Time-resolved fluorescence anisotropy measurements of the adsorption of Rhodamine-B and a labelled polyelectrolyte onto colloidal silica

 The application of time-resolved fluorescence anisotropy measurements (TRAMS) to the investigation of the adsorption of the dye Rhodamine B and a Rhodamine B-labelled cationic polyelectrolyte onto colloidal silica (Ludox) is described. For Rhodamine B the time-resolved fluorescence anisotropy behavior observed can be interpreted using a model consisting of fluorophores with two distinct fluorescence decay lifetimes and two rotational correlation times corresponding to the fluorophore free in solution and bound to the Ludox. Details of the binding obtained from a global analysis of the data are reported. Restricted motion of the fluorescently labelled polyelectrolyte is also observ-ed on adsorption. The considerations for the general application of TRAMS for monitoring adsorption behavior are discussed. Received: 8 July 1998 Accepted: 10 August 1998     

Adsorption of polyelectrolyte multilayers and complexes on silica and cellulose surfaces studied by QCM-D

The adsorption of polyelectrolyte (PE) multilayers and complexes, obtained from both high- and low-charge polyelectrolytes, was studied on silica and on cellulose model surfaces by quartz crystal microbalance with dissipation (QCM-D). The film properties acquired with the different strategies were compared. When polyelectrolytes were added on an oppositely charged surface in sequence to form multilayers both the change in frequency and dissipation increased. The changes in frequency and dissipation were clearly higher if low-charge PEs were used in the multilayer formation. The substrate, silica or cellulose, did not affect the adsorption behaviour of low-charge PEs and only minor differences were seen in the adsorbed amounts and changes in dissipation of high-charge PEs between SiO₂ and cellulose. The complexes formed by low-charge PEs had higher changes in frequency and dissipation at low ionic strength on both surfaces, while the complexes formed from high-charge polyelectrolytes adsorbed more at high salt concentration. The complexes of low-charge polyelectrolytes adsorbed more on silica, while the complexes formed by high-charge PEs formed thicker layers on cellulose. The charge ratio had a significant effect on the adsorption and the highest changes in frequency and dissipation were obtained in the anionic/cationic charge ratio of 0.5–0.6. Generally, the multilayers and complexes formed by low-charge polyacrylamides adsorbed highly and formed rather thick layers on both surfaces, unlike the high-charge PEs which formed thin layers using either one of the addition techniques.     

Polyelectrolyte adsorption layers studied by streaming potential and particle deposition

Adsorption of a cationic polyelectrolyte, polyallylamine hydrochloride (PAH), having a molecular weight of 70,000 on mica was characterized by the streaming potential method and by deposition of negative polystyrene latex particles. Formation of PAH layers was followed by determining the apparent zeta potential of surface zeta as function of bulk PAH concentration. The zeta potential was calculated from the streaming potential measured in the parallel-plate channel formed by two mica plates precovered by the polyelectrolyte. The experimental data were expressed as the dependence of the reduced zeta potential zeta/zeta0 on the PAH coverage Theta(PAH), calculated using the convective diffusion theory. It was found that for the ionic strength of 10(-2) M, the dependence of zeta/zeta0 on Theta(PAH) can be reflected by the theoretical model formulated previously for surfaces covered by colloid particles. The electrokinetic measurements were complemented by particle deposition experiments on PAH-covered mica surfaces. A direct correlation between the polymer coverage and the initial deposition rate of particles, as well as the jamming coverage, was found. For ThetaPAH > 0.3 the initial deposition rate attained the value predicted from the convective diffusion theory for homogeneous surfaces. The initial deposition rates for surfaces modified by PAH were compared with previous experimental and theoretical results obtained for heterogeneous surfaces formed by preadsorption of colloid particles. It was revealed that negative latex deposition occurred at surfaces exhibiting negative apparent zeta potential, which explained the anomalous deposition of particles observed in previous works. It was suggested that the combined electrokinetic and particle deposition methods can be used for detecting adsorbed polyelectrolytes at surfaces for coverage range of a percent. This enables one to measure bulk polyelectrolyte concentrations at the level of 0.05 ppm.     

Conformation of adsorbed layers of polyNIPAM on silica in a binary solvent

The conformation of poly( N-isopropylacrylamide) chains adsorbed at a silica interface was studied as a function of concentration in the methanol-water binary solvent mixture. Both water and methanol are good solvents for PNIPAM; however, in certain mixtures cononsolvency is induced by a lowering of the LCST. This led to a decrease in the extent of the PNIPAM layer away from the interface as measured using the colloidal probe technique in the poor solvent region. At low methanol concentrations but still in the good solvent region capillary bridging between the silica surfaces with adsorbed PNIPAM layers was observed due to the increased methanol concentration in this interfacial region over that of the bulk. Furthermore, adsorption measurements showed that PNIPAM adsorbed only weakly to the silica interface with a low surface excess on the order of 0.23 mg/m (2), which allowed study of the behavior of the immobilized PNIPAM chains under highly dilute conditions using the quartz crystal microbalance. As the concentration of methanol increased toward the phase transition boundary, a slight contraction followed by an expansion of the PNIPAM was observed, which is in agreement with previous predictions from theory for polymers in solution.     

Intelligent colloidal hybrids via reversible pH-induced complexation of polyelectrolyte and silica nanoparticles

We present novel intelligent colloidal polymer/silica nanocomposites, in which the complexation of cationic silica nanoparticles and a weak anionic polyelectrolyte can be manipulated simply by pH change through a hydrogen-bonding interaction and ionic complexation caused by hydrogen-transfer interactions between the constituents. Special silica particles which have nanometer size (diameter approximately 3.0 nm) and two independent proton-accepting sites were developed in this study. Both the silica and poly(acrylic acid) form transparent colloidal solutions in water, while a white turbid dispersion was obtained just after mixing the two solutions due to the complexation. The pH-induced association-dissociation behavior was confirmed by the turbidity and potentiometric titration measurements. The assembled structures of the hybrids were visualized by scanning force microscopy.     

Two different approaches to XPS quantitative analysis of polyelectrolyte adsorption layers

X-ray photoelectron spectroscopy (XPS) was employed to quantify adsorption of polyelectrolytes from aqueous solutions of low ionic strength onto mica, glass, and silica. Silica surfaces were conditioned in base or in acid media as last pre-treatment step (silica-base last or silica-acid last, respectively). Consistency in the determined adsorbed amount, Γ, was obtained independent of the choice of XPS mode and with the two quantification approaches used in the data evaluation. Under the same adsorption conditions, the adsorbed amount, Γ, varied as Γ_mica > Γ_{silica-base last} ≈ Γ_glass > Γ_{silica-acid last}. In addition, the adsorbed amount increased with decreasing polyelectrolyte charge density (100% to 1% of segments being charged) for all substrates. Large adsorbed amount was measured for low-charge density polyelectrolytes, but the number of charged segments per square nanometer was low due to steric repulsion between polyelectrolyte chains that limited the adsorption. The adsorbed amount of highly charged polyelectrolytes was controlled by electrostatic interactions and thus limited to that needed to neutralize the substrate surface charge density. For silica, the adsorbed amount depended on the cleaning method, suggesting that this process influenced surface concentration and fraction of different silanol groups. Our results demonstrate that for silica, a higher density and/or more acidic silanol groups are formed using base, rather than acid, treatment in the last step.     

Overcharging of polyelectrolyte complexes by the guest polyelectrolyte studied by fluorescence spectroscopy

Polyion complexes (PICs) of anionic block copolymer poly(ethylene oxide)-block-poly(sodium methacrylate), PEO-block-(PMA)Na, and a cationic homopolymer, poly((methacryloyloxyethyl)trimethylammonium chloride), PMOTAC, have been studied by fluorescence spectroscopy. Pyrene and naphthalene singly labeled block copolymers were used with two different sodium methacrylate block lengths. The chain exchange between the stoichiometric PICs at the equilibrium state and the formation of the negatively charged PICs on addition of excess PEO-block-(PMA)Na to stoichiometric PIC solution were of interest. The chain exchange between the stoichiometric complexes was observed to occur via two mechanisms. The faster chain exchange occurs via insertion and expulsion of single chains, while merging and splitting of the PIC particles is behind the slower chain exchange event. Incorporation of an excess amount of the guest polyion into a stoichiometric PIC took place on further addition of the PEO-block-(PMA)Na. The same mechanisms were recognized in the overcharging process of the PICs as in the chain exchange between the stoichiometric PICs.     

Multilayered adsorption of macrocations and macroanions on colloidal spheres as studied by dynamic light scattering measurements

The electrophoretic light scattering data on the thickness of the alternate multiple adsorption layers of macrocations and macroanions on the surfaces of colloidal spheres, which have been published by the authors in Colloid and Polymer Science (1999) 277;813, (2000) 278:380 and (2002) 280:533, are reexamined with help of the dynamic light scattering measurements. Colloidal silica spheres (110 nm in diameter) and monodispersed polystyrene spheres (220 nm) are used as colloidal spheres. The macrocations used are poly(4-vinyl- N- n-butylpyridinium bromide and poly(allylamine). Sodium poly(styrene sulfonate) and sodium polyacrylate are used as macroanions. It was clarified in the previous work that a very small amount of the large aggregates of the macroions coexists for most of the suspensions and the thickness values reported are large compared with the true values. The corrected thickness values support the continuous thin layer's growing adsorption of the macroions on the colloidal surfaces but do not support the expansion–contraction-type adsorption.     

Dynamics of order formation by colloidal adsorption onto a substrate studied with Brownian dynamics

Colloidal adsorption and spontaneous ordering of adsorbed particles on a substrate was simulated using a three-dimensional simulation model for colloidal dispersion system with an adsorptive surface under a specified bulk concentration, where the particle-particle and particle-substrate interactions were modeled on the DLVO theory. The key process for order formation is considered to be the adsorption of a particle that induces the transition from incomplete order to perfect order, and is found to involve a stochastic nature due to an energy barrier which must be overcome for the system to reach ordered state. Also, a model was developed to predict the energy barrier for order formation based on direct observation of the key process. Further, a model to describe the stochastic nature of the process was developed and its quantitative validity was demonstrated. Through the examination of the key process, it is concluded that the mechanism of the order formation is composed of two successive processes and the rate-determining step varies depending on the ionic strength.     

Protein adsorption on to low-temperature isotropic carbon 4. Competitive adsorption on carbon and silica studied by two-dimensional electrophoresis

Two-dimensional polyacrylamide gel electrophoresis (2D PAGE) was applied to the study of competitive protein adsorption from diluted human plasma. We obtained the depletion (adsorption) of some 25 plasma proteins in the presence of low-temperature isotropic carbon (LTIC) or silica powders. The depletion data are used as a measure of protein adsorptivity. Generally, proteins of lowest abundance have the highest tendency to associate with the two solid surfaces studied. The adsorptivity of a protein is largely determined by its solubility. Most proteins detected exhibit similar depletion behavior on both adsorbents, suggesting a multilayer adsorption process. Three proteins, hemopexin, apolipoprotein A I, and apolipoprotein A II, are depleted differently in the presence of LTIC and silica powders.     

The structure and adsorption properties of colloidal silica used in toothpastes

The dispersity, specific surface area, porosity, and pore size distribution are determined for samples of colloidal silica and calcium o-phosphate, toothpaste constituents. The results obtained show that adsorbents have the developed mesoporous structure. It was found that the adsorption of sodium lauryl sulfate and a nonionic surfactant, glyceryl monocaprylate, at the aqueous surfactant solution-colloidal silica interface is small and has the unusual character. The reasons for a low adsorption of surfactants and their mixtures on the surface of the studied adsorbents are discussed.     

Correlation between colloidal stability and surfactant adsorption/association phenomena studied by light scattering

The stability of a colloidal system composed of styrene-acrylate copolymer particles and potassium stearate (KS) anionic surfactant molecules has been determined in terms of the Fuchs stability ratio, W, as a function of the surfactant concentration, by measuring the initial aggregation kinetics using the small-angle light scattering (SALS) technique. The structure of the particle surface is peculiar, being irregularly patterned, and thus represents a model system to investigate colloidal stability of nonsmooth colloidal particles. From the SALS kinetic experiments, it is found that the stability increases dramatically with KS concentration until the saturation of the available surface occurs. At concentrations higher than the saturation concentration, the W value decreases markedly with KS, as a consequence of attractive depletion forces induced by formation of micelles in the water phase. The adsorption isotherm, determined through the surface tension technique, agrees with the W vs KS behavior, with respect to the onset of saturation and the surface-per-molecule value, and it can be described by the two-step Langmuir isotherm. Static light scattering spectra of the particles at different adsorbed amounts of KS have been fitted by means of the Lorenz-Mie theory and accounting for the experimentally determined particle size distribution. The increase in the particle diameter imputable to KS adsorption is sizable. Stability data measured under high fluid shear in a turbulent capillary (in the absence of any screening salt) fit well into this scenario. However, depletion forces are shown to be noncooperative with turbulent shear in the absence of screening electrolytes.     

Thickness and morphology of polyelectrolyte coatings on silica surfaces before and after protein exposure studied by atomic force microscopy

Analyte–wall interaction is a significant problem in capillary electrophoresis (CE) as it may compromise separation efficiencies and migration time repeatability. In CE, self-assembled polyelectrolyte multilayer films of Polybrene (PB) and dextran sulfate (DS) or poly(vinylsulfonic acid) (PVS) have been used to coat the capillary inner wall and thereby prevent analyte adsorption. In this study, atomic force microscopy (AFM) was employed to investigate the layer thickness and surface morphology of monolayer (PB), bilayer, (PB-DS and PB-PVS), and trilayer (PB-DS-PB and PB-PVS-PB) coatings on glass surfaces. AFM nanoshaving experiments providing height distributions demonstrated that the coating procedures led to average layer thicknesses between 1 nm (PB) and 5 nm (PB-DS-PB), suggesting the individual polyelectrolytes adhere flat on the silica surface. Investigation of the surface morphology of the different coatings by AFM revealed that the PB coating does not completely cover the silica surface, whereas full coverage was observed for the trilayer coatings. The DS-containing coatings appeared on average 1 nm thicker than the corresponding PVS-containing coatings, which could be attributed to the molecular structure of the anionic polymers applied. Upon exposure to the basic protein cytochrome c, AFM measurements showed an increase of the layer thickness for bare (3.1 nm) and PB-DS-coated (4.6 nm) silica, indicating substantial protein adsorption. In contrast, a very small or no increase of the layer thickness was observed for the PB and PB-DS-PB coatings, demonstrating their effectiveness against protein adsorption. The AFM results are consistent with earlier obtained CE data obtained for proteins using the same polyelectrolyte coatings.     

Water vapor adsorption on activated carbon preadsorbed with naphtalene

The adsorption of water vapor on a microporous activated carbon derived from the carbonization of coconut shell has been studied. Preadsorption of naphthalene was used as a tool to determine the location and the influence of the primary adsorbing centers within the porous structure of active carbon. The adsorption was studied in the pressure range p/p0=0-0.95 in a static water vapor system, allowing the investigation of both kinetic and equilibrium experimental data. Modeling of the isotherms using the modified equation of Do and Do was applied to determine the effect of preadsorption on the mechanism of adsorption.     

Interaction of oxygen with preadsorbed hydrogen on magnesium oxide studied by temperature-Programmed desorption method

The interaction of oxygen with preadsorbed hydrogen on thermally activated magnesium oxide has been studied by temperature-programmed desorption (TPD) method. It was found that oxygen at 450 K reacts with preadsorbed hydrogen and the resulting hydroxylation suppresses the adsorptive capacity of magnesia with respect to oxygen.

---

. , 450 K , .

     

Dilational surface visco-elasticity of polyelectrolyte/surfactant solutions: formation of heterogeneous adsorption layers

Recent application of the methods of surface dilational rheology to solutions of the complexes between synthetic polyelectrolytes and oppositely charged surfactants (PSC) gave a possibility to determine some steps of the adsorption layer formation and to discover an abrupt transition connected with the formation of microaggregates at the liquid surface. The kinetic dependencies of the dynamic surface elasticity are always monotonous at low surfactant concentrations but can have one or two local maxima in the range beyond the critical aggregation concentration. The first maximum is accompanied by the generation of higher harmonics of induced surface tension oscillations and caused by heterogeneities in the adsorption layer. The formation of a multilayered structure at the surface for some systems leads to the second maximum in the dynamic surface elasticity. The hydrophobicity and charge density of a polymer chain influence strongly the surface structure, resulting in a variety of dynamic surface properties of PSC solutions. Optical methods and atomic force microscopy give additional information for the systems under consideration. Experimental results and existing theoretical frameworks are reviewed with emphasis on the general features of all studied PSC systems.     

Dynamics of counterions in polyelectrolyte multilayers studied by electro-optics

The electro-optical behavior of a multilayer constructed via layer-by-layer deposition of poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) onto ellipsoidal β-FeOOH particles is examined using electric light scattering method. For fully charged polymers (at pH 4.5), the electro-optical effect is found to increase with polyelectrolyte layer number, showing a tendency to saturation in the linear growth regime. The effect is greater and of lower frequency of relaxation for the films ending with PAH in comparison to those with top PSS layer. Evidence is given that polarization of “condensed” counterions along the chains of the last-adsorbed polymer is mainly responsible for the observed electro-optical behavior of the polyelectrolyte multilayer. Although incorporation of “condensed” small ions into the film bulk seems probable for the PSS/PAH multilayer, their participation in the electro-optical effect is found negligible. The structural changes in the PSS/PAH multilayer due to the PAH deprotonation at pH 7.5 and the corresponding changes in the electro-optical effect confirm the key role of the last-adsorbed polymer for the behavior of the entire PSS/PAH film.     

Formation of weak polyelectrolyte multilayers studied by spin labeling

Multilayers of alternately adsorbing poly(allylamine) (PAH) and poly(acrylic acid) (PAA) of opposite charges on silica have been studied by the spin labeling technique, as a function of pH. The two polyelectrolytes have been labeled independently by a nitroxide free radical. Its electron paramagnetic resonance spectrum is mainly sensitive to the local Brownian motion and shows lines typical of two different environments, namely, loops protruding in solution with a fast motion and trains adsorbed on the solid with a hindered motion. These two parts have been evaluated for each of the polymer layers separately, and the thickness of the coatings has been described more precisely by characterizing the four contributions existing, for example, for a bilayer. Complexation is demonstrated by the loss of loops and tails belonging to the first polyelectrolyte. The overall picture emerging from the data is explained in terms of compensation of charges and entropy of confinement.     

Dumbbell-shaped polyelectrolyte brushes studied by depolarized dynamic light scattering

We present the synthesis and comprehensive characterization of dumbbell-shaped polyelectrolyte brushes (DPB). The core of these particles consists of poly(methyl methacrylate) (PMMA) and poly(styrene) onto which a dense brush shell of poly(styrene sulfonate) is grafted. The morphology of DPB particles is studied in solution by cryogenic-transmission electron microscopy. We demonstrate that well-defined DPB are generated that react to external stimuli such as surfactant and salt concentration. The rotational diffusion and collective relaxations of the DPB particles were monitored by depolarized dynamic light scattering (DDLS). Here we found a new relaxation mode in the DDLS-signal that can be ascribed to collective fluctuations of the polyelectrolyte layer affixed to the surface of the dumbbells.