(464c) Effect of Ion Sterics on Diffusiophoresis in Concentrated Electrolytes

Khair, A. S., Carnegie Mellon University
Diffusiophoretic migration of a charged colloidal particle in an electrolyte gradient has been known since the pioneering work of Derjaguin et al. in 1947. Recent interest in diffusiophoresis has been motivated by new microfluidic measurement techniques, and applications in membrane filtration, mineral replacement reactions, and colloidal self-propulsion, for example. Here, we revisit the fundamental problem of predicting the diffusiophoretic velocity of a spherical, uniformly charged colloid. All existing work in this regard utilizes the Poisson-Nernst-Planck (PNP) model for point-like, non-interacting ions. However, it is well known that the PNP model can lead to impossibly large ion concentrations near even moderately charged surfaces. Thus, we consider modified PNP equations that account for steric repulsion between finite-sized ions, via the Bikerman, Carnahan-Starling, and BMCSL equations of state. We compute the diffusiophoretic velocity using an (experimentally relevant) asymptotic scheme for thin Debye layers. Ion steric effects have a dramatic influence: e.g, while the PNP model predicts a reversal in the diffusiophoretic velocity in certain electrolytes (e.g. KCl) with increasing particle zeta potential â?? at precisely the zeta potentials at which the validity of the PNP model is questionable â?? we show that ion sterics essentially eliminates this reversal. This finding is of particular relevance to diffusiophoretic migration in high salinity solutions, where the effects of ion sterics are expected to be most prominent.