Abstract:
Grand-canonical Monte Carlo and microcanonical molecular dynamics methods have been used to simulate an ultrathin monatomic film confined to a slit-pore [i.e., between solid surfaces (walls)]. Both walls comprise atoms rigidly fixed in the face centered cubic (100) configuration; one wall is smooth on a nanoscale and the other is corrugated (i.e., scored with regularly spaced rectilinear grooves one to several nanometers wide). Properties of the film have been computed as a function of the lateral alignment (registry), with the temperature, chemical potential, and distance between the walls kept constant. Changing the registry carries the film through a succession of equilibrium states, ranging from all solid at one extreme to all fluid at the other. Over a range of intermediate registries the film consists of fluid and solid portions in equilibrium, that is fluid-filled nanocapillaries separated by solid strips. The range of registries over which such fluid-solid equilibria exist depends upon the width of the grooves and the frequency of the corrugation. For grooves of width comparable to the range of the interatomic potential, fluid and solid phases cease to coexist. In the limit of very wide grooves the character of the film is similar to that of the film confined by strictly smooth walls. The rich phase behavior of the confined film due to the coupling between molecular (registry) and nano (corrugation) scales has obvious implications for boundary lubrication. © 1994 American Institute of Physics.
Abstract:
The relationship between the interlayer structure of octamethylcyclotetrasiloxane (OMCTS) lubricant and the atomic structure of confining mica surfaces is examined by Monte Carlo simulation. Results exhibited a sufficient coupling between OMCTS and mica surfaces causing a fluid lattice distortion. The coupling resulted in a nonzero shear stress which is in good agreement with experimental shear stress results.
Abstract:
We investigated the stability of an octadecyltriethoxysilane (OTE) monolayer self-assembled on plasmatreated and untreated mica using a surface forces apparatus by measuring the thickness of the water layer that is adsorbed from vapor. The OTE monolayers are initially highly hydrophobic, but contact angle hysteresis indicates that water interacts favorably with the monolayers on prolonged exposure. Defects in the monolayer most likely make it possible for the water to reach the hydrophilic region between the silane headgroups and the mica. This explains why there has been very little success in measuring hydrophobic forces between OTE-coated mica surfaces. Hydrophobic forces between OTE surfaces have been successfully measured only with silica as the substrate. Even though these monolayers are not suitable for studies of the so-called hydrophobic force, they are ideal for studies designed to probe the interactions between the silane headgroups and mica. For a given relative humidity, the water film thickness is always less if the surfaces are plasma-treated before the monolayer is deposited. In the untreated case, water penetrates into the hydrophilic region between the monolayer and the mica. This does not occur in the plasma-treated case because the monolayer is more firmly anchored, most likely through covalent bonding.
Abstract:
The nonlinear Poisson-Boltzmann equation is solved with a field-dependent expression for the dielectric constant to determine the effect of dielectric saturation on swelling pressure in clays. Calculations including a scaled and an unscaled version of the approximate expression for the field-dependent dielectric constant first derived by Booth are compared. It is found that the swelling pressure decreases in all cases considered; however, the effect is significant only when the electric field at the surface is high enough to cause dielectric saturation. Nonlinear dielectric effects should be considered in models which predict surface electric fields in excess of 107 V·m-1.
Abstract:
We have investigated the effect of humidity on surface deformation and adhesion of mica surfaces coated with n-octadecyltriethoxysilane self-assembled monolayers using a surface forces apparatus. The Maugis model of contact elasticity based on linear elastic fracture mechanics is used to analyze the results. The Laplace pressure is assumed to act in the "Dugdale" zone outside the contact area to account for capillary condensation. We measure the radius of the contact area as a function of applied load and use the model to obtain the surface energy and elastic constant of these surfaces for humidities ranging from 0 to 99%. The limiting values in dry and near-saturated conditions are as expected from well-known theories. A significant result is that we also obtain the surface energy for intermediate humidities. Increasing humidity modifies the deformed shape of the surfaces in contact due to capillary condensation. The sharp bifurcation at the edge of the contact zone for low humidities (JKR-type contact) is replaced by rounded edges (DMT-type contact) with increasing humidity. This is predicted by the Maugis model and is experimentally observed using optical interference fringes of equal chromatic order. We are able to separate the capillary condensation and solid-solid contributions to the adhesive force because the Maugis model allows a direct calculation of the area on which the Laplace pressure acts. At humidities approaching saturation the forces due to capillary condensation dominate monolayer-monolayer adhesion. At lower humidities both capillary condensation and direct monolayer-monolayer interaction contribute to the overall adhesion. © 2003 American Chemical Society.
