XFNTR measurements A DHDP monolayer was formed in a Langmuir trough by spreading a DHDPchloroform solution on the surface of aqueous solutions containing NdCl3 (or GdCl3) and DTPA, then compressed by a barrier to maintain a constant surface pressure of 10 mN/m. The trough vapor was purged of oxygen for roughly a half hour prior to measurements by flowing helium that had passed through an aqueous bubbler into the trough. Samples were exposed by reflecting 10 keV X-rays at small incident angles to achieve wave vector transfers of either Qz = 0.018 Å -1 or Qz = 0.025 Å -1 , where 𝑄 # = (4𝜋/𝜆) sin 𝜃 for angle of incidence 𝜃 and X-ray wavelength 𝜆 . The X-ray fluorescence spectrum was recorded by a Vortex60-EX silicon drift energy dispersive detector placed perpendicularly above the surface. The X-ray fluorescence spectrum was normalized to the incident X-ray beam intensity and corrected for detector dead time. After normalization, the L⍺1 X-ray fluorescence peak (Nd: 5255 eV; Gd: 6053 eV) was fit to a Gaussian function and a linear background to calculate the peak area, which provides the fluorescence signal.