Excess entropy scaling of transport properties of Lennard-Jones chains

Excess-entropy scaling relationships for diffusivity and viscosity of Lennard-Jones chain fluids are tested using molecular dynamics simulations for chain sizes that are sufficiently small that chain entanglement effects are insignificant. The thermodynamic excess entropy S-e is estimated using self-associating fluid theory (SAFT). A structural measure of the entropy S-2 is also computed from the monomer-monomer pair correlation function, g(m)(r). The thermodynamic and structural estimators for the excess entropy are shown to be very strongly correlated. The dimensionless center-of-mass diffusivities, D*(cm), obtained by dividing the diffusivities by suitable macroscopic reduction parameters, are shown to conform to the excess entropy scaling relationship, D*(cm) =A(n) exp(alpha S-n(e)), where the scaling parameters depend on the chain length n. The exponential parameter alpha(n) varies as -(1/n) while A(n) varies approximately as n(-0.5). The scaled viscosities obey a similar relationship with scaling parameters B-n and beta(n) where beta(n) varies as 1/n and B-n shows an approximate n(0.6) dependence. In accordance with the Stokes - Einstein law, for a given chain length, alpha(n)=-beta(n) within statistical error. The excess entropy scaling parameters associated with the transport properties therefore display a simple dependence on chain length. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2995990]