Temperature-dependent structure of 1-propanol/water mixtures: X-ray diffraction experiments and computer simulations at low and high alcohol contents

TL;DR

This study combines X-ray diffraction experiments and molecular dynamics simulations to analyze the temperature-dependent structure of 1-propanol/water mixtures across various compositions, revealing insights into hydrogen bonding and cyclic structures.

Contribution

It provides the first combined experimental and computational analysis of 1-propanol/water mixtures over a range of compositions and temperatures, highlighting the temperature dependence of hydrogen bonding networks.

Findings

Semi-quantitative agreement between simulations and experiments at low and high alcohol contents.

Identification of temperature-dependent changes in hydrogen bonded networks and cyclic entities.

Observation of the percolation threshold and ring size variations with temperature.

Abstract

Aqueous mixtures of 1-propanol have been investigated by high-energy synchrotron X-ray diffraction upon cooling. X-ray weighted total scattering structure factors of 6 mixtures, from 8 mol% to 89 mol% alcohol content, as well as that of pure 1-propanol are reported from room temperature down to the freezing points of the liquids. Molecular dynamics simulations have been performed, in order to interpret measured data. The all atom OPLS-AA potential model was used for 1-propanol, combined with both the SPC/E and the TIP4P/2005 water models: both combinations provide a semi-quantitative description of the measured total structure factors at low and high alcohol contents, while the agreement is qualitative for the mixture with 71 mol% of 1-propanol. From the simulated particle configurations, partial radial distribution functions were calculated. Furthermore, detailed description of the hydrogen bonded network is provided, in terms of hydrogen bond numbers, analysis of proton donor-acceptor ratios, size distributions of hydrogen bonded clusters and ring size statistics. Strong temperature dependence of the percolation threshold, as well as of the participation of the number of doubly hydrogen bonded molecules in cyclic entities, has been found for the mixture with 89 mol% of 1-propanol. Above an alcohol content of 20 mol%, 5-fold rings are the most frequent cyclic entities, with a strong temperature dependence in terms of the number of rings.