The rheology of n-decane and 4-propyl heptane by non-equilibrium molecular dynamics simulations

In a recent paper [Mol. Sim., 16, 229 (1996)], we reported non-equilibrium molecular dynamics (NEMD) simulation of planar Couette flow for normal (n-butane) and isomeric butane (i-butane) molecules using two collapsed atom models and an atomistically detailed model. It was found that the collapsed atom models predict the viscosity of the n-butane quite well, but the viscosity of i-butane by those models is underpredicted. It was also found that the atomislicalIy detailed model does not yield quantitative agreement with the viscosity of either the n-butane or i-butane, but it does have the one positive feature that the calculated viscosity of i-butane is higher than that of n-butane (branching increases viscosity) as observed experimentally. In the present paper, we present results of NEMD simulations of planar Couette flow for normal decane and 4-propyl heptane molecules using the same models. The results show exactly the same branching effect on the viscosity as for the case of n- and i-butanes. We find that the viscosity of n-decane predicted by the collapsed models is in excellent agreement with the experimental value and the calculated viscosity of 4-propyl heptane by those models is smaller than that of n-decane. For the atomistically detailed model, the quantitative agreement with the experiments is poor fur the viscosity of either the n-decane or 4-propyl heptane, but the calculated viscosity of 4-propyl heptane is higher than that of n-decane (branching increases viscosity). In this case, since the experimental value for the viscosity is not known, we cannot conclude that the role of interactions including H atoms plays in the right way. However, the significant difference on the branching effect on the viscosity in the collapsed models and the atomistically detailed model is once more observed following in the case of n- and i-butane.