Hard-wall Repulsion Research Articles

Interactions between Mutant Bacterial Lipopolysaccharide (LPS-Ra) Surface Layers: Surface Vesicles, Membrane Fusion, and Effect of Ca2+and Temperature.

Lipopolysaccharides (LPS) are a major component of the protective outer membrane of Gram-negative bacteria. Understanding how the solution conditions may affect LPS-containing membranes is important to optimizing the design of antibacterial agents (ABAs) which exploit electrostatic and hydrophobic interactions to disrupt the bacteria membrane. Here, interactions between surface layers of LPS (Ra mutants) in aqueous media have been studied using a surface force apparatus (SFA), exploring the effects of temperature and divalent Ca2+ cations. Complementary dynamic light scattering (DLS) characterization suggests that vesicle-like aggregates of diameter ∼28-80 nm are formed by LPS-Ra in aqueous media. SFA results show that LPS-Ra vesicles adsorb weakly onto mica in pure water at room temperature (RT) and the surface layers are readily squeezed out as the two surfaces approach each other. However, upon addition of calcium (Ca2+) cations at near physiological concentration (2.5 mM) at RT, LPS multilayers or deformed LPS liposomes on mica are observed, presumably due to bridging between LPS phosphate groups and between LPS phosphates and negatively charged mica mediated by Ca2+, with a hard wall repulsion at surface separation D0 ∼ 30-40 nm. At 40 °C, which is above the LPS-Ra β-α acyl chain melting temperature (Tm = 36 °C), fusion events between the surface layers under compression could be observed, evident from δD ∼ 8-10 nm steps in the force-distance profiles attributed to LPS-bilayers being squeezed out due to enhanced fluidity of the LPS acyl-chain, with a final hard wall surface separation D0 ∼ 8-10 nm corresponding to the thickness of a single bilayer confined between the surfaces. These unprecedented SFA results reveal intricate structural responses of LPS surface layers to temperature and Ca2+, with implications to our fundamental understanding of the structures and interactions of bacterial membranes.

Contact mechanics of and Reynolds flow through saddle points: On the coalescence of contact patches and the leakage rate through near-critical constrictions

We study numerically local models for the mechanical contact between two solids with rough surfaces. When the solids softly touch either through adhesion or by a small normal load L, contact only forms at isolated patches and fluids can pass through the interface. When the load surpasses a threshold value, , adjacent patches coalesce at a critical constriction, i.e., near points where the interfacial separation between the undeformed surfaces forms a saddle point. This process is continuous without adhesion and the interfacial separation near percolation is fully defined by scaling factors and the sign of . The scaling leads to a Reynolds flow resistance which diverges as with . Contact merging and destruction near saddle points become discontinuous when either short-range adhesion or specific short-range repulsion are added to the hard-wall repulsion. These results imply that coalescence and break-up of contact patches can contribute to Coulomb friction and contact aging.

Density profiles of Ar adsorbed in slits of CO2: Spontaneous symmetry breaking revisited

A recently reported symmetry breaking of density profiles of fluid argon confined by two parallel solid walls of carbon dioxide is studied. The calculations are performed in the framework of a nonlocal density functional theory. It is shown that the existence of such asymmetrical solutions is restricted to a special choice for the adsorption potential, where the attraction of the solid-fluid interaction is reduced by the introduction of a hard-wall repulsion. The behavior as a function of the slit's width is also discussed. All the results are placed in the context of the current knowledge on this matter.

Hard-wall potential function for transport properties of alkali metal vapors

This study demonstrates that the transport properties of alkali metals are determined principally by the repulsive wall of the pair interaction potential function. The (hard-wall) Lennard-Jones (LJ) (15-6) effective pair potential function is used to calculate the transport collision integrals. Accordingly, reduced collision integrals of K, Rb, and Cs metal vapors are obtained from the Chapman-Enskog solution of the Boltzmann equation. The law of corresponding states based on the experimental transport reduced collision integral is used to verify the validity of a LJ(15-6) hybrid potential in describing the transport properties. LJ(8.5-4) potential function and a simple thermodynamic argument with the input PVT data of liquid metals provide the required molecular potential parameters. Values of the predicted viscosity of monatomic alkali metal vapor are in agreement with typical experimental data with average absolute deviations of 2.97% for K in the range of 700-1500 K, 1.69% for Rb, and 1.75% for Cs in the range of 700-2000 K. In the same way, the values of predicted thermal conductivity are in agreement with experiment within 2.78%, 3.25%, and 3.63% for K, Rb, and Cs, respectively. The LJ(15-6) hybrid potential with a hard-wall repulsion character conclusively predicts the best transport properties of the three alkali metal vapors.

A simulation study of the effects of adsorbent energy on the diffusion of molecules in slit pores: 1 commensurate slit widths

Equilibrium molecular dynamics simulation has been used to study the self-diffusion coefficients (from correlation of the molecular velocity) and the collective, or centre of mass diffusion coefficients (from correlation of the streaming velocity) of a Lennard-Jones fluid in model slit pores. The slit widths were chosen to be integer multiples of the Lennard-Jones adsorbate diameter, and therefore are close to being commensurate with layered adsorbate structures. Slits of reduced width H* = 3 and 5 were examined at a reduced temperature of T* = 1.0. The adsorbate densities ranged from 0.3 to 0.9 in reduced units. The adsorbent adsorbate interaction was modelled as a simple potential with inverse 4th power attraction plus hard wall repulsion, and systems with reduced parameter u0* ranging from −5 to +5 were studied. Molecule-wall scattering was represented by a diffuse reflection algorithm. The density distributions show strong layering in the attractive system, but this is absent in the most repulsive slits, except at very high densities. Self-diffusion is only weakly dependent on u0* and slit width at high densities, but a strong dependence on u0* appears at low densities. The collective diffusion coefficient is less easy to calculate with high accuracy; nevertheless, it is clear that there is a strong dependence of this property on u0* Trajectory plots show zones in which the particles are more or less strongly localized, but undergo irregular oscillatory motion corresponding to regions of high density in the single-particle distributions.

Effects of Time and Compression on the Interactions of Adsorbed Polystyrene Layers in a Near-ϑ Solvent

We have investigated the effects of molecular weight, adsorption time, force-measuring rate, and repeated compression−decompression cycles (previous history) on the interactions between high molecular weight polystyrenes adsorbed on mica from dilute cyclopentane solutions under near-ϑ solvent conditions. On the first slow approach, a long-range bridging attraction, which needs several minutes to fully develop for each incremental movement of the surfaces, is found at separations below 6Rg. The interaction becomes a steep hard-wall repulsion at separations below 0.5Rg. With increasing molecular weight the thickness of the adsorbed layers increases, and the attractive bridging force on approach and adhesion on separation decreases. Self-consistent field theory underestimates the onset location and depth of the attractive minima but accurately predicts the location of the hard-wall repulsion. For shorter (nonequilibrium) adsorption times, the bridging attraction and adhesion are stronger and occur at smaller...

Solvent hydrogen-bond network in protein self-assembly: solvation of collagen triple helices in nonaqueous solvents.

Forces between type I collagen triple helices are studied in solvents of varying hydrogen-bonding ability. The swelling of collagen fibers in reconstituted films is controlled by the concentration of soluble polymers that are excluded from the fibers and that compete osmotically with collagen for available solvent. The interaxial spacing between the triple helices as a function of the polymer concentration is measured by x-ray diffraction. Exponential-like changes in the spacing with increasing osmotic stress, qualitatively similar to the forces previously found in aqueous solution, are also seen in formamide and ethylene glycol. These are solvents that, like water, are capable of forming three-dimensional hydrogen-bond networks. In solvents that either cannot form a network or have a greatly impaired ability to form a hydrogen-bonded network, strikingly different behavior is observed. A hard-wall repulsion is seen with collagen solvated by ethanol, 2-propanol, and N,N-dimethylformamide. The spacing between helices hardly changes with increasing polymer concentration until the stress exceeds some threshold where removal of the solvent becomes energetically favorable. No solvation of collagen is observed in dimethoxyethane. In solvents with an intermediate ability to form hydrogen-bonded networks, methanol, 2-methoxyethanol, or N-methylformamide, the change in spacing with polymer concentration is intermediate between exponential-like and hard-wall. These results provide direct evidence that the exponential repulsion observed between collagen helices at 0-8-A surface separations in water is due to the energetic cost associated with perturbing the hydrogen-bonded network of solvent molecules between the collagen surfaces.

Interactions between adsorbed molecules: CO on Ni(111)

We report measurements of the isothermal desorption rate for the CONi(111) chemisorption system. Values for microscopic lateral interaction parameters were determined by fitting the isotherms in the coverage regime (0.015 < θ < 0.10) to a lattice-gas model making the quasi-equilibrium approximation and using a transfer matrix grand partition function. The parameters determined from this analysis are compatible with a wide range of experimental data for this system, and indicate a system that is weakly interacting outside a short-range hard-wall repulsion. Low-coverage Arrhenius parameters were determined by fitting the desorption waveforms below θ = 0.015. The Arrhenius prefactor and activation energy for desorption are determined to be log( A) = 15.1 ± 0.7 and E d = 130 ± 5 kJ mol −1, respectively, in good agreement with previous results. Comparisons will be made to the system COPt(111) where appropriate. We discuss a simple model for substrate-mediated lateral interactions based upon relief of surface tensile stress which can qualitatively explain the differences between these two systems.

Membrane Fluctuation and Polymer Adsorption

We investigate adsorption behaviors and desorption transitions of a flexible but self-avoiding polymer on a fluctuating (undulating) membrane (or interface). Incorporating hard-wall repulsion with polymer-membrane attraction of finite range, we find that the mem- brane fluctuation gives rise to an effective repulsion and enhances unbinding (desorption) tran- sition of polymer, lowering the transition temperature. Employing hard square-well model as an example, we calculate the desorption transition temperature and the critical behaviors of polymer and membrane fluctuations. Our results are qualitatively different from those of Garel, Kadar and Orland (Europhys. Leti. 29 (1995) 303) who employed a contact attractive potential which allows the polymer to permeate the Other side of the surface.

Selective-adsorption structures in the inelastic scattering of He-graphite (0001)

Structures were observed in the angular distribution of helium atoms inelastically scattered by the graphite (0001) surface; they were clearly identified as bound-state resonance processes (selective adsorption) assisted by phonons. To better understand the mechanism of the inelastic process, detailed measurements taken at different incident angles for the structure corresponding to a well-defined resonance are presented. Some unexpected behavior, as the change of shape and intensity of an inelastic resonance observed at definite incident angles, suggests the occurrence of multiple resonant transitions between different energy levels of the gas-surface potential. The experimental data are analyzed theoretically in terms of single-phonon inelastic processes, on the assumption of a model potential with a time-dependent short-range repulsion and a stationary long-range attraction. Explicit formulas for inelastic selective adsorption are obtained in the eikonal approximation, assuming a corrugate hard-wall repulsion. Different ways to go resonantly in a bound state are considered, with or without energy exchange with the surface. The calculated behavior of resonances shows reasonably good agreement with the experimental data on He-graphite. From the angular position of phonon-assisted resonances preliminary information on the graphite (0001) surface dynamics has been obtained, in agreement with calculated phonon spectra.

Eigenstates of excitons near a surface

The exact eigenstates and energies of an electron and a hole of equal effective masses, with an attractive $\ensuremath{\delta}$-function interaction and hard-wall repulsion at the surfaces of a solid, are classified and obtained explicitly for a solid of arbitrary thickness. Both bound and scattering states of the exciton are significantly quantized for thin films.

Addendum

Forces between type I collagen triple helices are studied in solvents of varying hydrogen-bonding ability. The swelling of collagen fibers in reconstituted films is controlled by the concentration of soluble polymers that are excluded from the fibers and that compete osmotically with collagen for available solvent. The interaxial spacing between the triple helices as a function of the polymer concentration is measured by x-ray diffraction. Exponential-like changes in the spacing with increasing osmotic stress, qualitatively similar to the forces previously found in aqueous solution, are also seen in formamide and ethylene glycol. These are solvents that, like water, are capable of forming three-dimensional hydrogen-bond networks. In solvents that either cannot form a network or have a greatly impaired ability to form a hydrogen-bonded network, strikingly different behavior is observed. A hard-wall repulsion is seen with collagen solvated by ethanol, 2-propanol, and N,N-dimethylformamide. The spacing between helices hardly changes with increasing polymer concentration until the stress exceeds some threshold where removal of the solvent becomes energetically favorable. No solvation of collagen is observed in dimethoxyethane. In solvents with an intermediate ability to form hydrogen-bonded networks, methanol, 2-methoxyethanol, or N-methylformamide, the change in spacing with polymer concentration is intermediate between exponential-like and hard-wall. These results provide direct evidence that the exponential repulsion observed between collagen helices at 0–8-A surface separations in water is due to the energetic cost associated with perturbing the hydrogen-bonded network of solvent molecules between the collagen surfaces.

Calculations of atomic scattering from surfaces

Abstract The scattering of a monoenergetic beam of classical atoms from a crystal lattice is considered. A simple model of the gas-solid interaction is proposed for the rigid solid and an analytic expression for the differential scattering probability is obtained that exhibits many of the features observed in experimental studies of the scattering of neon from LiF and in the computer simulations of that system. The model potential consists of a square-well attractive energy plus a sinusoidal hard-wall repulsion. The model is solved for two dimensional scattering as well as for in-plane and out-of-plane scattering from a simple square lattice. A brief discussion is given of the comparison between previous results and the theoretical curves calculated in this work for specific choices of the parameters of the model. Although the quality of the agreement obtained is encouraging, several refinements are suggested that should improve the model considerably at the cost of losing the analytic character of the calculation.