Critical constants of saturated aliphatic hydrocarbons

Through the use of group contributions the van der Waals' constants, a and b , were estimated for a number of saturated aliphatic hydrocarbons from a knowledge of the chemical structure of these compounds and were used to define the critical temperature and pressure of these substances. By the use of methane as the base group, both van der Waals' constants were estimated for a number of saturated aliphatic hydrocarbons of considerable size and complexity through the additive contribution of methyl groups in the seccessive substitution of hydrogen until the desired structure of the substance was obtained. For the normal saturated hydrocarbons these contributions were found to be additive for the evaluation of a 0.626 and b 0.76 up through n ‐octane, and these exponents have been assumed to apply in the scaling up of larger normal and isomeric hydrocarbon molecules for which experimental data are lacking. The volume van der Waals' constant b alone serves to define the critical volume of these compounds through the expression γ c = 3 β b , where β represents a factor which has been found to depend on the size and arrangement of the molecule. By following this procedure the critical temperatures, pressures and volumes of the normal saturated hydrocarbons through eicosane ( C 20 H 42 ), inclusive, and all the isomeric hydrocarbons up to and through the nonanes were calculated and compared, whenever possible, with values already available in the literature with an agreement of 0.43% for the estimation of the critical temperature, 0.69% for the critical pressure, and 0.86% for the critical volume. A combined consideration of these average deviations points to the estimation of the critical constants of the aliphatic saturated hydrocarbons with an average error of 0.7%.