Understanding alcohol aggregates and the water hydrogen bond network towards miscibility in alcohol solutions: graph theoretical analysis

Under ambient conditions, methanol and ethanol are miscible in water at all concentrations, while n-butanol is partially miscible. This is the first study to quantitatively examine the miscibility of butanol and compare with miscible alcohols by employing molecular dynamics simulations and graph theoretical analysis of three water-alcohol mixtures at various concentrations. We show how distinct alcohol aggregates are formed, thereby affecting the water structure, which established the relationship between the morphological structure of the aggregates and the miscibility of the alcohol in aqueous solution. The aggregates of methanol and ethanol in highly concentrated solutions form an extended H-bond network that intertwines well with the H-bond network of water. n-Butanol tends to self-associate and form large aggregates, while such aggregates are segregated from water. Graph theoretical analysis revealed that the alcohol aggregates of methanol and ethanol solutions have a morphological structure different from that of n-butanol, although there is no significant difference in morphology between the three pure alcohols. These two distinct alcohol aggregates are classified as water-compatible and water-incompatible depending upon their interaction with the water H-bond network, and their effect on the water structure was investigated. Our study reveals that the water-compatible network of alcohol aggregates in methanol and ethanol solutions disrupts the water H-bond networks, while the water-incompatible network of n-butanol aggregates does not considerably alter the water structure, which is consistent with the experimental results. Furthermore, we propose that miscible alcohols form water-compatible networks in binary aqueous systems while partially miscible alcohols form water-incompatible networks. The bifurcating hypothesis on the alcohol aggregation behavior in liquid water is of critical use to understand the fundamental issues such as solubility and phase separation in solution systems.