The refractive index of colloidal carbon

Abstract

A critical evaluation of published data on complex refractive index leads to the conclusion that for absorbing paracrystalline colloidal carbons (carbon blacks) no accurate determination of this significant fundamental material property has been made. It is readily demonstrated that the usual scheme of matching the generalized Fresnel equations to two (or more) catoptrically measured data points leads to results which, although reproducible, are purely formal artifacts because the particulate nature of the material precludes preparation of optically smooth and homogeneous test pieces, without which the Fresnel equations do not hold. An alternative approach is to derive the index from measurements of dilute suspension optical properties by way of the Mie equations, but this is rendered difficult by the effects of polydispersity and particle nonsphericity. Nevertheless, progress has proved possible through extinction measurements made on a specially made carbon black comprised of very small (and consequently also relatively paucidisperse) particles. At visible wavelengths (350–1000 nm) a result near m 0 = 2.0 − 1.0 i has been obtained for this material. It is foreseen that additional refinement of this estimate will be feasible by use of the same technique on further improved (e.g., sizefractionated) samples, but for the present this result appears to be a needed improvement over others published before.