Adsorption of Polyelectrolytes at Oppositely Charged Surfaces

Investigating forces between charged particles in the presence of oppositely charged polyelectrolytes with the multi-particle colloidal probe technique

Direct force measurements are used to obtain a comprehensive picture of interaction forces acting between charged colloidal particles in the presence of oppositely charged polyelectrolytes. These measurements are achieved by the multi-particle colloidal probe technique based on the atomic force microscope (AFM). This novel extension of the classical colloidal probe technique offers three main advantages. First, the technique works in a colloidal suspension with a huge internal surface area of several square meters, which simplifies the precise dosing of the small amounts of the polyelectrolytes needed and makes this approach less sensitive to impurities. Second, the particles are attached in-situ within the fluid cell, which avoids the formation of nanobubbles on the latex particles used. Third, forces between two similar particles from the same batch are being measured, which allows an unambiguous determination of the surface potential due to the symmetry of the system. Based on such direct force measurements involving positively and negatively charged latex particles and different polyelectrolytes, we find the following forces to be relevant. Repulsive electrostatic double-layer forces and attractive van der Waals forces as described by the theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO) are both important in these systems, whereby the electrostatic forces dominate away from the isoelectric point (IEP), while at this point they vanish. Additional non-DLVO attractive forces are operational, and they have been identified to originate from the electrostatic interactions between the patch-charge heterogeneities of the adsorbed polyelectrolyte films. Highly charged polyelectrolytes induce strong patch-charge attractions, which become especially important at low ionic strengths and high molecular mass. More weakly charged polyelectrolytes seem to form more homogeneous films, whereby patch-charge attractions may become negligible. Individual bridging events could be only rarely identified from the retraction part of the force profiles, and therefore we conclude that bridging forces are unimportant in these systems.

Interactions of a cationic cellulose derivative with an ultrathin cellulose support

The adsorption behavior of cellulose-4-[N-methylammonium]butyrate chloride (CMABC) on two hydrophilic substrates is studied, namely nanometric cellulose model thin films and silicon dioxide substrates. The adsorption is quantified in dependence of electrolyte concentration and pH value using a quartz crystal microbalance with dissipation (QCM-D). In case of CMABC, at high ionic strengths (25-100 mM NaCl) high adsorption is observed at pH 7 (Δf(3): -15 to -17 Hz) while at lower ionic strengths (1-10 mM) less CMABC (Δf(3): -2 to -12 Hz) is deposited on the cellulose surfaces as indicated by the frequency changes using QCM-D. A change in pH value from 7 to 8 reveals an increase in adsorption. Atomic force microscopy shows that the coating of cellulose thin films with CMABC changes the morphology from a fibrillar to a particle like structure on the surface. The surface wettability with water increases with an increasing amount of CMABC on the surface compared to neat cellulose model films. At lower pH values (3 and 5), CMABC does not adsorb onto the cellulose model thin films. XPS is used to validate the results and to determine the nitrogen content of the surfaces. In addition, adsorption of CMABC onto another hydrophilic and negatively charged substrate, silicon dioxide coated quartz crystals, cannot be detected at different pH values and electrolyte concentrations as proven by QCM-D.

Double charge inversion in polyethylenimine-decorated liposomes

The study of the interaction of a cationic polymer as PEI with phospholipids membranes is of special relevance for gene therapy because the PEI is a potential nonviral vector to transfer DNA in living cells. We used light scattering, zeta potential, and electron transmission microscopy to characterize the interaction between DMPG and DOPC liposomes with PEI as a function of the charge molar ratio, pH, temperature, initial size of the liposomes, and headgroup of the lipids. Unexpectedly, a double charge inversion and two different ranges of PEI-liposome concentrations where an aggregation occurs were found, when the proper pH and initial size of the liposomes were chosen. The interaction is analyzed in terms of the interaction potential proposed by Velegol and Thwar for colloidal particles with a nonuniform surface charge distribution. Results show a remarkable dependence of the stability on pH and the initial size of the liposomes, which explains the low reproducibility of the experiments if no special care is taken in preparing the samples. Comparatively small changes in the pH or in the liposomes size lead to a completely different stability behavior.

Conformational transitions of flexible hydrophobic polyelectrolytes in solutions of monovalent and multivalent salts and their mixtures

Conformations of cationic polyelectrolytes (PEs), a weak poly(2-vinylpyridine) (P2VP) and a strong poly(N-methyl-2-vinylpyridinium iodide) (qP2VP), adsorbed on mica from saline solutions in the presence of counterions of different valences are studied using in situ atomic force microscopy (AFM). Quantitative characteristics of chain conformations are analyzed using AFM images of the adsorbed molecules. The results of the statistical analysis of the chain contour reveal collapse of the PE coils when ionic strength is in a range from tens to hundreds of millimoles per kilogram and re-expansion of the coils with a further increase of ionic strength up to a region of the saturated saline solutions. The competition between monovalent and multivalent counterions simultaneously present in solutions strongly affects conformations of PE chains even at a very small fraction of multivalent counterions. Shrinkage of PE coils is steeper for multivalent counterions than for monovalent counterions. However, the re-expansion is only incremental in the presence of multivalent counterions. Extended adsorbed coils at low salt concentrations and at very high concentrations of monovalent salt exhibit conformation corresponding to a 2D coil with 0.95 fraction of bound segments (segments in "trains") in the regime of diluted surface concentration of the PE. Shrunken coils in the intermediate range of ionic strength resemble 3D-globules with 0.8 fraction of trains. The incrementally re-expanded PE coils at a high ionic strength remain unchanged at higher multivalent salt concentrations up to the solubility limit of the salt. The formation of a strong PE complex with multivalent counterions at high ionic strength is not well understood yet. A speculative explanation of the observed experimental result is based on possible stabilization of the complex due to hydrophobic interactions of the backbone.

Structure of adsorbed polyelectrolyte monolayers investigated by combining optical reflectometry and piezoelectric techniques

Polyelectrolyte monolayers on solid substrates are studied with optical reflectivity and the quartz crystal microbalance (QCM). In particular, we investigate the adsorption of anionic poly(styrene sulfonate) (PSS) on amino-functionalized silica as well as cationic poly(allylamine hydrochloride) (PAH) and poly-L-lysine (PLL) on bare silica. By comparing the dry and wet masses measured on identical substrates with these two techniques, we obtain information on the layer thickness and water content of these layers. Monolayers typically feature an adsorbed dry mass of about 0.1-2 mg/m(2), a layer thickness of 0.5-2 nm, and a water content of 20-50%. One finds that the layer thickness increases with increasing concentrations of monovalent salts and polyelectrolytes.

Polyelectrolyte adsorption: electrostatic mechanisms and nonmonotonic responses to salt addition

The main question addressed in this work is as follows: Under pure electrosorption conditions, that is, disregarding nonelectrostatic effects, how does the net adsorption of a polyelectrolyte at an oppositely charged surface respond to the addition of simple salt? Previous simulations and mean-field calculations have suggested that the polymers will desorb. However, we will demonstrate that an increased adsorption also is possible, even for pure electrosorption, at low and intermediate levels of salt. As this is a correlation-driven effect, mean field approaches will fail to capture it. Using simulations, one will in general need to simulate large systems and relatively long polymers. Also important is the presence of a proper bulk solution, with a finite and well-defined polyelectrolyte concentration. We have performed a theoretical study of polyelectrolyte adsorption, assuming screened Coulomb interactions between monomers; that is, the salt is implicit. This work focuses on the effects from ionic screening and polymer length. Specifically, the adsorption at a weakly charged colloidal particle, with a diameter of 200 nm, is monitored for various salt concentrations, in the presence of highly charged chains. Using simulations, we investigate polymers with two different degrees of polymerization: 40 and 160, respectively. These simulations are complemented by predictions from classical polymer density functional theory, utilizing a recently developed correlation-correction (Forsman, J.; Nordholm, S. Langmuir, in press). The agreement with corresponding simulations is semiquantitative, and because the calculations run many orders of magnitude faster than the simulations, longer and more realistic polymers could be studied with this approach. However, switching off the correlation-correction leads to a mean-field theory, which fails to even qualitatively reproduce the simulated response to screening.

Adsorption of cationic hydroxyethylcellulose derivatives onto planar and curved gold surfaces

The adsorption of two positively charged hydroxyethylcellulose derivatives with 7 and 60 mol % positively charged groups and a cationic, hydrophobically modified hydroxyethylcellulose containing 1 mol % hydrophobic groups and 7 mol % charged groups onto flat and spherical citrate-coated gold surfaces of different sizes has been investigated. The planar surfaces were studied by means of the quartz crystal microbalance with dissipation monitoring, whereas nanoparticle suspensions were examined using dynamic light scattering and UV-vis spectroscopy. Two different driving forces for adsorption have been evaluated: the electrostatic interaction between the positive charges on the polymers and the negatively charged gold surfaces and the affinity of the polymers for gold due to hydrophobic interactions. The comparison between the data obtained from curved and planar surfaces suggests a strong correlation between surface curvature and adlayer conformation in the formation of the hybrid polymer-gold nanoparticles. The influence of particle size on the amount of adsorbed polymer has been evaluated for the different polymers. The impact of the ionic strength on polymer adsorption has been explored, and the adsorbed polymer layer has been found to protect the gold nanoparticles from aggregation when salt is added to the solution. The addition of salt to a mixture of gold particles and a charged polymer can induce a thicker adsorbed layer at low salinity, and desorption was found at high levels of salt addition.

Polyelectrolyte-induced aggregation of liposomes: a new cluster phase with interesting applications

Different charged colloidal particles have been shown to be able to self-assemble, when mixed in an aqueous solvent with oppositely charged linear polyelectrolytes, forming long-lived finite-size mesoscopic aggregates. On increasing the polyelectrolyte content, with the progressive reduction of the net charge of the primary polyelectrolyte-decorated particles, larger and larger clusters are observed. Close to the isoelectric point, where the charge of the adsorbed polyelectrolytes neutralizes the original charge of the particles' surface, the aggregates reach their maximum size, while beyond this point any further increase of the polyelectrolyte-particle charge ratio causes the formation of aggregates whose size is progressively reduced. This re-entrant condensation behavior is accompanied by a significant overcharging. Overcharging, or charge inversion, occurs when more polyelectrolyte chains adsorb on a particle than are needed to neutralize its original charge so that, eventually, the sign of the net charge of the polymer-decorated particle is inverted. The stability of the finite-size long-lived clusters that this aggregation process yields results from a fine balance between long-range repulsive and short-range attractive interactions, both of electrostatic nature. For the latter, besides the ubiquitous dispersion forces, whose supply becomes relevant only at high ionic strength, the main contribution appears due to the non-uniform correlated distribution of the charge on the surface of the polyelectrolyte-decorated particles ('charge-patch' attraction). The interesting phenomenology shown by these system has a high potential for biotechnological applications, particularly when the primary colloidal particles are bio-compatible lipid vesicles. Possible applications of these systems as multi-compartment vectors for the simultaneous intra-cellular delivery of different pharmacologically active substances will be briefly discussed.

Phase controllable transfer printing of patterned polyelectrolyte multilayers

We introduce a simple and robust method for varying the phase of the patterns in transfer printing of polyelectrolyte multilayers. Simply transferred positive patterns, edge-defined intermediate patterns, and negatively engraved patterns were obtained by manipulating the capillarity of the plasticized polymeric layer from weak to strong. A phase diagram of the pattern transfer was suggested to account for the influences of the temporal and spatial factors in the experiments. In addition, the main role of adhesion at the interfaces in the successful realization of multilayer transfer printing was investigated. It is anticipated that the strategy and ability to control the phase of the patterns transferred may offer a variety of opportunities for functional devices in optics, organic electronics, membranes, and microfluidics.

Transition from completely reversible to irreversible adsorption of poly(amido amine) dendrimers on silica

The adsorption and desorption behavior of poly(amido amine) (PAMAM) dendrimers at the water-silica interface was investigated by optical reflectometry. Polymer desorption upon dilution was studied as a function of generation (i.e., molecular mass), solution pH, and ionic strength. Three distinct adsorption regimes upon flushing with dendrimer-free solutions were identified. (i) Completely reversible adsorption refers to rapid and complete desorption. (ii) Partially reversible adsorption is characterized by rapid but partial desorption and a remaining irreversibly bound fraction. (iii) Irreversible adsorption refers to the case where there is no detectable change in the adsorbed mass. The system tends to be completely reversible for low generations, low pH values, and high ionic strengths, while it tends to be irreversible for high generations, high pH values, and low ionic strengths. The parameters for which these regimes are found are summarized in corresponding adsorption maps.

Effect of temperature on the reentrant condensation in polyelectrolyte-liposome complexation

Interactions of oppositely charged macroions in aqueous solution give rise to intriguing aggregation phenomena, resulting in finite-size, long-lived clusters, characterized by a quite narrow size distribution. Particularly, the adsorption of highly charged linear polyelectrolytes on oppositely charged colloidal particles is strongly correlated and some short-range order arises from competing electrostatic interactions between like-charged polymer chains (repulsion) and between polymer chains and particle surface (attraction). In these systems, in an interval of concentrations around the isoelectric point, relatively large clusters of polyelectrolyte-decorated particles form. However, the mechanisms that drive the aggregation and stabilize, at the different polymer/particle ratios, a well-defined size of the aggregates are not completely understood. Nor is clear the role that the correlated polyion adsorption plays in the aggregation, although the importance of "patchy interactions" has been stressed as the possible source of attractive interaction term between colloidal particles. Different models have been proposed to explain the formation of the observed cluster phase. However, a central question still remains unanswered, i.e., whether the clusters are true equilibrium or metastable aggregates. To elucidate this point, in this work, we have investigated the effect of the temperature on the cluster formation. We employed liposomes built up by DOTAP lipids interacting with a simple anionic polyion, polyacrylate sodium salt, over an extended concentration range below and above the isoelectric condition. Our results show that the aggregation process can be described by a thermally activated mechanism.

Complexation of ferric oxide particles with pectins of different charge density

The effect of polyelectrolyte charge density on the electrical properties and stability of suspensions of oppositely charged oxide particles is followed by means of electro-optics and electrophoresis. Variations in the electro-optical effect and the electrophoretic mobility are examined at conditions where fully ionized pectins of different charge density adsorb onto particles with ionizable surfaces. The charge neutralization point coincides with the maximum of particle aggregation in all suspensions. We find that the concentration of polyelectrolyte, needed to neutralize the particle charge, decreases with increasing charge density of the pectin. The most highly charged pectin presents an exception to this order, which is explained with a reduction of the effective charge density of this pectin due to condensation of counterions. The presence of condensed counterions, remaining bound to the pectin during its adsorption on the particle surface, is proved by investigation of the frequency behavior of the electro-optical effect at charge reversal of the particle surface.

Normal and frictional forces between surfaces bearing polyelectrolyte brushes

Normal and shear forces were measured as a function of surface separation, D, between hydrophobized mica surfaces bearing layers of a hydrophobic-polyelectrolytic diblock copolymer, poly(methyl methacrylate)- block-poly(sodium sulfonated glycidyl methacrylate) copolymer (PMMA- b-PSGMA). The copolymers were attached to each hydrophobized surface by their hydrophobic PMMA moieties with the nonadsorbing polyelectrolytic PSGMA tails extending into the aqueous medium to form a polyelectrolyte brush. Following overnight incubation in 10 (-4) w/v aqueous solution of the copolymer, the strong hydrophobic attraction between the hydrophobized mica surfaces across water was replaced by strongly repulsive normal forces between them. These were attributed to the osmotic repulsion arising from the confined counterions at long-range, together with steric repulsion between the compressed brush layers at shorter range. The corresponding shear forces on sliding the surfaces were extremely low and below our detection limit (+/-20-30 nN), even when compressed down to a volume fraction close to unity. On further compression, very weak shear forces (130 +/- 30 nN) were measured due to the increase in the effective viscous drag experienced by the compressed, sliding layers. At separations corresponding to pressures of a few atmospheres, the shearing motion led to abrupt removal of most of the chains out of the gap, and the surfaces jumped into adhesive contact. The extremely low frictional forces between the charged brushes (prior to their removal) is attributed to the exceptional resistance to mutual interpenetration displayed by the compressed, counterion-swollen brushes, together with the fluidity of the hydration layers surrounding the charged, rubbing polymer segments.

Adsorption of highly charged polyelectrolytes onto an oppositely charged porous substrate

The adsorption behavior of highly charged cationic polyelectrolytes onto porous substrates is electrostatic in nature and has been shown to be highly dependent on the polyelectrolyte properties. Copolymers of acrylamide (AM) and diallyldimethylammonium chloride (DADMAC) were synthesized to have a range of macromolecular properties (i.e., charge density and molecular mass). Traditional titration methods have been complemented by fluorescence labeling techniques that were developed to directly observe the extent that fluorescently labeled poly(AM- co-DADMAC) adsorbs into the pore structure of a cellulosic substrate. Although contributing to the electrostatic driving force, the charge density acts to limit adsorption to the outermost surface under electrolyte-free conditions. However, adsorption into the pores can occur if both the molecular mass and charge density of poly(AM- co-DADMAC) are sufficiently low. Adsorption initially increases as the electrolyte concentration is increased. However, the electrostatic persistence length of poly(AM- co-DADMAC) restricts the polyelectrolyte from entering the pores. Therefore, changes in the adsorption behavior at moderate electrolyte concentrations have been attributed to swelling of the polyelectrolyte layer at the fiber exterior. The adsorption behavior changes again at high electrolyte concentrations such that poly(AM- co-DADMAC) could adsorb into the pore structure. This occurred when the electrolyte concentration was sufficient to screen the electrostatic persistence length of poly(AM- co-DADMAC), provided that the entropic driving force for adsorption still existed. It is suggested that adsorption into the pore structure is a kinetic process that is governed by localized electrostatic interactions between poly(AM- co-DADMAC) and the charges located within the pores.

Adsorption of low charge density polyelectrolytes to an oppositely charged porous substrate

The adsorption behavior of a low charge density cationic polyelectrolyte to cellulosic fibers has been studied. Cationic dextran served as a model polyelectrolyte, as it can be prepared over a range in molecular mass and charge density. The adsorption behavior of the cationic dextran was measured in electrolyte-free conditions using polyelectrolyte titration techniques. By fluorescent labeling the cationic dextran, the extent to which adsorption occurs inside the porous structure was further determined by fluorescent confocal laser scanning microscopy. Cationic dextran having a sufficiently low charge density adsorbed into the pores, although the extent the cationic dextran adsorbed was governed by the molecular mass. The adsorption behavior of the cationic dextran was also studied in various electrolyte concentrations. The adsorbed mass monotonically decreased with increasing electrolyte, as the electrostatic interaction with the substrate was more effectively screened. This behavior also suggests that the interactions between adsorbed polyelectrolyte chains, i.e. lateral correlation effects, are negligible for low charge density polyelectrolytes. Finally, the effect of having a preadsorbed layer of cationic dextran on the adsorption behavior was determined in electrolyte-free conditions using fluorescent double staining techniques. The preadsorbed cationic dextran had almost no effect on the adsorption of low molecular mass fractions. Low molecular mass fractions directly adsorbed into the pore structure, as opposed to adsorbing to a free surface and diffusing into the pores. It was also shown that cationic dextran can be selectively adsorbed to different locations, such that the surface of a porous substrate can be treated uniquely from the bulk.

Electro-optics of colloid–polyelectrolyte complexes: Counterion condensation on free and adsorbed sodium carboxymethyl cellulose

Electrical properties of sodium carboxymethyl cellulose (NaCMC) in aqueous solution and after addition to a dilute suspension of beta-ferric hydrous oxide particles (beta-FeOOH) are studied by means of electric birefringence method. We established extended conformation of the NaCMC chains in a solution at concentration 10(-2) g dm(-3), which is found high enough to assure overcharging of the particle surface. The frequency behavior of the electric birefringence of NaCMC solution with concentration 10(-2) gdm(-3) is found similar to the behavior of the suspension of beta-FeOOH particles containing same amount of polyelectrolyte. The observed decrease in the relaxation frequency of the electro-optical effect in both systems is attributed to polarization of condensed counterions near to the polyion surface. These results are in line with our previous estimation, showing that the condensed counterions are not released from NaCMC because of its adsorption onto weakly charged particle surface. They also reveal that, at overcompensation of the particle charge, the electrical properties of the adsorbed polyion dominate the electro-optical behavior of the weakly charged particle.

Adsorption of Weakly Charged Polyelectrolytes onto Oppositely Charged Spherical Colloids

The adsorption of a long weakly charged flexible polyelectrolyte in a salt solution onto an oppositely charged spherical surface is investigated. An analytical solution for Green's function is derived, which is valid for any sphere radius and consistently recovers the result of a planar surface in the limit of large sphere radii, by substituting the Debye-Hückel potential via the Hulthén potential. Expressions for critical quantities like the critical radius and the critical surface charge density are provided. In particular, we find a universal critical line for the sphere radius as a function of the screening length separating adsorbed from desorbed states. Moreover, results for the monomer density distribution, adsorbed layer thickness, and the radius of gyration are presented. A comparison of our theoretical results with experiments and computer simulations yields remarkably good agreement.

Radiofrequency dielectric loss relaxation in polyion-induced liposome aggregates

In this note, we present a set of dielectric loss relaxation measurements of aqueous charged liposome suspensions during the whole aggregation process induced by oppositely charged adsorbing polyions. The system experiences two concomitant effects known as "reentrant condensation" and "charge inversion," resulting in the formation of liposome aggregates whose average size reaches a maximum in the vicinity of the electroneutrality condition, accompanied to a progressive reduction of their overall electrical charge. Far from the neutrality, from both sides, polyion-coated liposomes exist with a charge of opposite sign. The dielectric loss relaxation in these complex aggregating systems has never been measured so far and we report here, for the first time, the dielectric loss behavior of liposomes built up by a cationic lipid and stuck together by poly(acrylate), which is a flexible oppositely charged polyion. The data are analyzed in the framework of standard electrokinetic model theory. The evolution of the aggregation process as a function of the polyion content is mainly characterized by a counterion polarization effect, governed by the surface charge density of the aggregates and hence by the zeta-potential.

Thin adsorbed films of a strong cationic polyelectrolyte on silica substrates

The adsorption of poly(diallyldimethyl ammonium chloride) (DADMAC) on planar silica substrates was examined as a function of ionic strength and pH. The study was carried out with reflectometry in an impinging-jet cell and complemented by atomic force microscopy (AFM) and ellipsometry investigations. The adsorption process is initially transport limited, whereby the adsorption rate increases somewhat with increasing ionic strength. This effect is caused by a simultaneous decrease of the hydrodynamic radius of the polymer. After a transient period, the adsorption process saturates and leads to an adsorption plateau. The plateau value increases strongly with increasing ionic strength. This increase can be explained by progressive screening of the electrostatic repulsion between the adsorbing polyelectrolyte chains, as can be rationalized by a random sequential adsorption (RSA) model. The adsorbed amount further increases with increasing pH, and this effect is probably caused by the corresponding increase of the surface charge of the silica substrate.

Bending moduli of charged membranes immersed in polyelectrolyte solutions

We study the contribution of polyelectrolytes in solution to the bending moduli of charged membranes. Using the Helfrich free energy, and within the mean-field theory, we calculate the dependence of the bending moduli on the electrostatics and short-range interactions between the membrane and the polyelectrolyte chains. The most significant effect is seen for strong short-range interactions and low amounts of added salt where a substantial increase in the bending moduli of order 1 is obtained. For short-range repulsive membranes, the polyelectrolyte contribution to the bending moduli is small, of the order the 0.1 up to at most 1 . For weak short-range attractions, the increase in membrane rigidity is smaller and of less significance. It may even become negative for a large enough amount of added salt. Our numerical results are obtained by solving the adsorption problem in spherical and cylindrical geometries. In some cases, the bending moduli are shown to follow simple scaling laws.

Effect of chain length and charge density on the construction of polyelectrolyte multilayers on colloidal particles

Two combinations of sodium poly(4-styrene sulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) of different chain length and charge density are employed to construct multilayer films. The polyelectrolytes are assembled layer-by-layer on colloidal particles in the absence of salt. We have investigated the formation and electrical characteristics of the films by using electric light scattering technique. The results show that the film thickness is independent of the chain length when fully charged PAH (at pH 4.6) is combined with fully charged PSS. When the films are prepared with less charged PAH (at pH 6.7) and fully charged PSS, lower thickness is found for the film with shorter polymer chains. In all cases, the thickness increment realized on addition of the polymer with lower molar concentration is partially lost on exposure to the solution with higher concentration of the oppositely charged partner. When the film growth is regular (at equal molar concentrations of the fully charged polyelectrolytes), the ratio of PSS to PAH charge, estimated from the electro-optical effect values, exceeds 1. The electro-optical effect is also higher for the films ending with PSS when fully charged PSS is combined with less charged PAH (at pH 6.7). This reveals the key role of the charge in the last-adsorbed layer for the electro-optical behavior of the whole film.

Self-assembly of acridine orange dye at a mica/solution interface: formation of nanostripe supramolecular architectures

Optical waveguide spectroscopy and atomic force microscopy (AFM) have been used to characterize the supramolecular architectures of acridine orange (AO) dye self-assembled at a mica/aqueous solution interface. Under the saturated adsorption conditions, optical waveguide spectroscopy revealed that the dye formed H-type aggregates at the interface. In situ AFM visualized interesting morphology of the dye aggregates showing nanosized meandering stripes with the width of approximately 1.5 nm (or brightness periodicity of approximately 3 nm). Electrostatic adsorption of the dye cations onto a mica surface as well as the intermolecular pi-pi stacking brought about the ordered nanostructures. We propose an interfacial aggregation model that shows a meandering staircase structure with the intermolecular slip angle of 60 degrees. According to the model, the AO molecule occupies a surface area of about 1.0 nm2.

Phase diagram for the adsorption of weak polyelectrolytes at a soft charged surface

We have experimentally studied the adsorption of polyelectrolytes at oppositely charged surfaces. A weak flexible polyelectrolyte, poly(acrylic acid), was adsorbed from dilute solutions on a Langmuir film of a cationic amphiphile, dimethyldioctadecylammonium bromide. The polymer surface coverage, Gamma, at equilibrium was measured by two reflectivity techniques-ellipsometry and polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS)-as a function of the surface charge density, sigma, and of the polymer ionization degree, alpha. Different adsorption regimes were evidenced. For weakly charged surfaces, sigma < sigma sat, Gamma increases with sigma and with 1/alpha, as expected for a neutralization of the surface by the adsorbed polymers. For highly charged surfaces, sigma > sigma sat, the adsorption of polyelectrolytes saturates. The mean orientation of the adsorbed chains also depends on the value of sigma: it is parallel to the surface for sigma < sigma (< sigma sat) and orthogonal to the surface for sigma > sigma. We have measured the values of sigma sat and sigma as a function of alpha and compared the results with existing theories.

Adsorption of randomly annealed polyampholytes at the silica-water interface

We have investigated the adsorption of randomly annealed polyampholytes containing [2-(dimethylamino)ethyl methacrylate)] (DMAEMA), methacrylic acid (MAA), and [3-(2-methylpropionamido)propyl] trimethylammonium chloride (MAPTAC) with various molar compositions. The adsorption was performed from dilute aqueous solutions onto silicon substrates. The adsorbed layers were characterized by reflectivity techniques such as reflectometry, ellipsometry, and neutron specular reflection. As expected for annealed polyampholytes, the adsorption was found to depend strongly on the pH, with a maximum within the isoelectric domain of the polyampholyte. The monomer volume fraction profiles of the adsorbed layers were determined from neutron specular reflection measurements. In the isoelectric domain, the polyampholyte chains adopt a compact conformation, with a layer thickness of about 60 A. The polyampholyte layer is as dense as the adsorbed layer of fully charged polyelectrolyte but much thicker. Finally, we found that changing the ratio of neutral units along the polyampholyte chain in the isoelectric domain had no significant effect on the concentration profile of the adsorbed layer.

Phase behavior of polyelectrolyte-surfactant complexes at planar surfaces

We investigate theoretically the phase diagram of an insoluble charged surfactant monolayer in contact with a semidilute polyelectrolyte solution (of opposite charge). The polyelectrolytes are assumed to have long-range and attractive (electrostatic) interaction with the surfactant molecules. In addition, we introduce a short-range (chemical) interaction which is either attractive or repulsive. The surfactant monolayer can have a lateral phase separation between dilute and condensed phases. Three different regimes of the coupled system are investigated depending on system parameters. A regime where the polyelectrolyte is depleted due to short range repulsion from the surface, and two adsorption regimes, one being dominated by electrostatics, whereas the other by short range chemical attraction (similar to neutral polymers). When the polyelectrolyte is more attracted (or at least less repelled) by the surfactant molecules as compared with the bare water-air interface, it will shift upwards the surfactant critical temperature. For repulsive short-range interactions the effect is opposite. Finally, the addition of salt to the solution is found to increase the critical temperature for attractive surfaces, but does not show any significant effect for repulsive surfaces.