# Sequestration of Small Ions and Weak Acids and Bases by a Polyelectrolyte Complex Studied by Simulation and Experiment

## TL;DR

This paper studies how small ions and weak acids/bases are separated by a polyelectrolyte complex using simulations and experiments.

## Contribution

A new coarse-grained model for the Grand-reaction method is introduced to study phase separation in polyelectrolyte complexes.

## Key findings

- Monovalent ions partition equally between phases, while divalent ions accumulate in the PEC phase.
- Weak diprotic acid ionization is enhanced in the PEC phase, increasing with pH.
- Model parameters can be fine-tuned to achieve near-quantitative agreement with experiments.

## Abstract

Mixing of oppositely charged polyelectrolytes can result
in phase
separation into a polymer-poor supernatant and a polymer-rich polyelectrolyte
complex (PEC). We present a new coarse-grained model for the Grand-reaction
method that enables us to determine the composition of the coexisting
phases in a broad range of pH and salt concentrations. We validate
the model by comparing it to recent simulations and experimental studies,
as well as our own experiments on poly(acrylic acid)/poly(allylamine
hydrochloride) complexes. The simulations using our model predict
that monovalent ions partition approximately equally between both
phases, whereas divalent ones accumulate in the PEC phase. On a semiquantitative
level, these results agree with our own experiments, as well as with
other experiments and simulations in the literature. In the sequel,
we use the model to study the partitioning of a weak diprotic acid
at various pH values of the supernatant. Our results show that the
ionization of the acid is enhanced in the PEC phase, resulting in
its preferential accumulation in this phase, which monotonically increases
with the pH. Currently, this effect is still waiting to be confirmed
experimentally. We explore how the model parameters (particle size,
charge density, permittivity, and solvent quality) affect the measured
partition coefficients, showing that fine-tuning of these parameters
can make the agreement with the experiments almost quantitative. Nevertheless,
our results show that charge regulation in multivalent solutes can
potentially be exploited in engineering the partitioning of charged
molecules in PEC-based systems at various pH values.