Analysis of the subcritical carbon dioxide-water interface

We follow the evolution of the H(2)O/CO(2) interface at 300 K from the low pressure limit to near-critical pressures in molecular dynamics simulations using the SPC water and EPM2 carbon dioxide models. The intrinsic structure of the interface is elucidated by accumulating density profiles relative to the fluctuating capillary wave surface. Our main finding is that a carbon dioxide film of increasing density and thickness grows in two stages at the interface while the structure of the water surface barely changes. At low density, the entire film density profile grows linearly with the bulk CO(2) density. This regime continues up to a bulk CO(2) density of roughly 0.00095 Å(-3). At pressures above this point, we observe a distinct second peak in the CO(2) density, along with a tail of excess density that decays exponentially with distance from the interface. The decay length of the exponential tail diverges with increasing CO(2) pressure according to an inverse power law decay. Over the entire range of pressures, the CO(2) film had no detectable effect on the orientational order of the water surface. As expected, when the film of excess CO(2) at the interface grows, we find that the surface tension drops with increasing pressure. This is in qualitative accord with existing measurements, although the rate at which the surface tension falls with increasing pressure according to the SPC and EPM2 models is too small, indicating that the surface excess of CO(2) is underestimated by these models.