Composition dependence of thermodynamic, dynamic and dielectric properties of water–methanol model mixtures. Molecular dynamics simulation results with the OPLS-AA model for methanol

Radial distribution and hydrogen bonded network graphs of alcohol-aniline binary mixture

CONTEXT

Hydrogen bonds play a vital role in the stability and functioning of biomolecules. Suitable binary liquids are often used as prototypes for the study of biologically significant hydrogen bond studies and their intricate networks. Often, such systems show deviations in their physico-chemical properties from ideal conditions. As a continuation of our research interest in biologically important hydrogen-bonded systems, this paper reports the classical molecular dynamic studies on mixtures of aniline with 8 primary alcohols (C_RH_2R+1-OH, R = 1 to 8) for the complete concentration range. The energetics results indicate the predominance of OH--O interactions over other hydrogen bonds. Structures in the network are analyzed using radial distribution function (RDF), hydrogen bond statistics, and graph theoretical analysis (GTA). Coordination numbers, hydrogen bond statistics, and GTA show a bunching of alcohol-alcohol hydrogen bonds for lower aniline concentrations, while the aniline-aniline interactions are not affected by changes in the concentration.

METHODS

Interaction energies are calculated using B3LYP/6-311G++(d, p) density functional theory using Gaussian-09. The molecular dynamics simulations are carried out using GROMACS (V 2020.6) with the OPLS/AA force field and the simulation box is visualized using VMD. The NetworkX Python package is used for GTA calculation.

Partition Constant of Binary Mixtures for the Equilibrium between a Bulk and a Confined Phase

It is well-known that the thermodynamic, kinetic and structural properties of fluids, and in particular of water and its solutions, can be drastically affected in nanospaces. A possible consequence of nanoscale confinement of a solution is the partial segregation of its components. Thereby, confinement in nanoporous materials (NPM) has been proposed as a means for the separation of mixtures. In fact, separation science can take great advantage of NPM due to the tunability of their properties as a function of nanostructure, morphology, pore size, and surface chemistry. Alcohol-water mixtures are in this context among the most relevant systems. However, a quantitative thermodynamic description allowing for the prediction of the segregation capabilities as a function of the material-solution characteristics is missing. In the present study we attempt to fill this vacancy, by contributing a thermodynamic treatment for the calculation of the partition coefficient in confinement. Combining the multilayer adsorption model for binary mixtures with the Young equation, we conclude that the liquid-vapor surface tension and the contact angle of the pure substances can be used to predict the separation ability of a particular material for a given mixture to a semiquantitative extent. Moreover, we develop a Kelvin-type equation that relates the partition coefficient to the radius of the pore, the contact angle, and the liquid-vapor surface tensions of the constituents. To assess the validity of our thermodynamic formulation, coarse grained molecular dynamics simulations were performed on models of alcohol-water mixtures confined in cylindrical pores. To this end, a coarse-grained amphiphilic molecule was parametrized to be used in conjunction with the mW potential for water. This amphiphilic model reproduces some of the properties of methanol such as enthalpy of vaporization and liquid-vapor surface tension, and the minimum of the excess enthalpy for the aqueous solution. The partition coefficient turns out to be highly dependent on the molar fraction, on the interaction between the components and the confining matrix, and on the radius of the pore. A remarkable agreement between the theory and the simulations is found for pores of radius larger than 15 Å.

The liquidus temperature curve of aqueous methanol mixtures: a numerical simulation study

The liquidus temperature curve that characterizes the boundary between the liquid methanol/water mixture and its coexistence with ice Ih is determined using the direct-coexistence method. Several methanol concentrations and pressures of 0.1 MPa, 50 MPa, and 100 MPa are considered. In this study, we used the TIP4P/Ice model for water and two different models for methanol: OPLS and OPLS/2016, using the geometric rule for the Lennard-Jones cross interactions. We compared our simulation results with available experimental data and found that this combination of models reproduces reasonably well the liquidus curve for methanol mole fractions up to xm=0.3 at p=0.1 MPa. The freezing point depression of these mixtures is calculated and compared to experimental results. We also analyzed the effect of pressure on the liquidus curve, and we found that both models also reproduce qualitatively well the experimental decreasing of the liquidus temperatures as the pressure increases.

Activity Coefficients and Solubilities of NaCl in Water-Methanol Solutions from Molecular Dynamics Simulations

We obtain activity coefficients and solubilities of NaCl in water-methanol solutions at 298.15 K and 1 bar from molecular dynamics (MD) simulations with the Joung-Cheatham, SPC/E, and OPLS-AA force fields for NaCl, water, and methanol, respectively. The Lorentz-Berthelot combining rules were adopted for the unlike-pair interactions of Na⁺, Cl^-, and the oxygen site in SPC/E water, and geometric combining rules were utilized for the remainder of the cross interactions. We found that the selection of appropriate combining rules is important in obtaining physically realistic solubilities. The solvent compositions studied range from pure water to pure methanol. Several salt concentrations were investigated at each solvent composition, from the lowest concentrations permitted by the system size used up to the experimental solubilities. We first calculated individual ion activity coefficients (IIACs) for Na⁺ and Cl^- from the free energy change due to the gradual insertion of a single cation or anion into the solution, accompanied by a neutralizing background. We obtained the salt solubilities by comparing the chemical potentials in solution with solid NaCl chemical potentials calculated previously using the Einstein crystal method. Mean ionic activity coefficients obtained from the IIACs are in reasonable agreement with experimental data, with deviations increasing for solutions of higher methanol content. Predictions for the salt solubility are in surprisingly good agreement with experimental data, despite well-known challenges in the simultaneous calculation of activity coefficients and solubilities with classical MD simulations. The present study demonstrates that good predictions for these two important phase equilibrium properties can be obtained for mixed-solvent electrolyte solutions using existing nonpolarizable models and further suggests that the previously proposed single ion insertion technique can be extended to complex mixed-solvent solutions as well.

Structure and dynamics of a water/methanol mixture confined in zeolitic imidazolate framework ZIF-8 from atomistic simulations

A classical atomistic simulation study is reported for the microscopic structure and dynamics of a water/methanol mixture confined in flexible nanoporous zeolitic imidazolate framework ZIF-8. Both the radial density distribution and vivid two-dimensional density profile demonstrate that methanol molecules can roughly be viewed as "embedded" between two layers of water molecules to form a "sandwich" structure. The reason for the formation of such a specific structure is explained based on the hydrogen-bonding state and the strength of various hydrogen bonds. The investigation of guest molecular diffusion shows that the self-diffusion coefficient of confined water is generally one to two orders of magnitude smaller than that of bulk water. In addition, the dependence of the self-diffusion coefficient on loading is non-monotonic: the self-diffusion coefficient firstly shows a significant increase and then decreases at higher loading. Moreover, both the structure and dynamics of the hydrogen bond (HB) network of confined water molecules are investigated in a spatially resolved manner. The results indicate that both the HB structure and dynamics of water molecules near the ZIF-8 surface deviate significantly from those of bulk water. However, while water molecules located at the pore center are relatively similar to bulk water molecules with respect to the HB structure, they exhibit strong slowdown in HB dynamics when compared with bulk water. This simulation study elucidates in detail the structural and dynamical properties of a water/methanol mixture in nanoscopic ZIF-8 confinement, which is expected to provide a deep insight into the role of porous fillers, such as ZIF-8, in improving the performance of the dehydration of alcohols via pervaporation and other related processes.

Molecular-scale origins of solution nanostructure and excess thermodynamic properties in a water/amphiphile mixture

The molecular and nanoscale origins of nonideality in excess thermodynamic properties are essential to understanding cosolvent mixtures, yet they remain challenging to determine. Here, we consider a binary mixture of water and an amphiphile, N,N,N',N'-tetramethylmalonamide (TMMA), which is characterized by strong hydrogen bonding between the two components and no hydrogen bonding between amphiphiles. Using molecular dynamics simulation, validated with excess volume measurements and X-ray scattering, we identify three distinct solution regimes across the composition range of the binary mixture and find that the transition between two of these regimes, marked by the water percolation threshold, is closely correlated with minima in the excess volume and excess enthalpy. Structural analysis of the simulations reveals an interplay between local interactions and solution nanostructure, determined by the relative strength of the water-water and water-amphiphile hydrogen bonding interactions. By comparison with other amphiphiles, such as linear alcohols, the relative strength of like and unlike interactions between water and amphiphile affects the relationship between thermodynamics and structural regimes. This provides insight into how molecular forces of mutual solvation interact across length scales and how they manifest in excess thermodynamic properties.

Pressure-Dependent Structure of Methanol-Water Mixtures up to 1.2 GPa: Neutron Diffraction Experiments and Molecular Dynamics Simulations

Total scattering structure factors of per-deuterated methanol and heavy water, CD₃OD and D₂O, have been determined across the entire composition range as a function of pressure up to 1.2 GPa, by neutron diffraction. The largest variations due to increasing pressure were observed below a scattering variable value of 5 Å⁻¹, mostly as shifts in terms of the positions of the first and second maxima. Molecular dynamics computer simulations, using combinations of all-atom potentials for methanol and various water force fields, were conducted at the experimental pressures with the aim of interpreting neutron diffraction results. The peak-position shifts mentioned above could be qualitatively reproduced by simulations, although in terms of peak intensities, the accord between neutron diffraction and molecular dynamics was much less satisfactory. However, bearing in mind that increasing pressure must have a profound effect on repulsive forces between neighboring molecules, the agreement between experiment and computer simulation can certainly be termed as satisfactory. In order to reveal the influence of changing pressure on local intermolecular structure in these “simplest of complex” hydrogen-bonded liquid mixtures, simulated structures were analyzed in terms of hydrogen bond-related partial radial distribution functions and size distributions of hydrogen-bonded cyclic entities. Distinct differences between pressure-dependent structures of water-rich and methanol-rich composition regions were revealed.

Molecular Clustering in Formaldehyde-Methanol-Water Mixtures Revealed by High-Intensity, High-q Small-Angle Neutron Scattering

Methanol-Water (mw) mixtures, with or without a solute, display a nonideal thermodynamic behavior, typically attributed to the structure of the microphase. However, experimental observation of the microphase structures at the molecular length scale has been a challenge. We report the presence of molecular clusters in mw and formaldehyde-methanol-water (fmw) mixtures using small-angle neutron scattering (SANS) experiments and molecular dynamics (MD) simulations. Hydrophobic clusters of methanol in mw and formaldehyde-methanol in fmw mixtures were observed at low methanol compositions (x_m ≤ 0.3). A three-dimensional hydrogen-bonded network of water with the solute is observed at x_m = 0.5. Linear chains of methanol surrounding the formaldehyde and water molecules were observed at high methanol compositions (x_m ≥ 0.7). The calculated size of the molecular clusters (r ≈ 0.5 nm, spherical) from the SANS data and their volume fraction closely matched the MD simulation results.

The Effect of the Methanol–Water Interaction on the Surface Layer on Titanium in CH3OH-H2O-LiClO4 Solutions

The purpose of this study was to explain the mechanism of formation and to examine the composition of the anodic film formed on the surface of titanium in an anhydrous neutral methanol solution of electrolytes. In an environment deprived of water molecules, the growth of a 3D-phase titanium oxide layer is not possible. Electrochemical investigations demonstrated that the Ti surface in CH3OH-LiClO4 solutions experienced a pseudo-passivation with the formation of a methoxy layer, which resulted from the reaction of the metal surface with alcohol molecules. The presence of this methoxy surface film was confirmed through XPS and in situ FTIR measurements. The layer blocked the Ti anodic dissolution at the potential range corresponding to the stability of methanol and methoxy ions (i.e., <0.55 V). At potentials over 0.55 V, the methoxy layer was oxidised, which caused the “depassivation” of the metal surface and the etching of titanium. The addition of water changed the properties of Ti in CH3OH-LiClO4 solutions, but only with a water content above 0.2 mole fraction. Below this concentration of water, titanium behaved like it would in an anhydrous solution of methanol. In the range of water concentration of 0.2 to 0.7 mole fraction, the structure of the solution is strengthened because both components of the solvent formed separate percolating networks. The strengthening of the solution structure resulted in a strengthening of the surface layer of Ti(OH)m(OCH3)n. Such a layer had strong barrier properties similar to the properties of an organic polymer film. The formation and growth of a stable layer of TiO2 were possible only in a solvent when the water concentration was higher than ≈0.7 mole fraction.

Polarization Effects in Simulations of Kaolinite-Water Interfaces

Computational models of clay minerals and their interactions with the surrounding medium are highly valuable to study adsorption processes at an atomistic resolution, which may be relevant in different areas such as chromatography, environmental chemistry, and so forth. In this work, we analyzed the effect of the treatment of long-range interactions on the polarization of kaolinite-water interfacial systems in terms of structural, electric and dynamic properties, and hydrogen bonds. When using conventional three-dimensional (3D) Ewald summation, water molecules were more structured on the alumina interface of the kaolinite compared to simulations, in which the periodicity perpendicular to the plane was effectively removed. By applying an external electric field to the latter simulations, we were able to reproduce results obtained with the 3D Ewald summation. We proved that the induced polarization promotes adsorption processes of polar and charged species from aqueous solutions using a glyphosate molecule (widely used herbicide) as a particular example. The polarization phenomena raised from the simulation setup should be accounted for carefully in simulations of adsorption processes, which involve periodic images of polar interfaces. An inappropriate treatment of long-range electrostatic interactions can easily lead to artifacts and/or erroneous results.

New Force Field Model for Propylene Glycol: Insight to Local Structure and Dynamics

In this work we developed a new force field model (FFM) for propylene glycol (PG) based on the OPLS all-atom potential. The OPLS potential was refined using quantum chemical calculations, taking into account the densities and self-diffusion coefficients. The validation of this new FFM was carried out based on a wide range of physicochemical properties, such as density, enthalpy of vaporization, self-diffusion coefficients, isothermal compressibility, surface tension, and shear viscosity. The molecular dynamics (MD) simulations were performed over a large range of temperatures (293.15-373.15 K). The comparison with other force field models, such as OPLS, CHARMM27, and GAFF, revealed a large improvement of the results, allowing a better agreement with experimental data. Specific structural properties (radial distribution functions, hydrogen bonding and spatial distribution functions) were then analyzed in order to support the adequacy of the proposed FFM. Pure propylene glycol forms a continuous phase, displaying no microstructures. It is shown that the developed FFM gives rise to suitable results not only for pure propylene glycol but also for mixtures by testing its behavior for a 50 mol % aqueous propylene glycol solution. Furthermore, it is demonstrated that the addition of water to the PG phase produces a homogeneous solution and that the hydration interactions prevail over the propylene glycol self-association interactions.

Decreasing temperature enhances the formation of sixfold hydrogen bonded rings in water-rich water-methanol mixtures

The evolution of the structure of liquid water-methanol mixtures as a function of temperature has been studied by molecular dynamics simulations, with a focus on hydrogen bonding. The combination of the OPLS-AA (all atom) potential model of methanol and the widely used SPC/E water model has provided excellent agreement with measured X-ray diffraction data over the temperature range between 298 and 213 K, for mixtures with methanol molar fractions of 0.2, 0.3 and 0.4. Hydrogen bonds (HB-s) have been identified via a combined geometric/energetic, as well as via a purely geometric definition. The number of recognizable hydrogen bonded ring structures in some cases doubles while lowering the temperature from 298 to 213 K; the number of sixfold rings increases most significantly. An evolution towards the structure of hexagonal ice, that contains only sixfold hydrogen bonded rings, has thus been detected on cooling water-methanol mixtures.