ENTROPY OF ATTRACTIVE FORCES AND MOLECULAR NONSPHERICITY IN REAL LIQUIDS - A MEASURE OF STRUCTURAL ORDERING

Attempts are made to unravel the entropy of real liquids into its comp onent parts, a topic of general interest in liquid chemistry. The meth od is based on a comparison of the experimentally-determined entropies of vaporization of a liquid with the entropies of vaporization calcul ated using three different models for the reference systems: (I) hard spheres, (II) dipolar hard spheres, and (III) dipolar-polarizable hard spheres. For the first and second reference systems, accurate equatio ns of state are known. In the third case, a somewhat less accurate exp ression based on the mean spherical approximation (MSA) is available. The calculations are performed on a set of 87 liquids covering most of the chemically relevant solvent classes. The excess entropies thus ca lculated are a measure of the degree of order in real liquids, above t hat in the corresponding reference system. This excess order is the re sult of attractive forces (as in the case of strongly dipolar or assoc iated liquids) as well as more efficient packing of elongated molecule s, relative to hard spheres (as is the case for compounds containing l onger hydrocarbon chains). The appreciable excess entropy of the longe r-chain hydrocarbons, however, cannot be explained solely in terms of nonsphericity of the repulsive core. Other effects have to be invoked such as the intertwining of the chains. The numerical values of all th e excess entropies are critically dependent on the choice of the hard sphere diameter sigma, given that the molecules to be modeled are neit her hard nor spherical. A method that uses the liquid isothermal compr essibility as the parameter probing intermolecular repulsion as a rout e to the determination of sigma is employed and found to be consistent with an independent route based on inert gas solubilities.