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Volume 129, Issue 24, 28 December 2008
129(2008); http://dx.doi.org/10.1063/1.3050315View Description Hide Description
We show that a system of particles interacting through the exp-6 pair potential, commonly used to describe effective interatomic forces under high compression, exhibits anomalous melting features such as reentrant melting and a rich solidpolymorphism, including a stable BC8 crystal. We relate this behavior to the crossover, with increasing pressure, between two different regimes of local order that are associated with the two repulsive length scales of the potential. Our results provide a unifying picture for the high-pressuremelting anomalies observed in many elements and point out that, under extreme conditions, atomic systems may reveal surprising similarities with soft matter.
129(2008); http://dx.doi.org/10.1063/1.3055596View Description Hide Description
Fluorescence measurements with single-molecule sensitivity are used to measure the hydrodynamic size and local of a weak polyelectrolyte, poly-2-vinyl pyridine end labeled with -sensitive dye, the polyelectrolyte having concentration so low (nanomolars) that molecular properties are resolvable only from fluorescence experiments and cannot be accessed by light scattering. We find that the local near the dye, inferred from its brightness, is consistently three orders of magnitude higher than the bulk . Upon varying the bulk , we measure the collapse point at which hydrophobic attraction overwhelms electrostatic repulsion between charged elements along the chain, and conclude that adding monovalent salt shifts this coil-to-globule collapse to higher than in the absence of salt. The influence of salt appears to shift the ionization equilibrium of this weak polyelectrolyte in the direction of the chain possessing enhanced electric charge at a given . Phenomenologically, this is opposite to the case for strong polyelectrolytes, although the mechanism differs.