On the hydrogen bond networks in the water–methanol mixtures: topology, percolation and small-world

Abstract

Statistical mechanics based topological analysis and island (or cluster) statistics were used to study the hydrogen bond (H-bond) networks in the water-methanol mixtures with the following methanol mole fractions (x(m)): 0.00, 0.10, 0.20, 0.25, 0.28, 0.30, 0.32, 0.36, 0.38, 0.42, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00. NPT-Monte Carlo simulations were performed at room conditions using the TIP5P model potential for water and united-atoms (OPLS) for methanol to generate the H-bond networks. We have found evidence for non-ideal behavior of mixtures with x(m) ≈ 0.3. Several structural and topological properties present strong dependence with the mixture composition. Island statistics indicate a change from the percolated to non-percolate regime at x(m) ≈ 0.5. Statistical analysis of the islands' nature (homo-clusters: same type of molecules × hetero-clusters: two types of molecules) yields a preferential formation of homo-clusters that quantifies the local composition and preferential solvation ("microimmiscibility"). The topology of the hydrogen bond networks was characterized by local (clustering coefficients, average degrees), semi-global (path lengths) and global (spectral densities) properties. Small-world patterns (highly clustered and small path lengths) appear for x(m) in the range 0.40-0.70, and the momenta in the spectral densities correlate quite well with previous analysis based on rings, chains and branched chains topologies. It also seems that small quantities of methanol in water cause disruption of the continuous fully connected H-bond networks formed by water molecules.

A molecular dynamics study of the equation of state and the structure of supercooled aqueous solutions of methanol

“…57 This is a well-established model for methanol and it has been widely used in the literature on simulations of aqueous mixtures. [58][59][60][61][62][63] It reproduces well the thermodynamic experimental behavior of methanol. The OPLS force field also reproduces the solid-solid and solid-fluid equilibria of pure methanol.…”

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

“…The properties of solutions containing less than 0.2 mole fraction of water (i.e., viscosity and conductivity) were practically the same as those observed in the case of the anhydrous methanolic solutions ( Figure S3). The slight minimum observed of the viscosity curve, and the maximum observed in the conductivity curve after the addition of a small amount of water (x = 0.01), could be associated with the relaxation of the methanol structure [56]. The maximum strength of the mixed solvent that was observed in viscosity measurements occurred at a water content of x = 0.7, which is consistent with the literature [45,46,74,75].…”

“…Of particular interest is the concentration region of the water mole fraction in methanol between 0.4 and 0.7, where both components appear to form separate, percolating networks. This is the concentration range where many transport properties and thermodynamic excess functions reach extreme values [38,45,46,50,56,57].…”

Computational Vibrational Spectroscopy of HDO in Osmolyte–Water Solutions

“…These quantitative differences in the morphological structures of osmolyte-aggregates were noticed and emphasized with our spectral graph analysis method 33 that is a theoretical tool to probe the global changes in various composite systems. [61][62][63][64][65] In fact, these morphological differences between the protecting and destabilizing osmolytes are reflected in the experimentally measured data of the vibrational stretch mode and reorientation dynamics of HDO that provide information on the local water H-bonding network.…”