Statistical thermodynamics of short chain molecular liquids: n-Nonane

Abstract

The hole theory previously discussed primarily in connection with amorphous high polymers and polymer melts is considered for systems with finite chainlengths. It is shown that the reduced equation of state approaches the limiting expressions for infinite chainlength only slowly. However, isotherms and isobars are superimposable over the whole range of molecular weights, except at the upper range of temperatures. Application of the theory to the detailed experimental information provided by Grindley and Lind on n‐nonane over a range of 120 deg and up to 8 kbar indicates a generally good agreement at atmospheric as well as elevated pressure. Only at moderate pressures and elevated temperatures appear numerically small but systematic deviations. These may be ascribed to a slow variation in the number of effectively external degrees of freedom, and hence of the scaling temperature with increasing temperature. Increasing pressures compensate for this effect. These discrepancies are also reflected in the behavior of the isothermal derivatives of the entropy and energy. The theory, however, predicts correctly the explicit dependence of these functions on temperature as well as volume.