Acta Phys. -Chim. Sin. ›› 2010, Vol. 26 ›› Issue (10): 2807-2812.doi: 10.3866/PKU.WHXB20100941


Effect of the Hydrodynamic Radius of Colloid Microspheres on theEstimation of the Coagulation Rate Constant

DU Xuan, XU Sheng-Hua, SUN Zhi-Wei, AA Yan   

  1. Key Laboratory of Microgravity, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, P. R. China
  • Received:2010-04-02 Revised:2010-05-19 Published:2010-09-27
  • Contact: SUN Zhi-Wei
  • Supported by:

    The project was supported by the National Natural Science Foundation of China (10972217, 10932012) and Knowledge Innovation Program of the Chinese Academy of Sciences (KJCX2 -YW-L08).


Experimental values of the coagulation rate constant for colloidal particles are known to be much lower than the theoretical values. Only the particle's geometric radii are used in the theoretical derivation of coagulation rate constant. However, it should actually be the hydrodynamic radius (larger than the geometric radius) that determines the particles' diffusion speed and thus the coagulation rate. Therefore, it is one of the reasons that cause the experimental coagulation rate constant lower than the theoretical one. Many factors affect the hydrodynamic radius and among them the electric double layer can significantly swell the hydrodynamic radius, which lowers the coagulation rate. The thickness of the electric double layer changes with the ionic strength of the solution. To correct the error caused by ne-glecting the difference between the geometric and hydrodynamic radius, a correction factor, which is the ratio of geometric radius to hydrodynamic radius, is introduced in this study. The geometric radius and the hydrodynamic radius were determined by scanning electron microscopy (SEM) and dynamic light scattering (DLS), respectively. Two different sized polystyrene microspheres were used to investigate the effect of ionic strength on the difference between the experimental coagulation rates and the theoretical ones. The results show that for slow aggregation, the rate constant calculated by using the hydrodynamic radius can be lower than its theoretical value by about 8% for microspheres with radius of 30 nm. This difference decreases with the increase of ionic strength. The effect of the hydrodynamic radius on the coagulation rate is negligible for fast aggregation.

Key words: Polystyrene microspheres, Ionic strength, Hydrodynamics radius, Geometric radius, Coagulation rate constant