Self-assembly in mixtures with competing interactions

TL;DR

This study investigates the complex phase behavior of a binary mixture with competing short-range attraction and long-range repulsion interactions, using combined theoretical and molecular dynamics approaches to reveal microsegregation and layered structures.

Contribution

The paper develops a combined density functional and field-theoretic theory to predict structural and thermodynamic properties of mixtures with competing interactions, validated by MD simulations.

Findings

Coexistence of disordered and layered ordered phases.

Phase diagram qualitatively altered by concentration fluctuations.

Ordered phases form prolate monocrystals aligned with concentration oscillations.

Abstract

A binary mixture of particles interacting with spherically-symmetric potentials leading to microsegregation is studied by theory and molecular dynamics (MD) simulations. We consider spherical particles with equal diameters and volume fractions. Motivated by the mixture of oppositely charged particles with different adsorption preferences immersed in near-critical binary solvent, we assume short-range attraction long-range repulsion for the interaction between like particles, and short range repulsion long-range attraction for the interaction between different ones. In order to predict structural and thermodynamic properties of such complex mixtures, we develop a theory combining the density functional and field-theoretic methods. We show that concentration fluctuations in mesoscopic regions lead to a qualitative change of the phase diagram compared to mean-field predictions. Both theory and MD simulations show coexistence of a low-density disordered phase with a high-density phase with alternating layers rich in the first and the second component. The density and the degree of order of the ordered phase decrease with increasing temperature, up to a temperature where the theory predicts a narrow two-phase region with increasing density of both phases for increasing temperature. MD simulations show that monocrystals of the solid and liquid crystals have a prolate shape with the axis parallel to the direction of concentration oscillations, and the deviation from the spherical shape increases with increasing periodic order.