The structural properties of model two-dimensional (2D) ionic liquids are examined, with a particular focus on the charge ordering process, with the use of computer simulation and integral equation theories. The influence of the logarithmic form of the Coulomb interaction, versus that of a 3D screened interaction form, is analysed. Charge order is found to hold and to be analogous for both interaction models, despite their very different form. The influence of charge ordering in the low density regime is discussed in relation to well known properties of 2D Coulomb fluids, such as the Kosterlitz-Thouless transition and criticality. The present study suggests the existence of a stable thermodynamic labile cluster phase, implying the existence of a liquid-liquid "transition" above the liquid-gas binodal. The liquid-gas and Kosterlitz-Thouless transitions would then take place inside the predicted cluster phase.

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Mixtures of 1-alkanols are a textbook example of the concept of ideal mixtures. Yet, such mixtures have a very strong local order due to the hydrogen bonding interactions, with a strong tendency for chain formation. Despite this apparent non-ideality, the Kirkwood-Buff integrals of such system exhibit near ideal behaviour. This dual property can be used to test the calculations of the Kirkwood-Buff integrals in a controlled mixing situation, and clarify many points, in particular the statistical problems that can be encountered. By studying the methanol-ethanol mixtures, we uncover an interesting physical asymmetry between low methanol and low ethanol concentrations, which can produce statistical artifacts in the calculation of Kirkwood-Buff integrals, illustrating and exemplifying some of the difficulties encountered in such calculations. Finally, liquid state integral equations results for these mixtures are reported. They help demonstrate that thermodynamic ideality hides complex correlations and microscopic non-ideality.

Hydrogen bonded liquids are associated liquids and tend to exhibit local inhomogeneity in the form of clusters and segregated sub-nano domains. It is an open question as to whether Hbonded clusters in pure water have common features with the water segregated pockets observed in various aqueous binary mixtures, such as water–alcohol mixtures, for example. In the present study, we demonstrate through classical molecular dynamics studies of the lifetime distributions of the hydrogen bonds in different types of binary mixtures, that these lifetimes exhibit the same universal features in the case of the pure liquids, independently of the species concentrations. The same types of three distinct lifetimes are observed, all of them in the sub picosecond regime. The primary lifetime concerns that of Hbonded dimers, and strongly depends on Hbonding criteria such as the bonding distance. The two others are independent of bonding criteria and appear as universal accross many liquids and mixtures. The secondary lifetime ($$\tau _1 \approx 20$$ τ 1 ≈ 20 fs) concerns Hbonded cluster lifetimes, while the tertiary lifetime ($$\tau _2 \approx 50$$ τ 2 ≈ 50 fs) concerns the topology of these clusters, such as chains or globules, for example. This surprizing separation in three distinct lifetimes suggests the existence of associated three distinct kinetic mechanisms in the very short sub-picosecond time scales, with, in addition, an appealing connection to the concepts of local energy and entropy.