Interfacial properties of aqueous solutions of butanol isomers and cyclohexane

Phase behavior at water-oil and water-air interfaces has received much attention in the past sixty years. The study of detergents as surfactants and their applications to industry is widely known and under constant study. Other molecules, such as alcohols, can also act like surfactants between water and nonpolar molecules. Hydrotropes are amphiphilic molecules, with both a hydrophobic and a hydrophilic section, that also reduce the interfacial tension between two liquids or a vapor and a liquid. Tert-butyl alcohol (TBA) is a well-studied hydrotrope, but its behavior is not completely understood at low molar compositions in aqueous solution. In addition, few studies have examined the influence of different butanol isomers on the interfacial dynamics of mixtures. We used molecular dynamics to study the behavior of all four butanol isomers at the interface of water and cyclohexane. We observed a rapid decrease in surface tension at very low hydrotrope concentrations, < 1 mol% in water, in agreement with previous experimental and simulation studies. The effect of different butanol isomers on the interfacial properties between an aqueous and an organic phase was examined looking at its dependence on hydrotrope concentration and chemical structure. We characterized the position of the hydrotrope molecules at the interface using atom number density profiles, radial distribution functions, and hydroxyl tilt angles with respect to the z-axis (perpendicular to the water-cyclohexane interface). There is statistical difference on the effect of some isomers on the disruption of the interface at low hydrotrope concentrations. Our studies show the chemical structure of these molecules affects the water-oil interface in different ways, which could potentially affect the use of one isomer over the other in different applications.