Variation of ion-exchange capacity, zeta potential, and ion-transport selectivities with the number of layers in a multilayer polyelectrolyte film

Diffusiophoretic Behavior of Polyelectrolyte-Coated Particles

Diffusiophoresis, the movement of particles under a solute concentration gradient, has practical implications in a number of applications, such as particle sorting, focusing, and sensing. For diffusiophoresis in an electrolyte solution, the particle velocity is described by the electrolyte relative concentration gradient and the diffusiophoretic mobility of the particle. The electrolyte concentration, which typically varies throughout the system in space and time, can also influence the zeta potential of particles in space and time. This variation affects the diffusiophoretic behavior, especially when the zeta potential is highly dependent on the electrolyte concentration. In this work, we show that adsorbing a single bilayer (or 4 bilayers) of a polyelectrolyte pair (PDADMAC/PSS) on the surface of microparticles resulted in effectively constant zeta potential values with respect to salt concentration throughout the experimental range of salt concentrations. This allowed a constant potential model for diffusiophoretic transport to describe the experimental observations, which was not the case for uncoated particles in the same electrolyte system. This work highlights the use of simple polyelectrolyte pairs to tune the zeta potential and maintain constant values for precise control of diffusiophoretic transport.

Stability of Properties of Layer-by-Layer Coated Membranes under Passage of Electric Current

Electrodialysis with layer-by-layer coated membranes is a promising method for the separation of monovalent and polyvalent ions. Since the separation selectivity is significantly reduced in the presence of defects in the multilayer system, the stability of the modifiers becomes an important issue. This article reports the i-V curves of layer-by-layer coated membranes based on the heterogeneous MK-40 membrane before and after 50 h long electrodialysis of a solution containing sodium and calcium ions at an underlimiting current density, and the values of concentrations of cations in the desalination chamber during electrodialysis. It is shown that the transport of bivalent ions through the modified membranes is reduced throughout the electrodialysis by about 50%, but the operation results in decreased resistance of the membrane modified with polyethylenimine, which may suggest damage to the modifying layer. Even after electrodialysis, the modified membrane demonstrated experimental limiting current densities higher than that of the substrate, and in case of the membrane modified with polyallylamine, the limiting current density 10% higher than that of the substrate membrane.

Tuning the charge of polyelectrolyte complex membranes prepared via aqueous phase separation

In this work, polyelectrolyte mixing ratio is studied as a tuning parameter to control the charge, and thus the separation properties of polyelectrolyte complex (PEC) membranes prepared via Aqueous Phase Separation (APS). In this approach, various ratios of poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC) are mixed at high salinity and the PEC-based membranes are then precipitated using low salinity coagulation baths. The monomeric ratio of PSS to PDADMAC is varied from 1.0 : 0.8 through to 1.0 : 1.2. Obtained membranes have an asymmetric structure and function as nanofiltration membranes with on average 1 L m-2 h-1 bar-1 pure water permeance and <400 Da molecular weight cut-off (MWCO); except for the 1.0 : 1.2 membrane, where the water permeance was much higher (>20 L m-2 h-1 bar-1) with a similarly low MWCO. For the first time, we report the formation of both negatively and positively charged PSS-PDADMAC based APS membranes, as determined by both streaming potential and salt retention measurements. We hypothesize that the salt type used in the APS process plays a key role in the observed change in membrane charge. The point where the membrane charge transitions from negative to positive is found to be between the 1.0 : 0.9 and 1.0 : 1.0 PSS : PDADMAC ratios. The polyelectrolyte ratio not only affects membrane charge, but also their mechanical properties. The 1.0 : 0.9 and 1.0 : 1.0 membranes perform the best amongst the membranes prepared in this study since they have high salt retentions (up to 90% Na2SO4 and 75% MgCl2, respectively) and better mechanical stability. The higher permeance of the more charged, and thus more swollen, 1.0 : 0.8 and 1.0 : 1.2 membranes provide a relevant new direction for the development of APS-based PEC membranes.

Polyelectrolytes as Building Blocks for Next-Generation Membranes with Advanced Functionalities

The global society is in a transition, where dealing with climate change and water scarcity are important challenges. More efficient separations of chemical species are essential to reduce energy consumption and to provide more reliable access to clean water. Here, membranes with advanced functionalities that go beyond standard separation properties can play a key role. This includes relevant functionalities, such as stimuli-responsiveness, fouling control, stability, specific selectivity, sustainability, and antimicrobial activity. Polyelectrolytes and their complexes are an especially promising system to provide advanced membrane functionalities. Here, we have reviewed recent work where advanced membrane properties stem directly from the material properties provided by polyelectrolytes. This work highlights the versatility of polyelectrolyte-based membrane modifications, where polyelectrolytes are not only applied as single layers, including brushes, but also as more complex polyelectrolyte multilayers on both porous membrane supports and dense membranes. Moreover, free-standing membranes can also be produced completely from aqueous polyelectrolyte solutions allowing much more sustainable approaches to membrane fabrication. The Review demonstrates the promise that polyelectrolytes and their complexes hold for next-generation membranes with advanced properties, while it also provides a clear outlook on the future of this promising field.

Theoretical Evaluation of Polyelectrolyte Layering during Layer-by-Layer Coating of Ultrafiltration Hollow Fiber Membranes

Layer-by-layer (LbL) modification of porous membranes for water filtration has become an active research field in the past few years. Different mechanisms regarding polyelectrolyte film growth, swelling and smoothing, transport through these films, etc., have been studied. Although there are conjectures, it is not yet fully understood where the polyelectrolyte layering takes place when modifying porous membranes, either within the pores or on top of the porous material. This study presents a theoretical approach to investigate the dominant layer buildup regime between pore-dominated vs. layer-dominated growth of polyelectrolytes on porous membranes without mechanically interfering or damaging the membrane material. For this, fouling mechanism processes are used as an analogy. The presented approach gives a new insight into layering conformation and might be helpful to investigate the interaction between the membrane surface and the PE film. Moreover, the MgSO₄ rejection behavior of two types of modified membranes was investigated: one with an initial pore-dominated layer growth followed by a layer-dominated film growth; the other one with a completely layer-dominated film growth. The data confirm that a rejection for MgSO₄ could only be achieved in the regime of layer-dominated film growth. Additionally, when layer-dominated film growth prevails from the early stages of the coating process, permeability values are higher at similar MgSO₄ rejection rates compared to an initial pore-dominated and then layer-dominated film growth. Accordingly, the interaction between the membrane pore size and molecular weight of the polyelectrolytes in the coating solutions plays an important role during LbL coating.

Characterization and Performance of LbL-Coated Multibore Membranes: Zeta Potential, MWCO, Permeability and Sulfate Rejection

The characterization of membranes is suitable to investigate changes in the membrane properties caused by Layer-by-Layer (LbL) modification. Besides permeability, rejection, and molecular-weight cut-off (MWCO), which give information about the modification of the separation behaviour of the membrane, the zeta potential is capable of describing the surface charge of the membrane and its variation impacted by the properties of the polyelectrolyte multilayers (PEM). In this study, a new method for zeta potential measurement of hollow fibre membranes with several capillaries was developed and further investigations on the LbL modification of such membranes were performed. The results showed that an LbL coating with 8 DL PDADMAC/PSS led to a significant increase in the membrane charge of more than 20 mV. The coating with a different number of polyelectrolyte (PE) layers showed a zig-zag behaviour, comparable to data from flat sheet studies. However, in contrast to most flat sheet membranes, the charge curve assumes a totally negative trajectory at neutral pH. Further experiments on the MWCO of the LbL-modified membrane showed a reduction in the pore diameter from approx. 20 nm to less than 2 nm, reaching the range of nanofiltration membranes. With information on both the zeta potential and the MWCO, it was found that the rejection mechanism in LbL-modified multibore membranes is a complex interplay between the sieving effect due to reduction in the pore diameter and the repulsion effect of the charged membrane.

Polyelectrolyte Complex Membranes via Salinity Change Induced Aqueous Phase Separation

Polymeric membranes are used on very large scales for drinking water production and kidney dialysis, but they are nearly always prepared by using large quantities of unsustainable and toxic aprotic solvents. In this study, a water-based, sustainable, and simple way of making polymeric membranes is presented without the need for harmful solvents or extreme pH conditions. Membranes were prepared from water-insoluble polyelectrolyte complexes (PECs) via aqueous phase separation (APS). Strong polyelectrolytes (PEs), poly(sodium 4-styrenesulfonate) (PSS), and poly(diallyldimethylammonium chloride) (PDADMAC) were mixed in the presence of excess of salt, thereby preventing complexation. Immersing a thin film of this mixture into a low-salinity bath induces complexation and consequently the precipitation of a solid PEC-based membrane. This approach leads to asymmetric nanofiltration membranes, with thin dense top layers and porous, macrovoid-free support layers. While the PSS molecular weight and the total polymer concentrations of the casting mixture did not significantly affect the membrane structure, they did affect the film formation process, the resulting mechanical stability of the films, and the membrane separation properties. The salt concentration of the coagulation bath has a large effect on membrane structure and allows for control over the thickness of the separation layer. The nanofiltration membranes prepared by APS have a low molecular weight cutoff (<300 Da), a high MgSO4 retention (∼80%), and good stability even at high pressures (10 bar). PE complexation induced APS is a simple and sustainable way to prepare membranes where membrane structure and performance can be tuned with molecular weight, polymer concentration, and ionic strength.

Assessment of Layer-By-Layer Modified Nanofiltration Membrane Stability in Phosphoric Acid

Nanofiltration (NF) can enable P recovery from waste streams via retaining multivalent impurities from spent pickling acid. However, with the currently available membranes, an economically feasible process is impossible. Layer-by-layer modified NF membranes are a promising solution for the recovery of P from acidic leachate. LbL membranes show a high level of versatility in terms of fine tuning for ion retention, which is necessary to achieve sufficient phosphorus yields. However, the stability of layer-by-layer modified membranes during phosphoric acid (H₃PO₄) filtration needs to be further investigated. In our study, we show that a polyethersulfone hollow fiber membrane modified with four or eight bi-layers was stable during immersing and filtering of a 15% H₃PO₄ solution. A sulfonated polyethersulfone (sPES)-based hollow fiber LbL membrane was only stable during filtration. Thus, we show the importance of applying real process conditions to evaluate membranes. Another important aspect is the influence of the high ionic strength of the feed solution on the membrane. We show that a high ionic strength led to a decrease in Mg retention, which could be increased to 85% by adjusting the process parameters.

Characterization of MK-40 Membrane Modified by Layers of Cation Exchange and Anion Exchange Polyelectrolytes

Coating of ion exchange membranes used in electrodialysis with layers of polyelectrolytes is a proven approach that allows for the increasing of the limiting current, the suppressing of sedimentation, the controlling of the intensity of generation of H⁺ and OH⁻ ions, and also the improving of monovalent selectivity. However, in the case when two materials with the opposite sign of the charge of fixed groups come in contact, a bipolar boundary is created that can cause undesirable changes in the membrane properties. In this work, we used a MK-40 heterogeneous membrane on the surface of which a layer of polyethyleneimine was applied by adsorption from a solution as a model of heterogeneous membranes modified with oppositely charged polyelectrolyte. It was found that, on one hand, the properties of modified membrane were beneficial for electrodialysis, its limiting current did not decrease and the membrane even acquired a barrier to non-selective electrolyte transport. At the same time, the generation of H⁺ and OH⁻ ions of low intensity arose, even in underlimiting current modes. It was also shown that despite the presence of a layer of polyethyleneimine, the surface charge of the modified membrane remained negative, which we associate with low protonation of polyethyleneimine at neutral pH.

Synthesis of cross-linked poly(acrylic acid) nanogels in an aqueous environment using precipitation polymerization: unusually high volume change

For the first time, by using precipitation polymerization in an aqueous solution, a cross-linked poly(acrylic acid)-(pAA) nanogel was synthesized. pAA was synthesized and cross-linked with N,N'-methylenebisacrylamide (BIS) at 70°C in an acidified environment (pH 2) and containing 0.7 M NaCl using potassium persulfate as the initiator. Ionized pAA was soluble in water. The use of sodium chloride at low pH caused a decrease in the solubility of pAA and led to its precipitation and formation of cross-linked pAA nanogel. By using electron microscopies and light scattering techniques, the morphology, pH sensitivity and zeta potential of the obtained p(AA-BIS) nanogel were evaluated. The polymerization in an aqueous environment resulted in a very big swelling/shrinking coefficient (of approx. 4000) in response to pH and exhibited an unusually high negative zeta potential (of approx. -130 mV). These properties make the nanogel a very interesting sorbent and a construction material.

Nafion-Based Low-Hydration Polyelectrolyte Multilayer Membranes for Enhanced Water Purification

The increase of micropollutant concentration in both surface and groundwater is an emerging concern for the environment and human health. Most of such small organic molecules (medicines, hormones, and plasticizers) enter the environment via our wastewater, because they are not sufficiently removed by the current techniques applied in wastewater treatment plants. A possible solution to remove micropollutants is the usage of polyelectrolyte multilayer (PEM) based membranes. PEM membranes have received a growing interest in the past decade due to their high chemical and physical stability and their high permeability and selectivity. A popular polyelectrolyte pair to make dense PEM membranes with high salt retentions is the combination of poly(allylamine hydrochloride) (PAH) and poly(sodium 4-styrenesulfonate) (PSS). Unfortunately, smaller micropollutants (such as bisphenol A, sulfamethoxazole, naproxen, and bezafibrate) still show significant permeation through this membrane. In this study, for the first time, a single final layer of Nafion is applied on the PEM to increase the density of the PEM membrane. It is shown that when terminating with Nafion, the swelling of the multilayer decreases by 50%. These pronounced changes in layer structure are reflected by changes in membrane performance, such as a lower molecular weight cutoff (MWCO) and an increasing hydraulic membrane resistance. Furthermore, we show that the Nafion content of the multilayer can be increased by constructing a Nafion/PAH multilayer on top of the existing PSS/PAH multilayer, thereby lowering the MWCO. Although hydraulic resistance increases, these PSS/PAH/Nafion-based multilayers show excellent performance in rejecting difficult-to-remove micropollutants that have low molecular weight (200-650 Da) and different charges. Overall, a cocktail of eight small micropollutants can be removed up to 97% by these membranes, allowing strongly enhanced water purification.

Viscoelastic Properties of Polyelectrolyte Multilayers Swollen with Ionic Liquid Solutions

Polyelectrolyte multilayers (PEM) obtained by layer-by-layer assembly can be doped with ionic liquid (IL) via the swelling of the films with IL solutions. In order to examine the mechanical properties of IL-containing PEM, we implement a Kelvin-Voigt model to obtain thickness, viscosity and elastic modulus from the frequency and dissipation shifts determined by a dissipative quartz crystal microbalance (QCM-D). We analyze the changes in the modeled thickness and viscoelasticity of PEI(PSS/PADMAC)4PSS and PEI(PSS/PAH)4PSS multilayers upon swelling by increasing the concentration of either 1-Ethyl-3-methylimidazolium chloride or 1-Hexyl-3-methylimidazolium chloride, which are water soluble ILs. The results show that the thickness of the multilayers changes monotonically up to a certain IL concentration, whereas the viscosity and elasticity change in a non-monotonic fashion with an increasing IL concentration. The changes in the modeled parameters can be divided into three concentration regimes of IL, a behavior specific to ILs (organic salts), which does not occur with swelling by simple inorganic salts such as NaCl. The existence of the regimes is attributed to a competition of the hydrophobic interactions of large hydrophobic ions, which enhance the layer stability at a low salt content, with the electrostatic screening, which dominates at a higher salt content and causes a film softening.

Dynamic Self-Assembly of Polyelectrolyte Composite Nanomaterial Film

The aim of this study is not only to investigate the feasibility of using PAH (polyallylamine hydrochloride) and PSS (poly styrene-4-sulfonic acid sodium salt) to prepare a film via a layer by layer self-assembly process entrained with silver nanoparticles, but also to show that the silver nanoparticles crystalline structure can be defined and deposited on the surface of the substrate in the desired alignment structure and manner, which is of great help to research on the LBL method in the cellulose field. The effect of outermost layer variation, assembly layers, and composition of multilayers on the formation of the LBL structure on a nanofibrillated cellulose (NFC)/polyvinyl alcohol (PVA) substrate was investigated. The deposition of PAH and PSS was monitored by Fourier-transform infrared spectroscopy (FT-IR). The morphology of the LBL film layers was observed by scanning electron microscope (SEM) and atomic force microscope (AFM). Furthermore, thermal degradation properties were investigated by thermogravimetric analysis (TGA), and physical properties of multilayer films were tested by a universal mechanical tester. The results reveal that PAH and PSS can be readily deposited on a NFC/PVA substrate by using LBL methodology to prepare self-assembled polyelectrolyte multilayer films. The surface morphology of the LBL composite changed from negative to positive charged depending on the final LBL treatment. Also, according to SEM and AFM analysis, silver nanoparticles were well dispersed in the (PAH/PSS) film, which significantly improved the thermal stability of the composite films.

Intrinsic Properties of Polyelectrolyte Multilayer Membranes: Erasing the Memory of the Interface

Polyelectrolyte multilayers (PEMUs) are ultrathin membranes made by alternating adsorption of oppositely charged polyelectrolytes on substrates. Although PEMUs have shown exceptional selectivity for certain ion-filtering applications, they usually contain an excess of one of the polyelectrolytes due to the history- and condition-dependent mode of PEMU assembly. This excess charge provides fixed sites for ion exchange, enhancing the concentration of oppositely charged ions. Thus, the ion-permselective properties of PEMUs cannot be compared unless they are assembled under identical conditions. This work demonstrates the enhanced permeability of PEMUs as-made from poly(diallyldimethylammonium) (PDADMA), and poly(styrene sulfonate) (PSS) to ferricyanide as an example of an anion. Annealing by NaCl followed by pairing of excess PDADMA with additional PSS produces an almost stoichiometric film that better reflects the intrinsic transport properties of PEMUs. This pairing, observed in real time using electrochemical methods, occurs at the PEMU/solution interface under countercurrent transport of PSS from solution and excess PDADMA paired with a counterion, termed PDADMA*, from the PEMU bulk. A quantitative comparison of PSS and PDADMA* diffusion reveals the conditions under which PEMU assembly depends on PSS molecular weight and concentration.

Recent Advances in the Fabrication of Membranes Containing "Ion Pairs" for Nanofiltration Processes

In the face of serious environmental pollution and water scarcity problems, the membrane separation technique, especially high efficiency, low energy consumption, and environmental friendly nanofiltration, has been quickly developed. Separation membranes with high permeability, good selectivity, and strong antifouling properties are critical for water treatment and green chemical processing. In recent years, researchers have paid more and more attention to the development of high performance nanofiltration membranes containing “ion pairs”. In this review, the effects of “ion pairs” characteristics, such as the super-hydrophilicity, controllable charge character, and antifouling property, on nanofiltration performances are discussed. A systematic survey was carried out on the various approaches and multiple regulation factors in the fabrication of polyelectrolyte complex membranes, zwitterionic membranes, and charged mosaic membranes, respectively. The mass transport behavior and antifouling mechanism of the membranes with “ion pairs” are also discussed. Finally, we present a brief perspective on the future development of advanced nanofiltration membranes with “ion pairs”.

Macroion adsorption-electrokinetic and optical methods

Recent studies on macroion adsorption at solid/liquid interfaces evaluated by electrokinetic and optical methods are reviewed. In the first section a description of electrokinetic phenomena at a solid surface is briefly outlined. Various methods for determining both static and dynamic properties of the electrical double layer, such as the appropriate location of the slip plane, are presented. Theoretical approaches are discussed concerning quantitative interpretation of streaming potential/current measurements of homogeneous macroscopic interfaces. Experimental results are presented, involving electrokinetic characteristics of bare surfaces, such as mica, silicon, glass etc. obtained from various types of electrokinetic cells. The surface conductivity effect on zeta potential is underlined. In the next section, various theoretical approaches, proposed to determine a distribution of electrostatic potential and flow distribution within macroion layers, are presented. Accordingly, the influence of the uniform as well as non-uniform distribution of charges within macroion layer, the dissociation degree, and the surface conductance on electrokinetic parameters are discussed. The principles, the advantages and limits of optical techniques as well as AFM are briefly outlined in Section 4. The last section is devoted to the discussion of experimental data obtained by streaming potential/current measurements and optical methods, such as reflectometry, ellipsometry, surface plasmon resonance (SPR), optical waveguide lightmode spectroscopy (OWLS), colloid enhancement, and fluorescence technique, for mono- and multilayers of macroions. Results of polycations (PEI, PAMAM dendrimers, PAH, PDADMAC) and polyanions (PAA, PSS) adsorption on mica, silicon, gold, and PTFE are quantitatively interpreted in terms of theoretical approaches postulating the three dimensional charge distribution or the random sequential adsorption model (RSA). Macroion bilayer formation, experimentally examined by streaming current measurements, and theoretically interpreted in terms of the comprehensive formalism is also reviewed. The utility of electrokinetic measurements, combined with optical methods, for a precise, in situ characteristics of macroion mono- and multilayer formation at solid/liquid interfaces is pointed out.

Quantifying and controlling the cation uptake upon hydrated ionic liquid-induced swelling of polyelectrolyte multilayers

Controlling the uptake of specific ions in polyelectrolyte multilayers is of interest for various fields of application. Here, we quantify the amount of cation of an ionic liquid, namely 1,3-bis(cyanomethyl)imidazolium chloride, incorporated into polyelectrolyte multilayers upon contact with an ionic liquid solution. The ion partition equilibrium is determined depending on concentration in solution, employing attenuated total reflection infrared spectroscopy. Generating an excess charge in multilayers by post-preparative manipulation of their charge balance, one can control the incorporated amount. Three multilayer systems are assembled for this purpose, i.e., PSS/PDADMAC, PSS/PAH and PAA/PDADMAC, employing poly(styrene sulfonate) (PSS), poly(diallyldimethylammonium chloride) (PDADMAC), poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). The charge balance of the latter two films is manipulated by an external pH stimulus generating an excess positive or negative internal charge, respectively. The concentration of cations in PEM amounts to 30% to 100% of the bulk concentration and scales as PAA/PDADMAC > PSS/PDADMAC > PSS/PAH. Thus, post-preparative pH treatment may be a future tool to create ion-conductive polymer gel films with a desired concentration of small cations.

Layer-by-layer deposition on a heterogeneous surface: Effect of sorption kinetics on the growth of polyelectrolyte multilayers

Surface functionalization by means of controlled deposition of charged polymers or nanoparticles using the layer-by-layer (LbL) approach has been used to modify mostly engineered materials with well-defined surface chemistry and morphology. In this regard, natural and inhomogeneous interfaces have gained very little attention. Furthermore, natural substrates are susceptible to alterations by factors commonly used to control the growth of multilayers, such as pH, temperature and ionic strength. Here, we study the impact of sorption kinetics of a bilayer system (Poly(diallyldimethylammonium chloride) (PDDA) and Poly(sodium 4-styrenesulfonate) (PSS)) on a natural heterogeneous wood surface at neutral pH, without salt addition, on the multilayer buildup. To overcome analytical limitations we introduce a complementary approach based on UV reflectance spectroscopy, atomic force microscopy (AFM) and zeta potential measurements. Compared to immersion times used for ideal substrates, we found that a high surface coverage requires relatively long immersion, approximately 30min, into polyelectrolyte solutions, while a sufficient removal of polyelectrolyte excess during the washing step, requires even longer, about 100min. Based on these findings, we show that film growth can be controlled kinetically. Long immersion times provide well-defined and regular multilayers. The obtained data points to specific requirements to be considered when LbL treatments are applied to rough, porous and heterogeneous surfaces, and thereby sets a basis for a successful transfer of various surface functionalization approaches already shown on ideal surfaces.

Surface modification of PEN and PET substrates by plasma treatment and layer-by-layer assembly of polyelectrolyte multilayer thin films and their application in electroless deposition

Surface modification of PEN and PET substrates by plasma-treatment and LbL assembly of polyelectrolyte multilayers and subsequent electroless nickel deposition.

Zeta potential of polyelectrolyte multilayers using the spinning disk method

Zeta potentials of surfaces bearing stable mono- or multilayers of polyelectrolyte were determined using the spinning disk method recently described by Sides et al. (Langmuir 2004, 20, 11493-11498). In this technique, the streaming potential difference between two electrodes, one at the disk surface, is quantitatively related to zeta potential. Variables such as rotation speed, electrolyte concentration, and electrode distance from the disk surface were explored and used to validate the recently-described theory, which emphasizes minimal contribution to net potential from surface conductivity. Layer-by-layer oscillations in sign and magnitude of the zeta potential were observed, in accord with prior work using electrophoretic mobility of multilayer-coated particles and other streaming potential measurements. The open geometry and the excellent mass transport of the spinning disk allowed in-situ observation of surface charge switching during the addition of a layer. As with all zeta potentials, especially those recorded at soft interfaces, translating results to quantitative densities of fixed surface charge is a challenge.

Building polyzwitterion-based multilayers for responsive membranes

We systematically investigate the assembly of multilayers based on a polyzwitterion (PSBMA) and a polycation (PDADMAC) for the development of ionic strength responsive membranes. Although the polyzwitterion is essentially charge neutral, we show that specific electrostatic interactions with the PDADMAC allow for the formation of stable multilayers. The growth of this LbL system is monitored on model surfaces (silica) via optical reflectometry for different pH values and ionic strengths. While no effect of pH on the layer growth is observed, we did observe a strong dependence on the ionic strength. Upon increasing the ionic strength during deposition from 0.005 to 0.5 M NaCl, the adsorbed amount is significantly decreased, a behavior that is opposite to classical LbL systems. Similar results to those obtained on silica are also observed on top of classical LbL systems and on polymeric membranes. This demonstrates that the growth of the polyzwitterion multilayers is independent of the substrate. Coating these polyzwitterion multilayers on hollow fiber membranes via dip-coating yields membranes that are stimuli responsive toward the ionic strength of the filtration solution, with an increase in permeability of up to 108% from 0 to 1.5 M NaCl. We show that the fabrication of the polyzwitterion multilayers is an easy and controlled way to provide surfaces, such as membranes, with the specific functionalities of polyzwitterions.

Adhesion property profiles of supported thin polymer films

Polymer coatings are frequently utilized to control and modify substrate properties. The performance of the coatings is often determined by the first polymer layers between the substrate and the bulk polymer material, which are termed interphase. Standard methods have failed to completely characterize this interphase, because its properties change significantly over a few nanometers. Here we determine the spatially resolved adhesion properties of the interphase in polyelectrolyte multilayers (PEMs) by desorbing a single polymer covalently bound to an atomic force microscope cantilever tip from PEMs with varying thickness. We show that the adhesion properties of the first few layers (up to three double layers) is dominated by the surface potential of the substrate, while thicker PEMs are controlled by cohesion in between the PEM polymers. For cohesion, the local film conformation is the crucial parameter. This finding is generalized by utilizing oligoelectrolyte multilayer (OEM) as coatings and both hydrophilic and hydrophobic polymers as polymeric force sensors.

Functional polyelectrolyte multilayer membranes for water purification applications

A diverse set of supported multilayer assemblies with controllable surface charge, hydrophilicity, and permeability to water and solute was fabricated by pressure driven permeation of poly(sodium 4-styrenesulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDDA) solution through poly(ethylene terephthalate) (PET) track-etched membranes. The polyelectrolyte multilayer fabrication was confirmed by means of FTIR, SEM, AFM, ellipsometry, zetapotential, and contact angle characterization. The prepared membranes were characterized in terms of their pure water permeability, flux recovery, and resistance to organic and biofouling properties. The antifouling behavior of the membranes was assessed in terms of protein adsorption and antibacterial behavior. Finally, the membranes were tested for rejection of selected water soluble dyes to establish their usefulness for organic contaminant removal from water. The membranes were highly selective and capable of nearly complete rejection of congo red with sufficiently high fluxes. The feasibility of regenerating the prepared membranes fouled by protein was also demonstrated and good flux recovery was obtained. In summary, the multilayer approach to surface and pore modification was shown to enable the design of membranes with the unique combination of desirable separation characteristics, regenerability of the separation layer, and antifouling behavior.

Fundamentals of selective ion transport through multilayer polyelectrolyte membranes

Membranes composed of multilayer poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films on porous alumina supports exhibit high monovalent/divalent cation selectivities. Remarkably, the diffusion dialysis K(+)/Mg(2+) selectivity is >350. However, in nanofiltration this selectivity is only 16, suggesting some convective ion transport through film imperfections. Under MgCl(2) concentration gradients across either (PSS/PAH)(4)- or (PSS/PAH)(4)PSS-coated alumina, transmembrane potentials indicate Mg(2+) transference numbers approaching 0. The low Mg(2+) transference numbers with both polycation- and polyanion-terminated films likely stem from exclusion of Mg(2+) due to its large size or hydration energy. However, these high anion/cation selectivities decrease as the solution ionic strength increases. In nanofiltration, the high asymmetry of membrane permeabilities to Mg(2+) and Cl(-) creates transmembrane diffusion potentials that lead to negative rejections (the ion concentration in the permeate is larger than in the feed) as low as -200% for trace monovalent cations such as K(+) and Cs(+). Moreover, rejection becomes more negative as the mobility of the trace cation increases. Knowledge of single-ion permeabilities is vital for predicting the performance of polyelectrolyte films in the separation and purification of mixed salts.

Homogeneity, modulus, and viscoelasticity of polyelectrolyte multilayers by nanoindentation: refining the buildup mechanism

Atomic force microscopy, AFM, and nanoindentation of polyelectrolyte multilayers, PEMUs, made from poly(diallyldimethylammonium), PDADMA, and poly(styrene sulfonate), PSS, provided new insight into their surface morphology and growth mechanism. A strong odd/even alternation of surface modulus revealed greater extrinsic (counterion-balanced) charge compensation for fully hydrated multilayers ending in the polycation, PDADMA. These swings in modulus indicate a much more asymmetric layer-by-layer growth mechanism than previously proposed. Viscoelastic properties of the PEMU, which may contribute to cell response, were highlighted by variable indentation rates and minimized by extrapolating to zero indentation rate, at which point the surface and bulk equilibrium moduli were comparable. Variations in surface composition were probed at high resolution using force mapping, and the surface was found to be uniform, with no evidence of phase separation. AFM comparison of wet and dry films terminated with PSS and PDADMA revealed much greater swelling of the PDADMA-terminated PEMU by water, with collapse of surface roughness features in dry conditions. Dynamic and static contact angle measurements suggested less rearrangement for the glassy PSS surface.

Reactive layer-by-layer deposition of poly(ethylene imine) and a precursor of TiO2: influence of the sodium chloride concentration on the film growth, interaction with hexacyanoferrate anions, and particle distribution in the film

Films prepared according to a layer-by-layer (LBL) manner find increasing importance in many applications such as coatings with dedicated optical or electronic properties, particularly when including nanomaterials. An alternative way to prepare such hybrid layer-by-layer coatings is to perform sol-gel chemistry in a layer-by-layer manner. In this article, we highlight the importance of the NaCl concentration as a parameter to control the growth as well as the properties of LBL films made from poly(ethylene imine) as the organic counterpart and titanium IV (bisammoniumlactato)dihydroxyde ([Ti(lac)(2)(OH)(2)](2-)) as the precursor of TiO(2). An increase in the sodium chloride concentration leads to the faster growth of the film and to a decrease in the number of hexacyanoferrate anions remaining in the film after a buffer rinse. This may be due to a progressive increase in the fraction of negatively charged TiO(2) as suggested by transmission electron microscopy. In the presence of 0.5 M NaCl, the fraction of TiO(2) is close to 60% in mass. As a surprising finding, the films produced from 0.15 M NaCl are not homogeneously filled with TiO(2) even if the film is produced in an LBL fashion. The increased concentration of TiO(2) at the film-solution interface could constitute a barrier for the incorporation of the negatively charged redox probe.

Dynamic aspects of films prepared by a sequential deposition of species: perspectives for smart and responsive materials

The deposition of surface coatings using a step-by-step approach from mutually interacting species allows the fabrication of so called "multilayered films". These coatings are very versatile and easy to produce in environmentally friendly conditions, mostly from aqueous solution. They find more and more applications in many hot topic areas, such as in biomaterials and nanoelectronics but also in stimuli-responsive films. We aim to review the most recent developments in such stimuli-responsive coatings based on layer-by-layer (LBL) depositions in relationship to the properties of these coatings. The most investigated stimuli are based on changes in ionic strength, temperature, exposure to light, and mechanical forces. The possibility to induce a transition from linear to exponential growth in thickness and to change the charge compensation from "intrinsic" to "extrinsic" by controlling parameters such as temperature, pH, and ionic strength are the ways to confer their responsiveness to the films. Chemical post-modifications also allow to significantly modify the film properties.