High salinity effects on the depletion attraction in colloid-polymer mixtures

Depletion attraction induced by polymers can be employed across multiple disciplines. Previous studies implied that besides screening the electrostatic repulsion between colloids, adding salt may also affect the polymers even in a nonpolar solvent. Here, we study the depletion-induced gelation of a colloid-polymer model system, focusing on the salt effects on the depletion attraction.

Confocal microscopy was used to quantitatively characterize the colloidal gels formed by the polymethylmethacrylate (PMMA)/polystyrene (PS) model system. The attraction experienced by colloids was estimated by correlating the colloidal dynamics with the local structure. Correspondingly, static light scattering was employed to systematically investigate the polymers. The resulting radius of gyration R_g and osmotic pressure were used to evaluate the depletion attraction offered by polymers.

Salt was discovered to lower the strength of inter-particle attraction, which can be attributed to the salt-induced decrease in R_g. The depletion attraction grew sublinearly with c, owing to the considerable decrease in R_g in the good solvent as c increased. We demonstrated how the close form equations in the framework of renormalization group theory can be employed to predict the depletion interaction using the properly determined zero-concentration radius of gyration.