Counterions – A new approach to control the degree of ionization of polyelectrolytes in layer-by-layer films

The atomic composition of layer-by-layer (LbL) films of the conductive polyelectrolyte poly(o-methoxyaniline) (POMA) alternated with the strong polyelectrolyte poly(styrene sulfonate) (PSS) was obtained from X-ray photoelectron spectroscopy (XPS), in order to study the influence of sodium chloride concentration of PSS aqueous solutions on the degree of ionization of polyelectrolytes in the film structure. The results revealed that the degree of ionization for both weak and strong polyelectrolytes decreases with NaCl concentration while the relative amount of counterions/co-ions increases. A larger number of ions, in comparison with the number of polylectrolytes ionic groups, indicate the formation of NaCl nanocrystals in the LbL film. However, results showed that some Cl atoms are being used in an extra chlorination chemical reaction with POMA. The number of water molecules retained in the films decreases with salt at a rate lower than that of the degree of ionization indicating that water molecules are close to salt ions. Moreover, the ratio water molecules per sodium assumes a value of 4 for films prepared without salt, which is the typical hydration number value for NaCl in solution, and decreases linearly with salt concentration pointing out to nanocrystals increase in size with salt concentration.

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► The degree of ionization in weak and strong polyelectrolytes is controlled by salt.
► The amount of counterions/co-ions in the films increases with NaCl concentration.
► Nanocrystals are being formed with ions which are not compensating the ionic groups.
► Some Cl atoms are being used in an extra chlorination chemical reaction with POMA.
► The water molecules retained in the films decrease with the salt concentration.