The effect of light absorption of particle and medium on the particle size estimation by turbidimetry

Abstract

A theoretical study was undertaken in order to investigate the role of particle and medium light absorption on the estimation of the particle size distribution (PSD) by NUV (200-300 nm) and NIR (1300-1580 nm) turbidimetries. Styrene/butadiene and polystyrene latexes (i.e. polymer particles suspended in water) were selected. The absorption bands of polystyrene in the NUV (200-235 nm) and water in the NIR (1300-1580 nm) were studied. In the NUV turbidimetry, the studied distributions had logarithmic shapes and a width of 46 nm. Light absorption improved the quality of the PSD estimation for distributions with a mean diameter lower than 170 nm; and for the large sizes no improvement was observed. Particle light absorption was avoided reducing the NUV region to the range [235,300] (a transparent NUV spectrum); and for the large sizes, the calculated distributions were very similar to those estimated using the whole NUV spectrum. For these sizes, the particle light absorption can be avoided. In the NIR turbidimetry, valuable information about the PSD is generated by the absorption of light into an opaque medium. The method is only applicable for small particles with diameters lower than 100 nm; and the estimated distributions do not reproduce all the fine details of the distributions. The PSD estimation of small particles could be enhanced by combining the information obtained by the NIR turbidimetry and other optical methods (as, for example, dynamic light scattering).