Colloidal refractometry: meaning and measurement of refractive index for dispersions; the science that time forgot

This paper introduces a new way of investigating the microstructure of high phase volume dispersions. It is a light scattering method where, instead of scattering intensities, one obtains refractive index, n, of a dispersion using Mie theory (the so-called direct Mie problem). The measured values of n on undiluted dispersions can provide a measure of the volume-weighted average particle size. The accurate measurement of n for turbid fluids is not trivial and total internal reflection refractometers that rely on the visual identification of the degraded cut-off, or critical, edge of a dark-light border (or even most of modern automatic ones) cannot cope with this task. Here a total internal reflection refractometer is described that relies on differentiating the light profile for accurate identification of the degraded cut-off edge. It can cope with a wide range of concentrations and size distributions. The sizing results for polystyrene latices and liposome dispersions compare well with conventional methods.