Study of the Compressions of Several High Molecular Weight Hydrocarbons

Isothermal compressions were measured for thirteen high‐purity liquid hydrocarbons and two binary mixtures of liquid hydrocarbons. These hydrocarbons have a molecular weight range of 170 to 351 and included normal paraffins, cycloparaffins, aromatics, and fused ring compounds. The pressure range for these measurements was from atmospheric to as high as 10 000 bars, being limited to lower values for some compounds to avoid possible solidification of the liquid. The volume changes due to pressure were measured at six temperatures spaced about equally in the range 37.8°C to 135.0°C. The volume changes and pressures were measured by methods similar to those of P. W. Bridgman. Pressure‐volume isotherms can be described adequately by the Tait equation, v₀—v=C log(1+P/B), or for pressures above a certain minimum, whose value depends on the compound, by the Hudleston equation $$\log {\mathrm{[v}}^{\text{2/3}}{\mathrm{P/(v}}_0^{\text{1/3}}{\mathrm{-v}}^{\text{1/3}}{\mathrm{)]=A+B(v}}_0^{\text{1/3}}{\mathrm{-v}}^{\text{1/3}}\mathrm{).}$$ For the Tait equation the parameter C can be predicted for hydrocarbon liquids from the relation C=0.2058 v₀. Compressibility for a given hydrocarbon decreases with increasing pressure at constant temperature and increases with increasing temperature at constant pressure. The compression, and the compressibility, of liquid hydrocarbons are strongly dependent on molecular structure. Cyclization introduces a rigidity of molecular shape which decreases the compressibility markedly. Furthermore, fused ring cyclization as exemplified by naphthyl and decalyl structures has a considerably greater effect in decreasing compressibility than cyclization to nonfused rings such as cyclopentyl, cyclohexyl, or phenyl, even at equivalent carbon atom in ring percentages. Isobars and isochores were drawn and studied over the full range of temperature and pressure. The coefficient of thermal expansion, (1/v₀) (δv/δT) _P, for a given hydrocarbon, decreases with increasing pressure at constant temperature. (δ²v/δT²) _P undergoes a sign change at a certain pressure, whose value depends on the compound; (δv/δT) _P increases with increasing temperature below this pressure and decreases with increasing temperature above this pressure. The pressure coefficient, (δP/δT) _v, is not a function of volume alone but is also dependent on the temperature and pressure. (δE/δv) _T and (δE/δP) _T go to zero and then reverse sign for compounds that can be studied to sufficiently high pressures.