  Biphasic interfaces are complex but fascinating regimes that display a number
of properties distinct from those of the bulk. The CO$_2$-H$_2$O interface, in
particular, has been the subject of a number of studies on account of its
importance for the carbon life cycle as well as carbon capture and
sequestration schemes. Despite this attention, there remain a number of open
questions on the nature of the CO$_2$-H$_2$O interface, particularly concerning
the interfacial tension and phase behavior of CO$_2$ at the interface. In this
paper, we seek to address these ambiguities using ab initio-quality
simulations. Harnessing the benefits of machine-learned potentials and enhanced
statistical sampling methods, we present an ab initio-level description of the
CO$_2$-H$_2$O interface. Interfacial tensions are predicted from 1-500 bar and
found to be in close agreement with experiment at the pressures for which
experimental data is available. Structural analyses indicate the build-up of an
adsorbed, saturated CO$_2$ film forming at low pressure (20 bar) with
properties similar to those of the bulk liquid, but preferential perpendicular
alignment with respect to the interface. CO$_2$ monolayer build-up coincides
with a reduced structuring of water molecules close to the interface. This
study highlights the predictive nature of machine-learned potentials for
complex macroscopic properties of biphasic interfaces, and the mechanistic
insight obtained into carbon dioxide aggregation at the water interface is of
high relevance for geoscience, climate research, and materials science.
