Compressive Behavior of Some Globular Hydrocarbons and Their Effective Intermolecular Interaction Potentials

Abstract

The compressive behavior of neopentane, bicyclo[2.2.2]octane, and adamantane has been measured at room temperature in a piston cylinder apparatus. A phase transition was found in each system. The results for phase I (low pressure) for the average of compression and decompression runs were fit to an equation of the form − ΔV / V0 = a0 + a1P + a2P2. Literature results for methane (at 77°K) were also fit to this equation. The following results were obtained (per kilobar): methane, phase I, a0 = 3.64 × 10−3, a1 = 4.98 × 10−2, a2 = − 3.46 × 10−3, − ΔV / V0 (transition) = 0.022; neopentane, phase I, a0 = 0.000, a1 = 4.08 × 10−2, a2 = − 2.60 × 10−3, − ΔV / V0 (transition)=0.049 at 6.5 ± 0.5 kbar; bicyclo[2.2.2]octane, phase I, a0 = 0.000, a1 = 2.65 × 10−2, a2 = − 1.04 × 10−3, − ΔV / V0 (transition) = 0.022 at 9.1 ± 0.7 kbar. adamantane, phase I, a0 = 0.000, a1 = 2.59 × 10−2, a2 = − 2.04 × 10−3, − ΔV / V0 (transition) = 0.014 at 4.1 ± 0.5 kbar. The well-depth, minimum distance, and repulsive exponents of a “N-6” potential derived from heats of sublimation, crystal lattice parameters, and compressibilities are, respectively: methane, ε / k = 142°K, rm = 4.27 Å, n = 11.2; neopentane, 436°K, 6.32 Å, 16.0; bicyclo[2.2.2]octane, 737°K, 6.58 Å, 14.2; adamantane, 915°K, 6.82 Å, 13.8. The volume thermal expansions of phase I for bicyclo[2.2.2]octane and adamantane are 4.4 ± 0.2 × 10−4 °K−1 and 4.7 ± 0.5 × 10−4 °K−1, respectively.