Transport properties of polyelectrolytes in solution, such as electrophoretic mobilities, are strongly related to the distribution of the counterions in the vicinity of the molecules. When the charge density of the polyelectrolyte is above a critical value, a certain amount of counterions will condense on the polyelectrolyte and neutralize part of its charge (Manning condensation). We investiged the dynamics and the distribution of the counterions within a distance of one Bjerrum length from a polyelectrolyte molecule using molecular dynamic simulations. We found that counterion condensation is a dynamic process in which individual ion enters and leaves the vicinity of the molecule independently and randomly on the time scale of the diffusion time across a Debye layer. On this time scale, we found that the distribution of the accumulated counterions along the polyeletrolyte is not uniform, and we can associate each condensed conterion to its nearest bead in the polymer molecule and consider a bead-ion pair. We found that these bead-ion pairs are strongly correlated at short length scale, but are independent when the separation is large. Our simulations provide information on the detail arrangement of the condensed counterions along a flexible polyelectrolyte, which is useful in understanding the electrophoretic behavior of, as well as the interaction between, these molecules.
Journal: TechConnect Briefs
Volume: 3, Technical Proceedings of the 2007 NSTI Nanotechnology Conference and Trade Show, Volume 3
Published: May 20, 2007
Pages: 284 - 287
Industry sector: Sensors, MEMS, Electronics
Topics: Micro & Bio Fluidics, Lab-on-Chip