Surface chemistry-based continuous separation of colloidal particles via diffusiophoresis and diffusioosmosis

The separation of colloidal particles is of great importance in many fields, such as purification, sensing, and bioanalysis. However, separating particles based on their surface physico-chemical properties remains challenging. This study demonstrates through experimental and theoretical analyses that diffusiophoresis and diffusioosmosis enable the continuous separation of carboxylate polystyrene particles with similar sizes and zeta potentials but distinct surface concentrations of carboxyl groups. In the proposed approach, the particles are exposed to salt concentration gradients generated in a double-junction microfluidic device, fed with low and high electrolyte concentration streams. As the particles move across environments with varying salinity levels, their dynamics are affected by the sensitivity of their electrophoretic mobility - and consequently, their apparent zeta potential, which is proportional to it - to the local salt concentration. The apparent zeta potential, measured via electrophoretic light scattering, and its sensitivity to salt concentration are influenced by the ionic conduction occurring near the particle surface whose intensity depends, in turn, on the concentration of surface carboxyl groups. By harnessing these effects, colloids with comparable apparent zeta potentials but different surface concentrations of carboxyl groups are separated with high efficiency when they exhibit opposite apparent zeta potential sensitivities to salt. This simple approach, which relies on an easy-to-operate device with no external energy source, has discipline-spanning potential for the continuous separation of colloids distinguished solely by surface properties like roughness, permeability, heterogeneity, and chemical composition that influence the sensitivities of their electrophoretic mobility and, thus apparent zeta potential, to the salt concentration.