A Neutral Polyampholyte in an ionic solution

TL;DR

This paper models the behavior of a neutral polyampholyte in ionic solutions using the DHBjF theory, revealing temperature-dependent phase transitions between extended and collapsed states influenced by salt concentration.

Contribution

It provides a theoretical analysis of polyampholyte conformations in ionic solutions, incorporating electrostatic interactions and phase transitions with a comprehensive model.

Findings

High salt and temperature favor extended states.

Decreasing temperature induces collapse into a microelectrolyte.

Low salt densities lead to persistent collapsed states with phase transition at low temperatures.

Abstract

The behavior of a neutral polyampholyte ($PA$) chain with $N$ monomers, in an ionic solution, is analyzed in the framework of the full Debye-H$\ddot u $ckel-Bjerrum-Flory $(DHBjF)$ theory. A $PA$ chain, that in addition to the neutral monomers, also contains an equal number of positively and negatively charged monomers, is dissolved in an ionic solution. For \underline{high} concentrations of salt and at high temperatures, the $PA$ exists in an extended state. As the temperature is decreased, the electrostatic energy becomes more relevant and at a $T=T_{\theta}$ the system collapses into a dilute globular state, or microelectrolyte. This state contains a concentration of salt higher than the surrounding medium. As the temperature is decreased even further, association between the monomers of the polymer and the ions of the salt becomes relevant and there is a crossover from this globular state to a low temperature extended state. For \underline{low} densities of salt, the system is collapsed for almost all temperatures and exhibits a first-order phase transition to an extended state at an unphysical low temperature.