Refractive-index measurement of turbid colloidal fluids by transmission near the critical angle

We describe a modified Abbe refractometer for measuring the optical transmittance across a plane interface from a diffusely scattering turbid sample into a prism. When the prism refractive index exceeds that of the sample, a critical angle is present even when the sample contains particles which are wavelength sized or larger. Hence, the sample refractive index may be measured. However, measurements of transmittance near the critical angle show a poor definition of the critical angle for some samples, with a consequent loss of experimental precision in the refractive index. The method is applied to several polymer latex dispersions and some glass ballotini. For all lattices the measured refractive index varies linearly with latex volume fraction. For all dispersions except one the dependence of refractive index on volume fraction is in good agreement with the theory of refractive index based on Mie scattering by spherical particles.