  Diffusiophoresis is the spontaneous motion of particles under gradients of
solutes. In electrolyte-driven diffusiophoresis, the zeta potential of the
particles is an important surface property that characterizes diffusiophoretic
mobility. However, the zeta potential is not a fixed material property and
colloidal surfaces often show varying potentials depending on the physical
chemistry of the surrounding fluid, e.g., solute type, ionic strength, and pH.
In this article, we study experimentally and theoretically pH-dependent
diffusiophoresis of polystyrene particles using a dead-end pore geometry. In
particular, the influence of the isoelectric point (pI) on diffusiophoresis is
demonstrated in the absence and presence of wall diffusioosmosis. Throughout
the paper, we show with experiments and model calculations how the pH-dependent
diffusiophoresis and diffusioosmosis influence the particle motion in dead-end
pore configurations, including changes that occur when there is a sign change
in the zeta potential near the pI.
