Monika Schönhoff

Self-assembled polyelectrolyte multilayers

Recent developments in the field of self-assembly of polyelectrolyte multilayers are presented and discussed. Fundamental studies of multilayers on planar or colloidal supports have in the past few years advanced from structural studies towards questions of the internal composition, dynamics, hydration, and interactions in multilayers. This is accompanied by new theoretical concepts and models for the multilayer assembly. Furthermore, in the past years the development of coating procedures onto colloidal templates instead of planar substrates has had a large impact on the field, leading either to the formation of new types of nanostructures, such as hollow shells with large application potential or colloid coating offered additional possibilities for fundamental multilayer studies involving volume methods. Finally, permeability studies are today an essential tool in providing an understanding of transport processes in these semi-permeable membranes, which are essential for applications such as drug encapsulation or membrane separation.

Layered polyelectrolyte complexes: physics of formation and molecular properties

Multilayers of charged polymers can be formed by the alternating adsorption of polyanions and polycations. By this so-called layer-by-layer self-assembly method, planar surfaces or colloidal templates can be coated. This method is reviewed here with respect to the basic physical principles governing multilayer formation. Particular emphasis is put on the adsorption process of a single polyelectrolyte layer to a multilayer surface as the process controlling the charge complexation and the local molecular structure. Then, the implications for the properties of the multilayer assembly are discussed. In particular, molecular properties such as the internal stoichiometry, the local interactions, and the conformation on a molecular scale are reviewed.Multilayers of charged polymers can be formed by the alternating adsorption of polyanions and polycations. By this so-called layer-by-layer self-assembly method, planar surfaces or colloidal templates can be coated. This method is reviewed here with respect to the basic physical principles governing multilayer formation. Particular emphasis is put on the adsorption process of a single polyelectrolyte layer to a multilayer surface as the process controlling the charge complexation and the local molecular structure. Then, the implications for the properties of the multilayer assembly are discussed. In particular, molecular properties such as the internal stoichiometry, the local interactions, and the conformation on a molecular scale are reviewed.

Enhanced lithium transference numbers in ionic liquid electrolytes.

Ion transport processes in mixtures of N-butyl- N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide (BMP-TFSI) and lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) were characterized by ac impedance spectroscopy and pulsed field gradient NMR. Molar ratios x = n Li-TFSI/( n Li-TFSI + n BMP-TFSI) up to 0.377 could be achieved without crystallization. From the bulk ionic conductivity and the individual diffusion coefficients of cations and anions we calculate the Haven ratio and the apparent lithium transference number. Although the Haven ratio exhibits typical values for ionic liquid electrolytes, the maximal apparent lithium transference number is higher than found in other recent studies on ionic liquid electrolytes containing lithium ions. On the basis of these results we discuss strategies for further improving the lithium transference number of such electrolytes.

The influence of a viscous fluid on the vibration dynamics of scanning near-field optical microscopy fiber probes and atomic force microscopy cantilevers

The influence of a viscous fluid on the dynamic behavior of a vibrating scanning near-field optical microscopy fiber tip is investigated both theoretically and experimentally. A continuum mechanical description of a cylindric cantilever is used to calculate the resonance frequencies and the widths of the resonance bands. The linearized Naviers–Stokes equations are analytically solved and describe the interaction of the beam with the viscous fluid. The contribution of the liquid to the shift and the broadening of the resonance lines is summarized by two constants that can be derived from a master function and the kinetic Reynolds number. The theoretical values are compared with experimental data collected from an optical fiber which is used as a probe in a scanning near-field microscope. Agreement, with a relative error of less than 1%, is achieved. The theory is further developed for the application to atomic force microscopy cantilevers with a rectangular cross section. Experimental data taken from literature are in good agreement with the theory.

1H NMR of thermoreversible polymers in solution and at interfaces: the influence of charged groups on the phase transition.

The phase transition of thermoreversible polymers occurring at the lower critical solution temperature (LCST) is investigated by 1H NMR. Poly-N-isopropylacrylamide (PNIPAM) shows such a coil to globule transition at 32°C in aqueous solution. To study the effect of charged polymer segments on the phase transition, the temperature dependent properties of PNIPAM and of a charged PNIPAM-copolymer, containing 10% carboxylic groups, are investigated in solution. Experiments are performed by 1H spectra and PFG-NMR diffusion measurements at different polymer concentrations. The 1H liquid signal is sharply decreasing at the phase transition temperature. The transition is found to be equally sharp for the copolymer as for the homopolymer at concentrations below and around the overlap concentration, whereas the transition is broadened at higher concentrations. Diffusion measurements prove that the conformation of the polymer coils is maintained with increasing temperature until close to the phase transition, apart from a minor decrease of the hydrodynamic radius at about 2°C below the LCST. All data indicate identical phase transition properties of the copolymer as compared with the homopolymer. The introduction of charged groups (3% of monomers dissociated) has thus not altered the transition. Therefore, the copolymer is a suitable candidate for exhibiting a phase transition under electrostatic coupling conditions in layers. Both polymers are adsorbed to colloidal silica (Cab-O-Sil) and investigated by 1H NMR in order to monitor the phase transition in the restricted geometry of an adsorption layer. The liquid 1H intensities of both polymers are decreasing with temperature, this is interpreted as a phase transition of the loops and tails. The transition is substantially broader than in solution, especially at low surface coverage. Significant differences between the copolymer and the homopolymer are observed, since above the transition temperature a liquid signal from loops and tails of the copolymer is still observed. This is interpreted as a comparatively mobile arrangement of the copolymer layer, arising from electrostatic repulsion from the surface and between polymer segments, which is partly hindering globule formation.

Unconventional Layer‐by‐Layer Assembly: Surface Molecular Imprinting and Its Applications

Layer-by-layer assembly (LbL) is a rich, versatile, and powerful technique for fabricating multilayer thin films with controlled architecture and functions. Singly charged, uncharged, or water-repellent molecules cannot be used directly in conventional LbL assembly. This problem can be solved with unconventional LbL methods, by employing the preassembly of building blocks in solution and the use of these assemblies for LbL formation at the interface. This Concept summarizes different methods of unconventional LbL assembly, including electrostatic complex formation, hydrogen-bonded complexes, block-copolymer micelles, and π-π interaction complexes. These preassembly treatments endow the building blocks with enhanced abilities for advanced functionality, in particular, surface molecular imprinting, a new concept emerging from unconventional LbL. Molecular imprinting approaches are thus conceptually described based on different types of interactions and their great potential in applications is demonstrated by examples such as selective surface patterning and selective filtration.

Melting Behavior and Ionic Conductivity in Hydrophobic Ionic Liquids

Four room-temperature ionic liquids (RTILs) based on the N-butyl-N-methyl pyrrolidinium (Pyr(14)(+)) and N-methyl-N-propyl pyrrolidinium cations (Pyr(13)(+)) and bis(trifluoromethanesulfonyl)imide (TFSI(-)) and bis(fluorosulfonyl)imide (FSI(-)) anions were intensively investigated during their melting. The diffusion coefficients of (1)H and (19)F were determined using pulsed field gradient (PFG) NMR to study the dynamics of the cations, anions, and ion pairs. The AC conductivities were measured to detect only the motion of the charged particles. The melting points of these ionic liquids were measured by DSC and verified by the temperature-dependent full width at half-maximum (FWHM) of the (1)H and (19)F NMR peaks. The diffusion and conductivity data at low temperatures gave information about the dynamics at the melting point and allowed specifying the way of melting. In addition, the diffusion coefficients of (1)H (D(H)) and (19)F (D(F)) and conductivity were correlated using the Nernst-Einstein equation with respect to the existence of ion pairs. Our results show that in dependence on the cation different melting behaviors were identified. In the Pyr(14)-based ILs, ion pairs exist, which collapse above the melting point of the sample. This is in contrast to the Pyr(13)-based ILs where the present ion pairs in the crystal dissociate during the melting. Furthermore, the anions do not influence the melting behavior of the investigated Pyr(14) systems but affect the Pyr(13) ILs. This becomes apparent in species with a higher mobility during the breakup of the crystalline IL.

Diffusion exchange NMR spectroscopic study of dextran exchange through polyelectrolyte multilayer capsules

Diffusion exchange of dextran with molecular weights 4.4 and 77 kDa through polyelectrolyte multilayer (PEM) hollow capsules consisting of four bilayers of polystyrene sulfonate/polydiallyldimethylammonium chloride has been investigated using two-dimensional nuclear-magnetic-resonance methods: diffusion-diffusion exchange spectroscopy (DEXSY) and diffusion-relaxation correlation spectroscopy (DRCOSY). Results obtained in DRCOSY experiments show that the diffusion process of dextran 77 kDa exhibits an observation time dependence suggesting a diffusion behavior restricted by confinement. We find evidence for both single capsule and capsule aggregate states, with a partitioning of the 77-kDa dextran between the free and capsule states much larger than that suggested by volume fraction alone. Results from DEXSY experiments show that dextran 77 kDa is in diffusive exchange through the capsules with an exchange time of around 1 s. In contrast, the capsules have no detectable influence on the diffusion process of the dextran 4.4 kDa. This quantitative information may be used in designing PEM capsules as drug carriers.

Photoinduced optical anisotropy in organic molecular films controlled by an electric field

Abstract The photoinduced reorientation of dye molecules in molecular films on solid substrates has been controlled with external dc electric fields, leading to solid state structures, which are macroscopically polar, temporally stable at room temperature, and well-defined at the molecular level. A simulation based on the excitation-driven rotational diffusion of the molecules in the potential of their neighbors yielded a realistic model and indicated a collective character of the process. This shows that we are close to a quantitative comprehension of the molecular interactions within these films.

Surface Molecular Imprinting in Layer-by-Layer films on Silica Particles

An improvement to molecular imprinting in polymers, where bulk systems often suffer from slow dynamics of release and uptake, is the formation of thin films with imprinting sites that are more rapid to access by guest molecules. Based on our previous development of surface molecular imprinting layer-by-layer (LbL) films (SMILbL), the present paper presents selective imprinted sites in a surface film on dispersed silica particles, thus designing a SMILbL system with maximized active area and in addition allowing studies with bulk techniques. The multilayer is designed to include the template during the LbL buildup and to form a cross-linked network upon UV-irradiation for enhanced stability. A theophylline moiety is grafted to poly(acrylic acid) as the template, while a UV-sensitive diazo polycation cross-links the polymers after irradiation. Electrophoretic measurements prove the successful buildup of the multilayers by an alternating sign of the zeta potential. Template release is achieved by cleavage of the grafted template. The released amount of template is quantified in solution by (1)H NMR spectra and is in good agreement with the prediction from surface coverage calculations. Rebinding studies of template to the now empty imprinted binding sites show a high affinity for a theophylline derivative with a rebound amount on the order of the original template content. In contrast to theophylline, caffeine with a very similar chemical structure-only differing in one functional group-shows very different binding properties due to a thiol moiety in the binding site. Thus, a particle system with very selective molecular imprinting sites is demonstrated.