Thermodynamic properties and interfacial tension of a model water-carbon dioxide system

We performed molecular dynamics (MD) simulations of liquid-liquid and liquid-vapor interfaces between bulk water and carbon dioxide. Interfacial systems, constructed from periodically replicated slabs, were studied at different pressures and temperatures by means of npT and nVT MD. Constant-pressure runs of 3, 1.2, and 0.3 nanoseconds were used to estimate the water-CO2 interfacial tension at 284.5 K and 298 K for a liquid-liquid system comprising 108 SPC water molecules and 108 three-site CO2 molecules. The liquid-vapor interface was studied under nVT conditions using 108 water molecules and 32 CO2 molecules. Interfacial tension was obtained from the difference between pressure components normal and tangential to the interface. The results showed a surprisingly (CO2 potential used has been never optimized for water-CO2 interaction) good agreement with experimental data; our model system also reproduced the pressure temperature relationship of the interfacial tension. A second liquid liquid system of 256 SPC water and 108 CO2 molecules was tested for temperature persistence of the interface at higher pressures (100 atm and 300 atm). The results of the simulation prove the feasibility of using the model system to predict the key properties of liquid-liquid water carbon dioxide interface under widely varying conditions, including those relevant for deep-sea disposal of carbon dioxide.