Theoretical examination of hexanol-water interfaces

The energetics, structure, and molecular interactions have been examined for model interfacial systems consisting of 1-hexanol/water mixtures in monolayer, bilayer, and double bilayer (biphasic) arrangements. Monte Carlo statistical mechanics simulations were carried out for the three systems in the NPT ensemble at 25°C and 1 atm. The computations predict minimal water penetration into the hydrophobic regions from the analyses of denisty profiles and radial distribution functions. However, water molecules are found to interact freely with the hydroxyl groups of hexanol in the region ca. 7-14 Å from the center of the water unit. Hydrogen-bonding analyses revealed that an average of ca. 2 water molecules are hydrogen bonding with each hexanol molecule in the monolayer, while ca. 1.6 are observed for the bilayer and biphase. The number of hydrogen bonds between the 1-hexanol pairs is ca. 1.0 in the aqueous interfacial region of the bilayer and biphasic systems and 0.8 in the monolayer. A significant increase in the population of trans conformations about bonds in the hexanol molecules is also found in the interfacial systems. The population shift helps facilitate the formation of hydrogen bonds in the interfacial regions and helps maintain the dense packing of the amphiphiles. Many of the structural findings are consistent with previous experimental and theoretical results that support the picture of relatively narrow interfaces with minimal water penetration beyond the head-group region in amphiphilic aggregates.