonded molecules(the interested reader may refer, e.g., to Figure 1 of Ref.

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