Assembly modulates dissociation: electrokinetic experiments reveal peculiarities of the charge formation at monolayer films

Self-assembled monomolecular films of alkanethiols chemisorbed on gold were used as a two-dimensional model system to study the effects of spatial confinement of surface functional groups on their acid-base behaviour; reduction of the surface charge density by mixed assemblies revealed an increase of the acidity of carboxy groups; surfaces with low amounts of charged groups show the interplay of dissociation of functional groups and preferential adsorption of ions in terms of surface charge formation.     

Thermal expansion and Grüneisen parameters of Ba(Fe(1-x)Co(x))2As2: a thermodynamic quest for quantum criticality

Thermal expansion data are used to study the uniaxial pressure dependence of the electronic-magnetic entropy of Ba(Fe(1-x)Co(x))2As2. Uniaxial pressure is found to be proportional to doping and, thus, also an appropriate tuning parameter in this system. Many of the features predicted to occur for a pressure-tuned quantum critical system, in which superconductivity is an emergent phase hiding the critical point, are observed. The electronic-magnetic Grüneisen parameters associated with the spin-density wave and superconducting transitions further demonstrate an intimate connection between both ordering phenomena.     

Interactions between multivalent ions and exponentially growing multilayers: dissolution and exchange processes

The interactions between multivalent ions (small ions or polyelectrolytes) and two exponentially growing polyelectrolyte multilayers, namely, (HA-PLL)(n)() and (HA-PAH)(n)() films, are investigated (HA = hyaluronic acid, PLL = poly-l-lysine, PAH = poly(allylamine)). Ferrocyanide and ferricyanide ions are used as small ion probes. The most striking finding is that, even though these two ions differ only by one charge unit, the ferrocyanide ions induce a partial dissolution of both multilayers whereas these films remain stable in the presence of ferricyanide ions. The dissolution process of (HA-PLL)(n)() films is more rapid than that of (HA-PAH)(n)() films, indicating a stronger interaction between HA and PAH compared to HA and PLL. This is confirmed by polyelectrolyte exchange experiments: when an (HA-PLL)(n)() multilayer film is put into contact with a PAH solution, PLL is quantitatively exchanged with the PAH chains and transformed into an HA-PAH film, whereas an (HA-PAH)(n)() multilayer remains stable in the presence of a PLL solution.     

Hydrogel characteristics of electron-beam-immobilized poly(vinylpyrrolidone) films on poly(ethylene terephthalate) supports

A novel strategy for the preparation of thin hydrogel coatings on top of polymer bulk materials was elaborated for the example of poly(ethylene terephthalate) (PET) surfaces layered with poly(vinylpyrrolidone) (PVP). PVP layers were deposited on PET foils or SiO2 surfaces (silicon wafer or glass coverslips) precoated with PET and subsequently cross-linked by electron beam treatment. The obtained films were characterized by ellipsometry, X-ray photoelectron spectroscopy, infrared spectroscopy in attenuated total reflection, atomic force microscopy (AFM), and electrokinetic measurements. Ellipsometric experiments and AFM force-distance measurements showed that the cross-linked layers swell in aqueous solutions by a factor of about 7. Electrokinetic experiments indicated a strong hydrodynamic shielding of the charge of the underlying PET layer by the hydrogel coatings and further proved that the swollen films were stable against shear stress and variation of pH. In conclusion, electron beam cross-linking ofpreadsorbed hydrophilic polymers permits a durable fixation of swellable polymer networks on polymer supports which can be adapted to materials in a wide variety of shapes.     

Layer-by-layer composite films of polyaniline and vanadium pentoxide sol gels studied by electrochemical surface plasmon spectroscopy

Electroactive multilayer films of polyaniline (PAni) and vanadium pentoxide (V₂O₅) sol-gels were prepared on gold surfaces by means of the layer-by-layer (LBL) method. The stepwise buildup and the optoelectrochemical behaviour of the composite films were investigated in-situ by electrochemical surface plasmon spectroscopy (ESPS). A thickness increment of 2.3 nm per deposited bilayer was obtained by modelling the surface plasmon spectra. The composite multilayer films display charging-discharging characteristics similar to a bare vanadium pentoxide xerogel network along with an optoelectrochemical response which is dominated by the conducting polymer component. Electrochemical impedance spectroscopy (EIS) experiments confirm the findings from ESPS with regard to the presence of different conductive states of the composite.     

Importance of in-plane anisotropy in the quasi-two-dimensional antiferromagnet BaNi2V2O8

The phase diagram of the quasi-two-dimensional antiferromagnet BaNi(2)V(2)O(8) is studied by specific heat, thermal expansion, magnetostriction, and magnetization for magnetic fields applied perpendicular to c. At micro(o)H* approximately 1.5 T, a crossover to a high-field state, where T(N)(H) increases linearly, arises from a competition of intrinsic and field-induced in-plane anisotropies. The pressure dependences of T(N) and H* are interpreted using the picture of a pressure-induced in-plane anisotropy. Even at zero field and ambient pressure, in-plane anisotropy cannot be neglected, which implies deviations from pure Berezinskii-Kosterlitz-Thouless behavior.     

Light-induced structural changes in sodiumnitroprusside (Na2(Fe(CN)5NO)·2D2O) at 80 K

Groundstate and electronic excited state (MS_I) of deuterated sodiumnitroprusside (Na₂(Fe(CN)₅NO)·2D₂O) have been investigated by neutron diffraction as well as by optical and Mössbauer techniques. Significant structural changes occur predominantly in the O–N–Fe–C-bond. It has been shown, that the N–O bond-length is not the order parameter, as expected from other studies. We found an increase in the bond lengths Fe–N4 of 0.019(2) Å and N–O of 0.004(4) Å respectively, which is in qualitative agreement with changes determined by Raman spectroscopy and predictions based on diatomic correlations (Badger/Herschbach/Laurie). Additionally we observed a change in the Fe–C1 bond length of 0.012(3) Å in agreement with Raman meaurements.