Diffusiophoresis of charged particles in the presence of electrolytes has been extensively studied in the literature. However, in these setups, particles typically move in a single direction, either up or down the electrolyte gradient. Here, we theoretically investigate the conditions under which a particle can reverse its diffusiophoretic direction within the same setup, leading to the formation of a focusing band under steady-state concentration gradients. Using multi-ion diffusiophoresis calculations, we simulate particle transport in an acid-base reaction system where salt is added alongside the acid. For a range of salt concentrations, particles focus within the channel. Our analysis reveals that a pH-dependent zeta potential is necessary for this focusing to occur, and determines where the particles focus, i.e., on or off the acid-base reaction front. We consider two cases: (i) modest pH-driven variation in the magnitude of a negative zeta potential, and (ii) sign reversal across an isoelectric point. For modest variations in negatively charged particles, we report qualitative agreement with prior experimental observations and derive semi-analytical conditions governing particle focusing, highlighting the delicate balance between concentration gradients and zeta potential variations. For quantitative analysis, we create focusing phase diagrams for both types of zeta-potential variations and compare their focusing regimes. The work elucidates the crucial physics of pH-dependent zeta potential and opens new avenues for exploring diffusiophoresis in acid-base systems, with implications for microfluidic design and biophysical transport processes.

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• Diffusion
• Diffusiophoresis
• Electrokinetic flows