Influence of concentration and temperature on the dynamics of water in the hydrophobic hydration shell of tetramethylurea

We study the influence of the amphipilic compound tetramethylurea (TMU) on the dynamical properties of water, using dielectric relaxation spectroscopy in the regime between 0.2 GHz and 2 THz. This technique is capable of resolving different water species, their relative fractions, and their corresponding reorientation dynamics. We find that the reorientation dynamics of water molecules in the hydration shell of the hydrophobic groups of TMU is between 3 (at low concentrations) and 10 (at higher concentrations) times slower than the dynamics of bulk water. The data indicate that the effect of hydrophobic groups on water is strong but relatively short-ranged. With increasing temperature, the fraction of water contained in the hydrophobic hydration shell decreases, which implies that the overall effect of hydrophobic groups on water becomes smaller.     

Quantitative multiplex CARS spectroscopy in congested spectral regions

A novel procedure is developed to describe and reproduce experimental coherent anti-Stokes Raman scattering (CARS) data, with particular emphasis on highly congested spectral regions. The approach, exemplified here with high-quality multiplex CARS data, makes use of spontaneous Raman scattering results. It is shown that the underlying vibrational Raman response can be retrieved from the multiplex CARS spectra, so that the Raman spectrum can be reconstituted, provided an adequate signal-to-noise ratio (SNR) is present in the experimental data and sufficient a priori knowledge of the vibrational resonances involved exists. The conversion of CARS to Raman data permits a quantitative interpretation of CARS spectra. This novel approach is demonstrated for highly congested multiplex CARS spectra of adenosine mono-, di-, and triphosphate (AMP, ADP, and ATP), nicotinamide adenine dinucleotide (NAD+), and small unilamellar vesicles (SUVs) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Quantitative determination of nucleotide concentrations and composition analysis in mixtures is demonstrated.     

Adsorption and dissociation of NO on stepped Pt (533)

We present an experimental and theoretical investigation of the adsorption, desorption, and dissociation of NO on the stepped Pt (533) surface. By combining temperature programmed desorption and reflection absorption infrared spectroscopy, information about the adsorption sites at different temperatures is obtained. Surprisingly, metastable adsorption structures of NO can be produced through variation of the dosing temperature. We also show that part of the NO molecules adsorbed on the step sites dissociates around 450 K. After dissociation the N atoms can desorb either by combining with an O fragment, or with another N atom, resulting in NO and N(2). The N(2) production can be enhanced by coadsorbing CO on the surface: CO scavenges the oxygen atom, thereby suppressing associative recombinative desorption of N and O atoms. Density functional theory calculations are used to reveal the adsorption energies and vibrational frequencies of adsorbed NO as well as barriers for dissociation of NO and for diffusion of N atoms. The combined experimental results and theoretical calculations reveal that dissociation of NO is the rate limiting step in the formation of N(2).     

Polar properties of hydrothermally synthesized BiFeO3 thin films

Multiferroic bismuth ferrite (BiFeO₃) has attracted considerable attention due to applications related to the bulk photovoltaic effect in which the direction of polarization determines the direction of the photocurrent. Epitaxial thin films are produced by means of techniques that usually require high temperature processes. The hydrothermal method can be seen as an alternative route to obtain highly textured thin films in quantities compatible with batch processing; nevertheless, the structural, dielectric and electric properties are generally affected by the presence of hydrogen and other reaction by-products. In this work, functional and highly textured BiFeO₃ films were successfully produced on metallic SrTiO₃:Nb (0.5 wt.%) (100) substrates via hydrothermal synthesis. X-ray diffraction (XRD) and Atomic Force Microscopy (AFM) were used to analyze the structural properties of the films. Piezoresponse Force Microscopy (PFM) and Photoconductive Atomic Force Microscopy (Pc-AFM) were used to determine their functional properties. We show the polarization switching and confirm the presence of the bulk photovoltaic effect for the first time in hydrothermally synthesized BiFeO₃.     

Ultrafast reorientation of dangling OH groups at the air-water interface using femtosecond vibrational spectroscopy

We report the real-time measurement of the ultrafast reorientational motion of water molecules at the water-air interface, using femtosecond time- and polarization-resolved vibrational sum-frequency spectroscopy. Vibrational excitation of dangling OH bonds along a specific polarization axis induces a transient anisotropy that decays due to the reorientation of vibrationally excited OH groups. The reorientation of interfacial water is shown to occur on subpicosecond time scales, several times faster than in the bulk, which can be attributed to the lower degree of hydrogen bond coordination at the interface. Molecular dynamics simulations of interfacial water dynamics are in quantitative agreement with experimental observations and show that, unlike in bulk, the interfacial reorientation occurs in a largely diffusive manner.     

Morphology and persistence length of amyloid fibrils are correlated to peptide molecular structure

The formation of amyloid fibrils is a self-assembly process of peptides or proteins. The superior mechanical properties of these fibrils make them interesting for materials science but constitute a problem in amyloid-related diseases. Amyloid structures tend to be polymorphic, and their structure depends on growth conditions. To understand and control the assembly process, insights into the relation between the mechanical properties and molecular structure are essential. We prepared long, straight as well as short, worm-like β-lactoglobulin amyloid fibrils and determined their morphology and persistence length by atomic force microscopy (AFM) and the molecular conformation using vibrational sum-frequency generation (VSFG) spectroscopy. We show that long fibrils with near-100% β-sheet content have a 40-times higher persistence length than short, worm-like fibrils with β-sheet contents below 80%.     

Vibrational response of hydrogen-bonded interfacial water is dominated by intramolecular coupling

Using the surface-specific vibrational technique of vibrational sum-frequency generation, we reveal that the double-peaked structure in the vibrational spectrum of hydrogen-bonded interfacial water molecules originates from vibrational coupling between the stretch and bending overtone, rather than from structural effects. This is demonstrated by isotopic dilution experiments, which reveal a smooth transition from two peaks to one peak, as D2O is converted into HDO. Our results show that the water interface is structurally more homogeneous than previously thought.     

Nanoscale Distribution of Sulfonic Acid Groups Determines Structure and Binding of Water in Nafion Membranes

The connection between the nanoscale structure of two chemically equivalent, yet morphologically distinct Nafion fuel‐cell membranes and their macroscopic chemical properties is demonstrated. Quantification of the chemical interactions between water and Nafion reveals that extruded membranes have smaller water channels with a reduced sulfonic acid head group density compared to dispersion‐cast membranes. As a result, a disproportionally large amount of non‐bulk water molecules exists in extruded membranes, which also exhibit larger proton conductivity and larger water mobility compared to cast membranes. The differences in the physicochemical properties of the membranes, that is, the chemical constitution of the water channels and the local water structure, and the accompanying differences in macroscopic water and proton transport suggest that the chemistry of nanoscale channels is an important, yet largely overlooked parameter that influences the functionality of fuel‐cell membranes.