Elucidating Trivalent Ion Adsorption at Floating Carboxylic Acid Monolayers*

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). Neodymium ions compensate the surface charge of the arachidic acid without any charge reversal. However, the interfacial water goes under significant reorganization, even after the charge compensation. Controlling the Debye length with excess background salt, we reconcile the apparent contradictions between X-ray and surface spectroscopy experiments. 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.