STRUCTURE AND THERMODYNAMICS OF CHAIN MOLECULAR FLUID MIXTURES

NVT molecular dynamics simulations have been used to investigate the effects of attractive forces on the structure and thermodynamics of n-pentane in solution with spheres at various concentrations from 66.7% to 6.25% mole fraction of pentane and density of 0.4426 g cm-3 and temperature of 430 K. Simulations were performed using first the repulsive part of the potential as formulated by Weeks, Chandler, and Anderson (WCA potential) and then the full Lennard-Jones potential. The bond lengths for the n-pentane were constrained to be 0.153 nm, while the bond angles were allowed to vary slightly from the equilibrium angle of 109.7-degrees by a realistic potential. The LJ potential parameters were sigma(LJ) = 0.3923 nm and epsilon/k = 72 K for both the n-pentane sites and spheres. The effect of attractive forces on the fluid structure was pronounced and most significant at low concentrations of n-pentane. The chains clustered among themselves while cavitating the spheres at high concentrations of n-pentane, indicating microscopic separation of the two components. The chain-sphere correlation was suppressed at high concentrations of n-pentane and increased as the solution became dilute in n-pentane. This behaviour is compared with solute-solvent and solute-solute clustering of dilute solution of spheres in spheres, and spheres in diatomics near the critical point of the solvent.