Abstract
Structuring of aqueous suspensions of colloidal silica nanoparticles near an isolated planar silica-water interface is studied by specular neutron reflectivity. The reflectivity data clearly show that the suspensions develop a damped, oscillatory concentration profile in the normal direction to the interface. The wavelengths of these oscillations agree well with those independently determined by direct force measurements in the slit-geometry. The reflectivity data further demonstrate that the oscillatory structure persists over several layers and that the first particle layer is separated from the interface by a particle-free region.
Highlights
Interactions between colloidal particles and water–solid interfaces are relevant in a broad range of applications, in material science, food processing, medicine, or environmental engineering.[1,2,3,4,5,6] Several aspects in these processes remain poorly understood, and this topic continues to represent an active research field
This shoulder is a signature of a decaying oscillatory structure near the interface, and the corresponding wavelength l can be estimated from the Bragg condition as l = 2p/q E 250 Å
The Debye length, which characterizes the thickness of the electrical double layer, increases with the inverse of the square root of the ionic concentrations, and this relation might rationalize the exponent of À1/2.41 Our observation of the particle-free layer is in line with recent quartz crystal microbalance (QCM) measurements of suspensions of negatively charged polystyrene nanoparticles.[46]
Summary
Interactions between colloidal particles and water–solid interfaces are relevant in a broad range of applications, in material science, food processing, medicine, or environmental engineering.[1,2,3,4,5,6] Several aspects in these processes remain poorly understood, and this topic continues to represent an active research field. Direct force measurements have recently revealed that concentrated colloidal particle suspensions self-organize when sandwiched between like-charged interfaces, thereby forming layered structures with a characteristic oscillatory profile that a Department of Inorganic and Analytical Chemistry, University of Geneva, Sciences II, 30 Quai Ernest-Ansermet, 1205 Geneva, Switzerland.
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