  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.
