Solvation pressure in a Lennard-Jones fluid by molecular dynamics simulation

We present molecular dynamics simulations of a simple dimer immersed in a Lennard-Jones LJ fluid to test the validity of the solvation pressure model in a system where we attempt to isolate the general and ubiquitous solvation pressure. The solvated dimer acts as a simple pressure gauge in a system where the cohesive energy density CED of the solvent is fixed. We study the dimer bond length as a function of uniform hydrostatic pressure and as a function of a scaling parameter x which changes the attractive component of the LJ solventsolute interaction. For small x, or weak solvent-solute attractive interactions, the liquid bond lengths are dominated by repulsive interactions, but at higher values of x strong solvent-solute interactions produce a “packing effect” that is the dominant factor. We find that the change in bond length between vapor and liquid is consistent with the solvation pressure model for only a narrow range of x. Despite the simplicity of the system, departure from the solvation pressure model and an increase in the dimer bond length with increasing pressure are observed which is consistent with experimental observations of real liquids and normally attributed to masking effects. The existence and impact of these effects are explained in terms of CED, axial forces, and axial pressures.