Optical properties of monodispersed hematite hydrosols.

The experimentally determined extinction spectra of colloidal hematite, consisting of spherical particles of 0.10, 0.12, 0.13, 0.15, 0.16, and 0.51 microm in modal diameter, respectively, are compared with the corresponding theoretically calculated spectra using the Lorenz-Mie theory. These spectra are most sensitive to the particle size in the 0.10-0.20-microm range due to the damping effects which broaden the resonance at the expense of its height. For large particles (0.51 microm) the extinction becomes independent of the wavelength because of the broader size distribution of the hematite dispersion. As the small particles aggregate, a shoulder appears at 550 nm corresponding to the onset in fluctuations in the extinction efficiency of various sized particles. This shoulder may suggest the existence of certain surface electromagnetic modes caused by aggregation. A comparison is made in the size-dependent extinction efficiency at various wavelengths between colloidal hematite and gold sols.