Low angle light scattering technique for the study of coagulation

Abstract

A new low angle light scattering technique for the measurement of absolute coagulation rate constants of monodisperse spherical sols has been developed. Measurements have been made on a series of polystyrene latices under both rapid and slow coagulation conditions. The results were compared with calculated values which incorporate (i) a hydrodynamic correction due to Spielman and Honig et al., (ii) an estimate of the electrostatic repulsion between the particles based on constant potential calculations and electrophoretic measurements and (iii) an estimate of the van der Waals attraction based on the Hamaker theory. From the rapid rate measurements (accurate to within ± 13%) the value of the Hamaker constant is expected to lie within the range, 3 × 10^–21 to 1.6 × 10^–21 J. The slow coagulation results suggest that the van der Waals attraction is salt dependent: in 0.2 M NaCl the measured Hamaker constant is ∼4 × 10^–21 J which is the value expected from the classical Hamaker theory, in 0.09 M NaCl agreement between theory and experiment requires this value to have increased to 1.1 × 10^–20 J. These findings are in qualitative agreement with the recent theoretical predictions of Parsegian and Ninham which are based on the macroscopic Lifshitz theory of van der Waals interactions.