Statistical Thermodynamic Theory for the Melting of n-Alkanes from Their Rotator Phases

: Most liauid n-alkanes comprised of 9–43 carbons freeze to an equilibrium “rotator” phase a few degrees above the temperatures at which they fully crystallize. We develop statistical mechanical theory to account for the thermodynamic properties of melting from this rotator phase. Chain configurations are taken into account through the rotational isomeric state approximation. Volume-dependent interactions are approximated through adaptation of a classical equation of state for liquid alkanes. The anisotropy of the short-range repulsive forces, which must be the principal source of ordering in alkanes, is taken into account through appropriate modification of a chain pair potential. We also present a critical study of available experimental data. With a minimum of free parameters, the theory predicts enthalpies, volume changes, and temperatures of melting for the alkanes that have rotator phases at atmospheric pressure. For alkanes from 44 carbons to poly(methylene), which have no stable rotator phases at 1 atm, the theory predicts rotator-phase melting temperatures that are nearly identical with the experimentally observed crystalline melting temperatures.