Effect of acid-base equilibria on the donnan potential of layer-by-layer redox polyelectrolyte multilayers

In polymer films carrying an excess of fixed charge the electrostatic penalty to bring ions of same charge from the bathing electrolyte into the film sets a membrane potential (Donnan Potential) across the film-electrolyte interface. This potential is responsible for the ionic permselectivity observed in polyelectrolyte membranes. We have used electrochemical measurements to probe the dependence of the Donnan potential on the acid-base equilibrium in layer-by-layer self-assembled polyelectrolyte multilayers. The voltammperogram peak position of the Os(III)/Os(II) couple in self-assembled polyelectrolyte multilayers comprised of poly(allylamine) derivatized with Os(bpy)(2)PyCl+ and poly(vinylsulfonate) was recorded in solutions of increasing ionic strength for different assembly and testing solution pH. Protonation-deprotonation of the weak redox poly(allylamine) changes the fixed charge population in the as prepared (intrinsic) self-assembled redox polyelectrolyte multilayers. For films assembled in solutions of pH higher than the test solution pH, the Donnan plots (E(app) vs log C) exhibit a negative slope (anionic exchanger) while for films assembled at lower pH than that of the test solution positive slopes (cationic exchanger) are apparent. The ion exchange mechanism has been supported by complementary electrochemical quartz crystal microbalance. X-ray photoelectron spectroscopy and infrared reflection-absorption spectroscopy experiments demonstrated that the as prepared films have a memory effect on their protonation state during assembly, which leads to the observed dependence of the Donnan potential on the adsorption pH.     

Donnan permselectivity in layer-by-layer self-assembled redox polyelectrolye thin films

Redox polyelectrolyte multilayers have been assembled with use of the layer-by-layer (LBL) deposition technique with cationic poly(allylamine) modified with Os(bpy)(2)ClPyCHO (PAH-Os) and anionic poly(styrene)sulfonate (PSS) or poly(vinyl)sulfonate (PVS). Different behavior has been observed in the formal redox potential of the Os(II)/Os(III) couple in the polymer film with cyclic voltammetry depending on the charge of the outermost layer and the electrolyte concentration and pH. The electrochemical quartz crystal microbalance (EQCM) has been used to monitor the exchange of ions and solvent with the external electrolyte during redox switching. At low ionic strength Donnan permselectivity of anions or cations is apparent and the nature of the ion exclusion from the film is determined by the charge of the topmost layer and solution pH. At high electrolyte concentration Donnan breakdown is observed and the osmium redox potential approaches the value for the redox couple in solution. Exchange of anions and water with the external electrolyte under permselective conditions and salt and water under Donnan breakdown have been observed upon oxidation of the film at low pH for the PAH-Os terminating layer. Moreover, at high pH values and with PVS as the terminating layer EQCM mass measurements have shown that cation release was masked by water exchange.     

Dynamics of ion exchange between self-assembled redox polyelectrolyte multilayer modified electrode and liquid electrolyte

A probe beam deflection (PBD) study of ion exchange between an electroactive polymer poly(allylamine)-bipyridyl-pyridine osmium complex film and liquid electrolyte is reported. The PBD measurements were made simultaneously to chronoamperometric oxidation-reduction cycles, to be able to detect kinetic effects in the ion exchange. Layer-by-layer (LbL) self-assembled redox polyelectrolyte films with osmium bipyridyl complex covalently attached to poly(allylamine) (PAH-Os) and poly(styrene sulfonate) (PSS) have been built by alternate electrostatic adsorption from soluble polyelectrolytes. The ionic exchange during initial conditioning of the film ("break-in") undergoing oxidation-reduction cycles and recovery after equilibration in the reduced state have shown an exchange of anions and cations with time lag between them. The effect of the nature of cation on the ionic exchange has been investigated with dilute HCl, LiCl, NaCl, and CsCl electrolytes. The ratio of anion to cation exchanged at the film-electrolyte interface has a strong dependence on the nature of charge in the topmost layer, that is, when negatively charged PSS is the capping layer, a larger proportion of cation exchange is observed. This demonstrates that the electrical potential distribution at the redox polyelectrolyte multilayer (PEM)/electrolyte interface determines the ionic flux in response to charge injection in the film.     

Layer-by-layer electrostatic self-assembly of single-wall carbon nanotube polyelectrolytes

We have used anionic and cationic single-wall carbon nanotube polyelectrolytes (SWNT-PEs), prepared by the noncovalent adsorption of ionic naphthalene or pyrene derivatives on nanotube sidewalls, for the layer-by-layer self-assembly to prepare multilayers from carbon nanotubes with polycations, such as poly(diallyldimethylammonium) or poly(allylamine hydrochloride) (PDADMA or PAH, respectively), and polyanions (poly(styrenesulfonate), PSS). This is a general and powerful technique for the fabrication of thin carbon nanotube films of arbitrary composition and architecture and allows also an easy preparation of all-SWNT (SWNT/SWNT) multilayers. The multilayers were characterized with vis-near-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), surface plasmon resonance (SPR) measurements, atomic force microscopy (AFM), and imaging ellipsometry. The charge compensation in multilayers is mainly intrinsic, which shows the electrostatic nature of the self-assembly process. The multilayer growth is linear after the initial layers, and in SWNT/polyelectrolyte films it can be greatly accelerated by increasing the ionic strength in the SWNT solution. However, SWNT/SWNT multilayers are much more inert to the effect of added electrolyte. In SWNT/SWNT multilayers, the adsorption results in the deposition of 1-3 theoretical nanotube monolayers per adsorbed layer, whereas the nominal SWNT layer thickness is 2-3 times higher in SWNT/polyelectrolyte films prepared with added electrolyte. AFM images show that the multilayers contain a random network of nanotube bundles lying on the surface. Flexible polyelectrolytes (e.g., PDADMA, PSS) probably surround the nanotubes and bind them together. On macroscopic scale, the surface roughness of the multilayers depends on the components and increases with the film thickness.     

Combined effect of spin speed and ionic strength on polyelectrolyte spin assembly

Polyelectrolyte spin assembly (PSA) of multilayers is a sequential process featuring adsorption of oppositely charged polyelectrolytes from dilute solutions undergoing spin-coating flow. Here, we report on the dependence of PSA multilayer buildup of poly(sodium 4-styrenesulfonate) and poly(allylamine hydrochloride) on solution ionic strength and spin speed. We observed that at a given spin speed, the PSA coating growth rate (thickness/bilayer) and polymer surface coverage shows a nonmonotonic dependence on salt concentration, first increasing and then decreasing with increasing solution ionic strength. This is argued to be a manifestation of two competing mechanisms responsible for the layer formation. At low salt concentrations, the electrostatic interactions control the multilayer assembly process, while at high salt concentrations it is dominated by shear flow. We explain this nonmonotonic behavior in the framework of a Flory-like theory of multilayer formation from polyelectrolyte solution under shear flow. Additionally, the PSA process led to multilayer coatings with a radial dependence on thickness at lower spin speed in the shear-dominated regime. On increasing spin speed, such radial dependence subsided, eventually leading to uniform coatings by planarization. The surface topography of the multilayered coatings adsorbed at salt concentration less than 0.1 M was flat and featureless for all studied spin speeds. Unique morphological features in the films were formed at salt concentration higher than 0.1 M, the size of which depended on the spin speed and solution ionic strength.     

Polyelectrolyte spin assembly: Influence of ionic strength on the growth of multilayered thin films

Layer-by-layer (LbL) assembly of polymer electrolyte multilayers is now a well-established method for the fabrication of thin films by sequential adsorption of alternating layers of oppositely charged polyelectrolytes. Most commonly, such adsorptions have been from quiescent solutions of varying ionic strength and pH. Here, we report results on an alternative processing route for the achievement of polymeric multilayer assemblies of poly(sodium-4-styrene sulfonate) and poly(allylamine hydrochloride) that utilizes conventional spin coating. We investigated and describe herein the dependence of multilayer film buildup on solution ionic strength for comparison with similar dependence in quiescent adsorption. Using UV-Vis spectroscopy we monitored the growth of the multilayered films, while with Atomic Force Microscopy (AFM) we examined the surface features and measured coating thicknesses at different salt concentrations. AFM and UV-Vis data reveal two regimes of behavior with increasing salt: strong salt-dependence at low salt contents, and weak salt-dependence for high salt contents. To explain this observation, we introduce the relevance of the dimensionless group De = τ, the local Deborah Number, to the problem. As ionic strength increases, τ increases so that spin-assembly flow influences adsorbed conformation, and thus LbL growth rate. Our results indicate the ability to design and control polyelectrolyte multilayered structures prepared via spin assembly by varying solution properties that influence the conformation of deposited polymer chains. Additionally, our studies reveal the need for study of the fundamental mechanisms of polyelectrolyte adsorption within complex flow fields. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3654–3666, 2004     

Assembly of multilayer films from polyelectrolytes containing weak and strong acid moieties

Multilayer films were assembled from a copolymer containing both weakly and strongly charged pendant groups, poly(4-styrenesulfonic acid-co-maleic acid) (PSSMA), deposited in alternation with poly(allylamine hydrochloride) (PAH). The strongly charged groups (styrene sulfonate, SS) are expected to form electrostatic linkages (to enhance film stability), while the weakly charged groups (maleic acid, MA) can alter multilayer film properties because they are responsive to external pH changes. In this study, we varied several assembly conditions such as pH, SS/MA ratio in PSSMA, and the ionic strength of the polyelectrolyte solutions. The multilayer films were also treated by immersion into pH 2 and 11 solutions after assembly. Quartz crystal microgravimetry and UV-visible spectrophotometry showed that the thickness of PSSMA/PAH multilayers decreases with increasing assembly pH regardless of whether salt was present in the polyelectrolyte solutions. When no salt was added, the multilayers are thinner, smoother, and grow less regularly. Atomic force microscopy images indicate that the presence of salt in polyelectrolyte solutions results in rougher surface morphologies, and this effect is especially significant in multilayers assembled at pH 2 and pH 11. When both polyelectrolytes are adsorbed at conditions where they are highly charged, salt was necessary to promote regular multilayer growth. Fourier transform infrared spectroscopy studies show that the carboxylic acids in the multilayers are essentially ionized when assembled from different pHs in 0.5 M sodium chloride solutions, whereas some carboxylic acids remain protonated in the multilayers assembled from solutions with no added salt. This resulted in different pH stability regimes when the multilayers were exposed to different pH solutions, post assembly.     

Assembly of poly(N-isopropylacrylamide)-co-acrylic acid microgel thin films on polyelectrolyte multilayers: Effects of polyelectrolyte layer thickness, surface charge, and microgel solution pH

Temperature- and pH-sensitive poly(N-isopropylacrylamide)?Cco-acrylic acid (pNIPAm-co-AAc) microgels were deposited on glass substrates coated with polyelectrolyte multilayers composed of the polycation poly(allylamine hydrochloride) (PAH) and the polyanion poly(sodium 4-styrenesulfonate) (PSS). The microgel density and structure of the resultant films were investigated as a function of: (1) the number of PAH/PSS layers (layer thickness); (2) the charge on the outer layer of the polyelectrolyte multilayer film; and (3) the pH of microgel deposition solution. The resultant films were studied by differential interference contrast optical microscopy, atomic force microscopy, and scanning electron microscopy. It was found that the coverage of the microgels on the surface was a complex function of the pH of the deposition solution, the charge on the outer layer of the polyelectrolyte thin film and the PAH/PSS layer thickness; although it appears that microgel charge plays the biggest role in determining the resultant surface coverage.     

Adsorption of polyelectrolyte-nanoparticle systems on silica: influence of ionic strength

In this work the effect of ionic strength on the adsorption behavior of cationic polyelectrolyte (acrylamide-acrylamidopropyltrimethylammonium chloride) and negatively charged silica particles has been studied by means of ellipsometry. The adsorption of the polyelectrolyte was observed to increase with increasing salt concentration, a behavior typical for polyelectrolytes with a screening-reduced solvency and a nonelectrostatic affinity for the surface. A similar dependence on the ionic strength was observed when studying the electrolyte effect on the nanoparticle adsorption to the preadsorbed polyelectrolyte film, suggesting that the polyelectrolyte surface conformations largely govern the binding capacity of the particles to the surface.     

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.     

Functionalization of organic membranes by polyelectrolyte multilayer assemblies: application to the removal of copper ions from aqueous solutions

The functionalization of an organic polyethersulfone membrane (PES) was performed by alternating deposition of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrene sulfonate) (PSS), leading to the formation of a polyelectrolyte multilayer film (PEM). The resulting assembly was characterized by tangential streaming potential measurements to determine the charge of the modified membranes as a function of the polyelectrolyte solution concentration and as a function of the immersion time of the membrane in the polyelectrolyte solutions. Then, the modified membranes were used to perform the ultrafiltration of aqueous solutions containing copper(II) ions. Different operating conditions were tested including: polyelectrolyte concentration, polyelectrolyte nature, thickness of the PEM film or pH of the Cu(2+) solutions. These filtration experiments demonstrated that it was possible to obtain a satisfactory retention of the copper ions (88%), thus proving that this type of assembly can be useful for the removal of copper ions from contaminated aqueous solutions.     

The pH inside a swollen polyelectrolyte gel: poly(N-vinylimidazole)

The pH inside a dissolved polyelectrolyte coil or a swollen ionic polymer network is not accessible to direct measurement. It is here calculated through a simple model, based on Donnan equilibrium, counterion condensation (for charge density exceeding the critical value), and balance of mobile ions, without any assumption on the pKa of the ionizable groups. The data needed for the calculation with this model are polymer concentration, pH value in the initial solution, and pH value in the bath at equilibrium. All three can be determined experimentally by a batch method where the polymer is immersed in a different pot for each starting pH. The model is applied to a sample system, namely, chemically cross-linked poly(N-vinylimidazole) immersed in acidic baths of different pH values. The imidazole units are basic and become protonated by the acid, thus changing the pH of the initial bath. The model shows how the pH developed inside the swollen gel is several units higher than the pH of the bath at equilibrium, both with or without the correction for counterion condensation. Consequently, when the pKa of the polyelectrolyte is determined in the usual way (with the pH measured in the external bath), it gives an apparent value that is several units below the pKa determined from the actual pH inside the swollen gel at equilibrium. The inclusion of the counterion condensation decreases very slightly the polymer basicity. Surface effects and intramolecular association between protonated and unprotonated imidazole rings are discussed to explain the pKa behavior in the limit of low degree of ionization.     

Multicompartment films made of alternate polyelectrolyte multilayers of exponential and linear growth

The layer by layer deposition process of polyelectrolytes is used to construct films equipped with several compartments containing "free polyelectrolytes". Each compartment corresponds to a stratum of an exponentially growing polyelectrolyte multilayer film, and two consecutive compartments are separated by a stratum composed of a linearly growing multilayer that acts as a barrier preventing polyelectrolyte diffusion from one compartment to another. We use hyaluronic acid/poly(L-lysine) as the system to build the compartments and the poly(styrene sulfonate)/poly(allylamine) system for the barrier. Using confocal microscopy, it is shown that poly(L-lysine) diffuses only within the compartment in which it was initially introduced during the film construction and is thus unable to cross the barriers. Using fluorescein isothiocyanate as a pH indicator, it is also shown that although poly(styrene sulfonate)/poly(allylamine) multilayers act as a barrier for polyelectrolytes, they do not prevent proton diffusion through the film. Such films open the route for multiple functionalization of biomaterial coatings.     

Polyamine Colloids Cross-Linked with Phosphate Ions: Towards Understanding the Solution Phase Behavior

Ionically crosslinked poly(allylamine)/phosphate (PAH/Pi) colloids consist of self-assembled nanostructures stabilized by supramolecular interactions. Under physiological conditions, these interactions should be present at high ionic strength and only in a narrow pH window to be effective as drug delivery agents. In this work we study the effect of the pH and ionic strength in the chemical behaviour of inorganic phosphate (Pi), poly(allylamine hydrochloride) (PAH) and their mixture in aqueous solution (PAH−Pi). By combination of experimental measurements and a theoretical model, we demonstrate that the driving force that leads to the formation of colloids is the electrostatic pairing between the positively charged amino groups in PAH and negatively charged HPO₄²⁻ ions. Increasing the ionic strength of the system by addition of KCl weakens the PAH−Pi interactions and narrows the pH stability window from 4 to 1.8 pH units. In addition, a fully reversible system was obtained in which the colloids assemble and disassemble by changing the pH between 6.8 and 7.1 at high ionic strength, making them suitable for use as pH-responsive nanocarriers.     

Electropreparation of Poly(benzophenone‐4) Film Modified Electrode and Its Electrocatalytic Behavior Towards Dopamine,Ascorbic Acid and Nitrite

The oxidation of benzophenone‐4 (2‐hydroxy‐4‐methoxybenzophenone‐5‐sulfonic acid) at glassy carbon electrode gives rise to stable redox active electropolymerized film during repetitive potential cycling between 0 to 1.3 V (Ag/AgCl). Cyclic voltammogram of poly(benzophenone‐4) film shows a redox couple with well‐defined peaks. The redox response of the modified electrode was found to be depending on the pH of the contacting solution. The peak potentials were shifted to a less positive region with increasing pH and the dependence of the peak potential was found to be 51 mV/pH. The electrocatalytic behavior of poly(benzophenone‐4) film modified electrode towards oxidation of dopamine, ascorbic acid and reduction of nitrite was investigated. The oxidation of dopamine and ascorbic acid occurred at less positive potential on poly(benzophenone‐4) film compared to bare glassy carbon electrode. For dopamine, the overpotential was reduced about 180 mV. Feasibility of utilizing poly(benzophenone‐4) film coated electrode in analytical estimation of dopamine, ascorbic acid and nitrite was also demonstrated.     

Influence of solvent quality on the growth of polyelectrolyte multilayers

The effect of solvent conditions on the growth of polyelectrolyte (PE) multilayer films comprising poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate sodium salt) (PSS) on planar substrates was investigated by means of surface plasmon resonance spectroscopy (SPRS), quartz crystal microbalance (QCM), and atomic force microscopy techniques. The solvent quality was varied by the addition of ethanol to the PE solutions used for deposition of the layers, thus tuning the relative strength of electrostatic and secondary intermolecular and intramolecular interactions. Experiments were performed with PE solutions both without added electrolyte and containing 0.5 M NaCl. Decreasing the solvent quality (i.e., increasing the amount of ethanol in the adsorption solution) resulted in a marked increase of both the multilayer film thickness and mass loading, as determined from the SPRS spectra and QCM frequency shifts, respectively. With the solution composition approaching the precipitation point, thick PAH/PSS films were formed due to the screening of the electrostatic intra- and interchain repulsions and enhanced hydrophobic interactions between the polyelectrolyte chains. However, the films formed from water/ethanol mixtures remained stable upon subsequent exposure to water or salt-containing solutions: no significant film desorption occurred after up to 24 h of exposure to water or 0.5 M NaCl solutions. In addition, the effect of postdeposition exposure to water/ethanol mixtures was investigated for PE multilayers assembled from aqueous solutions. In this case, the optical thickness of the films was determined during exposure to water/ethanol mixtures, and instead of swelling, the polyelectrolyte films collapse to the surface as a result of the unfavorable segment-solvent interactions.     

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.     

Electrochemical Preparation of Poly(Malachite Green) Film Modified Nafion‐Coated Glassy Carbon Electrode and Its Electrocatalytic Behavior Towards NADH,Dopamine and Ascorbic Acid

Poly(malachite green) film modified Nafion‐coated glassy carbon electrodes have been prepared by potentiodynamic cycling in malachite green solution. The pH of polymerisation solution has only minor effect on film formation. Electrochemical quartz crystal microbalance (EQCM) was used to monitor the growth of the poly(malachite green) film. Cyclic voltammogram of the poly(malachite green) film shows a redox couple with well‐defined peaks. The redox response of the modified electrode was found to be depending on the pH of the contacting solution. The peak potentials were shifted to a less positive region with increasing pH and the dependence of the peak potential was found to be 56 mV per pH unit. The electrocatalytic behavior of poly(malachite green) film modified Nafion‐coated glassy carbon electrodes was tested towards oxidation of NADH, dopamine, and ascorbic acid. The oxidation of dopamine and ascorbic acid occurred at less positive potential on poly(malachite green) film compared to bare glassy carbon electrode. In the case of NADH, the overpotential was reduced substantially on modified electrode. Finally, the feasibility of utilizing poly(malachite green) film electrode in analytical estimation of ascorbic acid was demonstrated in flow injection analysis.     

Peculiarities of polyelectrolyte multilayer assembly on patterned surfaces

The layer-by-layer assembly of poly(diallyldimethylammonium chloride) and poly(sodium 4-styrenesulfonate) is studied on templates with imprinted arrays of microwells ranging from 2 to 25 μm and different aspect ratios. The thickness and microstructure of polyelectrolyte multilayers (PEMs) are measured using scanning electron microscopy. At 0.2 M ionic strength, the PEM film evenly coats the template both inside and outside the microwells. If the film is thinner than the critical value of about 400 nm, PEM microstructures collapse upon dissolving the template. Euler's model of critical stress is used to describe the collapse. At 2 M ionic strength, a substantially thinner PEM film is assembled inside the 25 μm wells than outside. If the well diameter is reduced to 7 and 2 μm, a much thicker PEM film is formed inside the microwells. These observations have been attributed to the changing of polyelectrolyte conformation in the solutions.