| Summary: | We present a systematic study of the thermodynamic properties of a polydisperse fluid mixture. The size of the particles, σ, is assumed to be distributed according to a continuous distribution function f<sub>Σ</sub>(σ), for which we have chosen a Γ-distribution. The interatomic potentials are given by a hard core repulsion plus an adjacent attractive tail in the form of a square-well or a Yukawa potential; for the size-dependence of the attraction strength we have assumed different models. The properties of the mixture are calculated using the optimized random phase approximation (ORPA), a thermodynamic perturbation theory which is known to give reliable results in the case of simple liquids. To take into account polydispersity we combine the ORPA with the orthogonal decomposition technique where all σ-dependent functions (i.e., the correlation functions and the interatomic potentials) are expanded in terms of orthogonal polynomials p<sub>i</sub>(σ) associated with the weight function f<sub>Σ</sub>(σ).
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