Neat methanol and tert-butanol are studied by molecular dynamics with the focus on the microstructure of these two alcohols. The site-site radial distribution functions, the corresponding structure factors, and an effective local one-body density function are shown to be the appropriate statistical quantities that point in a complementary manner towards the same microstructure for any given liquid. Methanol is found to be a weakly associated liquid forming various chainlike patterns (open and closed) while tert-butanol is almost entirely associated and forms micellelike primary pattern. The presence of stable local microheterogeneity within homogeneous disordered phase appears as a striking feature of these liquids. The absence of any such apparent clustering in water--a stronger hydrogen bonding liquid--through the same two statistical quantities is analyzed.

Formation of microstructure in homogeneous associated liquids is analysed through the densitydensity pair correlation functions, both in direct and reciprocal space, as well as an effective local one-body density function. This is illustrated through a molecular dynamics study of two neat alcohols, namely methanol and tert-butanol, which have a rich microstructure: chain-like molecular association for the former and micelle-like for the latter. The relation to hydrogen bonding interaction is demonstrated. The apparent failure to find microstructure in water -a stronger hydrogen bonding liquid-with the same tools, is discussed.Liquids are generally thought as macroscopically homogeneous when they are considered far from phase transitions and interfacial regions. From a statistical mechanical point of view, homogeneity is expressed by the fact that the order parameter, in this case the one body density, which formally depends on both the position and orientation of a single particle 1 (as in a crystal or a liquid crystal, for example), is a constant throughout the sample: ρ (1) (1) = ρ = N/V , where N is the number of particles per volume V. As a consequence, the microscopic description of the structure of a neat liquid starts from the two-body density function ρ (2) (1, 2) = ρ (1) (1)ρ (1) (2)g(1, 2) that expresses the density correlations between particles 1 and 2, and reduces in this case to ρ 2 g(1, 2) , where g(1, 2) is the pair distribution function. Associated liquids, such as water and alcohols, for example, belong to a special class because of the particularity of the hydrogen bonding (HB) that is highly directional, and tend to enhance the structure the liquid locally. One particularly interesting example of this phenomena is the microheterogeneous nature of aqueous mixtures, which has attracted a recent upsurge of interest [1,2,3,4,5,6,7]. Perhaps the most remarkable reported fact is that watermethanol mixtures show local immiscibility at microscopic level, while being miscible at macroscopic level [1,2]. In order to appreciate this result it is interesting to compare it to microemulsions where bicontinuous phases are usually observed, and micro-immiscibility operates with domain sizes ranging from 100 nanometers to few micrometers, while those mentioned here are around few nanometers-that is about few molecular diameters. In addition, it is important to note that bicontinuous phases in microemulsions arise after a phase transition has occurred from disordered to ordered phase; while in the former case, we are still in a genuinely homogeneous and disordered liquid phase. From these facts, microheterogeneity in aqueous mixtures can be considered as both obvious and mysterious, obvious because the mechanism behind it is the strong directionality of the hydrogen bonding, and mysterious because of the existence of stable microimmiscibility of water and solute in a macroscopically homogeneous sample. In contrast to the situation for mixtures, neat water do not seem to exhibit any micro phase separation betwe...

Several combinations of existing classical water and acetone models are studied by molecular dynamic simulations in order to sort out which models can reproduce available experimental data: enthalpies, pressure, densities, diffusion coefficients, and Kirkwood-Buff integrals. It turns out that all these properties, but the last, are rather well reproduced by all models, and with little numerical effort. By contrast, trials to measure by simulations the Kirkwood-Buff integrals lead to very long simulation times, thus revealing unexpected divergent behavior between the different models, such as phase separation, for example, and ultimately leading to a failure of any models combinations to reproduce these properties according to the experimental tendencies. It is argued herein that these deficiencies provide, in fact, an insightful picture of the microscopic structure of the solution, particularly into the relation between the hydrogen-bond network and the concentration fluctuations, as well as the role played by the solute in their spatial organization.