J. Phys. II France
Volume 5, Numéro 5, May 1995
Page(s) 677 - 695
DOI: 10.1051/jp2:1995157
J. Phys. II France 5 (1995) 677-695

Flory Theory of a Polyampholyte Chain

Andrey V. Dobrynin1 and Michael Rubinstein2

1  Department of Physics and Astronomy, Unversity of Rochester, Rochester, New York 14627-0011, USA
2  Imaging Research and Advanced Development, Eastman Kodak Company, Rochester, New York 14650-02116, USA

(Received 8 August 1995, revised 12 January 1995, accepted 8 February 1995)

The behavior of a polyampholyte chain is analyzed in the framework of the two-parameter Flory theory. We consider a N-mer containing N+ positively charged monomers and N- negatively charged ones. There are three different regimes for polyampholytes with charge asymmetry |N+ -N-|>(N+ +N-)1/2 depending on the effective temperature $t=\frac{N}{u(N_++N_-)}$, where u is the ratio of the Bjerrum length $l_{\rm B}$ to the bond size b: (i) Unperturbed regime: at high effective temperatures t>t1 the electrostatic interactions are unimportant and polyampholyte configuration is controlled by the solvent quality for the uncharged backbone, (ii) Polyelectrolyte regime: at intermediate effective temperatures t2<t<t1 the repulsion between uncompensated charges e(N+-N-) dominates over the charge-fluctuation-induced attraction. The chain is stretched into an array of electrostatic blobs, (iii) Polyampholyte regime (t<t2): when the Debye screening length due to the charges on the chain becomes smaller than the electrostatic blob size, the charge-fluctuation-induced attraction collapses the polyampholyte into an elongated globule with aspect ratio $\frac{(N_+-N_-)^2}{(N_+ +N_-)}$ independent of the temperature. The nearly symmetric chains with $\vert N_+-N_-\vert\leq (N_+ +N_-)^{1/2}$ directly cross over from regime (i) to regime (iii), but stay nearly spherical.

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