Ethanol is a hydrogen bonding liquid. When mixed in small concentrations with water or alkanes, it forms aggregate structures reminiscent of, respectively, the direct and inverse micellar aggregates ...found in emulsions, albeit at much smaller sizes. At higher concentrations, micro-heterogeneous mixing with segregated domains is found. We examine how different statistical methods, namely correlation function analysis, structure factor analysis and cluster distribution analysis, can describe efficiently these morphological changes in these mixtures. In particular, we explain how the neat alcohol pre-peak of the structure factor evolves into the domain pre-peak under mixing conditions, and how this evolution differs whether the co-solvent is water or alkane. This study clearly establishes the heuristic superiority of the correlation function/structure factor analysis to study the micro-heterogeneity, since cluster distribution analysis is insensitive to domain segregation. Correlation functions detect the domains, with a clear structure factor pre-peak signature, while the cluster techniques detect the cluster hierarchy within domains. The main conclusion is that, in micro-segregated mixtures, the domain structure is a more fundamental statistical entity than the underlying cluster structures. These findings could help better understand comparatively the radiation scattering experiments, which are sensitive to domains,
versus
the spectroscopy-NMR experiments, which are sensitive to clusters.
Snapshots of the difference in complex disorder, with analogy with direct (ethanol-water) and inverse (ethanol-alkanes) emulsions.
The Kirkwood-Buff integrals (KBIs) provide a link between structuring on an atomic scale accessed by simulations and thermodynamics, which makes it a powerful tool for bridging the gap between ...microscopic and macroscopic quantities. Here we present the simulation study of the concentration dependence of KBIs for various types of binary ethanol mixtures which range from simple to complex. The thermodynamically ideal but extensively clustered methanol-ethanol mixture shows linearity in KBIs. The non-ideal mixtures, ethanol-water and ethanol-benzene, have the extrema in KBIs which are linked to the formation of the domains due to the association of molecules of the same species. We also explore some of the issues in KBI calculation, as KBI, a property of an open ensemble, is usually obtained by using the running integrals of a pair correlation function, calculated from the simulation of constant number of particles placed in a finite simulation box. The study includes a multifaceted analysis of the LP shift (J. Mol. Liq. 159, 52, 2011) and Ganguly and van der Vegt correction (J. Chem. Theory Comput. 9(3), 1347, 2013) of the radial distribution function, and compares different approaches in the computation of running KBI (J. Phys. Chem. Lett. 4(2), 235, 2013), both of which compensate for finite-size and ensemble effects. The results show how the best approach to the KBI calculation varies depending on the structural complexity of the simulated system.
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•Analysis of RDF corrections and integral computations used in KBI calculations is given.•KBI concentration dependencies of various mixtures are provided.•Mechanisms of associations in ethanol mixtures are discussed.•Structural properties are related to KBI features and running KBI convergence issues.
Two binary mixtures with methanol as common component, namely water–methanol and methanol–acetone are studied by Molecular Dynamics simulations. Thermodynamical properties such as enthalpies, excess ...enthalpies, volumes and excess volumes are compared. Structural properties are studied through the various site–site pair correlation functions and the associated Kirkwood–Buff integrals. While the thermodynamical properties are relatively well calculated, we show that structural properties are affected by two severe problems. The first is an inherent property of the asymptote of the correlation function in finite systems, which affects their integrals, the second is the micro-heterogeneous structure of these hydrogen bonding mixtures, which affects the medium-to-long-range part of the distribution functions. These two problems conspire to make computer simulation unreliable to study this precise part of the structural properties of such mixtures. We propose pathways to correct this situation and demonstrate how it serves to considerably improve the calculated Kirkwood–Buff Integrals. Finally, we compare the two mixtures relative to their tendencies to form local heterogeneity. The analysis demonstrates that the micro-structure of neat methanol is better preserved in acetone than in water.
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 disks 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.
•Simple model of methanol is developed.•Model is based on Mercedes - Benz (MB) model of water.•Thermodynamics and structural properties of the model are studied.•Comparison was done with experimental results and with new simulations done for 3D models of methanol.
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.
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 behavior. 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.
•Molecular dynamics simulation of methanol-ethanol mixtures•Detailed Kirkwood-Buff Integral calculations to test ideality•Asymmetry of the ideality: statistics dependence•Integral-equation theory results with model bridge function
Substituting benzene for water in computer simulations of binary mixtures allows one to study the various forms of disorder, without the complications often encountered in aqueous mixtures. In ...particular, we study the relationship between the local order generated by different types of molecular interactions and the nature of the global disorder, by analyzing the relationship between the concentration fluctuations and the correlation functions and the associated structure factors. Alkane-benzene mixtures are very close to ideal mixtures, despite appreciable short range shape mismatch interactions, acetone-benzene mixtures appear as a good example of regular mixtures, and ethanol-benzene mixtures show large micro-segregation. In the latter case, we can unambiguously demonstrate, unlike in the case of water, the appearance of domain-domain correlations, both in the correlation functions and the structure factor calculated in computer simulations. This finding helps to confirm the existence of a pre-peak in the structure factor associated with the micro-heterogeneity, which was speculated from several of our previous simulations of aqueous-alcohol mixtures. The fact that benzene as a solvent allows us to solve some of the problems that could not be solved with water points towards some of the particularities of water as a solvent, which we discuss herein. The concept of molecular emulsion put forward in our earlier work is useful in formulating these differences between water and benzene through the analogy with direct and inverse micellar aggregates.
Substituting benzene for water in computer simulations of binary mixtures, allows one to study the various forms of disorder, without the complications often encountered in aqueous mixtures.
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