Elucidating Trivalent Ion Adsorption at Floating Carboxylic Acid Monolayers: Charge Reversal or Water Reorganization?

We study trivalent neodymium adsorption on floating arachidic acid films at the air/water interface by two complementary surface specific probes, sum frequency generation (SFG) spectroscopy and X-ray fluorescence near total reflection (XFNTR). In the absence of background ions, neodymium ions compensate the surface charge of the arachidic acid film at 50 M bulk concentration without any charge reversal. Increasing the bulk concentration to 1 mM does not change the neodymium surface coverage but affects the interfacial water structure significantly. In the presence of a high concentration of background salt, NaCl, there is overcharging at 1mM of Nd3+ i.e., 30% more Nd3+ than needed to compensate the surface charge. The overcharging trend with and without salts is different than those observed at mineral/water interface, emphasizing the importance of surface properties. These results show that the total coverage of neodymium ions is not enough to describe the complete picture at the interface, and interfacial water and ion coverage needs to be considered together to understand more complex ion adsorption and transport processes.