Free energy of adhesion of lipid bilayers on titania surfaces.

The adhesion strength between a flexible membrane and a solid substrate (formally the free energy of adhesion per unit area) is difficult to determine experimentally, yet is a key parameter in determining the extent of the wrapping of a particle by the membrane. Here, we present molecular dynamics simulations designed to estimate this quantity between dimyristoylphosphatidylcholine (DMPC) bilayers and a range of low-energy titanium dioxide cleavage planes for both anatase and rutile polymorphs. The average adhesion strength across the cleavage planes for rutile and anatase is relatively weak ∼-2.0 ± 0.4 mN m-1. However, rutile has two surfaces (100 and 101) displaying relatively strong adhesion (-4 mN m-1), while anatase has only one (110). This suggests a slightly greater tendency for bilayers to wrap rutile particles compared to anatase particles but both would wrap less than amorphous silica. We also estimate the adsorption free energies of isolated DMPC lipids and find that only the rutile 101 surface shows significant adsorption. In addition, we estimate the adhesion enthalpies and infer that the entropic contribution to the adhesion free energy drives adhesion on the rutile surfaces and opposes adhesion on the anatase surfaces.

Free energy of adhesion of lipid bilayers on silica surfaces.

The free energy of adhesion per unit area (hereafter referred to as the adhesion strength) of lipid arrays on surfaces is a key parameter that determines the nature of the interaction between materials and biological systems. Here we report classical molecular simulations of water and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) lipid bilayers at model silica surfaces with a range of silanol densities and structures. We employ a novel technique that enables us to estimate the adhesion strength of supported lipid bilayers in the presence of water. We find that silanols on the silica surface form hydrogen bonds with water molecules and that the water immersion enthalpy for all surfaces varies linearly with the surface density of these hydrogen bonds. The adhesion strength of lipid bilayers is a linear function of the surface density of hydrogen bonds formed between silanols and the lipid molecules on crystalline surfaces. Approximately 20% of isolated silanols form such bonds but more than 99% of mutually interacting geminal silanols do not engage in hydrogen bonding with water. On amorphous silica, the bilayer displays much stronger adhesion than expected from the crystalline surface data. We discuss the implications of these results for nanoparticle toxicity.

Effect of oxidation on the wettability of poly(dimethylsiloxane) surfaces.

The wetting of amorphous poly(dimethylsiloxane) (PDMS) surfaces by water has been studied using molecular dynamics simulations. PDMS surfaces were generated by compressing a long PDMS chain between two elastic boundaries at atmospheric pressure. Oxidation of the PDMS surface, achieved in real systems by exposure to air plasma or corona discharge, was modeled by replacing methyl groups on the PDMS chain with hydroxyl groups. Three surfaces of varying degrees of oxidation were characterized by measuring the water contact angle and the roughness. The dependence of the microscopic contact angle on drop size was measured from time averaged density profiles. The macroscopic contact angle was measured directly using a cylindrical drop of infinite length with zero contact line curvature. The measured macroscopic contact angle ranged from approximately 125 degrees on the untreated surface to 75 degrees on the most oxidized surface studied. The line tension was found to increase with increasing degree of oxidation, from a negligible value on the untreated surface to approximately 5x10(-11) J m(-1) on the most heavily oxidized surface.

Wetting of nanopatterned surfaces: the hexagonal disk surface.

Metropolis Monte Carlo simulations are used to investigate the wetting of chemically nanopatterned surfaces, for the case of hexagonal disk patterns where liquid wishes to wet high-energy circular patches but not wet the background surface. We calculate the density profiles of saturated liquid adsorbed on a variety of such substrates, spanning the nanoscale to atomic scale patterns. In addition, statistical mechanical sum rules are used to obtain interfacial order parameters and interfacial free energies. We observe that Cassie's law is typically obeyed, together with an associated breakdown of the mechanical interpretation of Young's equation, for pattern wavelengths greater than 15 molecular diameters. Here, the adsorbed fluid exists as an array of hemi-drops. At about half this wavelength, the breakdown of Cassie's law lies within realistic energy scales and is associated with the unbending of the outer surface of adsorbed films. For atomic scale patterns, the usual interpretation of Young's equation is restored for films thicker than one monolayer. At high chemical contrast, when the monolayer in contact with high-energy regions would prefer to be crystalline, we observe a variety of exotic interfacial phenomena that may have technological significance.

Films driven by surface tension gradients.

Fingering instabilities are often observed at the contact line of drops of surfactant solutions spreading spontaneously on solid surfaces. It has been recognised recently that a usual linear stability analysis predicts stable behaviour in contrast to the observed instability. It now seems the instability arises from short-time transients, where the thickness of the film ahead of the main drop is a crucial parameter for amplification. We reconsidered previous experiments and performed new ones along these lines. The strengths and weaknesses of the available models were analysed.