Polyelectrolyte-coated Surfaces Research Articles

Adsorption of Polyions on Flat TiO2 Surface

In this study, the surface properties of Ti/TiO2 substrate before and after the adsorption of polyelectrolytes were investigated. As model polyelectrolytes, strongly charged polycation poly(diallyldimethylammonium) (PDADMA) and strongly charged polyanion poly(4-styrenesulfonate) (PSS) were used. Initially, the bare titanium substrate was characterized by means of ellipsometry, atomic force microscopy (AFM), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and measurements of inner surface potential using crystal electrode (CrE). It was shown that the substrate surface is very smooth with the roughness of 3.5 nm and oxide layer thickness of 3.8 nm. After the adsorption of PDADMA and PSS, polyelectrolyte-coated titanium surface was examined using the above-mentioned methods under the same conditions. It was found that both PDADMA cations and PSS anions form a stable polymeric nanofilm on Ti/TiO2 surface that partially covers the surface, without significant impact on the surface roughness. The corrosion protection effectiveness values indicate that the corrosion properties were greatly enhanced upon polyion adsorption and polyelectrolyte coating formation on the flat TiO2 surface. The obtained results were additionally confirmed by inner surface potential measurements. According to the methods employed, PDADMA nanofilm modification offers enhanced corrosion protection to the underlying titanium material in sodium chloride electrolyte solution.

Oil droplet behavior on model nanofiltration membrane surfaces under conditions of hydrodynamic shear and salinity

HypothesesOil droplet stability and electrical charge, and membrane’s affinity for oil govern droplet attachment to a membrane surface. Moderate droplet-surface affinity encourages surface coalescence and removal of droplets to help maintain the membrane relatively oil-free. ExperimentsDroplet attachment onto model nanofiltration membranes was studied, in situ and in real time, using the Direct Observation Through the Membrane method. Optically transparent nanofiltration membranes were designed by forming polyelectrolyte multilayer films, with either positively or negatively charged surfaces, on Anopore ultrafilters. Crossflow across the membrane surface employed hexadecane-in-water emulsions stabilized by an anionic surfactant (sodium dodecylsulfate) in model sea water or aqueous solutions containing NaCl or MgSO4. FindingsModerate affinity between oil and the polyelectrolyte-coated surface promotes crossflow controlled coalescence to remove droplets larger than a critical size, ddropcrit, in the crossflow shear. The torque balance on a sessile oil droplet in a linear shear field overpredicted ddropcrit pointing to a need for more accurate estimates of lift and drag forces on a droplet. In the presence of divalent cations, lower electrostatic repulsion between droplets facilitated droplet-droplet adhesion and led to rapid coalescence that resulted in membrane fouling. The most significant fouling appeared in tests with positively charged and less oleophobic coatings.

Virus deposition onto polyelectrolyte-coated surfaces: A study with bacteriophage MS2

HypothesesBy selecting constituent polyelectrolytes and controlling conditions of their deposition, the resulting polyelectrolyte multilayers can be designed as surface coatings with controlled adhesive properties with respect to viruses. Charge and hydrophilicity of the polyelectrolyte multilayers govern virus adhesion. ExperimentsFour surfaces of different charges and hydrophobicities were designed using a layer-by-layer assembly of poly(styrene-4-sulfonate) and poly(dimethyl diallyl ammonium chloride). Contact angle measurements gave an estimate of MS2 hydrophilicity in terms of free energy of interfacial interaction in water. Experimental results on MS2 adhesion obtained using quartz crystal microbalance with dissipation monitoring were compared with predictions by the extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory. FindingsMS2 deposition onto polyelectrolyte multilayers occurred in two phases: an early phase defined by virus-surface interactions and a later phase with virus-virus interactions controlling deposition kinetics. Principal component analysis showed that the deposition rates in the two phases were independent one of another and that each was correlated to the depth of the secondary minimum of the corresponding XDLVO energy profile. Hydrophobic and electrostatic interactions governed the deposition process: short range hydrophilic repulsion prevented deposition into the primary minimum while electrostatic interactions defined the dependence of the deposition kinetics on the ionic strength. Different surfaces showed distinct kinetics of and capacities for MS2 deposition pointing to the potential of polyelectrolyte multilayers as easy-to-apply coatings for regulating virus adsorption, inactivating viruses via the virucidal action of cationic polyelectrolytes and reducing human exposure to viruses.

Electrowetting of Weak Polyelectrolyte-Coated Surfaces.

Polymer coatings are commonly used to modify interfacial properties like wettability, lubrication, or biocompatibility. These properties are determined by the conformation of polymer molecules at the interface. Polyelectrolytes are convenient elementary bricks to build smart materials, given that polyion chain conformation is very sensitive to different environmental variables. Here we discuss the effect of an applied electric field on the properties of surfaces coated with poly(acrylic acid) brushes. By combining atomic force microscopy, quartz crystal microbalance, and contact angle experiments, we show that it is possible to precisely tune polyion chain conformation, surface adhesion, and surface wettability using very low applied voltages if the polymer grafting density and environmental conditions (pH and ionic strength) are properly formulated. Our results indicate that the effective ionization degree of the grafted weak polyacid can be finely controlled with the externally applied field, with important consequences for the macroscopic surface properties.

Electro-responsive polyelectrolyte-coated surfaces.

The anchoring of polymer chains at solid surfaces is an efficient way to modify interfacial properties like the stability and rheology of colloidal dispersions, lubrication and biocompatibility. Polyelectrolytes are good candidates for the building of smart materials, as the polyion chain conformation can often be tuned by manipulation of different physico-chemical variables. However, achieving efficient and reversible control of this process represents an important technological challenge. In this regard, the application of an external electrical stimulus on polyelectrolytes seems to be a convenient control strategy, for several reasons. First, it is relatively easy to apply an electric field to the material with adequate spatiotemporal control. In addition, in contrast to chemically induced changes, the molecular response to a changing electric field occurs relatively quickly. If the system is properly designed, this response can then be used to control the magnitude of surface properties. In this work we discuss the effect of an external electric field on the adhesion and lubrication properties of several polyelectrolyte-coated surfaces. The influence of the applied field is investigated at different pH and salt conditions, as the polyelectrolyte conformation is sensitive to these variables. We show that it is possible to fine tune friction and adhesion using relatively low applied fields.

Comment on “Physicochemical controls on adsorbed water film thickness in unsaturated geological media” by Tetsu K. Tokunaga

Comment on “Physicochemical controls on adsorbed water film thickness in unsaturated geological media” by Tetsu K. Tokunaga

Lubrication by organized soft matter

The AFM-colloidal probe technique has been used to explore surface interactions and friction forces between polyelectrolyte-coated surfaces immersed in aqueous solutions in the absence and presence ...

Voltammetric investigation of surface-confined proteins

Development of sensitive and selective methods to detect proteins at trace levels is of great biological importance. Via derivatization with a bifunctional cross-linker 4-maleimidobutyric acid N-hydroxysuccinimide ester (GMBS) and an electrochemical marker 11-ferrocenyl-1-undecanethiol (Fc-SH), voltammetric determination of surface-confined proteins electrostatically adsorbed onto the polyelectrolyte of poly (sodium 4-styrenesulfonate) (PSS) or poly(allylamine hydrochloride) (PAH)-covered surfaces could be realized. The utilization of PSS or PAH was anticipated to reduce the nonspecific adsorption of the proteins on the surface. Two kinds of proteins with no redox activity or exhibiting complex or ill-defined voltammetric peak(s), i.e. the positively charged lysozyme and negatively charged metallothionein (MT) were demonstrated. Due to the incorporation of the bifunctional reagent GMBS and the redox active Fc groups onto the protein-modified electrodes, well-defined voltammetric peaks of high signal intensity were obtained. The anodic peak heights were found to be dependent on the surface density of the proteins electrostatically binded to the polyelectrolyte-coated surface. The present method can measure lysozyme concentration as low as 0.1 nM.

Patterning of proteins and cells on functionalized surfaces prepared by polyelectrolyte multilayers and micromolding in capillaries

A method for protein and cell patterning on polyelectrolyte-coated surfaces using simple micromolding in capillaries (MIMIC) is described. MIMIC produced two distinctive regions. One contained polyethylene glycol (PEG) microstructures fabricated using photopolymerization that provided physical, chemical, and biological barriers to the nonspecific binding of proteins, bacteria, and fibroblast cells. The second region was the polyelectrolyte (PEL) coated surface that promoted protein and cell immobilization.The difference in surface functionality between the PEL region and background PEG microstructures resulted in simple patterning of biomolecules. Fluorescein isothiocyanate-tagged bovine serum albumin, E. coli expressing green fluorescence protein (GFP), and fibroblast cells were successfully bound to the exposed PEL surfaces at micron scale. Compared with the simple adsorption of protein, fluorescence intensity was dramatically improved (by about six-fold) on the PEL-modified surfaces. Although animal cell patterning is prerequisite for adhesive protein layer to survive on desired area, the PEL surface without adhesive proteins provides affordable microenvironment for cells.The simple preparation of functionalized surface but universal platform can be applied to various biomolecules such as proteins, bacteria, and cells.

Decomposing Bridging Adhesion between Polyelectrolyte Layers into Single Molecule Contributions

A novel approach to analyze the force response of multiple polymer strands, which are bridged between two surfaces, is proposed. The response of single polymer strands is experimentally accessible by measuring the force upon separation of two polymer-coated surfaces with the atomic force microscope. Our approach is based on the decomposition of the stretching and desorption sequence into contributions of independently bridged chains and of the elimination of loops formed on the opposite surface during contact. This approach was applied to investigate the bridging adhesion of surfaces coated with poly(vinylamine) (PVA). The force response of single PVA molecules was described on the basis of a recently proposed model, which accounts for the discrete chain character of the polymer at higher extension forces. As exemplary results, we determined the length distributions of the individual chains and the loop number distribution of these bridging chains on the polyelectrolyte-coated surfaces. The former were compared with scaling theories of polymer adsorption.

Polyelectrolytes as adhesion modifiers

Adsorbed layers of polyelectrolytes have been studied with atomic force microscopy (AFM) and the interferometric surface force apparatus (SFA). Particular emphasis was put on determining the effect of the polyelectrolyte charge density on surface topography, and the effect of the polyelectrolyte coating on the adhesive properties. The AFM was employed to image individual polymer chains at low adsorption densities and to characterize the layer topography and coverage at higher adsorption densities. The adhesive properties between two polyelectrolyte-coated surfaces in air were determined as a function of the number of contacts made at any given spot. The data provide evidence for formation of electrostatic bridges, particularly when highly charged polyelectrolytes are used. Further, material transport between the surfaces is observed when the polyelectrolyte is either highly charged or have a very low charge density. For intermediate charge densities we could not observe any indication of material transfer. The adhesion between one polyelectrolyte-coated surface and one bare surface was initially higher than that between two polyelectrolyte-coated surfaces. However, due to material transfer between the two surfaces the adhesion decreased significantly with the number of times that the surfaces were driven into contact. For the polyelectrolytes of the lowest charge density the results suggest that entanglement effects contribute to the adhesive interaction. The modification of the adhesion by polyelectrolytes in practical systems such as in the case of dry-strength additives to improve paper resistance is also considered.

Direct Measurements of Interactions between Hydrophobically Anchored Strongly Charged Polyelectrolyte Brushes

We investigated the nature and the range of interactions between negatively charged polyelectrolyte-coated surfaces as a function of salt concentration using the surface forces apparatus. The measured force profiles (interaction forces versus separation distance) are purely repulsive and show long-range electrostatic and short-range steric interactions. The measured range of interaction in salt-free as well as in low ionic strengths extends well beyond the contour length of the polyelectrolyte chain. It is shown that the interaction range between the ionic brush layers and the grafting density depend on the ionic strength of the solution. In salt-free solution, the counterions associated with polyelectrolyte cause the chains to stretch and give rise to long-range double-layer electrostatic repulsions between the opposing chains. When salt is added to the system, the electrostatic interactions are partially screened and the polymer chain regains its flexibility and therefore the range of interactions is reduced. The measured total range of interaction exhibits relatively weaker dependence on the salt concentration. We find that our force−distance profiles with added salt in a compressed regime can be very well described by the Pincus scaling model.

The Effect of Salt Concentration on Adsorption of Low-Charge-Density Polyelectrolytes and Interactions between Polyelectrolyte-Coated Surfaces

In this investigation surface force, X-ray photoelectron spectroscopy and ellipsometry techniques have been used to study the adsorption of a low-charge-density cationic polyelectrolyte on negatively charged surfaces. It is shown that the low cationicity of this polyelectrolyte induces an adsorption behavior which is limited by steric factors rather than by the substrate surface charge or potential. It is also established that an increase in ionic strength of the solution results in desorption of the polyelectrolyte accompanied by an increase in layer thickness. This phenomenon is typical of a screening-reduced adsorption regime where electrostatic interactions predominate in the adsorption process. An increase in layer thickness most often occurs as a result of an increased adsorbed amount. Here, however, the increase in layer thickness occurs despite a reduction in the adsorbed amount. This can be understood as resulting from a reduced polyelectrolyte–surface affinity and a swelling of the adsorbed layer. Finally, it is demonstrated that the employed techniques complement each other and reveal new information on the interaction forces and conformation of polyelectrolytes at the solid–liquid interface.

Surface properties of poly(ethylene imine)-coated mica surfaces—salt and pH effects

The adsorption of a branched weak polybase, poly(ethylene imine) (PEI), on the basal plane of muscovite mica was investigated using surface force measurements, electro-osmotic measurements, and ESCA. The use of this combination of techniques allows us to obtain information about adsorbed amount, interfacial charges, average structure of adsorbed layers and the most important types of forces acting between PEI-coated surfaces. The system was studied under two different conditions. The adsorbed layer adopts a flat conformation at low ionic strength and at low pH owing to the predominance of electrostatic forces. The interfacial properties of PEI-coated surfaces are compared with those of strong polyelectrolytes as well as with a linear weak polybase studied previously under similar conditions. At higher pH-values the adsorbed amount increases owing to a reduction in the polyelectrolyte charge density. This results in a thicker layer and in the increased importance of repulsive steric forces. Finally, the interaction between one polyelectrolyte-coated surface and a bare mica surface was investigated and compared with recent theoretical predictions.

Electro-osmosis on a Thermosensitive-Hydrogel Surface

The electro-osmotic velocity has been measured on a poly(N-isopropylacrylamide) hydrogel plate at various temperatures. The results are analyzed on the basis of a theory for the electro-osmosis on a "soft" surface (i.e., a polyelectrolyte-coated surface) to determine the charge density and the "softness" of the hydrogel layer. It is found that as the temperature is raised, the charge density increases and the softness of poly(N-isopropylacrylamide) hydrogel decreases. These changes are most remarkable around 33°C. This is probably because poly(N-isopropylacrylamide) hydrogel shrinks above its LCST (33°C) and swells below it. This behavior is in good agreement with our previous results obtained from electrophoretic mobility measurements for poly (N-isopropylacrylamide)-coated latex particles. The obtained values of the charge density in the hydrogel layer are compared with those that are estimated by potentiometric titration and the swelling ratio measurement of the hydrogel plate. Agreement is excellent, suggesting that the electro-osmosis measurements should be appropriate for analyzing the physicochemical properties of a soft surface.

Forces between polyelectrolyte-coated surfaces: relations between surface interaction and floc properties

The forces acting between negatively charged mica surfaces in the presence of some different cationic polyelectrolytes have been measured using a surface force apparatus. The different polyelectrolytes used have between 10% and 100% of the monomers charged. Particular emphasis has been given to the forces acting between the polyelectrolyte-coated surfaces when the ionic strength of the solution is low, i.e. when electrostatic forces predominate the interaction between the surface and the polyelectrolyte. Under these conditions the most highly charged polyelectrolytes adsorb in a very flat conformation and only a weak recharging is observed even at polyelectrolyte concentrations considerably above that needed to neutralise the mica surface charge. The surface force data are discussed in relation to the floc structures and properties obtained when latex particles are flocculated by polyelectrolytes. These studies are carried out by using a device that is based on three photometric dispersion analysers. The floc structures formed in the presence of various polyelectrolytes can to a large extent be rationalised by considering the interactions measured in the surface force apparatus provided that the difference in charge density of the mica surface and the latex surface is kept in mind.

Effects of ambient conditions on adsorbed surfactant and polymer monolayers

The physical properties of surfactant-coated and polyelectrolyte-coated surfaces in adhesive contact in air have been studied using the surface forces apparatus technique. Various physisorbed monolayers with different head groups and chains (or polymer segments) were prepared both by adsorption form solution (self-assembly) and by the Langmuir-Blodgett deposition technique. The results show that many monolayer properties depend on the atmospheric conditions such as the relative humidity or presence of organic vapors and that these properties can further change when two monolayer-coated surfaces are brought into contact. These properties include monolayer composition and structure, thickness and compressibility, fluidity and phase state (i.e., whether solid, gel, or liquid), and the adhesion between two monolayer-coated surfaces. In addition, we find that both out-of-plane and in-plane (lateral) phase transitions can be induced in certain adsorbed monolayers when they are subjected to a compressive stress. The results provide new insights into molecular ordering and dynamics in physisorbed monolayers and how monolayers are affected when they are exposed to vapors or when they interact with other surfaces. 18 refs., 13 figs., 4 tabs.