ATTRACTIVE-FORCE EFFECTS IN CHAIN MOLECULAR FLUIDS

Molecular-dynamics simulations are used to investigate the effects of attractive forces on the structure of two chain molecular fluids: n-butane and n-pentane. Simulations are performed using the complete site-site Lennard-Jones potential at state points identical with those in previous simulations that used just the repulsive part of the potential. The bond angles are constrained at 109-47° and the bond lengths at σ-153 nm. The potential parameters are σLJ = 0.3923 nm and the eJk = 72 K. Five densities are simulated for butane and four for pentane, all at a constant temperature of 430 K. Comparisons of pressures and the average site distribution functions from simulations for both potential models show an increased effect of attractive forces at low density, very similar to atomic fluids. The magnitude of the attractive-force effect appears to be much greater for chains than for atomic fluids, however, and the magnitude increases with increasing chain length. The simulation results are briefly compared with an approximate RISM theory, but the theory appears to be less accurate for attractive-force than for repulsive effects. © 1990 Taylor & Francis Group, LLC.