1. Opportunities In Separations Science using Advanced Synchrotron X-ray Methods
G. BRIAN Stephenson
Materials Science Division, Argonne National Laboratory
A Research Agenda for a New Era in Separations Science
August 22-23, 2018, Washington, DC

2. OUTLINE Introduction – Synchrotron X-ray Opportunities
Upgrades to sources
Examples of Current Synchrotron Studies in Separations Science
X-ray reflectivity, surface fluorescence, grazing- incidence scattering, small-angle scattering
New Opportunities using Coherent X-ray Methods
Nanoscale dynamics using X-ray photon correlation spectroscopy (XPCS)

3. synchrotron X-ray Capabilities: Spectroscopy, Scattering, Imaging
X-ray Spectroscopy:
Photon energy matches atomic energy levels and gives chemical composition
Relevant methods: EXAFS, XANES, surface fluorescence (XFNTR)
X-ray Scattering:
Short wavelength allows the atomic structure to be precisely determined
Reflectivity, small-angle scattering (SAXS), pair distribution (PDF), grazing-incidence (GIXS and GISAXS); all can be resonant
New coherent methods (XPCS, CDI)
X-ray Imaging:
Large penetration depth of hard x-rays reveals internal structure
Many imaging modes: nanoprobe, phase contrast, coherent diffraction, ...

4. Opportunity: Light Source UPGRADES
Revolutionary new capabilities
Upgrades are occurring at all of the US X-ray light sources
The APS Upgrade will increase its brightness by 100 to 1000X, as big a revolution as the original building of APS in 1996
APS-U will turn on in 2023, and we are already planning and developing the new methods enabled:
photon correlation spectroscopy (XPCS)
nanometer x-ray probes
coherent x-ray imaging
I didn't show it on the plot, but the Cornell synchrotron operated by NSF is currently undergoing an upgrade that will make all it beamlines roughly equivalent to the current APS by next year
Now I'd like to show some examples of synchrotron experiments currently being conducted at APS
These are related to liquid-liquid extractant systems
Brightness as a function of x-ray energy for the current APS and new NSLS-II sources, and the future APS Upgrade

5. example: resonant x-ray reflectivity
Adsorption of chlorometalate anions
Resonant X-ray reflectivity provides the total metalate adsorption in Stern layer in addition to the total electron density profile with sub-nm resolution
Pt Cl 6 anions
DPTAP monolayers
Red and blue are above and below Pt L3 absorption edge
Inset shows difference, Pt building up in Stern layer close to interface
Figure 2. (a) Anomalous XR data (symbols) and fits (solid lines) of DPTAP monolayers on a subphase with 0.5 M LiCl and varying PtCl62– concentrations. The plot contains XR measurements at the platinum L3 absorption edge (blue, keV) and 250 eV below the edge (red). Data for each concentration are shifted by two decades for clarity. (b) Electron density profiles (EDPs) derived from the fits to the XR data in (a) (blue and red) and from the MD simulations (green). EDPs at each concentration are shifted by 0.1 e/Å3 for clarity. The cartoon depicts the approximate positions of DPTAP molecules and PtCl62– ions at the air/water interface corresponding to the EDPs. The inset shows the difference between the red (off-edge) and the blue (on-edge) EDPs for each concentration. The differential EDPs in the inset are compared to an appropriately scaled PtCl62– distribution from MD simulations (dashed green curve).
A. Uysal, W. Rock, B. Qiao, W. Bu, and B. Lin, J. Phys. Chem. C, 121(45), 25377, 2017

6. Example: surface X-ray fluorescence
Adsorption of chlorometalate anions
X-ray fluorescence near total reflection (XFNTR) provides the total metalate adsorption both in Stern layer and in diffuse layer.
Also studied flurescence
By changing incidence angle, can control penetration depth; below critical angle, only get surface contribution
Cobine with reflectivity to distinguish Stern and diffuse layers
Figure 3. Depiction of XFNTR measurements below (a) and above (b) the critical angle. While only surface-adsorbed ions (both diffuse and Stern layers) fluoresce below the critical angle, the ions in the bulk are also excited above the critical angle. (c) Concentration-dependent fluorescence signal at fixed q = Å–1 below the critical angle. (d, e) Concentration-dependent XFNTR data (symbols) calculated by integrating the area under the curves in (c) and their counterparts at various q values. Solid lines show the fits to the data as described in the text. The 20 mM sample in (d) is measured with (squares) and without (circles) DPTAP monolayer. All measurements in (e) are done with DPTAP.
Figure 4. Coverage of PtCl62– ions as a function of the bulk concentration. The total coverage is calculated from XFNTR measurements (black squares). The contribution of the PtCl62– ions in the Stern layer is calculated from a-XR measurements (red circles). The solid black and red lines are Langmuir adsorption fits to the corresponding data sets. The diffuse layer contribution (blue line) is calculated as the difference between the fits to the total and the Stern layer coverage. The inset cartoons visualize the adsorption behavior in the Stern and diffuse layers at low and high bulk concentrations.
A. Uysal, W. Rock, B. Qiao, W. Bu, and B. Lin, J. Phys. Chem. C, 121(45), 25377, 2017

7. EXAMPLE: GRAZING INCIDENCE X-RAY SCATTERING
Adsorption of chlorometalate anions
Grazing incidence x-ray scattering (GIXS) provides the information about in-plane packing and tilt structure of the extractants.
Finally, used GIXS to study changes in the structure of the extractant molecular layer
Get diffraction peaks that allow determination of the spacing and tilt angle
Figure 5. (a) GID data from DPTAP on a 1 mM PtCl62– solution. One in-plane and one doubly degenerate out-of-plane peak are the signature of nearest-neighbor (NN) tilt. The position of the out-of-plane peak changes with the concentration as shown in the inset. (b) Tilt angle of the DPTAP molecules from the surface normal (blue circles, left axis) and their molecular area (orange squares, right axis) as a function of the metalate concentration.
A. Uysal, W. Rock, B. Qiao, W. Bu, and B. Lin, J. Phys. Chem. C, 121(45), 25377, 2017

8. example: combining X-ray Studies with SFG measurements
Unique interfacial water structure
Have also studied this system with Vibrational Sum Frequency Generation, gives a complementary view, sensitive to water structure which is largely invisible with X-rays
Now I'd like to change gears, and talk about emerging opportunities using coherent X-ray methods
Three different types of water molecules can be identified around a PtCl62- complex adsorbed at a quaternary amine monolayer at the air/water interface by using VSFG and MD simulations.
While x-rays provide information about the metal ions, SFG studies elucidate the interfacial water structure.
W. Rock, B. Qiao, T. Zhou, A. Clark, and A. Uysal, arXiv: , 2018

9. ADDITIONAL INFORMATION USING A COHERENT BEAM
scattering
sample with disorder (e.g. nanostructure in a liquid)
Incoherent Beam: Diffuse Scattering
Measures average two-point correlations, e.g. average size, spacing, anisotropy
Coherent Beam: Speckle
Speckle depends on exact nanostructure arrangement
In principle, contains complete structural information
Can observe equilibrium dynamics

10. transversely coherent
X-RAY PHOTON CORRELATION SPECTROSCOPY (XPCS) t1 t2 t3
sample
transversely coherent
X-ray beam
monochromator 1
Intensity auto-correlation function:
“movie” of speckle
recorded by CCD
XPCS is the ‘movie’ of speckles recorded in time.
Time correlations of the speckle pattern give the dynamics of atomic-scale fluctuations
Intensity fluctuations of the speckle pattern reflect sample dynamics.
Q dependence indicates nature of dynamics (e.g. diffusive, relaxational)
Review of XPCS: O. Shpyrko, J. Synchr. Rad. 21, 1057 (2014) 10

11. XPCS Allows Study of Equilibrium Dynamics Down To The Atomic SCale
Improvements in coherent flux and detector speed are moving XPCS studies into the length and time scale ranges needed to study dynamics of complex fluids
Leheny, Curr. Opin. Colloid Interface Sci. 2012, 17, 3

12. Converging experiments, Simulations, and Separations science
New territories in XPCS and MD
future signal limit
current signal limit
new detector limit
ions
to-date MD
micelles
mesophases
to-date detector limit
new MD methods
Expand region of XPCS and look at how capabilities align with problems of interest
Kind of complicated diagram
Most relevant here is comparison of XPCS method and dynamics in liquid extraction systems
Green lines show how time scales of diffusion of various species in a complex fluid vary depending on their size
ions, individual micelles, aggegates of micelles
Red lines show limit on time scale based on readout speed of pixel array detectors, recently increases from millisecond to microsecond range
Black lines show limit of XPCS studies based on typical signal levels from complex fluids, using the current APS source, or the future APS upgrade
Pink region shows how the new detectors allow coverage of the length and time scale region relevant to complex fluids
Also shown by the blue lines is the region that can be modeled based on traditional MD methods, and the improvements that our collaborators have proposed
The developments in both XPCS and MD are now overlapping right in the region of interest for liquid extractant systems
Leheny, Curr. Opin. Colloid Interface Sci. 2012, 17, 3

13. initial XPCS study of micelle dynamics in complex fluid for ion separation
Self-assembling reverse micelles of Ce(NO3)3 in dodecane with malomamide extractant
Ce phase diagram ■
Show some very recent results on a liquid-liquid extractant system
Had previously been characterized by a variety of methods, including SAXS
Shown to have a phase diagram that behaves like a critical point, with increased small-angle fluctuations as the concentration of Ce ions is increased at room T
We chose this system to see if we could measure the dynamics using XPCS with the new detector
Ellis, Antonio, Langmuir 2012, 28, 5987
SAXS provides good characterization of structure and phase behavior
XPCS wlll reveal micelle aggregation dynamics

14. Approaching Critical point by changing Temperature
Power-law shape of SAXS extending to low Q is typical of critical fluctuations near a second-order transition
I ~ Q -1.7
Here we started with the dodecane phase loaded with inverse micelles near the critical point, and varied the temperature
Stephenson, Antonio, et al. unpublished 2018

15. Wavenumber dependence of fluctuation correlation time
Can observe correlation times of micelle clusters down to ~10 us
Further detector and source improvements will allow study of individual micelle dynamics at ~1 ns
Q dependence supports model of micelle aggregation as Tc is approached
Opens the way for combined SAXS, XPCS and MD studies of structure and dynamics of complex fluids used in separations
~ Q -3.0
This shows the dynamics of the fluid near the critical point
Stephenson, Antonio, et al. unpublished 2018

16. Summary
Advanced synchrotron x-ray methods are already having and impact in separations science e.g. through studies of adsorption at interfaces and complex fluid structure
The revolutionary coherent x-ray and nanoprobe capabilities provided by new sources and detectors will extend these capabilities e.g. into fast dynamics in fluids and nanoscale imaging, providing new opportunities for separations science
water
oil
Qiao, Muntean, Olvera de la Cruz, Ellis, Langmuir 2017, 33, 6135