Impact of hydrophilic/hydrophobic surface chemistry on hydration forces in the absence of confinement.

The oscillatory force profile, observed in liquids due to molecular ordering at interfaces, has been extensively investigated by means of atomic force microscopy, but it remains unclear whether molecular ordering is present at the tip apex. Using a displacement-sensitive, low-noise atomic force microscope (AFM) operated in dynamic mode, with a tip of radius < 1 nm, we have investigated the force profile between two approaching surfaces of the same or different hydrophilic and hydrophobic character. By directly comparing different surface chemistry interactions, we have been able to elucidate whether an oscillatory force profile is due to structured water layers adjacent to the surface, the tip, or a combination of the two. We have found that an oscillatory force profile is observed when the surface is hydrophilic in nature, irrespective of whether the tip is hydrophilic or hydrophobic. When the surface is hydrophobic, an oscillatory force profile is not measured, but rather a monotonic repulsive or a short-range attractive force is observed for interactions with a hydrophilic or a hydrophobic tip, respectively. Thus, we attribute the measurement of an oscillatory force profile, in the absence of lateral confinement effects, solely to water layers adjacent to a hydrophilic surface rather than the structuring of water at the tip apex. This is the first direct evidence that solvation forces occur solely as a result of water layers adjacent to the substrate.

The morphology of decorated amyloid fibers is controlled by the conformation and position of the displayed protein.

Self-assembled structures capable of mediating electron transfer are an attractive scientific and technological goal. Therefore, systematic variants of SH3-Cytochrome b(562) fusion proteins were designed to make amyloid fibers displaying heme-b(562) electron transfer complexes. TEM and AFM data show that fiber morphology responds systematically to placement of b(562) within the fusion proteins. UV-vis spectroscopy shows that, for the fusion proteins under test, only half the fiber-borne b(562) binds heme with high affinity. Cofactor binding also improves the AFM imaging properties and changes the fiber morphology through changes in cytochrome conformation. Systematic observations and measurements of fiber geometry suggest that longitudinal registry of subfilaments within the fiber, mediated by the interaction and conformation of the displayed proteins and their interaction with surfaces, gives rise to the observed morphologies, including defects and kinks. Of most interest is the role of small molecule modulation of fiber structure and mechanical stability. A minimum complexity model is proposed to capture and explain the fiber morphology in the light of these results. Understanding the complex interplay between these factors will enable a fiber design that supports longitudinal electron transfer.

Direct imaging of salt effects on lipid bilayer ordering at sub-molecular resolution.

The interactions of salts with lipid bilayers are known to alter the properties of membranes and therefore influence their structure and dynamics. Sodium and calcium cations penetrate deeply into the headgroup region and bind to the lipids, whereas potassium ions only loosely associate with lipid molecules and mostly remain outside of the headgroup region. We investigated a dipalmitoylphosphatidylcholine (DPPC) bilayer in the gel phase in the presence of all three cations with a concentration of Ca²+ ions an order of magnitude smaller than the Na+ and K+ ions. Our findings indicate that the area per unit cell does not significantly change in these three salt solutions. However the lipid molecules do re-order non-isotropically under the influence of the three different cations. We attribute this reordering to a change in the highly directional intermolecular interactions caused by a variation in the dipole-dipole bonding arising from a tilt of the headgroup out of the membrane plane. Measurements in different NaCl concentrations also show a non-isotropic re-ordering of the lipid molecules.

The influence of polymer structure and morphology on talc wettability.

Advancing water contact angles were measured on freshly cleaved talc faces as well as on talc particles. The intrinsic hydrophobicity of talc was shown to be due to the dominance of the apolar components of the work of adhesion. Polyacrylamides and polysaccharides adsorb onto the surface of talc, displaying strikingly different morphologies. Adsorbed amount, apparent layer thickness, and polymer structure control talc wettability.

Morphology of adsorbed polymers and solid surface wettability.

The adsorption of a polyacrylamide (MW 14600) and two polysaccharides (MW 9260 and 706 x 10(3)) onto model silica surfaces of different hydrophobicities was investigated. In all cases, adsorption adhered to the Freundlich isotherm, reflecting the heterogeneous character of the solid substrates. The latter strongly influenced the character of the adsorbed polymer, with morphologies from chainlike structures to thin films and patches being observed. Surface roughness, polymer type, and molecular weight also play roles in controlling adsorbed polymer morphology. Surface wettability is strongly influenced by the thickness of the adsorbed layer.

A new family of water-swellable microgel particles.

In this study a new family of microgel particles is investigated which contain methylmethacrylate (MMA), ethylacrylate (EA), acrylic acid (AA), glycerol propoxytriacrylate (GPTA), and Emulsogen (Em). GPTA is a trifunctional crosslinking monomer, whereas Em is a polymerisable alcohol ethoxylate surfactant. TEM and PCS data reveal that the extent of microgel swelling originates from a pH-independent contribution (due to Em) as well as a pH-dependent contribution (due to AA). The major contribution to swelling comes from pH-independent swelling. Consideration of the equations governing particle swelling allows the effective pK(a) of the incorporated AA groups to be estimated. There is evidence of a shift of the pK(a) for the AA groups from 4.5 to ca. 9.5 when the microgel particles containing AA also contain Em. This suggests intraparticle hydrogen bonding between AA and ethylene oxide segments at low pH.