Working fluids play a relevant role in solvent screening, which is required to analyze the performance of cool and power generation. In this contribution, a solidly thermodynamic-based criteria has been developed in order to provide a rational guide, capable of qualitatively and quantitatively predicting the effects of adding solvents. The latter with the aim of modify the performance of a given fluid by the addition of key agents, thus seeking to induce or inhibit some desirable or undesirable properties. This development is rigorous and applicable directly to any model through systematic applications of Legendre transforms. Both, slope and curvature of the temperature vs. entropy boundary have been analytically developed and applied to binary mixtures. Moreover, a dimensionless function which depends only on configurational properties is presented. Foremost, this theoretical framework was applied to van der Waals fluids due to their qualitative accuracy. For this approach, the behavior of simple and azeotropic mixtures is shown and an untold phenomenon constituted by multiple isentropic transitions for a single mixture has also been described. For quantitative descriptions, PC-SAFT EoS has been selected in its simplest form in order to render the behavior of mixtures composed by linear hydrocarbons and fluorocarbons. (C) 2018 Elsevier Ltd. All rights reserved.