Interfacial Behavior of Perchlorate versus Chloride Ions in Aqueous Solutions

In recent years, theoretical as well as experimental studies have presented a novel view of the aqueous interface, wherein hard and/or multiply charged ions are excluded from the interface but large polarizable anions show interfacial enhancement relative to the bulk. The observed trend in the propensity of anions to adsorb at the air/water interface appears to follow an inverse order of the Hofmeister series for anions. This study focuses on experimental and theoretical examination of the partitioning behavior of perchlorate (ClO4−) and chloride (Cl−) ions at the air/water interface. We have used ambient pressure X-ray photoelectron spectroscopy to directly probe the interfacial concentrations of ClO4− and Cl− ions in sodium perchlorate and sodium chloride solutions, respectively. In the case of ClO4− ion, experimental observations are compared with molecular dynamics simulations utilizing both first principles based interaction potentials as well as polarizable classical force fields. Both the experimental and the theoretical results show enhancement of ClO4− ion at the interface, compared with the absence of such enhancement in the case of the Cl− ion. Our observations are in agreement with the expected trend in the interfacial propensity of anions based on the Hofmeister series.