Simple models for the trapping of charged particles and macromolecules by diffusiophoresis in salt gradients

We study the trapping of charged particles and macromolecules (such as DNA) in salt gradients in aqueous solutions. The source for the salt gradient can be as simple as a dissolving ionic crystal, as shown by McDermott et al. [Langmuir 28, 15491 (2012)0743-746310.1021/la303410w]. Trapping is due to a competition between localization due to diffusiophoresis in the salt gradient, and spreading out by diffusion. The size of the trap is typically 1-100µm. We further predict that, at steady state, the particle (macromolecule) number density is a power law of the salt concentration, with an exponent that is the ratio of the diffusiophoretic mobility to the diffusion coefficient of the trapped species. This ratio increases with size and typically becomes ≫1 for particles or macromolecules with hydrodynamic radii of hundreds of nanometers and above. Thus, large particles or macromolecules are easily caught and trapped at steady state by salt gradients.