Hydrophobic and hydrophilic interactions in aqueous mixtures of alcohols at a hydrophobic surface

Aqueous solutions of alcohols are interesting because of their anomalous behavior that is believedto be due to the molecular structuring of water and alcohol around each other in solution. The interfacialstructuring and properties are significant for application in alcohol purification processesand biomolecular structure. Here we study aqueous mixtures of short alcohols (methanol, ethanol,1-propanol, and 2-propanol) at a hydrophobic surface using interfacial statistical associating fluidtheory which is a perturbation density functional theory. The addition of a small amount of alcoholdecreases the interfacial tension of water drastically. This trend in interfacial tension can beexplained by the structure of water and alcohol next to the surface. The hydrophobic group of anadded alcohol preferentially goes to the surface preserving the structure of water in the bulk. Fora given bulk alcohol concentration, water mixed with the different alcohols has different interfacialtensions with propanol having a lower interfacial tension than methanol and ethanol. 2-propanol isnot as effective in decreasing the interfacial tension as 1-propanol because it partitions poorly tothe surface due to its larger excluded volume. But for a given surface alcohol mole fraction, all thealcohol mixtures give similar values for interfacial tension. For separation of alcohol from water,methods that take advantage of the high surface mole fraction of alcohol have advantages comparedto separation using the vapor in equilibrium with a water-alcohol liquid