College Park, Maryland      June 6 - 10 , 2004

M4-D3 (4:30 PM): SANS Microstructure and rheology of sterically stabilized colloidal dispersions with depletion attractions

L.N. Krishnamurthy, N.J. Wagner (Center for Molecular and Engineering Thermodynamics, Department of Chemical Engineering, University of Delaware, Newark, DE 19716)

Colloidal particles are employed in a wide range of industries and products. A thorough understanding of dispersion rheology and stability is critical for successful formulation and processing of colloidal dispersions for practical applications. These properties are macroscopic manifestations of the microscopic interparticle interactions acting between the colloidal particles. The microscopic forces that can arise from polymer adsorption include bridging, steric, and depletion forces with a strongly coupled electrosteric force if the polymer is a polyelectrolyte. Rational design of stable dispersions requires the characterization of these interparticle interactions.

Rheological and small angle neutron scattering measurements are presented for a model dispersion of silica nanoparticles in a buffered, aqueous gelatin solution over a range of particle and gelatin concentrations. There is debate in the literature so to whether free polymer in an adsorbing polymer-colloid mixture can result in an attractive depletion interaction force. The adsorbed polymer layer and the resulting large excluded volume clearly dominate the rheology. However, comparison of SANS measurements with analysis of the rheology demonstrates the presence of weak attractions, which has a profound influence on the stability and rheology of these systems.

We model the attractive interactions within the classic Asakura-Yukawa framework for the depletion interactions. We propose a predictive semi-empirical model to describe the rheology of colloidal systems with attractive interactions. Excellent quantitative predictions of zero shear viscosity and SANS for a broad range of particle and polymer concentration validate our model.

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