Time-scale ordering in hydrogen- and van der Waals-bonded liquids

Area of Science:

• Condensed Matter Physics
• Physical Chemistry
• Materials Science

Background:

• Structural relaxation governs liquid dynamics and is influenced by molecular interactions.
• Understanding relaxation time scales is crucial for predicting material properties.
• Secondary relaxation processes can complicate the analysis of structural dynamics.

Purpose of the Study:

• To investigate the time scales of structural relaxation using multiple response functions.
• To compare relaxation dynamics in a hydrogen-bonded liquid (1,2,6-hexanetriol) and a van der Waals liquid (squalane).
• To determine if molecular interactions affect the ordering of relaxation time scales.

Main Methods:

• Analysis of five different response functions: shear modulus, bulk modulus, dielectric permittivity, longitudinal thermal expansivity coefficient, and longitudinal specific heat.
• Derivation of time scales from inverse peak frequencies and terminal relaxation modes.
• Temperature-dependent measurements and analysis.

Main Results:

• A consistent ordering of time scales (shear modulus < bulk modulus < dielectric permittivity < thermal expansivity < specific heat) was observed for both liquids.
• Temperature-independent time scale ratios were found for 1,2,6-hexanetriol.
• While squalane showed decoupling of time scales initially, an alternative analysis revealed temperature-independent ratios, even with significant secondary relaxation.

Conclusions:

• The observed time-scale ordering is independent of the type of intermolecular interaction (hydrogen-bonded vs. van der Waals).
• Intermolecular interactions play a subordinate role in determining the relative time scales of structural relaxation.
• The process of structural relaxation appears to be generic across different types of liquids.