Joseph B. Hubbard

Self assembly driven by hydrophobic interactions at alkanethiol monolayers: mechanism of formation of hybrid bilayer membranes

The mechanism for the formation of biomimetic model cell membranes consisting of bilayers composed of alkanethiols and phospholipids was probed with a kinetic study using surface plasmon resonance. The kinetics of formation of a monolayer of phospholipid from vesicles in solution onto a hydrophobic alkanethiol monolayer is described by a model that takes into account the lipid concentration, diffusion, and a surface reorganization rate constant. Monomer phospholipid apparently does not play a direct role in determining the kinetics of bilayer formation. Expressions for the limiting cases of this model describe the behavior of two distinct vesicle concentration conditions. At high concentrations of lipid vesicles the formation of the bilayer appears to be limited by the diffusion of vesicles to the surface; at lower concentrations of vesicles, the rate-limiting step is apparently the surface reorganization of lipid. This kinetic model can also be used to describe the formation of a biomimetic bilayer from an alkanethiol monolayer and cell membranes.

A Brownian dynamics algorithm for calculating the hydrodynamic friction and the electrostatic capacitance of an arbitrarily shaped object

An algorithm originally devised for calculating the diffusion‐controlled reaction rate toward an arbitrarily shaped object is adapted to calculate the scalar translational hydrodynamic friction and the electrostatic capacitance of the object. In this algorithm Brownian particles are launched from a spherical surface enclosing the object. Each particle is propagated until it either hits the enclosed object or crosses the starting surface. In the latter case the particle is allowed to escape to infinity with an analytically known probability. If the particle does not escape to infinity, it is put back on the starting surface with the correct distribution density and the process is repeated. The scalar friction or capacitance of the ‘‘probed’’ object is proportional to the fraction of particles that hit the object. This algorithm is illustrated on a dumbbell made of two equal‐size spheres, a cube, and a phantom spherical shell having random distributed beads embedded in its surface.

Real-time measurement of spontaneous antigen-antibody dissociation.

We report observations in real time of thermally driven adhesion and dissociation between a monoclonal IgE antibody and its specific antigen N-epsilon-2,4-dinitrophenyl-L-lysine. Both molecules were attached to the surfaces of different polystyrene microspheres trapped by optical tweezers. Monitoring spontaneous successive attachment and detachment events allowed a direct determination of the reaction-limited detachment rate k(off) for a single bond and also for multiple bonds. We observed both positive and negative cooperativity between multiple bonds depending on whether the antigen was linked to the microsphere with or without a tether, respectively.

Dielectric friction and dielectric dispersion in electrolyte solutions with spin

A self‐consistent continuum model of dielectric friction on a moving ion is developed in which the only adjustable parameters are the hydrodynamic boundary conditions and a ’’rotational viscosity’’ hR. Ion mobility and the solution dielectric properties are calculated as a function of ion size, ion charge, low and high frequency dielectric constants of the solvent, dielectric relaxation time of the solvent, rotational viscosity, and solvent shear viscosity. The consequences of varying hR are investigated: For hR→∞ the continuum theory analyzed by Hubbard and Onsager (HO) is recovered; for small hR, however, the polarization and ’’spin’’ fields in the solvent near the ion differ significantly from the HO model. While this has remarkably little effect on calculated ionic mobilities, the solution dielectric properties are noticeably altered. In particular, the kinetic dielectric decrement diverges as ln hR as hR vanishes.

A first-passage algorithm for the hydrodynamic friction and diffusion-limited reaction rate of macromolecules

Many important properties of a macromolecule can be expressed in terms of averages over the trajectories of diffusing particles that begin in the medium surrounding the molecule and terminate at its surface. These properties include its translational hydrodynamic friction coefficient and the Smoluchowski rate constant for diffusion-limited reactions. In this paper we introduce a first-passage algorithm (FPA) for calculating such quantities. This algorithm uses certain exact Green’s functions, or propagators, for the Laplace equation to eliminate the need to construct explicitly those portions of a diffusing particle’s trajectory that are not near an absorbing object. The algorithm is especially efficient for studying objects that contain large voids or have very irregular surfaces, such as macromolecules. Diffusion algorithms were previously shown to give accurate results for the quantities we study. In this paper, we show that first-passage methods make these algorithms more accurate and efficient. In fu...

Reaction diffusion in a medium containing a random distribution of nonoverlapping traps

The transient reaction diffusion kinetics in a system containing a random distribution of stationary spherical traps is analyzed. It is shown that recently obtained results concerning the long‐time behavior of the trap‐averaged density at the origin ρ(o,t) may be readily extended to the cases of partially absorbing and nonoverlapping traps, independently of the number density of traps. We also estimate the size of the relative fluctuations about ρ(o,t) and show that these fluctuations diverge at long times.

Hydrodynamics of magnetic and dielectric colloidal dispersions

We discuss the hydrodynamic behavior of colloidal suspensions, the particles of which possess electric or magnetic moments. Particular attention is given to the case where an external electric or magnetic field acts on a system in which the polarization does not relax instantaneously, so that reorientational Brownian motion is coupled to both the field and to hydrodynamic degrees of freedom. Magnetosonic and magnetoviscous effects are derived, with emphasis on anisotropy with respect to the external field.

Dielectric saturation and dielectric friction in electrolyte solutions

Electrohydrodynamic equations developed by Hubbard and Kayser to account for the combined effects of dielectric saturation and dielectric friction on ionic motion in polar solvents are solved numerically to yield ionic conductances. Dielectric saturation is incorporated into this continuum treatment through a phenomenological relationship between the electric permittivity and field strength. The results of the analysis are critically tested by comparison with experimental conductance data.

Degradation of organic coatings on steel : mathematical models and predictions

Abstract Conceptual and mathematical models are developed for blistering processes induced by the corrosion of defect-containing coatings on steel substrate exposed to electrolytes. Models were developed based on cation transport along the coating/steel interface from the anodic sites at the defect to the cathodic sites under the coatings. The models accounted for both an initial induction period before cations have broken through into the blister and a subsequent propagation period when ions accumulate within the blister. The mathematical models are solved analytically to predict cation fluxes and concentrations along the interface and within the blisters. Solutions of the models are expressed in terms of dimensionless parameters. Model variables include blister size, distance between the blister and defect, ion diffusivity and potential gradients. The conditions under which ion transport is likely to be controlled by either potential or concentration gradients are established. Model results indicate that large blisters are more likely to grow than small ones because higher cation concentrations can build up within them. Implications of this conclusion for maintaining the integrity of organic coatings are discussed.

Molecular dynamics study of a dipolar fluid between charged plates. II

Recent experiments and computer simulations of thin films have observed the segregation of nonpolar molecules into layers or sheets parallel to the confining walls. We discuss a molecular dynamics study of a thin film of Stockmayer molecules between Lennard‐Jones plates and find that, in the absence of an electric field, the dipoles are mainly oriented parallel to the plates in each layer. The component of the dipole autocorrelation function in this plane decays to zero more slowly than the component perpendicular to the walls. The polarization density profile, with an electric field perpendicular to the plates, is also studied, and is found to oscillate from layer to layer, with a magnitude that is in excess of what is predicted by the Debye theory of dielectric saturation by a factor nearly equal to the ratio of the local density to the average bulk density.