Confined thin films of a linear and branched octane. A comparison of the structure and solvation forces using molecular dynamics simulations

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.