Rapid Desorption of Polyelectrolytes from Solid Surfaces Induced by Changes of Aqueous Chemistry

The short-term desorption induced by changes of aqueous chemistry of predeposited polyelectrolyte layers on solid surfaces was studied with reflectometry. The behavior of a strong polycation, polydiallydimethylammonium chloride (PDADMAC), interacting with flat silica was investigated in detail. Results showed that partial desorption of preadsorbed polymer chains can be quickly triggered by changes in ionic strength and pH. When lowering these parameters in the PDADMAC-silica system, the increased lateral repulsive potential of neighboring chains drove the desorption of some of the polymer. Furthermore, layer desorption was favored when electrostatic interactions between a polyelectrolyte and the underlying surface became less attractive or switched to being repulsive. At the investigated timescales (<1 h), adlayer desorption was always partial and often incomplete. When initiating desorption from a condition of large adsorbed mass, desorption effects did not result in the plateau mass obtained by adsorption on a clean surface: an excess mass remained deposited. The results thus suggest that a relatively large energy barrier needs to be overcome to induce redissolution of predeposited chains and that this barrier may be a function of the number of polymer-surface interactions, which are in turn correlated with polymer molecular mass. These mechanisms have important implications for environmental processes and colloidal systems because they imply that, once adsorbed, polymeric chains may be redissolved but only to a limited degree at typical engineering timescales.