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This project is working towards solving the problem of acquiring and interpreting light scattering data from polymeric and colloidal solutions that absorb light. Light absorption heats samples during scattering experiments causing temperature gradients and thermal lensing. This results in convection within the scattering volume and in changes to the experimental geometry. The limitations imposed by absorption have largely limited the application of scattering techniques using visible wavelengths to transparent, colorless solutions. Nonetheless there are systems of immense technological interest; nanotubes, colloidal nanoparticles, proteins, and conducting polymers, where characterization of their solutions is highly desirable but, in many instances, they absorb light.

The convection observed in highly absorbing solutions gives rise to homodyne correlation functions with an oscillatory component. The funding from the Petroleum Research Fund has supported our effort to study model systems with known size distributions where the absorbance is controlled with added dyes. Thus far we are able to reproduce every feature seen in the intensity-intensity autocorrelation functions obtained from colloidal and polymeric systems that absorb light and exhibit aggregation.

Model solutions comprise hydrophobically modified silica nanoparticles, polystyrene, and boronal dye in various concentrations with either tetrahydrofuran (THF) or N-methyl pyrrolidone (NMP). Prior work in our group has shown that a flux equation that incorporates diffusion and convection for a bimodal distribution provides a theoretical basis for fitting the qualitative features of the correlation functions obtained in these experiments. This approach falls short of providing the hydrodynamic radii of the scatters in question because of difficulties in establishing the temperature (and hence the viscosity) in the scattering volume. The hydrodynamic radii of silica nanoparticles are relatively insensitive to changes in temperature and will serve as an internal calibration in the model studies.

The presence of oscillations in the slowly decaying tail of the autocorrelation functions substantiates the theory that, in homodyne DLS experiments of light absorbing solutions, the oscillatory term requires the presence of two distinct species having different diffusion coefficients as well as different velocities from convection. This theory is corroborated by the absence of oscillation when any element is missing. In the dye solution without scatterers there is no correlation at all which indicates that the dye does not aggregate. The non-light absorbing solutions of scatterers provide simple sums of exponentials and can be dealt with using standard techniques. Lastly, the narrow polydispersity scatterers with added dye also show simple exponential behavior even though heating and convection are observed which indicates that bimodality is essential to produce the oscillations. This sensitivity to the size distribution even at low concentrations of the larger scattering species indicates that these oscillating correlation functions will be useful for analyzing solutions of carbon nanotubes and aggregating colloids.