Effect of X-ray exposure on DHDP and DTPA The measurements in Fig. /E can be explained as the result of two aspects of X-ray exposure: unbinding of ions from the DHDP monolayer and unbinding of ions from DTPA. XFNTR measurements of the temporal evolution of the Nd surface density from samples without DTPA in the aqueous phase are shown in Figure . These measurements demonstrate that the surface density of Nd bound to the DHDP monolayer decreases with time when the sample is continuously exposed to X-rays. It is important to note that other authors have used XFNTR to measure stable surface densities of lanthanide and other ions bound to DHDP monolayers on the surface of water and we also present evidence of stable measurements in the SI (Fig. ). Stable measurements can be obtained by introducing absorber foils into the X-ray beam to reduce the X-ray flux below a critical value. The measurements in this paper used X-ray beam fluxes of 3 x 10 10 photons/s which were roughly four times this critical value to achieve the measured effects (Section S2).