THE SUITABILITY OF SCANNING ANGLE REFLECTOMETRY FOR COLLOIDAL PARTICLE SIZING

We investigated the suitability of reflectometry as a sizing method fo r polystyrene latex particles. This technique, where the particles are adsorbed at a quartz/water interface, involves the analysis of the re flection around the Brewster angle of light polarized in the plane of incidence. A computation of a full reflectivity curve containing about 20 datapoints, taking into account all electromagnetic interactions, is extremely complex and may take up to 100 h on an Indy R4600 compute r (Silicon Graphics, Mountain View, CA). To use the complete electroma gnetic theory for analysis of experiments therefore is impractical. We analyzed the experimental reflectivity curves using an approximate th eory (free-sphere model), which neglects all electromagnetic interacti ons between different particles and their images in the substrate. To assess the errors introduced by neglecting these interactions, we have calculated reflectivity curves using the full electromagnetic theory for this system, and analyzed them with the free-sphere model. We foun d that the diameters determined with the free-sphere model deviate at most 2% from the actual values. The surface coverage determined with t he free-sphere model is systematically smaller than the actual coverag e. The difference can be as large as 15% at a coverage of 9% We also s how that size polydispersity of the sample plays a minor role in the a nalysis of the data. We illustrate the usefulness of the free-sphere m odel, by analyzing measurements in which polystyrene latex particles a re swollen by an organic solvent that is miscible with water. (C) 1996 American Institute of Physics.