Franjo Sokolić

Concentration fluctuations and microheterogeneity in aqueous amide mixtures

The relationship between concentration fluctuations and the microheterogeneous status of aqueous amide mixtures is addressed through the molecular dynamics study of three different amides, namely, formamide, N-methylformamide, and dimethylformamide. The computer simulations provide structural evidence that these mixtures exhibit considerable microheterogeneity, in apparent contrast to the experimentally obtained Kirkwood-Buff integrals which indicate that these mixtures should be near ideal. This contradiction is addressed by distinguishing microheterogeneity from concentration fluctuations. The former is the result of mixing H-bonding species under specific constraints due to various bonding possibilities between the molecules, while the second is related to the average relative distribution of the molecules. The relationship between these two different quantities is analyzed and illustrated in terms of the partial site-site structure factors. Small wave-number prepeaks relate to the microheterogeneity while zero wave-number value relates to the concentration fluctuations. A simple analytical statistical model for the microheterogeneity is formulated, which allows to discuss the small wave-number behavior of these structure factors in terms of the kinetics of the transient cluster formation, as observed in the computer simulations.

A comparison of force fields for ethanol–water mixtures

Aqueous ethanol mixtures are studied through molecular dynamics simulations with the focus on exploring how various force field models reproduce the association and its influence on selected thermo-physical properties of these mixtures. The most important conclusion seems to be the inadequacy of all classical force fields to reproduce the very peculiar shape of the excess enthalpy of these mixtures, as a function of the ethanol concentration, neither quantitatively nor qualitatively. The Kirkwood–Buff (KB) integrals calculated using the simulation data follow the same trends as the experimental ones. This suggests complicated correlation of the excess enthalpy with the concentration fluctuation and clustering in these mixtures. The KB force field shows better overall agreement with experimental results than the other studied models.

The microscopic structure of cold aqueous methanol mixtures

The evolution of the micro-segregated structure of aqueous methanol mixtures, in the temperature range 300 K-120 K, is studied with computer simulations, from the static structural point of view. The structural heterogeneity of water is reinforced at lower temperatures, as witnessed by a pre-peak in the oxygen-oxygen structure factor. Water tends to form predominantly chain-like clusters at lower temperatures and smaller concentrations. Methanol domains have essentially the same chain-like cluster structure as the pure liquid at high concentrations and becomes monomeric at smaller ones. Concentration fluctuations decrease with temperature, leading to quasi-ideal Kirkwood-Buff integrals, despite the enhanced molecular interactions, which we interpret as the signature of non-interacting segregated water and methanol clusters. This study throws a new light on the nature of the micro-heterogeneous structure of this mixture: the domain segregation is essentially based on the appearance of linear water clusters, unlike other alcohol aqueous mixtures, such as with propanol or butanol, where the water domains are more bulky.

Water-like structure with repulsive double-core interactions

The soft-core repulsive interaction together with a Gaussian repulsive interaction are used to reproduce major features of the structure of liquid water, both in direct and reciprocal space, by Monte Carlo and integral equation theories. The study reveals that the structure of liquid water is determined, within the model studied here, by the competition of the two repulsive cores, which results in a two-fold spatial distribution, very reminiscent of the two-state water model proposed by many authors. The fact is that many of the structural features of water could be reproduced without any recourse to direct attractive interactions, such as directional hydrogen bonds, and appear to be the result of long-range competing packing correlations, as witnessed by the particular features of the structure factor. The Hypernetted-Chain integral equation is able to reproduce very accurately the most important features of the experimental structure of room temperature water, whereas the Percus–Yevick approximation fails to reach this state point. A high-temperature study shows that this failure is related to the insufficient diagrammatic structure of this closure.

Micro-Heterogeneity in Complex Liquids

Liquids are fundamentally thought to be disordered systems (with few exceptions such as liquids crystals).We feel that aqueous mixtures belong to a very special type of disorder, caracterised by the micro-heterogeneity, and where fluctuations in the number of particles in a given volume play an important role. It is this problem that is not well described by finite size simulations, for the very simple reason that such system contains two scales of description, the original microscopic scale related to molecular size, and the newly emerged mesoscopic scale related to the segregated domains.

A re-appraisal of the concept of ideal mixtures through a computer simulation study of the methanol-ethanol mixtures

Methanol-ethanol mixtures under ambient conditions of temperature and pressure are studied by computer simulations, with the aim to sort out how the ideality of this type of mixtures differs from that of a textbook example of an ideal mixture. This study reveals two types of ideality, one which is related to simple disorder, such as in benzene-cyclohexane mixtures, and another found in complex disorder mixtures of associated liquids. It underlines the importance of distinguishing between concentration fluctuations, which are shared by both types of systems, and the structural heterogeneity, which characterises the second class of disorder. Methanol-1propanol mixtures are equally studied and show a quasi-ideality with many respect comparable to that of the methanol-ethanol mixtures, hinting at the existence of a super-ideality in neat mono-ol binary mixtures, driven essentially by the strong hydrogen bonding and underlying hydroxyl group clustering.

A Simple Two Dimensional Model of Methanol

Methanol is the simplest alcohol and possible energy carrier because it is easier to store than hydrogen and burns cleaner than fossil fuels. It is a colorless liquid, completely miscible with water and organic solvents and is very hygroscopic. Here, simple two-dimensional models of methanol, based on Mercedes-Benz (MB) model of water, are examined by Monte Carlo simulations. Methanol particles are modeled as dimers formed by an apolar Lennard-Jones disk, mimicking the methyl group, and a sphere with two hydrogen bonding arms for the hydroxyl group. The used models are the one proposed by Hribar-Lee and Dill (Acta Chimica Slovenica, 53:257, 2006.) with the overlapping discs and a new model with tangentially fused dimers. The comparison was done between the models, in connection to the MB water, as well as with experimental results and with new simulations done for 3D models of methanol. Both 2D models show similar trends in structuring and thermodynamics. The difference is the most pronounced at lower temperatures, where the smaller model exhibits spontaneous crystallization, while the larger model shows metastable states. The 2D structural organization represents well the clustering tendency observed in 3D models, as well as in experiments. The models qualitatively agree with the bulk methanol thermodynamic properties like density and isothermal compressibility, however, heat capacity at the constant pressure shows trend more similar to the water behavior. This work on the smallest amphiphilic organic solute provides a simple testing ground to study the competition between polar and non-polar effects within the molecule and physical properties.