Intramolecular variations in chain molecular fluid structure

Molecular Dynamics simulations have been performed for 5-site chains mixed with single site spheres. Two separate simulations were performed at each set of conditions first with a repulsive Weeks-Chandler-Anderson (WGA) potential (Weeks et al. 1971) and then a full Lennard-Jones potential. Constant site density (0.4426 g cm³) and temperature (430 K) were assumed while studying two different concentrations of single site fractions of 0.3330 and 0.9375. The 5-site chains were formed to simulate the united atom model of pentane with σ_LJ = 0.3923 nm and ε{lunate}/k = 72 K. The spheres in the mixtures assumed the same values of σ_LJ and ε{lunate}/k. The bond lengths in n-pentane were fixed to be 0.153 nm while the bond angles were allowed to vary around 109.47° by a realistic potential. The attractive effects on the mixture system were studied by taking the difference between the full LJ and the repulsive WCA simulation at various concentrations. The results show a strong 'cooperative attraction' effect between chains especially at low pentane concentrations. Analysis of this effect in terms of different sites in the chain shows that the chain ends are the key to this phenomenon which is enhanced largely due to chain connectivity. Reference Interaction Site Model (RISM) theory was also applied to predict the repulsive potential correlation. Attractive effects were difficult to predict for the mixture system because the RISM method tended to overestimate the magnitude of the attractive effects. An empirical correction to RISM was therefore developed based on the comparison to fluid structures at low density for the pure fluids. With this correction, it is possible to predict the fluid structure with reasonable accuracy for all mixtures.