Charge order, domain order, ideal mixing and absence of demixing in 2D binary mixtures of alcohols

TL;DR

This study uses computer simulations to explore charge ordering and mixing behaviors in 2D binary alcohol mixtures, revealing unexpected mixing and microstructure phenomena influenced by charge interactions.

Contribution

It uncovers the role of charge ordering in 2D alcohol mixtures, explaining microstructure and mixing behaviors not solely due to fluctuations, contrasting with 3D systems.

Findings

Short and long alcohol mixtures are well mixed in 2D, unlike in 3D.

Ideal mixing and micro phase separation compete within polar head aggregates.

Long-range correlations show non self-averaging behavior similar to real systems.

Abstract

Binary mixtures of two dimensional, site-based models of alcohols are investigated by computer simulations, with a focus on ideal mixing, local clustering and miscibility trends. Four representative systems are considered: methanol/ethanol, butanol/pentanol, methanol/pentanol, and methanol/octanol. The models retain chemical specificity, while allowing to investigate dimensional constraints and uncover non/trivial micro/structurations. Two unexpected results are observed. First, mixtures of short and long alcohols are well mixed, instead of the macroscopic phase separation found in their three-dimensional counterparts. Second, ideality and micro phase separation compete within the chain like polar head aggregates. These behaviors cannot be explained solely by enhanced fluctuations in two dimensions, and instead point to a key role of charge ordering in shaping the local structure. The resulting interplay between concentration fluctuations and micro heterogeneous aggregation is analyzed through snapshots, site/site distribution functions, structure factors and Kirkwood Buff integrals. In particular, the analysis reveals that the domain correlations in the long range part of the correlations have an intriguing non self averaging behaviour, similar to that found in the real systems, indicating that mixtures of associating molecules are not ruled by conventional fluctuations.