Structural Interpretations of Static Light Scattering Patterns of Fractal Aggregates

Information on the size and structure of aggregates is critical in predicting the formation kinetics, settling velocities, and reactivity of particle aggregates. For some systems, however, accessing this information may be very difficult. Light scattering measurements are among the most useful techniques for accessing such information. In the case of large primary particles forming aggregates, the common Rayleigh approximation is not valid. Instead, Mie scattering must be used and multiple scattering must be accounted for. Moreover, size polydispersity and structure of aggregates are combined in the scattered intensity measurements. This work presents an experimental validation of a new theoretical approach for extracting information on both aggregate structure and size when multiple scattering cannot be neglected. The chemically controlled aggregation of 0.8-µm latex particles demonstrates the following: (1) Polydispersity effects prevent the interpretation of data to obtain structural information from the Structure factor S(q). (2) The calculated optical contrast decreasing during the aggregation can be correlated with the structural changes in the growing aggregates independently of size polydispersity. We have shown that a strict correlation can be obtained between the fractal dimension D(f) and the scatterers' mean optical contrast calculated at large scattering angles. (3) The changes in the Form factor (F(q)) due to multiple scattering when particles are close together yield a predicted structure that is in agreement with expected fractal dimension values and therefore S(q) can be described in term of both structure and size polydispersity. Copyright 2000 Academic Press.