Looking for the Azeotrope: A Computational Study of (Ethanol)₆–Water, (Methanol)₆–Water, (Ethanol)₇, and (Methanol)₇ Heptamers

Abstract

Considering that a molecular-level understanding of the azeotropic ethanol−water system can contribute to the search of new methodologies and/or modifications of industrial separation methods, this study tries to provide some clues to understand why azeotropes should be expected for ethanol, but not for methanol. Our exploration of the potential energy surface of (ethanol) 6 −water heteroheptamers, carried out at the B3LYP-D3/6-311++G(d,p) level, shows these heteroclusters to exhibit a cyclic structure where the cooperativity effects between the OH•••O HBs is a fundamental ingredient. An analysis of this cooperativity clearly indicates that ethanol−water systems will exhibit a similarly high stability as the heterocluster size approaches the azeotrope. However, a similar behavior should not be expected for the methanol-containing analogues. A comparison between (ethanol) 7 , (ethanol) 6 −water, (methanol) 7 , and (methanol) 6 −water shows the ethanol-containing systems to be significantly more stable than the methanol-containing analogues. This result is probably due to the fact that the OH•••O HBs are weaker than those found between ethanol molecules. However, our atoms in molecule (AIM) and noncovalent interaction (NCI) analyses unambiguously show that important contributors to the enhanced stability of the ethanol-containing clusters are the secondary van der Waals interactions between ethyl groups, which are not observed between methyl groups. Hence, while the formation of stable azeotropes is expected for the case of ethanol, for the methanol-containing analogues, the relative stability of the clusters is significantly smaller, and its formation is accompanied by an increase of the free energy.

Microwave heating of water–ethanol mixtures

“…Such splitting is in line with the suggestions by Bao et al [11] and Petong et al [12[, who recognized two main relaxation ranges, predicting some correspondence with peculiar molecular arrangements. This hypothesis finds a confirmation in a number of theoretical and experimental investigations [13][14][15][16] which indeed support the view that These results show a clear tendency: ethanol − water systems would exhibit a high stability, as the hetero-cluster size approaches the ratio of nine molecules of ethanol and one of water, namely the azeotropic composition. According to molecular dynamics simulations of liquid ethanol-water mixtures [16], hetero-decamers (EtOH) 9 .H 2 O would indeed appear at the azeotropic composition.…”

“…This means that the low DP value comes from a poor energy transfer toward the core and the surface of the sample, rather than from evaporation. Since such transfer is sustained by molecular displacements, the low DP value could be the result of a heavy molecular arrangement of azeotropic mixture, like that attained with the formation of (EtOH) 9 -H 2 O decamers [16].…”

“…These molecular clusters would be already present at room temperature, involving the whole liquid phase, and persist up to the boiling point that is the lowest of the EtOH-H 2 O binary, as they imply weak van der Waals interactions between ethyl groups [16] that are easily loosen on raising T. Such interactions are not possible for methanol: this would be why no azeotrope appears in the methanol-water binary [16]. For X EtOH > 0.90, the liquid binary would host decamers and more stable molecular arrangements.…”

“…Since the experimental approach used in the present work is much easier than a thorough spectroscopic [11,12] or theoretical [13][14][15][16] analysis, it may be of help in preliminary investigations on new materials, including novel food products [17], allowing a tentative ranking of their MW susceptibility.…”

A Theoretical Analysis of Interaction Energies and Intermolecular Interactions between Amphotericin B and Potential Bioconjugates Used in the Modification of Nanocarriers for Drug Delivery

“…On the other hand, it has been reported that H---H interactions are usually longer and weaker than C-H---O interactions in other types of clusters, such as those observed between ethanol or methanol molecules or clusters of those alcohols with water molecules [ 26 ]. For the (methanol) 5 -water system, the difference between the topological parameters shows that C-H---O has a magnitude greater than H---H interactions, indicating that interactions such as C-H---O are of great importance in the formation of these clusters [ 27 ].…”