Absence of demixing in 2D binary mixtures of alcohols
Binary mixtures of two-dimensional, site-based models of alcohols are investigated by computer simulations, focusing on ideal mixing, local clustering and miscibility trends. Four systems are considered: methanol-ethanol, butanol-pentanol, methanol-pentanol, and methanol-octanol, the first two being expected to be ideal mixtures because of the proximity of the molecules, while the last two are examples of mixing distant alcohols. The models retain chemical specificity of their 3D counterparts, thus allowing one to investigate dimensional constraints and non-trivial micro-structurations. It is found that mixtures of short and long alcohols remain miscible, in contrast with the corresponding macroscopic phase separation observed in three dimensions. This issue is analyzed through snapshots, site-site distribution functions, structure factors and Kirkwood-Buff integrals (KBI). It is found that the short range part features are dominated by charge ordering of the hydroxyl head groups, while the long range part develops persistent oscillatory features affecting the determination of the KBI, and suggesting the presence of anomalous domain-like concentration fluctuations. All these findings suggest a complex interplay between charge ordering of the hydrophilic head groups and the hydrophobic tail groups.