In situ nano- to microscopic imaging and growth mechanism of electrochemical dissolution (e.g., corrosion) of a confined metal surface.

Reactivity in confinement is central to a wide range of applications and systems, yet it is notoriously difficult to probe reactions in confined spaces in real time. Using a modified electrochemical surface forces apparatus (EC-SFA) on confined metallic surfaces, we observe in situ nano- to microscale dissolution and pit formation (qualitatively similar to previous observation on nonmetallic surfaces, e.g., silica) in well-defined geometries in environments relevant to corrosion processes. We follow "crevice corrosion" processes in real time in different pH-neutral NaCl solutions and applied surface potentials of nickel (vs. Ag|AgCl electrode in solution) for the mica-nickel confined interface of total area ∼0.03 mm2 The initial corrosion proceeds as self-catalyzed pitting, visualized by the sudden appearance of circular pits with uniform diameters of 6-7 µm and depth ∼2-3 nm. At concentrations above 10 mM NaCl, pitting is initiated at the outer rim of the confined zone, while below 10 mM NaCl, pitting is initiated inside the confined zone. We compare statistical analysis of growth kinetics and shape evolution of individual nanoscale deep pits with estimates from macroscopic experiments to study initial pit growth and propagation. Our data and experimental techniques reveal a mechanism that suggests initial corrosion results in formation of an aggressive interfacial electrolyte that rapidly accelerates pitting, similar to crack initiation and propagation within the confined area. These results support a general mechanism for nanoscale material degradation and dissolution (e.g., crevice corrosion) of polycrystalline nonnoble metals, alloys, and inorganic materials within confined interfaces.

Critical and off-critical miscibility transitions in model extracellular and cytoplasmic myelin lipid monolayers.

Monolayers based on the composition of the cytoplasmic (CYT) or extracellular (EXT) sides of the myelin bilayer form coexisting immiscible liquid phases similar to the liquid-ordered/liquid-disordered phases in phospholipid/cholesterol monolayers. Increasing the temperature or surface pressure causes the two liquid phases to mix, although in significantly different fashion for the CYT and EXT monolayers. The cerebroside-rich EXT monolayer is near a critical composition and the domains undergo coalescence and a circle-to-stripe transition along with significant roughening of the domain boundaries before mixing. The phase transition in the cerebroside-free cytoplasmic side occurs abruptly without domain coalescence; hence, the cytoplasmic monolayer is not near a critical composition, although the domains exhibit shape instabilities within 1-2 mN/m of the transition. The change in mixing pressure decreases significantly with temperature for the EXT monolayer, with dΠ(crit)/dT ∼ 1.5 mN/m/°C, but the mixing pressure of the CYT monolayer varies little with temperature. This is due to the differences in the nonideality of cholesterol interactions with cerebrosides (EXT) relative to phospholipids (CYT). EXT monolayers based on the composition of white matter from marmosets with experimental allergic encephalomyelitis (EAE), an animal model of multiple sclerosis, remain phase-separated at higher surface pressures than control, while EAE CYT monolayers are similar to control. Myelin basic protein, when added to the CYT monolayer, increases lipid miscibility in CYT monolayers; likely done by altering the dipole density difference between the two phases.

Forces between surfaces in liquids.

Recent developments in the direct measurements of forces between surfaces in liquids at the ångstrom resolution level are reviewed. The results reveal a rich variety of interactions and interaction potentials that depend on the nature of the surfaces and intervening liquids. These results also shed new insights into liquid structure adjacent to surfaces and the interactions occurrig in complex systems, with implications in many different areas of chemical physics, biology, and technology. The origin of some important fundamental interactions, such as repulsive "hydration" forces and attractive "hydrophobic" forces, are still not understood and offer a challenge for experimental and theoretical work in this area.

Dynamic properties of molecularly thin liquid films.

An experimental technique is described for simultaneously measuring the static and dynamic interactions of very thin liquid films between two surfaces as they are moved normally or laterally relative to each other. Film thickness can be measured and controlled to 1 angstrom. Initial results are presented of the transition in the physical properties of liquid films only one molecular layer thick to thicker films whose properties are practically indistinguishable from the bulk. In particular, the results show that two molecularly smooth surfaces, when close together in simple liquids, slide (shear) past each other while separated by a discrete number of molecular layers, and that the frictional force is "quantized" with the number of layers.

Molecular mechanisms and forces involved in the adhesion and fusion of amphiphilic bilayers.

The surface forces apparatus technique was used for measuring the adhesion, deformation, and fusion of bilayers supported on mica surfaces in aqueous solutions. The most important force leading to the direct fusion of bilayers is the hydrophobic interaction, although the occurrence of fusion is not simply related to the force law between bilayers. Bilayers do not need to "overcome" some repulsive force barrier, such as hydration, before they can fuse. Instead, once bilayer surfaces come within about 1 nanometer of each other, local deformations and molecular rearrangements allow them to "bypass" these forces.

Long-range attraction and molecular rearrangements in receptor-ligand interactions.

A surface force apparatus was used to measure a long-range attractive protein-ligand force at separations D less than 85 angstroms. This force may effectively "steer" ligand trajectories, resulting in a greater than 27-fold enhancement of the association rate. A much stronger specific attraction is measured at contact (D less than 4 angstroms). A sevenfold increase in intermembrane adhesion resulted from increased lateral mobility of the receptors and molecular rearrangements in membranes above the solid-fluid transition temperature.

Direct measurement of a tethered ligand-receptor interaction potential.

Many biological recognition interactions involve ligands and receptors that are tethered rather than rigidly bound on a cell surface. A surface forces apparatus was used to directly measure the force-distance interaction between a polymer-tethered ligand and its receptor. At separations near the fully extended tether length, the ligands rapidly lock onto their binding sites, pulling the ligand and receptor together. The measured interaction potential and its dynamics can be modeled with standard theories of polymer and colloidal interactions.

Van der Waals Epitaxial Growth of agr-Alumina Nanocrystals on Mica.

Lattice mismatch stresses, which severely restrict heteroepitaxial growth, are greatly minimized when thin alumina films are grown by means of van der Waals forces on inert mica substrates. A 10-nanometer-thick epitaxial film exhibits crystallographic sixfold symmetry, a lattice constant close to that of the basal plane [0001] of alpha-alumina (sapphire), and an aluminum: oxygen atomic ratio of 1:1.51 +/- 0.02 (measured by x-ray photoelectron spectroscopy), again the same as for bulk sapphire. The film is free of steps and grain boundaries over large areas and appears to be an ideal model system for studying adhesion, tribology, and other surface phenomena at atomic scales.

Impact of polymer tether length on multiple ligand-receptor bond formation.

The promoters of cell adhesion are ligands, which are often attached to flexible tethers that bind to surface receptors on adjacent cells. Using a combination of Monte Carlo simulations, diffusion reaction theory, and direct experiments (surface force measurements) of the biotin-streptavidin system, we have quantified polymer chain dynamics and the kinetics and spatial range of tethered ligand-receptor binding. The results show that the efficiency of strong binding does not depend solely on the molecular architecture or binding energy of the receptor-ligand pair, nor on the equilibrium configuration of the polymer tether, but rather on its "rare" extended conformations.

The X-ray Surface Forces Apparatus: Structure of a Thin Smectic Liquid Crystal Film Under Confinement.

An x-ray surface forces apparatus for simultaneously measuring forces and structures of confined complex fluids under static and flow conditions is described. This apparatus, combined with an intense synchrotron x-ray source, allows investigation of molecular orientations within a thin liquid crystal film confined between two shearing mica surfaces 3900 angstroms apart. The layer-forming smectic liquid crystal 8CB (4-cyano-4'-octylbiphenyl) adopted a series of distinct planar layer orientations, including the bulk flow-forbidden b orientation.

Refinement of the fluid-mosaic model of membrane structure.

Certain molecular packing criteria previously employed in a quantitative analysis of lipid micelles and bilayers are here extended to biological membranes. The inclusion of both thermodynamic and packing considerations point to a highly complex self-assembly mechanism in which the organization of lipids and proteins is highly coupled, with far reaching consequences as regards the structure and function of biological membranes.

A model for the packing of lipids in bilayer membranes.

A number of known structural properties of mixed lipid bilayer membranes and monolayers are accounted for by a model in which lipids pack into bilayers and monolayers like building blocks, each characterized by a surface head group area and characteristic solid angle. In phospholipids above the melting transition the head group area (at a given temperature and degree of hydration) is fairly invariant while the hydrocarbon region may be liquid-like so long as the molecule is not compressed beyond its characteristic solid angle. Phosphatidylcholine and phosphatidylserine are tapered lipids, i.e. their surface head group areas are greater than their non-polar end areas; cholesterol is frayed, i.e. its polar end area is less than its non-polar end area; while phosphatidylethanolamine is almost cylindrical. The "condensing" effect of cholesterol in mixed phospholipid-cholesterol films is seen as a taper-fray accommodation. The lipid distribution in erythrocyte membrane is shown to be conducive to a stable strain-free membrane.

Lipid packing and transbilayer asymmetries of mixed lipid vesicles.

Predictions, based on a previously developed theory, of the radii and asymmetric lipid distribution of mixed phosphatidylcholine/lysophosphatidylcholine and phosphatidylcholine/cholesterol vesicles of variable composition are presented. The results compare well with available experimental data, except for cis-unsaturated phosphatidylcholine/cholesterol vesicles at high concentrations of cholesterol. It is concluded that specific lipid-lipid interactions need not be invoked for saturated and trans-unsaturated phosphatidylcholine mixed with lysophosphatidylcholine or cholesterol. A discussion of the effect of packing stresses on induced flip-flop and non-spherical vesicles is also presented.

Theory of self-assembly of lipid bilayers and vesicles.

A simple theory is developed that explains the formation of bilayers and vesicles and accounts quantitatively for many of their physical properties: Properties including vesicle size distributions and bilayer elasticity emerge from a unified theory that links thermodynamics, interaction free energy, and molecular geometry. The theory may be applied to the analysis of more complicated membrane structures and mechanisms.

Mussel-Inspired Adhesives and Coatings.

Mussels attach to solid surfaces in the sea. Their adhesion must be rapid, strong, and tough, or else they will be dislodged and dashed to pieces by the next incoming wave. Given the dearth of synthetic adhesives for wet polar surfaces, much effort has been directed to characterizing and mimicking essential features of the adhesive chemistry practiced by mussels. Studies of these organisms have uncovered important adaptive strategies that help to circumvent the high dielectric and solvation properties of water that typically frustrate adhesion. In a chemical vein, the adhesive proteins of mussels are heavily decorated with Dopa, a catecholic functionality. Various synthetic polymers have been functionalized with catechols to provide diverse adhesive, sealant, coating, and anchoring properties, particularly for critical biomedical applications.

Role of hydrophobic forces in bilayer adhesion and fusion.

With the aim of gaining more insight into the forces and molecular mechanisms associated with bilayer adhesion and fusion, the surface forces apparatus (SFA) was used for measuring the forces and deformations of interacting supported lipid bilayers. Concerning adhesion, we find that the adhesion between two bilayers can be progressively increased by up to two orders of magnitude if they are stressed to expose more hydrophobic groups. Concerning fusion, we find that the most important force leading to direct fusion is the hydrophobic attraction acting between the (exposed) hydrophobic interiors of bilayers; however, the occurrence of fusion is not simply related to the strength of the attractive interbilayer forces but also to the internal bilayer stresses (intrabilayer forces). For all the bilayer systems studied, a single basic fusion mechanism was found in which the bilayers do not "overcome" their short-range repulsive steric-hydration forces. Instead, local bilayer deformations allow these repulsive forces to be "bypassed" via a mechanism that is like a first-order phase transition, with a sudden instability occurring at some critical surface separation. Some very slow relaxation processes were observed for fluid bilayers in adhesive contact, suggestive of constrained lipid diffusion within the contact zone.

Polymer-induced membrane contraction, phase separation, and fusion via Marangoni flow.

Experiments have shown that the depletion of polymer in the region between two apposed (contacting or nearly contacting) bilayer membranes leads to fusion. In this paper we show theoretically that the addition of nonadsorbing polymer in solution can promote lateral contraction and phase separation of the lipids in the outer monolayers of the membranes exposed to the polymer solution, i.e., outside the contact zone. This initial phase coexistence of higher- and lower-density lipid domains in the outer monolayer results in surface tension gradients in the outer monolayer. Initially, the inner layer lipids are not exposed to the polymer solution and remain in their original "unstressed" state. The differential stresses on the bilayers give rise to a Marangoni flow of lipid from the outer monolayers in the "contact zone" (where there is little polymer and hence a uniform phase) to the outer monolayers in the "reservoir" (where initially the surface tension gradients are large due to the polymer-induced phase separation). As a result, the low-density domains of the outer monolayers in the contact zone expose their hydrophobic chains, and those of the inner monolayers, to the solvent and to each other across the narrow water gap, allowing fusion to occur via a hydrophobic interaction. More generally, this type of mechanism suggests that fusion and other intermembrane interactions may be triggered by Marangoni flows induced by surface tension gradients that provide "action at a distance" far from the fusion or interaction zone.