Temperature-Dependent Segregation in Alcohol–Water Binary Mixtures Is Driven by Water Clustering

Abstract

Previous neutron scattering work, combined with computer simulated structure analysis, has established that binary mixtures of methanol and water partially segregate into water-rich and alcohol-rich components. It has furthermore been noted that, between methanol mole fractions of 0.27 and 0.54, both components, water and methanol, simultaneously form percolating clusters. This partial segregation is enhanced with decreasing temperature. The mole fraction of 0.27 also corresponds to the point of maximum excess entropy for ethanol-water mixtures. Here, we study the degree of molecular segregation in aqueous ethanol solutions at a mole fraction of 0.27 and compare it with that in methanol-water solutions at the same concentration. Structural information is extracted for these solutions using neutron diffraction coupled with empirical potential structure refinement. We show that ethanol, like methanol, bi-percolates at this concentration and that, in a similar manner to methanol, alcohol segregation, as measured by the proximity of neighboring methyl sidechains, is increased upon cooling the solution. Water clustering is found to be significantly enhanced in both alcohol solutions compared to the water clustering that occurs for random, hard sphere-like, mixing with no hydrogen bonds between molecules. Alcohol clustering via the hydrophobic groups is, on the other hand, only slightly sensitive to the water hydrogen bond network. These results support the idea that it is the water clustering that drives the partial segregation of the two components, and hence the observed excess entropy of mixing.

Ab Initio Molecular Dynamics Study of Methanol-Water Mixtures under External Electric Fields

“…Because of such an amphiphilic nature, the microscopic structures of hydrated methanol in liquid water have attracted noticeable attention. Several experimental approaches have shown that the anomalous thermodynamics of alcohol-water systems arises mainly from the incomplete mixing at the molecular level, which exhibits partially segregated microstructures, both into water-rich and alcohol-rich components depending on the relative concentrations [4][5][6][7][8][9][10][11][12]. In order to supplement or corroborate the experimental results, many computer simulation studies have been performed at different levels of sophistication.…”

Linking excess order to co-solvent aggregation in solvent mixtures

“…Overall, from this geometrical analysis, it can be envisaged the presence of two competing effects at play in the solvent mixture: (i) the requirement to satisfy all of the hydrogen bonds available sites, a job that is mainly left to water molecules, that are small and flexible as they form slightly longer HBs among themselves and with the solutes and (ii) the requirement to accommodate and increasing number of very weakly interacting polar groups (so weakly, that in the scale that we can detect with the combination of neutrons and EPSR simulations, it doesn't make any difference if charges are present or not 7 ).…”

“…5 The problem of the causes of conon-solvency therefore intersects and has implications for the long-standing debate on the hydrophobic effect and the observed excess order in alcohol/water mixtures. 6 In these 17 years of publications and particularly in a very recent re-visitation of the topic, 7 the full power of combining neutron diffraction with isotopic substitution (NDIS) 8 and a simulation 2 method developed specifically for the interpretation of total scattering data from molecular liquids, the empirical potential structure refinement (EPSR) 9 has been exploited. The main findings can be summarised as:…”

“…1. there is no sign of a water structure enhancement (in the form of straighter, shorter hydrogen bonds) around the hydrophobic groups of the alcohol in the alcohol water/mixture; 5 2. there is an approximate concentration range (0.27-0.54 MeOH molar fraction x m ) where the solvent mixture undergoes a quite dramatic change in its structure, which becomes a bi-percolating mixture; 10 3. outside of these range, only one of the two components is percolating and the other components forms small clusters that are not interconnected; 11 4. the alcohol/alcohol structuring is mainly sterically driven (it is not affected by the removal of Coulombic interaction between the molecules in the EPSR simulation); 7 5. the water/water interaction is affected by the removal of Coulombic interaction in the EPSR simulation; 7 6. the strength of hydrogens bonds among water molecules (and not around the hydrophobic groups) is therefore enhanced at certain concentrations in the alcohol water mixture; 11 With the present study, the powerful combination of neutron diffraction with isotopic substitution and EPSR simulation has been applied to studying the aggregation and solvation of an amphiphilic compound (TBA) in a water/methanol mixtures at various concentrations. ).…”

Diffusion in Binary Aqueous Solutions of Alcohols by Molecular Simulation

“…In this work, the influence of structural characteristics and hydrogen bonds on kinetic characteristics will not be revealed. A similar analysis with the use of the all-atom potential for water-alcohol rats was carried out in [28][29][30][31][32][33][34][35][36][37].…”