College Park, Maryland      June 6 - 10 , 2004

M4-D4 (4:45 PM): Highly Monodispersed Silica Nanoparticles: Controlled Formation and EDL Characterization by SANS

W. Wang (Oak Ridge National Laboratory), L. Porcar (NIST Center for Neutron Research), W. A. Hamilton (Oak Ridge National Laboratory), P. D. Butler (Oak Ridge National Laboratory; NIST Center for Neutron Research), L. Liang (Oak Ridge National Laboratory; Cardiff University, Cardiff CF24 0YF, UK), B. Gu (Oak Ridge National Laboratory)

Suspensions of monodispersed colloidal particles offer many scientific opportunities for new applications in emerging nanotechnologies, such as advanced coatings, photonic crystals and photonic band-gap materials. The goal of this project is to develop and apply small angle neutron scattering (SANS) to quantify the electric double layer (EDL) structure associated with silica nanoparticles in an aqueous solution containing organic electrolytes. The amount and the orientation of the ionic organic molecules present in the Stern layer, and the resulting thickness of the EDL all contribute to interaction energies during particle encounter.

In order to detect small changes in particle size due to sorption of the organic ions, a high monodispersity of colloidal particles is crucial. Although the preparation of silica nanoparticles has been extensively studied, synthesis of highly monodispersed silica colloids with sizes <100 nm remains a challenge. We optimized synthesis and purification procedures to produce monodispersed silica colloids in size range of 20-100 nm by hydrolysis of tetraethoxysilane. The particle sizes and polydispersity could be controlled by chemical reagent concentrations, reaction temperature, reaction time, or surfactant template media.

These monodispersed silica colloids were studied by SANS over a wide range of scattering vectors. These organic counterions, such as tetraalkylammonium and alkyltrimethylammonium ions, functionally serve as monovalent simple ions with a high volume density and therefore provide a model system to study ion adsorption and distribution on silica surfaces by SANS. The adsorption layer thicknesses of various organic cations at the silica nanoparticle-water interface were studied with contrast match technique.This deuteration accentuated different domains relative to others to provide the needed discrimination for specific domain structures such as particle core size, adsorbed monolayer or bilayer thicknesses, and solvent penetration in the organic layers. We fit the scattering spectra by PolyCoreShell Model to deduce the diameter, the size polydispersity, and the thickness of adsorbed organic layer on the spherical silica particles.

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