Structural, Rheological and Dynamic Aspects of Hydrogen-Bonding Molecular Liquids: Aqueous Solutions of Hydrotropic tert-Butyl Alcohol

TL;DR

This study combines experimental and computational methods to explore how molecular structure, viscosity, and dynamics are interconnected in aqueous tert-butanol solutions, revealing four distinct compositional regimes and the role of hydrogen bonds.

Contribution

It introduces a comprehensive scheme integrating SWAXS, MD simulations, and the complemented-system approach to analyze structure-viscosity-dynamics relationships in hydrogen-bonded liquids.

Findings

Four composition ranges with different structures identified

Successful correlation of viscosity and diffusion with structural details

Hydrogen bonds significantly influence aggregate formation and dynamics

Abstract

Hypothesis: The structural details, viscosity trends and dynamic phenomena in t-butanol/water solutions are closely related on the molecular scales across the entire composition range. Utilizing the experimental small- and wide-angle x-ray scattering (SWAXS) method, molecular dynamics (MD) simulations and the complemented-system approach method developed in our group it is possible to comprehensively describe the structure-viscosity-dynamics relationship in such structurally versatile hydrogen-bonded molecular liquids, as well as in similar, self-assembling systems with pronounced molecular and supramolecular structures at the intra-, inter-, and supra-molecular scales. Experiments: The SWAXS and x-ray diffraction experiments and MD simulations were performed for aqueous t-butanol solutions at 25 {\deg}C. Literature viscosity and self-diffusion data were also used. Findings: The interpretive power of the proposed scheme was demonstrated by the extensive and diverse results obtained for aqueous t-butanol solutions across the whole concentration range. Four composition ranges with qualitatively different structures and viscosity trends were revealed. The experimental and calculated zero-shear viscosities and molecular self-diffusion coefficients were successfully related to the corresponding structural details. The hydrogen bonds, that were, along with hydrophobic effects, recognized as the most important driving force for the formation of t-butanol aggregates, show intriguing lifetime trends and thermodynamic properties of their formation.