Pressure dependence of the dielectric loss minimum slope for ten molecular liquids

Abstract

We present a comprehensive study of data for the dielectric relaxation of ten glass-forming organic liquids at high-pressure along isotherms, showing that the primary (α) high-frequency relaxation is well-characterized by the minimum slope and the width of the loss peak. The advantage of these two parameters is that they are model independent. For some materials with β processes in the mHz and kHz range, the high-frequency slope tends to be with pressure increase. In addition, the two parameters capture the relaxation shape invariance at a given relaxation time but different combinations of pressure and time.

Keywords:

• compression
• time pressure temperature super position
• minimum slope
• width
• dielectric

Acknowledgements

This work was supported by a grant from the Danish National Research Foundation (DNRF) for funding the Centre for Viscous Liquid Dynamics ‘Glass and Time’.

Notes

1. If one believes in a simple additive relation between the primary relaxation and DC conductivity, then it is easy to show mathematically that ϵ″ → ∝ω^−β) for large ω. The cases where α process and DC decouple, but if all temperature curves are superimposed TTS is obeyed around the loss peak like in the situation for DisoBP. This means that the α process has no significant contribution from DC.

2. In our case, the construction of the capacitor for pressure investigations ensures an excellent separation of the sample from the pressure transmitting liquid–silicon oil. The observed DC conductivity results from some ions existing in every measured liquid irrespective of the purification procedure. The noise below 1 Hz is due to relatively small (below 0.1) values of ϵ″ of the presented samples and limitation experimental time.