Diffusiophoresis of a soft, pH-regulated particle in a solution containing multiple ionic species

The diffusiophoresis of a soft, pH-regulated particle comprising a rigid core and a polyelectrolyte (PE) layer in an aqueous salt solution containing multiple ionic species is modeled theoretically. We show that the diffusiophoretic behavior of a soft particle can be different appreciably from that of a rigid one. In particular, both the sign and the magnitude of the mobility of a soft particle can vary with the friction coefficient of its PE layer. In an aqueous KCl solution, if that coefficient is sufficiently large, the particle always moves towards the opposite direction as that of the applied concentration gradient, regardless of the pH level, an interesting observation which has not been reported previously. In an aqueous NaCl solution, if pH is sufficiently low, the particle mobility increases with increasing friction coefficient, an interesting and unexpected finding. The results obtained provide valuable and necessary information for both interpreting experimental data and design diffusiophoresis devices.