research - charged systems

charged systems

charged polymers
electrical double layers

	statics and dynamics of charged polymers Charged soft matter in external fields exhibits a very rich and complex behavior that is determined by the interplay between electrostatic, elastic and hydrodynamic interactions. Electric fields are important in many technological and biological application (most noteworthy electrophoresis of DNA and proteins). Experimentally, it is known that charged polymers show conformational changes in electric fields, which in turn influence their mobility. Such non-equilibrium phenomena are studied with Brownian molecular dynamics simulations. We indeed find that at strong enough fields a polyelectrolyte unfolds. This unfolding is most abrupt if the polymer is collapsed by the action of multivalent counterions. This transition might be relevant for capillary electrophoresis, since the mobility should change drastically in the vicinity of this transition which might lead to enhanced separation of polyelectrolytes of different length. The polarizability of the ionic clouds of rod-like polymers leads to an orientation of the polyelectrolyte that can be detected by optical methods (birefringence). We study the influence of the salt and polymer concentration, and the effect of hydrodynamic interactions and hydrodynamic screening. With similar methods we also investigate the diffusion and electrophoretic motion of various charged colloidal systems, e.g. two-dimensional layers of oppositely charged particles in electric fields as models for membrane-based two-dimensional electrophoresis. related publications: X. Schlagberger, J. Bayer, J. O. Rädler and R. R. Netz Diffusion of a single semiflexible charged polymer Europhys. Lett. (2006) (doi) V. Lobaskin, B. Duenweg, M. Medebach, T. Palberg, C. Holm Electrophoresis in colloidal dispersions in the counterion dominated screening regime cond-mat/0601588 (pdf) X. Schlagberger and R.R. Netz Orientation of Elastic Rods in Homogeneous Stokes Flow Europhysics Letters 70 (1), 129-135 (2005) (doi) M. Manghi and R.R. Netz Variational theory for a single polyelectrolyte chain revisited Eur. Phys. J. E, 14, 67 (2004) (doi) R.R. Netz Non-Equilibrium Unfolding of Polyelectrolyte Condensates in Electric Fields Phys. Rev. Lett. 90, 128104 (2003) (doi) R.R. Netz Polyelectrolytes in Electric Fields J. Phys. Chem. B, 107, 8208 (2003) (doi) R.R. Netz Conduction and diffusion in two-dimensional electrolytes Europhys. Lett. 63, 616 (2003) (doi) top


	statistics and dynamics of electrical double layers Almost all biological and many technologically relevant surfaces are charged. The distribution of ion-densities and the electrostatic potential at charged surfaces are important for the understanding and description of these simple model systems and were in the past obtained on the mean-field level with the Poisson-Boltzmann equation. Correlation effects in solutions have been described using the linearized Debye-Hückel theory. We recently introduced a field-theoretic framework within which correction terms to the Poisson-Boltzmann theory are systematically included via a loop-wise expansion around the saddle point and which acts as a merger of Poisson-Boltzmann and Debye-Hückel theories. In the future, this theory will be applied to other geometries (spherical and cylindrical) and more complicated systems (many spheres, salt ions, additional interactions), allowing for a description of biologically relevant electrostatic processes. In parallel to this theory, we developed a strong-coupling theory for counter-ion distributions This theory becomes exact in the limit of highly valent ions, low temperatures, large charges, which is the opposite limit from the one where the mean-field description becomes accurate. Using these methods, we calculated the counter-ion density profile at a charged wall, which gives quantitative agreement with extensive numerical simulation. Since experimentally one is very often in the strong-coupling limit, there is a whole bunch of experimentally relevant applications of this theory which I presently pursue (different geometries, etc.). We also looked at the interaction between two similarly charged surfaces. We find, in quantitative agreement with simulations, that the two surfaces attract each other at small separations and for large enough coupling parameter. Such attractions between similarly charged objects are presently at the core of intense and controversial discussions in the community. Our results not only show (in an asymptotic exact fashion) that such attractions exist, they also allow for a simple and intuitively instructive description and explanation of these attractions. related publications: C. Fleck and R.R. Netz Counterions at disordered charged planar surfaces Europhysics Letters 70, 341 (2005) (doi) C. Fleck and R.R. Netz Counterion Density Profiles at Charged Flexible Membranes Physical Review Letters 95, 128101 (2005) (doi) Andre G. Moreira and Roland R. Netz Simulations of Counterions at Charged Plates European Physical Journal E 8, 33 (2002) (doi) Andre G. Moreira and Roland R. Netz Strong-Coupling Theory for Counter-Ion Distributions Europhysics Letters 52, 705 (2000) (doi)

top