The Rheology ofn-Butane andi-Butane by Non-Equilibrium Molecular Dynamics Simulations

We present non-equilibrium molecular dynamics (NEMD) simulations of planar Couette flow for models for normal (n-butane) and isomeric butane (i-butane) molecules. Two of the three classes of models used here are collapsed atom models while the third class is an atomistically detailed model. In general agreement with previous workers, we find that the collapsed atom models predict the viscosity of the n-butane quite well. However, if the collapsed atom models are applied to the isomer by simply changing the geometry of the molecule, one finds that the viscosity is underpredicted. The atomistically detailed model considered here is an amalgam of several models. We find that our atomistically detailed model does not yield quantitative agreement with the viscosity of either the n-butane or its isomer. However, the atomistically detailed model does have the one positive feature that if it is applied to the isomer by simply changing the geometry of the molecule, one finds that the viscosity increases in agreement with experiment. This suggests that atomistically detailed models may hold the key to developing a set of group-group interactions which can be used equally well to predict the viscosity of normal and isomeric alkane fluids.