The macroscopic phenomenology of hydrophobicity - the segregation of oil and water - is well studied. However, its theoretical understanding in a molecular level is still incomplete. Recent progress in X-ray diffraction allowed for the first time to determine the structure of the oil/water interfaces with atomic resolution, yielding information on the molecular-scale origin of the hydrophobic interaction. Although bare oil/water interfaces occur only rarely, these interfaces are often decorated by surfactants, which modify the hydrophobic interaction. Oil/water interfaces, modified by non-ionic, electrically neutral, alcohol surfactants have been already studied. However, molecular resolution measurements for the more complex electrically-charged ionic surfactants are not available in the literature. Thus, a key ingredient in the fundamental understanding of the relation between ionic surfactants and the hydrophobic interactions is still missing. Here, we employ X-ray diffraction and surface tension techniques in order to investigate the structure and thermodynamics of these surfactant decorated oil/water interfaces and their hydrophobic interactions.
