Transport in a binary mixture of non-spherical molecules: Is hydrodynamics at fault?

TL;DR

This study investigates how molecular shape influences transport properties in dense liquids, revealing that traditional hydrodynamic models fail to accurately predict rotational and translational diffusion in a binary mixture of non-spherical molecules.

Contribution

It introduces a new model of a binary mixture of prolate and oblate ellipsoids and systematically tests the validity of hydrodynamic predictions for their transport properties.

Findings

Hydrodynamic predictions break down for both translational and rotational diffusion.

Prolate and oblate molecules exhibit different dynamical behaviors.

No phase separation observed except at low temperatures and pressures.

Abstract

We consider a new class of model systems to study systematically the role of molecular shape in the transport properties of dense liquids. Our model is a liquid binary mixture where both the molecules are non-spherical and characterized by a collection of parameters. Although in the real world most of the molecules are non-spherical, only a limited number of theoretical studies exist on the effects of molecular shapes and hardly any have addressed the validity of the hydrodynamic predictions of rotational and translational diffusion of these shapes in liquids. In this work, we study a model liquid consisting of a mixture of prolate and oblate (80:20 mixture) ellipsoidals with interactions governed by a modified Gay-Berne potential for a particular aspect ratio (ratio of the length and diameter of the ellipsoids), at various temperature and pressure conditions. We report calculations of transport properties of this binary mixture by varying temperature over a wide range, at a fixed pressure. We find that for the pressure-density conditions studied, there is no signature of any phase separation, except transitions to the crystalline phase at low temperatures and relatively low pressure (the reason we largely confined our studies to high pressure). We find that for our model binary mixture, both the stick and slip hydrodynamic predictions break down in a major fashion, for both prolates and oblates and particularly so for rotation. Moreover, prolates and oblates themselves display different dynamical features in the mean square displacement and in orientational time correlation functions.