Interaction Forces between Hydrophobic and Hydrophilic Self-Assembled Monolayers.

An investigation is presented of the interaction of charged self-assembled monolayers (SAMs) with a monoprotic ionizable acid functional group (-COOH) and uncharged SAMs with a methyl terminated functional group (-CH(3)). The strength of the interactions are determined using an atomic force microscope. For all electrolyte conditions investigated the interactions are not well described by a summation of van der Waals attractions and electrostatic repulsions in a manner suggesting that van der Waals attractions are screened. The repulsions are accurately described as corresponding to two surfaces of different charge interacting with surface charges that are independent of separation (i.e., the constant charge model). A small adhesion force was observed under all conditions and its magnitude increased with NaCl concentration. Copyright 2000 Academic Press.

Crystal Nucleation Rates for Particles Experiencing Short-Range Attractions: Applications to Proteins.

A kinetic model for the nucleation of a crystalline phase consisting of particles experiencing short-range attractions is developed. Of particular significance is the proximity of the metastable fluid/fluid phase boundary. The model incorporates self-consistent thermodynamics, changes in gradient diffusivity, and density fluctuations in the vicinity of the critical point. Density fluctuations associated with the spinodal of this metastable phase transition greatly enhance nucleation rates, suggesting that experimental conditions may be found where rapid nucleation and slow crystal growth can be achieved by moving the metastable critical point relative to the solubility boundary. Copyright 2000 Academic Press.

The Equilibrium Properties and Microstructure of Mixtures of Colloidal Particles with Long-Range, Soft Repulsions.

Osmotic pressure and elastic moduli of bimodal suspensions of particles experiencing long-range, soft repulsions were measured. At fixed osmotic pressure, the total suspension volume fraction, φ, varies linearly as the mixing ratio φl/φ is increased from 0 to 1. Here φl is the volume fraction of large particles based on total suspension volume. This result suggests the suspensions studied here are phase separated into domains containing primarily small and primarily large particles and is not expected for hard sphere suspensions where, at fixed osmotic pressure, φ passes through a maximum as φl/φ is increased. Elastic moduli are well described by a model based on a composite microstructure where the domains of pure large and small particles must have the same osmotic pressure which fixes the local particle volume fraction and hence the elastic modulus in each phase. The existence of phase separation is supported by electron micrographs taken on samples prepared by rapidly drying suspensions with volume fractions near 0.6. Copyright 1999 Academic Press.

The Rheology of Bimodal Mixtures of Colloidal Particles with Long-Range, Soft Repulsions.

The rheological properties of dense suspensions of bimodal mixtures of colloidal particles with long-range, soft repulsions were investigated. Suspensions of particles suspended in 10(-4) M KCl with volume fractions ranging from 0.3-0.6 were studied for volume fraction ratios of large to small particles of 0, 0.25, 0.5, 0.75, and 1.0. Latex particles of diameters ranging between 105 to 544 nm were used. These particles were stabilized by a combination of electrostatic and short range steric repulsions. Four separate mixtures were investigated with size ratios (large/small) of 1.2-5. At volume fractions investigated, the suspensions displayed dynamic yield stresses, tauy, and shear thinned with increasing stress or shear rate. The yield stress was found to be proportional to the suspension's elastic modulus, with a constant of proportionality lying between 0.015 and 0.03 as has been reported for a wide range of monodisperse suspensions. The functional dependence of stress on shear rate could be reduced to a single master curve which was independent of volume fraction, particle size ratio, and mixing ratio by scaling tauy on G, and the shear rate on G/etac where etac is the continuous phase viscosity. In bimodal suspensions shear thickening accompanied by irreversible aggregation was observed at volume fractions substantially below that measured for monodisperse suspensions. The stress and shear rate at thickening decreased rapidly as the volume fraction of the mixed suspension was increased. These results are substantially different than what has been reported for well-mixed suspensions of particles experiencing "hard" repulsions in that as the fraction of large particles is increased no viscosity minimum is seen at low and intermediate shear rates. Copyright 1999 Academic Press.

Effect of Solvents on Interactions between Hydrophobic Self-Assembled Monolayers.

An atomic force microscope was used to measure forces between self-assembled monolayers (SAMs) of hexadecanethiol as a function of solvent type. Solvents alter interactions between surfaces by changing the wetting properties of the surface. Adhesive forces decrease with solvent dielectric constant and vary monotonically for mixtures of water and ethanol, ethanol and methanol, and water and ethylene glycol. This study demonstrates that interactions and adhesion between hydrophobic SAMs can be manipulated through changes in the contact angle of the surface. Copyright 1999 Academic Press.

The Electrostatic Interaction of Rigid, Globular Proteins with Arbitrary Charge Distributions.

We use the linearized Poisson-Boltzmann equation to construct an analytical estimate of the electrostatic interaction energy for two rigid, globular proteins with arbitrary charge distributions represented by spherical multipole moments. Our analysis avoids invoking the superposition approximation and therefore is not limited to weakly interacting double layers. The interaction energy is a function of the protein properties (radii, multipole moments, and dielectric constant); the geometry (protein separation and orientations); and the solution properties (Debye screening length and dielectric constant). The proteins may be different. This is an extension of previous treatments for constant surface charge or for constant surface potential particles that are more applicable to isotropic colloids. We give example calculations for the interactions of Ribonuclease A molecules interacting in an electrolyte. These results suggest that electrostatic effects may be largely responsible for attractions between proteins, even if they are like charged. A set of routines using MATHEMATICA is available for performing the interaction energy and electrostatic potential calculations. Copyright 1998 Academic Press.

Consistent Surface Potentials from Bulk Suspension Properties

The equilibrium mechanical properties of face centered cubic (FCC) colloidal crystals interacting through the Yukawa potential are investigated. We investigate the use of a screened Coulomb interaction potential to define a single, electrostatic surface potential capable of predicting both the elastic modulus and osmotic pressure of colloidal crystals. Correlations are developed which capture the volume fraction, particle size, and ionic strength dependencies of these properties. Copyright 1997 Academic Press. Copyright 1997Academic Press