Transfer of relatively short polycation chains (guest polyelectrolyte, GPE) from their polyelectrolyte complex (PEC) with a relatively long polyanion (host poly electrolyte, HPE) to another polyanion of the same chemical structure and chain length but tagged with a fluorescent group has been investigated by the luminescence quenching technique. It has been shown that if only one polycation chain, on average, is transferred, the reaction follows irreversible second-order kinetics. The irreversibility of the reaction is due to additional selective nonelectrostatic interaction between the polycations and polyanion chains, carrying pyrenyl groups. The rate constant of the exchange reaction is independent of polyanion chain length and increases sharply with an increase of ionic strength of water-salt solution, a decrease of polycation chain length, and a decrease of the charge density. In accordance with the proposed mechanism, the rate-limiting step of the reaction is the transfer of a polycation chain from one polyanion to the other one in the united coil HPE-GPE-HPE*. The polyion addition reaction, i.e. the complexation between two oppositely charged polyanions, also has been studied by luminescence quenching and quasielastic light scattering techniques. The reaction can be considered as a multistage process, consisting first of the rapid coupling of the oppositely charged polyions with the formation of a nonequilibrium interpolyelectrolyte network followed by the slow relaxation process, leading finally to the formation of the individual PEC.
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 SO 3or SO 4groups show the strongest interpolyelectrolyte binding, resulting in inhibited chain exchange within PECs and/or PEMs. With weakly bound polyelectrolyte pairsspolycations containing quaternary ammonium groups and carboxylate polyanionsswater-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.
Thin films obtained from a layer-by-layer deposition of a weak polycarboxylic acid and a positively charged globular protein were studied by in situ ATR-FTIR. The system was chicken egg lysozyme (Lys), bovine pancrease ribonuclease A (RNase), or bovine gamma-globulin (IgG) self-assembled with polycarboxylic acids. When the pH value was lowered below a critical point, the growth of films and their tolerance to decomposition by added sodium chloride improved dramatically. Stabilization of protein/polyacid films in salt solutions at lower pH values occurred due to the onset of nonelectrostatic interactions to intermolecular binding within protein/polyacid multilayers and was controlled by polyacid ionization within the film rather than the pH of the external solution. A fractional ionization of polyacid in the pH-stabilization region was lower with protein-containing films than for polyacid/linear polycation films, reflecting hindrance of the inter-association of protonated carboxylic groups by protein globules. Practical ramifications of the pH-stabilization effect might extend to areas of biotechnology and biomaterials.
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