High-Speed XPCS Studies of Composition Fluctuations in Liquids

High-Speed XPCS Studies of Composition Fluctuations in Liquids

Abstract

G. Brian Stephenson^1,*, Allison Peroutka², Dina Sheyfer³, Jyotsana Lal², Qingteng Zhang³, Eric M. Dufresne³, Suresh Narayanan³, Michael Servis²

¹Materials Science Division, Argonne National Laboratory, Lemont, IL 60439, USA

²Chemical Science and Engineering Division, Argonne National Laboratory, Lemont, IL 60439, USA

³X-ray Science Division, Argonne National Laboratory, Lemont, IL 60439, USA

*gbs@anl.gov

High-speed X-ray photon correlation spectroscopy (XPCS) using new coherent X-ray sources and fast detectors opens new avenues to explore fluctuation dynamics in fluids. We have been studying systems relevant to liquid-liquid extraction processes, where an organic solution of extractant molecules is used to separate ions from an aqueous solution by formation of nanoscale molecular complexes. The organic phase exhibits incipient phase separation, and critical fluctuations play a key role in the structure of the molecular complexes [1-5]. Here we present XPCS studies of microsecond timescale composition fluctuations within 5 K of the critical temperature T_C, carried out at APS beamline 8ID. With the 500 times higher coherent X-ray flux that is now available from the new multi-bend achromat synchrotron sources and higher-speed detectors such as the TEMPUS [6], it should be possible to observe fluctuation dynamics much further away from T_C. This will enable exploration of the crossover from Ising to mean-field behavior, as well as the changes in dynamics expected at the Widom line (the locus of fluctuation maxima extending from the critical point into the single-phase region). We will present preliminary progress along these lines, as well as behavior calculated from models.

Work supported by U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division, Separation Science Program, under Contract DE-AC02-06CH11357. This research used resources of beamlines 12-ID-C and 8-ID-I at the Advanced Photon Source, a DOE Office of Science User Facility operated by Argonne National Laboratory.

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[5] T. Rahman et al., J. Mol. Liq. 393, 123625 (2024).

[6] J. Correa, et al., J. Synchrotron Rad. 31, 1209 (2024).