Counterions and water in polyelectrolyte multilayers: a tale of two polycations

Attenuated total internal reflectance Fourier transform infrared, ATR-FTIR, spectroscopy was used to compare the water uptake and doping within polyelectrolyte multilayers made from poly(styrene sulfonate), PSS, and a polycation, either poly(allylamine hydrochloride), PAH, or poly(diallyldimethylammonium chloride), PDADMAC. Unlike PDADMA/PSS multilayers, whose water content depended on the solution ionic strength, PAH/PSS multilayers were resistant to doping by NaCl to a concentration of 1.2 M. Using (infrared active) perchlorate salt, the fraction of residual counterions in PDADMA/PSS and PAH/PSS was determined to be 3% and 6%, respectively. The free energy of association between the polymer segments, in the presence of NaClO4, was about 5 kJ mol-1 and -10 kJ mol-1, respectively, for PDADMA/PSS and PAH/PSS, indicating the relatively strong association between the polymer segments in the latter relative to the former. Varying the pH of the solution in contact with the PAH/PSS multilayer revealed a transition to a highly swollen state, interpreted to signal protonation of PAH under much more basic conditions than the pKa of the solution polymer. The increase in the multilayer pKa suggested an interaction energy for PAH/PSS in NaCl of ca. 16 kJ mol-1.     

Unusual collapse of highly hydrated polyelectrolyte multilayers with the ionic strength

Highly hydrated polyelectrolyte multilayers (PEMs) were fabricated by “layer by layer” (LBL) assembly of poly (diallyl dimethyl ammonium chloride) (PDADMAC) and poly (sodium 4‐styrene sulfonate) (PSS) in 0.5 M NaCl. Both thickness and hydration of the film were determined in situ as the multilayer was assembled by means of the quartz crystal microbalance with dissipation (QCM‐D) and the Spectroscopic Ellipsometry techniques combined in a single device. For PEMs of 17 total layers in water, a final thickness of up to 300 nm and a hydration of 69% were measured. The response towards the ionic strength was then studied by means of QCM‐D. PEMs of 17 layers, with PDADMAC as last layer, shrunk dramatically and lost water when exposed to aqueous NaCl solutions of increasing concentration. Indeed, a thickness variation up to 100 nm and reduction in the 50% of the water content were observed when the PEM was exposed to 1 M NaCl. On the contrary, PEMs where PSS appears on top showed no measurable change upon the variation in the ionic strength. This brings the possibility to control the responsive character of the PEMs simply by selecting the last polyelectrolyte layer (PDADMAC or PSS) deposited. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Effect of ionic strength and type of ions on the structure of water swollen polyelectrolyte multilayers

This study addresses the effect of ionic strength and type of ions on the structure and water content of polyelectrolyte multilayers. Polyelectrolyte multilayers of poly(sodium-4-styrene sulfonate) (PSS) and poly(diallyl dimethyl ammonium chloride) (PDADMAC) prepared at different NaF, NaCl and NaBr concentrations have been investigated by neutron reflectometry against vacuum, H(2)O and D(2)O. Both thickness and water content of the multilayers increase with increasing ionic strength and increasing ion size. Two types of water were identified, "void water" which fills the voids of the multilayers and does not contribute to swelling but to a change in scattering length density and "swelling water" which directly contributes to swelling of the multilayers. The amount of void water decreases with increasing salt concentration and anion radius while the amount of swelling water increases with salt concentration and anion radius. This is interpreted as a denser structure in the dry state and larger ability to swell in water (sponge) for multilayers prepared from high ionic strengths and/or salt solution of large anions. No exchange of hydration water or replacement of H by D was detected even after eight hours incubation time in water of opposing isotopic composition.     

Controlling the nanofiltration properties of multilayer polyelectrolyte membranes through variation of film composition

We report the use of a variety of polyelectrolyte multilayers (PEMs) as selective skins in composite membranes for nanofiltration (NF) and diffusion dialysis. Deposition of PEMs occurs through simple alternating adsorption of polycations and polyanions, and separations can be optimized by varying the constituent polyelectrolytes as well as deposition conditions. In general, the use of polycations and polyanions with lower charge densities allows separation of larger analytes. Depending on the polyelectrolytes employed, PEM membranes can remove salt from sugar solutions, separate proteins, or allow size-selective passage of specific sugars. Additionally, because of the minimal thickness of PEMs, NF pure water fluxes through these membranes typically range from 1.5 to 3 m3/(m2 day) at 4.8 bar. Specifically, to separate sugars, we employed poly(styrene sulfonate) (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC) films, which allow 42% passage of glucose along with a 98% rejection of raffinose and a pure water flux of 2.4 m3/(m2 day). PSS/PDADMAC membranes are also capable of separating NaCl and sucrose (selectivity of approximately 10), while high-flux chitosan/hyaluronic acid membranes [pure water flux of 5 m3/(m2 day) at 4.8 bar] may prove useful in protein separations.     

Swelling and shrinking of polyelectrolyte microcapsules in response to changes in temperature and ionic strength

Swelling and shrinking of polyelectrolyte microcapsules consisting of poly(styrene sulfonate, sodium salt) (PSS) and poly(diallyldimethyl ammonium) chloride (PDADMAC) multilayers have been observed in response to temperature and electrolyte exposure, respectively. Heat-induced capsule swelling and capsule wall volume reduction were observed by confocal laser scanning microscopy (CLSM) and scanning force microscopy (SFM). On the other hand, pronounced shrinking in diameter induced by exposure to an electrolyte was observed in parallel to increases in the thickness of the capsule wall. The estimated wall volume was reduced to two thirds of the control for the salt-exposed capsules and one half for the salt-exposed and simultaneously annealed capsules. This reduction in volume was supposedly mainly caused by the compression of the capsule wall due to the ionic screening from the electrolyte. The highly porous microstructure of the multilayers and loosely bound PSS/PDADMAC complex are thought to be responsible for the structure of the PSS/PDADMAC capsules being easily modulated upon annealing and salt-exposure.     

Formation of polyelectrolyte multilayers on fibres: influence on wettability and fibre/fibre interaction

Polydimethyldiallylammonium chloride (PDADMAC) and polystyrene sulfonate (PSS) have been used to build-up polyelectrolyte multilayers (PEMs) on chemical soft wood fibres and on SiO2 at various electrolyte concentrations. Adsorption onto SiO2 was studied using a stagnation point adsorption reflectometer (SPAR), and the adsorbed amount of PDADMAC and PSS on the fibres was determined using nitrogen analysis and Sch?niger burning, respectively. The adsorption onto the two substrates was then compared. Paper testing showed that the tensile index (TI) increased by about 90% when 11 layers had been adsorbed, and that there was a correlation between the adsorbed amount and the increase in TI. It was also shown that the particular polymer present in the outermost layer significantly influenced the TI, and that PDADMAC produced a higher TI. A correlation between the adsorbed amount and the TI was also found. Individual fibres were partly treated with a PEM and analysed using a dynamic contact angle analyser (DCA) and environmental scanning electron microscopy (ESEM).     

Responsive Polyelectrolyte Multilayers Assembled at High Ionic Strength with an Unusual Collapse at Low Ionic Strength

Responsive polyelectrolyte multilayers (PEMs) of poly(diallyl dimethyl ammonium chloride) (PDADMAC) and poly(styrene sodium sulfonate) (PSS) with thicknesses between 350 and 400 nm for 11 deposited polyelectrolyte layers were fabricated assembling the polyelectrolytes at 3 M NaCl. When the 3 M NaCl bulk solution is replaced by water, the PEMs release water, approximately a 46% of the total mass, and experience a thickness reduction of more than 200 nm. Changes in thickness and water content are fully reversible. The film recovers its original thickness and water content when it is exposed again to a 3 M NaCl solution. A responsive polymer film is achieved with the capability of swelling at high ionic strength and collapsing in water with variations in thickness of hundred of nanometers.     

The influence of secondary interactions during the formation of polyelectrolyte multilayers: layer thickness, bound water and layer interpenetration

With X-ray and neutron reflectivity, the structure and composition of polyelectrolyte multilayers from poly(allyl amine) (PAH) and poly(styrene sulfonate) (PSS) are studied as function of preparation conditions (salt concentration and solution temperature, T). The onset of a temperature effect occurs at 0.05 M NaCl (Debye length approximately 1 nm). At 1 M salt, the film thickness increases by a factor of 3 on heating the deposition solution from 5 to 60 degrees C. The PAH/PSS bilayer thickness is independent of the kind of salt (NaCl or KCl), yet its composition is different (more bound water for NaCl). At low T, the internal roughness is 33% of the bilayer thickness; it increases to 60% at high T. The roughening is accompanied by a total loss of bound water. At which temperature the roughening starts is a function of the kind of salt (50 degrees C for NaCl and 35 degrees C for KCl). The strong temperature dependence and the eventual loss of bound water molecules may be attributed to the hydrophobic force; however, there is an isotope effect, since the loss of bound water is less pronounced in the deuterated layers.     

Polyelectrolyte multilayers on wood fibers: influence of molecular weight on layer properties and mechanical properties of papers from treated fibers

This paper compares the influence of the molecular weight of polylelectrolytes forming polyelectrolyte multilayers (PEM) on wood fibers on adhesion and paper strength. Sheets were made from fibers treated with poly(allylamine hydrochloride) (PAH)/poly(acrylic acid) (PAA) of molecular mass 70,000 and 240,000, respectively, and of poly(dimethyldiallylammonium chloride) (PDADMAC)/poly(styrene sulfonate) (PSS) of molecular mass 30,000 and 80,000, respectively. The results were compared to what has recently been reported for PEM formation on fibers using a low-molecular-mass combination of PAH and PAA and a high-molecular-mass combination of PDADMAC/PSS. There was a less significant improvement in the case of the low-molecular-mass PDADMAC/PSS and the high-molecular-mass PAH/PAA. The adsorbed amounts of PAH and PDADMAC were also determined, showing a lower adsorbed amount of the low-molecular-mass PAH than of the high-molecular-mass PDADMAC. The amount of low-molecular-mass PDADMAC was similar to that found for high-molecular-mass PDADMAC/PSS. Individual fibers were partly treated and studied, showing a less significant decrease in wettability with low-molecular-mass PDADMAC/PSS than with the high-molecular-mass combination. The effect of the molecular weight on the adhesion was discussed in terms of the structure and wettability of the PEMs.     

Controlled Stripping of Polyelectrolyte Multilayers by Quaternary Ammonium Surfactants

The quartz crystal microbalance with dissipation technique (QCM‐D) and atomic force microscopy (AFM) have been employed to study the interaction of N‐tetradecyl trimethyl ammonium bromide (TdTmAB) with polyelectrolyte multilayers containing poly(sodium 4‐styrene sulfonate) (PSS) as the polyanion and either poly(allylamine hydrochloride) (PAH) or poly(diallyl dimethyl ammonium chloride) (PDADMAC) as the polycations. The multilayers were exposed to aqueous solutions of TdTmAB. This resulted in a selective removal of PDADMAC PSS layers while layers with PAH as polycation remained stable. It is suggested that PDADMAC/PSS multilayers can be employed as strippable protecting layers.

     

Effect of ionic strength and protein concentration on the transport of proteins through chitosan/polystyrene sulfonate multilayer membrane

The influence of ionic strength and protein concentration on the transport of bovine serum albumin (BSA), ovalbumin and lysozyme through chitosan (CHI)/polystyrenesulfonate (PSS) multilayers on polyether sulfone supports are investigated under ultrafiltration conditions. The percentage transmission and flux of BSA, ovalbumin and lysozyme were found to increase with increase in salt concentration in the protein. The percentage transmission of BSA through 9 bilayer membrane was found to increase from 5.3 to 115.6 when the salt concentration was varied from 0 to 1 M. It was observed that 0.1 M NaCl in BSA solution is capable of permeating all the BSA. When the salt concentration in BSA was further increased, a negative solute rejection (solute enrichment in permeate) was found to take place. With 9 bilayer membrane, the percentage transmission of ovalbumin was found to increase from 23.3 to 125.8 when the salt concentration in protein was increased from 0 to 0.05 M. The effect of protein concentration on protein transport is studied taking BSA as a model protein. BSA was rejected by the multilayer membrane at all the studied concentrations (0.25, 0.5, 1 and 2 mg/ml). With increase in feed concentration, maximum rejection of protein occurred at higher number of CHI/PSS bilayers. BSA solution flux was found to decrease with an increase in BSA concentration. This study indicates that it is possible to fine tune the transport properties of proteins through multilayer membranes by varying the concentration and ionic strength of protein solutions.     

Modulating the pattern quality of micropatterned multilayer films prepared by layer-by-layer self-assembly

Patterned multilayer films composed of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS) were prepared using dip and spin self-assembly (SA) methods. A silicon substrate was patterned with a photoresist thin film using conventional photolithography, and PAH/PSS multilayers were then deposited onto the substrate surface using dip or spin SA. For spin SA, the photoresist on the substrate was retained, despite the high centrifugal forces involved in depositing the polyelectrolytes (PEs). The patterned multilayer films were formed by immersing the PE-coated substrates in acetone for 10 min. The effect of ionic strength on the pattern quality in dip and spin multilayer patterns (line-edge definition and surface roughness of the patterned region) was investigated by increasing the salt concentration in the PE solution (range 0-1 M). In dip multilayer patterns, the presence of salt increased the film surface roughness and pattern thickness without any deformation of pattern shape. The spin multilayer patterns formed without salt induced a height profile of about 130 nm at the pattern edge, whereas the patterns formed with high salt content (1 M) were extensively washed off the substrates. Well-defined pattern shapes of spin SA multilayers were obtained at an ionic strength of 0.4 M NaCl. Multilayer patterns prepared using spin SA and lift-off methods at the same ionic strength had a surface roughness of about 2 nm, and those prepared using the dip SA and lift-off method had a surface roughness of about 5 nm. The same process was used to prepare well-defined patterns of organic/metallic multilayer films consisting of PE and gold nanoparticles. The spin SA process yielded patterned multilayer films with various lengths and shapes.     

Characterization of poly(3,4-ethylenedioxythiophene)/poly(styrene sulfonate) (PEDOT:PSS) adsorption on cellulosic materials

The interaction between poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS) and cellulosic fibers was characterized in order to obtain further understanding of the conductivity properties of the modified cellulosic fiber material. Microcrystalline cellulose (MCC) was used as a model surface to study the adsorption behavior at various pH and salt concentrations, while samples of low-conductivity paper, normally used for the production of electrical insulation papers, were dipped into PEDOT:PSS dispersion and air-dried for X-ray photoelectron spectroscopy (XPS) studies. The results showed a strong interaction between the MCC and PEDOT:PSS, which implied a broad molecular distribution of the conducting polymer. With increasing pH, less amount of the conducting polymer was adsorbed whereas the amount adsorbed passed through a maximum value with varying salt concentration. Zeta potential measurement and polyelectrolyte titration were used to determine the surface charge of both suspended MCC particles and dispersed PEDOT:PSS at various pH levels and salt concentrations. Dip-coated paper samples exhibited two peaks in the S(2p) XPS spectra at 168–169 and 164–165 eV which correspond to the sulfur signals of sulfonate (in PSS) and in thiophene (in PEDOT), respectively. It was found that the PEDOT:PSS with a ratio of 1:2.5 was adsorbed more in the base paper than that with a ratio of 1:6. The PEDOT:PSS ratio on the surface of the cellulosic material was higher than that in the bulk liquid for all samples. The results indicated that PEDOT was preferentially adsorbed rather than PSS. The degree of washing of the conducting polymer did not significantly affect the PEDOT enhancement on the surface.     

Influence of assembly pH on compression and Ag nanoparticle synthesis of polyelectrolyte multilayers

Influence of assembly pH on compression and Ag nanoparticle synthesis of polyelectrolyte multilayers was studied using poly(diallyldimethylammonium chloride) (PDADMAC) and poly(4-styrenesulfonic acid-co-maleic acid, 1:1SS:MA) sodium salt (PSSMA 1:1) as the building blocks. The thickest multilayers turned out at pH 4. A homogeneous compression by a silicone rubber stamp increased significantly the water contact angle to a same value which was independent on the original assembly pH anymore. The multilayers assembled at pH 4 could be maximumly compressed to a ratio of 70% by a silicone rubber stamp with linear patterns, which was considerably larger than those assembled at other pHs (the compression ratio ~50%). The Ag nanoparticles were then synthesized inside the multilayers either flat compressed or not. The results showed that the compression reduced significantly the amount of Ag nanoparticles for the multilayers assembled at pH 2 and pH 4. The particle amount was also decreased significantly when the multilayers were assembled at higher pH, pH 6, for example, regardless of the compression. Substantial alteration of the multilayers in terms of the surface morphology, thickness and refractive index was found during the reduction of Ag(+) containing multilayers by NaBH(4) solution.     

Characteristics of polyelectrolyte multilayers: effect of PEI anchoring layer and posttreatment after deposition

The influence of a first (anchoring) layer and film treatment on the structure and properties of polyelectrolyte multilayer (PEM) films obtained from polyallylamine hydrochloride (PAH) and polysodium 4-styrenesulfonate (PSS) was studied. Branched polyethyleneimine (PEI) was used as an anchoring layer. The film thickness was measured by ellipsometry. Complementary X-ray reflectometry and AFM experiments were performed to study the change in the interfacial roughness. We found that the thickness of the PEM films increased linearly with the number of layers and depended on the presence of an anchoring PEI layer. Thicker films were obtained for multilayers having PEI as the first layer comparing to films having the same number of layers but consisting of PAH/PSS only. We investigated the wettability of PEM surfaces using direct image analysis of the shape of sessile water drops. Periodic oscillations in contact angle were observed. PAH-terminated films were more hydrophobic than films with PSS as the outermost layer. The effect of long time conditioning of PEM films in solutions of various pH's or salt (NaCl) concentrations was also examined. Salt or base solutions induced modification in wetting properties of the polyelectrolyte multilayers but had a negligible effect on the film thickness.     

Polyelectrolyte complexation between poly(methacrylic acid,sodium salt) and poly(diallyldimethylammonium chloride) or poly[2‐(methacryloyloxyethyl) trimethylammonium chloride]

Polyelectrolyte complexes between poly(methacrylic acid, sodium salt) and poly(diallyldimethylammonium chloride) (PDADMAC) or poly[2‐(methacryloyloxyethyl)trimethylammonium chloride] (PMOETAC) form gels, liquid phases, or soluble complexes depending on charge ratio, total polymer loading, polymer molecular weight, and ionic strength. Increasing the ionic strength of the medium led most polyelectrolyte pairs to transition from gel through liquid complexes (complex coacervate) to soluble complexes. These transitions shift to higher ionic strengths for higher molecular weight polymers, as well as for PMOETAC compared to PDADMAC. The complex phases swelled with increasing polymer loading, ultimately merging with the supernatant phase at a critical polymer loading. The isolated liquid complex phases below and above this critical loading were temperature‐sensitive, showing cloud points followed by macroscopic phase separation upon heating. Incorporating 5 mol % lauryl methacrylate into the polyanion led to increased complex yield with PDADMAC, and increased resistance to ionic strength. In contrast, incorporating 30 mol % of oligo(ethylene glycol) methacrylate into the polyanion led to decreased complex yield, and to lower resistance to ionic strength. Two polyelectrolyte systems that produced liquid complexes were used to encapsulate hydrophobic oils, and in one case were used to demonstrate the feasibility of crosslinking the resulting capsule walls. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 4129–4143, 2007     

Counterions in poly(allylamine hydrochloride) and poly(styrene sulfonate) layer-by-layer films

The amount of counterions in layer-by-layer (LBL) films of poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS) has been determined with X-ray photoelectron spectroscopy (XPS) for films prepared from solutions with various NaCl concentrations. Sodium and chloride counterions are present in LBL films produced from salt solutions, which are located at the surface and in the bulk of the films. The percentage of bulk counterions increases with the ionic strength of the polyelectrolyte before reaching a constant value. The bulk sodium/sulfur percentage ratios tend to 0.8 for samples washed with pure water and for samples washed with NaCl aqueous solutions, while the bulk chlorine/nitrogen percentage ratios tend to 0.5 for the same samples. The ratio between the percentages of polyelectrolyte ionic groups lies close to unity for all samples, indicating that counterions do not contribute to charge compensation in the polyelectrolyte during the adsorption process. The presence of counterions in LBL films is explained by Manning condensation near the polyelectrolyte ionic groups, leading to inter-polyelectrolyte ionic bondings via ionic networks. It is believed that condensation leads to the formation of NaCl crystallites in these LBL films, which was confirmed by X-ray diffraction measurements.     

Oscillations in solvent fraction of polyelectrolyte multilayers driven by the charge of the terminating layer

We have investigated polyelectrolyte multilayers of poly(styrene sulfonate) (PSS) and poly(allylamine hydrochloride) in contact with D2O by neutron reflectometry. The study particularly focuses on the changes in the solvent fraction of the system upon addition of a layer. When the layers are deposited at a low salt concentration (0.25 M NaCl), no significant changes in the solvent fraction are detected. In contrast, at a larger salt concentration (1 M NaCl), oscillations in the solvent fraction are detected when a new layer is deposited. In this case, addition of PSS systematically increases the solvent volume fraction, and addition of PAH decreases the solvent fraction. The results suggest that one of the parameters driving the oscillations in solvent fraction is the uncompensated charges present in the layers. This study opens new perspectives on results previously published by other authors: in addition to polymer desorption, water uptake or release might contribute to the different regimes of multilayer growth reported in the literature (linear, asymmetric, or exponential growth). In addition, comparison to NMR results previously reported allows for conclusions about the mobility of the solvent in the multilayers: the average rotational correlation time of the water molecules in the polyelectrolyte layers decreases upon addition of PSS and increases upon addition of PAH.     

Separation of fluoride from other monovalent anions using multilayer polyelectrolyte nanofiltration membranes

Nanofiltration (NF) is an attractive technique for reducing F- concentrations to acceptable levels in drinking water, but commercial NF membranes such as NF 270 and NF 90 show minimal Cl-/F- selectivity. In contrast, simple layer-by-layer deposition of 4.5-bilayer poly(styrene sulfonate) (PSS)/poly(diallyldimethylammonium chloride) (PDADMAC) films on porous alumina supports yields NF membranes that exhibit Cl-/F- and Br-/F- selectivities>3 along with solution fluxes that are >3-fold higher than those of the commercial membranes. Fluoride rejection by (PSS/PDADMAC)4PSS membranes, which is >70%, is independent of pressure over a range of 3.6 to 6.0 bar, suggesting that the primary transport mechanism in these films is convection. Moreover, the fact that Br-/F- selectivity is 12% higher than Cl-/F- selectivity suggests that discrimination among the monovalent ions is based on size (Stokes radius). Chloride/fluoride selectivities are essentially constant over Cl-/F- feed ratios from 1 to 60, so these separations will be viable over a range of conditions. Interestingly, PSS/protonated poly(allylamine) films show little Cl-/F- selectivity, and the selectivity of PSS/PDADMAC membranes is a strong function of the number of deposited layers, indicating that NF properties are very sensitive to film structure.     

Effect of thiocyanate counterion condensation on poly(allylamine hydrochloride) chains on the buildup and permeability of polystyrenesulfonate/polyallylamine polyelectrolyte multilayers

In this study, we investigate the buildup of PEI-(PSS-PAH)(n) polyelectrolyte multilayers at pH 7.4 in the presence of either NaCl or NaSCN as a supporting electrolyte. It appears that in the presence of increasing thiocyanate concentrations (from 0.1 to 0.5 M), the thickness increment, obtained from optical waveguide lightmode spectroscopy experiments, increases whereas it stays practically constant for increasing sodium chloride concentrations (between 0.1 and 0.5 M). The hydration of the films differs also markedly between both electrolyte solutions. The differences in the construction of the polyelectrolyte multilayers in the presence of both supporting electrolytes are rationalized in terms of strong SCN(-) condensation on the PAH chains. The occurrence of this ion condensation is indirectly demonstrated by means of zeta potential measurements and directly demonstrated by means of attenuated total internal reflection infrared spectroscopy on the multilayer films. Moreover when the films are built up in the presence of SCN(-), these ions are only slowly exchanged by the Cl(-) ions introduced in the bulk. Conversely the thick films obtained from 0.5 M NaSCN solutions do not deswell when the buffer solution is replaced by a 0.5 M NaCl containing buffer. The permeability of the films constructed in the presence of both sodium salts is also studied by means of cyclic voltametry and is found to be markedly different in the case of films made from five bilayers at 0.5 M salt concentration. This difference is due to the different morphology and porosity of the films constructed in the presence of 0.5 M NaCl and 0.5 M NaSCN.