Equation of State of Acetic Acid Derived by Molecular-Dynamics Simulations on a Rigid-Molecule Model

Abstract

The equation of state of an acetic acid fluid was derived by molecular-dynamics simulations on a rigid-molecule model. The optimized potential for liquid simulations (OPLS) given by Briggs, Nguyen, and Jorgensen was assumed. Only the Z form was adopted in the two conformers in order to save central processing unit (CPU) time to perform the molecular-dynamics simulation at many state points. Microcanonical simulations were performed for 729 states. The system had 256 molecules in the cubic basic cell. The equation of state was obtained by a least-squares fitting. The critical point and liquid-vapor phase boundary were compared with the observed values. Dimerization was also observed by 27 long runs in the gaseous phase around the liquid-vapor phase boundary. The main structure is hydrogen-bonded chains in the liquid phase. The excess entropy after subtracting the ideal gas term is -10R at low temperatures. This is compared with -8R in liquid water. These comparisons show that the present model can be used to simulate the hydrogen-bonded fluid of acetic acid.