Chain Model for Polyelectrolytes. III. Equimolar Polyampholytes of Regularly Alternating Structure

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Abstract
The chain model for polyelectrolytes developed in the preceding papers of this series is extended to polyampholytes composed of a regular alternation of acidic and basic groups. Included in this model are the effects of counter ion binding, dissociation equilibria among the acidic and basic groups, and electrostatic interactions among the charges of the polymer. The electrostatic free energy of interaction between neighboring charges of the chain was computed including interactions between both nearest and next nearest neighboring charge sites by a simple extension of the methods previously employed. The equilibrium conditions are then expressible as a set of simultaneous equations which may be solved by a convenient iterative method. The end‐to‐end extension may be calculated by previously described methods except in the vicinity of the isoelectric point (i.p.), near which point the effects of charge fluctuations become relatively more important. It is shown how both charge fluctuation and the distribution of monomer units along the polymer chain are related to its end‐to‐end extension. However, for a regularly alternating copolymer the skeletal distribution effect vanishes. Calculations for a hypothetical polymer indicate that the expansion is nearly independent of ionic strength, and that the fluctuations produce a negligibly small contraction at the i.p. However, it is found that added salt acts, primarily by reducing the electrostatic forces, to alter the titration curves in the same manner as has been observed experimentally in dilute protein solutions.