Quasi-static surface layer Bulk 1 of surface ion depletion. This assumption of surface ion depletion is similar to the interfacial partitioning of ions which has been extensively investigated and recognized in literature. For the case of sulfate salts, such as Na 2 SO 4 and (NH 4 ) 2 SO 4 , both cations and anions were found to be significantly depleted from the air-water interface, and the interface depth of such depletion could be in an order of sub nm.