Effect of the Supporting Electrolyte Anion on the Thickness of PSS/PAH Multilayer Films and on Their Permeability to an Electroactive Probe

Abstract

Quartz crystal microbalance and cyclic voltammetry are used to investigate the influence of the supporting salt of polyelectrolyte solutions on the buildup and the structure of PSS/PAH polyelectrolyte multilayers (PSS: poly(4-styrene sulfonate); PAH: poly(allylamine hydrochloride)). This film constitutes a model polyelectrolyte multilayer system. The supporting electrolytes were sodium salts where the nature of the anion was changed by following the Hofmeister series from cosmotropic to chaotropic anions (F-, Cl-, NO3-, ClO4-). For all the investigated anions, the film thickness increases linearly with the number of deposition steps.Wefind that chaotropic anions lead to larger thickness increments per bilayer during the film buildup than cosmotropic ones, confirming results found on PSS/PDADMA multilayers (PDADMA:poly(diallyldimethylammonium)). Films constituted by more than nine PSS/PAH bilayers are still permeable to hexacyanoferrate(II) ions, Fe(CN)(6)4-, whatever the nature of the supporting salt anion. On the other hand, these films are impermeable to ruthenium(II) hexamine ions, Ru(NH3)(6)2+, after the third PAH layer in the presence of NaF, NaCl, or NaNO3. These results are explained by the presence of an excess of positive charges in the film, which leads to a positive Donnan potential. We find that this potential is more positive when more chaotropic anions are used during the film buildup. We also find that a film constructed in the presence of chaotropic anions swells and becomes more permeable to Fe(CN)(6)4- ions when the film is brought into contact with a solution containing more cosmotropic anions. All our experimental findings can be explained by a strong interaction between chaotropic anions with the NH3+groups of PAH that is equivalent, as far as the multilayer buildup and electrochemical response is concerned, to a deprotonation of PAH as it is observed when the film is constructed at a higher pH. We thus arrive to a coherent explanation of the effect of the nature of the anions of the supporting electrolyte on the polyelectrolyte multilayer. We also find that great care must be taken when investigating polyelectrolyte multilayer films by electrochemical probing because electrochemical reactions involving the probes can appreciably modify the multilayer structure.

Polyelectrolyte Complex Membranes for Immobilizing Biomolecules, and Their Applications to Bio-analysis

“…LBL membranes have been applied to the separation of specific gases, such as methyl mercaptan 11 and a type of smoke. 12 Ionic species in solution, such as chloride/phosphate, 13 sodium/calcium, 14 chloride/fluoride, 15 halogen ions 16 and ionic dyes, 17 have also been separated using LBL membranes. Similar separation systems can be obtained using PIC membranes.…”

Sequential coating of nanopores with charged polymers: A general approach for controlling pore properties of self-assembled block copolymer membranes

“…In addition, when a new solution was made of PEI·HCl between two sets of membrane depositions, the slope of the decrease in layer thickness changed considerably. So, as is seen for polyelectrolyte multilayers on surfaces, 24 a large number of very small changes can contribute to the layer thickness within the nanopore.…”

Electrochemical nanoarchitectonics and layer-by-layer assembly: From basics to future

“…Because these ions diffuse within the polyelectrolyte complexes their concentration must be extremely small. This conclusion is not unexpected if one refers to the findings of El Haitami et al who found an excess of fixed positive charges in a PSS/PAH multilayer [53]. As far as the anions (Cl − in their case) are concerned, no conclusion can be drawn because it seems that in these polyelectrolyte complexes Cl − ions do not contribute to ionic conductivity [57].…”

“…Anson showed that the Donnan potential between a polyelectrolyte film deposited on an electrode and the contacting electrolyte solution contributes to the formal potential measured for redox couples incorporated in the polyelectrolyte coating [51,52]. For example, El Haitami et al showed that the apparent redox potential of ferrocyanide (Fe(CN) 6 4− ) ions in a PSS/PAH multilayer film depends upon the kind of salt used for the buildup [53]. This variation is due to the Donnan potential in the film, indicating an extrinsic charge imbalance in PSS/PAH multilayers.…”

“…Such a phenomenon should be rather general, leading to a Donnan potential inside the films and a resulting exclusion of ions of the same charge as the fixed charges within the film. For instance, cyclic voltametry measurements with ruthenium(III) hexaamine (Ru(NH 3 ) 6 3+ ) lead to no current, in contrast to ferrocyanide ions, confirming the presence of excess ''fixed'' positive charges within the PSS/PAH film [53]. However, these probes lead to similar currents with PSS/PolyDADMAC films having less than 10 bilayers, proving that both species could penetrate to the film, replacing either Cl − or Na + ions, and suggesting the absence of Donnan potential [54].…”