Mesoscopic theory for inhomogeneous mixtures

TL;DR

This paper develops a mesoscopic density functional theory for inhomogeneous mixtures, analyzing phase stability and structure formation, including effects beyond mean-field approximation, with explicit results for two-component systems.

Contribution

It introduces a systematic coarse-graining approach to derive mesoscopic theories for inhomogeneous mixtures, extending analysis beyond mean-field to include fluctuation effects.

Findings

Identification of spinodal and λ-surfaces in phase diagrams.

Demonstration of fluctuation effects shifting stability boundaries.

Explicit expressions for two-component system phase stability.

Abstract

Mesoscopic density functional theory for inhomogeneous mixtures of sperical particles is developed in terms of mesoscopic volume fractions by a systematic coarse-graining procedure starting form microscopic theory. Approximate expressions for the correlation functions and for the grand potential are obtained for weak ordering on mesoscopic length scales. Stability analysis of the disordered phase is performed in mean-field approximation (MF) and beyond. MF shows existence of either a spinodal or a $\lambda$-surface on the volume-fractions - temperature phase diagram. Separation into homogeneous phases or formation of inhomogeneous distribution of particles occurs on the low-temperature side of the former or the latter surface respectively, depending on both the interaction potentials and the size ratios between particles of different species. Beyond MF the spinodal surface is shifted, and the instability at the $\lambda$-surface is suppressed by fluctuations. We interpret the $\lambda$-surface as a borderline between homogeneous and inhomogeneous (containing clusters or other aggregates) structure of the disordered phase. For two-component systems explicit expressions for the MF spinodal and $\lambda$-surfaces are derived. Examples of interaction potentials of simple form are analyzed in some detail, in order to identify conditions leading to inhomogeneous structures.