Modeling thermodynamic properties of pure fluids and mixtures is a scientific and technological challenge.The simple model that used for hard chain molecules is freely jointed chain fluid. Each molecule consists of freely jinted tangent spheres, each sphere interacts with the other spheres through the hard sphere potential. In the first part of this thesis, the vapour- liquid equilibria of square-well chain fluid, at different well width, calculated and compared with molecular dynamic simulations. It is found that the efficiency of equation of state in predicting vapour-liquid equilibria improve by increasing the well-width. Also, the enthalpy of vaprozaition of square-well chain fluid at three different well width ( =1.3, 1.5, 1.8) calculated from obtained vapour pressure and compared with simulation data. In the second part of this thesis, to improve the prediction of the equation of state, An analytical expression for the average site – site radial distribution function of hard sphere chain fluid is derived. This expression is derived by Global fitting the numerical values of radial distribution function obtained from Chiew’s Integral equation. Using the obtained radial distribution function in the framework of equation of state, again, the thermodynamic properties of square-well and Yukawa chain fluids incluiding, compressibility factor, critical constants, vapour-liquid equilibria calculated and compared with simulation date. The results show that the using of radial distribution function improve the results considerably in comparssion with molecular dynamic simulation. The hard-sphere Yukawa chain (HSYC) fluid is a simple model for chain fluids and polymers in which each molecule is composed of freely-jointed tangent spheres. The spheres each have a hard core and an attractive interaction, written as a single Yukawa potential or as a sum of Yukawa potentials.
