Induced phase transformation in ionizable colloidal nanoparticles

TL;DR

This paper investigates how pH-induced charge regulation causes phase transitions in ionizable colloidal nanoparticles, revealing a shift from ordered to disordered states due to increased charge polydispersity.

Contribution

It models charge regulation effects in colloids using Brownian simulations and maps the phase diagram as a function of pH, highlighting the transition mechanisms.

Findings

Particles transition from FCC to disordered states with increasing pH

Charge polydispersity increases as pH rises

High mean charge leads to amorphous colloidal solids

Abstract

Acid-base equilibria directly influence the functionality and behavior of particles in a system. Due to the ionizing effects of acid-base functional groups, particles will undergo charge exchange. The degree of ionization and their intermolecular and electrostatic interactions are controlled by varying the pH and salt concentration of the solution in a system. Although the pH can be tuned in experiments, it is hard to model this effect using simulations or theoretical approaches. This is due to the difficulty in treating charge regulation and capturing the cooperative effects in a colloidal suspension with Coulombic interaction. In this work, we analyze a suspension of ionizable colloidal particles via Brownian simulations and derive a phase diagram of the system as a function of pH. It is observed that as pH increases, particles functionalized with acid groups change their arrangement from face-centered cubic (FCC) packing to a disordered state. We attribute these transitions to an increase in the degree of charge polydispersity arising from an increase in pH. Our work shows that charge regulation leads to amorphous solids in colloids when the mean nanoparticle charge is sufficiently high.