The particle size of substances is important in pre-formulation studies. Manufacturing a stable liquid formulation and improving the sedimentation rate becomes a challenging experience. It is difficult to disperse the particles without the occurrence of sedimentation. For the present study, pumice is used as a model because of its non irritant, non interfering and non reactive nature. In order to calculate and compare particle size effects, different techniques such as sieving, microscopy, sedimentation, scanning electron microscopy and laser diffraction techniques were used.
Hindered settling is used to evaluate the sedimentation by calculating the floccule size for pumice. The study was performed using different concentrations of pumice and surfactants in water and glycerin mixtures as a dispersion medium. The surfactants used for the studies belonged to different categories which included Tween 20® (non ionic), benzalkonium chloride (cationic) and sodium lauryl sulfate (anionic). In the present study the floccule size was calculated using three different mathematical models, namely the Richardson and Zaki, Steinour and Dollimore and McBride equations.
The particle size for the floccules obtained using sodium lauryl sulfate surfactant was the largest amongst the three surfactants. It is also possible that the sedimentation rate was delayed by the dispersion medium. There is no direct relationship known between the pumice and the surfactants behavior.
Laser Diffraction technique is more accurate in particle size calculation. However it fails to calculate the particle size of pumice retrained on sieve # 300. Significant difference was observed in the results obtained when using different techniques.