1.Key Laboratory of Microgravity, Institute of Mechanics, Chinese Academy of Sciences, Beijing 100190, China 2.School of Engineering Science, University of Chinese Academy of Sciences, Beijing 100049, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 11672295, 11572322)
Received Date:04 September 2020
Accepted Date:12 November 2020
Available Online:25 February 2021
Published Online:05 March 2021
Abstract:The effective surface charge of colloid particles is an important parameter that determines the colloidal properties. However, it is still unclear whether the solvent environment (such as the electrolyte concentration) can affect the effective surface charge. Due to complicated effects relevant to the effective surface charge, such as the exchange of dissociable ions between the electrolyte and surface groups of polystyrene particles, the coupling effect of incomplete ionization of the surface groups of the particles and the adsorption of ions by colloidal particles, etc., it is rather difficult to accurately measure the surface charge and understand the mechanism of charge variation with solvent environment. To solve this problem, we measure the conductivities of polystyrene colloidal particles of carboxyl groups and sulfonic acid groups at various particle number densities and HCl concentrations. Since the cations generated from the two kinds of particles and HCl solution are all H+ cations, the surface charge can be obtained by the conductivity-number density method (migrant method), no matter whether the cation exchanges occur between ionized positive ions of the electrolyte and colloidal particles. Based on the experimental results, the influences of HCl concentration and particle number density on the surface charge of colloidal particles are detected, and the reasons of the influence are analyzed. It is found that the change of the surface charge of the particles of carboxyl group with HCl concentration is faster than that of sulfonic acid group with the HCl concentration. For the same electrolyte concentration, the effective surface charge of carboxyl modified colloidal particles is related to the particle number density, while the charge of sulfonic modified particles is not. Considering the fact that the sulfonic acid group and carboxyl group are strong and weak acid groups respectively, the ionization of H+ cations of the two different groups have profound influences on the cation replacement process, and affect the trend of the curve of the conductivity-particle number density. This effect further results in different change tendencies of effective surface charge with HCl concentration and particle number density. According to the theoretical model as described in this study, all experimental results are well explained. The mechanisms described in this article will be useful for stating the influencing factors of the surface effective charge, and the application of the effective charge to different phenomena relating to interparticle interactions with different parameters of solutions. Keywords:colloidal particles/ effective surface charge/ electrolyte concentration/ particle number density
图2给出了磺酸基/羧基修饰聚苯乙烯胶体在不同HCL浓度下的电导率与数密度的关系曲线, 对同一HCl溶液浓度, 磺酸基修饰颗粒的电导率均与粒子数密度呈线性增长关系(图2(a)), 符合公式(9)关于表面电荷不随粒子数密度发生变化的假设, 而羧基修饰胶体颗粒的电导率随粒子数密度的变化曲线(图2(b))在某一HCl溶液浓度下并不完全线性, 数密度较低时, 电导率增长较快, 增势逐渐减缓, 直到数密度到达某一数值后, 电导率与数密度才呈线性关系. 图 2 (a)磺酸基和(b)羧基修饰聚苯乙烯胶体的电导率-数密度关系曲线 Figure2. Conductivity of (a) sulfonic acid and (b) carboxyl-modified polystyrene as function of the number density.