Combinatorial Entropy of Mixing Molecules that Differ in Size and Shape. A Simple Approximation for Binary and Multicomponent Mixtures

For liquid solutions, the combinatorial entropy of mixing is commonly calculated using either (a) the ideal-solution model (suitable for mixtures of simple spherical molecules) or (b) the Flory lattice model (suitable for mixtures of chainlike molecules). Based on the lattice theories of Huggins, Staverman, Tompa, and Lichtenthaler for molecules of arbitrary size and shape, this work proposes an adhoc generalization of Flory's formula for the combinatorial entropy of mixing in binary and multicomponent solutions. The generalized Flory model yields results that are always intermediate between those obtained from (a) and (b). Parameters re quired for the calculations are found from molecular structure data (bond angles and distances). Illustrative calculations are given for the system carbon tetrachloride – octamethylcyclotetrasiloxane.