Microstructure of nonideal methanol binary liquid mixtures

The nonideality of binary mixtures is often related to the nature of the interactions between both liquids and of the heterogeneity at the nanoscale-named microstructure. When one of the liquids is a hydrogen bonds former and the second is aprotic, the progressive diluting of the hydrogen-bonding network leads to a clustering and nanophases. By considering two mixtures, toluene-methanol and cyclohexane-methanol, the nonideality and its connection with the structure at the nanoscale and the intermolecular interactions are numerically investigated. Contrary to the toluene that is fully miscible in methanol, cyclohexane presents a high range of immiscibility which makes it a relevant system to study the nucleation (local segregation) and its propagation. In both mixtures, the deviation from the ideal behavior is observed. In the case of the toluene-methanol mixture, the initial hydrogen-bonding network corresponding to a homogenous structure is locally broken due to the favorable toluene-methanol interactions leading to the spatial heterogeneity at the origin of the nonideality. In the range of miscibility of the cyclohexane-methanol mixtures, the formation of hydrophobic nanophases of larger size is observed due to the unfavorable interactions between both components leading to a self-organizing of cyclohexane molecules. The immiscibility of cyclohexane and methanol are then correlated to the formation of nanophases and their propagation, which are also at the origin of the spatial heterogeneity. In the pure methanol, we highlight the disconnection between the clustering and the heterogeneity. We shed light on the fact that the prepeak observed in the structure factor is independent of the degree of heterogeneity, but is connected to the presence of cyclic clusters.

Figure 1

Simulated and experimental density (a) and the excess density (b) of the CHX-MeOH and TOL-MeOH mixtures as a function of $x_{\mathrm{MeOH}}$ at 300 K and 1 bar. The uncertainties about the density are too small to be represented. (c) Snapshots of binary mixture at $x_{\mathrm{MeOH}}=0.1$, 0.5, and 0.9 such that methanol is represented in red spacefill, and CHX is represented in stick cyan.

Figure 2

(a) Profiles of the total pressure along the $z$ direction for three methanol concentrations of the CHX-MeOH mixtures and for water at 1 bar and 300 K. (b) HOP as a function of the molar fraction in MeOH for both CHX-MeOH and TOL-MeOH mixtures.

Figure 3

Snapshot of the CHX-MeOH mixture at $x_{\mathrm{MeOH}}=0.2$ such that oxygen atoms and the methyl groups of the methanol are represented in red and black spacefill, respectively. CHX is represented in stick cyan.

Figure 4

Radial distribution functions between centers of mass of the aprotic component for both CHX-MeOH (a) and (b) TOL-MeOH mixtures. The vertical dashed line represents the position of the first peak.

Figure 5

Radial distribution functions between centers of mass of aprotic component and methanol for both CHX-MeOH (a) and TOL-MeOH (b) mixtures in the miscible regions. The dashed circles highlights the peaks located around 3.5 Å.

Figure 6

Radial distribution functions between the center of mass of the aprotic molecules and the hydrogen atoms of OH (HOC) and ${\mathrm{CH}}_3$ (${\mathrm{H}}_3\mathrm{C}$) groups of methanol for $x_{\mathrm{MeOH}}=0.9$.

Figure 7

(a) nHB per methanol molecule as a function of the methanol concentration. (b) KBIs as a function of the methanol concentration for both TOL-MeOH and CHX-MeOH mixtures. In panel (b) the right axis corresponds to the MeOH-MeOH KBI for the CHX-MeOH mixture.

Figure 8

Cluster size probability as a function of the MeOH concentration in logarithmic scale in both TOL-MeOH (a) and CHX-MeOH (b).

Figure 9

Illustrations of opened and closed clusters highlighted with the yellow solid lines. Red and cyan correspond to the oxygen atoms and methyl groups, respectively.

Figure 10

Cluster number as a function of the MeOH concentration for the dimers, trimers, tetramers, and pentamers in both TOL-MeOH (a) and CHX-MeOH (b). (c) Number of higher clusters for both TOL-MeOH and CHX-MeOH mixtures.

Figure 11

Total structure factor of the TOL-MeOH (a) and CHX-MeOH (b) mixtures as a function of methanol concentration.

Figure 12

Partial structure factor of the oxygen atom of the OH group of methanol, ethanol, and tert-butanol liquids at 1 bar and 300 K.