Depletion-driven four-phase coexistences in discotic systems

TL;DR

This paper uses free volume theory to predict complex phase coexistences in discotic systems, revealing conditions for four-phase equilibria involving isotropic, nematic, and columnar phases.

Contribution

It derives analytical expressions within free volume theory to systematically identify multi-phase and four-phase coexistences in discotic colloidal systems.

Findings

Identification of four-phase coexistence conditions involving all three phase states

Prediction of multiple two-phase isostructural transitions

Validation of theoretical diagrams against computer simulations

Abstract

Free volume theory (FVT) is a versatile and tractable framework to predict the phase behaviour of mixtures of platelets and non-adsorbing polymer chains in a common solvent. Within FVT, three principal reference phases for the hard platelets are considered: isotropic (I), nematic (N), and columnar (C). We derive analytical expressions that enable us to systematically trace the different types of phase coexistences revealed upon adding depletants and confirm the predictive power of FVT by testing the calculated diagrams against phase stability scenarios from computer simulation. A wide range of \textit{multi}-phase equilibria is revealed, involving two-phase isostructural transitions of all phase symmetries (INC) considered as well as the possible three-phase coexistences. Moreover, we identify the system parameters, relative disk shapes and colloid-polymer size ratios, at which four-phase equilibria are expected. These involve a remarkable coexistence of all three phase states commonly encountered in discotics including isostructural coexistences I$_1$-I$_2$-N-C, I-N$_1$-N$_2$-C, and I-N-C$_1$-C$_2$.