Determination of the Structures of Colloidal Suspensions by Scattering: Structural Arrest Transitions, Equilibrium Clusters and Attraction between Like-Charged Colloids

Wei-Ren Chen, Department of Chemistry, University of Tennessee

Most of the presently available knowledge about the microstructure and dynamics of colloidal suspensions is the result of scattering experiments. In this talk, three examples are given to report some latest development in investigating the colloidal interaction by neutron and light scattering techniques: First, in L64/D2O solutions, a PEO-PPO-PEO triblock copolymer micellar system, the kinetic glass transition can be triggered by increasing the volume fraction. By using Small Angle Neutron Scattering (SANS) and Photon Correlation Spectroscopy (PCS) measurements, we report the first observation of the glass-to-glass transition in a colloidal system, which is predicted by recent mode coupling calculations.

The second topic of this talk is the cluster formation in cytochrome C and lysozyme protein systems. The formation of equilibrium clusters in protein systems by changing the pH value, volume fraction of protein and ionic strength are discovered by SANS measurement. The structure factors of two Yukawa and Adhesive Hard Sphere Yukawa (AHSY) potentials are derived to analyze SANS intensity distributions. Surprisingly, SANS intensity distributions show a rising intensity at a very small wave vector Q, which implies the presence of a weak long-range attraction between protein molecules.

Inspired by the protein experiment. The effective interaction between PAMAM G4 dendrimers in dilute D2O solutions is investigated by SANS and DLS. Upon increasing the ionization of the terminal amines by adding DCl, upturns appear in the SANS intensity distributions at low Q within a certain range of pH value, then surprisingly disappear again when pH=2. Detailed data analysis suggests the upturns can be interpreted in terms of the presence of an effective long-range attraction between similarly charged dendrimeric colloids. Such long-range weak attractive interactions, probably due to the dynamic interaction between the counterions and the charged dendrimeric colloids, can not be explained by the well-accepted DLVO theory of colloidal stability.

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