From Hydrophobic to Hydrophilic: The Structure and Density of the Hexadecane Droplet/Alkanol/Water Interface

The molecular structure of the hexadecane droplet/alkanol/water interface has been investigated using vibrational sum frequency scattering and second harmonic scattering. This combination of methods allows us to investigate the interfacial alkyl chain conformation of the oil and several alkanols, ranging from 1-pentanol to 1-dodecanol, the orientational distribution of the methyl groups, the surface density of the alkanols, as well as the orientational alignment of water. For the hexadecane/1-dodecanol/water interface, dodecanol alkyl chains form a fluid layer with a wide distribution of tilt angles of the terminal CH3 groups. Indistinguishable spectra are recorded for the alkanol alkyl chains of 1-pentanol, 1-hexanol, 1-octanol, and 1-dodecanol, and alkanols with chain length longer than 6 C atoms all form films with similar densities. In contrast, the alkyl chains of the oil phase are relatively more distorted with respect to the pure oil/water interface for alkanols with shorter chain lengths. The projected surface area of a saturated film of hexanol is 29 +/- 5 angstrom(2), which requires a free energy of adsorption of Delta G = -26.3 +/- 0.7 kJ/mol. In addition, with increasing alkanol density the interfacial water structure loses its initial orientational alignment, which matches with the added number of interfacial 1-hexanol molecules. The found structures differ significantly from those reported on the alkanol/water and alkanol/air interface and charged surfactant/oil/water interface.