# # arev:/usr/local/unicat/unireq/beamreq/req00108.txt # UNICAT Member Beam Time Request #108 # created Thu Mar 30 11:40:50 CST 2000 # collaboration: Yes days: 1 description: The broad goal of our SAXS research is to obtain an accurate description of the potential distribution at the particle-water interfaces in environmental systems. This information is essential because the electrostatic repulsion (and thus the fate of environmental particles in natural systems) arises from interactions of two overlapping electric double layers when particles encounter one another. We seek to describe the interface in terms of counter- ion distribution and morphology of macromolecules near the mineral surface. The success of this ambitious goal hinges on our ability to measure the thickness and charge density of the condensed counterion layer around a charged particle in water. However, these measurement have, to the best of our knowledge, never been attempted in a system of charged mineral particles in an aqueous solution. The focused objective of the initial study proposed here is to detect and measure the thickness of a condensed layer of Tl+1 counterions around an iron oxide colloid using anomalous x-ray scattering approaches. We propose USAXS measurements in an aqueous system containing the spherical, crystalline iron- oxide colloid, hematite. The hematite we have synthesized is monodisperse with a diameter of approximately 60 nm. The pH of the solution will be above the pH_zpc of the hematite so that the particle has a negative surface potential, and Tl+1 will be used as the counterion because of its L_III absorption edge (12658 eV). Anomalous scattering techniques will be used to identify the contribution of the hollow shell of Tl counterions to the total scattering intensity of the hematite-Tl system. Results will be analyzed to determine the thickness of the condensed counterion layer (Stern layer) around the colloid. equipment+required: USAXS, liquid sample cells experiment: counter-ion distribution around an aquatic colloidal particle hazards: No hazards. Samples will be prepared at ORNL prior to experiments. Samples will be loaded into liquid cells in UNICAT chem lab. name: Pete Jemian new+request: on nonmembers: John F. McCarthy, ORNL station: 33ID-D unacceptable+dates: I am unavailable May 10 - May 14 and evening of May 24. This experiment is best scheduled in conjunction with a "batch" of USAXS experiments. Such as immediately following request #105. #QUERY_STRING: #REMOTE_HOST: user9.uni.aps.anl.gov #REMOTE_ADDR: 164.54.216.59 #CONTENT_LENGTH: 2553 #HTTP_REFERER: http://www.uni.aps.anl.gov/unireq.htm #HTTP_USER_AGENT: Mozilla/4.72 [en] (WinNT; I)