Molecular Dynamics Simulation of the Water/2-Heptanone Liquid-Liquid Interface

Molecular dynamics simulations were performed to study the structural and dynamic properties of the water/2-heptanone (HPT2) liquid/liquid interface. It was found that HPT2 forms a bilayer structure at the interface, pointing its polar heads into the aqueous phase. Water molecules penetrate the hydrophilic head-group region but not the hydrophobic core. At the hydrophilic region water molecules establish hydrogen bonds with the ketone oxygen of the HPT2 molecule. Behind that zone, the water molecules show a preference in keeping their dipoles in the interfacial plane and these orientations remain in two or three molecular layers. The water dipole distribution is slightly asymmetric, having an average excess in the resulting component normal to the interfacial plane. The water dipoles point toward the aqueous phase for waters in the aqueous side of the interface and into the organic phase for water molecules in the organic side of the interface. The water structure remains almost unchanged at the Gibbs dividing surface. The HPT2 structure is not so robust, and near the interface it is distorted by the presence of the aqueous phase. Self diffusion exhibits long range anisotropy, diffusion toward the interface being slower than diffusion in the interfacial plane. The water orientational dynamics is slowed down near the interface. The HPT2 reorientation becomes anisotropic at the interface as reorientations perpendicular to the interface are slower than those in the interfacial plane. The interface was found to be sharp, highly corrugated, and broadened by capillary waves. I