Session J46: Invited Session: Electrostatic Interactions in Polymeric and Biological Systems

Ion Specific Effects at Interfaces

Availability of highly reactive halogen ions at the surface of aerosols has tremendous implications for the atmospheric chemistry. Yet neither simulations, experiments, nor existing theories are able to provide a fully consistent description of the electrolyte-air interface. In this talk a new theory will be presented which allows us to explicitly calculate the ionic density profiles, the surface tension, and the electrostatic potential difference across the solution-air interface [1,2]. The theory takes into account both ionic hydration and polarizability [3]. The theoretical predictions are compared to experiments and are found to be in excellent agreement. Finally, the implications of the present theory for stability of lyotropic colloidal suspensions will be considered [4], shedding new light on one of the oldest puzzles of physical chemistry --- the Hofmeister effect.\\[4pt] [1] Y. Levin, A.P. dos Santos, and A. Diehl, Phys. Rev. Lett. {\bf 103}, 257802 (2009). \\ \noindent [2] A. P. dos Santos, A. Diehl, and Y. Levin, Langmuir {\bf 26}, 10778 (2010)\\ \noindent [3] Y. Levin, Phys. Rev. Lett. {\bf 102}, 147803 (2009) \\ \noindent [4] A. P. dos Santos and Yan Levin, Phys. Rev. Lett. {\bf 106}, 167801 (2011)   

Solvation of ions in bulk and at interfaces: What can density functional theory teach us?

Insights from molecular simulation have influenced both experiment and theory regarding the understanding of the specific ion effect. Although there seems to be a consensus that large polarizable anions exist at the air-water interface, the understanding of the precise molecular interactions that give rise to surface adsorption remain elusive. I will present our work on the adsorption of iodide at the air-water interface using density functional theory (DFT) based interaction potentials. I will discuss similarities and differences of the results obtained using different descriptions of molecular interaction. Last, we are able to reconcile the results obtained with molecular simulation using standard empirical potentials with the dielectric continuum theory of Levin and co-workers.\\[4pt] In collaboration with Marcel Baer, Pacific Northwest National Laboratory; Douglas Tobias, Abe Stern, University of California, Irvine, CA; and Yan Levin, Instituto de F\'isica, UFRGS, Porto Alegre, RS, 91501-970, Brazil.   

Simulation of single molecule stretching experiments on denatured ssDNA

We have performed simulations of stretching denatured ssDNA using a bead-spring model to compare with recent single molecule experiments. Each bead represents a singly charged base or monomer in the ssDNA. The salt and counterions are explicitly treated. An equal and opposite force is applied to the two terminal beads, and the force-extension curves are calculated at a range of forces. In denatured ssDNA, the bases of ssDNA are blocked from pairing making the flexible polyelectrolyte model applicable. The recent single molecule stretching experiments\footnote{McIntosh and O.A. Saleh, Macromolecules {\bf 44}, 2328 (2011).} on denatured ssDNA have studied the effect of added salt treating both 1:1 and 2:1 salts. These experiments found force-extensions curves exhibit two regimes: at low forces, the extension $R \sim f^\gamma$, where $\gamma = 0.60 - 0.69 $ and a high force regime where $R \sim \log f$. The force-extension curves can be scaled to produce overlap for the salt dependence. The force at the crossover between the two regimes $f_c$ scales as $c_s^{1/2}$ for 1:1 salt, but as $I^3$ for 2:1 salt, where is $I$ is the ionic strength. The simulation data reproduce the experimental overlap for both salt cases. The two regimes including the logarithmic behavior at large forces are present in the simulation results. The simulation results imply that the behavior is due to a competition between electrostatics, entropy and the applied force and that other molecular interactions can be neglected. Thus, the standard theoretical methods are not missing an important term in the free energy, although approximation level may be an issue. We will present simulation data on the conformations as a function of applied force and discuss the implications for analytic theory.   

Charge-regularization effects on polyelectrolytes

When electrically charged macromolecules are dispersed in polar solvents, their effective net charge is generally different from their chemical charges, due to competition between counterion adsorption and the translational entropy of dissociated counterions. The effective charge changes significantly as the experimental conditions change such as variations in solvent quality, temperature, and the concentration of added small electrolytes. This charge-regularization effect leads to major difficulties in interpreting experimental data on polyelectrolyte solutions and challenges in understanding the various polyelectrolyte phenomena. Even the most fundamental issue of experimental determination of molar mass of charged macromolecules by light scattering method has been difficult so far due to this feature. We will present a theory of charge-regularization of flexible polyelectrolytes in solutions and discuss the consequences of charge-regularization on (a) experimental determination of molar mass of polyelectrolytes using scattering techniques, (b) coil-globule transition, (c) macrophase separation in polyelectrolyte solutions, (c) phase behavior in coacervate formation, and (d) volume phase transitions in polyelectrolyte gels.   

Effect of electrostatic interactions on lubrication in polymeric and biological systems

Many connective tissues, such as cartilage demonstrate excellent lubrication and wear characteristics. Cartilages in mammalian joints can withstand pressures of the order of ten atmospheres and have remarkably low friction coefficient in the range of 0.001-0.03. The surface of the cartilage is covered with bottle-brush-like polyelectrolyte layer consisting of glycoproteins. This brush layer, which faces a similar layer on the opposing cartilage, is sheared as two surfaces slide passing each other during joint motion. We have performed molecular dynamics simulations of charged and neutral bottle-brush macromolecules tethered to substrates to understand the role of the electrostatic and hydrodynamic coupling between brush layers on the lubricating properties in biological and polymeric systems. Glycoprotein layers were modeled as two opposing layers of highly charged bottle-brush macromolecules composed of Lennard-Jones particles grafted to a substrate. Simulations have shown that charged bottle-brush systems have lower friction under shear and weaker dependence of the disjoining pressure on substrate separation than neutral bottle-brush systems. This was explained by formation of lubricating layer with excess of counterions located in the middle between brush-bearing surfaces. In overlapping brush layers the disjoining pressure between brush-bearing surfaces is controlled by the bottle-brush bending rigidity. Under shear, the main deformation mode of the charged bottle-brush layers is associated with the bottle-brush backbone deformation resulting in backbone deformation ratio and shear viscosity being universal functions of the Weissenberg number. In the case of neutral bottle-brush systems there is coupling between backbone and side chain deformation. This violates universality in backbone deformation ratio and manifests itself in shear viscosity dependence on the shear rate. The shear viscosity as a function of the shear rate for the neutral bottle-brush systems has two plateaus and two shear thinning regimes.