Computer simulation of adhesion between hydrophilic and hydrophobic self-assembled monolayers in water

Shear-Induced Detachment of Polystyrene Beads from SAM-Coated Surfaces

In this work we experimentally and theoretically analyze the detachment of microscopic polystyrene beads from different self-assembled monolayer (SAM) surfaces in a shear flow in order to develop a mechanistic model for the removal of cells from surfaces. The detachment of the beads from the surface is treated as a thermally activated process applying an Arrhenius Ansatz to determine the activation barrier and attempt frequency of the rate determing step in bead removal. The statistical analysis of the experimental shear detachment data obtained in phosphate-buffered saline buffer results in an activation energy around 20 kJ/mol, which is orders of magnitude lower than the adhesion energy measured by atomic force microscopy (AFM). The same order of magnitude for the adhesion energy measured by AFM is derived from ab initio calculations of the van der Waals interaction energy between the polystyrene beads and the SAM-covered gold surface. We conclude that the rate determing step for detachment of the beads is the initiation of rolling on the surface (overcoming static friction) and not physical detachment, i.e., lifting the particle off the surface.

From hydration repulsion to dry adhesion between asymmetric hydrophilic and hydrophobic surfaces

Using all-atom molecular dynamics (MD) simulations at constant water chemical potential in combination with basic theoretical arguments, we study hydration-induced interactions between two overall charge-neutral yet polar planar surfaces with different wetting properties. Whether the water film between the two surfaces becomes unstable below a threshold separation and cavitation gives rise to long-range attraction, depends on the sum of the two individual surface contact angles. Consequently, cavitation-induced attraction also occurs for a mildly hydrophilic surface interacting with a very hydrophobic surface. If both surfaces are very hydrophilic, hydration repulsion dominates at small separations and direct attractive force contribution can-if strong enough-give rise to wet adhesion in this case. In between the regimes of cavitation-induced attraction and hydration repulsion we find a narrow range of contact angle combinations where the surfaces adhere at contact in the absence of cavitation. This dry adhesion regime is driven by direct surface-surface interactions. We derive simple laws for the cavitation transition as well as for the transition between hydration repulsion and dry adhesion, which favorably compare with simulation results in a generic adhesion state diagram as a function of the two surface contact angles.

Computer simulation of water-mediated adhesion between phospholipid bilayer and solid support functionalized with self-assembled monolayers

An attempt is made to estimate, via computer simulation of the force-distance relation, the free energy of adhesion between a phosphatidylethanolamine bilayer and an alkanethiolate self-assembled monolayer (SAM) in aqueous medium. The simulations are performed using the grand canonical Monte Carlo technique and atomistic force fields. The bilayer adhesion free energy is predicted to be -22 ± 3 mJ/m(2) (-1.4 ± 0.2 kcal/mol) on a hydrophilic carboxyl-terminated SAM and -1 ± 1 mJ/m(2) (-0.06 ± 0.06 kcal/mol) on a hydrophobic methyl-terminated SAM.