Indian Institute of Science Education and Research (IISER), Pune-411008, India.
J Phys Condens Matter. 2010 Dec 15;22(49):494102. doi: 10.1088/0953-8984/22/49/494102.
We study the variation of electrophoretic mobility μ of highly charged spherical colloidal macroions for varying surface charge density σ on the colloid using computer simulations of the primitive model for charged colloids. Hydrodynamic interactions between ions are incorporated by coupling the primitive model of charged colloids to the lattice Boltzmann model (LB) of the fluid. In the highly charged regime, the mobility μ of the colloid is known to decrease with the increase of bare charge Q of the colloid; the aim of this paper is to investigate the cause of this. We have identified that the two main factors contributing to the decrease of μ are counterion charge condensation on the highly charged colloid and an increase in effective friction of the macroion-counterion complex due to the condensed counterions. Thus the established O'Brien and White theory, which identified the dipolar force originating from distortion of the electric double layer as the cause of decreasing μ, seems to break down for the case of highly charged colloids with σ in the range of 30-400 µC cm (- 2). To arrive at our conclusions, we counted the number of counterions q0 moving along with the spherical macroion. We observe in our simulations that q0 increases with the increase of bare charge Q, such that the effective charge Qeff = Q - q0 remains approximately constant. Interestingly for our nanometer-sized charged colloid, we observe that, if surface charge density σ of the colloid is increased by decreasing the radius RM of the colloid but fixed bare charge Q, the effective charge Q - q0 decreases with the increase of σ. This behavior is qualitatively different when σ is increased by increasing Q keeping RM fixed. Our observations address a controversy about the effective charge of a strongly charged macroion: some studies claim that effective charge is independent of the bare charge (Alexander et al 1984 J. Chem. Phys. 80 5776; Trizac et al 2003 Langmuir 19 4027) whereas others claim that Qeff decreases with increasing bare charge Q (dos Santos 2009 J. Chem. Phys. 130 124110; Diehl et al 2004 J. Chem. Phys. 121 12100; Groot et al 1991 J. Chem. Phys. 95 9191) at relatively high values of σ.
我们使用带电胶体的原始模型和格子玻尔兹曼模型（LB）对离子的动力学相互作用进行了耦合，来研究在不同表面电荷密度σ下，高度带电的球形胶体大分子的电泳迁移率μ的变化。在高电荷状态下，胶体的迁移率μ已知随着胶体的裸电荷 Q 的增加而降低；本文的目的是研究导致这种情况的原因。我们已经确定，导致μ降低的两个主要因素是：高度带电胶体上的抗衡离子电荷凝聚和由于凝聚抗衡离子而导致的大分子-抗衡离子复合物的有效摩擦力增加。因此，O'Brien 和 White 建立的理论认为，双电层变形产生的偶极力是μ降低的原因，但对于表面电荷密度σ在 30-400 µC cm（- 2）范围内的高度带电胶体，该理论似乎失效。为了得出我们的结论，我们计算了与球形大分子一起移动的抗衡离子 q0 的数量。我们在模拟中观察到，q0 随着裸电荷 Q 的增加而增加，使得有效电荷 Qeff = Q - q0 保持大致恒定。有趣的是，对于我们的纳米级带电胶体，如果通过减小胶体的半径 RM 但固定裸电荷 Q 来增加胶体的表面电荷密度σ，则有效电荷 Q - q0 会随着σ的增加而减小。当通过保持 RM 固定而增加 Q 来增加σ时，这种行为在定性上是不同的。我们的观察结果解决了关于强带电大分子有效电荷的争议：一些研究声称有效电荷与裸电荷无关（Alexander 等人，1984 年，J. Chem. Phys. 80 5776；Trizac 等人，2003 年，Langmuir 19 4027），而另一些研究则声称 Qeff 随着裸电荷 Q 的增加而减小（dos Santos，2009 年，J. Chem. Phys. 130 124110；Diehl 等人，2004 年，J. Chem. Phys. 121 12100；Groot 等人，1991 年，J. Chem. Phys. 95 9191），在相对较高的σ值下。
