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.

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.

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.

Tuning of solid phase in supracrystals made of silver nanocrystals.

Decanethiol-passivated silver nanocrystals are shown, by small-angle X-ray diffraction, to organize into hexagonal close packed or face centered cubic (fcc) structures depending on the substrate temperature. When the nanocrystals are passivated by dodecanethiols, fcc and body centered cubic lattices as well as disordered arrangements are observed. The different crystalline phases correspond to thermodynamic equilibrium states. The passivant chain length is shown to control the interactions between the nanocrystals and consequently the superlattice structure.

Friction between two weakly adhering boundary lubricated surfaces in water.

The sliding of adhesive surfactant-bearing surfaces was studied with a surface forces apparatus nanotribometer. When the surfaces are fully immersed in an aqueous solution, the dynamic behavior is drastically different and more varied than under dry conditions. In solution, the shear stress exhibits at least five different velocity regimes. In particular, the sliding may proceed by an "inverted" stick-slip over a large range of driving velocities, this regime being bounded by smooth (kinetic) sliding at both lower and higher driving velocities. The general behavior of the system was studied in detail, i.e., over a large range of experimental conditions, and theoretically accounted for in terms of a general model based on the kinetics of formation and rupture of adhesive links (bonds) between the two shearing surfaces with an additional viscous term.

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.

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.

Dynamic phase transitions in confined lubricant fluids under shear.

A surface force apparatus was used to measure the transient and steady-state friction forces between molecularly smooth mica surfaces confining thin films of squalane, C30H62, a saturated, branched hydrocarbon liquid. The dynamic friction "phase diagram" was determined under different shearing conditions, especially the transitions between stick-slip and smooth sliding "states" that exhibited a chaotic stick-slip regime. The apparently very different friction traces exhibited by simple spherical, linear, and branched hydrocarbon films under shear are shown to be due to the much longer relaxation times and characteristic length scales associated with transitions from rest to steady-state sliding, and vice versa, in the case of branched liquids. The physical reasons and tribological implications for the different types of transitions observed with spherical, linear, and branched fluids are discussed.

Direct observation of shear-induced orientational phase coexistence in a lyotropic system using a modified x-ray surface forces apparatus.

The second generation x-ray surface forces apparatus (XSFA-II) allows for the first time simultaneous in situ small-angle x-ray scattering and surface force measurements. We have used the XSFA-II to monitor shear-induced orientational transitions in a lyotropic model lubricant system. Upon applying small shear amplitudes (approximately 20 micrometer) to a relatively thick (approximately 800 micrometer) film, we observed evidence for the formation of an orientational boundary layer at the shearing surface. Time-resolved x-ray diffraction revealed the gradual transition to shear-favored orientation by growth of the boundary layer.

Polyethylene glycol-coated biocompatible surfaces.

Surfaces covered with polyethylene glycol (PEG; HO-(CH(2)-CH(2)-O)(n)-H) have been shown to be biocompatible because PEG's properties yield nonimmunogenicity, nonantigenicity, and protein rejection. To produce a biocompatible surface coating, we have developed a method for grafting PEG onto activated silica films. We first deposited an amorphous silica film by plasma-enhanced chemical vapor deposition from SiH(4) and O(2) gases, which provided the flexibility to coat diverse materials with different chemistries and shapes. The silica films were activated by exposure to water plasma, increasing the number of silanol groups (Si-OH) on their surface. The surface silanol groups were then chemically reacted with the hydroxyl end of PEG to form an ester bond, Si-O-C, and to cover the surface with PEG. The surface reactions were monitored using attenuated total reflection Fourier transform infrared spectroscopy. The vibrational absorption bands of the C-O and -CH(2) bonds increased with time and saturated, indicating that PEG was adsorbed to saturation coverage on the surface. Simultaneously, the Si-OH absorption band decreased, showing that the surface silanols reacted with PEG and were depleted. The PEG-covered surfaces were physically characterized by atomic force microscopy, Auger electron spectroscopy, ellipsometry, and contact angle measurements. These characterization techniques provided additional evidence for the existence of chemically bonded PEG on the surfaces. Efficacy of protein rejection on PEG-covered surfaces was studied through measurements of the fluorescence intensity of Texas red-labeled bovine serum albumin brought in contact with such surfaces in solution. Significantly less protein adsorption was observed on surfaces covered with PEG compared to uncovered surfaces.

Polymer-cushioned bilayers. I. A structural study of various preparation methods using neutron reflectometry.

This neutron reflectometry study evaluates the structures resulting from different methods of preparing polymer-cushioned lipid bilayers. Four different techniques to deposit a dimyristoylphosphatidylcholine (DMPC) bilayer onto a polyethylenimine (PEI)-coated quartz substrate were examined: 1) vesicle adsorption onto a previously dried polymer layer; 2) vesicle adsorption onto a bare substrate, followed by polymer adsorption; and 3, 4) Langmuir-Blodgett vertical deposition of a lipid monolayer spread over a polymer-containing subphase to form a polymer-supported lipid monolayer, followed by formation of the outer lipid monolayer by either 3) horizontal deposition of the lipid monolayer or 4) vesicle adsorption. We show that the initial conditions of the polymer layer are a critical factor for the successful formation of our desired structure, i.e., a continuous bilayer atop a hydrated PEI layer. Our desired structure was found for all methods investigated except the horizontal deposition. The interaction forces between these polymer-supported bilayers are investigated in a separate paper (Wong, J. Y., C. K. Park, M. Seitz, and J. Israelachvili. 1999. Biophys. J. 77:1458-1468), which indicate that the presence of the polymer cushion significantly alters the interaction potential. These polymer-supported bilayers could serve as model systems for the study of transmembrane proteins under conditions more closely mimicking real cellular membrane environments.

Polymer-cushioned bilayers. II. An investigation of interaction forces and fusion using the surface forces apparatus.

We have created phospholipid bilayers supported on soft polymer "cushions" which act as deformable substrates (see accompanying paper, Wong, J. Y., J. Majewski, M. Seitz, C. K. Park, J. N. Israelachvili, and G. S. Smith. 1999. Biophys. J. 77:1445-1457). In contrast to "solid-supported" membranes, such "soft-supported" membranes can exhibit more natural (higher) fluidity. Our bilayer system was constructed by adsorption of small unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles onto polyethylenimine (PEI)-supported Langmuir-Blodgett lipid monolayers on mica. We used the surface forces apparatus (SFA) to investigate the long-range forces, adhesion, and fusion of two DMPC bilayers both above and below their main transition temperature (T(m) approximately 24 degrees C). Above T(m), hemi-fusion activation pressures of apposing bilayers were considerably smaller than for solid-supported bilayers, e.g., directly supported on mica. After separation, the bilayers naturally re-formed after short healing times. Also, for the first time, complete fusion of two fluid (liquid crystalline) phospholipid bilayers was observed in the SFA. Below T(m) (gel state), very high pressures were needed for hemi-fusion and the healing process became very slow. The presence of the polymer cushion significantly alters the interaction potential, e.g., long-range forces as well as fusion pressures, when compared to solid-supported systems. These fluid model membranes should allow the future study of integral membrane proteins under more physiological conditions.

A neutron reflectivity study of polymer-modified phospholipid monolayers at the solid-solution interface: polyethylene glycol-lipids on silane-modified substrates.

The structure of polymer-decorated phospholipid monolayers at the solid-solution interface was investigated using neutron reflectometry. The monolayers were composed of distearoylphosphatidylethanolamine (DSPE) matrixed with varying amounts of DSPE-PEG (DSPE with polyethylene glycol covalently grafted to its headgroup). Mixed lipid monolayers were Langmuir-Blodgett deposited onto hydrophobic quartz or silicon substrates, previously hydrophobized by chemically grafting a robust monolayer of octadecyltrichlorosilane (OTS). We show that this method results in homogeneous and continuous phospholipid monolayers on the silanated substrates and determine that the grafted PEG chains extend away from the monolayers into the solvent phase as a function of their density, as expected from scaling theories. In addition, ligands were coupled to the end of the PEG chains and selective binding was demonstrated using fluorescence microscopy. Our results demonstrate that these constructs are ideal for further characterization and studies with well-defined monomolecular films.

Structural studies of polymer-cushioned lipid bilayers.

The structure of softly supported polymer-cushioned lipid bilayers, prepared in two different ways at the quartz-solution interface, were determined using neutron reflectometry. The polymer cushion consisted of a thin layer of branched, cationic polyethyleneimine (PEI), and the bilayers were formed by adsorption of small unilamellar dimyristoylphosphatidylcholine (DMPC) vesicles. When vesicles were first allowed to adsorb to a bare quartz substrate, an almost perfect bilayer formed. When the polymer was then added to the aqueous solution, it appeared to diffuse beneath this bilayer, effectively lifting it from the substrate. In contrast, if the polymer layer is adsorbed first to the bare quartz substrate followed by addition of vesicles to the solution, there is very little interaction of the vesicles with the polymer layer, and the result is a complex structure most likely consisting of patchy multilayers or adsorbed vesicles.

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.

Role of hydration and water structure in biological and colloidal interactions.

The conventional explanation of why hydrophilic surfaces and macromolecules remain well separated in water is that they experience a monotonically repulsive hydration force owing to structuring of water molecules at the surfaces. A consideration of recent experimental and theoretical results suggests an alternative interpretation in which hydration forces are either attractive or oscillatory, and where repulsions have a totally different origin. Further experiments are needed to distinguish between these possibilities.