为考察电解质溶液中不同离子在膜表面的吸附作用,采用自制的截留相对分子质量40 000的PVDF中空纤维超滤膜进行电性能和渗透性能表征.由于PVDF无可离子化基团或固有电荷,可大大简化模型,因而考察了膜在1 mmol/L四种不同类型电解质(阴阳离子价态)KCl(1:1),K2SO4(1:2),MgCl2(2:1)和MgSO4(2:2)溶液中不同pH下的zeta电势.结果发现:在pH 3～9之间,膜在MgCl2,MgSO4,KCl和K2SO4溶液中的等电点分别为7.4,7.0,6.9和5.5.这是由于膜表面吸附离子的迁移速率不同造成的.离子的化合价和水合离子半径对离子迁移速率的影响非常大.迁移速率更快的离子可在膜表面吸附得更多,并产生一层过量的离子层,对膜表面电荷的影响力更大.不同离子的迁移速率决定了膜不同的等电点.膜在电解质溶液中的截留结果与相应的zeta电势吻合.在等电点附近的截留为零,而往两侧逐渐升高.膜的通量在等电点处最大,往两侧逐渐降低.
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