Dispersion of depolarisation of light-scattering in colloids. Part II. Silver sols

Abstract

Mesurements of the depolarisation factors ρ_u, ρ_v and ρ_h and the extinction coefficient of a series of six silver sols are made for different wave-lengths from 2500 A. U. to 7000 A. U. As in the case of gold sols, so also in silver sols in the region of the characteristic absorption the depolarisation factors show an enormous increase. The optical anisotropy of the silver particles in the colloidal state is rather low in the ultra-violet and the red regions, whereas it assumes rather high values in the violet region where the absorption is maximum. Compared with the sols prepared according to other methods Bredig's sol behaves anomalously. Depolarisation measurements indicate that the nuclear sol contains particles of very small size, while the sols prepared by using formalin as reducing agent and that got by employing gold nuclear solution and hydrogen peroxide as reducing agent contain particles of size comparable with the wave-length of light. In the other sols the partic1es are of intermediate sizes. The observed values of the extinction coefficient and the depolarisation factor ρ_u are compared with the values calculated according to Gans' theory. From both these considerations it is inferred that the particles in silver sols behave optically like elongated ellipsoids with the axial ratio equal to 0.75. It is further suggested that the colloidal particles of silver may be in the form of minute octahedra which can be considered as optically equivalent to a prolate spheroid of the axial ratio stated above. The negative streaming double refraction and the negative electric double refraction observed by the earlier investigators are explained on the basis of the elongated ellipsoidal shape of the silver particles.