Theory & Bio-Systems, Max Planck Institute of Colloids and Interfaces, Science Park Golm, D-14424 Potsdam, Germany.
Langmuir. 2013 Jun 25;29(25):7939-48. doi: 10.1021/la400342m. Epub 2013 Jun 7.
Electrophoresis is an experimental method widely used to study electrostatic properties of interfaces. Here, we question the validity of the macroscopic theory for the planar geometry by Helmholtz and Smoluchowski by considering a POPC bilayer in an aqueous solution with 500 mM NaCl, using molecular dynamics simulations. We find that POPC shows positive electrophoretic mobility due to adsorption of sodium ions at the lipid headgroups. The theory assumes that the region in which the water density undergoes a transition from the bulk value to zero (interfacial width) is small compared to the Debye screening length. This separation of length scale is not fullfilled in the present case. Hence, contrasting the theory, we observe that the surface is not sharply defined, continuum hydrodynamics is not applicable, the effective viscosity in the double layer is increased compared to the bulk, and the zeta potential is dominated by the dipole potential. Our results might have widespread implications for interpretation of electrokinetic studies in general.
电泳是一种广泛用于研究界面静电特性的实验方法。在这里，我们通过分子动力学模拟，对亥姆霍兹和斯莫卢霍夫斯基提出的用于平面几何的宏观理论的有效性提出质疑。我们发现，由于钠离子在脂质头部的吸附，POPC 表现出正电泳迁移率。该理论假设，水密度从体相值到零（界面宽度）发生转变的区域与德拜屏蔽长度相比很小。在目前的情况下，这种长度尺度的分离并不完全满足。因此，与理论相反，我们观察到表面不是清晰定义的，连续流体动力学不适用，双层中的有效粘度比体相增加，并且 ζ 电位主要由偶极势决定。我们的结果可能对普遍解释电动研究具有广泛的意义。
