The Electrophoretic Mobility of PPI Dendrimers:  Do Charged Dendrimers Behave as Linear Polyelectrolytes or Charged Spheres?

The electrophoretic mobilities μ0 of charged polypropyleneimine dendrimers were measured by capillary electrophoresis as a function of ionization α, ionic strength I, and hydrodynamic radius a. For the two generations examined, 3 and 5, μ0 trends reminiscent of a linear polyelectrolyte are found in α but not in I and a. Strikingly, all three trends accurately follow standard electrokinetic model predictions for a charged, ion-impenetrable dielectric sphere of radius a. Although the sphere depiction is physically inexact, the meshing of theory to experiment requires no fitting of parameters. For highly charged dendrimers across the accessible aκ range (0.2 < aκ < 2), the standard electrokinetic model quantitatively captures a large observed relaxation effect. No evidence for nonspecific ion binding was uncovered. Commercial availability and conformity to sphere theory suggest that charged dendrimers might serve as model electrophoresis systems. Limitations and alternatives to the sphere depiction are discussed.