Diffusiophoresis in acid-base reaction fronts with and without an isoelectric point: When, why, and where the particles focus

Electrolytic diffusiophoresis refers to the movement of charged particles in electrolytes. This motion typically proceeds either up or down an electrolyte concentration gradient. However, when multiple electrolyte gradients are present, such as an acid-base reaction, the direction may be reversed, inducing the formation of a focal band. While the results were reported experimentally, an understanding of the phenomena has remained elusive. Here, we computationally show that a pH-dependent zeta-potential is required for focusing to occur. Our model provides an intuitive understanding of the governing physics and a compelling match to prior experimental reports.