Contribution of the two liquid phases to the interfacial tension at various water-organic liquid-liquid interfaces

Abstract

The interfacial tension at liquid-liquid interfaces emerges ultimately from the uneven interaction between its two components. In simple cases, where one of the two components is polar, and the other is not, it is possible to employ empirical relations, such as the Antonov rule, to estimate, to some extent, the interfacial tension of the binary fluid from those of the liquid/vapour interfaces of its components. However, these empirical rules are only approximate and fail as soon as some degree of mixing of the two components is present. Here, we use atomistic molecular dynamics simulation to access the contribution of each species (as well as its distribution across the interface) to the interfacial tension at the interface of five different organic liquids, i.e., hexane, cyclohexane, hexanol, dichloromethane, and carbon tetrachloride, with water. Our results reveal that the organic component contributes 20–30% to the total interfacial tension, and this result is independent from the temperature, pressure, water model used, and also from the type of the organic molecule as long as it does not interact strongly and, consequently, does not mix in a considerable extent with water. Among the chosen organic liquids, hexanol is the only one that exhibits partial miscibility with water to an extent accessible by computer simulation, due to the possible hydrogen bonding between the water and hexanol molecules. Here we show how this partial miscibility, leading to the complete breakdown of the Antonov rule, is associated to a negative contribution of the hexanol molecules, and also that of the hexanol-rich mixed phase, to the total interfacial tension, consistent with the tendency of the hexanol molecules to mix with water.