Surface Properties of Saponin-Chitosan Mixtures.

The surface properties of saponin and saponin-chitosan mixtures were analysed as a function of their bulk mixing ratio using vibrational sum-frequency generation (SFG), surface tensiometry and dilational rheology measurements. Our experiments show that saponin-chitosan mixtures present some remarkable properties, such as a strong amphiphilicity of the saponin and high dilational viscoelasticity. We believe this points to the presence of chitosan in the adsorption layer, despite its complete lack of surface activity.

Dynamic Wetting of Photoresponsive Arylazopyrazole Monolayers is Controlled by the Molecular Kinetics of the Monolayer.

Smart surfaces that can change their wettability on demand are interesting for applications such as self-cleaning surfaces or lab-on-a-chip devices. We have synthesized arylazopyrazole (AAP) phosphonic acids as a new class of photoswitchable molecules for functionalization of aluminum oxide surfaces. AAP monolayers were deposited on α-AlO(0001) and showed reversible / photoswitching that can trigger contact angle changes of up to ∼10°.

pH effects on the molecular structure and charging state of β-Escin biosurfactants at the air-water interface.

Saponins like β-escin exhibit an unusually high surface activity paired with a remarkable surface rheology which makes them as biosurfactants highly interesting for applications in soft matter colloids and at interfaces. We have applied vibrational sum-frequency generation (SFG) to study β-escin adsorption layers at the air-water interface as a function of electrolyte pH and compare the results from SFG spectroscopy to complementary experiments that have addressed the surface tension and the surface dilational rheology. SFG spectra of β-escin modified air-water interfaces demonstrate that the SFG intensity of OH stretching vibrations from interfacial water molecules is a function of pH and dramatically increases when the pH is increased from acidic to basic conditions and reaches a plateau at a solution pH of > 6.

Ultrafast dynamics of the dipole moment reversal in a polar organic monolayer.

Pyridine layers on Cu(110) possess a strong electric field due to the large dipole of adsorbed pyridine. This electric field is visible as an enhanced sum frequency response from both the copper surface electrons and the aromatic C-H stretch of pyridine via a third order susceptibility. In response to a visible pump pulse, both surface electron and C-H stretch sum frequency signals are reduced on a subpicosecond time scale.

Quantifying Double-Layer Potentials at Liquid-Gas Interfaces from Vibrational Sum-Frequency Generation.

Vibrational sum-frequency generation (SFG) spectroscopy is demonstrated as a fast method to quantify variations of the electric double-layer potential ϕ at liquid-gas interfaces. For this, mixed solutions of nonionic tetraethyleneglycol-monodecylether (CE) and cationic hexadecyltrimethylammonium bromide (CTAB) surfactants were investigated using SFG spectroscopy and a thin-film pressure balance (TFPB). Derjaguin-Landau-Verwey-Overbeek analysis of disjoining pressure isotherms obtained with the TFPB technique provides complementary information on ϕ, which we apply to validate the results from SFG spectroscopy.

Charge-Controlled Surface Properties of Native and Fluorophore-Labeled Bovine Serum Albumin at the Air-Water Interface.

Proteins at interfaces are important for protein formulations and in soft materials such as foam. Here, interfacial stability and physicochemical properties are key elements, which drive macroscopic foam properties through structure-property relations. Native and fluorescein isothiocyanate-labeled bovine serum albumin (BSA) were used to modify air-water interfaces as a function of pH.

Effects of water on low-overpotential CO reduction in ionic liquid studied by sum-frequency generation spectroscopy.

We used vibrational sum-frequency generation spectroscopy (SFG) to investigate low-overpotential CO reduction on a polycrystalline Ag electrode using room temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium tetrafluorborate (EMIM-BF) electrolyte mixtures with 0.3 mol%, 45 mol% and 77 mol% water. Adding water dramatically increases CO reduction efficiency up to 87.

Hot hole-induced dissociation of NO dimers on a copper surface.

We use reflection-absorption infrared spectroscopy (RAIRS) to study the photochemistry of NO on Cu(110) in the UV-visible range. We observe that the only photoactive species of NO on Cu(110) is the NO dimer, which is asymmetrically bound to the surface. RAIRS shows that photoinduced dissociation proceeds via breaking of the weak N-N bond of the dimer, photodesorbing one NO(g) to the gas phase and leaving one NO(ads) adsorbed on the surface in a metastable atop position.