Where polyelectrolyte multilayers and polyelectrolyte complexes meet

We summarize existing knowledge and present some new results on the relationship between polyelectrolyte multilayer (PEM) growth and phase behavior of polyelectrolyte complexes (PECs) in solution. Detailed understanding of competition between surface and solution as applied to PEMs requires selective labeling of polymers and/or the application of techniques that allow chemically specific monitoring of film components, such as in-situ ATR-FTIR spectroscopy. The trends observed with multilayers directly follow from the properties of PECs in solution. Effects of a number of parameters, such as the type of interacting polyelectrolyte chains, the ratio of their lengths, and ionic strength and pH of deposition solutions, on the likelihood of the multilayer stability or erosion are considered. Polycations with high density of primary amino groups and polyanions with SO3- or SO4- groups show the strongest interpolyelectrolyte binding, resulting in inhibited chain exchange within PECs and/or PEMs. With weakly bound polyelectrolyte pairspolycations containing quaternary ammonium groups and carboxylate polyanionswater-soluble PECs are easily formed, often resulting in erosion of PEMs. For the latter case, we report a full phase diagram of polycation/polyanion/NaCl aqueous mixtures and show how ionic strength can be used to tune the deposition of PEMs at surfaces. In addition, we present that the phase behavior of PECs in solution also controls pH response of PEMs at surfaces. Better knowledge of the relationships between the PEMs and PECs allows rational prediction and control of deposition of a wide range of weak or permanently charged polyelectrolytes at surfaces.