The effect of cycling X-ray exposure on the surface adsorption of Nd 3+ . A monolayer of DHDP is held at a surface pressure of 10 mN/m on the surface of water containing 30 μM DTPA, which is a chelating agent, and 10 μM NdCl3. (A) DHDP (dihexadecyl phosphoric acid) and DTPA (diethylenetriamine pentaacetic acid). (B) Illustration of the X-rays-on steady state with a high surface density of ions. Ions are colored magenta, DHDP monolayer at the surface with red phosphoric acid headgroups and black alkyl chains, and DTPA in the bulk (black cages). Red background indicates that X-rays expose the surface and bulk. (C) Illustration of the X-rays-off steady state with a low surface density of ions and most ions bound to DTPA in the bulk aqueous solution shown in green. (D, E) Temporal evolution of the Nd surface density measured for three cycles at aqueous pH of (D) 3.0 and (E) 4.5. Each cycle consists of an X-ray exposure ("X-rays on") and subsequent recovery ("X-rays off"). X-rays-on refers to continuous exposure of the sample to X-rays with ongoing XFNTR measurements of 5 seconds duration. X-rays-off refers to a 5-second XFNTR measurement every 5 minutes without X-ray exposure between measurements. Times marked "0" indicate when the X-rays-on or X-rays-off step started. The color strip at the bottom of each panel also identifies the X-ray exposure (red for on, green for off). X-ray exposures and measurements were made at Qz = 0.025 Å -1 , which exposes both the surface and the bulk aqueous phase within a few micrometers of the surface. Lines are fits described later in the text.