Adsorption of a polyampholyte chain on a charged surface

We discuss the adsorption of a single polyampholyte chain with degree of polymerization N carrying Nf charges, (Nf(+) positive and Nf(-) negative charges) on a charged surface with a charges per unit area. The adsorption is due to the electrostatic polarization of the polyampholyte chain, which acquires a dipole moment in the surface electric field. Symmetric Gaussian polyampholytes with equal numbers of positive and negative charges adsorb if the surface charge density sigma is larger than (1/alpha l(B)N).(1/f(1/2)) (l(B) is the Bjerrum length and a is the bond size). Polyampholyte chains carrying a charge of the same sign as the surface can also adsorb if the surface charge is large enough. For a typical asymmetry \f(-)-f(+)\approximate to(f/N)(1/2), the adsorption threshold is sigma > (1/alpha l(B)N(3/4))(1/f(1/4)). The effect of the added salt on the adsorption is considered. Finally, we determine the adsorption threshold of a collapsed polyampholyte globule.