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arXiv · 2607.29230

Weak-electrolyte diffusiophoresis for rigid colloids

Subrata Majhi·Huanshu Tan
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Abstract

We develop a model for the diffusiophoresis of a chemically inert, rigid spherical colloid with fixed surface charge in a monovalent weak electrolyte, in which a neutral solute reversibly dissociates into ions. A weak far-field gradient is imposed in the neutral-species concentration. In the fast-reaction limit, local mass action and bulk electroneutrality determine the far-field ionic gradients, while the bulk zero-current condition determines the diffusion-potential gradient. We solve the coupled Nernst-Planck, Poisson and Stokes equations for arbitrary double-layer thickness, linearising in the gradient strength while retaining the nonlinear Poisson-Boltzmann equilibrium. In the Debye-H\"uckel limit, the mobility consists of one half of the matched fully dissociated response and a finite-double-layer correction due to neutral-ion coupling; the correction vanishes in both the H\"uckel and Smoluchowski limits. Beyond this limit, numerical solutions for the representative systems reveal a branch-selective response as the surface potential magnitude increases. When the counterion is slower than the co-ion, dissociation-association weakens a retarding concentration-polarisation layer, allowing the mobility to exceed the fully dissociated value. When the counterion is faster, the response remains close to the one-half scaling set by mass action. This reaction-polarisation coupling cannot be reproduced by adjusting only the bulk ionic strength, and hence the Debye length, in a fully dissociated model.

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Subrata Majhi, Huanshu Tan. 2026-07-31. Weak-electrolyte diffusiophoresis for rigid colloids. https://arxiv.org/abs/2607.29230

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