A double square-well potential function is proposed to describe the interaction between unbounded particles. This model is constructed based on the Lennard-Jones potential function and has three adjustable parameters. Based on the two-layers local composition model, a coordination number model for this double square-well fluid is developed and then by using the generalized van der Waals partition function a new expression for the attractive part of equation of state is derived. Two new equations of state are presented by sum of the attractive term and repulsive expressions of Carnahan-Starling and van der Waals. These models have three adjustable parameters, which are obtained by simultaneously fitting vapor pressures and liquid densities of pure substances. The capability of these two models in correlation of the vapour pressure and liquid density and in prediction of the vapour molar volume and heat of vaporization of pure compounds is investigated. Good results obtained especially with the new EOS in which Carnahan-Starling repulsive term has been used. The results of this EOS are excellent even for large molecules such as long- chain alkanes from C10 to C20.

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