流体力学半径对估算胶体微球聚集速率常数的影响

胶体粒子聚集速率常数实验值远低于理论值一直是被普遍关注的问题.聚集速率常数的理论推导是基于粒子的几何半径来考虑的,但决定粒子扩散速率及聚集速率的应该是粒子的流体力学半径(大于几何半径),因而它是使聚集速率常数实验值低于理论值的因素之一.影响流体力学半径的因素很多,其中,带电粒子在溶液中因表面存在双电层,会明显增大流体力学半径,造成聚集速率减慢.而双电层的厚度又随溶液中离子强度的不同而改变.本工作在聚集速率的公式中引入了修正因子,即几何半径与其流体力学半径之比,以修正由于用几何半径代替流体力学半径带来的误差.其中几何半径和流体力学半径可以分别用扫描电镜(SEM)和动态光散射(DLS)来测定.以两种粒径的聚苯乙烯带电微球为例,考察了在不同离子强度下,该误差的大小.结果发现,对于半径为30 nm的微球,用流体力学半径计算的慢聚集速率常数比理论值偏低约8%.该误差随离子强度增加而减少.对于快聚集情况,流体力学半径对聚集速率基本没有影响.

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

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.