Dispersion of depolarisation of light-scattering in colloids. Part I. Gold sols

Abstract

The earlier investigations (theoretical as well as experimental) on the light scattering and absorption of gold sols have been briefly reviewed. It is pointed out that in order to determine the size and shape of colloidal particles, it is important to make comparative studies of the dispersion of depolarisation of the transversely scattered light with the incident light in three diflerent states of polarisation, namely, unpolarised, vertically polarised and horizontally polarised. Measurements of the depolarisation factors ρ_u and ρ_v and the extinction coefficient of a series of six gold sols are made for different wave-lengths from 2500 A.D. to 7000 A.D. The value of ρ_h is calculated from the observed values of ρ_u and ρ_v using the reciprocity relation. In the region of the characteristic absorption the depolarisation factors show an enormous increase. The optical anisotropy of the gold particles in the colloidal state is rather low in the shorter wavelength region, whereas it assumes rather high values in the green region where the absorption is maximum. The observed values of the extinction coefficient and. the depolarisation factor ρ_u are compared with the values calculated according to Gans's theory. From both of these considerations, it is inferred that the particles in the gold sols behave optically like elongated ellipsoids with the axial ratio equal to about 0.75. The values of ρ_h indicate that the size of the particles in the nuclear sols is small compared with the wave-length of light, while the blue sol contains particles of size comparable with the wave-length of light, The negative streaming double refraction and its dispersion with wave-length observed by the earlier investigators are explained on the basis of the elongated ellipsoidal shape of the particles and the dispersion of their optical anisotropy.