Stratford upon Avon, UK
Particle Size Distribution by Space or Time Dependent Extinction Profiles obtained by Analytical Centrifugation
T. Detloff, T. Sobisch and D. Lerche
L. U.M. Gesellschaft für Labor-, Umweltdiagnostik & Medizintechnik mbH
Rudower Chaussee 29 (OWZ) 12489 Berlin / Germany
e-mail info@lum-gmbh.de, www.lum-gmbh.com
ABSTRACT
The particle size distribution of suspensions of fine particles were determined by a photocentrifuge from the transmission / extinction profiles. Photocentrifuges provide a useful tool for the characterization and quality control of suspensions. While the variation of the light extinction curves caused by centrifugal segregation allows a qualitative description, a rigorous formulation can give access to a quantitative characterization. The present work reports on the determination of the particle size distribution using space and time resolved extinction profiles. The particle size distribution is obtained by analysing the variation of the extinction at any point of the sample over centrifugation time or by analysing the extinction variation over the entire sample length at any time span of centrifugation. The advantage of the second method is its potential to save time.
Keywords: particle size analysis, sedimentation, analytical centrifugation, extinction profiles
1 INTRODUCTION
The characterization of the dispersed state of suspensions and emulsions is essential for technical applications in diverse fields such as nanomaterials, coatings, paper making, ceramics, cosmetics, sludge dewatering, to name only a few. Techniques are preferable, which avoid dilution, thus do not modify dispersion properties. In this respect analytical centrifugation has a great potential, however, it was seldom exploited. Today the measurement of the particle size distribution (PSD) of dispersions by sedimentation in a centrifugal field is conducted by analysing the variation of the extinction at one position over the measurement time using line start or homogeneous technique. This method (Constant Position) leads to an integral which only can be solved approximately (all particles have to be homogenously distributed in the dispersion at the beginning). It is also necessary that all particles have passed the detector (ISO 13318, Allen (1999)). One measurement could take a long time, depending on the material properties of the dispersion. An alternative faster solution is provided by analysing the dispersion concentration over the entire sample length at least at one time. This method (Constant Time) has the advantage that an analytical solution with algebraic equations is possible and enables to reduce the time of measurement considerably, because it is not required that all particles are settled out.
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