Polyelectrolyte-compression forces between spherical DNA brushes.

Optical tweezers are employed to measure the forces of interaction within a single pair of DNA-grafted colloids, dependent on the molecular weight of the DNA chains, and the concentration and valence of the surrounding ionic medium. The resulting forces are short range and set in as the surface-to-surface distance between the colloidal cores reaches the value of the brush height. The measured force-distance relation is analyzed by means of a theoretical treatment that quantitatively describes the effects of compression of the chains on the surface of the opposite-lying colloid. Quantitative agreement with the experiment is obtained for all parameter combinations.

Fluid-fluid demixing transitions in colloid-polyelectrolyte star mixtures.

We derive effective interaction potentials between hard, spherical colloidal particles and star-branched polyelectrolytes of various functionalities f and smaller size than the colloids. The effective interactions are based on a Derjaguin-like approximation, which is based on previously derived potentials acting between polyelectrolyte stars and planar walls. On the basis of these interactions we subsequently calculate the demixing binodals of the binary colloid-polyelectrolyte star mixture, employing standard tools from liquid-state theory. We find that the mixture is indeed unstable at moderately high overall concentrations. The system becomes more unstable with respect to demixing as the star functionality and the size ratio grow.

From sea-urchins to starfishes: controlling the adsorption of star-branched polyelectrolytes on charged walls.

We demonstrate that star-branched polyelectrolytes are a versatile soft component forming a variety of well-characterised conformations upon electrostatic complexation on oppositely charged planar surfaces. We discover and characterise five complexation shapes, described by a two-dimensional morphological order parameter. These shapes are distinguished on the basis of their deviations from sphericity, the orientation of the polyelectrolyte arms, and the equilibrium centre-to-surface distance of the stars. Conformational transformations can be steered by changing the electrostatic surface-star coupling and/or the star functionality. For low functionalities and/or strong external fields, full collapse of the star onto the wall, forming a starfish configuration, is observed. Otherwise, partially adsorbed states with a fraction of chains pointing away from the surface and with the star core being detached from the wall are formed.

Polyelectrolyte stars in planar confinement.

We employ monomer-resolved molecular dynamics simulations and theoretical considerations to analyze the conformations of multiarm polyelectrolyte stars close to planar, uncharged walls. We identify three mechanisms that contribute to the emergence of a repulsive star-wall force, namely, the confinement of the counterions that are trapped in the star interior, the increase in electrostatic energy due to confinement as well as a novel mechanism arising from the compression of the stiff polyelectrolyte rods approaching the wall. The latter is not present in the case of interaction between two polyelectrolyte stars and is a direct consequence of the impenetrable character of the planar wall.

Comment on "A centroid molecular dynamics study of liquid para hydrogen and ortho deuterium" [J. Chem. Phys. 121, 6412 (2004)].

We show that the two phase points considered in the recent simulations of liquid para hydrogen by Hone and Voth lie in the liquid-vapor coexistence region of a purely classical molecular dynamics simulation. By contrast, their phase point for ortho deuterium was in the one-phase liquid region for both classical and quantum simulations. These observations are used to account for their report that quantum mechanical effects enhance the diffusion in liquid para hydrogen and decrease it in ortho deuterium.(c) 2005 American Institute of Physics.