Dielectric constant of disordered phases of the smallest monoalcohols : evidence for the hindered plastic crystal phase

TL;DR

This study investigates the dielectric properties of disordered phases in small monoalcohols, revealing evidence for a hindered plastic crystal phase and dynamic disorder influenced by external conditions.

Contribution

It provides novel evidence for hindered plastic crystal phases in monoalcohols and introduces a new method to separate electrode polarization effects in dielectric measurements.

Findings

Dielectric contribution near solidification is about half of the liquid's dielectric constant.

Hindered plastic crystal phase exists in ethanol at high pressure, similar to ambient methanol.

Dynamic disorder increases near order-disorder transitions and is affected by external conditions.

Abstract

With gradual temperature increase in premelting regions of solid phase of methanol and high pressure phase of ethanol, and using novel procedure of separation of electrode polarization effects, we are able to register the contribution of relaxation process to low-frequency dielectric constant. This contribution is about half the liquid's dielectric constant near temperature of solidification , and is almost an order of magnitude higher than reported earlier for ambient pressure phase of methanol. As opposed to dielectric constant of water at ambient pressure, which does not change much during crystallization, our finding indicates the hindrance of molecule rotation in orientationally disordered phases of monoalcohols. Similar dielectric responses of ambient pressure methanol and high-pressure phase of ethanol imply existence of hindered plastic crystal phase of ethanol (not observed at low pressures). We also have found some dynamic disorder in nominally fully ordered phase of these monoalcohols ($\alpha$-phase of methanol and low-pressure phase of ethanol), the contribution of this disorder being dependent on external conditions (e.g. temperature), and increasing at approaching the order-disorder transition. On the other hand, the amplitude of dielectric responce in hindered plastic crystal phases is almost independent of temperature.