Molecular structure and hydrophobic solvation thermodynamics at an octane-water interface

We present results from atomically detailed molecular dynamics simulation of an octane-water liquid-liquid interface. We specifically focus on water structure, orientation, coordination numbers, and hydrogen bonding at the interface. In addition, we probe the interface through insertions of different nonpolar solutes at various locations in the system. Several interesting details of the interface emerge from our calculations. We find that the number density profiles of both water and octane vary monotonically through the interface in a sigmoidal fashion over approximately 1 nm 1-99 interfacial width. Interestingly, the overall heavy-atom density profile shows a distinct minimum in the interfacial region that reflects the hydrophobic nature of the hydration at the octane-water interface. Furthermore, calculations of excess chemical potentials of attractive Lennard-Jones and purely repulsive hydrophobic solutes display an interfacial minimum, indicating the relative ease of cavity formation at the interface. The inhomogeneous nature of the interface affects the water structure and hydrogen-bonding properties at the interface. We find that water coordination number as well as the number of hydrogen bonds water molecules make with their neighbors decreases through the interface as we move from bulk water to the octane phase. As a result, we find populations of water with low coordination numbers, including monomeric water species in the interfacial region. Although the number of hydrogen bonds per water is low in the interfacial region, a larger fraction of coordination waters is hydrogen bonded to the central water in the interfacial region. (C) 2003 American Institute of Physics.