Water Interfaces Group – UIC Department of Physics

Experimental Tools

Mark’s group uses a variety of experimental techniques to investigate liquid surfaces and interfaces, though much of the research relies on X-ray scattering studies carried out at synchrotron X-ray sources. A separate set of experiments uses a Brewster angle microscope at UIC. Supporting these experimental investigations of interfacial structure are measurements of surface and interfacial tension by quasi-elastic light scattering and Wilhelmy plate techniques. Electrochemical methods, including voltammetry and impedance spectroscopy, are used to establish electric fields at interfaces and to characterize their electric and electrochemical properties.

Synchrotron X-ray Scattering from Liquid Surfaces and Interfaces

X-ray scattering from liquid surfaces and interfaces requires specialized equipment and high-brilliance X-ray beams. Brilliant beams are available from synchrotron X-ray sources, like the Advanced Photon Source (APS) at Argonne National Laboratory. Mark has designed, fabricated and helped to manage the operations of instrumentation for liquid surface/interface scattering at beamline X19C of the National Synchrotron Light Source, which is no longer in operation, and at ChemMatCARS, Sector 15 at the APS (Figure 1).

APS liquid surface reflectometer

Figure 1. The liquid surface reflectometer at Sector 15 (Advanced Photon Source - Argonne National Laboratory), circa 2012.

A general liquid surface reflectometer, like that illustrated in Figure 1, can execute multiple types of surface scattering techniques.

X-ray reflectivity probes the electron density profile in the direction perpendicular to the plane of the interface. It is used to characterize the ordering of molecules and nanoparticles at interfaces. A typical example is shown in Figure 2. See the pages Ions at Interfaces, Solvent Extraction, and Amphiphiles at Water Interfaces for a few other examples. X-ray reflectivity is particularly useful for probing the non-crystalline ordering of molecules and particles that is typical of soft matter.

DHDP monolayer

Figure 2. Electron density profile produced by an X-ray reflectivity measurement from a monolayer of DHDP molecules (di-hexadecyl phosphate) at the liquid-liquid interface between water and dodecane and an accompanying cartoon of the interfacial arrangement of molecules. The red line shows the result of the measurement. The dashed line shows the underlying intrinsic profile for which the effects of thermal (capillary wave) fluctuations of the interface have been removed. The cartoon illustrates the inferred headgroup and tailgroup ordering, but not the effect of the lower density at the end of the tail [3].

For a basic introduction to X-ray reflectivity from liquid surfaces, see this presentation from the APS 2016 School on Liquid Surface X-ray Scattering: Data Analysis.

Grazing-incidence X-ray diffraction probes 2-dimensional crystalline order of molecules or particles within the plane of the interface. An illustration of this type of data is shown on the Nanoparticles page.

X-ray off-specular diffuse scattering probes inhomogeneities within the surface. These could be structural inhomogeneities (such as the formation of domains), chemical inhomogeneities, or interfacial fluctuations such as thermally-induced capillary waves.

X-ray fluorescence can be used to measure the interfacial density of a specific element (Figure 3). This technique was used to measure the density of Er ions in the interfacial structure shown in the cartoon on the Solvent Extraction page.


Figure 3. Illustration of a sample geometry that allows for the detection of fluorescence from a layer of ions beneath an amphiphilic monolayer [4].

Other X-ray scattering techniques used for studying liquid surfaces and interfaces are described in this recent handbook article and in the book described here.

Brewster Angle Microscopy

The reflection of p-polarized light from an ideal interface is extinguished at the Brewster angle. Variations from ideality, such as the presence of a layer of amphiphiles at the interface produce a weak reflection. Inhomogeneities in this layer are observed with an in-plane spatial resolution of a few micrometers, as illustrated in Figure 4. For other examples, see the Amphiphiles at Water Interfaces page. A picture of the BAM instrument is shown in Figure 5.

Labyrinth monolayer

Figure 4. A partial monolayer of partially fluorinated dodecanol at the water-hexane interface. Dark lines are the bare liquid-liquid interface between water and hexane. Brigher background is a single layer of partially fluorinated dodecanol molecules. The scale bar is 100 micrometers long [5].

BAM picture

Figure 5. Picture of our Brewster angle microscope flanked by its developer, Adam Schuman (right), and his assistant, Thomas Bsaibes (left).


[1] A Synchrotron X-Ray Liquid Surface Spectrometer, Mark L. Schlossman, Dennis Synal, Yongmin Guan, Mati Meron, Grace Shea-McCarthy, Zhengqing Huang, Anibal Acero, Scott Williams, Stuart A. Rice, and P. James Viccaro, Rev. Sci. Instrum. 68, 4372-4384 (1997)

[2] The Liquid Surface/Interface Spectrometer at ChemMatCARS Synchrotron Facility at the Advanced Photon Source, Binhua Lin, Mati Meron, Jeff Gebhardt, Tim Graber, Mark L. Schlossman, P. James Viccaro, Physics B 336, 75-80 (2003)

[3] Observation of a Rare Earth Ion-Extractant Complex Arrested at the Oil-Water Interface During Solvent Extraction, Wei Bu, Hao Yu, Guangming Luo, Mrinal K. Bera, Binyang Hou, Adam W. Schuman, Binhua Lin, Mati Meron, Ivan Kuzmenko, Mark R. Antonio, L. Soderholm, Mark L. Schlossman, Journal of Physical Chemistry B 118, 10662-10674 (2014)

[4] X-ray Studies of Interfacial Strontium-Extractant Complexes in a Model Solvent Extraction System, Wei Bu, Miroslav Mihaylov, Daniel Amoanu, Binhua Lin, Mati Meron, Ivan Kuzmenko, L. Soderholm, Mark L. Schlossman, Journal of Physical Chemistry B 118, 12486-12500 (2014)

[5] Microphase Formation at a 2D Solid-Gas Phase Transition, Adam W. Schuman, Thomas S. Bsaibes, and Mark L. Schlossman, Soft Matter 10, 7353-7360 (2014)