Colloids and Surfaces, 19 (1986) 337--349 337 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands PREPARATION AND OPTICAL PROPERTIES OF HOMODISPERSE HAEMATITE HYDROSOLS N.H.G. PENNERS* and L.K. KOOPAL** Laboratory for Physical and Colloid Chemistry, Agricultural University,De Dreyen 6, 6703 BC Wageningen (The Netherlands) (Received 11 October 1984; accepted in final form 12 February 1986) ABSTRACT The principle of heterogeneous nucleation has been used to develop a method for the preparation of homodisperse haematite hydrosols with extremely narrow size distri- butions over a wide range of diameters. The particles formed are monocrystalline and exhibit the crystallographic features of a-Fe203. Conditions which give rise to the for- mation of nearly spherical particles are very strict; slight alterations in these conditions may change the particle morphology significantly. The turbidity measurements carried out at wavelengths from 400 to 800 nm, using hydrosols with particle diameters ranging from 30 to 500 nm, are compared with calcu- lations based upon Mie's scattering theory for homogeneous isotropic spheres. The agreement between experimental and calculated values is good for all particle sizes and wavelengths, despite the monocrystalline nature of the scatterers and the resulting devi- ation from sphericity. The experiments support the dependence of the absorption co- efficient on wavelength as proposed by Kerker et al. [J. Colloid Interface Sci., 71(1979) 176]. INTRODUCTION Among the family of inorganic colloidal compounds, the ferric hydrous oxides are of special importance because of their appearance in nature as soil constituents and minerals, and their widespread applications in industry. They are used as pigments, catalysts, coatings, fillers and in the production of magnetic tapes. The preparation of ferric hydrous oxides sols has been a subject of re- search for more than 100 years. However, in most cases, the prepared hydrosols were rather poorly defined with regard to size and shape. Only a few examples of relatively homodisperse systems are recorded [1--7] in the early literature. *Present address: Netherlands Energy Research Foundation ECN, Petten, The Nether- lands. **To whom correspondence should be addressed. 0166-6622/86/$03.50 © 1986 Elsevier Science Publishers B.V.