Overcharging and reentrant condensation of thermoresponsive ionic microgels

TL;DR

This study explores how thermoresponsive ionic microgels interact with cationic polylysine, revealing temperature-triggered overcharging and reentrant condensation phenomena driven by electrostatic interactions.

Contribution

It demonstrates the temperature-dependent overcharging and reentrant condensation of microgels with polylysine, highlighting the electrostatic nature of these effects and their reversibility.

Findings

Microgel overcharging occurs above the volume phase transition temperature.

Reentrant condensation can be induced by temperature or polycation density.

Electrostatic interactions are key to the observed phenomena.

Abstract

We investigated the complexation of thermoresponsive anionic poly(N-isopropylacrylamide) (PNiPAM) microgels and cationic $\epsilon$-polylysine ($\epsilon$-PLL) chains. By combining electrophoresis, light scattering, transmission electron microscopy (TEM) and dielectric spectroscopy (DS) we studied the adsorption of $\epsilon$-PLL onto the microgel networks and its effect on the stability of the suspensions. We show that the volume phase transition (VPT) of the microgels triggers a large polyion adsorption. Two interesting phenomena with unique features occur: a temperature-dependent microgel overcharging and a complex reentrant condensation. The latter may occur at fixed polyion concentration, when temperature is raised above the VPT of microgels, or by increasing the number density of polycations at fixed temperature. TEM and DS measurements unambiguously show that short PLL chains adsorb onto microgels and act as electrostatic glue above the VPT. By performing thermal cycles, we further show that polyion-induced clustering is a quasi-reversible process: within the time of our experiments large clusters form above the VPT and partially re-dissolve as the mixtures are cooled down. Finally we give a proof that the observed phenomenology is purely electrostatic in nature: an increase of the ionic strength gives rise to the polyion desorption from the microgel outer shell.