THERMODYNAMICS OF CHAIN FLUIDS FROM ATOMISTIC SIMULATION - A TEST OF THE CHAIN INCREMENT METHOD FOR CHEMICAL-POTENTIAL

A formulation is presented for the calculation of the excess chemical potential mu(ex)(n(test)) of n(test)-mer chains mixed at infinite dilu tion with a bulk n-mer fluid and of the excess segmental chemical ex p otential mu(seg)(ex)=mu(ex)(n + 1) - mu(ex)(n) from detailed atomistic simulations. The formulation is applied for n(test) = 6 to 16 in n-he xadecane (C-16, n = 16) in the liquid (P = 50 atm) and vapor (P = 1.02 atm) states at T = 580 K using a configurational bias Monte Carlo (MC ) scheme. Two different reference states (ideal gas and continuous unp erturbed chains) are examined for the definition of mu(ex), and simula tions are conducted with two united-atom model representations from th e recent literature. In parallel, mu(ex) and mu(seg)(ex) with referenc e to the ideal gas are derived from two cubic equations of state (EoS) for the same systems and conditions. Both the MC and the EoS calculat ions for both models and reference states examined give a linear depen dence of mu(ex)(n(test)) on n(test), confirming that chemical potentia ls for long chains can be reliably estimated from small test chain and test segment insertions. This confirmation of the ''chain increment a nsatz'' is of great practical value for phase equilibrium calculations in long-chain systems. Predictions for the structure of the C-16 liqu id and vapor are in good agreement with existing experimental and simu lation evidence. Chain conformations in the liquid and vapor are indis tinguishable from unperturbed and ideal gas chain conformations, respe ctively. In lower temperature liquids (T = 450 K, P = 20 atm), inserti ons of long test chains cannot provide adequate sampling, but the chai n increment ansatz remains useful for estimating chemical potentials.