Theory of polyzwitterion conformations

TL;DR

This paper develops a theoretical framework to understand the conformations of polyzwitterions in water, considering electrostatic interactions, salt effects, and counterion adsorption, revealing conditions for chain collapse and expansion.

Contribution

It introduces analytical relations for polyzwitterion size considering various electrostatic and ionic factors, advancing understanding of their solution behavior.

Findings

Attractive dipole interactions cause chain collapse without small ions.

Salt screening and counterion adsorption influence chain size and conformation.

Charge-dipole and dipole-dipole interactions significantly affect polyzwitterion behavior.

Abstract

Conformational characteristics of polyzwitterionic molecules in aqueous solutions are investigated using the variational method. Analytical relations are derived for the radius of gyration of a single polyzwitterionic chain as a function of the chain length, electrostatic interaction strength, added salt concentration, dipole moment and degree of ionization of the zwitterionic monomers. In the absence of the small ions (counterions and coions) near the polyzwitterionic chain, attractive dipole-dipole interactions are shown to induce a collapse of the polyzwitterionic chain. However, in the presence of the small ions, the radius of gyration is shown to be an interplay of the screening of the electrostatic interactions and the counterion adsorption on the zwitterionic sites. In addition to the well-known Debye-Huckel screening of the charge-charge interactions, screening of the charge-dipole and dipole-dipole interactions are found to play important roles in determining the size of the chain. Functional forms for the screened charge-dipole and dipole-dipole interaction potentials are presented. Furthermore, counterion adsorption on the zwitterionic monomers is predicted to be asymmetric depending on the nature of the added salt and the zwitterionic groups. Qualitative remarks regarding the solubility of these molecules in aqueous solutions along with the classical "anti-polyelectrolyte" effect (increase in the solubility in water with the addition of salt) are presented.