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

2H and 17O NMR studies of solvent dynamics related to the cononsolvency of poly(N-isopropyl acrylamide) in ethanol-water mixtures

Although the thermoresponsive polymer poly(N-isopropylacrylamide) (pNIPAM) is well soluble in both ethanol and water, it shows a miscibility gap in ethanol-water mixtures, an effect termed cononsolvency. We use 2H and 17O nuclear magnetic resonance (NMR) together with appropriate isotope labelling to selectively study reorientation dynamics of ethanol and water related to the cononsolvency effect over the whole range of solvent compositions from pure ethanol to pure water. At low ethanol concentrations (≤30 vol%), spin-lattice (T1) and spin-spin (T2) relaxation times show a step-like decrease when heating across the lower critical solution temperature for the respective solvent composition. However, the drop is notably stronger for ethanol (2H NMR) than for water (17O NMR) in the solvent mixtures. These observations show that the coil-to-globule transition of pNIPAM is accompanied by a slowdown of average solvent dynamics, which is more prominent for ethanol than for water. The different degree of slowdown of the solvent components implies that preferential interaction with the polymer plays a significant role for cononsolvency. Field-cycling relaxometry reveals a low-frequency T1 dispersion above the coil-to-globule transition, indicating that the average solvent dynamics is slower because a major free solvent fraction is accompanied by a minor bound solvent fraction, which shows strongly retarded dynamics. From intermediate to high ethanol concentrations (>50 vol%), the T1 and T2 relaxation times yield no evidence for significant changes in ethanol and water dynamics when crossing an expected upper critical solution temperature.

Exploring the structural feature of water, alcohols, and their binary mixtures with concrete atomic charge assignments in Dreiding forcefield

The water, alcohols, and their binary mixtures are widely used in molecular simulations. However, the Dreiding force field lacks a generally accepted method for assigning atomic charges to these solvents during simulations. In this study, we propose a universal charge assignment for water and eight water-miscible alcohols in Dreiding. Through extensive molecular simulations, we demonstrate the good accuracy of our charge assignments in displaying characteristic of these solvents and their mixtures, including liquid density and structure. Moreover, we investigate equilibrium snapshot, radial distribution function, coordination number and hydrogen bonding, all of which confirm the miscibility of alcohols with water or ethanol. Notably, we reveal that the structure diversity among different mixtures can be attributed to distinctive characteristic of alcohols, including molecular volume, as well as the number and position of hydroxyls.

Molecular diffusion in aqueous methanol solutions: The combined influence of hydrogen bonding and hydrophobic ends

Although the nonmonotonic variation in the diffusion coefficients of alcohol and water with changing alcohol concentrations in aqueous solutions has been reported for many years, the underlying physical mechanisms remain unclear. Using molecular dynamics simulations, we investigated the molecular diffusion mechanisms in aqueous methanol solutions. Our findings reveal that the molecular diffusion is co-influenced by hydrogen bonding and the hydrophobic ends of methanol molecules. A stronger hydrogen bond (HB) network and a higher concentration of hydrophobic ends of methanol molecules both enhance molecular correlations, thereby slowing molecular diffusion in the solution. As methanol concentration increases, the HB network weakens, facilitating molecular diffusion. However, the increased concentration of hydrophobic ends counteracts this effect. Consequently, the diffusion coefficients of water and methanol molecules exhibit nonmonotonic changes. Previous studies have only focused on the role of HB networks. For the first time, we have identified the impact of the hydrophobic ends of alcohol on molecular diffusion in aqueous alcohol solutions. Our research contributes to a better understanding and manipulation of the properties of aqueous alcohol solutions and even liquids with complex compositions.

Pressure-Induced Aggregation of Associating Liquids as a Driving Force Enhancing Hydrogen Bond Cooperativity

The behavior of hydrogen bonds under extreme pressure is still not well understood. Until now, the shift of the stretching vibration band of the X-H group (X = the donor atom) in infrared spectra has been attributed to the variation in the length of the covalent X-H bond. Herein, we combined infrared spectroscopy and X-ray diffraction experimental studies of two H-bonded liquid hexane derivatives, i.e., 2-ethyl-1-hexanol and 2-ethyl-1-hexylamine, in diamond anvil cells at pressures up to the GPa level, with molecular dynamics simulations covering similar thermodynamic conditions. Our findings revealed that the observed changes in the X-H stretching vibration bands under compression are not primarily due to H-bond shortening resulting from increased density but mainly due to cooperative enhancement of H-bonds caused by intensified molecular clustering. This sheds new light on the nature of H-bond interactions and the structure of liquid molecular systems under compression.

Non-monotonic composition dependence of the breakdown of Stokes-Einstein relation for water in aqueous solutions of ethanol and 1-propanol: explanation using translational jump-diffusion approach

A series of experimental and simulation studies examined the validity of the Stokes-Einstein relationship (SER) of water in binary water/alcohol mixtures of different mixture compositions. These studies revealed a strong non-monotonic composition dependence of the SER with maxima at the specific alcohol mole fraction where the non-idealities of the thermodynamic and transport properties are observed. The translational jump-diffusion (TJD) approach elucidated the breakdown of the SER in pure supercooled water as caused by the jump translation of molecules. The breakdown of SER in the supercooled water/methanol binary mixture was successfully explained using the same TJD approach. To further generalize the picture, here we focus on the non-monotonic composition dependence of SER breakdown of water in two water/alcohol mixtures (water/ethanol and water/propanol) for a broad temperature range. In agreement with previous studies, maximum breakdown of SER is observed for the mixture with alcohol mole fraction x = 0.2. Diffusion of the water molecules at the maximum SER breakdown point is largely contributed by jump-diffusion. The residual-diffusion, obtained by subtracting the jump-diffusion from the total diffusion, approximately follows the SER for different compositions and temperatures. We also performed hydrogen (H-)bond dynamics and observed that the contribution of jump-diffusion is proportional to the total free energy of activation of breaking all H-bonds that exist around a molecule. This study, therefore, suggests that the more a molecule is trapped by H-bonding, the more likely it is to diffuse through the jump-diffusion mechanism, eventually leading to an increasing degree of SER breakdown.