Charged Collapse

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Science  08 Nov 2002:
Vol. 298, Issue 5596, pp. 1139
DOI: 10.1126/science.298.5596.1139a

In dilute solution, a polymer chain in a good solvent exists as a random coil. If the solvent quality is lowered, the chain will collapse to form a globule. For stiffer molecules, the collapse will be an abrupt first-order transition, whereas for more flexible molecules, the collapse will be gradual and resemble a second-order transition. However, for stiff molecules, coils and globules can coexist, so that when an ensemble of chains is studied, the transition may appear to be continuous.

Kiriy et al. probed this transition for polyelectrolyte chains whose behavior is complicated by electrostatic interactions and by the tendency for chains to aggregate. They adsorbed the chains onto a mica surface, while preserving their radial configurations, and then measured the chain dimensions by atomic force microscopy. When salt was added, the chains collapsed to form “pearl necklaces,” in which small globules are connected by extended segments. With increasing salt concentration, the pearls coalesced to form larger clusters. This type of transition is thus intermediate between that of flexible and stiff polymers, and can be described as occurring by a cascade of first-order transitions. — MSL

J. Am. Chem. Soc. 10.1021/ja0261168 (2002).

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