Experiments using the surface force apparatus indicate that the nature of the forces acting on two surfaces separated by liquid film less than 5.0 nm thick are extremely sensitive to the chemical structure of the molecules in the liquid. Using molecular dynamics simulations we probe the structure and solvation forces upon two model surfaces separated by liquid hydrocarbons. We examine two cases—films composed of n-octane and films composed of iso-octane (2-methyl heptane). At certain surface separations the liquid films are strongly layered with chains lying parallel to the surface. These films produce a repulsive force equivalent to as much as 100 MPa on the surfaces, while exhibiting liquidlike self-diffusion constants with no evidence of solidification or glassification. A plot of this force, known as the solvation force, as a function of surface separation reveals strong oscillations with a period of about 0.4–0.5 nm for both the linear and branched molecules. Although the branching in the iso-octane significantly reduces the intensity of the layering, its solvation force curve shows oscillations which are just as strong as those seen in the films of the linear octane. This contradicts experimental results showing that the addition of a single methyl side chain significantly reduces the oscillatory nature of the solvation forces.
Molecular fluids in confined spaces exhibit behavior that cannot be properly described by continuum theories. We examine the structure of n-octane confined between crystalline surfaces and in equilibrium with the bulk liquid at 1 atm and 297 K using molecular dynamics simulations of a chemically realistic model of the molecules.We also compute the solvation forces acting upon the surfaces as a function of their separation. The films are studied between solid surfaces separated by gaps of 1 .O-2.4 nm. At 297 K, all films exhibit liquid-like dynamics and are strongly layered, although the sharpness of the layering oscillates with gap size. In addition, the solvation forces are oscillatory functions of the surface separations. An especially interesting anomaly appears at a gap size of 1.25 nm, barely enough to allow the molecules to extend lengthwise from one surface to the other. There is a strong tendency of the molecules to orient themselves normal to the solid surfaces, in contrast to molecules in other gap sizes and in similar systems studied by other researchers. Furthermore, the film readily freezes to a solid monolayer when cooled to 250 K in contrast to the 1 .S-nm-thick film which remains a liquid at that temperature. IntroductionIn problems ranging from adsorption to adhesion and lubrication, one often encounters situations in which molecular liquids are confined to regions with length scales of the order of only a few molecular diameters. In such situations, experimental evidence suggests that the normal continuum treatments of liquids break down, and a molecular level view is needed to understand the behavior of the fluids.One class of experiments which illustrates the importance of such situations to adhesion and forces between particles in a colloidal suspension is adhesive force measurements with the surface force apparatus.' In a typical surface force apparatus experiment, two cylindrical surfaces formed by bending perfectly cleaved mica sheets around the face of a cylinder are immersed in the liquid with their long axes perpendicular to each other. Sensitive equipment controls the forces acting upon the sheets while interferometry is used to measure the length of the narrowest section of the gap between the cylinders. Because the radii of curvature of the cylinders are much larger than the size of the gap, at a molecular level the walls of the gap appear as two flat surfaces-forming a geometry known to many as a "slit pore".The idealized "slit pore" is bounded by two flat surfaces of infinite area separated by a distance referred to as a pore width. Hence, simulations or theories of fluids in slit pores are often used to model these systems as well as adsorption phenomena. Many of the earlier simulations and theories of fluids in such pores are reviewed in a recent article by Evans.2For small spherical molecules, simulations indicate that when the pore width is less than about five atomic diameters, the molecules order in layers perpendicular to the sheets.2-5 Density functional and integral equati...
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