Molecular structure of the water-supercritical CO2 interface

We report the results on the structure of the binary dense CO2-water interface at 20 MPa and 318 and 338 K and 28 MPa and 318 K, as investigated by molecular dynamics Computer simulations. Realistic potential models are used to describe the interactions, and the Ewald summation technique is employed to account for the long range electrostatic interactions. It is shown that the interface is molecularly sharp with distortions from a flat interface due to the presence of capillary waves induced by thermal fluctuations. The use of a local dynamic interface definition(1) provides a revealing density profile in which interfacial packing of fluids on both sides of the interface is observed. Atomic radial distribution functions, orientational probability distribution functions, and H-bond analysis are used to probe the nature of the bulk to interface transition. Specific attractive interactions between CO2 and water due to Coulombic interactions are evident. The interfacial tension is determined from the pressure tensor analysis and from capillary wave theory, and the results are compared to the experimental values obtained in our laboratories.