Solvent properties of supercritical fluid depend upon their molecular physical dimensions and the extent of their molecular interactions. The nonideality of fluids in the supercritical region, to some extent, can be attributed to molecular associations which, in turn, affect solubility behavior. An association model in terms of cluster formation is developed to explain fluid compressibility behavior far above the supercritical region. The development incorporates a continuous specie association

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... together with mass balance to provide agreement with observed gas compressibility behavior. This approach incorporates two general characteristics of gas isothermal compressibility, large values at low pressures and small values at high pressures, into a simple two specie system which can account for fluid P-V-T behavior over wide ranges of temperature and pressure in the supercritical regions where such fluids find practical uses as solvents in certain commercial applications. Excellent agreement is obtained using reported compressibility data for methane in the ranges of 360-470 K and 0-1000 MPa.