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x-ray Surface Scattering Studies of Molecular Ordering at Liquid-Liquid Interfaces

Published online by Cambridge University Press:  10 February 2011

Mark L. Schlossman
Affiliation:
Department of Physics, University of Illinois, Chicago, IL 60607, schloss@uic.edu
Ming Li
Affiliation:
Department of Physics, University of Illinois, Chicago, IL 60607, schloss@uic.edu
Dragoslav M. Mitrinovic
Affiliation:
Department of Physics, University of Illinois, Chicago, IL 60607, schloss@uic.edu
Aleksey M. Tikhonov
Affiliation:
Department of Physics, University of Illinois, Chicago, IL 60607, schloss@uic.edu
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Abstract

We present our recent progress in using synchrotron x-ray surface scattering techniques to study several different aspects of ordering at liquid-liquid interfaces. (1) We present measurements of the interfacial width at the water-alkane interface for a series of different chain length alkanes. The width for the shortest chain length studied is in agreement with capillary wave theory. However, significant deviations occur for longer chain lengths, indicating the presence of molecular ordering at the interface. (2) Under appropriate conditions, a surfactant monolayer forms at the interface between water and a hexane solution of a fluorinated surfactant. Reflectivity measurements that probe the electron density profile normal to the interface provide information about the surfactant ordering. This monolayer undergoes a solid to gas transition as a function of temperature. Diffuse scattering near the transition reveals the presence of islands. (3) Equilibrium interfaces between two aqueous phases containing PEG (polyethylene glycol) and potassium phosphate salts can be studied. We present studies of coherent capillary wave fluctuations between two interfaces of a thin film of this biphase system. We also demonstrate that biological macromolecules can be trapped and studied at this aqueous-aqueous interface.

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