Electrophoresis of a charged porous sphere normal to an air-water interface.

Electrophoretic behavior of a charged porous sphere normal to an air-water interface is investigated theoretically. The charged porous sphere is a very popular model of DNA and proteins, among other polyelectrolytes, based on the experimental observation of the DNA and protein conformations in a free solution. Two major motion-deterring effects are thoroughly investigated in particular: the boundary effect due to the presence of an air-water interface and the double layer polarization effect due to the convection-induced ion flux redistribution. The presence of the air-water interface is found to reduce the particle mobility in general, especially when the double layer is very thick or the particle is close to the interface. This boundary effect diminishes as the double layer gets very thin. However, an interesting phenomenon is observed that a particle closer to the interface may actually move faster than a farther one under some circumstances. The reason behind this phenomenon is attributed to the interaction between the boundary and polarization effects. This is demonstrated with evidence of corresponding contour plots. Convenient charts of correction factors are provided to facilitate the usage by interested experimental researchers.