Specific anion effect in swelling of polyelectrolyte multilayers

The well-known strong polyelectrolytes poly(diallyldimethylammonium) cation (PDADMA) and poly(4-styrenesulfonate) anion (PSS) form multilayers (PEMs) where formally the small counterions do not participate in any way. Previously, we have shown that by replacing the conventional chloride as the counteranion of PDADMA by other small anions occupying different positions in the Hofmeister series the resulting PEMs had widely different thicknesses in the dry state [Langmuir 2004, 20,3679] and widely different stiffnesses in the wet state [Macromolecules 2004, 37,9585]. In this work we show that even more striking differences are observed in the swelling behavior when comparing PSS/PDADMA multilayers made in the presence of fluoride or bromide. PSS/PDADMA-F shows nearly no swelling at different concentrations of NaF whereas PSS/PDADMA-Br shows strong swelling, especially at low concentrations of NaBr. The similar difference appears also when using the Solutions of NaF, HCOONa, NaBrO3, NaCl, NaClO3, NaNO3, and NaBr as swelling agents. Again swelling of PSS/PDADMA-F is negligible but PSS/PDADMA-Br swells, the extent of swelling having some correlation with the hydration entropy of anion; i.e., the more negative is the entropy, the stronger is swelling.