Stoichiometry of the Components and Polyion Molecular Weight

A multilayer film has to be totally neutral, that is, a stoi-chiometry of charged groups in neighboring polycation and polyanion layers has to be 1:1. In many cases, we confirmed this (for PSS/PDDA, PSS/PEI, PSS/chitosan, DNA/PAH multilayers), but for PSS/PAH a deviation from 1:1 stoi-chiometry was found [34]. This is probably due to the incomplete dissociation of the polyions. It is difficult to control polyion dissociation because it depends on a concentration and on the presence of oppositely charged compounds in solution [97]. The complex stoichiometry did not depend on the ionic strength of polyion solutions, which indicates an absence of low molecular weight ions (K+, Na+, Cl-) in the films.

For deposition from water solutions, the PSS/PAH growth step was similar for PSS molecular weights varying from 40,000 to 1,000,000 [21]. The influence of the polymerization degree on the formation of stable interpolyelectrolyte bulk complexes was analyzed by Kabanov [97]. He found that such complexes were stable when a polyion contained more than 20 charged groups in the sequence. We used polyions, such as PSS, PVS, PDDA, PEI with molecular weights of 50,000-200,000. They contain hundreds of ionized groups in the pH range from 3 to 9. A cooperative electrostatic interaction between the polycations and polyanions used is strong and prevents the dissolution of the multilayers even in high ionic strength solvents.

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