Acid-base equilibrium directly influences the functionality and behavior of molecules in many chemical and biological processes. Here, we study charge regulation effects in a suspension of colloidal nanoparticles functionalized with acid-base groups via simulations. The degree of ionization and their intermolecular and electrostatic interactions are controlled by varying the pH and salt concentration of the solution. When the chemical reaction occurs, the nanoparticles can acquire two states: uncharged state and charged state. The nanoparticles are surrounded by cations and anions. The cations represent the release particle (H⁺) and the salt cations (S⁺) and the red particles represent the hydroxyl (OH^-) and the salt anions (S^-) 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 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. We contrast our results with the formation of amorphous solids in mixtures of oppositely charged nanoparticles.

Fig. 1. Charge regulation of colloidal nanoparticles at two different pH values. Each nanoparticle contains N<sub>A</sub>=110 dissociable sites on the surface. The values of the pKa=6.5 and the salt concentration is 10mM. When the chemical reaction occurs, the nanoparticles can acquire two states: uncharged state (small white particles) and charged state (small blue particles). The nanoparticles are surrounded by cations (green particles) and anions (red particles). The cations represent the release particle (H<sup>+</sup>) and the salt cations (S<sup>+</sup>) and the red particles represent the hydroxyl (OH<sup>-</sup>) and the salt anions (S<sup>-</sup>).

Fig. 1. Charge regulation of colloidal nanoparticles at two different pH values. Each nanoparticle contains N_A=110 dissociable sites on the surface. The values of the pKa=6.5 and the salt concentration is 10mM. When the chemical reaction occurs, the nanoparticles can acquire two states: uncharged state (small white particles) and charged state (small blue particles). The nanoparticles are surrounded by cations (green particles) and anions (red particles). The cations represent the release particle (H⁺) and the salt cations (S⁺) and the red particles represent the hydroxyl (OH^-) and the salt anions (S^-).

Monica Olvera De La Cruz

Northwestern Univ

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