Light Scattering from Interacting Brownian Particles.

Structural and dynamic behavior of interacting Brownian particles in solution was studied by laser light scattering. Structural information was obtained from measurements of the static structure factor at a fixed wavevector by static light scattering. Dynamic behavior of the interacting particles, as reflected in their effective diffusion coefficient, was determined from dynamic measurements of the fluctuations in the scattered light intensity. The particles in solution interacted by hard sphere, electrostatic, and hydrodynamic interactions. The electrostatic interaction potential energy was varied as both a function of charge on the particles and solution ionic strength. Increasing the charge or decreasing the ionic strength would increase the potential of interaction between particles, thus an enhancement of the diffusivity and a corresponding decrease in the static structure factor would be observed. For highly charged, low ionic strength systems, the diffusivity would increase by as much as a factor of 4.4, whereas the static structure factor would decrease by a factor of 8.0 from the non-interacting particle values. Recent theoretical predictions agreed well with the experimental data at high ionic strengths, but increasing discrepancies appeared at lower ionic strengths. The experimental data failed to reveal an inverse proportionality between the static structure factor and the effective diffusivity, as had been reported previously. Hydrodynamic interactions were shown to contribute significantly to the measured values of the diffusivity and altered the relationship irregardless of the strength of the interaction.