Synchrotron X-ray techniques for the investigation of structures and dynamics in interfacial systems
Graphical abstract
Introduction
This review focuses on recent results, and on the instrumental developments, which took place in the last years, in the diverse research fields interested by the application of synchrotron radiation to problems in interface and colloid science. Special emphasis is given to fluid interfaces (either air–water or liquid–liquid interfaces), to buried interfaces and to colloidal systems.
I shall begin the review discussing structural investigations. The main techniques in this case are X-ray reflectivity, (either in specular or in off-specular modes) and grazing incidence X-ray diffraction, often complemented successfully by grazing incidence X-ray fluorescence, which provides elemental sensitivity along the direction perpendicular to the interface. Section 2 is devoted to soft interfaces (either air/water or liquid/liquid) while Section 3 is devoted to films deposited on solid support.
In Section 4, the focus is on the dynamics of interfaces and of colloidal systems, accessed by X-ray photon correlation spectroscopy (XPCS) on the nanometer scale and on the timescale from the few milliseconds to several hours. XPCS is the equivalent of dynamic light scattering, but performed by X-rays. As such, it is able to follow the time evolution of correlation functions, and to characterize the evolution of systems undergoing aging. I will emphasize some recent works showing how XPCS proved able to access directly dynamical heterogeneities, which constitute an important signature of the different states of dynamically arrested systems. Finally, in Section 5, I shall discuss instrumental developments, some of which undertaken aiming at the implementation of these techniques at new microfocused beamlines and at the free-electron laser facilities that start now to be operational.
Section snippets
Structural investigations of systems at the air–water and liquid–liquid soft interfaces
Langmuir monolayers at the air–water interface are relatively simple systems, and have been studied for many years; though their structure is fairly well understood only in those cases where they are formed by simple molecules [1], [2]. On the contrary, much less is known about structure, dynamics and interfacial behavior of biologically relevant films made of proteins, and of macromolecular aggregates and colloidal layers which are of large technological relevance.
In this class, important
Structural investigation of buried interfaces and solid supported films
Moving from air–water to buried liquid–liquid interfaces represents an increase in experimental difficulty: this requires usage of higher energy X-ray beams, to overcome the attenuation of X-rays by the condensed liquid phases. However, this is justified by the importance of such interfaces: many technological processes, including food processing, foam stabilization, and many biological systems, can be understood, controlled and schematized to a good degree by this class of systems. A recent
Study of the dynamics by X-ray photon correlation spectroscopy (XPCS)
X-ray photon correlation spectroscopy (XPCS) is the extension of photon correlation spectroscopy (also called dynamic light scattering [36]) to the X-ray regime. It permits to access an otherwise inaccessible region of the space–time plane corresponding to localized and slow motions, e.g. taking place on the nm-scale and on the 10− 3–102 second time scales. XPCS technique was pioneered at the National Synchrotron Light Source at Brookhaven National Laboratory by Sutton, Mochrie and co-workers in
Instrumental developments
In recognition of the increasing importance of the application of X-rays to surface science, dedicated new beamlines are being built at the diverse facilities around the world. E.g. a dedicated beamline has been designed, constructed and optimized at beamline 8-ID-E at the Advanced Photon Source (Argonne National Lab) for high-resolution and coherent grazing incidence X-ray scattering (GIXS) experiments with applications for in situ and real-time studies of nanostructures and nano-composites at
Conclusions and outlook
In conclusion, a suite of sophisticated synchrotron-based techniques (see Fig. 6) are becoming of increasingly common use for the characterization of soft interfaces, often accessing otherwise unattainable information on the structure and the dynamics of these complex systems.
A promising evolution is the development of sophisticated sample environment including “ancillary equipment” especially designed to allow simultaneous characterization by different techniques, to cover the broad time- and
Acknowledgments
I am deeply indebted to Pietro Cicuta, Davide Orsi, Diego Pontoni and Beatrice Ruta for critical reading of the manuscript and for stimulating discussions, to Oleg Konovalov for initiating me to surface science by synchrotron radiation a long time ago, to Andrei Fluerasu for many interesting experiments together, and mostly to Anders Madsen for his introduction to the subtleties of the XPCS technique.
I also thank the co-authors of references [15], [63], [64], [67], [79•], [95••], [96], [98],
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