Thin films of n-octane confined between parallel solid surfaces: structure and adhesive forces vs film thickness from molecular dynamics simulations

Abstract

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...

Shear behavior of squalane and tetracosane under extreme confinement. III. Effect of confinement on viscosity

“…On the contrary, the molecules reportedly collapse into compact configurations. Wang et al 24 in their equilibrium simulation of noctane using a model similar to the one used in our study, also report the films retaining diffusive and conformational properties resembling a liquid even though strong layering in the films is observed. Experimental studies on squalane by Reiter et al 25 show that molecular thin liquid films of alkanes in the elastic rest state are very different from solids in the bulk; they are much weaker and more deformable than a molecular crystal or glass, which is in contrast to the beadspring simulation results.…”

Molecular dynamics simulations of organically modified layered silicates

“…The oscillating solvation force provides evidence of a strongly layered structure and a solid-like response when the mica separations become as small as a few methylene diameters. Concurrent computer modeling studies [2][3][4][5][6][7][8][9] have captured this macroscopic response of the normal force and have at the same time unveiled clearly layered density profiles normal to the confining surfaces. In addition, the computer studies revealed details such as a strong inhomogeneity of dynamics across the slit and dramatic increases of relaxation times near strongly physisorbing surfaces.…”

“…This information is of fundamental and practical importance and would greatly contribute to a deeper understanding of the behavior of fluids in the vicinity of confining surfaces, 16 as well as provide insight for material design for many applications. 14 Although many molecular dynamics and Monte Carlo simulations have been employed to study confined thin films of alkanes and alkanes at fluid-surface interfaces, [3][4][5][6][7][8][9] there are far fewer studies of grafted alkane oligomers. [17][18][19] Due to the extremely high grafting densities used in the latter studies, their results are not applicable to the systems of interest here.…”

Nanotribology

“…MD simulations of interfacial liquid junctions and confined films have been carried out to understand the physical properties and response of model lubricants and their molecular characteristics, such as chain length and molecular structure (for example, straight versus branched chains) (Thompson and Robbins, 1990a, b;Robbins and Thompson, 1991;Ribarsky and Landman, 1992;Thompson et al, 1992;Landman et al, 1993;Wang et al, 1993;Bhushan et al, 1995a;. The confinement of n-octane ranging in thickness from 1 to 2.4 nm, between parallel, crystalline solid walls (rigid Langmuir-Blodgett layers), was studied by Wang et al (1993).…”