Phase Behavior of Partially Charged Polyelectrolyte Solutions with Salt: A Theoretical Study

Abstract

AbstractWeakly (or partially) charged polyelectrolytes represent an important subset of the polyelectrolyte family. For instance, polycarboxylate‐based superplasticizers, or PCEs, are comb‐shaped polyelectrolytes possessing anionic backbones grafted with neutral side chains. PCEs play an important role in modern concrete production. However, their behavior in salt solutions remain poorly understood. The present work builds upon a recently developed liquid‐state theory for fully charged polyelectrolyte solutions and extends it to describe the phase behavior and salt partitioning of PCEs in a 2:1 salt solution. Previous studies have shown that there can be an additional short‐range attraction, often referred to as the “calcium‐binding” interaction between calcium ions and the negatively‐charged carboxylate groups of PCEs. Such a calcium‐binding interaction and how its strength affects the phase behavior are investigated. It is found that increasing the calcium‐binding strength expands the phase‐separated region and increases the critical extra salt concentration, and it also leads to a wider phase‐separated region for salting‐out and salting‐in phenomena. The structural parameters of PCEs also affect the phase behavior. Increasing the side‐chain length shrinks the phase‐separated region, while increasing the acid‐to‐ether ratio expands the phase‐separated region. Those results may find applications in molecular design of weakly charged polyelectrolytes like PCEs.