Enthalpies of vaporization of n-alkanes from the enlarged fused hard sphere model

An attempt is made to describe the state behavior of hard models of n-alkanes in terms of the geometry of hard convex bodies (HCB). In order to minimize “non-convexity” of hard models of n-alkane chains, the recently proposed enlarged fused hard sphere (EFHS) model is exploited. “The mean radii”, surface areas, and volumes of EFHS models of individual conformers were computed from a general analytical procedure. The bond lengths, hard sphere diameters of single groups and bond angles were taken from the recent study of Vega et al. The calculated geometric quantities of the conformers were used to evaluate the non-sphericity parameter and the packing fraction of the studied system and substituted into the HCB equation of state (EOS); the EOS was employed to evaluate the entropy change in a simple thermodynamic cycle. The entropy balance of the thermodynamic cycle allowed us to express the enthalpy of vaporization of individual conformers of a single n-alkane. The enthalpies of vaporization at the normal boiling point were evaluated for n-alkanes from ethane up to heptane. Fair agreement of the calculated enthalpies of vaporization with experimental data (deviations approx. 3% were found) indicates correctness of the EFHS models of n-alkanes in which no adjustable parameter was used.