A discussion of the constraints which lead to steady state electrophoretic boundaries

The electric field variation across moving boundaries in three component systems is analyzed with a model based on electromigration only (moving boundary equation and regulating function). A decrease in the electric field strength across a boundary in the direction of migration is the most general requirement for the establishment of steady state migrating boundaries. The formation of that electric field gradient is shown to be based on two fundamental constraints. These are (1) the higher net mobility of the leading component compared to that of the terminating constituent and (2) the higher concentration ratio of the leading component to the terminating component ahead of the moving boundary compared to that in the terminating zone. The relations are discussed with respect to strong and weak electrolytes where all three components are present on either side of the boundary. The predictions are verified experimentally. Limitations of the model based on migration only are discussed.