A QCM study of the immobilization of beta-galactosidase on polyelectrolyte surfaces: effect of the terminal polyion on enzymatic surface activity

This work describes the immobilization of beta-galactosidase onto polyelectrolyte multilayer assemblies of the polyanion poly[1-[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl, sodium salt] (PAZO) and the polycation poly(ethylenimine) (PEI) constructed by electrostatic self-assembly (ESA). A single layer of beta-galactosidase was deposited over a precursor film comprising up to five bilayers of the PEI/PAZO polyelectrolyte pair. The enzyme was deposited on both the polycationic (PEI) and the polyanionic (PAZO) surfaces. Quartz crystal microbalance with dissipation monitoring (QCM-D), single-wavelength ellipsometry, and UV-visible absorption spectroscopy revealed differences in both the amount of beta-galactosidase incorporated in each of the multilayer assemblies and the resulting enzyme packing density in the films. The enzymatic films were immersed in a reaction solution containing o-nitrophenyl-beta-d-galactopyranoside (ONPG), and absorbance measurements were used to monitor the concentration of o-nitrophenyl (ONP), the product of the beta-galactosidase catalyzed by hydrolysis of ONPG. Although our data indicate that comparable amounts of beta-galactosidase are incorporated onto both surfaces, enzymatic activity is substantially inhibited when the beta-galactosidase is immobilized on the polyanionic surface compared to the enzyme on the polycationic surface. The difference in catalytic activities reflects the different abilities of the two polyelectrolytes to screen the protein's active site from the substrate environment. In both assemblies, the protein interpenetrated the PEI/PAZO multilayer, disrupting the J-aggregated state of the PAZO chromophores. This work demonstrates that the charge, conformation, and composition of underlying polyelectrolyte cushions have a significant effect on the structure and function of an immobilized protein within functional nanoassemblies.     

Modifying the adsorption properties of anionic surfactants onto hydrophilic silica using the pH dependence of the polyelectrolytes PEI, ethoxylated PEI, and polyamines

The manipulation of the adsorption of the anionic surfactant, sodium dodecyl sulfate, SDS, onto hydrophilic silica by the polyelectrolytes, polyethyleneimine, PEI, ethoxylated PEI, and the polyamine, pentaethylenehexamine, has been studied using neutron reflectometry. The adsorption of a thin PEI layer onto hydrophilic silica promotes a strong reversible adsorption of the SDS through surface charge reversal induced by the PEI at pH 7. At pH 2.4, a much thicker adsorbed PEI layer is partially swelled by the SDS, and the SDS adsorption is now no longer completely reversible. At pH 10, there is some penetration of SDS and solvent into a thin PEI layer, and the SDS adsorption is again not fully reversible. Ethoxylation of the PEI (PEI-EO(1) and PEI-EO(7)) results in a much weaker and fragile PEI and SDS adsorption at both pH 3 and pH 10, and both polymer and surfactant desorb at higher surfactant concentrations (>critical micellar concentration, cmc). For the polyamine, pentaethylenehexamine, adsorption of a layer of intermediate thickness is observed at pH 10, but at pH 3, no polyamine adsorption is evident; and at both pH 3 and pH 10, no SDS adsorption is observed. The results presented here show that, for the amine-based polyelectrolytes, polymer architecture, molecular weight, and pH can be used to manipulate the surface affinity for anionic surfactant (SDS) adsorption onto polyelectrolyte-coated hydrophilic silica surfaces.     

Dual-beam polarization interferometry resolves mechanistic aspects of polyelectrolyte adsorption

The electrostatically driven binding dynamics of a polyelectrolyte multilayer (PEMU) film was investigated in real-time using dual-beam polarization interferometry (DPI) and independently supported by quartz crystal microbalance with dissipation monitoring (QCM-D) studies. Multilayer assemblies of the polyanions poly[1-[4[(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl sodium salt] (PAZO) and poly(styrene sulfonate) (PSS) were respectively constructed with the polycation poly(ethylenimine) (PEI) on anionic functionalized substrates using the layer-by-layer electrostatic self-assembly method. DPI measurements indicate that polyelectrolyte adsorption occurs in three distinct stages. In the first stage, for approximately 5 s, coil-like segments of polyanion partially tether to the surface of the oppositely charged PEI. In the second stage, these coils unfurl over a period of approximately 10 s to cover the surface resulting in an increase in average density of the film. During the final adsorption step, the surface-bound polyelectrolyte diffuses into the multilayer assembly, exposing the surface to further deposition. This last step occurs over a much longer time period and results in a highly interpenetrated film containing a charge-overcompensated region at the film surface.     

Concomitant adsorption of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers and sodium dodecyl sulfate at the silica-water interface

Upon addition of silica to aqueous solutions of poly(ethylene oxide)-b-poly(epsilon-caprolactone) copolymers (PEO-b-PCL) and sodium dodecyl sulfate (SDS), adsorption of the solutes occurs at the silica-water interface. The amount of the adsorbed constituents has been measured by the total concentration depletion method. Small-angle neutron scattering experiments (SANS) have been carried out to investigate the structure of the adsorbed layer. Although SDS is not spontaneously adsorbed onto hydrophilic silica, adsorption is observed in the presence of PEO-b-PCL diblocks, in relation to the relative concentration of the two compounds. Conversely, SDS has a depressive effect on the adsorption of the copolymer, whose structure at the interface is modified. Copolymer desorption is however never complete at high SDS content. These observations have been rationalized by the associative behavior of PEO-b-PCL and SDS in water.     

Selective Interaction Between Proteins and the Outermost Surface of Polyelectrolyte Multilayers: Influence of the Polyanion Type,pH and Salt

Protein adsorption was studied by in-situ ATR-FT-IR spectroscopy of consecutively deposited polyelectrolyte multilayer systems terminated either with poly(ethyleneimine) (PEI) or polyanions, such as poly-(acrylic acid) (PAC), poly(maleic acid-co-propylene) (PMA-P) or poly(vinyl sulfate) (PVS). The influence of the polyanion type, pH and ionic strength was investigated. Negatively charged human serum albumin (HSA) was strongly repelled by multilayers terminated with weak polyanions (PAC, PMA-P), whereas moderate attraction was observed for those terminated with the strong polyanion PVS. Changing the pH from 7.4 to 5 resulted in enhanced HSA adsorption onto PAC-terminated multilayers. An increase in ionic strength diminished the attractive HSA adsorption onto PEI-terminated multilayers. For the PEI/PAC system, the biomedically relevant adsorption of human fibrinogen (FGN) is determined via its isoelectric point in accordance with three other proteins.     

Fibronectin adsorption onto polyelectrolyte multilayer films

The Layer-by-layer deposition of positively and negatively charged macromolecular species is an ideal method for constructing thin films incorporating biological molecules. We investigate the adsorption of fibronectin onto polyelectrolyte multilayer (PEM) films using optical waveguide lightmode spectroscopy (OWLS) and atomic force microscopy (AFM). PEM films are formed by adsorption onto Si(Ti)O2 from alternately introduced flowing solutions of anionic poly(sodium 4-styrenesulfonate) (PSS) and cationic poly(allylamine hydrochloride) (PAH). Using OWLS, we find the initial rate and overall extent offibronectin adsorption to be greatest on PEM films terminated with a PAH layer. The polarizability density of the adsorbed protein layer, as measured by its refractive index, is virtually identical on both PAH- and PSS-terminated films; the higher adsorbed density on the PAH-terminated film is due to an adsorbed layer of roughly twice the thickness. The binding of monoclonal antibodies specific to the protein's cell binding site is considerably enhanced to fibronectin adsorbed to the PSS layer, indicating a more accessible adsorbed layer. With increased salt concentration, we find thicker PEM films but considerably thinner adsorbed fibronectin layers, owing to increased electrostatic screening. Using AFM, we find adsorbed fibronectin layers to contain clusters; these are more numerous and symmetric on the PSS-terminated film. By considering the electrostatic binding of a segmental model fibronectin molecule, we propose a picture of fibronectin adsorbed primarily in an end-on-oriented monolayer on a PAH-terminated film and as clusters plus side-on-oriented isolated molecules onto a PSS-terminated film.     

Adsorption of cationic cellulose derivative/anionic surfactant complexes onto solid surfaces. II. Hydrophobized silica surfaces

The effect of the anionic surfactant SDS (sodium dodecyl sulfate) on the adsorption behavior of cationic hydroxyethyl cellulose (Polymer JR-400) and hydrophobically modified cationic cellulose (Quatrisoft LM-200) at hydrophobized silica has been investigated by null ellipsometry and compared with the previous data for adsorption onto hydrophilic silica surfaces. The adsorbed amount of LM-200 is found to be considerably larger than the adsorbed amount of JR-400 at both surfaces. Both polymers had higher affinity toward hydrophobized silica than to silica. The effect of SDS on polymer adsorption was studied under two different conditions: adsorption of polymer/SDS complexes from premixed solutions and addition of SDS to preadsorbed polymer layers. Association of the surfactant to the polymer seems to control the interfacial behavior, which depends on the surfactant concentration. For the JR-400/SDS complex, the adsorbed amount on hydrophobized silica started to increase progressively from much lower SDS concentrations, while the adsorbed amount on silica increased sharply only slightly below the phase separation region. For the LM-200/SDS complex, the adsorbed amounts increased progressively from very low SDS concentrations at both surfaces, and no large difference in the adsorption behavior was observed between two surfaces below the phase separation region. The complex desorbed from the surface at high SDS concentrations above the critical micelle concentration. The reversibility of the adsorption of polymer/SDS complexes upon rinsing was also investigated. When the premixed polymer/SDS solutions at high SDS concentrations (>5 mM) were diluted by adding water, the adsorbed amount increased due to the precipitation of the complex. The effect of the rinsing process on the adsorbed layer was determined by the hydrophobicity of the polymer and the surface.     

Adsorption of polyelectrolyte/surfactant mixtures at the air-solution interface: poly(ethyleneimine)/sodium dodecyl sulfate

Neutron reflectivity and surface tension have been used to characterize the adsorption of the polyelectrolyte/ionic surfactant mixture of poly(ethyleneimine) (PEI) and sodium dodecyl sulfate (SDS) at the air-water interface. The surface tension behavior and adsorption patterns show a strong dependence upon the solution pH. However, the SDS adsorption at the interface is unexpectedly most pronounced when the pH is high (when the polymer is essentially a neutral polymer) and when the polymer architecture is branched rather than linear. For both the branched and the linear PEI polymer/surfactant complex formation results in a significant enhancement of the amount of SDS at the interface, down to surfactant concentrations approximately 10(-6) M. For the branched PEI a transition from a monolayer to a multilayer adsorption is observed, which depends on surfactant concentration and pH. In contrast, for the linear polymer, only monolayer adsorption is observed. This substantial increase in the surface activity of SDS by complexation with PEI results in spontaneous emulsification of hexadecane in water and the efficient wetting of hydrophobic substrates such as Teflon. In regions close to charge neutralization the multilayer adsorption is accentuated, and more extensively ordered structures, giving rise to Bragg peaks in the reflectivity data, are evident.     

Controlling layer thickness and photostability of water-soluble cationic poly(p-phenylenevinylene) in multilayer thin films by surfactant complexation

In this work we build on prior studies of the novel water-soluble cationic conjugated polymer known as "P2" (poly{2,5-bis[3-( N, N, N-triethylammonium bromide)-1-oxapropyl]-1,4-phenylenevinylene}) with a focus on its incorporation into thin films for such applications as photovoltaics or electroluminescent devices. Multilayer assemblies were constructed using P2, the anionic surfactant sodium dodecyl sulfate (SDS), and the polyanion poly(sodium 4-styrene-sulfonate) (PSS) using the technique of layer-by-layer electrostatic self-assembly (LBL-ESA). SDS was observed to affect the layer thicknesses and absorbance characteristics of the films. We show that the optical properties and photo-oxidative resistance can be improved by varying the SDS content in the assemblies. Specifically, the surfactant-complexed poly( p-phenylenevinylene) (PPV) shows an enhanced absorption at longer wavelengths as well as improved photostability. Therefore, our work may have broad implications on the development of stable PPV-based materials in general and their efficient integration into thin films technologies.     

Dynamics of adsorption of an oppositely charged polymer-surfactant mixture at the air-water interface: poly(dimethyldiallylammonium chloride) and sodium dodecyl sulfate

The dynamic adsorption behavior of mixtures of the cationic polymer poly(dimethyldiallylammonium chloride) [poly(dmdaac)] and the anionic surfactant sodium dodecyl sulfate (SDS) has been studied at the expanding liquid surface of an overflowing cylinder. A combination of ellipsometry and external reflection Fourier transform infrared spectroscopy was used to measure the adsorbed amounts of poly(dmdaac) and SDS as a function of the bulk surfactant concentration for various polymer concentrations in the range 0-0.2 g dm-3. Laser Doppler velocimetry was used to determine the surface age, which was approximately 1 s for solutions where the polymer adsorbed. The interfacial behavior is rationalized in terms of competition between surface activity and mass transport to the expanding surface. At low surfactant concentrations, adsorption of both poly(dmdaac) and SDS is enhanced as a result of the formation in solution of polymer-surfactant complexes that are more surface active than either component alone. The rate of adsorption of these complexes is diffusion-controlled, and their interfacial composition remains constant at three dmdaac units per SDS molecule over a 5-fold change in the surfactant concentration. For the higher polymer concentrations studied, the complexes saturate the air-water interface: the adsorbed amount is independent of the polymer concentration and remains constant also over a factor of 5 in the surfactant concentration. Once the number of bound surfactant molecules per dmdaac monomer exceeds 0.3, the complexes begin to form large aggregates, which are not surface active due to their slower mass transport. The adsorbed amount decreases rapidly on approach to the equivalence point (one SDS molecule per dmdaac monomer), and when it is reached, only a very small amount of material remains at the interface. At still higher surfactant concentrations, the free SDS adsorbs but there is no adsorbed poly(dmdaac). The dynamic adsorption data are compared with equilibrium measurements of the same system by Staples et al. (Langmuir 2002, 18, 5147), which show very different surface compositions and no significant change in surface coverage at the equivalence point.     

The electroactive multilayer films of polyelectrolytes and Prussian blue nanoparticles and their application for H2O2 sensors

Prussian blue (PB) nanoparticles were immobilized in polyelectrolyte (PE) multilayers of various compositions and thickness. Films containing nanoparticles and poly(allylamine hydrochloride) (PAH) were formed using the layer-by-layer adsorption method. A layer of branched poly(ethyleneimine) (PEI) was used to anchor the multilayer structure at the surface of a gold electrode. The films exhibited electroactive properties, increasing with the number of deposited PB layers. The properties of PEI/(PB/PAH) _n multilayers were then compared with the ones containing additionally the conductive polymer poly(3,4-ethylenedioxythiophene)–poly(styrenesulfonate) (PEDOT:PSS). We found that the addition of the conductive, water-soluble polymer enhances the electroactive properties of the multilayer films. It also increased sensitivity of the multilayer-covered electrodes for electrochemical detection of hydrogen peroxide.     

The effects of the addition of the polyelectrolyte, poly(ethyleneimine), on the adsorption of mixed surfactants of sodium dodecylsulfate and dodecyldimethylaminoacetate at the air-water interface

The effects of the addition of the polyelectrolyte, poly(ethyleneimine), PEI, on the adsorption of the mixed surfactants of sodium dodecylsulfate, SDS, and dodecyldimethylaminoacetate, dodecyl betaine, at the air-water interface have been investigated using neutron reflectivity and surface tension. In the absence of PEI the SDS and dodecyl betaine surfactants strongly interact and exhibit synergistic adsorption at the air-water interface. The addition of PEI, at pH 7 and 10, results in a significant modification of the surface partitioning of the SDS/dodecyl betaine mixture. The strong surface interaction at high pH (pH 7 and 10) between the PEI and SDS dominates the surface behavior. For solution compositions in the range 20/80-80/20 mol ratio dodecyl betaine/SDS at pH 7 the surface composition is strongly biased towards the SDS. At pH 10 a similar behavior is observed for a solution composition of 50/50 mol ratio dodecyl betaine/SDS. This strong partitioning in favor of the SDS at high pH is attributed to the strong ion-dipole attraction between the SDS sulfate and the PEI imine groups. At pH 3, where the electrostatic interactions between the surfactant and the PEI are dominant, the dodecyl betaine more effectively competes with the SDS for the interface, and the surface composition is much closer to the solution composition.     

Manipulation of the adsorption of ionic surfactants onto hydrophilic silica using polyelectrolytes

This paper demonstrates the use of polyelectrolytes to modify and manipulate the adsorption of ionic surfactants onto the hydrophilic surface of silica. We have demonstrated that the cationic polyelectrolyte poly(dimethyl diallylammonium chloride), poly-dmdaac, modifies the adsorption of cationic and anionic surfactants to the hydrophilic surface of silica. A thin robust polymer layer is adsorbed from a dilute polymer/surfactant solution. The resulting surface layer is cationic and changes the relative affinity of the cationic surfactant hexadecyl trimethylammonium bromide, C16TAB, and the anionic surfactant sodium dodecyl sulfate, SDS, to adsorb. The adsorption of C16TAB is dramatically reduced. In contrast, strong adsorption of SDS was observed, in situations where SDS would normally have a low affinity for the surface of silica. We have further shown that subsequent adsorption of the anionic polyelectrolyte sodium poly(styrene sulfonate), Na-PSS, onto the poly-dmdaac coated surface results in a change back to an anionic surface and a further change in the relative affinities of the cationic and anionic surfactants for the surface. The relative amounts of C16TAB and SDS adsorption depend on the coverage of the polyelectrolyte, and these preliminary measurements show that this can be manipulated.     

Interactions between Adsorbed Layers of a Low Charge Density Cationic Polyelectrolyte on Mica in the Absence and Presence of Anionic Surfactant

Interactions between two negatively charged mica surfaces across aqueous solutions containing various amounts of a 10% charged cationic polyelectrolyte have been studied. It is found that the mica surface charge is neutralized when the polyelectrolyte is adsorbed from a 10–50 ppm aqueous solution. Consequently no electrostatic double-layer force is observed. Instead an attractive force acts between the surfaces in the distance regime 250–100 Å. We suggest that this attraction is caused by bridging. Additional adsorption takes place when the polyelectrolyte concentration is increased to 100 and 300 ppm, and a long-range repulsion develops. This repulsive force is both of electrostatic and steric origin. The polyelectrolyte layer adsorbed from a 50 ppm solution does not desorb when the polyelectrolyte solution is replaced with an aqueous polyelectrolyte-free solution. Injection of sodium dodecyl sulfate (SDS) into the measuring chamber to a concentration of about 0.01 CMC (8.3 × 10⁻⁵M) does not affect the adsorbed layers or the interaction forces. However, when the SDS concentration is increased to 0.02 CMC (0.166 mM) the adsorbed layer expands dramatically due to adsorption of SDS to the polyelectrolyte chains. The sudden swelling suggests a cooperative adsorption of SDS to the preadsorbed polyelectrolyte layer and that the critical aggregation concentration between the polyelectrolyte and SDS at the surface is about 0.02 CMC. The flocculation behavior of the polyelectrolyte in solution upon addition of SDS was also examined. It was found that 0.16–0.32 mol SDS/mol charged segments on the polyelectrolyte is enough to make the solution slightly turbid.     

Adsorption behavior of statherin and a statherin peptide onto hydroxyapatite and silica surfaces by in situ ellipsometry

The salivary protein statherin is known to adsorb selectively onto hydroxyapatite (HA), which constitutes the main mineral of the tooth enamel. This adsorption is believed to be crucial for its function as an inhibitor of primary (spontaneous) and secondary (crystal growth) precipitation of calcium phosphate salts present in saliva. A fragment corresponding to the first 21 N-terminus amino acids of statherin (StN21) was previously found to reduce the rate of demineralization of HA. Therefore, the interfacial properties of this peptide and statherin onto silica, hydrophobized silica and HA discs was studied by in situ ellipsometry. Their reversibility induced by dilution and elutability induced by buffer and sodium dodecyl sulfate (SDS) was also determined. The results revealed that statherin adsorbed at a greater extent onto the HA as compared to StN21, suggesting that the hydrogen bonding between the uncharged polar residues at the C-terminal region of statherin and HA contributes to its adsorption. However, on both silica surfaces the peptide adsorption appeared to proceed in a similar way. Onto the hydrophobized silica the adsorption of both peptides was suggested to occur either via multilayer formation or adsorption of aggregates from solution, while onto the hydrophilic silica adsorption of peptide aggregates from solution was the suggested mechanism. Further, both peptides were observed to be strongly adsorbed onto HA, even after SDS treatment, in comparison to the layers adsorbed onto hydrophobized silica. Both peptide layers were found to be weakly adsorbed onto the hydrophilic silica surface as they were totally removed by buffer dilution.     

Interaction of Sodium Dodecyl Sulfate with Poly(ethyleneimine) in Bulk Solution and at the Air-Solution Interface

Interaction of sodium dodecyl sulfate (SDS) with the cationic polyelectrolyte poly(ethyleneimine) (PEI) was investigated in this study. Turbidity measurements were performed in order to analyze the interaction and complex formation in bulk solution as a function of polymer concentration and pH. Surface tension measurements were made to investigate the properties of SDS/PEI/water mixtures at air/solution interface. Results revealed that SDS/PEI complexes form in solution depending on the surfactant and polymer concentration. A decrease was observed in surface tension values in the presence of SDS/PEI mixtures compared to the values of pure SDS solutions. Both solution and interfacial properties exhibited pH dependent behavior. A shift was seen in the critical micelle concentration of SDS solutions as a function of PEI concentration and solution pH. Monovalent and divalent salt additions showed some influence on the interfacial properties of SDS solutions in the presence of PEI.     

Poly(acrylic acid)-poly(ethylene glycol) layers on positively charged surface coatings: molecular structure, protein resistance, and application to single protein deposition

A new copolymer (PAA-PEG2000) has been designed, consisting of a negatively charged poly(acrylic acid) (PAA) backbone to which poly(ethylene glycol) (PEG) side chains with a molecular weight of about 2 kDa were grafted in a molecular ratio of 3:10. It readily adsorbs to positively charged surfaces and may be considered to be the anionic counterpart of PEG-grafted poly(l-lysine) (PLL-PEG), which was first described by Kenausis et al. and is widely used to render negatively charged surfaces protein-resistant. The synthesis of PAA-PEG2000 can be carried out in aqueous solution at room temperature and does not require any sophisticated techniques such as handling in an inert gas atmosphere. Using ellipsometry and infrared reflection absorption spectroscopy (IRRAS), the film structure has been carefully analyzed for copolymer adsorption onto three different positively charged surfaces, namely, thin layers of poly(allylamine) (PAH), poly(ethyleneimine) (PEI) and (3-aminopropyl)triethoxysilane (APTES). Besides the film thickness, the conformation of the PEG chains and their orientation with respect to the surface normal appear to be important parameters for the protein resistance of the films. Although PAA-PEG2000 adsorbed to PAH and PEI renders the surfaces inert, only partial protein resistance has been observed if the copolymer is deposited on APTES. In a model application, we have generated heterogeneous surfaces composed of isolated small Au nanoparticles (AuNP's) embedded in a protein-resistant layer of PAA-PEG2000 and demonstrated that the AuNP's can serve as adsorption sites for single protein species. In the future, these nanopatterned surfaces may be used for the investigation of isolated proteins.     

Cationic amphiphilic polyelectrolytes and oppositely charged surfactants at the silica-aqueous interface

The influence of sodium dodecyl sulfate (SDS) on the interfacial behavior of two amphiphilic polyelectrolytes, which are copolymers of the cationic monomers triethyl(vinylbenzyl)ammonium chloride and dimethyldodecyl(vinylbenzyl)ammonium chloride, at the silica-aqueous interface was studied. The fraction of amphiphilic monomers was varied, where 0DT, 40DT, and 80DT contained 0, 40, and 80 mol % monomers with dodecyl side chains, respectively. We used in situ ellipsometry to follow the kinetics of adsorption, in terms of adsorbed amount and adsorbed layer thickness, as well as the response of the adsorbed layers to changes in ionic strength and surfactant concentration. Different results were obtained when surfactant was added to the preadsorbed layers compared to the cases when complexes were preformed in the solution prior to the adsorption. In the whole range of concentrations studied, SDS interacts with 40DT and 80DT noncooperatively, whereas for 0DT cooperativity of binding is observed. The amount adsorbed increased significantly as the SDS concentration was close to the cmc. At high SDS concentrations, a lowering of the layer density was observed. For the amphiphilic polyelectrolytes, 40 DT and 80DT, no desorption from the interface was detected for the range of SDS concentrations studied, while 0DT features a maximum in adsorbed amount at concentrations close to the cmc of SDS. Adsorption of 40DT and 80DT from their mixtures with SDS is found to be path dependent with respect to the variation in SDS concentration, where the reversibility decreases with increasing SDS concentration above the expected charge neutralization point. The coadsorption of 80DT and SDS is highly irreversible with respect to changes in the ionic strength within the time scale of the experiment. In this study, we attempt to illustrate both general mechanisms and specific effects. With regard to the general behavior, it is important to note the charge regulation of both the silica surfaces and the polyion/surfactant complexes; an interplay between the two charge-regulating effects is the key to understanding our observations.     

In situ kinetics of layer-by-layer assembled nonlinear-optical-active amphiphiles from dynamic surface force measurements

We report the synthesis and layer-by-layer (LBL) deposition of a class of azo-benzene surfactants with the polycation poly(ethylenimine) (PEI). The different surfactants of the type X-azo-(CH2)10-SO3-, where X = -NO2, -CN, and -COCH3 in the azo-benzene moiety, have decreasing electron-withdrawing strengths. We use dynamic surface force measurements to study the in situ kinetics of adsorption of the amphiphiles onto PEI. Ex situ kinetics data obtained by adsorption-paused UV-visible spectroscopy validate the surface force results. These measurements describe the first application of dynamic force measurements to follow adsorption in LBL systems. UV-visible spectroscopy, second harmonic generation (SHG), and single-wavelength ellipsometry were also used to characterize the films. The observed blue shift upon adsorption of the amphiphiles suggests H-type aggregation within the multilayer. Two of the surfactants studied within the LBL films follow Langmuir adsorption behavior with equilibrium adsorption times under 200 s. The SHG results are consistent with the expected trends in the hyperpolarizabilities of the amphiphiles.     

Film-forming microgels for pH-triggered capture and release

The pH-responsive behavior for a series of lightly cross-linked, sterically stabilized poly(tertiary amine methacrylate)-based latexes adsorbed onto mica and silica was investigated using in situ tapping mode AFM at room temperature. The adsorbed layer structure was primarily determined by the glass transition temperature, T(g), of the latex: poly[2-(diethylamino)ethyl methacrylate]-based particles coalesced to form relatively featureless uniform thin films, whereas the higher T(g) poly[2-(diisopropylamino)ethyl methacrylate] latexes retained their original particulate character. Adsorption was enhanced by using a cationic poly[2-(dimethylamino)ethyl methacrylate] steric stabilizer, rather than a nonionic poly(ethylene glycol)-based stabilizer, since the former led to stronger electrostatic binding to the oppositely charged substrate. Both types of adsorbed latexes acquired cationic microgel character and swelled appreciably at low pH, even those that had coalesced to form films. Fluorescence spectroscopy was used to study the capture of a model hydrophobic probe, pyrene, by these adsorbed latex layers followed by its subsequent release by lowering the solution pH. The repeated capture and release of pyrene through several pH cycles was also demonstrated. Since these poly(tertiary amine methacrylate) latexes are readily prepared by aqueous emulsion polymerization and adsorption occurs spontaneously from aqueous solution, this may constitute an attractive route for the surface modification of silica, mica and other oxides.