Statics and dynamics of strongly charged soft matter

Abstract

Soft matter materials, such as polymers, membranes, proteins, are often electrically charged. This makes them water soluble, which is of great importance in technological application and a prerequisite for biological function. We discuss a few static and dynamic systems that are dominated by charge effects. One class comprises complexation between oppositely charged objects, for example the adsorption of charged ions or charged polymers on oppositely charged substrates of different geometry. Here the main questions are whether adsorption occurs and what the effective charge of the resulting complex is. We explicitly discuss the adsorption behavior of polyelectrolytes on substrates of planar, cylindrical and spherical geometry with specific reference to DNA adsorption on supported charged lipid layers, DNA adsorption on oppositely charged cylindrical dendro-polymers, and DNA binding on globular histone proteins, respectively. In all these systems salt plays an important role, and some of the important features can already be obtained on the linear Debye-Hückel level. The second class comprises effective interactions between similarly charged objects. Here the main theme is to understand the experimental finding that similarly and highly charged bodies attract each other in the presence of multi-valent counterions. This is demonstrated using field-theoretic arguments as well as Monte-Carlo simulations for the case of two homogeneously charged bodies. Realistic surfaces, on the other hand, are corrugated and also exhibit modulated charge distributions, which is important for static properties such as the counterion-density distribution, but has even more pronounced consequences for dynamic properties such as the counterion mobility. More pronounced dynamic effects are obtained with highly condensed charged systems in strong electric fields. Likewise, an electrostatically collapsed highly charged polymer is unfolded and oriented in strong electric fields. All charged systems occur in water, and water by itself is not a very well understood material. At the end of this review, we give a very brief and incomplete account of the behavior of water at planar surfaces. The coupling between water structure and charge effects is largely unexplored, and a few directions for future research are sketched. On an even more nanoscopic level, we demonstrate using ab-initio methods that specific interactions between oppositely charged groups (which occur when their electron orbitals start to overlap) are important and cause ion-specific effects that have recently moved into the focus of interest.

The role of multipoles in counterion-mediated interactions between charged surfaces: strong and weak coupling

“…In the absence of a general approach that would cover thoroughly all the regions of the parameter space one has to take recourse to various partial formulations that take into account only this or that facet of the problem [15]. The traditional approach to these one-component Coulomb fluids has been the mean-field Poisson-Boltzmann (PB) formalism applicable at weak surface charges, low counterion valency and high temperature [13,16,17].…”

“…The traditional approach to these one-component Coulomb fluids has been the mean-field Poisson-Boltzmann (PB) formalism applicable at weak surface charges, low counterion valency and high temperature [13,16,17]. The limitations of this approach become practically important in highly-charged systems where counterion-mediated interactions between charged bodies start to deviate substantially from the mean-field accepted wisdom [15]. One of the most important recent advances in this field has been the systematization of these non-PB effects based on the notions of weak and strong coupling approximations.…”

Adsorption of Weak Polyelectrolytes on Charged Nanoparticles. Impact of Salt Valency, pH, and Nanoparticle Charge Density. Monte Carlo Simulations

“…Moreover, two dewrapping transitions occur at very low and very high salt concentrations. 31,32 Particular complex conformations such as rosette structures were analytically predicted with scaling theories in which the chain stiffness was shown to be the most important. 33,34 Indeed, polyelectrolyte wrapped to rosette transition occurs with an increase of the persistence length.…”

Sign of the Casimir-Polder interaction between atoms and oil-water interfaces: Subtle dependence on dielectric properties

“…An aug-cc-pV6Z basis set [25,26] was used for He, Ne, and Ar, and aug-cc-pV5Z was used for Kr [27]. Polarizabilities α(iξ ) in vacuum were transformed to excess polarizabilities α * (iξ ) in water via the relation for a dielectric sphere embedded in a dielectric medium [28,29], α * = R 3 ε a − ε w ε a + 2ε w .…”

“…An aug-cc-pV6Z basis set [25,26] was used for He, Ne, and Ar, and aug-cc-pV5Z was used for Kr [27]. Polarizabilities α(iξ ) in vacuum were transformed to excess polarizabilities α * (iξ ) in water via the relation for a dielectric sphere embedded in a dielectric medium [28,29], Here ε w is the dielectric function of water and R is the radius of the atom. ε a is the effective dielectric function of the atomic sphere, estimated from the atomic polarizability in vacuum as ε a = 1 + 4πα/V , where V is the volume of the atomic sphere.…”