Interfacial activity of nitric acid at the water supercritical CO2 interface:: a molecular dynamics investigation

We report a molecular dynamics study on a nitric acid water supercritical CO2 interface. Three extreme models are compared, where the acid is either all neutral HNO3, completely dissociated into NO3- and H3O+, or represented by a 1 : 1 mixture of both forms. The ionic species are found to be repelled by the neat interface, while the neutral HNO3 molecules are highly surface active. Similar features are observed with SC-CO2 or with chloroform as the organic phase, pointing to the generality of interfacial activity of the acid.