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Typically, laterally patterned films are fabricated by lithographic techniques, external fields, or di-block copolymer self-assembly. We investigate the self-patterning of polyelectrolyte multilayers, poly(diallyldimethylammonium) (PDADMA)/poly(styrenesulfonate) (PSS)short. The low PSS molecular weight (Mw(PSSshort) = 10.7 kDa) is necessary because PSSshort is somewhat mobile within a PDADMA/PSSshort film, as demonstrated by the exponential growth regime at the beginning of the PDADMA/PSSshort multilayer build-up. No self-patterning was observed when the PDADMA/PSS film consisted of only immobile polyelectrolytes. Atomic force microscopy images show that self-patterning begins when the film consists of seven deposited PDADMA/PSSshort bilayers. When more bilayers are added, the surface ribbing evolved into bands, and circular domains were finally observed. The mean distance between the surface structures increased monotonously with the film thickness, from 70 to 250 nm. Scanning electron microscopy images showed that exposure to vacuum resulted in thinning of the film and an increase in the mean distance between domains. The effect is weaker for PSSshort-terminated films than for PDADMA-terminated films. The mechanism leading to domain formation during film build-up and the effect of post-preparation treatment are discussed.

Surface forces are used to investigate the polymer conformation and the surface charge of polyelectrolyte multilayers. Films are prepared from strong polyelectrolytes with low and high linear charge density at 0.1 M NaCl, namely poly(diallyldimethylammonium) (PDADMA) and poly(styrenesulfonate) (PSS). The multilayer has two growth regimes: in the beginning, the film can contain as many positive as negative monomers. After about 15 deposited layer pairs, a linear growth regime characterized by an excess of cationic PDADMA monomers occurs. Independent of the film composition, at preparation conditions, the film surface is flat, uncharged and partially hydrophobic. Surface force measurements at decreased ionic strength provide insight. For PSS-terminated films electrostatic forces are found. At the beginning of multilayer formation, the surface charge density is negative. However, in the linear growth regime it is positive and low (one charge per 200-400 nm

Polystyrene sulfonate (PSS) of different molecular weight M(w) is adsorbed to oppositely charged DODAB monolayers from dilute solutions (0.01 mmol/L). PSS adsorbs flatly in a lamellar manner, as is shown by X-ray reflectivity and grazing incidence diffraction (exception: PSS with M(w) below 7 kDa adsorbs flatly disordered to the liquid expanded phase). The surface coverage and the separation of the PSS chains are independent of PSS M(w). On monolayer compression, the surface charge density increases by a factor of 2, and the separation of the PSS chains decreases by the same factor. Isotherms show that on increase of PSS M(w) the transition pressure of the LE/LC (liquid expanded/liquid condensed) phase transition decreases. When the contour length exceeds the persistence length (21 nm), the transition pressure is low and constant. For low-M(w) PSS (7 kDa) the LE/LC transition of the lipids and the disordered/ordered transition of adsorbed PSS occur simultaneously, leading to a maximum in the contour length dependence of the transition enthalpy. These findings show that lipid monolayers at the air/water interface are a suitable model substrate with adjustable surface charge density to study the equilibrium conformation of adsorbed polyelectrolytes as well as their interactions with a model membrane.

Silver nanoparticles (4.5 nm average radius) are used as the first negatively charged layer of a polyelectrolyte multilayer prepared subsequently from poly(allylamine hydrochloride) (PAH) and poly(styrenesulfonate) (PSS) solutions with 1 M KCl. After adsorption of a PAH layer on top of the silver nanoparticle monolayer, particle aggregation occurs, as evidenced by the double peak of the UV-vis absorption spectrum and by the decreased number of the objects found with AFM. On adsorption of the first PSS layer on top of the Ag/PAH sandwich, the nanoparticles remain close as is indicated by a the reduced object count and the changed absorption spectrum. If the PAH layer covering the Ag nanoparticles is adsorbed from salt-free solution, the nanoparticles remain isolated. Apparently, the aggregation is mediated by the PAH adsorbing in coiled conformation. Additionally, UV-vis and X-ray reflectivity evidence is found for lateral yet not vertical nanoparticle movement when polyelectrolytes adsorb, even if the adsorbing coils do not touch the nanoparticles directly.

PEE212-PEO112 diblock copolymer monolayers are studied at the air/water interface. At large molecular areas, with X-ray reflectivity, PEE domains are observed, which are partly immersed into the water. The domain thickness increases on compression (28 to 40 A). With off-specular X-ray reflectivity, an average domain radius of 750 A is found, but there are also smaller domains. Due to these space constraints, most PEO blocks form a brush beneath the PEE domains. Only a few PEO blocks form a corona surrounding the domains and adsorb flatly onto the air/water interface. The PEO desorption transition is observed at the typical pressure of 9 mN/m, when the flatly adsorbed PEO is compressed at a domain fraction of 95%. It occurs at 6 A2/EO monomer, about half the value found for lipopolymers or diblock copolymers with NPEE approximately NPEO or NPEENPEO. Apparently, the thickness of the PEE domains is determined by the forces from the two interfaces, not by the PEO block, for which flat adsorption beneath the domain would be more favorable instead of formation of a PEO brush.

Experiments were carried out on a water-based ferrofluid (gamma-Fe2O3 with carboxydextran shell) using photon correlation spectroscopy (PCS), atomic force microscopy, and magnetic nanoparticle relaxation measurements. The experiments were designed with the aim to relate the Néel signals that are in theory generated by large single core particles with nanoscopic properties, that is, particle size, particle size distribution, shell properties, and aggregation. For this purpose, the ferrofluid was fractionated by magnetic fractionation and size exclusion chromatography. Nanoparticles adsorbed onto positively charged substrates form a two-dimensional monolayer. Their mean core diameters are in the range of 6 to about 20 nm, and particles above 10 nm are mostly aggregates. The hydrodynamic particle diameters are between 13 and 80 nm. The core diameter of the smallest fraction is confirmed by X-ray reflectometry; the surface coverage is controlled by bulk diffusion. Comparison with the hydrodynamic radius yields a shell thickness of 3.8 nm. Considering the shell thickness to be constant for all particles, it was possible to calculate the apparent particle diameter in the original ferrofluid from the PCS signals of all fractions. As expected, the small cores yielded no Néel relaxation signals in freeze-dried samples; however, the fractions containing mostly aggregates yielded Néel relaxation signals.

Amphiphilic block copolymers consisting of a fluid hydrophobic (poly(ethyl ethylene), PEE) and a hydrophilic (poly(ethylene oxide), PEO) block form monolayers at the air/water interface. With X-ray and neutron reflectivity, the density profile of PEE432−PEO484 was investigated. It was found that the polymer adsorption layer consists of a homogeneous PEE and a solubilized PEO block, which can be laterally compressed by a factor of 3. The PEE thickness increases in inverse proportion to the molecular area, and the PEO brush follows the scaling law predicted for a brush in a good solvent. However, the stretching of the two blocks roughens the PEE−PEO interface, causing a transition from a PEO monolayer adsorbed to a hydrophobic interface (0.8−1.1 nm thick) to a PEE−PEO/water interfacial layer of 3 nm thickness. This transition of the interfacial layer highlights the rich phase behavior of amphiphilic block copolymers, which resembles that of lipids and nonionic surfactants.

Polyelectrolyte adsorption onto an oppositely charged interface is determined by electrostatic and secondary interactions. Since polyelectrolytes precipitate at elevated temperatures, the secondary interactions are presumably temperature dependent. This idea is tested for poly(allylamine) hydrochloride/polystyrene sulfonate (PAH/PSS) films adsorbed from aqueous KCl solution (high salt conditions) at temperatures between 5 and 40 °C. KCl was chosen because the films were thicker than those obtained from NaCl or CsCl solutions indicating strong specific binding between K and PSS. The film thickness increases continuously with the adsorption temperature; the changes amount to 20−40%, depending on salt conditions. Furthermore, the roughness is increased, up to a factor of 5. The latter is attributed to the decreased percentage of strong electrostatic bonds within the polyelectrolyte multilayer. Another path to increased roughening is using low-weight polymers with a contour length similar to the thickness of ...

The method of X-ray reflectometry with electron density “labeling” of polyampholytes by heavy counterions has been demonstrated for measuring the adsorption profiles of gelatin at both hydrophobic ...

Colloidal gold adsorbed onto positively charged substrates forms a two-dimensional layer with a maximum coverage of 1 colloid/1000 nm2 as shown by optical absorption spectroscopy, small-angle X-ray scattering, and atomic force microscopy. Positively charged substrates are obtained by either chemisorption of end-group-functionalized silanes or by polycation adsorption. Bulk diffusion controls the gold adsorption; thus lower coverages can also be achieved. The optical spectra not only contain information on single colloids but also on their mutual interactions, as well as on the polymeric and solvent environments. Absorption spectra based on Mie and Maxwell−Garnett theory are calculated. The films remain stable on exchange of the external solvent. The local dielectric environment is shown to not only influence the position of the absorption band but also, more drastically, the intensity (by a factor of 2.5), in agreement with theoretical predictions (±15%). Particle aggregation induced by branched polycatio...

Monolayers of a statistical copolymer with a poly(methacrylate) chain and hydrophilic and hydrophobic side groups are investigated at the air/water interface. The isotherms suggest a fluid and a frozen phase. With in-situ X-ray reflectivity the monolayer thickness is determined to be 2.5 nm or less. The shear viscosity in the fluid phase is extremely high yet can be described in the framework of the free area model. However, the parameter which characterizes the overlap of holes available for a diffusing monomer unit is a factor of 2 higher than expected, suggesting local diffusion barriers formed by nanosized clusters. In the glassy phase single-molecule fluorescence shows anomalous diffusion in an ≈140 nm cage (cf. ref 1), indicating that the clusters touch. The aggregates are mainly formed by the hydrophilic monomers, as both the X-ray data and the isotherms suggest.

We present a computer-controlled technique to measure the distance-dependent forces in the surface forces apparatus. The power of our setup is shown by a measurement of the repulsive forces between mica surfaces immersed in a 0.01 M NaCl solution. At close distances we find an oscillatory force with a periodicity of 0.30 nm, which roughly corresponds to the diameter of a water molecule. For the distance determination we use the standard interferometric technique: the interferometer consists of a medium sandwiched between two mica sheets of equal thickness silvered on the backside. The surface separation is measured by comparing the resonance wavelength to the one obtained from contacting mica sheets. By substitution of the eye by a CCD-camera we improve the reproducibility of the method and we can determine the cavity parameters quantitatively.

Monolayers of the phospholipids 1,2-dipalmitoylphosphatidylethanolamine (DPPE) and 1,2-dipalmitoylphosphatidylcholine (DPPC) are studied at the water/air interface in contact with n-hexadecane, n-dodecane, and bicyclohexyl by null ellipsometry and X-ray reflectivity along the isotherms. From recent diffraction data we derive refractive indices nx and nz to calculate the thickness of the mixed films in the condensed phase by ellipsometry. The thickness is compared with the results from X-ray reflectivity measurements. Both methods reveal that the mixed lipid/hydrocarbon film is not covered by a thin hydrocarbon film as known from short chain alcanes like hexane. The incorporation of the hydrocarbon affects the film thickness by reducing the tilt angle of the lipids. This way DPPC, which due to the large head group does not exhibit a non tilted condensed phase at the water/air interface at all, enlarges its value for nz at the expense of nx, yielding a condensed mixed phase with zero tilt angle. The resulti...

For the first time, the formation and growth of a molecular superstructure within a lipopolymeric monolayer on the water surface is directly observed by small-angle grazing incidence diffraction. Nanoscopic stripes of ordered alkyl chains and solubilized polymer alternate.1 At the onset of the superstructure formation, the smallest stripe periodicity is measured. Then, the periodicity relaxes (up to 170 A), even if the monolayer is further compressed. This threshold behavior is attributed to a very high local pressure necessary for stripe alignment. On solid substrates, the superstructure periodicity is decreased down to 120 A because of dehydration of the polymer moieties.

A shift in plasmon surface resonance is measured for colloidal solutions of gold nanoparticles upon adsorption of polyampholyte (gelatin). It is demonstrated that the shift in wavelength of the absorption maximum can quantitatively yield measurements of the adsorbed amount as well as information about the structure of the adsorbed polymer layer. Semiquantitative agreement is found with calculations based on the Mie theory in the quasi-static limit. The method is demonstrated to provide quantitative measurements of the kinetics of biopolymer adsorption onto colloidal nanoparticles in solution.